Page 4 – Building for earthquake resistance
Engineers have a saying: ‘Earthquakes don’t kill people, buildings do.’ Destructive earthquakes have taught New Zealanders hard lessons in designing safe buildings. In early Wellington, buildings of brick and masonry collapsed in the 1848 earthquake. As a result, the town was largely rebuilt in wood, and suffered less damage during the magnitude 8.2 earthquake of 1855.
Laws and building codes
Widespread damage from the 1929 Murchison and 1931 Hawke’s Bay earthquakes had a profound effect on public perceptions of the hazard posed by earthquakes. Attention was focused on weaknesses in building construction, especially poor building standards and the lack of any provision for earthquake-resistant design. This led to a draft by-law in 1931, which was incorporated into a building code in 1935. The code recommended standards of design and construction so that buildings could resist the horizontal motions created by ground shaking. Masonry buildings had to be firmly bonded, with parts tied together so the structure would move as one unit.
Building codes in 1965, 1976, 1984 and 1992 have added requirements to accommodate changes in building materials and design. Rather than prescribing specific materials, designs or construction methods, the 1992 New Zealand code outlines how a building must perform to withstand the forces expected during an earthquake. This allows builders to use innovative design and construction methods to create earthquake-resistant buildings. For a moderate earthquake, the main aim is to protect a building from structural damage. For a major earthquake, however, the goal is to protect life by ensuring a building will not collapse and people can escape from it, even if the building itself is badly damaged.
Heritage or hazard?
Most older buildings in central Wellington were built between 1880 and 1930, and were not designed to resist earthquakes. In the 1970s the city council required such buildings to be strengthened. Many along Lambton Quay were replaced by new structures, and most of the remaining older buildings have now been upgraded. Such measures do not bring a building up to modern construction standards, but are aimed at avoiding collapse and minimising loss of life. It is anticipated that many older buildings would be damaged in a large earthquake and would ultimately need to be demolished.
Earlier building codes applied only to new construction, but current codes require many older buildings to be brought up to specified safety standards. A number of historic buildings have been strengthened, including New Zealand’s Parliament Buildings and the Wellington Town Hall.
Designing earthquake-resistant structures
The department of civil engineering at the University of Canterbury in Christchurch has gained international recognition for its research into the behaviour of reinforced and pre-stressed concrete in buildings and bridges during earthquakes. Their analysis and design methods have been used in structural design codes both in New Zealand and overseas. Books by New Zealand scientists and engineers have become standard texts.
Many buildings and bridges, both in New Zealand and overseas, are protected with lead dampers and lead and rubber bearings invented by a New Zealander, Bill Robinson. These devices in building foundations can reduce the motion caused by ground shaking. Te Papa Tongarewa, the national museum of New Zealand, and Parliament Buildings have been fitted with the bearings.
City lifelines – water, sewerage and drains, gas, electricity, telecommunications and transport networks – are also threatened by earthquakes. Lifeline engineering aims at reducing both the damage and the time needed to restore services. In New Zealand several measures have been undertaken to protect utilities. Flexible joints or ductile pipes have been used for water pipelines across unstable ground to prevent rupture. Similarly, gas pipelines have been welded to prevent breakage, or replaced by polythene. Some Wellington bridges and overpasses have bearings or dampers to reduce movement, preventing concrete decks from collapsing.
The Clyde Dam in Central Otago is built to withstand intense shaking, even though it is in a region where the probability of a major earthquake is low. The dam is built across the River Channel Fault, and has been constructed with a specially designed slip joint. If the land on either side of the fault moves during an earthquake, the joint will allow sections of the dam to shift up to 2 metres horizontally and 1 metre vertically without the dam failing.