For most materials density is important chiefly in relation to strength, but its importance varies with the type of structure and the nature and magnitude of the live loading. It directly determines the self-weight or dead load. Frequent significant variations in loading in relation to the maximum are usually undesirable since they may lead to cumulative damage known as fatigue. Similarly, large wind-loads in relation to vertical loads are undesirable since they will accentuate any existing tendency to buckle under the vertical loading even if they do not overturn the structure as a whole. From both these points of view, therefore, the greater the dead load and hence the greater the density, the better. On the other hand, if the dead load constitutes a large part of the total load, it will also make the major contribution to the total stresses. This is undesirable where these stresses approach the limits of strength of the materials

as in some large modern structures. From this point of view a low density is desirable, or rather a low ratio of density to strength provided that this does not entail too low a stiffness. It may also be desirable under earthquake conditions since a high density will result in high inertial loads.