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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, REIDsteel
Strengthening Existing 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.