Bridges are erected by a variety of methods. The choice of method in a particular case is influenced by type of structure, length of span, site conditions, manner in which material is delivered to the site, and equipment available. Bridges over navigable waterways are sometimes limited to erection procedures that will not inhibit traffic flow; for example, falsework may be prohibited.
Regardless of erection procedure selected, there are two considerations that override all others. The first is the security and stability of the structure under all conditions of partial construction, construction loading, and wind loading that will be encountered during erection.
The second consideration is that the bridge must be erected in such a manner that it will perform as intended. For example, in continuous structures, this can mean that jacks must be used on the structure to effect the proper stress distribution. These considerations will be elaborated upon later as they relate to erection of particular types of bridges.
Simple-beam bridges are often erected with a crawler or truck crane. Bridges of this type generally require a minimal amount of engineering and are put up routinely by an experienced erector. One problem that does occur with beam spans, however, and especially composite beam spans, arises from lateral instability of the top flange during lifting or before placement of permanent bracing. Beams or girders that are too limber to lift unbraced require temporary compression-flange support, often in the form of a stiffening truss. Lateral support also may be provided by assembling two adjacent members on the ground with their bracing or cross members and erecting the assembly in one piece. Beams that can be lifted unbraced but are too limber to span alone also can be handled in pairs. Or it may be necessary to hold them with the crane until bracing connections can be made.
Continuous-beam bridges are erected in much the same way as simple-beam bridges.
One or more field splices, however, will be present in the stringers of continuous beams.
With bolted field splices, the holes in the members and connection material have been reamed in the shop to insure proper alignment of the member. With a welded field splice, it is generally necessary to provide temporary connection material to support the member and permit adjustment for alignment and proper positioning for welding. For economy, field splices should be located at points of relatively low bending moment. It is also economical to allow the erector some option regarding splice location, which may materially affect erection cost. The arrangement of splices in Fig. 2.10a, for example, will require, if falsework is to be avoided, that both end spans be erected first, then the center spans. The splice arrangement shown in Fig. 2.10b will allow erection to proceed from one end to the other.
While both arrangements are used, one may have advantages over the other in a particular situation.
Horizontally curved girder bridges are similar to straight-girder bridges except for torsional effects. If use of falsework is to be avoided, it is necessary to resist the torques by assembling two adjacent girders with their diaphragms and temporary or permanent lateral bracing and erect the assembly as a stable unit. Diaphragms and their connections must be capable of withstanding end moments induced by girder torques.
Truss bridges require a vast amount of investigation to determine the practicability of a desired erection scheme or the limitations of a necessary erection scheme. The design of truss bridges, whether simple or continuous, generally assumes that the structure is complete and stable before it is loaded. The erector, however, has to impose dead loads, and often live loads, on the steel while the structure is partly erected. The structure must be erected safely and economically in a manner that does not overstress any member or connection.
Erection stresses may be of opposite sign and of greater magnitude than the design stresses. When designed as tension members but subjected to substantial compressive erection stresses, the members may be braced temporarily to reduce their effective length. If bracing is impractical, they may be made heavier. Members designed as compression members but subjected to tensile forces during erection are investigated for adequacy of area of net section where holes are provided for connections. If the net section is inadequate, the member must be made heavier.
Once an erection scheme has been developed, the erection engineer analyzes the structure under erection loads in each erection stage and compares the erection stresses with the design stresses. At this point, the engineer plans for reinforcing or bracing members, if required.
The erection loads include the weights of all members in the structure in the particular erection stage and loads from whatever erection equipment may be on the structure. Wind loads are added to these loads.
In addition to determining member stresses, the erection engineer usually calculates reactions for each erection stage, whether they be reactions on abutments or piers or on falsework.
Reactions on falsework are needed for design of the falsework. Reactions on abutments and piers may reveal a temporary uplift that must be provided for, by counterweighting or use of tie-downs. Often, the engineer also computes deflections, both vertical and horizontal, at critical locations for each erection stage to determine size and capacity of jacks that may be required on falsework or on the structure.
When all erection stresses have been calculated, the engineer prepares detailed drawings showing falsework, if needed, necessary erection bracing with its connections, alterations required for any permanent member or joint, installation of jacks and temporary jacking brackets, and bearing devices for temporary reactions on falsework. In addition, drawings are made showing the precise order in which individual members are to be erected.
Figure 2.11 shows the erection sequence for a through-truss cantilever bridge over a navigable river. For illustrative purpose, the scheme assumes that falsework is not permitted in the main channel between piers and that a barge-mounted crane will be used for steel erection. Because of the limitation on use of falsework, the erector adopts the cantilever method of erection. The plan is to erect the structure from both ends toward the center.
Note that top chord U13-U14, which is unstressed in the completed structure, is used as a principal member during erection. Note also that in the suspended span all erection stresses are opposite in sign to the design stresses.
As erection progresses toward the center, a negative reaction may develop at the abutments (panel point LO). The uplift may be counteracted by tie-downs to the abutment.
Hydraulic jacks, which are removed after erection has been completed, are built into the chords at panel points U13, L13, and U13. The jacks provide the necessary adjustment to allow closing of the span. The two jacks at U13 and L13 provide a means of both horizontal and vertical movement at the closing panel point, and the jack at U13 provides for vertical movement of the closing panel point only.