The tricks about bricks

The tricks about bricks
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If there’s one area where architectural aesthetics and engineering limitations often clash, it’s the treatment and behaviour of brick masonry.  With face brickwork making a welcome return to form and being popular again amongst residential architecture, now is as good a time as any to explore brick’s natural behaviour and to explain its limitations.

Until recently, architectural design and fashions meant brickwork was a strong, robust, and capable choice of building material that required very little engineering or consideration when used in houses.  Small spans, lots of return cross-walls, and small glazing elements meant houses could readily be built in brick with very few concessions or special requirements.  However, in the last decade or two, architectural trends have made it harder for bricks to work as effectively and efficiently as they once did.  For example, current trends for “open plan” living means there are now fewer return cross-walls to laterally restrain the side walls – which is a fancy way of saying, “There’s nothing to stabilise the walls against wind and earthquake loads”.  The trend for large glazing and window/door suites that open up on to decks and balconies mean there are now unsupported wall ends that terminate, yet they still have to support the horizontal loads imposed on the glazing when the wind blows.  And, in the case of high-end houses and architecture, we’re generally dealing with larger rooms, bigger spans, increased loads, plus longer, higher, and more slender brick wall panels – even double storey walls in the case of some atriums, stair voids, foyers, and “great rooms”.   In the past, such issues were dealt with by introducing engaged brick piers to provide localised strength and robustness where necessary.  However, since no one particularly wants a pier protruding into their living room, wall reinforcement these days (i.e. localised strengthening) will typically consist of steel columns and portal frames – ideally sleeved into the brickwork to conceal their presence.

For those who’ve been practicing architecture since the 70’s and 80’s and wonder why engineers are insisting on additional structure that wasn’t necessary 30 years ago, it’s not necessarily a case of us all being more conservative.  However, the reality is that aspects of the masonry design code (AS3700) have changed significantly (particularly with respect to slenderness limits and fire resistance), and modern architectural trends push brickwork further and impart higher stresses than was typically the case, say, 20-30 years ago.  (We’ll explore this further in just a moment).  As engineers, we love it when new architectural plans come into the office showing “full brick” 270mm cavity wall masonry construction…it means we’ll have strong supporting elements to hold up the floors.  But we still need to get out the calculator to work out what the wall can and cannot do.

Another item that can cause grief is control joints.  Over time, clay bricks will grow and concrete bricks will shrink.  Either way, provision has to be made in the structure to accommodate this movement.  Generally speaking, the shrinkage of concrete bricks is smaller and easier to deal with and is thus less of an issue.  However, concrete bricks are actually weaker than their clay counterparts and – let’s face it – not particularly attractive as a face brick, so we rarely encounter them in houses.  (They are better suited to partition walls in apartment building construction where they are either rendered or lined with gyprock).   The problem with clay bricks is that the brick growth can be significant and it is common to see bricks crack, bulge, or blow out at corners if provision is not made to accommodate the expansion – both horizontally and vertically.

Architecturally, vertical joints (which absorb horizontal expansion) can usually be located and worked in sympathetically with window or door openings, or they can be concealed behind downpipes.  However, horizontal joints (to absorb vertical expansion) are a bit harder to conceal or detail.  The vertical expansion of each individual brick is obviously extremely small, but by the time you’ve stacked enough of them on top of one another, the cumulative total expansion is significant and can cause serious issues.  Generally speaking, things get risky when the continuous height of brickwork exceeds two storeys (i.e. six metres) and so houses that feature side brick wall panels of three storeys or more are going to need a horizontal joint somewhere in the equation.

In less enlightened times, such joints were typically achieved by simply running the concrete slab out to the external face to break the run of brickwork, as illustrated in Figure 1 below.  However, very few architects are happy to express the concrete edge these days, and so alternative solutions need to be found.  It’s fine to stop the concrete on the inside skin of brickwork and let the outside skin “fly past” (see Figure 2), but you can only do this for up to two storeys if you want to avoid the risk of cracking or blow-outs.  In the case of buildings with three or more storeys, a popular solution these days is to introduce a shelf angle off the slab’s edge and break the run of brickwork that way.   (See Figure 3).  Of course, you’ll still read the break or differentiation in the external mortar joint along the line of the angle’s flange, but it’s a vast improvement on the old visible concrete slab solution.

As a brief aside, the above illustrations also show why brick walls are being asked to work harder these days than they were in the past.   In the detail shown in Figure 1 – which was widely used up until the mid-1990’s – the weight of the concrete floor slab and the walls above were shared across two supporting skins of brickwork.  However, in Figure 2, the weight of the floor slab is now being carried by the internal skin only – we’ve effectively doubled the load that the internal skin is carrying!  In the case of Figure 3, not only does the internal skin alone take the weight of the concrete floor slab, but it also has to support the outer skin of brickwork from the floor above, which is being transferred courtesy of the shelf lintel!  It is for these reasons and simple aesthetics – i.e. to conceal the slab edge – that brick walls today typically carry greater loads and are subject to much higher stresses than might have been the case just 20 years ago.   

Further consideration is needed when specifying the masonry for a rendered finish.  The vast majority of brick commons used these days are extruded, meaning they have core holes in their cross-section and also no frogs.  Whilst this is great for the bricklayers, many builders report that the presence of the cores means moisture in the wet render is absorbed unevenly into the face of the bricks, leading to blotchiness and crazing (fine cracking) once the render has cured.

For this reason, it may be desirable to specify dry-pressed commons for brickwork that will be rendered.  However, you’ll need to appreciate that dry-pressed commons are usually 25-33% weaker than their extruded counterparts, and so if the vertical load being carried by the wall is high, or if the wall is slender (when its height is many times greater than its thickness), additional strengthening  – e.g. steel columns – may be required.       

There’s an awful lot to consider when specifying, designing, and building with masonry.  It’s still usually the best option for when the big, bad wolf comes to huff and puff, and at least one out of three pigs got it right.  There are still many aspects we’ve not explored above, e.g. slip joints, fire resistance, bond patterns, cavity ties, mortar mixes, effluoresence, damp-proof courses, and the issues associated with either brick veneer or reverse brick veneer construction.  Nonetheless, the above is a starting point and hopefully gives some insight into the considerations that engineers take onboard when designing masonry walls.