Btech Masonry Design 4 Project

Contents Page

1. Introduction Page 2

2. Properties of bricks

2.1 Types of bricks Page 2

2.2 Compressive strength Page 2

2.3 Absorption Page 3

3. Connections Page 3

4. Movement in Masonry Walls Page 4

5. Wall ties Page 6

6. Shear strength and stresses Page 10

7. Lateral Support Page 10

8. Bonding

8.1 Stretcher Bond Page 12

8.2 English Bond Page 13

9. Header Course vs Wall Ties Page 15

10. Recommendations Page 18

11. Conclusion Page 19

12. References Page 20

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1. Introduction

The shape and size of bricks can vary a lot, and similarly the mortars that areused depends

on local material availability, but the basic form of construction for houseshas minor

geographical variations and has changed relatively little over time.A major problem is under

design of the masonry and mortar for certain wall requirements. Extensive care must be

taken for the binding together of different leaves. Many types of bonds have been developed

over centuries with English bond as one of the preferred bonds because of the alternating

header course to enhance strength and binding. Wall ties have also been introduced to

stretcher bond and cavity walls to enhance the strength of two leaves by supporting them to

act together as one single unit. The analysis of a typical masonry wall has to be done with

axial strain, moisture absorption, shrinkage, differential shortening and the different E-values

of different materials taken into account.

2. Properties of bricks

2.1 Types of bricks

Fig 1: Types of masonry units

2.2 Compressive strength

The compressive strength of masonry bricks can be determined accurately. It is found by

crushing 12 bricks individually until they fail or crumble. The pressure that was required to

crush the bricks is noted and the average compressive strength of the brick is stated as

Newton per mm² of surface area required to ultimately crush the brick. The crushing

resistance varies from about 3.5 N/mm² for soft facing bricks up to 140 N/mm² for

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engineering bricks. The bearing strength of a brick wall 215 mm thick is very much greater

than the loads a wall will usually carry. 

The average compressive strength of some bricks commonly used is:

Mild (soft) stocks 3.5 N/mm²

Hard stocks 17.5 N/mm²

Flettons 21 N/mm²

Southwater A 70 N/mm²

2.3 Absorption

The required thickness of an external brick wall is determined primarily by its ability to

absorb rainwater to the extent that water does not penetrate to the inside face of the wall. In

positions of moderate exposure to wind driven rain a brick wall 215 mm thick may absorb so

much water that it penetrates to the inside face.The current external wall to small buildings

such as houses is built as a cavity wall with a 102.5mm external leaf of brick, a cavity and an

inner leaf of block. The external leaf is sufficiently thick, with the cavity, to prevent

penetration of rain to the inside face and more than thick enough to support the loads it

carries.Many tests have been done to determine the amount of water absorbed by bricks

and also to determine the rate of absorption in order to establish a grading system for the

ability of bricks to resist the penetration of rain. Rain water tends to penetrate a small crack

in the mortar between bricks rather than being absorbed by the bricks itself. Also by soaking

a brick in water is a far from reliable guide to the amount of water they can absorb as air in

the poresmay prevent total absorption. But the amount of water a brick will absorb is a guide

to its density and therefore its strength in resisting crushing.

3. Connections

There are 4 types of connections that are crucial for the structural performance of brick

masonry elements:

1. The extent and quality of bond between bricks and mortar influences the integrity and

shear resistance of brick masonry walls. It is essential for the brickwork to be properly

constructed to allow for the best possible level to develop of bonding. It is also important to

ensure re-pointing of bed and head joints at regular time intervals so as to ensure the

maximum possible surface of contact.

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2. The connection between two leaves of brick walls is crucial to the stability and longevity.

Modern masonry construction standards require regularly spaced ties between the leaves of

a cavity wall to ensure monolithic behaviour and redistribution between the walls

3. The strength and stability of connections at corners and junctions depends on the specific

fabric of corner returns. Such connections ensure the redistribution of lateral forces among

walls.

4. The connection between the walls and the horizontal structures (floors and roof) are

important. This connection highly influences the seismic performance of the building.

4. Movement in Masonry Walls

Control joints are constructed into a wall to allow movements such as expansion and

contraction of the masonry. Control joints can either be vertical or horizontal joints.

Vertical control joints are 15 - 20mm in width.It accommodates horizontal movements. They

are not normally provided in base brickwork less than 600mm high or on the internal leaves

of clay brick cavity walls of small buildings.

Horizontal control joints controls the differential movement like for example when clay

brickwork is used as cladding or for infill purposes. Horizontal control joints are usually 10 –

15mm in width.

Differential movements in masonry walls may result from a number of factors:

Permanent changes in the size of the masonry walls

Shrinkage of the structure of reinforced concrete buildings to which masonry walls

are connected

Elastic deformation of load bearing members

Cyclical thermal movements

Failure to consider the above movements during the design process can result in

considerable structuraldamage that can be extremely costly to repair.Buildings that include

masonry elements should be designed to accommodate differentialmovement.The design

procedure is relatively simple with some basic design factors to consider.

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Basic Design Factors

All masonry walls and masonry structures should be designed to accommodate for

differential movement. Movements that must be allowed for can be outlined as follows:

Long term vertical and horizontal shrinkage of concrete blocks and calcium silicate

bricks

Long term vertical and horizontal expansion of clay bricks

Long term vertical shortening of load bearing frames and load bearing walls resulting

from such factors as elastic shortening, creep shortening and drying shrinkage

Long term horizontal shortening of reinforced concrete frames and floors form drying

shrinkage

Differential thermal movements. The outside cladding of a building is subjected to a

greater range of temperature than the internal structure it shields and the effects of

this must be considered at the time of design.

When a brick wall is subjected to a rise or fall in temperature, it will expand or contract. The

movement can be calculated with the following equation:

δ=α ×∆ t× L

Where: α is the coefficient of thermal expansion (See Table 1)

Δt is the change in temperature

L is the length of wall

Walling Material Coefficient of Thermal Expansion

(α) /°C

CLAY BRICKWORK

Horizontal 0.000006

Vertical 0.0000084

CALCIUM SILICATE 0.0000012

CONCRETE 0.0000012

Table 1: Coefficients of Thermal Expansion

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Example:

For a unit height of wall built with concrete brickwork with temperature change of

30°C:

δ=(12×10−6 )×30×1000

δ=0.36mm /unit height

5. Wall ties

Structural requirements in Masonry Walls is usually achieved with the introduction and

building in of Wall ties, designed to provide lateral stability and strength when used in correct

building practice.Metal wall ties are also used to anchor a wall to floors, roofs or to opposite

walls. The inertia force of the roof- and floor slabs is transferred to the wall through the wall

ties. When a wall is anchoreddirectly to an opposite wall, the metallic ties will pass under the

floor structure. Guidelines must be followed to ensure that the correct minimum amount of

wall ties is used and the distribution thereof in the walls has to be as specified in the referring

SANS codes. The load-deflection characteristics of ties have to be considered upon

assessment for the capability of ties to transmit the required forces. Fig 2 below shows

typical load deflection curves for some commercially available types of wall ties that are

clamped 75mm apart and subjected to axial tension.Butterfly ties are mostly used in

masonry wall construction. But for this assignment, crimp ties will be the main focus.

From a structural point of view, the main function of wall ties is to provide interaction

between the two leaves of a wall in such a degree that more loads can be carried by load-

sharing than by the two leaves acting separately. This interaction between the bricks and the

ties enables the leaves of a wall to act as a complete structural unit to resist compressive

and flexural forces.In some cases it also permits some differential longitudinal and vertical

movement between the leaves. Selection of a tie system to function properly in a wall is

further complicated by the vast number of tie types available. SABS 28: 1986 specifies

various types of ties.

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Fig 2: Axial tension – Deflection of wall ties

SIZE MASS

150mm 0.90kg

200mm 1.20kg

225mm 1.35kg

250mm 1.40kg

280mm 1.60kg

305mm 1.90kg

Table 2: Typical crimp tie sizes available

Table 3 and table 4 can be compared to see the difference between the compression

strengths of butterfly ties and crimp ties respectively:

Label/ Test No Load (N) Diameter Failure Mode of Tie

BC-1 813 3.15 mm Brick couplet collapse

BC-2 587 3.15 mm Brick couplet collapse

BC-3 623 3.15 mm Brick couplet collapse

Mean 674.33

Std Dev 121.43

Characteristic 473.97

Table 3: Butterfly tie compression strength

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Label/ Test No Load (N) Diameter Failure Mode of Tie

CC-1 986 3,15 mm Buckling

CC-2 1123 50 mm Buckling

CC-3 1235 50 mm Buckling

Mean 1114.67

Std Dev 124.71

Characteristic 908.89

Table 4: Crimped tie compression strength

Fig 3: Typical section through cavity wall showing tie connection

Fig 4: Typical crimp tie detail

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Minimum characteristic strength and stiffness of Wall Ties

1 2 3 4

ClassificationMin. characteristic strength, kN Min. characteristic

stiffness, kN/mmTension Compression

Light duty

Medium duty

Heavy duty

Extra-heavy duty

0. 25

0. 5

1. 25

2. 5

0. 3

0. 6

1. 5

3. 0

Not applicable

1. 0

2. 5

5. 0

Table 5:The min characteristic strength of wall ties

Fig 5: The function of wall ties

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6. Shear strength and stresses

Shear strength of masonry is defined as a combination of initial shear strength under zero

compressive load and increase in strength due to compressive stresses perpendicular to the

shear plane. The way a masonry wall will act under a load is comparable to any other

material under the same conditions.It will be subjected to the same basic stresses.

Fig 6: Stresses in walls

7. Lateral support

Masonry structures will tend to buckle when loads are applied on the wall. This failure is a function of the slenderness ratio. The slenderness ratio for masonry walls is calculated with the following equation:

Slenderness Ratio=SR=effective height (¿ length)effective thickness

SR=hef∨leftef

Given that:

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SR ≤ 20 for walls thinner than 90mm that exceeds two storeys

or

SR ≤ 27 for all other cases

Where walls are too slender, horizontal or vertical later support should be provided to prevent buckling. Lateral support is provided in many forms such as roofs, floors, piers, intersection walls etc. See Fig 7 for an illustration of the effect of lateral support.

Fig 7: Lateral support

Table 6 shows which support is relevant for certain heights of a specific wall.

Table 6: Lateral-support forwalls

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8. Bonding

Appearance is a big preference when choosing a type of bond, but the arrangement of the

bricks have to comply for the required strength abilities. If the foundations upon which a

masonry wall is built were to settle irregularly, the masonry wall will split from the top to the

bottom along one or more of the continuous vertical joints , according to the nature of the

settlement. This will cause the wall to simply act as a number of separate piers, with nothing

to bind them together but the adhesion of the mortar. If a half-brick that is cut longitudinally

(referred to as a closer), be inserted next to the smaller end of the corner brick and

continued through the thickness of the wall in every course, as in Fig. 8, it will be seen that

each brick overlaps the bricks with which it is in contact, thus entirely avoiding the

continuous vertical joints referred to above. In building a brick structure, therefore, care

should be taken to lay each brick so that it will overlap the bricks with which it is in contact

above and below, in such a manner that the whole wall will act as one complete mass. This

process of overlapping is called "Bonding."

Fig8.

8.1 Stretcher bond

The four faces of a brick which may be exposed in fairface brickwork are the two, long,

stretcher faces and the two header faces illustrated in Fig.9. The bed is referred to as the

face on which the brick is laid. Some bricks have an indent or frog formed in one of the bed

faces which purposeis to assist in compressing the wet clay during moulding. This also

serves as a reservoir of mortar on to which bricks in the course above may more easily be

bedded.

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Fig9: Stretcher bond detail

The length of a brick dictates the thickness of a wall. The length of bricks varies in many

cases due to the difference in materials and the conditions in which they are made. Clays

tend to shrink during firing.

The external leaf of a cavity wall is often built of brick for the advantage of the appearance of

brickwork. The most straightforward way of laying bricks in a thin outer leaf of a cavity wall is

with the stretcher face of each brick showing externally.

8.2English Bond

The following rules applyfor good bonding in walls built with English bond:

1. Bricks of the same size and shape have to be used throughout the whole wall.

2. A uniform arrangement of the brickwork should be kept throughout the wall.

3. Small portions of bricks should be used as little as possible.

4. The interior walls should be built with the bricks being laid as headers.

5. The lap should not be more than 58mm.

6. Vertical joints in courses should be vertically aligned over one another.

The facing bricks in an English bond wall are laid in alternate courses of headers and

stretchers. Queen-closers are inserted next to the quoin-headers to ensure that the bricks

will overlap properly. It does not matter whether the corners of English bond walls should

represent stopped ends because the external angle that are formed by the junction of two

walls’ appearance of face will be precisely the same all round.

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English bond is the strongest bond because it avoids continuous vertical joints in most cases

and should be recommended for general use because of its strength advantages. The use of

it on the ground can allow the penetration of damp through the numerous transverse joints

and its appearance may not present as good as Flemish bond brick walls. But the problem

with the damp can easily occur in any type of bond and is preventable through many

techniques and products on the market and with regards to the appearance of the wall there

is no reason why strength should be sacrificed for such a preference.

Fig. 10: Header and Flemish bond.

Fig. 11: English bond.

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9. Header Course vs Wall Ties

A 230mm thick wall, 2m high and with 0.8kN axial load that is carried by the wall. this

investigation was done to determine which bond would be stronger. Header course bonding

or wall tie bonding.

Fig A: Wall detail for calculations

Fig B: Illustration of wall bending with and without wall ties

Calculations:

Bending moment:

BM=Wk×Pf × h2

2

¿0.8×1.4× 22

2

¿2.24 kNm

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Wall with header course bonding. (No wall ties)

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Wall with wall ties

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This investigation was done to determine which bond would be stronger. Header course

bonding or wall tie bonding.

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It was determined that the moment of resistance for the wall with the ties is greater than that

of the wall bonded with header course. Therefore wall ties should be preferred above header

course construction.

Table 7 provides codes and guidelines for the spacing of wall ties and headers course bricks

in a double leaf wall.

Table 7: Spacing of Wall Ties and Headers in double leaf walls

10. Recommendations

It is clearly seen that the wall with the wall ties is stronger than the wall without any wall ties.

The wall ties transfer loads better than header course and when used correctly, can increase

the strength of the wall. Therefore it is recommended that wall ties should be considered

when large loads have to be carried by a wall when the load is not applied with zero

eccentricity and the wall is prone to buckling.

11.Conclusion

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Bonding between bricks and mortar is crucial to the unity of a wall but different factors play

an important role for enhancing the strength and stability of two leaves that need to work

together as a unit. Different bond-types have different strength and stability properties as

well as aesthetic value. English bond is a preferred bond type because of its ability to

enhance strength and binding but still does not provide the same enhanced properties as

wall ties. Wall ties can be introduced to stretcher bond and cavity walls to enhance the

strength. Both types of walls do not allow for header course construction and therefore wall

ties makes for a sound bonding tool. Crimped Ties have a greater resistance to buckling

under compressive loads than header course brickwork and therefore wall ties should rather

be considered during design and construction.

12.References

http://civilconstructiontips.blogspot.com/2011/06/stretcher-bond-walls.html

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http://www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm

http://theconstructor.org/structural-engg/walls-types-features-and-design-concept/819/

http://www.brunswicksales.com.au/masonry-flexible-anchors.html

SANS 10164-1

NHBRC (NATIONAL HOME BUILDERS REGISTRATION COUNCIL)

SABS 28:1986

AUSTRALIAN CODE (AS 2699- 1984)

STANDARDS ASSOCIATION OF AUSTRALIA . 1984. WALL TIES FOR MASONRY CONSTRUCTION. AS 2699. AUSTRALIA .

Dina, D. UNREINFORCED BRICK MASONRY CONSTRUCTION, University of Bath, United Kingdom

Watermeyer, R.B. Crofts, F.S. & Lane, J.W. THE STRUCTURAL USE OF MASONRY.

SABS 0164 – Parts 1 & 2. SAICE Lecture Course. 26 August 1992.

Parrot, G. REINFORCED CONCRETE & MASONRY DESIGN III. Study guide I. University

of South Africa.

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