It is, however, often necessary to strengthen a girder without adding to its dimensions, as, for instance, in a floor where too deep a girder would extend below the ceiling or lessen the height of the room below.
In such a case all extraneous aid afforded to the girder by means of trussing or otherwise, must be kept very nearly or quite within the limits of the wooden beam.
A beam may be improved by cutting it down the middle, reversing one of the halves (called "flitches"), end for end, and bolting them together with the sawn sides outwards, small slips of wood being introduced between the flitches to keep them an inch or two apart, so as to allow a free circulation of air between them.
This arrangement causes the timber to season more quickly and thoroughly, as the pieces are smaller; it also renders the heart of the wood visible, so that any decay can be detected; moreover, the reversal of the flitches, end for end, makes the beam of equal strength throughout, which is very seldom the case in a long balk, as the top of a tree is generally weaker than the butt.
Iron Flitches.1 - A beam thus cut down the middle is frequently strengthened by placing an iron plate or "flitch" between the halves, and bolting the whole together, as shown in the section Fig. 230.
1 Or Sandwich Beams.
The bolts are placed in the length of the beam so that the upper ones are over the centres of the intervals below the lower ones.
A writer in the Building News has shown that, when the depth and length of the iron plate is the same as that of the flitches, its thickness, in order that it may be effective, should be at least 1/8 of that of each flitch.
A rolled girder, as shown in Fig. 231, is, in some cases, used instead of the iron plate.
Beams have frequently been strengthened by a "truss" constructed within their own depth.
Such a truss may be formed by splitting a balk longitudinally clown the centre, and inserting between the flitches two cast-iron struts, s s. Along the bottom of the beam is a tension-plate, supported by a king bolt in the centre, and by a somewhat similar bolt, b, at each end, after which the "flitches" are bolted together as shown in Fig. 233.
Fig. 233.1 Plan.
Fig. 232 is a section through the centre of the iron truss, showing the farther flitch and bolt-holes in elevation.
The ends of the struts are secured as follows: -
The bolt b passes through the beam and secures the ends of the tension plates by means of the cog c; the lower part of the bolt is shaped so as to form an abutment for the struts s, and it is supported in the centre by the transverse bolt seen in section at t.
The truss adds to the strength of the girder so long as the bolts are screwed up, and all the parts are bearing accurately.
A similar truss may be formed as in Fig. 234, with two queen bolts instead of the king bolt.
The struts are sometimes formed of oak or other hard wood instead of iron.
Beams may be trussed with tension rods as shown in Fig. 235. The balk is split longitudinally into two, as before described, and the truss inserted between the flitches.
1 Modified from Newland's Carpenter's and Joiner's Assistant.
Fig. 235 is an elevation of the exterior of such a beam. The ends of the tension rod pass through cast-iron boxes capping the ends of the beam, and are there secured by nuts.
The cast-iron boxes are useful to protect the ends of the beam, especially when the latter are imbedded in masonry, but they are often dispensed with; the upper corners of the beam being cut off at right angles to the direction of the tension rod, and the nuts screwed up against a washer or plate, as in Fig. 236.
The centre of the rod bears against a cast-iron bar attached to the under side of the beam.
It will be seen that a shrinkage in the depth of this beam frees it from the tension rod, which becomes slack and plays no part whatever; it can, however, be again brought into action by screwing up the nuts at its ends.
In the meantime, however, the whole strain has come upon the timber, unassisted by the iron work.
The same description of truss may be used with two bearing points for the tension rod, as shown in Fig. 236.
Sir W. Fairbairn experimented upon beams with 4 feet 6 inches bearing, and 4 inches deep trussed as shown in Fig. 235, and found that the trussed beam was 1/5 stronger than a simple beam of the same dimensions.
Figs. 235, 236 show the position of the tension rods as they are usually placed, but Sir W. Fairbairn states 1 that "in the construction of truss beams, whether of wood or iron, the truss rod should never be carried to a greater height than the horizontal line passing through the centre of the beam."
Girders trussed within their own depth are objected to because the inevitable shrinkage of the timber slackens the iron work, and throws the whole strain upon the timber; which may therefore become crippled before the truss can be tightened up and the iron brought again into play.
Moreover, from the nature of the construction, especially when the girder is " cambered" in order to gain stiffness, the ends are subjected to great compression, which acting upon a small area is apt to crush the fibres.
This strain may be considerably reduced where there is sufficient depth by increasing the angle of inclination of the tension rods, as in girders of the form shown in Fig. 237, which are not so open to this objection.
The great difference in the nature of the materials composing the truss renders it almost impossible that the parts should act together in performing the work required of them.
Theoretically, they should be so adjusted that the members in compression are on the point of being crushed, at the exact moment that the parts in tension are about to tear asunder. To arrange this would require great skill and nicety in proportioning and fitting the parts; and, even if it were accomplished, slight changes caused by exposure to the atmosphere would soon utterly destroy the adjustment.
1 P. 350. Application of Iron to Buildings.
For these reasons beams trussed within their own depth are in some disrepute and are seldom used.
"Where circumstances do not limit the depth of the trussing, it may be used with great advantage.
Fig. 237 shows a form of truss frequently adopted for purlins of great length (see Part II.), for beams of long bearing used in gantries,1 and for travellers. This form of truss is, however, suitable only for a purlin or similar bearer carrying a uniform stationary load throughout its length; when it has to carry a moving weight it should be strengthened by cross braces as in Fig. 240.
Fig. 237. Beam with deep Trussing for Dead Load.
The extremities of the beam are enclosed in cast-iron boxes. These receive the ends of the tension rods, which pass through them and are secured by nuts.
The bearing surface of the nut should be at right angles to the direction of the tension rod; this may be effected by cutting off the ends of the beam obliquely at right angles to the tension rod, as in Fig. 238, and shaping the box accordingly - when the end of the beam and box is vertical a patch may be cast upon the box, or a splayed washer introduced so that the bearing may be at right angles to the rod.
The tension rod may pass through the centre of the beam as shown, or a rod may be used on each side.
Fig. 239. Trussed Ends of Beams.
In this case the cast-iron boxes have an ear on each side to receive the bars as shown in Fig. 239.
1 See Part II.
For smaller spans only one stay in the centre may be used instead of two as shown in Figs. 237, 240.
The truss is sometimes strengthened by diagonal ties, AD, BC, Fig. 240, which are necessary when the points A B are unequally loaded.
This form (Fig. 240) is therefore particularly adapted for the bearers of travellers, and others required to carry a moving load.
Fig. 240. Trussed Beam for Moving Load.