Rebuilding Canterbury requires cool heads and facts, argues an advocate for bricks, CATHY INGLIS. She writes that an engineering survey of over 1000 houses in the Christchurch area has provided some surprising results.
The recent decision to all but demolish Christ Church Cathedral, has reinforced for Cantabrians that the February 2011 earthquake and the subsequent earthquakes and aftershocks were like nothing that has hit the region in the past.
Buildings and building technologies that were thought to be invulnerable failed, some spectacularly. Not surprisingly, there have been calls for new building standards and new approaches, some out of frustration with the pace of rebuilding.
The recent call by NZ Wood (Perspective, March 14) for timber to be "the Canterbury rebuild material of choice" ignores both history and the realities of modern building design.
To better understand the situation and to provide quantifiable information, an engineering survey was conducted during June-July 2011 of 1084 residential dwellings scattered throughout the wider Christchurch area.
The research was undertaken by Associate Professor Jason Ingham of the University of Auckland who also supervised the door-to-door assessments conducted by engineers from the universities of Auckland and Adelaide. The investigation was sponsored by Think Brick New Zealand, which represents our brick manufacturers and resellers.
One-quarter of the inspected dwellings were constructed pre- 1996, the date when screw-fixed wall ties (more of which later) were mandated for all new brick veneer construction. About 30 per cent were of two-storey construction. The majority of the buildings surveyed - 88 per cent - were clad in clay brick, another 10 per cent in long concrete blocks, and the balance were mostly stone.
A summary is available for download at thinkbrick.co.nz. It makes interesting reading as it presents conclusions reached by scientific observation, not opinion or emotion.
Prominent among these is that, "Generally houses constructed since the 1990s tended to suffer lower levels of damage than those built earlier."
It is well accepted that older buildings, even one as massive as Christ Church Cathedral, were often badly damaged. But why should newer houses - and bear in mind the vast majority of those in the survey were constructed in brick - have a better rate of survivability? The report credits this to advances in the design of bricks and brick veneer systems.
Modern bricks are substantially lighter than older bricks, improving their potential performance in an earthquake. They are much narrower at 70mm wide and have larger core holes. These measures not only reduce weight but also allow some mortar to set in the holes, forming an improved "key" between bricks.
The structural solidity of older masonry walling systems, be they stone or brick, works against them in an earthquake because of their ungiving nature. Just as the top levels of skyscrapers waver slightly when under load, the modern brick veneer building is designed to accommodate ground movements without incurring major damage.
How does it work? The inner timber or steel frame is connected to the outer skin of brickwork by wall ties that are screw-fixed, a modern building standard requirement.
These ties allow the two elements of the structure - frame and brickwork - a degree of independent movement while maintaining the integrity or ultimate strength of the structure. In contrast, older brick ties were nailed to the frame and pulled out when shaken by the earthquake.
That's not to say even a modern brick veneer house could have survived being undermined by the severe soil liquefaction that occurred in some areas. The survey concludes the majority of cases of moderate to severe damage were concentrated close to river banks, mainly due to soil liquefaction and lateral spreading.
However, the survey does show that houses constructed in the newer, lighter-weight brick veneer performed much better than older brick homes, and suffered much lower levels of damage. It observes that structures using screw-fixed wall ties performed better than other types, with the majority exhibiting either no visible damage or minor damage at worst.
This is supported by comments made by Richard Dalman, Canterbury's representative on the New Zealand Institute of Architects Council in response to the earlier earthquake of September 2010. "A lot of the housing issues we have had have been with older houses with brick walls that are [not tied to] the structure or they haven't been sufficiently tied back to the structure," he said.
"The other problem has been where houses of all ages have been built on sandy soils, and it is the liquefaction that has resulted in the houses' foundations and floors being affected."
Of course, there is more to selecting the cladding for a building than its earthquake performance. One has to consider weathertightness, maintenance and durability.
Brick veneer construction has the advantage of being weathertight, thanks to its cavity design. Moisture entering the cavity is drained before it reaches the frame or the inner walling. For this reason brick veneer is now the "gold standard" in weathertight walling design which other cladding systems must achieve.
Low maintenance and zero finishing costs are also features of face brickwork. Fired clay bricks require no finishes such as painting to maintain their appearance and durability.
As well, paint, it must be remembered, is costly both in dollars and in the energy required for its manufacture.
Finally, there is the question of history and architectural character. Christchurch has long been renowned for the English character of its buildings and gardens, a legacy of its 1848 founders. Brickwork has been a key part of that character, with the Canterbury region embracing brick at a much higher rate than the rest of the country.
So it's not surprising that one of the earliest buildings to be completed in the Christchurch CDB is brick clad.
The three-storey apartment building has risen in Montreal St next to the Windsor Hotel B&B, which was damaged beyond repair in the February 2011 earthquake. The core of the building is precast concrete but the outer cladding is brickwork. This is in keeping with the character of the area but also creates a low maintenance, weathertight exterior.
The structural design principle of the Montreal St apartment building is identical to that in a single or double-storey house using a timber frame. The brickwork is connected to an inner concrete structure by screw-fixed wall ties that will allow the entire structure to move under load without separating.
Much of the reconstruction of the Canterbury region will be in single and two-storey housing and low to mid-rise commercial buildings. There is room for a range of structural solutions and exterior finishes.
The results of this survey show that brickwork is structurally sound using current building standards, materials and techniques. It also has a proven track record of being weathertight, low maintenance and durable. Rebuilding in brick will help retain the character that made Christchurch an architectural jewel in the south-western Pacific.
Cathy Inglis is technical adviser to Think Brick, representing New Zealand's clay brick and paver manufacturers.
- The Press