Back to
Information Area
How to Make Buildings Safer
in Tsunamis
What do we mean by a
Tsunami?
Why do buildings fail in Tsunamis?
So, how can we make buildings resist Tsunamis?
So, how can REIDSTEEL make Tsunami Resistant Buildings?
What do we mean by a Tsunami?
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.
An Earthquake is an enormous release of energy in the Earth's crust, and
sends out shock waves in all directions. In addition, the ocean floor
either side of a fault line can be forced up or down by a few feet (a
metre or so). This can raise or lower a mass of water hundreds of miles
long by a mile or two wide. The recent Tsunami along the coast of Sumatra
was just such an event, caused by an earthquake offshore, where a fault
line some 1000 miles long ruptured. There will be others along connecting
fault lines at some time.
A volcanic eruption in or close to the Ocean will also release an huge
amount of energy, and magma erupting will displace a lot of water in the
form of waves. The eruption of Krakatoa, 2 centuries ago, caused a Tsunami
throughout South East Asia, and the ripples travelled throughout the
world.
A large meteorite landing anywhere on earth will also cause Tsunamis.
There is no end to the energy they could poses. A little one would make a
splash. A big one could shatter the globe into fragments; there is not
much to gain by worrying about that one. But all sorts of meteorites land
on Earth daily, and occasionally some of these will create significant
Tsunamis.
A Landslip, most significantly from steep land into water, can suddenly
displace a large volume of water, causing a big wave to fan out.
Scientists in a Fjord in North West Canada were baffled when trees from
water level to 300 feet out of the water were found smashed down around
the whole inlet and on an island on the way out to sea. The reason only
became clear several years later. A cliff upstream slipped into the inlet.
A wave 300 feet high swept a boat, with 2 fishermen on the ride of a
lifetime, over the top of the 200 foot high island, out to sea. These
Landslip Tsunamis may be the most damaging: there is a large mountain-side
in the Canaries which may slip into the Atlantic, and the wave may
severely damage New York.
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 shelving beach, but will be
reduced over a rough surface, which tends to trip it over and make it
break earlier.
Back to Top
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 more 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 become 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.
Back to Top
So, how can we make buildings resist Tsunamis?
The answers are obvious from the above.
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 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.
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.
Back to Top
So, how can REIDSTEEL make Tsunami Resistant
Buildings?
How a building can resist flooding is best demonstrated by the recent
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.
Rollo Reid
C Eng FIStrucE, Director, Reid Steel.
You may like to read the articles on Hurricanes
and Earthquakes, too.
Back to Top
|
