How to Make Buildings & Structures Earthquake Proof
- What is an earthquake
- What makes a building or structure fail in earthquakes?
- How can we make buildings resistant to earthquakes?
- When looking at design and construction, how do we earthquake proof buildings?
- How can existing buildings be strengthened to resist earthquakes?
There are a wide variety of earthquake effects – these might include a chasm opening up or a drop of many metres across a fault line.
Therefore, it is not possible to design an earthquake proof building which is guaranteed to resist all possible earthquakes.However, it is possible during your design and construction process to build in a number of earthquake resistant features by applying earthquake engineering techniques, which would increase enormously the chances of survival of both buildings and their occupants.
What is an earthquake?
Both the seabed and the land that we inhabit are formed of a crusty skin of light rocks floating on the soft centre of the earth, which is made of heavier molten rock and molten iron. This crusty skin is not one solid piece but is made up of lumps, separated by faults and trenches, or pressed together into mountains.
These separate lumps and plates are not static but are moved in slow motion by convection forces in the molten core, gravitational forces from the Sun and Moon and centrifugal forces from the Earth’s rotation.
Some plates are moving apart, particularly in the Mid Ocean Trenches, where molten material pushes up and shoves the plates apart, whilst others are bumping into each other head on, these form mountains like the Himalayas (the whole of the Indian Sub Continent is moving northwards and hitting Asia, for example).
Some are sliding one over another, like the west coast of the Americas, where the land plates are sliding over the denser ocean bed plates, causing the Andes and the Rockies to be thrust upwards. Some plates are moving along past each other, sticking together a while at a fault line, often in combination with bumping or sliding under/over (As in San Francisco).
All of these movements cause earthquakes (and usually volcanoes as well). If the movement was steady, about a millimetre or so a year, no one would notice. But the plates tend to jam; the movement carries on, but the material where they touch is stretched, or compressed, or bent sideways.
The material deforms (like stretching or compressing or twisting a bit of plastic). At some stage it reaches the breaking point along all or part of the joint, then it breaks, and there is a sudden movement. The movement may be tiny or may be several feet; but enormous amounts of energy are released, far more than the biggest Nuclear Bombs.
The shock waves from this release of energy shoot out in all directions, like the ripples when you throw a stone in a pond: except that they travel faster through the land. They can be measured all around the whole world. This is an Earthquake.
Prior to the Earthquake there are often little warning shakes, where highly stressed bits break and the plate joints readjust themselves a little, but allow the main join to become more stressed. After the primary Earthquake when the main join has failed and moved, there is another readjustment, and further bits around the fault become overstressed too, and they fail.
These aftershocks can themselves be highly energetic Earthquakes. After the Earthquake, the area settles down again. But the movement carries on and the next Earthquake is already building up, remorselessly.
People forget and build buildings and structures that are going to kill their children next time when they could ensure that during the design and construction phase some earthquake proof measures have been incorporated.
What makes a building or structure fail in earthquakes?
An Earthquake moves the ground. It can be one sudden movement, but more often it is a series of shock waves at short intervals, like our ripples from the pebble in the pond analogy above. It can move the land up and down, and it can move it from side to side.
All buildings can carry their own weight (or they would fall down anyway by themselves). They can usually carry a bit of snow and a few other floor loads and suspended loads as well, vertically; so even badly built buildings and structures can resist some up-and-down loads.
But buildings and structures are not necessarily resistant to side-to-side loads, unless this has been taken into account during the structural engineering design and construction phase with some earthquake proof measures taken into consideration. This weakness would only be found out when the Earthquake strikes, and this is a bad time to find out. It is this side-to-side load which causes the worst damage, often collapsing poor buildings on the first shake.
The side-to-side load can be worse if the shocks come in waves, and some bigger buildings can vibrate like a huge tuning fork, each new sway bigger than the last, until failure. This series of waves is more likely to happen where the building is built on deep soft ground, like Mexico City. A taller or shorter building nearby may not oscillate much at the same frequency.
Often more weight has been added to a building or structure at most frequently at greater heights; say another floor and another over that; walls built round open balconies and inside partitions to make more, smaller, rooms; rocks piled on roofs to stop them blowing away; storage inside. This extra weight produces great forces on the structure and helps it collapse.
The more weight there is, and the higher this weight is in the building, the stronger the building and its foundations must be to be resistant to side earthquakes; many buildings have not been strengthened when the extra weight was added.
Often, any resistance to the sway loading of the building is provided by walls and partitions; but these are sometimes damaged and weakened in the Main Earthquake. The building or structure is then more vulnerable, and even a weak aftershock, perhaps from a slightly different direction, or at a different frequency, can cause collapse.
In a lot of multi storey buildings, the floors and roofs are just resting on the walls, held there by their own weight; and if there is any structural framing it is too often inadequate. This can result in a floor or roof falling off its support and crashing down, crushing anything below.
Often more weight has been added to building or structure at a higher level, for example another floor, extra walls and partitions, extra storage or even rocks piled on roofs to stop them blowing away. Small cracks appear in the concrete.
The bonding of the ‘stirrups’ (the small steel bars which bind the main reinforcement together) to the concrete weakens, the outer concrete crumbles (spalling), the main reinforcing bars can bend outwards away from the column and all strength disappears. This was beautifully demonstrated under the Oakland Freeway, where huge round concrete columns crumbled and crumpled.
They have now been reinforced with massive belts around them as a result of an earthquake engineering review and to improve structural dynamics.
In a lot of multi storey buildings the lower floor has more headroom (so taller columns); and it often has more openings (so less walls); and it is usually stood on ‘pinned’ feet with no continuity. So the ground-to-first floor columns, which carry the biggest loads from the weight and the biggest cumulative sideways loads from the earthquake, are the longest and the least restrained and have the least end fixity.
They are often the first to fail. It only takes one to fail for the worst sort of disaster, the pancake collapse so familiar to any one who has seen the results in Armenia, Mexico, Turkey, Iran, Peru, and now Pakistan and Kashmir. Sometimes buildings are built on soft soil; this can turn into quicksand when shaken about, leading to complete slumping of buildings into the soil.
Some tall buildings can stay almost intact but fall over in their entirety. The taller the building, the more likely this is to happen, particularly if the building can oscillate at the frequency of the shock waves, and particularly if some liquefaction of soft soil underneath has allowed the building to tilt.
How can we make buildings resistant to earthquakes with earthquake engineering?
To be earthquake proof, buildings, structures and their foundations need to be built to be resistant to sideways loads. The lighter the building is, the less the loads. This is particularly so when the weight is higher up. Where possible the roof should be of light-weight material. If there are floors and walls and partitions, the lighter these are the better, too.
If the sideways resistance is to be obtained from walls, these walls must go equally in both directions. They must be strong enough to take the loads. They must be tied in to any framing, and reinforced to take load in their weakest direction. They must not fall apart and must remain in place after the worst shock waves so as to retain strength for the after shocks.
If the sideways resistance comes from diagonal bracing then it must also go equally all round in both directions. Where possible, it should be strong enough to accept load in tension as well as compression: the bolted or welded connections should resist more tension than the ultimate tension value of the brace (or well more than the design load) and it should not buckle with loads well above the design load.
And the loads have got to go down to ground in a robust way. If the sideways load is to be resisted with moment resisting framing then great care has to be taken to ensure that the joints are stronger than the beams, and that the beams will fail before the columns, and that the columns cannot fail by spalling if in concrete. Again the rigid framing should go all around, and in both directions.
If the building earthquake resistance is to come from moment resisting frames, then special care should be taken with the foundation-to-first floor level. If the requirement is to have a taller clear height, and to have open holes in the walls, then the columns at this level may have to be much stronger than at higher levels; and the beams at the first floor, and the columns from ground to second floor, have to be able to resist the turning loads these columns deliver to the frame.
Alternatively, and preferably, the columns can be given continuity at the feet. This can be done with ‘fixed feet’ with many bolts into large foundations, or by having a grillage of steel beams at the foundation level able to resist the column moments. Such steel grillage can also keep the foundations in place.
If the beams in the frame can bend and yield a little at their highest stressed points, without losing resistance, while the joints and the columns remain full strength, then a curious thing happens: the resonant frequency of the whole frame changes. If the building was vibrating in time with shock waves, this vibration will tend to be damped out.
This phenomenon is known as ‘plastic hingeing’ and is easily demonstrated in steel beams, though a similar thing can happen with reinforced concrete beams as long as spalling is avoided.
All floors have to be connected to the framing in a robust and resilient way. They should never be able to shake loose and fall. Again all floors should be as light as possible. They should go all round each column and fix to every supporting beam or wall, in a way that cannot be shaken off.
One way of reducing the vulnerability of big buildings is to isolate them from the floor using bearings or dampers, but this is a difficult and expensive process not suitable for low and medium rise buildings and low cost buildings(though it may be a good technique for Downtown Tokyo). Generally it is wise to build buildings that are not too high compared to their width in Earthquake areas, unless special precautions are taken.
When looking at design and construction, how do we earthquake proof buildings?
When designing earthquake safe structures the first consideration is to make the highest bit, the roof, as light as possible. This is best done with profiled steel cladding on light gauge steel Zed purlins. This can also have double skin with spacers and insulation. It can have a roof slope between 3 and 15 degrees. If it is required to have a ‘flat’ roof, this could be made with a galvanised steel decking and solid insulation boards, and topped with a special membrane. Even a ‘flat’ roof should have a slope of about 2 degrees.
If it is required to have a ‘flat’ concrete roof, then the best solution is to have steel joists at about 2m, 6″, centres, and over these to have composite style roof decking. Then an RC slab can be poured over the roof, with no propping; the slab will only be say 110mm, 4 1/2″, and will weigh only about 180 kg/sqm. Such a slab will be completely bonded to the frame and will not be able to slip off, or collapse.
If the building or structure is a normal single storey, then any normal portal frame or other steel framed building, if the design and construction is competently done, will be resistant to Earthquake loads. If it is to have 2 or more stories, more needs to be done to ensure its survival in an earthquake. As with the roof, the floors should be made as light as possible. The first way to do this is to use traditional timber joists and timber or chipboard or plywood flooring. If this is done it is vital that the timber joists are firmly through bolted on the frames to avoid them slipping or being torn off. The frame needs them for stability and the floor must never fall down.
A better alternative is to substitute light gauge steel Zeds for the timber joists. These can span further and are easier to bolt firmly to the framework. Then, floor-boards or tongue-and-groove chipboard can easily be screwed to the Zeds. However in Hotels, Apartment buildings, Offices and the like, concrete floors may be needed. In such cases we should reduce the spans to the spanning capacity of composite decking flooring, and pour reinforced concrete slabs onto our decking. The decking is fixed to the joists, the joists into the main beams, the main beams into the columns and the concrete is poured around all the columns. There is simply no way that such floors can fall off the frame.
Proof building diagram
Once the floors are robustly fitted to the frames, the frames themselves must be correctly designed. Please look at the diagram above.
Start at the bottom. The frame should not be built on simple pinned feet at ground level. Outside earthquake zones it is normal to build a ‘nominally pinned footing’ under each column. This actually gives some fixity to the base as well as horizontal and vertical support. But in an earthquake, this footing may be moving and rotating, so rather than provide a bit of fixity, it can push to left or right, or up and down, and rotate the column base, helping the building to collapse prematurely.
Any pinned footing may actually be moving differently from other footings on the same building, and so not even be giving horizontal or vertical support, but actually helping to tear the building apart. So to earthquake proof the building REIDsteel would start with steel ground beams joining the feet together, and these should have moment resistance to prevent the bottoms of the columns from rotating.
These ground beams may well go outside the line of the building, thus effectively reducing the height-to-width ratio as well, helping to reduce total over-turning. This ground beam may be built on pads or piles or rafts as appropriate. On loose soils, the bearing pressure should be very conservatively chosen, to minimise effect of liquefaction.
By applying earthquake engineering techniques, REIDsteel would then fit the columns to these ground beams with strong moment connections. Either the connections should be strong in both directions, or some columns designed to resist loads in one direction and others in the other direction. The columns should not be the item that fails first: the ground beam should be able to rotate and form plastic hinges before either the connection or the column fails.
The reason is that a column failing could instigate a collapse; the connection failing could instigate the column failure. In comparison, the plastic hinging of the ground beam takes time, absorbs energy, and changes the resonant frequency of the frame while leaving the frame nearly full strength.
Next, REIDsteel would fix the main beams to the outer columns with full capacity joints. This will almost always mean haunched connections. Great care would be taken to consider the shear within the column at these connections. The connections should be equally strong in both up or down directions, and the bolt arrangement should never fail before the beam or the column. In extreme earthquake sway, the beams should always be able to form hinges somewhere, in one or two places, without the column with its axial load failing elastically.
In this way the frame can deflect, the plastic hinges can absorb energy; the resonant frequency of the structure is altered, all without collapse or major loss of strength. All this takes a little time until the tremor passes. The inner columns do not give a lot of sway resistance, but even so, should have connections which do not fail before the beam or the column. Then, the floors are fitted, Light-weight or conventional cladding is fitted to the frames, light-weight or thin concrete roofs are fitted as described above. You have a building that will behave very well in an earthquake with significant resistance to damage.
Nothing can be guaranteed to be fully resistant to any possible earthquake, but buildings and structures like the ones proposed here by REIDsteel would have the best possible chance of survival; and would save many lives and livelihoods, providing greater safety from an earthquake.
C Eng FIStrucE, Director, Reid Steel.