The most simple combination is that shown in Fig. 227. The two single timbers are bolted together side by side, with sometimes a small space between them. The bolts should be spaced about 2 feet apart and staggered as shown, so that two will not come side by side. Usually bolts three-quarters of an inch in diameter are used.
In Fig. 22S is shown a modification of this girder known as a "flitch-plate" girder. It has a plate of wrought iron or steel, inserted between the two timbers, and the whole is held firmly together by bolts. The size of the plate should be in proportion to the size of the timbers, so as to make the most economical combination. In general the thickness of the iron plate should be about one-twelfth of the combined thickness of the tim bers.
If we have two pieces of timber out of which we wish to make a compound girder, it is always possible to get a stronger combination by placing them one on top of the other, than by placing them side by side, This is because the strength of a beam varies as the square of its depth, but only directly as its width. For this reason most compound girders are composed of single pieces placed one above the other. The tendency is for each piece to bend independ ently, and for the two parts to slide by each other, as shown in Fig. 229. This tendency must be overcome and the parts so fastened together that they will act like a single piece. There are several methods in common use by which this object is accomplished.
Fig. 230 shows the most common method of building up a com pound girder. The timbers are placed together, as shown, and narrow strips of wood are nailed firmly to both parts. The strips are placed close against each other and have a slope of about forty five degrees, sloping in opposite directions, however, on opposite sides of the girder. It has been claimed that a built-up girder of this kind has strength ninety-five per cent as great as the strength of a solid piece of the same size but it is very doubtful whether this is true in most cases. Actual tests seem to indicate that such girders have an efficiency of only about seventy-five per cent. They usually fail by the splitting of the side strips, or the pulling out and bending of the nails, but seldom by the breaking of the main pieces. It is,
therefore, essential that the strips should be very securely nailed to each of the parts which make up the girder, and that they should also be carefully selected so that only those pieces which are free from all defects may be used. These girders are liable to considerable deflection, and should not be used in situations where such deflection would be harmful.
In Fig. 231 is shown another form of girder with the parts notched, as shown, so as to lock together. This prevents them from slipping by each other. Bolts are employed to hold the parts together, so, that the surfaces will always be in close contact. While this form of girder is very easily constructed, it has many disadvantages. A great deal of timber is wasted in cutting out the notches, as these must be deep enough to prevent crushing of the wood at the bearing surfaces, and thus the full strength of the timbers is not utilized. Moreover, it is apt to deflect a good deal, and its efficiency is not so great as that of other forms. On the whole it is inferior to the form previously described.
The compound beam which is almost universally considered the best is that shown in Fig. 232. This is known as a keyed beam, its characteristic feature being the use of keys to keep the parts from sliding on each other. The strength of a keyed beam has been found by actual experiment to be nearly ninety-five per cent of the strength of the solid timber, while the deflection when oak keys were used was only about one-quarter more than the deflection of the solid beam. By using keys of cast iron instead of wood this excess of deflection in the built-up girder can be reduced to a very small percentage. The keys should ke made in two parts, each shaped like a wedge, as explained in connection with the keys for tension splices, and should be driven from opposite sides into the holes made to receive them, so as to fit tightly. They should be spaced from S to 16 inches apart, center to center, according to the size of the timbers, and should be spaced more closely near the ends of the beam than near the middle. In the center of the span there should be left a space of 4 or 5 feet without any keys.