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.
- To accommodate differential
thermal movements in the masonry when compared with the support structure
(equal in all directions).
- To accommodate brick expansion
on moisture uptake (equal in all directions)
- To accommodate concrete or
silicate brick shrinkage on moisture loss (equal in all directions)
- To accommodate the shrinkage in
the elements supporting the masonry such as slabs beams or footings
(horizontal in the plane of the masonry).
- To accommodate creep in the
elements supporting the masonry such as prestressed slabs and beams
(horizontal in the plane of the masonry).
- To accommodate the vertical
creep, shrinkage and elastic shorting of support elements such as concrete
columns (vertical in the plane of the masonry).
- 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
4. Design of Concrete Masonry Buildings (in
accordance with AS3700 1998) CMMA and Standards Aust July 2000