Earthquakes have often been considered to be one of the greatest and unfortunate natural events that can take place. In the past, due to the lack of understanding and technology available there wasn’t much masons and architects could do. Today, structural engineers are trained to minimise damage and at times to try to eliminate it (if possible) though one must design especially for that.
The recent earthquake in Italy must have sent alarm bells ringing across the region. The damage was terrible and the shock even worse.
One often reads about earthquakes in far off regions, but when they occur in neighbouring countries one often wonders… what if?
The design of earthquake-proof buildings is complex indeed. There are various strengths of seismic activities and building codes often specify designs according to an expected worst-case scenario (over a long period of time). Obviously, when designing a high-rise building in say Japan or Greece, one would need to apply stiffer design guidelines and offer more protection than say in Malta.
The key word in the seismic design of buildings is flexibility. Tourists have often remarked how high-rise buildings have been seen to vibrate in Japan during seismic activities.
That vibration is good so long as it is controlled.
A flexible building will allow it to move at high frequencies but with small amplitudes.
When a building element is designed to deform gradually it cracks and starts to show its warning signs to the building occupants
Smaller amplitudes will help minimise damage.
The key issue is to design a flexible building rather than an overly heavy one. Heavier and less flexible buildings will oscillate at lower frequencies but with greater displacements, which in turn would lead to larger stresses and hence damage.
The actual design of concrete buildings in order to minimise damage (due to seismic activity) is actually quite interesting.
The worst possible scenarios during an earthquake would be for buildings to simply collapse without any warnings whatsoever.
This is similar to what happened to various buildings in Italy and usually results in the greatest loss of life and injuries. Structural engineers are taught to minimise loss of life, although to the layman this might seem impossible or easier said than done.
The approach an engineer would take would be to try and direct the modes of failure, which in a way is to design the collapse mechanisms and ultimately the order of failure for different building elements.
The way to do that is to avoid a brittle (or sudden) collapse and to promote a gradual one (with warning signs which come in the form of cracks).
When a building element is designed to deform gradually (rather than to simply collapse suddenly) it cracks and starts to show its warning signs to the building occupants who in turn will have time to vacate the building. This approach is a very popular one in seismic prone regions and has worked well.
Another aspect is to try and direct the order of collapse of various elements within a structure.
An obvious one would be to try and prevent a column failure, as this would arguably lead to a tragic scenario. Beam failures tend to be less tragic and would be more ideal during high seismic activity.
Currently in Malta not much attention is given to seismic design but as buildings are getting larger and more complex this is bound to change.
David Grima is an architect and chartered civil engineer specialised in structural engineering.