In designing a bearing block, must not he greater than the safe load for the masonry. The load is commonly brought on top of a bearing block through an iron plate, which should have such area that the pressure will not be more than one-tenth to one-tweith of the crushing strength of the stone. The bearing block must have sufficient thickness not to break under the transverse load imposed by the upward pressure of the masonry.
In designing a templet. which is to he built into a wall, the weight of wall resting on the top of the templet must he included in deter mining the pressure on its base.
53. Lintels and Corbels.—A stone lintel is a beam of stone span ning an opening in a wall. The strength of a lintel is determined by the ordinary beam formulas. The safe modulus of rupture may be taken at about one-twelfth to one-tenth of the ultimate modulus for the stone. Mean values of the safe modulus of rupture are about as follows: granite, 150 Limestone, 150; marble, 130; sandstone, 120 There are, however, certain tough sandstones, specially adapted to this use, which may be used with modulus of 250 to 300 lbs. in." Beams carrying live loads should not rest upon stone lintels. When the load upon a lintel is a solid masonry wall, it is common to assume that the masonry may arch over the opening, so that the actual weight upon the lintel is only that of a triangle whose height is about three-quarters of the span. This assumes that
the lintel will yield somewhat, and be relieved of stress before reach ing the maximum load. It is quite possible that in well-built masonry, with cement mortar, the lintel might be removed without the wall above yielding at all. If, however, there is no yielding of the lintel, the pressure upon its upper surface may be the same as at any other point in the same horizontal plane of the wall.
A corbel is a block of stone extending beyond the surface of a wall or pier for the purpose of carrying the end of a beam or an overhanging wall, see Fig. 32. The overhang of the corbel is a cantilever beam, which must have sufficient section at the surface of the wall to resist the bending moment clue to the load. The corbel must extend sufficiently into the wall, to give a resultant pressure within the middle third of the base of the corbel (R= P+W), as in the case of bearing blocks.
Double corbels may be used when necessary, each being separately treated in determining strength. When weight of wall above (W) is lack ing, the corbel must be anchored to the wall below by steel ties.