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How can we make buildings resist Tsunamis?
As rough ground reduces the effects of the wave, it is not a good idea to cut down all the vegetation and produce a smooth unprotected beach. Mangrove swamps are particularly good at stopping Tsunamis. Reefs too should be left intact, and not destroyed for shipping channels. It is better not to build buildings at low level on the shore line at the top of a smooth shallow beach. This is especially the case if on the sides of an inlet, which can channel and enhance the waves.
It is unlikely that the walls and frames could generally be designed to resist the water pressures in a breaking wave. If buildings have to be built, then it is better to make them higher, so that water can flow under them. They would then have suspended floors. If the suspended floors are concrete with suitable framing, their weight and integrity can combat some of the force of the wave.
Even if the building is above ground level, it will still be vulnerable to a bigger wave. It is possible to design the walls so that they can fail at ground-to-first floor level, but the frames must be strong enough to support the floors above without help from the walls. It helps if the building is not square on to the wave front. If diagonal, the wave hits the pointed corner first and is diverted around the sides. Pressure is much reduced. Buildings should not be close together in a way that makes a wider dam.
If roads have buildings all along both sides, the water is funnelled along the roadway, accumulating debris as it goes, and with no reduction in height or destructive force. It is much better if gaps are left between buildings out through which the water can dissipate. All the structural members have to be strongly fixed to the frame and then to the foundations, to prevent them floating off, and becoming missiles. If the soil is sandy, then the footings should be deep and bracing should go right down to the feet. Light soil will also be protected from erosion by tarmac or concrete surfacing, which should go right underneath the floor if it is raised.
As in seismic design, the most heavily loaded members, and the ones which take most bending, are the columns from the ground to suspended first floor. These usually have ‘pinned feet’, that is they are loosely fixed to concrete pads, or something. When the wave passes through, any such pad is scoured, and sinks or tips, so the effect, far from pinned, it is helping the building fail.
What you require is a grillage of steel beams, with moment connections to the columns, at or below ground level. And this grillage of steel beams should be enclosed in a concrete floor, which prevents tipping and scour. The weight of the water, at the same time that it is trying to push the building over or along, is also pushing this slab downwards, helping it resist the waves.
It is surprising that waves which can lift entire ships 30′ in the air do not destroy well made buildings around them.
Timber buildings are much liked in earthquake areas because they are light and thus reduce earthquake effects. But they are the worst possible choice in tsunami-prone areas; like the ships, they float, and there is nothing to hold them down.
As the water races around buildings it can erode the soil, particularly if it is loose sand, and the buildings can fall into the holes. It is a feature of many beaches that there is sandy soil.