Weight of Stone

For somewhat similar reasons the argillaceous limestone from which the Rosendale natural cement is made, gave a crushing strength of 40,875 pounds per square inch.

Crushing Strength of Slabs.

Only a few experiments have been made to determine the crushing strength of slabs of stone, that is, of specimens less in height than in width; and in the experiments most of the specimens had a thickness proportionally much greater than the blocks of stones employed in ordinary masonry. All of the experiments show that the strength per square inch of bed area is considerably greater for slabs than for cubes.

Prof. J. B. Johnson from experiments by Bauschinger deduces the formula* in which b = the least lateral dimension of the prism and h = its height. Eight experiments with the U. S. testing machine at Watertown t agree reasonably well with this formula.

When stones are used for lintels, etc., their transverse strength becomes important. The ability of a stone to resist as a beam depends upon its tensile strength, since that is always much less than its compressive strength. A knowledge of the relative tensile and compressive strengths of stones is valuable in interpreting the effect of different pressing surfaces in compressive tests, and also in determining the thickness required for lintels, sidewalks, cover stones for box culverts, thickness of footing courses, etc.

Owing to the small cross section of the specimen employed in determining the transverse strength of stones,—usually a bar 1 inch square,—the manner of dressing the sample affects the apparent transverse strength to a greater degree than the compressive strength (see 3: 13).

The following formulas are useful in computing the breaking load of a slab of stone. Let W represent the concentrated center load plus half of the weight of the beam itself, in pounds; and let 6, d, and l represent the breadth, depth, and length, in inches, respec tively. Let R = the modulus of rupture, in lb. per sq. in.; let C = the weight, in pounds, required to break a bar 1 inch square and 1 foot long between bearings; and let L = the length of the beam in feet. Then The equivalent uniformly distributed weight is equal to twice the concentrated center load.

According to tests made with the U. S. testing machine at Watertown, the transverse strength of each of the several classes of building stones in terms of its crushing strength is as follows:* Each result is the mean of four to six tests. The relatively small transverse strength of granite is evidence of the "grain" of that stone, the property which makes it easy to quarry prismoidal blocks of that material.

Table 3 gives the modulus of rupture of several kinds of stone as determined with the testing machine at the U. S. Arsenal at Water town, Mass., from 1883 to 1905. Each result is the mean of from one to four tests.

The question of what margin should be allowed for safety is one that can not be determined in the abstract; it depends upon the accuracy with which the maximum load is estimated, upon whether the live load is applied with or without shock, upon the care with which the stone was selected, etc. This subject will be discussed further in connection with the use of the data of the above table in subsequent parts of this volume.