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Strength of Stone Masonry

pressure, loads, center, mortar, joints, rubble and ashlar

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STRENGTH OF STONE MASONRY Compressive Strength.—Stone masonry varies widely in strength according to the character of the construction. The accuracy with which the joints are dressed, the strength of the mor tar, the bonding of the masonry and size of blocks of stone are more important than the strength of the stone itself.

No experimental data are available which show the actual strength of masonry as used. The mortar has usually much less strength than the stone, and in some experiments on brick piers, the mortar seemed to squeeze out, causing the failure of the brick in tension. The loads to which masonry is ordinarily subjected are much less than its actual strength, but when heavy loads are being carried by piers or arches, it is frequently necessary to proportion the sec tion to the load.

When the masonry is of cut stone xvith thin joints and cement mortar, the strength of the masonry may be proportioned to the strength of the stone. For rubble with thick joints, the strength of the stone has no material effect upon the strength of the masonry.

The loads used in practice vary quite widely according to the views of the designers. Building laws of the various cities differ considerably in the loads allowed. The following may be considered as conservative values for the limits of safe loading: Cut stone, with joints not more than a inch in first class Portland cement mortar: Tons per Square Foot.

Granite 50 to 60 Hard limestone or marble 35 to 40 Sandstone 25 to 30 The siliceous sandstones may have larger values, while the soft limestones should be reduced.

For ashlar of good quality as commonly laid with -inch joints in Portland cement: Tons per Square Foot.

Granite 40 to 45 Limestone, hard 35 to 40 Sandstone 25 to 30 masonry composed of large blocks of squared stone, 1-inch joints, in Portland cement mortar: Tons per Square Foot.

Sandstones or 10 to 20 Granite 20 to 30 Lncoursecl rubble: In cement mortar 5 to 8 In lime mortar 3 to 5 For an ashlar pier whose height exceeds ten times, or a rubble pier whose height exceeds five times, its least lateral dimension, these figures should be reduced. Piers of small dimensions carrying heavy loads should always be of ashlar. Hubble should not be

used for less thicknesses than 20 to 24 inches when it is necessary to develop the full strength of the masonry.

Failures of masonry inost frequently occur through defective foundation or workmanship. Masonry, to develop its full strength, must always be adequately supported, so that unequal pressures are not produced through settlement.

Weight of lllasanry.—In determining loads, it is usually necessary to estimate the weight of masonry. This depends upon the specific gravity of the stone and the closeness of the joints. The following table gives approximate weights for the different classes of stone masonry: Pounds per Cubic Foot.

Limestone, ashlar 155 to 165 Limestone, squared rubble 145 to 150 Limestone, rough rubble 135 to 140 Granite, ashlar 165 to 170 Granite, squared rubble 155 to 160 Sandstone, ashlar 135 to 150 Sandstone, rubble 120 to 140 52. Capstones and Templets.—When loads are to be transferred from the ends of beams or columns to masonry walls or piers, bearing blocks may be necessary properly to distribute the loads over the surface of the masonry. When used under a column or post, these blocks are called capstones; when used in walls to carry the ends of beams, they are templets.

In placing bearing blocks, the loads should always be centered on the top of the block, if possible, so as to produce uniform pressure upon the masonry below; in all cases, the center of pressure must be within the middle third of the base to avoid a tendency to open the joint between the bearing block and the masonry, In Fig. 31, let P= the vertical load at center of pressure; Icr = the pressure at edge nearest the center of pressure; the pressure at edge farthest from center of pressure; 1=the length of stone; x= distance from middle of block to center of pressure; distance from nearest edge to center of pressure; lo =distance from farthest edge to center of pressure; b" = width of block. Then, When x=0, and k2 = P; lb. When x=1/6, 11=7/3, lb and k2=0. If x becomes greater than 6, k2 is negative and a tension will be developed in the joint or it will open.

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