Seismic performance of 2-storey brick veneer

This Issue This is a part of the Building envelope feature

By - , Build 114

BRANZ earthquake simulations on 2-storey brick veneer have found it performed well, and BRANZ recommends that some of these building do not need specific design.

Figure 2: Veneer cracking on side 2 at peak imposed displacement (about 3.5 times design level).
Figure 1: Veneer cracking on side 1 at peak imposed displacement (about 3.5 times design level).
Figure 3: Failure during out-of-plane testing was from either collapse of the body of the veneer (left) or the top cantilevered portion of the veneer (right).

An article in Build 107 (August/September 2008, pages 82–83) discussed BRANZ seismic testing of single-storey clay-brick veneer buildings. Subsequently, a 2-storey brick veneer building was constructed in the BRANZ laboratory and cyclically displaced to replicate the movement expected in an extreme earthquake.

To do this, the building was pushed backwards and forwards at the roof and first floor levels. The slow speed loading did not induce the inertia forces on the veneer that would occur in a real earthquake – the test only simulated the in-plane and not the out-of-plane loading. However, the presence of the veneer on the face-loaded sides of the building and its interlocked connection to the in-plane loaded veneer allowed the true performance of the in-plane loaded veneer to be replicated.

Little damage at design level

At design level displacements, the cracking was fine and difficult to see, and the cracks almost completely closed up when the horizontal load was removed. Repair of the veneer after a design level earthquake would have only required repointing some of the cracked joints.

The cracking in the brick veneer when the roof and first floor of the building were displaced 143 mm and 69 mm respectively is shown in Figures 1 and 2. This is approximately 3.5 times the design displacement, so a lot of damage was expected, but although cracking was severe, only a few bricks fell.

Figure 1: Veneer cracking on side 1 at peak imposed displacement (about 3.5 times design level).
Figure 2: Veneer cracking on side 2 at peak imposed displacement (about 3.5 times design level).

Stood up well to out-of-plane tests

Out-of-plane brick veneer tests by other researchers have been supplemented with BRANZ shake-table tests. The major difference with the BRANZ tests is that the veneer was racked in-plane before being shaken to failure. The racking simulated the in-plane movement of the veneer that would likely occur in conjunction with the out-of-plane shaking in a real earthquake.

Failure modes were either collapse of the top cantilevered portion of the veneer or collapse of the body of the veneer (see Figure 3). While these failures may seem alarming, the tests showed modern brick veneer could sustain well in excess of the design earthquake in the out-of-plane direction before such failures eventuated.

Figure 3: Failure during out-of-plane testing was from either collapse of the body of the veneer (left) or the top cantilevered portion of the veneer (right).

NZS 3604 recommendations

Currently, 2-storey houses clad with brick veneer over the full height do not fall within the scope of NZS 3604:1999 Timber framed buildings and are therefore subject to specific design. Because the 2-storey veneer proved to be so resilient, BRANZ plans to recommend that the next revision of NZS 3604 allows (with provisos) certain 2-storey brick veneer construction without specific design. The provisos will relate mainly to the relative alignment of the upper and lower storey windows and also to the ‘cored’ style of bricks being used.

Measurements in both the BRANZ single- and 2-storey tests showed that the brick veneer carried most of the horizontal earthquake load, rather than the timber-framed wall bracing systems. Currently, NZS 3604 requires the full inertia loads from the brick veneer to be carried by the bracing walls. BRANZ will be recommending that NZS 3604 allows the veneer to carry its own in-plane seismic inertia loads. It has developed a simple procedure to calculate this resistance, which can be considered to be separate bracing elements.

Download the PDF

More articles about these topics

Articles are correct at the time of publication but may have since become outdated.

Figure 2: Veneer cracking on side 2 at peak imposed displacement (about 3.5 times design level).
Figure 1: Veneer cracking on side 1 at peak imposed displacement (about 3.5 times design level).
Figure 3: Failure during out-of-plane testing was from either collapse of the body of the veneer (left) or the top cantilevered portion of the veneer (right).

Advertisement

Advertisement