The appearance of these is familiar to all, and need not be illustrated.1
Screws securing work likely to be removed should, if used in damp places, be of copper or brass, otherwise they will rust, and be difficult to withdraw.1
1 They are described in Part III.
Bolts are often used in order to give additional security to joints, some forms of which, indeed, depend upon them altogether for strength.
In many cases bolts are most useful, from the facility with which they can be tightened up, by means of a screw and nut, after the work in which they are used has taken its bearing.
One end of the bolt is generally formed into a solid head, and the other with a screw, on which is fixed a movable nut.
Another way of securing a bolt when it is likely to be removed is by a "slot," or oblong hole, in one of its ends, through which a key or wedge is driven.
The size of bolts should be calculated according to the stresses upon them, and the quality of the iron used. Care should be taken that sufficient timber is left around them to prevent their tearing through in the direction of the strain.
"The following proportions will be found suitable for the bolts and nuts used in carpentry: -
Diameter of head and nut, rose-square or hexagon,' from side to side . . . . 1 3/4 x diameter of bolt.
Thickness of head ..
diameter of bolt.
Depth of nut ...
diameter of bolt." Hurst's Tredgold.
The application of bolts to framing of different kinds is illustrated in Figs. 179, 311, and others.
Washers are flat discs of iron placed under the nut of the bolt to prevent it from pressing into and injuring the timber.
Size of washer -
3 1/2 times
The thickness of washers should be equal to half that of the head of the bolt.
Plates are also used to prevent the sharp corners of the nut from pressing into and injuring the timber, and further, in order to strengthen joints by fishing (see Fig. 159, and others).
1 An excellent practice is to put goose grease or any non-acid grease upon the screws before driving them.
Straps are often used,instead of bolts,to strengthen or form joints. They have the great advantage of not cutting through and weakening the timber.
They are generally flat pieces of iron, about 1 1/2 to 2 inches in breadth, and with a thickness depending upon the quality of the iron and the stress upon them.
Straps should be fixed, as nearly as possible, so that the stress may come upon them in the direction of their length. Cross stresses should be avoided as much as possible, but they are necessarily incurred by straps such as that shown in Fig. 185.
Heel-steaps are used to secure the joints between inclined struts and horizontal beams, such as the joints between rafters and tie beams. They may be placed either so as merely to hold the beams close together at the joint (Fig. 199), or so as directly to resist the thrust of the inclined strut, and prevent it from shearing off the portion of the horizontal beam against which it presses (Fig. 325). Straps of the former kind are sometimes called kicking straps. Fig. 199 shows one form of strap for holding the foot of a rafter down to the tie beam. The screws and nuts on its extremities are prevented from sinking into the wood by the connecting plate B, and by them the strap may at any time be tightened up. A check plate is sometimes provided, as in Fig. 199, to prevent the strap from cutting into the under side of the tie beam, as in Fig. 202. When there is no ceiling, and the strap is therefore visible, the ends of the bearing plate are often rounded, instead of being left square as shown in the figure. The bearing plate is sometimes placed below the tie beam, as in Fig. 202.
A somewhat similar form of strap is shown in Fig. 181. A bearing plate or bar is passed through the holes in the strap across the back of the rafter, and the strap is tightened by wedges driven into the holes.
The straps shown in Figs. 189, 321 are placed so as to take the thrust of the rafter, but are not capable of being tightened up.
Fig. 199. Strap for Joint between Rafter and Tie Beam.
A bearing plate with screws and nuts may, however, be used with this form of strap, as shown in Fig. 325.
Straps of this description are sometimes placed so as to clip the rafter by a notch cut a few inches above the toe, so that they partially hold it down as well as resist its thrust (see Fig. 147).
Stirrup is a name given to a strap which supports a beam, as in Figs. 323 and 194, and to heel-straps of the form shown in Fig. 189. Stirrups, such as that shown in Fig. 194, are sometimes formed with a bearing plate below the supported beam, and tightening screws similar in principle to those in Fig. 199.
Tredgold s rule for straps supporting beams - If the longest unsupported part of the beam be
10 feet, strap should be 1 inch wide, 3/16 inch thick.
Straps which connect suspending pieces with beams may be formed with a slot, containing gibs and cotters, by which the joint may be tightened, as shown and explained at page 163.
When a strap embraces a built-up beam, it may be welded into a rectangular hoop, and driven on from the end, the beam being slightly tapered to facilitate this; or, if that is inconvenient, it may be made as shown in Fig. 200, the ends passing through an iron head, and being secured by nuts.
An iron strap bolt suitable for connecting two beams crossing one another is shown in Fig. 201.1 In both these methods the straps can be tightened by screwing up the bolts.
Branched Straps are frequently added to strengthen angle joints. They are subjected to cross strains when the framing settles.
Several forms of these are given (see Figs. 184, 185, 312, 323).
Cast-iron Shoes, Sockets, etc., are frequently used to protect the ends of beams from damp or fire (see Fig. 280), and also in themselves to form a joint between two beams.
Fig. 200. Strap round Built-up Beam.
Fig. 201. Strap Bolt for two Beams crossing.
1 From Seddon's Builders' Work.
These may be made of various forms (see Fig. 202, and also Fig. 338). While the plate protects the beam from the damp of the wall, it also forms a corbel to support it, and the upper part may be shaped so as to secure the pole plate above.
The foot of a strut or rafter may be received by a shoe instead of being tenoned into the beam (see Figs. 202, 204). The strap shown in this figure is hardly necessary, as the end of the rafter is held down by the shoe. Fig. 203 shows another form of shoe for a rafter when a tie-rod is used.
Fig. 202. Cast-iron Shoe for end of Tie Beam and foot of Rafter.
Fig. 203. Iron Shoe with Tie-rod at foot of Rafter.
Fig. 204. Double Shoe for foot of Struts.
Fig. 204 shows a cast-iron shoe adapted to receive a pair of struts in a framing, such as that of the roof shown in Fig. 338.
Sometimes the tenons at the head of rafters, or the heads themselves, are received in a cast-iron head, as in Fig. 205 (see also Fig. 329 and Fig. 331).
Fig. 205. Cast-iron Head for Wooden Rafters.