Shaking Induced Damage

Shaking induced structural damage was very minor, in almost all cases.

Other than chimney damage, the most severe shaking-induced structural damage was in a small number old buildings constructed of unreinforced structural brickwork.

Compared with Christchurch city, slightly more damage occurred in areas with high proximity to the fault rupture, such as West Melton, where cracking to gypsum board linings and brick veneer was common (see Figure 3a & 3b). The severity of shaking in West Melton caused some concrete slabs to move or crack.

Markedly different levels of damage were observed surrounding the fault rupture. Areas in-line with the fault trace, such as West Melton, suffered from noticeably more shaking induced damage than areas orthogonal to the fault trace, such as Darfield and Rolleston. Hence, the characteristics of the fault rupture (known as the directivity effect) appeared to significantly influence the seismic demand on buildings.

Shaking induced damage is discussed in five categories: internal linings, masonry veneers, chimneys, damage to contents and structural masonry.

Internal linings

Most houses in Christchurch had zero cracking to internal linings, and others had a few very minor cracks, usually near door or window openings. It can be inferred, due to limited wall damage, that the gypsum board wall linings worked well to provide most of the bracing to the buildings. Some buildings have additional plywood bracing panels, but it is not possible to visually determine the location of these in an undamaged house. Larger movements and cracks in some older buildings showed that lath and plaster is not as stiff as modern gypsum board as a bracing material. Buildings with heavy roofs tended to have more lining damage than those with light roofs.

There was generally very little ceiling damage other than minor cracking at wallceiling junctions. The only observations of local ceiling collapse were in a few most severely damaged buildings in liquefaction areas or very near the fault rupture.

Masonry veneers

The vast majority of unreinforced brick veneers suffered very little or no damage. Some had minor cracking in the mortar joints but no lateral displacement of the brickwork. In a small number of cases, mostly close to the epicentre, brick veneers became cracked or partially detached from the framing (see Figure 4a & 4b). In one particular building (see Figure 5), recycled brick veneer performed very poorly because the bricks had not been properly cleaned, and there was no adhesion of the mortar, reducing mechanical interlock between the new mortar and the old bricks.

Older heavier houses (mainly pre-1970s) with heavy brick veneer or concrete block veneer and concrete tiled roofs, appeared to suffer more structural damage than newer lighter buildings. This can be attributed to the greater mass of the structures. Modern brick veneer construction generally uses thinner bricks and lightweight roof cladding, giving improved seismic performance.

Chimneys

The most common type of damage for older buildings (more than 15 years old) was chimney collapse. This occurred in many thousands of buildings (see images 6a and 6b).
Falling chimneys could be interpreted as a violation of the ‘life-safety’ criterion required by New Zealand Standards (NZS1170.5:2004) for current building seismic design. Falling chimneys resulted in damage or piercing of the surrounding roof structure, damage to neighbouring properties, vehicles but (luckily) no loss of life.

Chimney collapse on to corrugated steel roofing often caused no further damage, depending on the height of the chimney, but some fell through the roof or caused rafter failure. Chimneys falling on to tile roofs (concrete or clay tiles, or slate roofs) more often fell through into the house, sometimes causing further structural damage and potential loss of life.

Damage to contents

Damage to contents varied greatly in different parts of Christchurch. In general, damage increased with increase severity of shaking. Many houses had little or no damage to contents. The most common damage to contents was broken glass-ware falling from in cupboards, draws and shelves. In areas of the strongest shaking such as West Melton, cabinets, bookshelves, televisions and stereos overturned. Washing machines, refrigerators and freezers moved, sometimes colliding with walls. There were some cases of hot water cylinder brackets breaking and pipes fracturing. Outside the houses, many block fences collapsed and cars were damaged from unrestrained items stored in garages.

Structural masonry

There was extensive damage and partial collapse of old (pre-1930) unreinforced brick masonry houses which do not comply with modern standards. The majority of these buildings consist of double brick structural walls and timber roof trusses, typical of early colonial construction. The Deans Homestead at Homebush is an example (see Figure 7). A number of unreinforced or minimally reinforced concrete block structures (garages and house) had very large diagonal cracks. Performance of unreinforced structural masonry buildings is described in a separate report.