SHEARING STRENGTH. Only occasionally is a stone used in such a position that its shearing strength is of any moment; but sometimes the shearing strength is important, for example, in a lintel or in a corbel. Not many experiments have been made on the shearing strength of stone, partly because of the relative unimportance of the matter and partly because of the difficulty in making the experiments to obtain a failure by shear independent of cross breaking.
The average of seventeen experiments on the U. S. testing machine at Watertown * seems to show that the shearing strength of stone is 11.4 per cent of its crushing strength, the range being from 7.9 to 22.1; while the average of eighteen experiments by Bauschinger t is 6.6 per cent, the range being from 5.7 to 19.1. The difference between the two sets of experiments is considerable; and is probably partly due to the use of different stones, but probably largely to differences in the method of making the experiments. In this connection see § 408.
Four to six tests upon each kind of stone, each test specimen coming from a different quarry, with the U. S. testing machine $ gave the shearing strength in terms of the compressive strength as follows: The modulus of elasticity of a stone is of value in computing the distortion under load of a monolith; and may throw a little light upon the distortion of stone masonry under load, although the yielding due to the mortar may be proportionally very large. The modulus of elasticity of all stones varies with the load, unlike that for steel, which is constant for all loads below the elastic limit. The granites, limestones, and marbles are nearly perfectly elastic for all working loads, but the sandstones take a permanent set for the smallest loads. Masonry is not usually subjected to loads of more than 100 to 1,000 pounds per square inch; and hence the values of the modulus of elasticity between these limits are given in Table 4.
Although the art of building has been practiced from the earliest times, and constant demands have been made in every age for the means of determining the best materials, yet the process of ascertaining the durability of stone appears to have received but little definite scientific attention, and the processes usually employed for solving this question are still in a very unsatisfactory state.
Hardly any department of technical science is so much neglected as that which embraces the study of the nature of stone, and all the varied resources of lithology in chemical, microscopical, and physical methods of investigation, wonderfully developed within the last quarter century, have never yet been properly applied to the selection and protection of stone used for building purposes." t Examples of the rapid decay of building stones have already been referred to, and numerous others could be cited, in which a stone which it was supposed would last forever has already begun to decay. In every way, the question of durability is of more interest to the architect than to the engineer; although it is of enough import ance to the latter to warrant a brief discussion here.
The destructive agents may be clas sified as mechanical, chemical, and organic. The last are unim portant, and will not be considered here.
The action of frost is usually one of the main causes of rapid decay. Two elements are involved,—the friability of the material *Tests of Metals, etc., 1894, p. 393-420; ibid., 1895, p. 341-372.
and its power of absorbing moisture. In addition to the alternate freezing and thawing, the constant variations of temperature from day to day, and even from hour to hour, give rise to molecular motions which affect the durability of stone as a building material. This effect is greatest in isolated masses, — as monuments, bridge piers, etc.