Earthquake force are compuyed by rules established in cheapte 7. Slab and round-head buttress dams are particularly efficient in resisting such force, because of small mass relative to rigidity for individual units, coupled with ability of the dam as a whole to yield to slight permanent displacement.
For general stability, the most unfavorable direction of motion for such dams is upstream horizontally. For the deck slabs, the maximum masonry inertia load is for motion normal to the face. Definite rules for the computation of the increased water pressure on the inclined face are lacking. The approximate rules established in Arts. Cross-stream acceleration is unimportant for straight-faced dams. Unreinforce buttresses may need checking for slab strength under lateral loading, but the buttresses cannot overturn sidewise.For multiple-arch dams, the important earthquake effect may come from transverse motion. The force resulting from the inertia of the masonry in the arch barrels is readily computed. Unless the buttresses are stable in themselves or are securely braced against lateral displacement, they must be held againt overturning sidewise by the arches. This introduces an accumulating transverse load, applied at the spring lines and transmitted to the abutments by the arches. The resulting stress may be of appreciable magnitude. The load is difficult to compute but may be approximated by dividing the the buttress into blocks and computing the force required to prevent overturning, proceeding step by step from the top downward. The small volume of water in the troughs over the piers adds to the uncertainly of increased water pressure from cross-stream acceleration. This effect has generally been ignored, although not on the side of safety.