Professor ANDY BUCHANAN takes a structural engineer's view of building damage in the Christchurch earthquakes.
Many people are asking "Why were so many modern buildings damaged beyond economic repair in the Christchurch earthquakes?"
The simple answer is that the ground shaking in Christchurch on February 22 was more than twice the level of shaking predicted by the Building Code used by the structural engineers who design modern buildings.
This article will focus on the causes of, and the responses to this damage caused by shaking.
The other main reason for damage is the unprecedented soil liquefaction, lateral spreading, and foundation failure, which can only be managed in the future by careful site investigation and high quality geotechnical advice for the design of all buildings and foundations.
The damage to buildings caused by ground shaking in Christchurch was largely as expected by structural engineers.
The old unreinforced masonry buildings were severely damaged. Moderately aged reinforced concrete and reinforced masonry buildings suffered significant damage, with two disastrous collapses.
Most well-designed houses and industrial buildings did not have major problems which cannot be repaired.
The biggest concern of structural engineers is with those modern multi-storey buildings which have been damaged beyond economic repair.
The seeds of this costly damage lie in the seismic design philosophy embedded in international building codes for seismic design, based on the principle that a minor earthquake should cause no damage, a moderate earthquake may cause repairable damage, and a huge earthquake can cause extensive damage but no collapse or loss of life.
As a very brief summary of the design process, when a structural engineer is designing a building for earthquake resistance it is necessary to provide the building structure with the three key attributes of strength, stiffness and ductility.
Strength is necessary so that the building can resist lateral forces without failure of the whole structure, or failure of any critical parts.
Increasing the strength of a structure costs money, but the required strength can be reduced if sufficient ductility is provided, as described below.
Stiffness is essential to limit the lateral deflections of the building during the earthquake, to ensure that secondary structural elements such as stairs, facades and ceilings are not damaged.
The stiffness (or flexibility) of a building is a measure of how much lateral movement will occur when it is subjected to lateral loads.
Modern building codes specify a maximum lateral deflection of about 75mm per floor under the highest expected earthquake loading.
Ductility is essential to avoid sudden failure after a building's strength limit is exceeded. Ductile materials like steel are often used locally in a building to increase the ductility of the whole building.
Ductile buildings are subjected to lower earthquake forces, making seismic design affordable, but they can be left with permanent structural damage. Ductility requires some toughness, or robustness, to ensure that a building hangs together under large movements caused by extreme loading.
A dilemma facing structural engineers is the trade-off between strength and ductility. Modern building codes provide for the design of safe but affordable buildings, by encouraging "capacity design" which allows for controlled damage in carefully selected ductile parts of the structure without exceeding the capacity of the most critical components.
In a severe earthquake, ductile buildings designed to minimum standards will have the most damage.
Many Christchurch buildings have such damage, as expected, and some will need to be demolished because repair is not economically viable.
This dilemma raises another question: "Can structural engineers economically design new buildings for no structural damage?"
There are three recognised strategies for limiting damage in a major earthquake, to provide both life safety and property protection. These three are overdesign, base isolation, and damage resistance.
The simplest and oldest method of limiting damage in a major earthquake is to overdesign the structure so that no damage occurs.
This can be achieved by increasing the design level well above the maximum expected earthquake. In this case little or no ductility is necessary.
Overdesign may be an economical solution for houses and low-rise buildings such as factories and schools, but for multi-storey buildings this solution is very expensive, and usually unaffordable, depending on the design level earthquake.
Base isolation is a second method of limiting damage in a major earthquake, reducing the response of the building by partially isolating it from the shaking ground.
This is done by placing the building on Kiwi-designed base-isolation units such as the lead-rubber bearings under Christchurch Women's Hospital, also used at Te Papa, and Parliament Buildings in Wellington.
These devices allow an economical building to be built on an expensive foundation, with the total cost being only a little more than conventional design.
Damage resistant design is the newest way of limiting damage in a major earthquake. Damage-resistant structures can be designed to absorb energy in a major earthquake, rocking back to an undamaged position.
This combines ductility to reduce the design forces with little or no residual damage. New Zealand engineers are contributing to international developments in this field, including the recently completed endoscopy building at Southern Cross Hospital in Christchurch, Te Puni Village at Victoria University in Wellington, and the new NMIT timber building in Nelson. Experimental research at the University of Canterbury has supported these developments, which will allow new damage-resistant buildings at no more cost than conventional designs.
The recent Christchurch earthquakes present a huge challenge and a huge opportunity to professional engineers.
Now is the time to show how Kiwi structural engineers and geotechnical engineers can contribute to a sustainable cityscape for the new Christchurch, designing attractive and safe modern buildings which will not suffer the fate of today's older buildings in future earthquakes.
The tools are available, with only a modest investment in building codes, education and research necessary to make it happen.
* Andy Buchanan is professor of Civil and Natural Resources Engineering at the University of Canterbury.
- The Press