How We
Make Structures Earthquake Resistant
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Exaggerated
deflecton diagram showing how a steel frame resists earthquake
loads without collapsing.
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It is surprisingly easy and cheap to produce realistic
resistance to typical seismic activity in buildings (though it's not possible to design
anything that would be resistant to 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 and typical, non earthquake resistant buildings. We can help. Seismic resistant 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 to which non earthquake resistant buildings are more susceptible. 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. Thus steel framed earthquake resistant buildings with their better structural behaviour help to solve these problems.
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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.
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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 earthquake resistant building,
however, would still be there. The best steel framed seismic resistant buildings
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.
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