Withstanding Seismic Risks

We often take for granted the idea that the buildings we use every day will remain standing. This becomes most apparent during seismic events when the structural capacity of the built environment is put to the ultimate test. Yet the damage caused by extreme earthquakes is highly variable, which is in fact best illustrated by the differences in the destruction caused by recent earthquakes in Haiti and Japan.

Earthquakes will continue to be a natural hazard as long as we continue to live in seismically active areas, so it is worth understanding the reasons why some buildings stay standing and others collapse when exposed to the same risks. Ultimately, that distinction is in the building codes governing construction practices, as well as how the buildings are maintained. Saeed Mirza, professor emeritus in Civil Engineering at McGill, has spent much of his career working on issues related to construction and society. “The devastation we saw in Haiti is what happens when there is no meaningful structural code,” he said. Even though that earthquake was 1,000 times less severe than the recent Japan event, over 200,000 people lost their lives and more than 90 per cent of buildings near the epicenter collapsed.

At their core, what structural codes do is allow designers to determine two things: expected loads, and buildings’ expected ability to resist those loads. Inherent in these estimates (which are based on the best available data and probability) is the risk that we will underestimate the applied loads and overestimate the resistance, leading to failure and possible loss of life. In practice, the way that design engineers minimize that risk is by making their resistance estimates as conservative as realistically possible. For earthquake resistance, the Canadian code ensures that structures will survive small to moderate earthquakes with little to no damage. In the case of larger earthquakes, the design philosophy emphasizes multiple layers of redundancy and types of ductile failures that allow maximum survivability (such as beams that stretch and sag instead of suddenly cracking in half, allowing occupants to escape). The code is strictest when it comes to ensuring that structures needed post-disaster – such as fire stations or hospitals – remain intact even when less critical structures collapse.

The corollary to this philosophy is the fact that less risk necessarily costs more money, and that this relationship is not linear. For post-disaster design, the additional material, design, and construction will increase building costs by 20 to 30 per cent, sometimes even more. Though the question of whether additional safety is worth the money is unpleasant to ask, structural codes exist to ensure that whatever the answer, loss of human life will hopefully be minimized.

How structural codes determine a feasible solution to this problem is highly dependent on location, since the forces that govern structural design are generally region-specific. Snow loads may be particularly troublesome in Eastern Canada – and often control design – but are extremely unlikely in places like Arizona. Likewise, the building code in Japan is notably conservative in respect to earthquake design since the country is particularly seismically active. All of Japan is potentially at risk, so the Japanese structural code requires measures not seen in other countries, such as base-isolated foundations, or multiple redundant structural systems. With these advanced construction practices comes a higher price tag for each building, but the Japanese government has decided that these higher prices are worth the lives saved.

Even the most conservative building codes are not 100 per cent effective, because there is always a chance that the design for “worst-case scenario” will be exceeded. In the wake of such events, the lessons learned help improve our codes, and make future buildings even safer. Though these changes do much to improve the safety of new buildings, they do not affect the state of existing construction. The absolute importance of these changes can be seen in the fact that earthquakes disproportionally effect older buildings, while their modern counterparts perform as intended.