Fig. 235.
Bridges.—Bridge building is not, strictly, a part of the carpenter's education at the present day, because most structures of this kind are now built of steel; but there are certain principles involved in bridge construction which the carpenter should master.
Self-supporting Roofs.—In putting up, for instance, self-supporting roofs, or ceilings with wide spans, and steeples or towers, the bridge principle of trussed members should be understood.
The most simple bridge or trussed form is the well-known A-shaped arch.
Common Trusses.—One form is shown in Fig. 235, with a vertical king post. In Fig. 236 there are two vertical supporting members, called queen posts, used in longer structures. Both of these [Pg 131] forms are equally well adapted for small bridges or for roof supports.
The Vertical Upright Truss.—This form of truss naturally develops into a type of wooden bridge known all over the country, as its framing is simple, and calculations as to its capacity to sustain loads may readily be made. Figs. 237, 238 and 239 illustrate these forms.
The Warren Girder.—Out of this simple truss grew the Warren girder, a type of bridge particularly adapted for iron and steel construction.
This is the simplest form for metal bridge truss, or girder. It is now also largely used in steel buildings and for other work requiring strength with small weight.
[Pg 132]
The Bowstring Girder.—Only one other form of [Pg 133] bridge truss need be mentioned here, and that is the bowstring shown in Fig. 240.
In this type the bow receives the entire compression thrust, and the chords act merely as suspending members.
Fundamental Truss Form.—In every form of truss, whether for building or for bridge work, the principles of the famous A-truss must be employed in some form or other; and the boy who is experimentally inclined will readily evolve means to determine what degree of strength the upper and the lower members must have for a given length of truss to sustain a specified weight.
There are rules for all these problems, some of them very intricate, but all of them intensely interesting. It will be a valuable addition to your knowledge to give this subject earnest study.
