Oaxiier

OAXIIER. Strictly, the center should be constructed with a camber just equal to the amount it will yield when loaded with the arch; but, since the load is indeterminate, it is impossible to compute accurately what this will be. Of course, the camber de pends upon the unit strain in the material of the center. The rule is frequently given that the camber should be one four-hundredth of the radius; but this is too great for the excessively heavy centers ordinarily used. It is probably better to build the centers true, and guard against undue settling by giving the frames great stiffness; and then if unexpected settling does take place, tighten the striking wedges slightly.

The two sides of the arch should be carried up equally fast, to prevent distortion of the center.

The Method.

The ends of the ribs or center-frames usually rest upon a timber lying parallel to, and near, the springing line of the arch. This timber is supported by wedges, preferably of hard wood, resting upon a second stick, which is in turn supported by wooden posts—usually one under each end of each rib. The wedges between the two timbers are used in removing the center after the arch is completed, and are known as striking wedges. They consist of a pair of folding wedges-1 to 2 feet long, 6 inches wide, and having a slope of from 1 to 5 to 1 to 10—placed under each end of each rib. It is necessary to remove the centers slowly, particularly for large arches; and hence the striking wedges should have a very slight taper, the larger the span the smaller the taper.

The center is lowered by driving back the wedges. To lower the center uniformly, the wedges must be driven back equally. This is most easily accomplished by making a mark on the side of each pair of wedges before commencing to drive, and then moving each the same amount.

Instead of separate pairs of folding wedges, as above, a compound wedge, Fig. 212, is sometimes employed. The pieces A and B are termed striking plates. The ribs rest upon the former, and the latter is supported by the wooden posts before referred to. The wedge C is held in place during the construction of the arch by the keys, K, K, etc., each of which is a pair of folding wedges.

To lower the center, the keys are knocked out and the wedge C is driven back. The piece C is usually as long as the arch, and supporta one end of all the ribs. With large arches, say 80 to 100 feet span, it is customary to support each rib on a compound wedge running parallel to the chord of the center (perpendicular to the axis of the arch). The piece C is usually made of an oak stick 10 or 12 inches square. The individual wedges are from 4 to 6 feet long.

For the larger arches, the compound' wedge is driven back with a heavy log battering-ram suspended by ropes and swung back and forth by hand. The inclined surfaces of the wedges should be lub ricated when the center is set up, so as to facilitate the striking.

An ingenious device, first employed in 1855 at the Pont d'Alma arch, Paris, France,-141 feet span and 28 feet rise,—con sisted in supporting the center-frames on wooden pistons or plungers, the feet of which rested on sand confined in plate-iron cylinders 1 foot in diameter and about 1 foot high. Near the bottom of each cylinder there was a plug which could be withdrawn and replaced at pleasure, by means of which the outflow of the sand was regulated, and consequently also the descent of the center. This method is particularly useful for large arches, owing to the greater facility with which the center can be lowered. For an example of its use, see Fig. 222, page 667.

The sand should be clean, fine, and dry; and the space between the plunger and the cylinder should be relatively small, or should be filled with a ring of neat cement mortar.

Another ingenious device for lowering arch centers has recently been employed several times in Austria.*. The special fea ture is a crushing block of soft wood of the shape shown in Fig. 213. The two feet of the block have sufficient bearing area to hold up the load during erection without sensible crushing; but it is so shaped that by sawing off the end portions of the block, the bearing area may be successively reduced, and thus cause the ends to crush down and allow the centers to settle away from the arch.

For still another ingenious method, see item 3 of § 1367.

The Time.

There is a great difference of opinion as to the proper time for striking the centers of voussoir arches. Some hold that the center should be struck as soon as the arch is completed and the spandrel filling is in place; while others contend that the mortar should be given time to harden. It is probably best to slacken the centers as soon as the keystone is in place, so as to bring all the joints under pressure. The length of time which should elapse before the centers are finally removed should vary with the kind of mortar employed, and also with its amount. In brick and rubble arches a large proportion of the arch ring consists of mortar; and if the center is removed too soon, the compression of this mortar might cause a serious or even dangerous deformation of the arch. Hence the centers of such arches should remain until the mortar has not only set, but has attained a considerable part of its ultimate strength,—this depending somewhat upon the maximum compression in the arch.

Frequently the centers of bridge arches are not removed for three or four months after the arch is completed; but usually the centers for the arches of tunnels, sewers, and culverts are removed as soon as the arch is turned and about half of the spandrel filling is in place.