w Uktfa Structural Guidance Platform New April 2011 20-09-2013 12.05.40

STRUCTURAL GUIDANCE FOR PLATFORM TIMBER FRAME UKTFA Special Project, May 2008

STRUCTURAL GUIDANCE FOR PLATFORM TIMBER FRAME2

Building Regulations RequirementRequirement from Part A3 of the Building Regulations 2000, updated 2004 for England & Wales:

‘The building shall be constructed so that in the event of an accident the building will not suffer collapse to an extent disproportionate to the cause’.

Requirement from Section C3 of the Technical Standards for compliance with the Building Standards (Scotland) - 6th Amendment 2001:

‘A building to which this standard applies must be designed and constructed so that in the event of damage occurring to any part of the building,

the extent of any resulting collapse will not be disproportionate to the cause of the damage.’

Code Requirement - relevant clausesThe following extracts are taken from British Standard

BS 5268-2:2002/Amd 1 August 2007

1.6.3 Accidental damage

1.6.3.1 General

In addition to designing a structure to support loads1 from normal use, there should be a reasonable probability that the structure will not collapse

catastrophically because of misuse or accident. No structure can be expected to be resistant to the excessive loads or forces that could arise from

an extreme cause, but it should not collapse to an extent that is disproportionate to the original cause.

TOPIC

REPORT DATE

KEYWORDS

PURPOSE

Design of Platform Timber Frame for Disproportionate Collapse provisions

February 2008

Platform Timber Frame, Disproportionate Collapse

BS 5268-2:2002/Amd 1 August 2007 includes new clauses providing guidance for complying with Part A3

of the Building Regulations 2000(updated 2004).

This Technical Note is aimed at interpreting this guidance as it applies to Platform Timber Frame buildings and

suggests methods for achieving compliance.

STRUCTURAL GUIDANCE FOR PLATFORM TIMBER FRAME

The general recommendations in 1.6.1.1 apply to all buildings. The measures required for each Class of building as defined in

the National Building Regulations or Standards are as follows:

a) Class 1 buildings: no additional requirements.

b) Class 2A buildings either:

1) Option 1. Effective anchorage of suspended floors to load-bearing walls in accordance with 1.6.3.2; or

2) Option 2. The provision of effective horizontal ties in accordance with 1.6.3.3.

c) Class 2B buildings either:

1) Option 1. Effective horizontal ties in accordance with 1.6.3.3 and vertical ties in accordance with 1.6.3.4; or

2) Option 2. Check for the notional removal of load-bearing elements in accordance with 1.6.3.5.

d) Class 3 buildings: The designer should carry out a risk assessment as required by the National Building Regulations

or Standards.

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Commentary1 The disproportionate collapse load case (as a result of misuse, accident or extreme cause) is never defined. It is a

theoretical ‘event’ that causes a force or removal of a load bearing element as defined in the clauses.


1.6.3.2 Effective anchorage of suspended floors

A suspended floor can be considered to be effectively anchored if the connection between the floor and load-bearing wall complies with either:

• Figure M.32; or

• BS 5628-1:2005, Annex D for timber floors supported by load-bearing masonry.

1.6.3.3 Effective horizontal ties3

All buildings should be effectively tied together at each principal floor level and at roof level. Horizontal ties should be provided as follows (see

also Figure M.I):

• Peripheral ties with a design capacity of 0.5F† should be provided around the whole perimeter of the building. Ties

should be anchored at external and re-entrant corners.

• Internal ties should be provided in two directions approximately at right angles. They should be effectively

continuous throughout their length and should be anchored at the periphery of the building. They may be

distributed evenly throughout the floor or may be concentrated at column positions. Internal ties should be

designed for a load of F†.

• External columns and load-bearing walls should be tied in by a tie perpendicular to the edge of the building. The

tie should be designed for the greater of Ft or 1% of the maximum design vertical dead and imposed load in the

column at that level. Corner columns should be tied in two directions approximately at right angles.

The basic tie force F† should be calculated as follows:

• For distributed ties: F† = 0.5(gk + qk)L kN/m but not less than 3.5 kN/m.

• For concentrated ties: F† = 0.5(gk + qk)S†L kN but not less than 10kN.

where

gk is the full dead load per unit area of the floor or roof (kN/m²).

qk is the full imposed floor or roof load per unit area (kN/m²).

s† is the mean spacing of ties transverse to the direction of the tie being considered (m).

L is the length of tie being considered (m).

When assessing the capacity of an element acting as a tie, in accordance with 1.6.3.8 or its connections in accordance with 1.6.3.9, the tie load

can be considered as an alternative load case to any other loads acting on that element.

2 Effective anchorage is provided by the horizontal structural junction strength of floor to wall connections as a ‘deemed

to satisfy’.

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3 Effective horizontal and vertical ties are the structural junction strengths of floor to wall connections as calculated.

The calculation formula for ties is based on similar force calculations taken from the light gauge steel industry and for the

design of restraint straps in masonry design codes. The tie force is an accidental load value.

Horizontal and vertical tie forces are not practical for most platform frame construction.

4 Key point: Notional removal relates to the imaginary removal of a defined area and is not an actual length of panel or length

of panel to a predetermined ‘weak junction’.

5 Key point: One wall at a time is notionally removed, not a number of walls together.

6 Some collapse is allowable within the limits set by the Building Regulations. ie 15% of the floor area of that storey or 70

m² whichever is the lesser and does not extend further than the immediate adjacent storeys. Storey area is the full building

plan area.

7 Vertical lateral restraints are structural walls of minimum width 1200mm and are not key elements.

1.6.3.4 Vertical ties

Each column or wall carrying vertical load should be tied continuously from the lowest to the highest level. The tie should be capable of resisting

a tensile force equal to the maximum design load received by the column or wall from any one storey. There should be an effective connection

between vertical ties and horizontal ties at each level.

When assessing the capacity of an element acting as a vertical tie, in accordance with 1.6.3.8 or its connections in accordance with 1.6.3.9, the

tie load can be considered as an alternative load case to any other loads acting on that element.

1.6.3.5 Notional removal4 of load-bearing element

The structure should be checked for the effect of the removal, within each storey, of each supporting column, or beam supporting column(s) or

load-bearing wall(s), or any nominal length of load-bearing wall, one at a time5, to ensure that disproportionate collapse does not occur. The portion

of the building at risk of collapse should not exceed the lesser of 15% of the floor area of that storey or 70 m2.6

If the area at risk exceeds the limits given then the column, beam or load-bearing wall should be designed as a key element in accordance with

1.6.3.6.

The nominal length of a load-bearing wall should be taken as:

• In the case of an external wall, the length measured between vertical lateral restraints7.

• In the case of an internal wall, the length measured between effective vertical lateral restraints but not exceeding

2.25h, where h is the height between horizontal restraints as shown in Figure M.2.

When considering the residual structure the loading should be as defined in 1.6.3.7. The capacity of any relevant elements should be calculated

in accordance with 1.6.3.8 and their connections should be calculated in accordance with 1.6.3.9.

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8 The principle of design for key elements or protected members is different from notional removal. Lengths of panel either

side of a key element are likely to be removed which may be significant leaving a ‘post and beam’ structural frame to

support the remaining building.

9 The wind loading requirement is to check against racking capacity of the structure after removal of a load bearing wall.

10 The permissible stress increase factor can be assumed to be a k3 = 2.25 duration of load factor.

1.6.3.6 Key element8

A key element should be designed for the accidental loading specified in BS 6399-1. Structural elements that provide lateral restraint vital to the

stability of a key element should also be designed as a key element. The accidental loading should be applied to the member from all horizontal and

vertical directions, in one direction at a time, together with the reactions from other building components attached to the member that are subject to

the same accidental loading but limited to the maximum reactions that could reasonably be transmitted, considering the capacity of such members

and their connections. The accidental loads should be considered as acting with the loads given in 1.6.3.7. The capacity of the element should be

calculated in accordance with 1.6.3.8 and its connections should be calculated in accordance with 1.6.3.9.

1.6.3.7 Design loads for the residual structure

When considering design of the residual structure the following loads should be considered, where appropriate:

• the dead load

• a third of the imposed load, except that in the case of buildings used predominantly for storage, or where the

imposed load is of a permanent nature, the full imposed load should be used.

A third of the imposed roof or snow load

• 100% of any ceiling storage loads

• a third of the wind load9

1.6.3.8 Permissible stresses for accidental load cases

When considering the probable effects of misuse, accident or particular hazards, or when computing the residual stability of the damaged structure,

the designer should normally multiply the values recommended in BS 5268-2 for all long-term permissible stresses by a factor of 2.25.10

6


11 The permissible fastener load increase factor can be assumed to be a k48,52 = 3.00 duration of load factor. The value is

higher for mechanical fasteners due to the factors already incorporated in the code.

1.6.3.9 Permissible fastener load for accidental load cases

When considering the probable effects of misuse, accident or particular hazards, or when computing the residual stability of the damaged structure,

the designer should normally multiply the values recommended in BS 5268-2 for all long-term permissible loads on fasteners by a factor of 3.011.

In the case of fastenings through particleboard the values recommended for long-term permissible loads should be increased by a factor of 4.0.

References to other Guidance1 UKTFA - Code of Practice for Engineered Wood Products 1st Ed. Jan 07

2 UKTFA - Technical Bulletin 3’ Design Guide for Disproportionate Collapse’ - March 2005

3 ‘Multi-storey timber frame buildings - a design guide’ - TRADA/BRE 2003

4 BS5268-2:2002 (Amd1) - August 2007

5 NHBC Technical Guidance Note - Part A3 Guidance - November 2004

Building Regulations RequirementsAll structures must comply with the minimum UK Building Regulation requirements for robustness. The building regulations address this aspect

of the design using the term ‘disproportionate collapse’. All buildings will be subject to the robustness check as required by Part A - Structure, of

the Building Regulations - amended December 2004.

Building Regulations Table 11 has classified buildings into 4 classes based on risk assessments dependent on the type of building and levels of

occupancy, as follows.

• Class 1, Single occupancy buildings from 1 to 4 storeys e.g. detached houses, town houses etc.

• Class 2A, Houses or apartments (residential nature), not exceeding 4 storeys.

• Class 2B, Houses or apartments and other residential buildings, exceeding 4 storeys but limited to 15 storeys.

Educational buildings not exceeding 15 storeys. Hospitals not exceeding 3 storeys.

• Class 3, Stadiums, sports grounds, or building subjected to high frequency of loading (crowd accumulation) etc,

subjected to full sensitivity analysis and has no specific mentioned design criteria.

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Class of Building for Disproportionate Collapse

Requirement A3 as a principle applies to all buildings. Previous regulations were aimed at specific heights of buildings. For example, for 5 storey

apartments disproportionate collapse provisions applied but for 4 storey apartments it did not.

The 2004 Regulations brings the UK in line with European Code proposals in terminology and while there are some differences, the general

5-storey or greater rule for disproportionate collapse-specific design still applies. For buildings of 4 storeys or less the code and regulations aim

at good practice to achieve robustness.

Some terminology refers to ‘progressive collapse’ which is a reference to what the design for disproportionate collapse is aimed at avoiding. This

document will refer to the term ‘disproportionate collapse’.

Reference should be made to NHBC Technical Guidance Note - Part A3 Guidance - November 2004(5) for complex buildings with

multiple uses to assess the relevant Class of building.

Elements of the building with differing uses or numbers of storeys may be classed independently for disproportionate collapse as long as they can

be shown to be independently stable for wind load effects.

Meeting A3 requirements

Disproportionate collapse is instigated by localized failure of one of the elements within the structure leading to significant failure of several floors

in the building. Disproportionate collapse can be reduced or minimized by providing some structural continuity (ties) within the elements of

structure or by ensuring a degree of structural ‘redundancy’ by considering notional removal of load-bearing elements.

BS5268-2:2002 (Amd1) - August 2007 guidance

The measures required for each Class of building are as follows:

Class 1: no additional requirement.

Class 2A: either:

• Option 1 - provision of effective anchorage of suspended floors to load-bearing walls as shown in Figure M3; or

• Option 2 - provision of effective horizontal ties as shown in Figure M1.

Class 2B: either:

• Option 1 - provision of effective horizontal and vertical ties as shown in Figure M1.; or

• Option 2 - check for the notional removal of load-bearing elements

Class 3: the designer should carry out a risk assessment as required by the Building regulations.

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Class 2A Buildings

The Building Regulations state that for Class 2A buildings, robustness will be achieved by providing effective horizontal ties, or effective anchorage

of suspended floors to walls.

In providing robustness for category 2A, minimum mechanical fixing specifications to provide anchorage of suspended floors to walls and notional

horizontal tying for platform timber frame structures are provided in UKTFA - Technical Bulletin 3’ Design Guide for Disproportionate

Collapse’ - March 2005(2) (see Appendix 1), UKTFA - CP for Engineered Wood Products 1st Ed. Jan 07(Fig 3.13) (1) and Figure M3

of BS5268-2:2002. (4)

For Class 2A buildings, the approach is to adopt good building practice of providing lateral restraint to walls and common anchorage details of

floors to walls. The design process will involve checking the capacity of the component interfaces (e.g. panel rail to soleplate, soleplate to floor

deck, floor joists to head binder and head binder to panel rail) against the variable horizontal wind forces. The timber frame designer should

therefore be providing a robust connection at each and every junction as part of the normal design process.

Fig 1: BS 5268 Figure M3 - exploded floor detail showing minimum nailing densities

Key points to note are:

• The fixings at each junction interface.

• The blockings at floor perimeters where joists are parallel to the wall.

Where these details are not applicable or cannot be adopted due to different framing arrangements, effective horizontal ties should be designed in

accordance with BS5268- 2:2002 Cl1.6.3.3. (4)

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Class 2B Buildings

The Building Regulations guidance state that for Class 2B buildings, robustness will be achieved by providing effective horizontal ties together with

effective vertical ties or by checking that upon notional removal of a load bearing wall (one at a time in each storey of the building) the building

remains stable and that the area of floor at any storey at risk of collapse does not exceed 15% of the floor area of that storey or 70sq.m, whichever

is the smaller, and does not exceed further than the immediate adjacent storeys.

Where the notional removal of lengths of walls would result in an extent of damage in excess of the above limit, then the use of a ‘key element’

design approach for an accidental design loading of 34 kN/sq.m applied in the horizontal and vertical directions (one at a time) to the ‘key element’

and any attached components (e.g. cladding) having regard to the ultimate strength of those components and their connections, should be adopted.

Consideration of notional panel removal and a ‘Sensitivity Analysis’ approach

In checking the robustness of timber frame buildings, Engineers are to apply judgmentbased thinking to the likely 3-dimensional structural

behaviour of a building, backed, where appropriate, with a 2-dimensional structural assessment of discrete elements. The TF2000 fullsize testing

has shown that this approach is conservative but appropriate to determining the robustness of platform frame construction in buildings such as

the medium-rise TF2000 building (3).

‘Multi-storey timber frame buildings - a design guide’ - TRADA/BRE 2003(3) provides guidance for the design process for Class 2B

buildings where notional removal of load bearing walls ispart of the design check to comply with Regulation A3:

A ‘Sensitivity Analysis’ should be carried out on primary supporting members to establish if their removal, one at a time in each storey, to check

that upon its removal the rest of the structure would bridge over the resulting lack of support, albeit in a substantially deformed condition, or that

the risk of collapse of the remaining structure due to the removal of the member is within the limits prescribed by the Building regulations.

If it is not possible to bridge over a missing member or to limit the area at risk, the member should be designed as a protected or key element.

Methods of Detailing for DC - Provision of ‘Bridging Elements’

The TF2000 test building provided proof of the inherent robustness and availability of secondary load paths in platform timber frame. Therefore,

sheathed walls with no openings designed to BS5268:Part 6.1 can be regarded as deep beams with vertical shear taken in the panel to panel

connections and tension taken out through the sheathing material in continuation with any timber framework across the panel junction e.g. rim

boards. Furthermore, the TF2000 tests demonstrated that the floor has additional strength through the transverse spanning capacity of the floor that

is supported on the walls parallel to the span ( 3).

It is possible to undertake structural calculations to prove that wall panels can be supported by ‘panel action’ but often large or numerous openings

occur, leading to a requirement for additional ‘bridging members’ to be provided as part of the robustness design.

Unfortunately the TF2000 tests were specific to that building floor and panel shape and size. The findings cannot be used as a general compliance

with the regulations and independent structural checks on buildings are required.

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The ‘Rim Beam’ Method

The provision of a continuous engineered timber ‘rim beam’ at every floor level (not generally required at roof level) at the end of all joist

spans ensures that structural continuity is achieved by providing vertical load transfer as a ‘bridging elements’ and horizontal continuity by

providing a nailing density at all interfaces in accordance with the recommendations of UKTFA - Technical Bulletin 3’ Design Guide for

Disproportionate Collapse’ - March 2005(2) and BS5268-2:2002 (Amd1) - August 200 - Figure M3. (4)

Fig 2: Indicative Rim Beam arrangement

This method allows joisted floor structures to be assembled in the factory as ‘cassettes’ with a ‘rim board’ used to connect the ends of the joists

together for transportation and which remains as a vertical load transfer element in the completed structure. A separate ‘rim beam’, which is usually

installed loose on site, spans between points of vertical lateral restraint (return walls) or ‘key elements’ and acts as a bridging member such that if

loadbearing walls are notionally removed between the walls or key elements, the resulting collapse will be limited to the maximum areas allowed

and any remaining structure will remain in place albeit with significant deformation being acceptable.

An example of the Rim Beam Structural Methodology is shown in Appendix 2

Intersecting return walls

Intersecting return walls must be of 1200mm minimum length in total (excluding framed openings). These walls can be non-load bearing in the

conventional sense but must be capable of transferring loads down through the structure. The use of lightweight partitions built off of floating floors

is not acceptable. The Rim Beams are supported at these wall intersections by corner stud groups. It is important that the Rim Beams supporting the

remaining structure have a full bearing on studs at the panel junctions and to achieve this, the wall panels should be lapped in the opposite manner

to the Rim Beams. If no stud clusters are present below the Rim Beam bearing, hangers or fixings are to be provided off of adjacent Rim Beams.

For external panels the minimum length of wall to be considered for notional removal is 2.4m, with no maximum length. Where the Rim Beams

cannot be designed to span the required distance between return walls, Key Element posts will be required to split the span of the Rim Beams.

For internal walls the maximum length of wall to be considered is 2.25H where H is the clear height of the panel between lateral supports.

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Rim Beam Design

The Rim Beams and their connections should be designed to withstand vertical loading comprising the full floor dead load plus one third of the

normal imposed load plus the weight of a single storey of wall plus any claddings or linings. A load duration factor, K3, of 2.25 and a deflection

limit of L/30 should be applied to timber members and a k48,52 = 3.00 for mechanical fixings.

Fig 3: Example Design of a typical Rim Beam supporting a floor and external wall only

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Assumptions for Rim Beam design

In designing Rim Beams, the assumptions listed below are applicable:

a) Rim beam design checks are carried out on the principal of notional removal of wall panels, one at a time, between

intersecting return walls ie: a wall is notionally removed, the structure is checked for adequacy and then the wall is replaced.

These checks are then repeated for other notionally removed panels.

b) For external panels the minimum length of wall to be considered is 2.4m, with no maximum length. For internal walls the

maximum length of wall to be considered is 2.25H where H is the clear height of the panel between lateral supports (the

top of the structural deck level below to the underside of the structural joist level above). For compartment walls, only one

leaf at a time is to be considered for removal.

c) Rim beams are designed to support the dead weight of the structure to be supported, 1/3rd of the imposed loads and a single

storey of wall panel with any supported claddings or linings, following a collapse event of the supporting wall panels below

being removed (one at a time). For this event, a duration of load factor of k3 = 2.25 and deflection limit of L/30 are applicable

for timber elements.

d) Rim beams are also to provide a horizontal tying action at all levels through the structure. The fixings presented for Category

2A buildings (BS5268-2:2002 (Amd1) - August 200 - Figure M3(4)) are the minimum fixings required for

robustness.

The Engineer is to determine the design approach for the Rim beam. The disproportionate collapse design does not require the building to be

serviceable after the event, merely safe for occupants to escape and emergency services to enter the building.

A typical Rim beam design can be seen in Fig 3.

Key Element design principles

The key element approach is not related to notional removal. The introduction of a key element is an alternative design approach. The difference

is easily explained when a key element is a column in a length of wall. This key element column is not considered as a lateral restraint to the wall

and a check of the notional removal of the wall either side of the column is not valid. The design of the column is for 34kN/sq.m in any horizontal

direction. The panels attached to the post on both sides are to be checked to see if they will remain attached to the column under this loading. If

they remain attached, then the column is designed to take the reaction from the wall panels as well as the load on the column itself.

Other considerations to achieve a ‘robust’ structure

The in-service design of the building must not be compromised by the disproportionate collapse design.

Key points to note are:

• A building should not be provided with intentional ‘weak’ points for notional panel removal. The term ‘notional’ is

deliberately used as a means of undertaking an imaginary design situation. The actual cause and practicality of

the ‘event’ is not defined or to be considered. Disproportionate collapse design is a methodology to enhance a

building robustness.

• As always, a check on the differential movement of the in-service condition of the building should be carried out.

• The support of external claddings should also be considered during an ‘event’.

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Appendix 1: Minimum recommended nailing densities to provide nominal horizontal tying for Class 2A& 2B buildings:

For conventional timber frame buildings of cellular plan form the UKTFA have recommended that the effective anchorage of floors to walls will be

achieved with a minimum density of nails as shown below.

Fig 4: Diagrammatic details of typical nailing density at all interfaces in accordance with the recommendations of BS5268: 2002

and UKTFA guidance dated December 2004

For more information regarding ‘robust’ junction connections for Platform Timber Frame buildings, refer to UKTFA Technical Note:

‘Robustness of platform timber frame and connectivity of the framing members.’

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Appendix 2: Disproportionate Collapse Philosophy - Example of Rim Beam Structural Methodology

Key to diagram:

E = external walls, I = internal load bearing walls, P = party wall (single skin), J = joists, H = clear height of the panel between lateral supports.

Design Check/D.C. Event 1: Notional removal of Internal wall panel I0 of maximum length 2.25H

Continuous joist spans J1-J5 avoid the need for rim beams on internal supports. On removal of the supporting wall I0 the joists act in double

span at each subsequent level and support the floor loads plus a single storey height of (now non-load bearing) wall panel I1-I5 supported off the

double-spanning joists. Ie. J1 supports I1, J2 supports I2 etc.

Design Check/D.C. Event 2: Notional removal of External wall panel E3 between intersecting return walls or defined key elements. (Party walls P0 to P5 similar)

Following removal of wall panel E3, unless the joists are ‘top-hung’ over the rim beam, joists J4 are assumed to collapse or cantilever and a check

should be carried out to ensure that the resulting floor collapse will constitute less than 15% of the floor area of that storey or 70sq.m, whichever

is the smaller. Rim beam R4 is designed to support panel E4 and floor joists J5 by ‘bridging’ over the notionally removed wall panel. Subsequent

rim beams R5 support wall panels E5 and roof joists J6. The rim beams are tied back to the floor diaphragm with the minimum nailing densities.

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Appendix 3: Disproportionate Collapse Philosophy - Cantilevered Joists Method

Notional removal of External wall panel E3 (Party walls similar) between intersecting return walls or defined key elements.

Following removal of wall panel E2 continuous joists J3 cantilever and support panel E3 (including any supported cladding). Subsequent joists

also cantilever and support a storey height of wall panel. A check should be carried out to ensure that there is sufficient holding down resistance at

the backspan of the cantilevered joists to resist uplift (especially at top storey level).

Continuous joists are designed to support a single storey of wall panel plus the full dead load plus 33% of the imposed loads on that floor. A

duration of load factor of k3 = 2.25 and deflection limit of L/30 are applicable for accidental load case.

1


Code Requirement - relevant clausesThe following extracts are taken from British Standard BS 5268-2:2002/Amd 1 August 2007

2.10.7 Deflection and stiffness

The dimensions of flexural members should be such as to restrict deflection within limits appropriate to the type of structure, having regard to

the possibility of damage to surfacing materials, ceilings, partitions and finishings, and to the functional needs as well as aesthetic requirments.

In addition to the deflection due to bending, the shear deflection may be significant and should be taken into account.

For most general purposes, this recommendation may be assumed to be satisfied if the deflection of the member when fully loaded does not exceed

0.003 of the span. For domestic floor joists, the deflection under full load should not exceed the lesser of 0.003 times the span or 14ζ mm, where:

ζ, = 0.86 for floors whose transverse stiffness is provided by the decking/ceiling.

= 1.00 for floors where there is additional transverse stiffness to that from the decking/ceiling.

This additional transverse stiffness may be provided by herringbone strutting or by blocking1 of depth at least 75% of the depth of the

joists or, in the case of transverse members which are continuous across the joists (i.e. joists with an open-webbed structure), by

timbers of depth at least 30% of the depth of the joists.

NOTE The 14ζ mm deflection is to avoid undue vibration under moving or impact loading. 2

Subject to consideration being given to the effect of excessive deformation, members may be precambered to account for the deflection under full

dead or permanent load, and in this case the deflection under live or intermittent load should not exceed 0.003 of the span

TOPIC

REPORT DATE

KEYWORDS

PURPOSE

Floor Serviceability Limits

February 2008

Platform Timber Frame, Floor joists, Deflection, Serviceability

Deflection of floors is classified as a serviceability issue. There have been investigations carried out by the

industry into the acceptable limits of floor deflection. In 2006 the NHBC changed their recommendations for

allowable deflection limits for I-joist engineered floor systems and BS 5268-2:2002 + Amd 1 updated

2007 has now also presented additional deflection criteria.

This report provides background to the changes and additional recommendations for floor serviceability

that Engineers may wish to follow.

The following notes provide some commentary on the interpretation of the new clauses:

1 The changes reflect the trend for blocking or strutting to be omitted on proprietary joist products. Although the British

Standard does not address I-joist or open web joists this clause is in fact aimed at this market.

2 This note refers to the fact that maximum

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The following extracts are taken from Eurocode 5 (BS EN 1995 -1 -1:2004) + UK National Annex:

NA to BSEN 1995-1-1:2004:

NA.2.5 Limiting values for deflections of beams [BS EN 1995-1-1:2004, 7.2(2)]

As stated in BS EN 1990:2002, Al.4.2(2), the serviceability criteria should be specified for each project and agreed with the client. 3 The

values in Table NA.4, which take into account creep deformations, are given for guidance.

Table NA.4 Limiting values for deflections of individual beams4

Type of member Limiting value for net final deflections of individual beams, Wnet,fin

A member of span, A member

l between two supports with a cantilever, l

Roof or floor members with a plastered or plasterboard ceiling l/250 l/125

Roof or floor members without a plastered or plasterboard ceiling l/150 l/75

NOTE When calculating' Wnet,fin W,fin should be calculated as Ufin in accordance with. BS EN 1995-1-1:2004,2.2.3(5).

NA.2.6 Vibrations in residential floors [BS EN 1995-1-1:2004, 7.3.3(2)] 5

NOTE For the value of the modal damping ratio, ζ, ,in BS EN 1995-1-1:2004, 7.8.1(3), a value of 0,02 has been found appropriate

for typical UK floors.

NA.2.6.1 BS EN 1995-1-1:2004, 7.3.3(2) is implemented nationally by using Table NA.5.

3 This allows for the Client to decide on the acceptable level of deflection appropriate to the building use and quality.

4 These limits are applicable to total deflection including creep deflection, something which the British Standard already

includes for. The two codes should not be mixed.

5 Vibration of residential floors is a complex area and one that requires a clear understanding of the construction mass and

product qualities.

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TOPIC

PARAMETER LIMIT

1,8 mm for l ≤ 4 000 mm

16 5OO/ l1.1 mm for l > 4 000 mm

where l = joist span in mm

for a ≤ 1 mm b = 180 - 60a

for a > 1 mm b = 160 - 40a

a, deflection of floor under a 1 kN point load

b, constant for the control of unit impulse velocity response

NOTE The formulae for b correspond, to BS EN 1995-1-1:2004, Figure 7.2. With a value of 0,02 far the modal damping ratio ζ, , the unit

impulse velocity response will not normally govern the size of floor joists in residential timber floors.

NA.2.6.2 The recommended limit on a may be compared with a corresponding floor deflection calculated as:

(NA.1) 1000 Kdist leq 3 Kamp ≤ a mm

48 (EI)joist

where

Kdist = proportion of point load acting on a single joist

leq = equivalent floor span in mm

Kamp = amplification factor to account for shear deflections in the case of solid timber and glued thin-webbed joists or joint slip hi the

case of mechanically-jointed floor trusses

(EI)joist = bending stiffness of a joist in Nmm² (calculated using Emean)

where

kdist = max Kstrut [0,38-0J08ln[14EIb / s4] ]

0,30

Kstrut = 0,97 for single or multiple lines of strutting, installed in accordance with reference NA.4.1, otherwise 1,0

(EI)b = floor flexural rigidity perpendicular to the joists in Nmm²/m

s = joist spacing in mm

leq = span, t, in mm, for simply supported single span joists = 0,9 1 for the end spans of continuous joists = 0,85 f for the internal

spans of continuous joists

Kamp = 1,05 for simply-supported solid timber joists

= 1,10 for continuous solid timber joists

= 1,15 for simply-supported glued thin-webbed joists

= 1,30 for continuous glued thin-webbed joists

= 1,30 for simply-supported mechanically-jointed floor trusses

= 1,45 for continuous mechanically-jointed floor trusses.

(EI)b is calculated as the flexural rigidity of the floor decking perpendicular to the joists, using

Emean for E. Discontinuities at the edges of floor panels or the ends of floor boards may be ignored.

{

19

Table NA.5 Limits for a and b in BS EN 1995-1-1:2004 expressions (7.3) and (7.4)


(EI)b may be increased by adding the flexural rigidity of plasterboard ceilings fastened directly to the soffit of the floor joists, assuming Eplasterboard =

2000N/mm².

(EI)b may be increased for open web joists with a continuous transverse bracing member fastened to all the joists within 0,1l of mid-span, by adding

the bending stiffness of the transverse member in Nmm² divided by the span l in metres.

The fundamental frequency f1 should not be less than 8 Hz unless a special investigation is made. In BS EN 1995-1-1 expression 7.5 the mass of

the floor should be the permanent actions only without including partition loads or any variable actions.

In calculating the equivalent plate bending stiffness (El) of floors, in which the decking is adhesively bonded to the joists, no allowance should

be made for composite action unless the floor is designed in accordance with 9.1.2 and with adhesives meeting the requirements of 3.6 and the

detailing and control provisions of 10.3.

IntroductionThe change in BS 5268-2: 2002 clause 2.10.7 Deflection & stiffness reflect changes by earlier NHBC guidelines. The NHBC changes occurred

before in depth research. The BS 5268-2 changes followed summary recommendations by Trada and the Code Committee.

Previous research into floor serviceability

It is known that Trada Technology Ltd. were commissioned by the NHBC to develop a simple design approach for lightweight floor systems using

engineered timber joists that would ensure an in-service performance for properly constructed floors comparable to that of traditional floors made

with solid timber joists. UKTFA have discussed the history of research carried out into deflections of floor joists with Trada and this guidance

provides a summary of this research.

Research was based mainly on Eurocode 5 and on five overseas sources of information, which related design methods for I-joisted floors to user

satisfaction. It was possible to compare the results of the design methods studied with the results given by BS 5268 and EC5, and to adjust the latter

two where it appeared that they deviated from the consensus view of other researchers, while still maintaining overall performance levels similar to

those which have proved acceptable in the UK for floors made with solid timber joists and strutting up to 4 meters in span.

As a result two sets of design recommendations have been made, one for designs based on BS 5268, and the other for designs based on EC5.

Design recommendations and advice

For design to BS 5268:

For designs to BS 5268 it was concluded that:

(i) When strutting is omitted from floors in which it would normally be fitted in accordance with current best practice, the

14 mm deflection limit under dead + imposed load should be reduced to 12.6 mm.

(ii) In addition, the stiffness required for all forms of timber joist should be increased for spans from 4 m to 8 m by a factor

increasing from 1.0 to 1.4 across this range.

(iii) No other special treatment for I-joists is required. Hence, while BS 5268 continues to be used, the following

recommendations are made for the design of floors made with solid timber or prefabricated glued I-joists:

20


Hence, while BS 5268 continues to be used, the following recommendations are made for the design of floors made with solid timber or

prefabricated glued I-joists:

The combined instantaneous bending and shear deflection of a single joist measured in mm should not exceed the lesser of 0.003ℓ and ulim where:

ℓ = joist span in mm

ulim = 18 - L mm for joists with strutting in accordance with current best practice

= 0.9(18-L) mm for joists without strutting

where L = joist span in m.

For design to Eurocode 5

Under Eurocode design protocols, deflection limits are advisory and should be agreed by the designer and client at the beginning of the design

process. The guidance given in the UK NA to EC5 is therefore advisory. In the light of this project(3) TRADA recommends that timber floors in the

UK should be designed to Eurocode 5 and its National Annex, but with the following changes.

(i) The point load deflection limit for spans above 4000 mm should be tightened to 131030/ℓ1.35.

(ii) While I-joist designers may prefer to use the final deflection limits given in the EC5 NA when calculating span tables in

order to be able to claim that their designs are in accordance with the UK NA, for everyday office design it is recommended

that curvature deflection limits on domestic floors be calculated as in BS 5268 in order to relate the limits more closely to

research results, and to reduce design time and the possibility of errors. The effect on floor joist stiffness will be very small.

(iii) For floors in which one end of the joists is supported on a beam the frequency of vibration of the floor system as a whole

should be calculated as:

f1,system = √( f²1,joist x f²1,beam/ f²1,joist + f²1,beam)

Where the frequencies of the joists and beam f1,joist and f1,beam, are calculated as stated in EC5, but using the stiffness of

a joist or beam and the mass of the floor supported by the joist of beam without imposed load.

Where a beam is securely attached to a permanent partition above or below it, it may be possible to regard it as a deep beam

which would have a very high fundamental frequency. In this case it could be considered to be a rigid support when

calculating the fundamental frequency of the floor as a whole.

21


Other points for consideration

(i) Increased stiffness

It is likely that perceived floor performance would improve if the resulting joist bending stiffness at all spans were increased

by an additional factor of 10%.

(ii) Site installation

It is recommended that continuing efforts be made to ensure that floor joists, decking, plasterboard and (where fitted)

strutting are installed correctly in properly conditioned members free from building dust and debris, since squeaks and

creaks are one of the most common causes of complaints about floors.

(iii) Multiple span joists

There are difficulties in maintaining close tolerances in multiple span joist supports. Particular care should be taken to

ensure that intermediate supports on multiple-span joists are installed at the correct height, with any necessary packing

being of adequate strength and stiffness. It is recommended that for multiple span joists the span ratios are kept

approximately equal to prevent short span uplift, especially where joists are not built-in but are supported in joist hangers.

(iv) Span table options

It is recommended that if prefabricated joist manufacturers wish to publish span tables giving options for more than one

performance level, then the “10% better” and “20-% better” spans should be calculated in accordance with the tightened

Eurocode 5 point load limit of 131030/ℓ1.35, and then be reduced by 0.90.25 and 0.80.25 respectively, i.e. to 0.974ℓ for

the “10% better” and 0.946ℓ for the “20% better” options. These reductions are broadly equivalent to increasing the joist

stiffness by 10% and 20% respectively.

(v) System deflection

It is recommended that for joists supported by a beam at one end which is also subject to deflection, then the combined

instantaneous bending and shear deflection of a single joist measured in mm should not exceed the lesser of 0.003ℓ and

ulim where:

ℓ = joist span in mm

ulim = 18 - L - (ubeam/2) mm for joists with strutting in accordance with current best practice

ulim = 0.9(18 - L - (ubeam/2)) mm for joists without strutting

where L = joist span in m

and ubeam = the combined instantaneous bending and shear deflection of the beam at the connection.

22


Code Requirement - relevant clausesRobustness of design is implied in the BS 5268 - 2- 2002(2007 edition) and referred to in the Eurocodes. However robustness as a design

principle is not always followed. The following provides clause references that should be considered.

BS 5268 - 2- 2002 - clause 1.6. ‘Design Considerations’ states the following:

1.6.1.1 The design and details of parts and components should be compatible, particularly in view of the increasing use of prefabricated

components such as trussed rafters and floors. The designer responsible for the overall stability of the structure should ensure this

compatibility even when some or all of the design and details are the work of another designer.

To ensure that a design is robust and stable

a) the geometry of the structure should be considered;

b) required interaction and connections between timber load bearing elements and between such elements and other parts of

the structure should be assured;

c) suitable bracing or diaphragm effect should be provided in planes parallel to the direction of the lateral forces acting on the

whole structure.

In addition, the designer should state in the health and safety plan any special precautions or temporary propping necessary at each and every stage

in the construction process to ensure overall stability of all parts of the structure.

1.6.1.2 With regard to the design process, design, including design for the construction durability and use in service, should be considered as

a whole.

NOTE Unless clearly defined standards for materials, production, workmanship and maintenance are provided and complied with the design

intentions may not be realized.

TOPIC

REPORT DATE

KEYWORDS

PURPOSE

Robustness and floor to wall connectivity of Platform Timber Frame

February 2008

Platform Timber Frame, Robustness, Fixings, Connectivity, Typical details, Disproportionate

Collapse, stability and serviceability.

To provide platform timber frame Engineers and Designers with guidance on the principles of robustness and

connectivity of walls to floors for a robust construction.

23


1.6.1.3 With regard to basic assumptions covering durability, workmanship and materials, the quality of the timber and other materials, and of

the workmanship as verified by inspections, should be adequate to ensure safety, serviceability and durability.

BS 5268 - 2- 2002 - clause 1.6.3 ‘Accidental damage’ refer to Technical Report ‘Design of Platform Timber Frame for Disproportionate

Collapse provisions’ for interpretation of this clause relating to accidental damage.

BS 5268-6.1:1996 clause 4.4 ‘Stability’ - refer to Technical Report ‘Stability of platform Timber Frame’ for interpretation of the new code

requirements for stability design of platform timber frame structures.

References to other Guidance1 ‘Multi-storey timber frame buildings - a design guide’ - TRADA/BRE 2003

2 BS5268-2:2002 (Amd1) - August 2007

3 Trada Technology Timber frame housing: UK Structural recommendations 2006

4 BS 5268-6.1:1996 incorporating amendments Nos. 1&2.

5 BS 5268-6.2:2001

6 Trada Technology - Timber frame construction: 3rd edition: 2001

Introduction‘Robustness’ of platform timber frame is the ability of the structure to withstand a range of variations in the predetermined design and construction

circumstances without sustaining loss of function or requiring remedial work.

In this document the terms Engineer and Designer are used to mean the following: Engineer - A suitably qualified Structural Engineer/Timber

Frame Engineer who is responsible for the numerical calculations associated with the design of the superstructure to resist the applied loadings.

Designer - A suitably qualified Structural Technician/Timber Frame Production Engineer who is responsible for the production of general

arrangement and fabrication drawings which are required to manufacture and build the structure in accordance with the Engineer’s design.

Whilst robustness and floor to wall connectivity are related subjects, this guidance is separated into two parts. Part 1 considers robustness as a

design principle and philosophy. Part 2 considers the connectivity of joists to walls as an example of good practice to be achieved at a

wall to floor junction.

24


Part 1 - Principles for achieving a robust building

Robustness for ‘serviceability’

The guidance contained in this report is about the provision of robustness for a structure during ‘normal’ use. The structure can be said to be ‘robust’

when it is not sensitive to slight variations in loading or as-built details or as a result of constructional tolerances. Robustness in this case is to

ensure the structure remains serviceable. This is in contrast to the new BS 5268 Part 2 clause on Disproportionate Collapse which is an ultimate

limit of robustness; i.e. the building exhibits a degree of robustness against collapse but can be unserviceable after the event causing the loss of

support.

A typical example of robustness is the ability of the structure to continue to function if, say 10% of the nails required to fix a junction have been

incorrectly installed.

TOPIC

PRINCIPLES ON BUILDING ROBUSTNESS PLATFORM TIMBER FRAME

Select a structural form which has low sensitivity to the hazards

considered.

Avoid as far as possible structural systems, which may collapse

without warning.

Provide structural forms that can be tied together.

Select a structural form and design that can survive adequately the

accidental removal of an individual element or a limited part of a

structure, or reasonable localised damage.

Ensure that layouts and plan arrangements provide returns and

intersecting walls and floors.

Adopt compatible materials used in the structure and ensure

adequate interaction.

Platform timber frame is inherently robust through the

interconnectivity of walls and floor panels. History of use, tests

and research has shown that correctly built frameworks achieve a

significant level of robustness.

The full range of the hazards or risks should be considered as

platform timber frame continues to be used for challenging

structures.

As new products for floors and walls get introduced into the build

process the structural integrity and ability to ensure robust

connections should be questioned at all stages.

Disproportionate collapse design principles have been established

and used successfully - see ‘Multi-storey timber frame

buildings - a design guide’ - TRADA/BRE 20031)

Platform timber frame is a structural form specifically for cellular

layouts and plan aspects greater than 2:1 will require additional

design and robustness to ensure its suitability as a building

solution.

Connectivity of materials used in the build process is essential

and normal timber frame components provide easy methods of

fastening together.

25


The factors of safety applied in the engineering design have long been claimed to be the process whereby robustness is achieved. However, while

this is partially correct, there are other elements of robustness that fall outside of the design code factors of safety. One of the fundamental principles

of robust engineering and design is to ensure practical, ‘buildable’ solutions are applied, taking into account the potential risks in the build process

(Note that risks are known or feasible events and not the unspecified events applicable to disproportionate collapse design). An example of a robust

design solution would be to detail all elements so that they can only physically be installed in the correct orientation or location so that errors

in interpretation can not be made. One of the mantras of robust design is that if a detail can be interpreted in more than one way it is not robust.

Platform Timber Frame design and build process and the potential for errors

To achieve robustness of a structure the platform timber frame design and build process needs to be understood and an appreciation of ‘good

practice’ is also required.

The platform timber frame process can be considered as comprising the following stages:

A Engineer to design the framework and assembly based on Code criteria and good practice.

B Designer to translate the Engineer’s solutions and guidance into fabrication drawings and erecting plans.

C Fabricator to construct the assemble components and package for transportation.

D Erector to assemble the comments into a framework on site that forms the structural shell of the building.

E Follow-on trades complete the building by dressing the structural shell with services and cladding.

At each stage in the process a lack of clarity of information and potential for misinterpretation can cause errors in the build which in turn can be

considered a lack of robustness. At each stage of the process the leader, at that point of the process, has to determine if the solution being

presented is robust. Robustness is not always a calculated engineered value or deliverable. Robustness is more to do with practical solutions that have

minimal risk of not being achieved in the final assembly and build of the structure.

The responsibility for robust solutions rests with the Engineer for the structural concepts and framework solution and with the Designer for the

appropriate details, clarity of information and ability to identify errors in the virtual on-screen build process.

Engineering Robustness

The Engineer will undertake calculations for the structure and detail junctions to transfer the applied forces with a predetermined factor of safety.

The Engineer will check a structure and its components under four conditions:

1 Standard design: to withstand the applied forces and to attain the agreed level of serviceability in accordance with the code

requirements by adopting the factors of safety within the codes.

2 Construction period design check: to ensure that the structure is capable of withstanding the construction loadings

and to be stable during various stages of the construction process.

3 Robustness for Serviceability: to ensure that the detail design of the assemblies and frameworks are compatible with

each element and that the junctions of members can be fitted and secured safely and practically.

4 Design against ‘Ultimate robustness’ or Disproportionate Collapse: to ensure that the building has sufficient

strength in accordance with the rules for the type and scale of building.

Condition 3 is where the robustness of the proposed engineering solutions is to be considered by the Engineer.

26


Robustness Checking for the Engineer

Each project will be specific to its own criteria but as a guide the following are examples of the checking procedure to be adopted:

a Are the fixing requirements clear and appropriate for the project?

b Are the junction details checked for the applied forces eg - sole plate junctions, floor to wall junctions, roof to wall

junctions?

c Has the compatibility of elements been considered - eg cladding to frame interface.

d Are the details for assembly of components presented? e.g. are fixings and assembly instructions clear.

e Are minimum requirements for mechanical fixings achieved and where additional fixings are required, is this clearly marked

e.g. areas of high racking forces clearly noted?

f Are areas of high risk of failure noted and the necessary information translated to the designer and client for possible

‘designing-out’ of these risk areas, e.g. where multiple trade components come together to form the structure without ad

equate coordination or checking by a structural engineer.

g Material specification for durability - e.g. is the correct preservative treatment or detailing to avoid moisture present?

h Component compatibility for shrinkage - e.g. will differential movement create additional stresses to the members

and framework?

i Component connectivity to other members and finishing trades e.g. is there sufficient width of stud for the cladding fixings?

j Is there more than one way of interpreting the construction drawings?

Design Robustness

The Designer will take the Engineer’s information and translate the information onto fabrication and erection general arrangement drawings. The

Designer has the unique ability to review the building in a virtual build sequence. Structural connectivity of the frameworks should be checked at

this virtual build stage.

The Designer may adopt standard details but it is essential that the Engineer has approved these details specifically for the project.

The Designer should check the details for the following:

a Practical alignment of the structural frames and report on areas of misalignment or lack of support.

b All details at junctions or references to standard details are provided.

c The drawings have references to all special items as instructed by the Engineer.

d Ensure co ordination of information from different engineering teams working on different aspects of the structure and

build items.

e Review details to ensure that there is no ambiguity or lack of information for the fabricator and erector.

27


Part 2 - Connectivity of the Framing members floors to wallsIntroduction

This section refers to a number of standard good practice connectivity details that are required to comply with the updated BS 5268 Parts 6.14) and

22), 2007. In particular, Figure M3 in BS 5268 Part 2 provides a ‘deemed to satisfy’ detail for robust horizontal tying of floors to walls.

For all platform timber frame structures, the approach is to adopt good building practice of providing lateral restraint to walls and common

anchorage details of floors to walls. The design process should involve checking the capacity of the component interfaces (e.g. panel rail to

soleplate, soleplate to floor deck, floor joists to head binder and head binder to panel rail) against the variable horizontal wind forces. The timber

frame designer should therefore be providing a robust connection at each and every junction as part of the normal design process.

Examples of ‘Good Practice’ to assist robust Platform Timber Frame

The following figures provide typical construction detailing between timber platform frame components for projects from 1 to 7 storeys to ensure

minimum levels of robustness. The Engineer can specify more or less fixings depending on the specific project criteria.

Index to Details:

Figure

1 Softwood joists - Wall/Floor intersection: exploded view based on Figure M3,BS 5268 -

2 2002

2 Softwood joists - alternative details at internal wall supports applicable to ‘loose’ floor construction.

3 Softwood joists - alternative details at load-bearing/Racking walls.

4 Softwood joists - details at Beams.

5 I-Joist Floors - Typical wall/floor intersection: exploded view

6 Open-web Floor joists - Typical wall/floor intersection: exploded view

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TOPIC

REPORT DATE

KEYWORDS

BACKGROUND

PURPOSE

Technical Report - Stability of Platform Timber Frame

February 2008

Platform Timber Frame, Racking Overturning check, Sliding

resistance check

The British Standard for the design of platform timber frame buildings is covered by BS 5268-6.1:1996 and

BS 5268-6.2:2001. The November 2007 edition of BS 5268-6.1:1996 has been updated to take account of

experience with this type of construction and the issue of relevant European standards. The scope has been

extended from 4 storeys to 7 storeys. The code contains significant clause changes affecting how the stability of

timber frame racking walls are to be considered for buildings in excess of three storeys tall.

This document provides guidance on the use of the new BS 5268- 6.1:1996 Clause 4.4 Stability including a

worked example for atypical four storey building.

The change in design approach to stability required by the new clauses is significant for dwellings above three

storeys in height.

In principle, the design approach for dwellings of three storeys or less and with a maximum height to width ratio

of 2:1 is as the previous edition of the code and it is accepted that ‘whole building stability’ can be adopted.

However, to keep in line with European codes and other material standards, an increased factor of safety of 1.4

against building overturning and sliding resistance is required.

To provide guidance and a worked example for the use of theStability clauses contained within the revised

BS 5268-6.1:1996 - Structural use of timber. Code of practice for timber frame walls. Dwellings not exceeding

seven storeys (AMD 9256) (AMD 17381) reissued November 2007.

The new code supersedes BS 5268-6.1:1988. Amendment 9256 dated June 1996. Amendment 17381 dated

November 2007.

35


Building Regulations or Code Requirement - relevant clausesThe following extracts are taken from British Standard BS 5268-6.1:1996 (+ Amnd No.1 & No.2)

4.4 Stability

4.4.1 General

The designer should ensure overall building stability by checking that it has adequate racking overturning1 and sliding resistance to lateral loads.

These checks should be made at critical levels for the completed building and for the various construction stages, when subjected to dead load,

zero imposed load, and both horizontal and vertical components of the wind load. 2

Stability is generally obtained from racking walls, set in two orthogonal directions. Unless demonstrated otherwise, walls with significant openings,

for example doors, should be considered as separate discrete walls. The racking resistance of each wall should be calculated in accordance with

4.7 for each direction.

4.4.2 Overturning

4.4.2.1 General

Subject to the limitations in 4.5, it may be assumed that floor diaphragms are capable of distributing the wind load to each racking wall in

proportion to its racking resistance. Due account should be taken of any significant eccentricity between the centroids of the wind load and the

aggregated wall racking resistance. 3

Commentary The following notes provide some commentary on the interpretation of the new clauses:

1 ‘Racking overturning’ is a deliberate statement to instruct the Engineer to consider the stability aspect of the walls that are

designed to carry lateral loads through the building (ie the chosen racking walls). Previously ‘whole building stability’

considered the racking forces to be evenly distributed through the structure such that the overturning and sliding forces

are shared throughout the assumed ‘boxtype’ structure

2 Engineering checks should be carried out at various build stages. The most critical loadcase is likely to be at full frame

completion but without roof finishes, plasterboard weight and floating floor or ceiling construction.

3 It is common sense that Engineers need to consider the distribution of lateral loads through asymmetric buildings to

individual racking walls. The Engineer is to consider the diaphragm action of the floors and roofs to distribute the forces

to the racking walls. Section 4.4.2.1 allows for the ceiling/decking of standard Platform Frame construction to provide this

diaphragm action without further checking for a maximum aspect ratio of 2:1.

36


The stability of each racking wall should be checked at the base as follows:

a) The overturning moment is the product of the apportioned wind load and the vertical distance between its centroid and the

wall base.4

b) The overturning resistance of a wall is the product of the dead load (reduced by any vertical component of the wind load)

and the horizontal distance between its centroid and the leeward corner.5

Additional dead load from return walls, where present, can be utilized but should be limited to an outstand distance equal

to the panel height or the distance to a door or window opening, whichever is the lesser (small openings as defined in 4.9.4

may be ignored). The connection between the return wall and the racking wall should be designed to transfer the shear loads

based on the resultant applied design Forces. 6 Tension fixings may also be used to mobilize dead load from the underlying

construction and their capacity added to the dead load as a contribution to the overturning resistance.

c The factor of safety of a racking wall against overturning is defined as the overturning resistance divided by the overturning

moment. For each racking wall, under its apportioned wind load, the factor of safety should be > 1.2.7

The factor of safety of the total racking wall resistance, under the total wind load, should be> 1.4.8

4 Key point: the lever arm for overturning checks is the vertical distance to the centroid of the lateral load and not the height

of the wall panel.

5 Engineering statics are used to calculate the inherent panel overturning resistance. Additional restoring forces can then

be considered as in the remainder of the clause.

6 The weight of walls perpendicular to the racking wall can be used within the limits given and when fixed appropriately to

the racking wall.

7 All racking walls require a minimum factor of safety of 1.2 against overturning.

8 The increased factor of safety of 1.4 for the aggregated overturning resistance of all racking walls brings the BS in line

with Eurocodes.

Key point: The factor of safety for racking resistance itself (ie FOS=1.0) remains unchanged with the factor of safety

inherent within the design values for basic racking resistance Rb from Table 2.

37


4.4.2.2 Overturning for dwellings of three or less storeys

For dwellings of three or less storeys, and with a maximum height to width ratio of 2:1. the overturning resistance of the building may be determined

as the product of its total dead load (reduced by any vertical component of the wind load) and the horizontal distance between the load centroid and

the leeward edge. The factor of safety, as defined in 4.4.2.1c) should be > 1.4.9

4.4.3 Sliding

The designer should ensure that there is a factor of safety of 1.4 against sliding at the top and bottom of each racking wall, and at sole plate level.

Friction, under dead load only, may be used in conjunction with metal fasteners when calculating the resistance to sliding10.

The coefficient of friction between timbers in contact or on the underside of the soleplate may, in the absence of other information, be taken as 0.3.11

4.5 Horizontal diaphragms

The design method for timber frame walls given in this British Standard assumes that, for the range of dwellings covered, the normal construction

of floors and roofs provides adequate diaphragm action, provided that, in the case of intermediate floors, a floor deck or sub-deck is fixed directly to

the top faces of the joists, or the floor is braced bysome other means. In the case of pitched roofs it is assumed that the plasterboard ceiling under

the roof, together with the roof bracing recommended in BS 5268-3 is sufficient to transfer applied wind forces to the resisting walls.12

9 For dwellings of three storeys or less the ‘whole building approach’ for checking overturning is acceptable - (see Note 1).

Note: The UKTFA recommend that for dwellings exceeding three storeys, a ‘whole building check’ for overturning should

still be carried out and the factor of safety should be >1.4.

10 Note: The BS allows friction and mechanical fixing capacity to be considered together. When considering the use of

mechanical fixings, the British Standard published values for timber to-timber fixings already include for a factor of safety

of 1.4. The UKTFA considered that it is not appropriate to apply an additional factor of safety for the mechanical fixings.

Alternatively the ultimate mechanical fixing capacities could be used with a factor of safety applied. For fixings into

foundation materials, the Engineer should ensure that a factor of safety > 1.4 is achieved.

11 The UKTFA consider that the coefficient of friction is 0.4 as an unfactored value.

12 The Engineer is to ensure the effectiveness of diaphragms for different construction types to transfer the horizontal loads

to the racking walls.

38


Due account should be taken of the eccentricity of the loading in relation to the wall panels providing resistance.13

References to other Guidance1) The Institution of Structural Engineers/TRADA - Manual for the design of timber building structures to Eurocode

5 - December 2007

For a worked example of overturning, sliding, racking and roof uplift checks for a dwelling of three storeys or less reference should be made

to 2) Trada Technology - Timber Frame housing: UK Structural recommendations 3rd ed.2006 Section 7.3 Overall Stability

calculations, except that an increased factor of safety of 1.4 for overturning and sliding should be adopted in accordance with the new code

requirements.

Overturning and Sliding worked example for 4 storey dwelling

The following worked example indicates the procedures to be adopted for checking timber frame buildings of more than three storeys in accordance

with BS5268-6.1:1996 (AMD 17381) Cl 4.4.2.1.

Overall Stability of Platform Timber Frame

4-Storey Building Worked Example to BS5268-6.1:1996 (incl Ammendments 1 & 2) November 2007 edition.

Consider the example of a residential building shown below:

13 This clause refers to the need for Engineers to consider the distribution of racking walls throughout a building (see

Note 3) ie the racking walls cannot be positioned on one side of a building without providing racking walls at right angles

to resist the resulting torsional effects.

Roof

Third

Second

First

Ground

Fig 1. Typical Section

Wall 2

Wall 1

Wall 1

Wall 2

Fig 2. Plan on typical storey

Width b (m)

Wind Direction 2

l/2

Span

l1

l/2

Win

d

Dire

ctio

n 1

Jois

t Spa

n

dire

ctio

n

Wal

l 3

Span

l2

l/2

Wal

l 3

Span

l3

l/2

h 2h 4

h 3

h sto

rey

/2

Floor Loads

Roof Loads

Floor Loads

Floor Loads

h 1

h sto

rey

h2/

stor

eyh s

tore

yh s

tore

y

Fig. 1 Typical Section

39


Calculation of Vertical Loads

Typical Unit Loads

Assume the following typical unit loads for the worked example:

Roof

Third

Second

First

Ground

Fig 1. Typical Section

Wall 2

Wall 1

Wall 1

Wall 2


Width b (m)

Wind Direction 2

l/2

Span

l1

l/2

Win

d

Dire

ctio

n 1

Jois

t Spa

n

dire

ctio

n

Wal

l 3

Span

l2

l/2

Wal

l 3

Span

l3

l/2

h 2h 4

h 3

h sto

rey

/2

Floor Loads

Roof Loads

Floor Loads

Floor Loads

h 1

h sto

rey

h2/

stor

eyh s

tore

yh s

tore

y


TOPIC

ITEM ESTIMATED UNIT DEAD LOADS (kN/m2)

Permanent Gk Temporary Gktemp (see note 3)

Roof Uplift (see note 4) -0.20 -0.10

Roof 1.50 0.50

Floors 1.00 0.50

Ext Walls (see note 2 0.40 0.10

Int Walls 0.60 0.05

40


Notes:

1 The above unit loads are provided as an example. Unit loads should be calculated for each individual building, taking into account the

construction and minimum imposed loads from BS6399-1:1996 and BS6399- 3:1998 or Eurocode BS EN 1991-1-1:2002 as

appropriate.

2 The 'Permanent ' Dead Load Gk refers to the in-service applied dead loads. In the case of external wall panel self weight, this should

include for the weight of any supported cladding type.

3 The 'Temporary ' Dead Load Gktemp refers to the expected dead loads during construction and should generally exclude the weight of

roof tiles, cladding, plasterboard, floating floors and ceiling constructions (the weight of plasterboard packs may be considered when

it is specified that a building is to be 'loaded-out' during construction.

4 Roof uplift pressures have the effect of reducing the effective dead load for resisting overturning and should be considered.

Load to individual Walls

In the example, the joists are indicated as 12m long continuous span floor joists.

In such cases the internal support reactions will be increased due to the continuous nature of the joists. Taking into account the effects of pattern

loading, it can be shown that the maximum internal reaction is approximately 1.25wL and the end span reactions are 0.45wL, where L is one span

and w the UDL on that span.

For floor joists parallel to external walls it is assumed that a load equivalent to half the joist spacing is carried by that wall.

Summary of 'Loading-Down'

In the example, the loads calculated at Ground Floor Level were as follows:

TOPIC

TOPIC

(kN/m2)

(kN)

Permanent Gk Temporary Gktemp (see note 3)

a) Internal Wall 1 = 28.90 10.20

b) External Wall 2 = 12.50 4.60

c) External Wall 3 = 6.09 1.77

d) Total Building weight = 540.00 194.40

41


Wind Loads acting on walls to BS6399-2:1997

Overall wind loads Pe should be calculated using BS6399-2:1997 Cl 2.1.3.6 for stability design. For simplicity, the worked example assumes that

the overall wall wind pressures calculated is:

BS6399-2:1997 Cl 2.1.3.6 Permanent overall pressure

ρfinal = 1.00 kN/sq.m

For simplicity, assume no masonry shielding is applicable:

BS5268-6.1:1996 Cl 3.2.3.1 Assume no masonry shielding

k100 = 1.00

BS6399-2:1997 Annex D

For temporary wind loads during construction, a factor of sd = 0.749 may be considered for wind loads with a probability of not being exceeded

during a period of 12 months duration. Overall wind pressures are reduced by the square of this factor.

BS6399-2:1997 Cl 2.1.2.1 Temporary overall pressure

ρtemp = 0.749 2 x 1.00

= 0.56 kN/sq.m

Wind Loads acting on Roofs to BS6399-2:1997

Wind loads acting on the inclined faces of a pitched roof (refer to BS6399 - 2:1997 )will also contribute to racking, sliding and overturning forces

acting on the building and should be considered.

For certain pitches of roof, BS6399 gives two sets of external pressure coeficients cpe, and it may be necessary to consider different combinations

of coefficients to identify the worst loadcase for stability and racking checks.

Roof uplift pressures have the effect of reducing the effective dead load for resisting overturning and should be considered. (For the worked example

the overturning effects of uplift presures on the flat roof are small and have been ignored for simplicity).

For an example of how to calculate the effects of roof wind pressures, reference should be made to Trada technology: UK Structural

recommendations: Section 1.2.2

Dimensional Checks

A/ Floor Diaphragm Check

The floor and roof diaphragms distribute wind loads acting on the elevations to the racking walls and can be assumed to be simply-supported

between racking walls.

BS5268-6.2:2001 Cl 6.5

Wind 1 direction:

Length/width = l/w = 4 ÷ 6 = 0.7

Wind 2 direction:

Length/width = l/w = 6 ÷ 12 = 0.5

42


Trada Technology ' Timber Frame housing: UK structural recommendations': 2006(2)

It can be assumed that conventional floors and flat roofs, in which a wood based panel product is fastened to timber joists, have adequate strength

and stiffness as horizontal diaphragms, provided that:

1 the diaphragm span: depth ratio does not exceed 2:1 in either wind direction.

2 the span does not exceed 12m between supporting walls.

3 the fixing around the edges of the panels complies with standard recommendations (e.g. 3.00mm diameter ringed shank

nails @ 150mm c/c for plywood or 3.35mm diameter ringed shank nails @ 300mm c/c for wood particleboard and OSB,

with a length equal to 2.5 times the board thickness)

4 the perimeter of the diaphragm is attached to the walls with fastenings of equivalent strength.

For diaphragms outside of the ranges given or in areas of high wind load (e.g. with a dynamic pressure exceeding 1500N/sq.) the required fastener

spacing should be checked in accordance with Section 1.1.2 of Trada Technology ' Timber Frame housing: UK structural recommendations': 2006

B/ Panel height Check to BS5268-6.1:1996 Cl 1.1

Storey height hstorey = 3000 mm

Subdeck thickness = 15 mm

Joist depth = 300 mm

Headbinder thickness = 38 mm

Soleplate thickness = 38 mm

Wall panel height hpanel = 2609 mm

As the wall panel height does not exceed 2.7m, the example is within the scope of BS5268-6.1:1996

Calculation of Horizontal Wind loads and distribution of Racking Loads to Walls

Vertical distribution of Loads:

When calculating the racking loads on the wall panels in a particular storey, it is assumed that the wind loads on the upper half of the panel is

applied as a racking load to the top of the panels, and the wind on the lower half of the panel is applied to the bottom of the panel where it is resisted

either by the panels in the storey below or by the foundations. The mechanical fixings at the interface with the adjacent storey or foundations must

therefore be proved for the racking and sliding forces to be carried at each level.

Therefore the total racking load on a timber frame wall is calculated as the racking load transferred from the roof or the storey above it, plus half

the wind load applied to the same storey.

Horizontal distribution of Loads:

BS5268-6.1:1996 Cl 4.4.2.1

For buildings which have an irregular arrangement of racking walls on plan, due account should be taken of any significant eccentricity between

the centroids of the wind load and the aggregated racking resistance.

43


The Engineer is to consider the horizontal load distribution to the racking walls. The horzontal load attracted to each racking wall is to be determined

to cary out the overturning check.

In reasonably symmetrical buildings it may be assumed that the overturning moment (and wind loads) are distributed between the racking walls

parallel to the wind direction in proportion to their racking strength, assuming that their stiffness is proportional to their strength. One method

therefore, is to assign horizontal loads in proportion to the racking strength of each wall.

In the worked example, the floor and roof diaphragms are assumed to distribute wind loads acting on the elevations to the racking walls and are

assumed to be simply-supported between racking walls. This approach is acceptable when the relative racking strengths of each wall are yet to be

determined.

It may be necessary for the Engineer to refine his assumptions once the racking resistance checks of the building are complete and the distribution

of racking resistance is finalised.

Summary of Horizontal Loading

In the example, the Racking loads calculated at Ground Floor Level were as follows:

TOPIC

TOPIC

WIND DIRECTION 1: (kN)

WIND DIRECTION 2: (kaN)

Permanent Frg Temporary Frg.temp

a) Internal Wall 1 = 42.00 23.56

b) External Wall 2 = 21.00 11.78

c) External Wall 3 = 126.00 70.69

c) External Wall 3 = 31.50 17.67

44


Stability Checks to BS5268-6.1:1996

BS5268-6.1:1996 Cl 4.4.1

The designer hould ensure overall building stability by checking that it has adequate racking, overturning and sliding resistance to lateral loads.

These checks should be made at critical levels for the completed building and for the various construction stages, when subject to dead load, zero

imposed load, and both horizontal and vertical components of the wind load.

Stability is generally obtained from racking walls, set in two orthogonal directions. Unless demonstrated otherwise, walls with significant openings,

for example doors, should be considered as separate discrete walls. The racking resistance of each wall should be calculated in accordance with

4 7 for each direction

Overturning at lower storey

Check 1) Whole Building Overturning to Cl 4.4.2

The designer should ensure that the total building resistance for overturning under the total wind load is >1.4 at every critical level (UKTFA

Guidance).

Loadcase

Permanent Temporary

Horizontal load causing overturning = Total Racking load to Ground Floor Panels Ftotal

Wind Load Ftotal (refer to Horizontal Load calculations) = 126.0 70.7 kN

Key Point: Note that the wind load acting on the lower half of the bottom storey has no effect on building overturning and is therefore ignored. The

lever arm H is the vertical distance between the centroid of the wind load and the wall base.

Key Point: The vertical load on the panel acts as a restraining moment. For each loadcase the minimum vertical load should be considered. Wind

uplift should also be considered. The vertical load acting at this level is assumed to include the self weight of the walls at this level. For whole

building checks, the walls perpendicular are also included in the total weight.

b

V

Ground Floor Wall Panel

Shear = F s0

Wind Load

F rg

h 1/2Le

ver A

rm H

1

45


Vertical Load to Panels V (kN/m):

Permanent Loadcase V = 0.9(Gkperm) = 0.9 x 540 = 486.00 kN

Temporary loadcase V = 1.0(Gktemp) = 1.0 x 194.4 = 194.40

Overturning Check:


Lever Arm H1 = h1/2+(h1/2 + h2 + h3 + h4)/2

= 3000/2 + (3000/2 + 3000 + = 6750 mm

3000 + 3000)/2

Overturning Moment = Frg x H1 = 126 x 6750 x 10E-3 = 850.5 kNm

= 70.7 x 6750 x 10E-3 = 477.1 kNm

Building width b = 6.00 m

Restoring Moment = V x b/2 = 486 x 6/2 = 1458.00 kNm

= 194.4 x 6/2 = 583.20 kNm

UKTFA recommend that the factor of safety of the total building resistance for overturning under the total wind load is >1.4

Factor of Safety against overturning = Restoring Moment = 1.71 1.22

Overturning Moment

FOS<1.4, Additional HD measures are required at foundations

Resistance of Whole Building to Overturning

EC0 requires the stability of a whole building and of its individual parts to be checked, regarding each as a rigid structure. The new design method

deals with the stability of individual parts of the building (racking walls) in respect of overturning but not the overturning stability of the building as

a whole. In theory the new method makes a whole building check unnecessary, since if the individual walls are stable with respect to overturning

then the building as a whole must be stable. However, since EC0 requires the whole building check to be carried out, this should be done in the

traditional manner and UKTFA recommend a FOS >1.4.

46


Additional holding down resistance required for FOS>1.4:

Rhd = 1.4(FtotalgH1/b) - (V/2) =

= 1.4(126 x 6750 x 10E-3/6) - (486/2) = -44.6 kN

= 1.4(70.7 x 6750 x 10E-3/6) - (194.4/2) = 14.1 kN

Where the resistance to overturning is inadequate (FOS<1.2 for racking wall or <1.4 overall), additional HD measures should be provided in the

form of galvanised anchor bolts, screws, soleplate fixings fixed to the concrete slab or galvanised or stainless steel holding down straps built into

the foundations. These fixings should be provided at 600-1200c/c along the walls perpendicular to the racking walls at both ends. These fixings

should not be considered to contribute to sliding resistance.

Check 2) Racking Wall Overturning to Cl 4.4.2 - Panel overturning check

BS5268-6.1:1996 Cl 4.4.2.1 The stability of each racking wall should be checked at the base as follows:

Internal Racking Wall I1

Consider an individual wall panel of length bpanel = 6.0 metres

Vertical Load V (kN/m)

Base Reactions

R1 = (Vb panel /2) - (F panel H 1/b panel R ) 2 = (Vb panel /2) + (F panel H 1/b panel )

or as a UDL

Rmin = V - (6F panel H 1/b panel2 R ) max = V + (6F panel H 1/b panel

2)

Sliding F s0

Wind Load

F panel

b panel

Leve

r Arm

H0

47


Loadcase

Permanent Temporary

"Apportioned" Racking Load to Panels

A racking wall which is broken into smaller racking panels by openings will carry a reduced proportion of the wind loads.

Reduction factor rf (0-1) = 1.0

Wind Load Fpanel = Frg x rf = 42 x 1 = 42.00 kN

23.6 x 1 = 23.56 kN


Permanent Loadcase V = 0.9(Gk) = 0.9 x 28.9 = 26.01 kN/m

Temporary loadcase V = 1.0(Gktemp) = 1.0 x 10.2 = 10.20 kN/m

Overturning Check:


Key Point: The lever arm H is the vertical distance between the centroid of the wind load and the wall base.

Lever Arm H1 = h1/2+(h1/2 + h2 + h3 + h4)/2

3000/2 + (3000/2 + 3000 + = 6750 mm

3000 + 3000)/2

Overturning Moment = Fpanel x H1 = 42 x 6750x 10E-3 = 283.5 kNm

= 23.6 x 6750x 10E-3 = 159.0 kNm

Restoring Moment = V x bpanel 2/2 = 26.01 x 6^2/2 = 468.18 kNm

= 10.2 x 6^2/2 = 183.60 kNm

BS5268-6.1:1996 Cl 4.4.2.1

For each racking wall under its apportioned wind load, the factor of safety for overturning should be >1.2

Factor of Safety against overturning = Restoring Moment = 1.65 1.15

Overturning Moment

FOS<1.2, Additional HD measures are required at the ends of the panels

48


Resistance to panel overturning

Where the resistance to overturning is inadequate (FOS<1.2 for racking wall or <1.4 overall), additional HD measures should be provided in the

form of galvanised anchor bolts, screws, soleplate fixings fixed to the concrete slab or galvanised or stainless steel holding down straps built into

the foundations. These fixings should be provided at 600-1200c/c along the walls perpendicular to the racking walls at both ends. These fixings

should not be considered to contribute to sliding resistance.

For the end panels, or for panels where further restraint is needed (ie where FOS<1.2 against overturning), the required additional uplift resistance

required is calculated as follows:

Vertical Load V (kN/m)

Holding Down Force reqd =

Rhd =1.2(F panel H 1/b panel ) - (Vb panel /2)

Net Base pressures =

Rmin = V - (6(F panel H 1-V HD b panel /2)/b panel2) Rmax = V + (6(F panel H 1-V HD b panel /2)/b panel

2)

Leve

r Arm

H0

Sliding F s0

Wind Load

F panel

b panel

Additional holding down resistance required for FOS>1.2:

Rhd = 1.2(FpanelH1/bpanel) - (Vbpanel/2) =

= 1.2(42 x 6750 x 10E-3/6) - (26.01 x 6/2) = -21.3 kN

= 1.2(23.6 x 6750 x 10E-3/6) - (10.2 x 6/2) = 1.2 kN

49


Net Base pressures:

AS A UDL =

Rmin = V - (6(FpanelH1-VHDbpanel/2)/bpanel 2)

= 26.01 - (6(42 x 6750 - 24.4 x 6/2)/6^2) -9.1 kN/m

= 10.2 - (6(23.6 x 6750 - 7.1 x 6/2)/6^2) -12.8 kN/m

Rmax = V + (6(FpanelH1-VHDbpanel/2)/bpanel2)

= 26.01 + (6(42 x 6750 - 24.4 x 6/2)/6^2) 61.1 kN/m

= 10.2 + (6(23.6 x 6750 - 7.1 x 6/2)/6^2) 33.2 kN/m

The resulting increased bearing stress in the bottom rail due to Rmax needs to be checked for very short term duration loading (k3 =1.75)

Where further restraint is needed (+ve figure), the additional dead load available from return walls or HD straps should be considered:

If the dead load available from return walls is still inadequate holding down straps or anchor bolts to the foundation at both ends of the end panels

capable of resisting the vertical shear force should be provided (more detailed information can be found in Trada Structural recommendations(2))

Forces in Edge Studs

The load available from a return wall panel has the effect of reducing the effective overturning moment on the panel and therefore reduces the base

pressures due to the wind moment:

Load available from return wall Vhd = Vreturnwall x 2 x hpanel or Vreturnwall x l/2 (whichever smaller)

= 6.09 x 4 = 24.4 kN

= 1.77 x 4 = 7.1 kN

check interface nailing/ check interface nai

Required Holding-down strap capacity = Rhd - Vhd =

= -21.3 - 24.36 = 0 kN

= 1.2 - 7.08 = 0 kN

Loadcase

Permanent Temporary

Loadcase

Permanent Temporary

50


Check 3) Aggregate Resistance to panel overturning

The factor of safety of the total racking wall resistance for overturning under the total wind load should be >1.4

Because the horizontal and vertical loads carried by individual walls differ, the factor of safety for overturning for different walls will also differ but

the aggregated factor of safety should be >1.4.

Racking Resistance:

The total design racking load which a wall assembly can resist is equal to the sum of the racking resistance of the constituent panels ie:

Fres total = ΣFres panel

The value of Fres panel depends on the shear strength of the panel and on the provision of adequate restraining force applied to the panel at its

windward end to prevent its overturning (see above)

The shear strength of the panel can be calculated using the methods descibed in BS 5268-6.1:1996, Cl 4.7, BS 5268-6.2:2001, Cl 6.7 and Eurcode

5 BS EN 1995-1-1:2004 Cl9.2.4 and is outside the scope of this document.

Sliding Resistance

BS5268-6.1:1996 Cl 4.4.3 - The designer should ensure that there is a factor of safety of 1.4 against sliding at the top and bottom of each racking

wall and at soleplate level.

51


Horizontal sliding force to racking wall (kN/m) =

Fs0 (refer to Horizontal Load calculations)/b = 48/6 = 8.00 kN/m

26.9/6 = 4.49 kN/m

(Note that the sliding force at the base of the panel also includes the wind load acting on the lower half of the panel and is therefore greater than

the racking force on the panel)

BS5268-6.1:1996 Cl 4.4.3 - Friction, under dead load only, may be used in conjunction with metal fasteners when calculationg the resistance to

sliding. The coeficient of friction between timbers in contact or on the underside of the soleplate may be taken as μ=0.3.

Soleplate to foundations and wall panel bottom rail to soleplate fixings need to be designed to resist this applied load.


Permanent Loadcase V = 0.9(Gk) = 0.9 x 28.9= 26.01

Temporary loadcase V = 1.0(Gktemp) = 1.0 x 10.2= 10.20 kN/m

Frictional resistance at base of panel f = V x μ = 26.0 x 0.30 = 7.80 kN/m

10.2 x 0.30 = 3.06 kN/m

This mechanical fixing capacity can be achieved using nails, screws or bolts as appropriate using k factors appropriate to very short term

duration in accordance with BS 5268-2:2002 Section 6.

For typical interface nailing at foundation and upper floor levels refer to 'Technical Report - Robustness and Connectivity of the framing members'

Loadcase

Permanent Temporary

Mechanical fixing capacity required for FOS > 1,4, Ffixings = 1.4(Fs0) - f =

= 1.4(8) - 7.8 = 3.40 kN/m

= 1.4(4.5) - 3.1 = 3.22 kN/m

52

Head officeThe UK Timber Frame Association The e-Centre Cooperage Way Alloa FK10 3LP

t: 01259 272140 f: 01259 272141 e: [email protected] w: www.uktfa.com

w Uktfa Structural Guidance Platform New April 2011 20-09-2013 12.05.40

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