In Part IV. will be shown the method of ascertaining the form and dimensions for plain timber beams, intended to support loads concentrated at different points or uniformly distributed.
It will be found that, for spans exceeding 20 feet, the girders to carry even moderate loads will require considerable sectional dimensions.
These difficulties have led to various expedients: -
1. For building up large beams out of pieces of smaller scantling.
-When beams are required of such a size that timber large enough for them cannot be procured without danger of defects, they may he built up of smaller pieces, arranged as shown in Figs. 225, 226, which are taken from Tredgold's Carpentry.
Fig. 22; shows a simple form of built beam consisting merely of two pieces of timber bolted together, and prevented from sliding by hard wood keys, having their grain at right angles to that of the timber.
Fig. 225. Keyed Beam.
1 Built-up beams and girders trussed within their depth are now not much used in this country - having been superseded by rolled iron beams and riveted girders - but they may be useful in new countries where iron girders cannot be obtained.
The upper portion may be cut into two, and the halves forced outward by a king bolt with bevelled sides, as shown in Fig. 226.
It is better, however, that no bolt or key should be placed in the centre, as at that point the transverse stress is in most cases the greatest.
The depths of all the keys added together should be more than 1 1/3 of the whole depth of the beam, and the breadth of each about twice the depth. - Tredgold.
The sliding may be prevented by indentations, as shown in Fig. 226, instead of keys. The upper layer of the beam is sometimes made in two pieces, a vertical wedge-shaped king bolt being inserted at the centre, with its narrow end downwards, so that by screwing up the nut the parts of the upper layer are forced outwards and the joints tightened.
The depth of all the indents added together should equal § depth of the beam. - Tredgold.
Fig. 226. Indented Beam.
When straps are used instead of bolts, the beam should be slightly tapered towards the ends to facilitate driving them on.
When a beam is built up in two layers, the upper may with advantage be of hard wood, as it will be compressed when a transverse stress comes upon the beam. The lower layer should be of tough straight-grained timber, to resist the tension to which it will be subjected.
For very long beams it may be necessary to have two or three pieces in the length as well as in the depth. These should, in the lower layer, be scarfed together as described in Part I.; in the upper layer they may simply butt against one another.
Several elaborate methods of building up such beams of considerable length and scantling are given in old books on carpentry, but these have been to a great extent superseded by the use of iron girders.
Curved Ribs are sometimes obtained from naturally curved timber sometimes artificially bent (see Part III.), or sometimes built up as follows.
Built Bibs are best constructed on a method invented by Philibert de l'Orme (see Figs. 227, 228).
Several layers of plank on edge are placed together so as to break joint, and are united by bolts or wedges passing through them.
If a curved rib be required, the corners abcde are rounded off.
A built rib of this sort properly constructed is nearly as strong as a solid rib of the same depth, and of a breadth less by the thickness of one layer. - Rankine.
Laminated Bibs are composed of layers of planks placed flatwise, breaking joint and bolted together, as in Fig. 229. They are easily made.
Fig. 228. Plan. Built Rib.
Fig. 229. Laminated Rib.
Their strength, compared to that of solid ribs, is as 1 to the number of layers of which they are composed. - Rankine,
Built and laminated ribs are used in some forms of roofs, which, however, do not fall within the limits of this Course.