Another arrangement consists in introducing headers so as to unite two half-brick rings wherever the joints of two such rings happen to coincide. The rings are sometimes thus united in consecutive pairs right through the thickness of the arch.
Thick Arches bonded throughout their Depth, as shown in Fig. 48, have sometimes been used for large spans.
The joints in the extrados are necessarily very wide, but the evil effects of this may be guarded against by using cement or quick-setting mortar, or by wedging up the joints of the outer portion with pieces of slate. In this latter case, however, the inner rings are apt to be relieved of pressure, and the stretchers are liable to drop out.
Arches of large Span, in whatever way they are bonded, should be built in good hydraulic mortar, setting moderately quickly, so that when the centres are struck the joints may be soft enough to adapt themselves to the inequalities of the bricks, and thus enable them to obtain a firm bearing. Hoop-iron bond is sometimes introduced between the rings parallel to the soffit.
In ordinary buildings, however, arches of large span are seldom required, and they need not, therefore, be further alluded to.
1 The block B is drawn in thicker lines to make it distinct, but its joints are of the same thickness as those of the rest of the arch.
In process of time they shrink, they rot, and, in case of fire, they burn away; in either instance the whole superincumbent weight of the wall is thrown upon a small portion of it, or again, they may absorb moisture, swell, and overthrow the masonry.
Ranging Bond consists of narrow horizontal pieces built into the joints of walling parallel to one another at intervals of about 18 inches, to form grounds for battening, etc. etc. The face of the pieces projects slightly from the wall, so that the battens may be clear of the masonry.
It has already been stated that timber in every shape and form should be kept out of brickwork and masonry as much as possible ; where it is absolutely necessary to insert it in order to form a hold for woodwork, etc., the pieces should be as small as practicable.
Hoop-iron Bond, consisting of strips of hoop-iron (about 1½ inch broad and 1/16 to 1/20 inch thick), tarred and sanded, and inserted in the joints as shown in Fig. 49, is far preferable to bond timbers, and is frequently used, especially in half-brick walls.2
The hoop-iron should in every case be thoroughly protected from the action of the atmosphere, or it will oxidise and destroy the masonry; if used in thin walls, they should always be built in cement.
The ends of the hoop-iron should be bent so as to hook one another at the joints in the length, and at corners where two walls meet, forming an angle.
Pieces of hoop-iron are often used to make a junction where the bond of the brickwork is defective (see Part I.)