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How We Make Structures Earthquake Resistant

Exaggerated deflecton diagram showing how a steel frame resists earthquake loads without collapsing.

It is surprisingly easy and cheap to produce realistic resistance to typical seismic activity (though it's not possible to design anything that would resist any conceivable earthquake). One death is a tragedy, but if the world's inhabitants had to choose between a few deaths and the thousands we know of in China, Mexico, Armenia, Turkey and so on, a few deaths would be the better choice. We should also consider the huge costs of disruption to the whole national economies of affected countries caused by earthquake damage. We can help. Steel framed structures offer the best solution to Earthquake problems. Single storey steel buildings if well designed are often strong enough at no extra cost, other than the checking of connections. Steel is light, resilient and ductile without loss of strength. The lightness reduces the earthquake's loads in the frames and the foundations. The resilience means they can bounce back from deformations. The ductility means they can deform and yield, absorbing energy, damping vibration, while still retaining good strength.

There are several killers in earthquakes. The first is horizontal or vertical acceleration of the ground, which moves suddenly sideways or up. If the frame has insufficient sway strength, it falls down there and then at the first big jerk. It's easy to design sway resistance in steel. The second is vibration from shock waves; like a tuning fork, a building will oscillate at its own frequency if relatively small shock waves come at the resonant frequency (often leaving taller or shorter structures nearby much less affected). Oscillation can build up and produce greater and greater sway loads until the building fails in sway or total overturning. This is where the ductility of the steel frame is so perfect; it deforms, absorbing energy and simultaneously changing the resonant frequency of the structure; both effects reduce oscillation.

The concrete cannot be torn off the deck, the deck can't be torn off the beams and the beams can't be torn off the columns.

The third killer is after-shock. Where buildings rely on internal walls or shear bracing for their sway resistance and such walls are damaged or displaced, the building can easily fail in a relatively small after-shock. A steel frame, however, would still be there. The best steel framed buildings for the job are designed with composite decking intimately connected to steel joists with full strength connections to steel main beams. The main beams are fully fixed by portalised connections to the columns to resist loads in reversal as well as the normal direction. The beams and connections are designed to yield plastically, protecting the columns, which are designed oversize to resist the haunched beam end moments elastically. There are no slabs to fall down. The joists tie the beams together. The beams can bend in plastic deformation and the columns remain elastic. These are the principles of REID earthquake resistant structure and they are surprisingly cost effective. (They resist hurricanes and blast too). They are in use in many seismic areas around the world.