Structural Design Report

Microsoft Word - J1696-Doc-02 Structural Design Report X3.docxWebb Yates Engineers Ltd 44-46 Scrutton Street, London, EC2A 4HH
020 3696 1550 [email protected] www.webbyates.co.uk
Structural Design Report
CONTENTS
1 INTRODUCTION.............................................................................................................................................................................. 4
3 EXISTING BUILDING ....................................................................................................................................................................... 4
4 MAIN STAIR ........................................................................................................................................................................................ 5
7 STAIR 2 ................................................................................................................................................................................................ 13
8 TEMPORARY WORKS ................................................................................................................................................................... 13
APPENDIX A – STONE DATA SHEET AND TESTING REPORT ............................................................................................... 14
APPENDIX B – STONE SPECIFICATION ........................................................................................................................................... 15
APPENDIX C – MAIN STAIR CALCULATION AND SKETCHES .............................................................................................. 16
APPENDIX D - STEEL GRILLAGE SCHEME ....................................................................................................................................... 17
APPENDIX E – LOADS FROM GRILLAGE TO EXISTING STRUCTURE ................................................................................. 18
APPENDIX F – MAIN LANDING BALUSTRADE CALCULATIOSN AND SKETCHES ....................................................... 19
APPENDIX G – STAIR 2 CALCULATIONS AND SKETCHES ..................................................................................................... 20
GENERAL NOTES
Only construction status documentation is to be constructed from. If you do not have a construction issue document
and you are about to build something, please contact Webb Yates Engineers. Ensure that you have the latest revision
prior to construction.
This document is strictly confidential to our client, or their other professional advisors to the specific purpose to
which it refers. No responsibility whatsoever is accepted to any third parties for the whole or part of its contents.
This document has been prepared for our client and does not entitle any third party to the benefit of the contents
herein.
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reproduced, stored or transmitted in any form without prior written permission from Webb Yates Engineers Ltd.
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REVISION HISTORY
Revisions indicated with line in margin.
Revision status: P = Preliminary, T = Tender, C = Construction, X = For Information
Revision Date Author Reviewer Approved Description
X1 29.11.13 RN SW SW Draft for comment
X2 11.12.13 RN SW SW Revised for information
X3 31.01.14 RN SW SW Updated with Final Information
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1 INTRODUCTION
Webb Yates Engineers Limited (WYE) have been appointed by Luc Tamborero of stair designers Mecastone to carry
out structural analysis and design of the stone stair and balustrade structures at St. James Place as part of the fit-out
contract. St. James Place is an existing Grade II* listed concrete framed building split into separate flats located in
Central London. The stone structures are to be installed into a duplex flat occupying the third and fourth floors
which are currently undergoing refurbishment.
These stone structures are contractor designed elements to be designed in line with requirements set out in Jamie
Fobert Architects outline drawings and specification and a design report by the primary building engineer, Arup.
There are four main elements:
- Main Stair (Stair 1) - to include stone stair and stone balustrade
- Steel transfer structure under main stair
- Main Landing Balustrade
- Small stair (Stair 2)
WYE are responsible for the structural design of the four main elements listed above. Stair support forces are to be
submitted by WYE to the main building engineer for approval. Detailed dimensions of the stair structures and any
sub-components are the responsibility of the stone contractor.
2 TERMS OF REFERENCE
- Jamie Fobert Architects stair drawings.
- Arup sketches ‘Stage D Drawings Structural Engineering 15 April 2013’.
- Arup structural note ‘Design Note 2 23 September 2013 Rev A’ - Briefing Note for new stair support.
3 EXISTING BUILDING
The existing building is an 8 storey concrete frame building in Central London built in the late 1950’s. The primary
structure consists of reinforced concrete slabs spanning between downstand beams supported by concrete columns
and walls. Overall stability is provided by a series of reinforced concrete shear walls.
The assessment of the primary structure for the imposed loads from the stair structures is the responsibility of the
main building engineer, Arup. The assessment and constraints are set out in Design note 2 23 September 2013 Rev A.
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4 MAIN STAIR
The main stair is to be installed in the centre of the main space linking the third and fourth floors. The architectural
aspiration is for a solid stone stair made from discrete blocks cut from larger blocks of quarried Roman Travertine
Limestone. Structurally the intention is to create a true loadbearing stone structure using the inherent material
properties and shape of the structure to provide adequate loadpaths.
By its nature the stair is a massive structure and so has a large dead load of the order of 9 tonnes. Importantly this
load cannot be supported off the existing third floor slab. The slab itself has been assessed by the main building
engineer and has been deemed to have insufficient capacity. The slab soffit is inaccessible and so no strengthening
work can be carried out from the underside. Also the risk of loading the slab and causing any slab deflection which
may cause damage to finishes in the demise below must be avoided. Therefore a transfer structure has been designed
to support the load and take it back to stiffer points; the core walls and downstand beams
Figure 4.1a) Existing structure and 4.1b) Proposed stair
4.1 GEOMETRY
The proposed geometry is shown on JFA drawings and consists of 4 main elements. A lower flight containing 6 treads
extends from the third floor up to a near semi-circular half landing. This landing with curved base turns through an
angle of 145 degrees to the point that the main flight starts. Eleven treads make up the main flight and take the stair
up to the main fourth floor landing. There is an arched balustrade on the inside of the treads which continues around
the re-entrant corner.
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Through a series of workshops with JFA and Mecastone (stone mason) this geometry has been fine tuned to create a
geometry and joint layout that strikes a balance between the architectural, structural and construction requirements.
A number of iterations have been required to reach this point with the process being informed by the original
architectural intent, the structural analysis, the size of the base blocks from which the pieces will be cut, weight of
each piece and an understanding of how the pieces will fit together to produce the most seamless visual finish in terms
of matching vein lines in the stone.
Figure 4.2 Extract from Rhino model showing joint locations
The geometry and joint layout have been drawn in Rhino, a 3D drawing software package from which cutting drawings
for each piece will be produced. This model and the final cutting drawings will require review and acceptance from
architect and engineer prior to commencement of cutting.
Final stair geometry and joint layouts are shown on Mecastone/ EDM drawings.
4.2 LOADING
Structural loading is calculated in accordance with BS EN 1991-1.
• Dead Load – Selfweight of stone based on density of 24.6kN/m3
• Superimposed Dead Load – No finishes are assumed
• Imposed Load – Sub-category A1 - 1.5kN/m2 or 2.0kN point load.
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• Horizontal balustrade load - 0.36kN/m applied at 1.1m above FFL or 0.5kPa applied to whole panel
4.3 STONE
The proposed stone is a Roman Travertine Limestone quarried in Italy. Travertine limestone is a sedimentary rock
created by the rapid precipitation of calcium carbonate from solution in ground and surface waters and geothermally
heated hot springs. It is characterised by horizontal bedding planes and porosity caused by organisms colonising the
surface and being pressurised under subsequent layers.
Figure 4.3a) Example of exposed rock face in quarry and 4.3b) Example of extracted blocks
In order to confirm the structural properties of the material strict testing procedures exist. Due to the variable and
inhomogeneous nature of this stone testing is required on samples taken from the specific blocks of stone to be used
in the stair construction. A partial material factor of safety of 3.0 is used in design. The full stone specification is
contained in Appendix B and the main points are summarised as follows:
• Stone is required to be tested to BS EN 13161:2008 (4pt) for flexural strength.
• Stone is required to be tested to BS EN 1926:2006 for compressive strength.
• Minimum of 10 sample specimens shall be selected from a homogenous batch for each loading direction to
determine the log-normal distribution lowest estimated value.
• Stone sampling and testing must be from a known source of the quarry and relevant to the stone used in the
stair design.
• Direction of the planes of anisotropy shall be marked on each specimen by at least two parallel lines.
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• If the use of the stone in respect of the position of the planes of anisotropy is known the test shall be carried
out with the force applied on the face that will be loaded during use.
Test data from the quarry has been received and the properties are as follows with full datasheet contained in
Appendix A:
Mean Uniaxial compressive strength - 78 MPa
Flexural Strength (load applied perpendicular to the bedding plane) - 11.6 MPa
NB: These values are taken from material data sheet provided by the quarry. The quarry is
responsible for ensuring that this data is relevant to the actual stone blocks used.
Further tests were required to determine the flexural strength with load applied parallel to the bedding plane (the
weakest direction) with samples cut from the actual blocks of stone to be used and the results are as follows with full
test report in the Appendix A.
Flexural Strength (load applied parallel to the bedding plane) - 1.1MPa
Shear Strength is taken to be equal flexural strength for the direction of loading considered.
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4.4 STRUCTURAL ANALYSIS
4.4.1 PHYSICAL MODEL
The geometry of the stair and therefore the loadpaths are complex. To aid the understanding of the structural
behaviour a 1:10 scale model of the stone stair was built. Each block was individually cut to reflect the joint layout
developed and the stair was built block by block to qualitatively prove the stability and loadpaths prior to the full
computer model analysis.
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4.4.2 STRUCTURAL BEHAVIOUR
The stair is a true loadbearing structure which requires no internal strengthening or reinforcement and works with
the material’s natural compressive and tensile strengths. The main flight and balustrade is shaped such that it acts as
one half of a three pin arch. It is horizontally restrained at the top and base to resist the arch thrust and also
vertically supported at the base. Joints between the blocks making up this arch are arrange such that they are
tangential to the arch thrust line and so theoretically only compressive forces are transferred through.
The soffit of this main flight is shaped such that approximately 2/3 of the flight width can act as an arch. The remaining
third of the treads are easily able to cantilever out the remaining distance. However in reality each tread is supported
by the tread beneath and so vertical load will track down towards the arches. The base of the arch then springs
directly from the supporting structure below.
The half landing is made up from segments with interlocking ‘torsion blocks’. These ensure that the segments cannot
rotate independently and are locked in place. At the edges vertical support is taken from the main arch base and the
lower flight.
The landing as a whole is prevented from overturning outwards from the centre by being notched into the centre
balustrade. This balustrade piece is further held down by the weight of the main flight. Due to the test data received
for the tensile strength of the stone in the weaker direction (load parallel to the bedding plane) this area has had to be
reinforced with the use of carbon fibre dowels to provide the required tensile capacity at this location.
The lower flight acts like a true cantilever stair in that the root of each tread is essentially fixed in torsion with vertical
load on the outer end of the tread tracking down to the base support.
4.4.3 ANALYSIS MODEL
The overall behaviour of the stair has been modelled using a 1-D finite element model in Oasys GSA. The discrete 1-
D elements represent theoretical elements within the stone. Connectivity between the separate pieces is modelled
by rigid arms with the relevant element releases which reflect the forces which can be transferred through each joint.
A thrust line analysis has also been carried out on the main arch to further show that the arch performs satisfactorily
both under self-weight and under patch imposed load cases.
Local stresses around a typical torsion blocks and nibs have been checked by hand to ensure that shear stresses are
within acceptable limits.
The results of these analyses show that the stair is stable under its own selfweight and the imposed loads (applied as
patch load cases) and that the stone is working within allowable compressive and tensile (and thus shear) stress limits.
Calculations and sketches are contained in Appendix C.
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5 STEEL TRANSFER STRUCTURE
A mentioned in the previous section, as the slab beneath the main stair does not have sufficient capacity the stair loads
must be transferred to the core wall and downstand beam at third floor level. To do this a steel transfer structure is
required which must span approximately 3.75m and fit within a 75mm structural zone which will be contained within
the floor finishes. A 10mm deflection zone is then allowed beneath this. This is a span/ depth ratio of nearly 50 and
so the steel structure will be governed by deflection.
Figure 5.1 Grillage structure
As the stair load is predominantly dead load from the selfweight the deflection due to this can be counteracted by
pre-cambering the structure by the required amount. Under dead load the deflection is of the order 20mm. The
grillage will be made with a pre-camber which is calculated based on the true vertical load distribution of the stair and
tested for sensitivity to ensure that with a range of load distributions the final level of the steel will be within the
finishes zone allowed.
15.00 mm
10.00 mm
5.000 mm
0.0 mm
-5.000 mm Case: C1 : Pre-camber
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The grillage will be formed with steel plates top and bottom approximately 3.7m long x 1.8m wide. A series of steel
web stiffeners made from back to back angle sections will be incorporated to give the structure adequate stiffness.
Plate will be grade S355 and between 6 and 10mm thick.
To construct the grillage with the specified pre-camber the bottom plate will be laid flat on shims to form the
required camber. The angles will be pre-rolled to the correct camber and then placed on top of the plate. The angles
will then be intermittently welded to the plate with the stiffness of the angles keeping the fabrication from distorting
however the fabricator will need to take care with the welding sequence to ensure that this effect is kept to a
minimum. The top plate can then be welded on in facetted sections short enough to enable welding between the
underside of the plate and the top of the web stiffeners until the fabrication is complete.
The sequence of construction is critical as any settlement of the stair support during erection should be minimized.
Therefore it is proposed that once grillage is pre-cambered, it is then pre-loaded to take up its final level position and
locked in place.
This will be done using a jack to load the grillage. Then, with the load applied, stiffening channels will be bolted to the
grillage at the ends only. Upon release of the jack load the grillage will push up against the bottom of the channels and
will be locked in place. The channels will deflect upwards 1-2mm but this is acceptable.
Following a discussion on site with the main contractor and steel fabricator it was decided that for ease of installation
the grillage would be built and welded up in-situ. All relevant site welding procedures will need to be in place and all
welds inspected following completion.
As each stone block is placed on the grillage the bearing between the grillage and stiffening channel will incrementally
reduce with negligible deflection. Once the weight of the whole stair is applied there should be no further stress at
the interface and the channels can then be removed.
At this point the stair load will be fully transferred to the supports at either side and a 10mm clear gap will remain
beneath. This will give visual verification that the slab is not loaded and that the grillage has performed it’s task. Any
further deflection will be caused by live load only. This will be of the order of 1-2mm under the full live load. This is
deemed to be acceptable however in normal service the live load will be much less than this.
Refer to Appendix D for sketches describing the steel grillage construction.
Loads from the grillage to the existing structure are given in Appendix E.
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6 MAIN LANDING BALUSTRADE
The balustrade to the main landing is formed using 100 thick stone panels. Lower panels are fixed back to the slab
edge with a joint at approximately finished slab level using resin anchored bolts and steel bearing angles with a detail
that allows tolerance to be taken out in all directions. Upper panels are then placed on top of the lower panels and
locked in position using shear keys and positive connections to existing columns and walls.
Structural sketches and calculations are contained in Appendix F.
7 STAIR 2
Stair 2 is much smaller and simpler in comparison to stair one. It consists of 3 main blocks each containing 3 treads.
The bottom tread is supported directly off the slab beneath. The middle block springs off the bottom block and in
turn supports the top block. It is restrained against torsion via a fixing back into the existing reinforced concrete wall.
The top block is supported between the concrete landing and the middle block.
Structural sketches and calculations are contained in Appendix G.
8 TEMPORARY WORKS
The Contractor will need to ensure that the structure is stable and adequately braced in the temporary condition and
until all permanent stability systems are place.
9 DESIGN STANDARDS AND REFERENCES
The structure will be designed to the Eurocodes including:
• BS EN 1990: Eurocode 0: Basis of structural design
• BS EN 1991-1-1: Eurocode 1: Actions on structures – Part 1-1: General actions – Densities, self-weight and
imposed loads
• BS EN 1993-1-1: Eurocode 3: Design of steel structures – Part 1-1: General rules and rules for buildings
• BS EN 1993-1-8: Eurocode 3: Design of steel structures – Part 1-8: Design of joints
• BS 6180: Barriers in and around buildings. Code of practice
• BS EN 13161:2001 Natural stone test methods. Determination of flexural strength under constant moment
• BS EN 1926:2006 Natural stone test methods. Determination of uniaxial compressive strength
Where Eurocodes are provided above, this is deemed to include the relevant National Annex together with main
Eurocode document.
APPENDIX A – STONE DATA SHEET AND TESTING REPORT
REPORT 50490/G
TESTING OF
OCEAN TRAVERTINE
Clapham High Street London SW4 7TD
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email: [email protected] web: www.sandberg.co.uk
Clapham High Street London SW4 7TD
Tel: 020 7565 7000 Fax: 020 7565 7101
email: [email protected] web: www.sandberg.co.uk
Clapham High Street London SW4 7TD
Tel: 020 7565 7000 Fax: 020 7565 7101
email: [email protected] web: www.sandberg.co.uk
Clapham High Street London SW4 7TD
Tel: 020 7565 7000 Fax: 020 7565 7101
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