Advanced Structural Designs

  

ACN 097 789 87    92 Vasey Cres CAMPBELL ACT 2612

Telephone  61612171         Facsimile 61612170

Email   mal@structuraldesigns.com.au

 

 

 
                                                                                                           

                                                             

 

How do I Design Joints in Brickwork and other Masonry?

 

There are generally only 7 reasons that you may want to have a joint in the brickwork.

 

  1. To accommodate differential thermal movements in the masonry when compared with the support structure (equal in all directions).
  2. To accommodate brick expansion on moisture uptake (equal in all directions)
  3. To accommodate concrete or silicate brick shrinkage on moisture loss (equal in all directions)
  4. To accommodate the shrinkage in the elements supporting the masonry such as slabs beams or footings (horizontal in the plane of the masonry).
  5. To accommodate creep in the elements supporting the masonry such as prestressed slabs and beams (horizontal in the plane of the masonry).
  6. To accommodate the vertical creep, shrinkage and elastic shorting of support elements such as concrete columns (vertical in the plane of the masonry).
  7. To control in plane stresses induced by foundation movements.

 

The following information will deal with items 1 to 6 as any requirement resulting from foundation movements should be independently advised and designed by your structural engineer.

 

While joints perform all of the above functions they also act to reduce the structural integrity of the structure and as such should always be designed in conjunction with the appropriate engineering advice.

 

Joint Design

 

Clay Bricks Vertical Joints

For the design of such joints a number of assumptions are made with regard to differential thermal movements, frame shortening and gap closures and the amount of restraint to growth afforded by the support structure.  These assumptions can be a little simplistic especially for prestressed structures but more detail on this can be found in reference 1.  The following information is therefore far from comprehensive but does represent the minimum consideration that should be given to meet current design practice standards.

 

e (sometimes referred to as em) is the Characteristic expansion (5 year unrestrained expansion) of a brick and is generally used to indicate the amount long term expansion the brick is likely to undergo in service.

 

Typical values for e may vary between 0.5 and 2.5 (mm/m) so we cannot emphasis enough the importance of sticking to the same or equivalent brick once the joints are designed.  Estimates of e are based on accelerated (4 hour steam expansion) tests, which should be carried out in accordance with AS1226.  Figure 1 above shows the long term expansion of bricks indicating that if it were possible to grass them (leave them lying around) for 6 months it would be possible to halve their long term expansion.

 

 

In the equations given below

 

Sv is the general spacing between vertical expansion joints in straight lengths of walls

 

Sp is the general spacing between vertical expansion joints in parapets or the distance from a return in the wall to the first joint.

 

Sv = 30/(e+0.8)

 

Sp = 15/(e+0.4)

 

The equations above assumes a 15mm gap closure which is the maximum allowed by AS3700 and sometimes not used on aesthetic grounds as an allowance needs to be made for sealant performance which may push the overall width out to 25mm.  It should be noted that although AS3700 allows gaps to close to 5mm most sealant manufacturers recommend 10mm.

 

The spacings obtained from the above equations are only a reference point however, as windows, doors and other discontinuities all act as stress concentrations.  Decisions need to be made as to the architecturally preferred locations and whether brick course reinforcement is required at discontinuities.  Detailing the slip requirements for lintels bridging these joints and accounting for the loss of arch action in the lintels are a couple of issues that are often missed.  Many architects design relief into the facades to hide the joints or hide at least some behind downpipes while others make a feature of them. 

 

A good example of well thought out jointing pattern can be seen on Canberra University’s Communication and Engineering building where Tim Halden Brown (from MGT) choose to express the joints to great affect.

 

Clay Brick Horizontal Joints

 

In concrete buildings the design closure K is given by

 

K=(e+0.7)Sh

 

Where Sh is the some multiple of the storey height.

 

It is generally good practice to set the outer brick skin 10mm below the inner brick skin so that the ties do not slope inwards and to isolate windows and doors from the outer skin so that the differential movement does not damage them.  If you decide not to set the outer skin down we suggest taking a close look at the cavity tie specification to ensure that the drip grove provisions are adequate.

 

Another consideration should always be the likely creep and shrinkage deflections of concrete edge beams between floors where the structural support system varies (most often at the ground floor).  This can add an extra 8 mm to the closure at mid-span.

 

Further Information

For further information on the design joints in concrete walls we refer you to reference 4 below. If you require information on calcium silicate joints, inner skin joints and detailing, diagnostic forensic engineering investigation in Canberra ACT or advice on repairs please contact Mal Wilson from Advanced Structural Designs on Ph 02 61612171.

 

References

1 Design Note 1D BDRI 1979

2 Technical Notes on Brick Construction 18A 1991

3 Australian Masonry Manual Deakin University Press 1991

4. Design of Concrete Masonry Buildings (in accordance with AS3700 1998) CMMA and Standards Aust July 2000

 

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