Perhaps the most thorough tests in this line were those carried on at the bridge at Chatellnault, Vienne, France. This bridge is 443 ft. long and composed of three arches whose spans are 135, 164, and 135 ft., respectively. On the removal of the forms this bridge was subjected to the following test.

1. Each day the spans were loaded, first over their total length, then on each half, then in the middle, with sand at the rate of 165 lbs. per sq. ft., on the roadway and 123 lbs. per sq. ft. on the sidewalk. The maximum deflections under these loads were, end spans 1/4 inch or 1/7300 of the span, centre arch 13/32 inch or 1/5000 of the span.

2. One 16-ton steam roller, two 16-ton two-axled carts, six 8-ton one-axled carts, total weight, including horses, 100 tons, passed at once over the bridge, the sidewalk at the same time carrying a load of 80 lbs. per sq. ft.

3. 250 soldiers (infantry) crossed the bridge, first in regular marching step, second in double time.

The maximum deflection under these tests did not exceed 1/9000 of the span, and all vibration ceased almost immediately on the removal of the load.

Arch Centres.-A centre is a temporary structure for supporting an arch while in process of construction. It is usually made of a number of circular ribs spaced a few feet apart, and lying in a plane perpendicular to the axis of the arch. These ribs are covered with narrow planks (lagging), running parallel to the axis of the arch, upon which the arch rests while in course of construction. In concrete arches, except in those that are very flat, provision for maintaining the extradosal line of the arch must also be made.

All centres should be made as strong and as rigid as possible, as any deformation of the centre due to insufficient strength or improper bracing will cause a corresponding change in the intrados of the arch, and consequently in the line of resistance, and may endanger the whole structure.

Arch centring in general may be divided into two classes. In the first class the ribs are supported by struts braced together so as to form transverse bents. These bents are spaced at convenient distances along the axis of the arch and braced longitudinally. Where the subsoil is sufficiently firm, the struts may rest on mud

sills, but in poorer soil temporary masonry or pile foundations are frequently used.

In the second form trusses are employed. These trusses may carry the lagging directly, in which case they must conform to the curve of the intrados of the arch, or they may support short braces which in turn support the ribs. Where trusses are used they should be cambered slightly so that after deflection the arch may be of the desired curvature.

The ribs are usually made of planks spiked together so as to break joints, and cut to a curve parallel to the intrados of the arch, but a sufficient distance below it so that the lagging, when applied,

shall coincide with the intrados of the arch. Sometimes the ribs are steel shapes bent to the desired curvature.

In order that the centres may be struck, or lowered, uniformly and without shock, either sand boxes or wedges are used under all of the supports.

The wedges usually consist of a pair of folding wedges, preferably of hard wood, having a slight taper. This taper should vary with the span of the arch, the longer the span the less the taper. To lower the centres equally the wedges should be driven back uniformly. To facilitate this, compound wedges are sometimes used. By driving the wedge all work resting on the wedge will be lowered uniformly.

Sand boxes usually consist of a steel cylinder in which sand is confined. A wooden plunger rests on the sand, and on these wooden plungers is carried the centring of the arch. Near the bottom of the cylinder is a plug which may be withdrawn and replaced at pleasure, by means of which the outflow of the sand is regulated. As the sand is allowed to escape, the centres will lower and the amount of this lowering can be easily controlled by the amount of sand allowed to escape. In using sand boxes particular care should be exercised first to secure a proper sand, and second to exclude all

foreign material from the boxes, which must also be properly sealed. Where any of these precautions are lacking trouble is likely to be experienced either through the sand flowing prematurely, or its failure to flow at the proper time.

The type of centres to be used in any case will depend entirely upon local conditions. Where it is desirable to obstruct the opening as little as possible, the trussed form of centring would probably be best adapted.

In other cases where the restriction of the opening is of little or no importance, bents would probably be the most economical and satisfactory.

Concreting the Arch. For convenience in concreting, an arch is frequently divided into a number of strips, which are practically arches in themselves. In all cases the concreting should be carried up from the springing line toward the crown, uniformly on each side of the arch. While this concreting is in progress the action of the centres should be carefully observed. Generally as the load on the haunch increases, the crown will tend to rise. If this tendency becomes excessive, it may be overcome by loading the crown with any material that is convenient, or by placing the concrete for that

FIG. 93.-Travelling Form for Roof Arch, New York Subway Tunnels.

portion of the arch before proceeding further with the haunches. It is well, if this method of placing the concrete is used, to so divide the arch that a complete ring may be placed without intermission.

Another method is to divide the arch in strips extending the full width of the arch. The strips are first placed near the springing line, then, to overcome the tendency of the crown to rise, the strip at the crown is placed and so on until the arch is completed.

Backfilling.-Backfilling is usually begun after the arch has hardened but before the centres are struck. The reason for this is obvious. If the filling were placed after the removal of the centres, it would be necessary to place the filling uniformly over the