STRENGTH OF STONE MASONRY. The results obtained by testing small specimens of stone (see § 16) are useful in determining the relative strength of different kinds of stone, but are of no value in determining the ultimate strength of the same stone when built into a masonry structure. The strength of a mass of masonry depends upon the strength of the stone, the size of the blocks, the accuracy of the dressing, the proportion of headers to stretchers, and the strength of the mortar. A variation in any one of these items may greatly change the strength of the masonry. The im portance of the mortar as affecting the strength of . masonry to resist direct compression is generally overlooked. The mortar acts as a cushion (§ 14) between the blocks of stone, and if it has insuf ficient strength it may squeeze out laterally and produce a tensile stress in the stone. It is certain that usually weak mortar causes the stare to fail either by direct tension or by tension due to flexure rather than by compression.
No experiments have ever been made upon the strength of stone masonry under the conditions actually occurring in masonry struc tures, owing to the lack of a testing machine of sufficient. strength. Experiments made upon brick piers (0 622) 12 inches square and from 2 to 10 feet high, laid in mortar composed of 1 volume portland cement and 2 sand, show that the strength per square inch of the masonry is only about one sixth of the strength of the brick. An increase of 50 per cent in the strength of the brick produced no appreciable effect on the strength of the masonry; but the sub stitution of cement mortar (1 portland and 2 sand) for lime mortar (1 lime and 3 sand) increased the strength of the masonry 70 per cent. The method of failure of these piers indicates that the mortar squeezed out of the joints and caused the brick to fail by tension. Since the mortar is the weakest element, the less mortar used the stronger the wall; therefore the thinner• the joints and the larger the blocks, the stronger the masonry, provided the surfaces of the stones do not come in contact.
It is generally stated that the working strain on stone masonry should not exceed one twentieth to one tenth of the strength of the stone; but it is clear, from the experiments on the brick piers re ferred to above, that the strength of the masonry depends upon the strength of the stone only in a remote degree. In a general way it
may be said that the results obtained by testing small cubes may vary 50 per cent from each other (or say 25 per cent from the mean) owing to undetected differences in the material, the cutting, and the manner of applying the pressure. Experiments also show that stones crack at about half of their ultimate crushing strength. Hence, when the greatest care possible is exercised in selecting and bedding the stone, the safe working strength of the stone alone should not be regarded as more than three eighths of the ultimate strength. A further allowance, depending upon the kind of struc ture, the quality of mortar, the closeness of the joints, etc., should be made to insure safety. Experiments upon even comparatively large monoliths give but little indication of the strength of masonry. The only practicable way of determining the actual strength of masonry is to note the loads carried by existing structures. How ever, this method of investigation will give only the load which does not crush the masonry, since probably no structure ever failed owing to the crushing of the masonry. After an extensive correspondence and a thorough search through engineering literature, the following list is given as showing the maximum pressure to which the several classes of masonry have been subjected.