Advanced Structural Designs

  

ACN 097 789 87    92 Vasey Cres CAMPBELL ACT 2612

Telephone  61612171         Facsimile 61612170

Email   mal@structuraldesigns.com.au

 

 

 
                                                                                                           

                                                                   

 

What causes Cracking in Brickwork or other masonry (esp brick house cracking) and who’s responsible?

 

The cause of cracks in brickwork and other masonry can generally be broken into 5 categories

 

*  Foundation movement.

*  Support structure deflection.

*  Thermal movement and moisture uptake or loss

*  Failure under external loads.

*  Arch failure.

 

The majority of cracking in houses in Canberra ACT is from foundation movement, whereas residential units and commercial premises tend to poor expansion joint design.  There is however no general answer to the problem and at times a number of the above causes act in unison making diagnosis more difficult.

 

Foundation movement

In Canberra many buildings are founded on reactive clays and these clays have a tendency to swell as they take up moisture.

 

In a building constructed on a flat block, founded on uniform soil and with no gardens close by, this swelling would lift the outside foundations uniformly and only have a tendency to crack inner walls.  This is because the moisture regime under the house would tend to stay the same while the perimeter would be affected by the usual seasonal variations.*  In normal circumstances however the ground moisture and soil parameters vary around the house and cracking can occur in any number of locations. 

 

It is the responsibility of the building designers to control the amount of cracking by judicious use of control joints and by appropriate footing design.  In this design consideration is given to the reactivity of the soil, the variability of the soil, the moisture history of the soil and any other constraints such as block boundaries, service trenches etc.

 

The building owner also has a responsibility to maintain plumbing services and to not unduly vary the moisture conditions around the property. Some guidance on what these responsibilities are for residential home owners is outlined in AS2870 Appendix B.  If you ring CSIRO publishing on 1800 – 645051 for a few dollars they can send you a Information sheet called “Guide to home owners on foundation maintenance and footing performance” which is also referenced by AS2870. To summarise if you are planting trees near the walls of the house or watering shrubs up against it you are likely to be contributing to the problem.

 

Foundation movement normally results in stepped cracking often emanating from the corners of windows and doors.  Such cracks are normally widest at the top.

 

For residential home owners considering negligence action against their engineer, classifier or builder they should be aware of the performance criteria in the AS2870 which acknowledges the risk of significant cracking to a small number of houses.  The point is, that if the engineer, classifier and builder have all acted in good faith, they are well protected by the code.

 

If on the other hand, any of the parties can be shown to have increased the probability of cracking beyond that deemed acceptable by the code there may be grounds to take action against them even if the cracking is within code limits.  This might be true if for example an engineer has allowed strip footings and infill slabs on a class M site and the infill slabs move causing cracking to internal masonry or Gyprock or causing windows to jam. 

 

If you are looking to arrest cracking resulting from soil swelling we normally recommend trying to control the moisture regime in the soil around the building first.  This can take a number of forms from watering in dry periods to introducing paths close to the building or installing root barrier to block tree roots.  If you are considering underpinning do not let anyone talk you into underpinning only the cracked portion of the building on a reactive clay site as this will normally only move the problem, and more often than not worsen it.  The exceptions to this rule in the local region is if the house has strip footings and is mostly on rock with a smaller portion on clay or poorly compacted soil, or if subsidence is occurring due to adjacent trenching.

 

It has recently been brought to our attention that one engineer practicing in the ACT is recommending partial underpinning of problem foundations on deep reactive clays.  This same engineer is failing to carry out even a rudimentary soil investigation and is pricing to carry out the work himself.  If you have such a recommendation we strongly suggest getting a second opinion before you waste a very large amount of money and potentially worsen the problem.  If you are currently suffering the results of this type of “repair” (which normally involves cracking immediately adjacent to the underpinned area) you will be well within your rights to seek retribution from the engineer involved. 

 

If the crack only partially closes with judicious watering it can often be given an extra nudge using resin grouting techniques such as those used by Uretek.  In this case the grout injected under the footing chemically expands and effectively jacks the footing back into place.  The procedure generally involves fewer risks than partial underpinning as the soil structure under the footing remains essentially the same.  It should be emphasised that this is not a “fix all” solution and it is one that works best when poor fill compaction was a contributing factor.  Controlling the soil moisture regime afterwards is still a very important consideration.  Another benefit of this system is that it is less disruptive and does not damage adjacent paths and gardens.

 

Another area that exhibits significant cracking is where house extensions are carried out and the new brickwork is toothed into the old.  Whilst this without a doubt provides the most attractive outcome in the short term (assuming the bricks match or it’s rendered) it never is in the long term if the house is founded on reactive clays.  This is because it is difficult to ensure adequate continuity of reinforcement in the footing.  Most builders opt to tie the footings together with a couple of dowels at the footing base which results in the footing being effectively hinged at the join.  We generally recommend dowelling top and bottom of footings but also specify a control joint over so that any rotation at the joint does not crack the brickwork.

 

The crack below is in a set of units in Aranda. Judging by the once fashionable streaks in the mortar I’d say built 1968-69.  An interesting point is that in almost any part of Aranda rock is within 1 m of the surface.  We can’t help but think this builder could have dug a little deeper.  As is often the case, the additional cost of the correct footing would be less than 1% of the loss in resale value of the property.

 

   

 

Moisture change in reactive clays are not the only cause of foundation movement as founding on poorly compacted fill or adjacent to deep trenches may also result in footing settlements.  Assessing the lay of the land and identifying services can often yield useful information when diagnosing a problem.

 

It is worth noting here that the old ACT building act prescribed shallow singly reinforced footings until 1986 when AS2870 (the Residential Slabs and Footings Code) was released.  Although we designed to this code from the day of it’s release many engineers did not, and the ACT building authorities did not move to correct this situation until 1988.  If your house was built after 1988 it should therefore have had soil tests carried out to determine the reactivity of the clay and should have had footings designed to suit the conditions.

 

Support Structure Deflection

Masonry may be supported in defined areas to create openings using lintel beams or may be completely supported by a series of steel or concrete beams carrying the load to the ground via concrete or steel columns.  The movement of the support structure can sometimes lead to cracking of the masonry. 

 

Contributing causes to this type of cracking can be:

 

  1. Poorly designed support structure (most common).
  2. Inadequate jointing (if specified).
  3. Overly rich mortar mix increases wall stiffness (especially true where lime is specified in the mix but not used).
  4. Rapid construction loading (incorrect design assumptions made).
  5. Support propping arrangements (props removed too late when brickwork is brittle).

 

It is always worth remembering that the concrete structure supporting masonry walls is not only deflecting but also shrinking and creeping.  Masonry can sometimes be overloaded because it is bounded by the columns connected to creeping and shrinking prestressed floors.  This is especially true if the masonry itself is trying to expand on moisture uptake, and can lead to some quite spectacular failures.

 

Thermal Movement and moisture uptake or loss

Clay bricks take up moisture and grow whereas silica and concrete bricks lose moisture and shrink over time.  To complicate matters the mortar between the joints is also shrinking which results in different vertical and horizontal movements and the amount of movement is affected by the degree of restraint or confinement.  These movements are generally of no concern if the expansion joints have been correctly designed (for more info see How do I design Expansion Joints.htm).

 

Where cracking from thermal movement and moisture variation does occur it is normally vertical and often worst at the building corners.  This type of cracking is more common in residential apartments and commercial buildings but as the plan area of residential houses increases the problem is becoming more widespread there as well. 

 

The location and detailing of expansion joints is generally on the architectural drawings and when the joints fail to accommodate movement the architect is often pursued for negligence.  Many architects ask that their structural engineer sign off on the design, which is certainly a service we provide free of charges on projects we are documenting.

 

The responsibility is however, not always with the architect or structural designer especially when not engaged to oversee construction. Joints are sometimes unable to function because of mortar dags in the joints, the use of non sliding wall ties or even non compressible fillers. Swapping brick sources are also potential reasons for expansion joint problems as the potential expansion of a different brick may drastically affect the required spacing of the joints.

 

Failure under External Loads.

Gravity loads, wind, earthquake or other external load such as soil pressures or tree roots can act to stress masonry beyond its acceptable limits.  Cracking from gravity, wind or soil pressure is often the result of a design or construction error. The most common failure of this type are retaining walls where the failures are generally due to poor design, incorrect bonding pattern or poor drainage.  The other fairly common failure type is freestanding walls where the failures are generally due to poor (or no) design.

 

Damage from earthquake or other seismic loads such as blasting is often seen as diagonal cracking in both directions.  This type of cracking is extremely rare in Canberra. If there is talk of blasting in the vicinity it is wise to have the façade of the building carefully photographed before any blasting commences.

 

Arch failure

 

Many archways detailed for houses around Canberra were not designed by structural engineers and were destined to failure from the outset.  Fortunately they are now out of fashion and rarely causing problems.  It is important to note that even correctly designed arches should not be detailed where footings rest on expansive soils as foundation movement is likely to compromise the integrity of the arch.  This recommendation can be found in the Cement and Concrete Associations’ TN 61 which is referenced by AS2780.

The above photograph shows a typical arch which has failed partly because the left abutment was not strong enough to carry the thrust from the arch to the ground.  Some relative movement of the left and right abutments have also contributed to the problem.  It is worth remembering that the shallow arches have high lateral forces.

 

We have had cause to look at masonry arch in Church building in Ainslie where it was overstressed almost to the point of causing a total roof collapse.  If you see a crack in an arch structure or its abutting walls, call a structural engineer to check it out.

 

Further Information

In most cases, causes of cracking in masonry can be determined and remedied or practices put in place to minimise their extent. When you are looking to spend money repairing cracks in masonry it is worth having an expert look at your particular problem.  If you have a particular problem in mind and need a structural engineers perspective in Canberra ACT or surrounding districts contact Mal Wilson from Advanced Structural Designs on 02 61612171.

 

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* This point is not strictly true as the house has a tendency to shade some areas more than others from the drying effects of the sun.