64 BUILD 127 December 2011/January 2012 RESEARCH Brick veneer stands the test Twisted wire ties are one reason many brick veneer houses failed in past earthquakes, such as Napier in 1931. However, modern brick veneer houses perform much better, and BRANZ research has identified the critical practices needed for this. By Stuart Thurston, BRANZ Senior Structural Engineer W hile many loadbearing brick buildings collapsed in the Canterbury earthquakes, giving brickwork a bad press, the performance of brick veneer was satisfactory, with no deaths attributed to its failure. Damage provides insight Liquefaction and lateral spreading from the earthquakes caused large ground differential movements and the distortion of entire houses. Sometimes, cracks formed in the foundation slabs and through the walls (see Figure 1). However, the choice of cladding is unlikely to have influenced this outcome, and despite the gross movements, there was little veneer shedding, which shows that the modern veneer performed well. During the 4 September 2010 7.1 magnitude earthquake, approximately 1.5 times design accelerations were experienced within 25 km of the epicentre. Most veneers suffered little or no damage due to this shaking. Three examples of veneer shedding were found within 15 km of the epicentre, but these were attributed to poor construction – in each case, brick ties had been omitted in the top half of the veneer. In another case, the shedding was attributed to weak powdery mortar, probably from construction during hot, dry weather. Cracking in the mortar joints was common in older veneer construction where some settlement may have occurred. Mainly older brick veneer damaged The 22 February 2011 Christchurch earthquake had higher accelerations in the city and Port Hills/Lyttelton areas than the September earthquake. The most severe shake damage to houses not influenced by liquefaction occurred in the Port Hills areas during this event. Significant brick veneer damage occurred in older houses in these suburbs, but with more modern homes, the veneer generally hung onto the frame. Although no ground motion recording stations were available on the Port Hills at the time, some had been installed before the severe 13 June 2011 shaking. An analysis of the combined data showed that the Port Hills houses experienced shaking approximately four times the design level shaking in February. Consequently, the brick veneer damage observed was well within the design expectations for such extreme shaking. Although the period of intense shaking was short, the high vertical accelerations would also have affected the veneer’s performance. Modern veneers stronger than historic Modern brick veneer construction should be significantly stronger than historic construction. In older construction: ❚ fewer ties per unit area were used ❚ ties were often bent wire, which corroded significantly after installation so that veneer fell away in discrete panels during the earthquake as the ties broke ❚ staples or nail fixings were used that withdrew easily from the timber framing ❚ the vibration from nail-fixing ties to the light timber framing substantially weakened the bond between the tie and the mortar ❚ split-stone veneer, commonly used about 50 years ago, often had a dusty surface when laid, resulting in a poor bond to the mortar. Modern construction: ❚ requires ties to be robustly screwed to the timber studs (since 1995) ❚ must meet minimum connection strength requirements between ties and veneer mortar from AS/NZS 2699.1 tests ❚ uses modern clay bricks with core holes, which are lighter (less seismic load), and the mortar penetrates core holes, interlocking the bricks. BRANZ mortar elemental tests Research over recent years by BRANZ measured the compressive and bond strengths of a range of brick veneer mortar mixes both on site and in the laboratory. A selection of mixes was used to construct brick veneer walls that were later tested under out-of-plane seismic shaking (see Figures 2 and 3). By examining the mortar properties that gave good wall performance, critical mortar properties for good seismic performance were determined. Figure 1: Brick veneer house damaged in the Christchurch earthquake. Figure 2: General set-up for the out-of-plane shake table tests.