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