Erection of Arch Bridges

Erection conditions vary so widely that it is not possible to cover many in a way that is generally applicable to a specific structure.
Cantilever Erection. For arch bridges, except short spans, cantilever erection usually is used. This may require use of two or more temporary piers. Under some conditions, such as an arch over a deep valley where temporary piers are very costly, it may be more economical to use temporary tiebacks.
Particularly for long spans, erection of trussed arches often is simpler than erection of solid-ribbed arches. The weights of individual members arc much smaller, and trusses are better adapted to cantilever erection. The Hell-Gate-type truss (Art. 14.2) is particularly suitable because it requires little if any additional material in the truss on account of erection stresses.
For many double-deck bridges, use of trusses for the arch ties simplifies erection when trusses are deep enough and the sections large enough to make cantilever erection possible and at the same time to maintain a clear opening to satisfy temporary navigation or other clearance requirements.
Control of Stress Distribution. For trussed arches designed to act as three-hinged, under partial or full dead load, closure procedures are simple and positive. Normally, the two halves of the arch are erected to ensure that the crown hinge is high and open. A top-chord member at the crown is temporarily omitted. The trusses are then closed by releasing the tiebacks or lowering temporary intermediate supports. After all dead load for the three-hinged condition is on the span, the top chord is closed by inserting the final member. During this operation consideration must be given to temperature effects to ensure that closure conditions conform to temperature-stress assumptions.

If a trussed arch has been designed to act as two-hinged under all conditions of loading, the procedure may be first to close the arch as three-hinged. Then, jacks are used at the crown to attain the calculated stress condition for top and bottom chords under the closing erection load and temperature condition. This procedure, however, is not as positive and not as certain of attaining agreement between actual and calculated stresses as the other procedure described. (There is a difference of opinion among bridge engineers on this point.)
Another means of controlling stress distribution may be used for tied arches. Suspender lengths are adjusted to alter stresses in both the arch ribs and the ties.
Fixed Bases. For solid-ribbed arches to be erected over deep valleys, there may be a considerable advantage in fixing the ends of the ribs. If this is not provided for in design, it may be necessary to provide temporary means for fixing bases for cantilever erection of the first sections of the ribs. If the structure is designed for fixed ends, it may be possible to erect several sections as cantilevers before it becomes necessary to install temporary tiebacks.

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