How to Make Buildings Safer in Tsunamis
- What do we mean by a Tsunami?
- Why do buildings fail in Tsunamis?
- How can we make buildings resist Tsunamis?
- How can REIDsteel make Tsunami Resistant Buildings?
What do we mean by a Tsunami?
It is normal to say 'nothing could resist a tsunami'; but this is not true. In places where boats and ships were deposited on top of buildings, well built buildings survive.
A tsunami is the Japanese for 'harbour-wave' and it means a big wave or a series of big waves. Common in the 'Ring of Fire', the seismic regions surrounding the Pacific, the Japanese fishermen would be at sea and not notice anything, but return to their harbours to find them destroyed by enormous waves. In English, the words Tidal Wave are some times used to mean Tsunami. They can be caused by any big disturbance in the Ocean or other body of water, such as earthquakes, volcanic eruptions, meteorites or landslips. Whatever causes the upset, a wave or series of waves is formed. The water is higher at the crest, lower in the trough. In deep water, the wave height may be very small, but the wave width and wave length may be very large. A wave 3 feet high, 2 miles long, 1000 miles wide, contains 10 billion tons of water. In deep water, the wave can travel very fast: maybe 600 mph, 1000km/hr - the speed of a jet airliner. But a boat might float up and down on this gentle giant of a swell without noticing it. The Fishermen in it would be astounded, on returning to their harbour, to find it destroyed.
When the wave comes close to land, a curious thing can happen. The water near the shore can, initially, be the trough of the coming wave: and the water may swirl out away from the shore. But as the leading edge of the fast moving wave comes into shallow water, it is slowed down. The water behind it doesn't know this, and carries on pushing forward. The edge of the wave gets higher. As more and more fast moving wave pushes into the slowing wave front, it gets higher and higher; and steeper and steeper. Eventually it can become a moving vertical wall of water, whose height depends on the geometry of the shore, and the characteristics of the Tsunami. The effect of the wave can be intensified, in particular if it enters a narrowing and shallow channel. In the same way that a tide 3 feet high in the Atlantic is confined by a triangular opening and produces 40 feet high tides at St Malo, Brittany, a wave pushing up a creek can get higher and steeper as it goes. Remember that many coastal settlements are up such inlets.
A wave will be bigger and stronger up a smooth shallow shelving beach, but will be reduced over a rough surface, which tends to trip it over and make it break earlier. If the ground slopes upwards away from the waterline, the power of the tsunami will be reduced. If the ground is low lying and level, like a lagoon, the wave can continue at its full strength for a long way, up to 5 kilometres or so. If the shore line is a bank, and then slopes down after the bank, the water will speed up and gain destructive energy.
Why do buildings fail in Tsunamis?
When a building stands in the path of the wave, the wall facing it tends to block the water, and the pressure here increases. It can overload walls, window, doors, columns or bracing systems, or push buildings completely over. Later on the water will swirl out again, loading the other side of the building. Water just 7 feet deep will have pressure of 450 pounds per sq ft, 21 kN per square metre, much more than any normal structure can withstand. If there is an opening in the side hit by the wave, the high pressure can find its way into a building. Here it will push through partitions, and far wall. The deeper the water, the greater the pressure.
There is a twist in the wave attack. As the water tries to escape from the dam, it rushes around the edges of the building, creating a series of small vortexes (or vortices). These are small ice-cream cone shaped spirals of water, which have intense suction at the tip. They tear away at the walls around every discontinuity. The debris from damaged buildings becomes weapons which attack other buildings, and are dangerous hazards to any one in the water. Hits from floating bits of building are a major cause of death and injury. 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.
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. The wood becomes weapons which destroy buildings and lives.
How can REIDsteel make Tsunami Resistant Buildings?
How a building can resist flooding is best demonstrated by the 2004 Tsunami. All the fragile shacks built at ground level were simply washed away. Multi-storey buildings that were weakly built with no side-sway resistance were badly damaged. Some multi-storey buildings had their lower wall pushed in on one side, and out on the other as the wave went through, but otherwise, survived. Some buildings were pushed along where they were not fixed firmly to firm ground. But well-built buildings survived in the middle of areas that were otherwise completely devastated.
To avoid wave surges, the building should be built out of the projected water path; and this may mean building it on legs with a suspended lower floor level. Even if the elevation of such a floor is modest, the forces from rushing water will be much less if the water can go under the building as well as round it. The buildings should be on a narrow front, with gaps between them, and preferably not at right angles to the Beach. Foundations may need to be deeper than usual and braced right down to the footings without counting on the soil around them for strength or stability. A frame which is of continuous construction in both directions is more likely to be able to survive loss of wall panels or even whole footings. The lower floor will be best in concrete to give some weight. The steel frames should be strong enough to resist substantial loads (the sort of loads needed to resist Hurricanes or Seismic loads for example). REIDsteel buildings, with columns, main beams, closely space steel joists, all bolted continuously together; and with the concrete poured on steel decking in such a way that it is trapped by the steel and cannot be dislodged: provide the best building method. Tsunami prone buildings are usually in Seismic areas anyway; and Beach-side developments are often in Cyclone or Hurricane areas too. The same REIDsteel construction methods are the best solution to all three problems.
There is no guarantee that any building could survive a Tsunami; but REIDsteel Tsunami resisting buildings will give the best chance possible, and would save many lives.
C Eng FIStrucE, Director, Reid Steel.