ENGINEERING FOR EARTHQUAKES
Why do buildings collapse?
Shaking causes support structures to shear and structural elements, such as beams, posts, and joists fail. Insufficiently secured stairwells collapse. Un-reinforced cladding such as brick or stonewalls give way and balloon outwards. In the Christchurch earthquakes, many people were killed by falling bricks when fleeing onto the street. Heavy tiled roofs shatter and fall inwards. Un-reinforced decorative items such as turrets topple. Deaths and injury in buildings also occur when heavy objects fall. Ground may liquefy, causing buildings to lean and topple.
What forces are we talking about?
Large earthquakes unleash energy many times greater than the atomic bombs that destroyed Hiroshima and Nagasaki, equivalent to millions of tons of TNT. The magnitude 6.3 February 22, 2011 Christchurch earthquake was assessed at between levels VIII and X plus on the Modified Mercalli Index (MMI), defined as “destructive” and “intense” to “extreme” and above. The scale runs from I (“instrumental”) to XII (“cataclysmic”). At a local level, this scale is a better indicator of destructive power than the widely used Richter scale. In Christchurch, peak ground acceleration (PGA) exceeded twice the acceleration of gravity in some areas – among the strongest ever recorded. Movement was both vertical and lateral.
Are tall buildings riskier?
Not necessarily, but they can be scarier. Many are designed to sway from side to side – as much as several feet in each direction – in a major earthquake. The higher up you are, the more they move. However, lower buildings of just a few stories can also be deadly (see above).
How do you make buildings safer?
There are two distinct approaches: First, is to make them stronger though the use of ultra-tough reinforcing such as heavyweight steel and concrete. The core of a building is designed to absorb shocks. This capability is known as ductility. The second is to make them flexible, using steel or timber framing. This is like rolling with the punches. Foundations must also be able to withstand earthquakes. Piles can be driven down – as much as 60 feet – to reach solid rock. Base isolators – which look like giant shock absorbers – may also be used (See below). An alternative is the use of floating foundations (used in some water houses in the Netherlands, for example).
Can old buildings be saved?
Sometimes, but doing so may prove to be uneconomic. Old buildings require substantial earthquake strengthening, principally by reinforcing walls, ceilings, and floors. One of the world’s largest examples of earthquake strengthening is the Hagia Sophia in Istanbul, built in the fourth century, first a church, then a mosque, and now a museum.