Hoop-iron Bond.—This consists in placing pieces of hoop iron longitudinally in the joints to strengthen the bond, the ends of the iron being turned down into vertical joints.
Pressed Brick Facing.—In applying a facing of pressed bricks to a wall of common bricks, it is quite common to lay all of the face bricks as stretchers. When this is done bond may be obtained by metal ties or by diagonal bond.
Metal Ties are sometimes used as shown in Fig. 37. When the joints in the face and backing cannot be brought to the same level, the metal tie may be bent, but this is not desirable, and frequent level joints should always be possible. These ties may consist of a thin piece of galvanized iron bent over a wire at the ends, or it may be a piece of galvanized wire bent into a loop at the ends to grasp the mortar.
Diagonal Bond consists in breaking off the back corners of face bricks and inserting bricks diagonally to bond with the face brick.
These bonds are not very strong, and the face bricks are not con sidered as adding to the strength of the wall or carrying any load. Stronger work is obtained by using occasional courses of headers, or courses of alternate headers and stretchers as in the Flemish bond. This is usually possible by using care in regulating the thickness of joints in the backing, even when the bricks are not of the same sizes.
Hollow Brick Walls.—For the purpose of providing air space in a wall to prevent the passing of moisture or changes of temper ature through it, hollow construction is sometimes adopted. This consists in building a double wall with a narrow air space between the outer and inner portions.
It is necessary for proper strength that the two portions of the wall be bonded in some way, either by occasional headers which span the opening or by metal ties. The headers constitute a con nection between the masonry of the two walls, and are sometimes objected to as likely to cause moisture to pass from one wall to the other. The metal tics may be provided with a drip at the middle which insures the complete isolation of the walls from each other. Such walls require more careful work and are more expensive to construct than solid walls. When loads are to be carried, one of the
walls must be capable of hearing them.
62. Strength of Brick Masonry.—In tests which have been made on the crushing strength of brick piers, failure occurred by the lateral bulging of the piers. When pressure is applied longitudinally upon the pier, a lateral expansion normal to the direction of pressure results. This causes tension upon the brickwork and the pier yields through breaking the bricks in tension and pulling apart of joints. The transverse strength of the bricks may also be called into play when they are not bedded with perfect evenness—a fact proven by a series of tests on brick piers at the Watertown arsenal in 1907, in which bricks set on edge gave somewhat higher strengths than when laid flat. Piers in which the joints were broken at every third or sixth course gave slightly better results than those breaking joints at every course, as was also observed in piers tested in 1884.
The strength of brickwork depends upon the bond as well as upon the adhesion of the mortar and the strength of the bricks. In masonry to he subjected to heavy loads, careful attention should be given to the bonding of the work and to the complete filling of the vertical joints in laying the masonry.
The advantage of using strong mortar in such work is demon strated by many tests made at Watertown arsenal and reported by the Ordnance Department of the United States Army in " Tests of Metals, etc." That the strength of brick masonry in piers is some what proportional to the strength of the bricks is also demonstrated by these tests.
A series of tests made by A. N. Talbot and D. A. Abrams at the University of Illinois Experiment Station in 190S gives very inter esting results. A summary of these results is given in Table VI.
In the testing of brick piers it has been found that the initial yielding of the pier usually occurs at about one-half the breaking load. The safe load should be taken at not more than one-tenth to one-twelfth of the breaking load, on account. of the many elements of uncertainty concerning the actual strength, chances for defective work, et c.