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18 Bearing rotation was also mentioned by respondents. For FINDINGS FOR ERECTORS tangent bridges on skewed supports, there is the potential for transverse rotation of the girder at the bearings, owing to dif- The erector should submit for review, acceptance, or approval ferential deflections as well as to skewed pier diaphragms. (based on the agency's practice) an erection procedure that Bearings for these types of structures should be designed to addresses all of the pertinent issues. This procedure should allow for this transverse rotation or, as a minimum, distortion- lead to a properly erected structure. The issues that follow forgiving bearings such as elastomeric pads should be used. should be included in the erection procedure, although they are listed separately here for emphasis. Other survey responses pertained to certification of fabri- cators and erectors. The owner should mandate that the fab- To ensure erection stability, the erector might take the fol- ricator and erector be certified by the American Institute of lowing several measures. Steel Construction or another suitable program. Furthermore, the owner should enforce submittal and review, accept, or · Check the ratio of member length-to-flange width for approve, according to agency practice, the erection proce- erection stability, dure prepared by the erector. · Install enough crossframes to avoid flange buckling owing to the dead load of steel and concrete, · Verify the stability of the partially erected structure for wind loading, and FINDINGS FOR FABRICATORS · Use falsework as appropriate. The fabricator should strive to understand the geometric fea- Geometry control should be maintained at all stages of tures and how they affect erection--particularly curvature, erection. This can be successfully accomplished by differential deflections, skew effects, tolerances, and mem- ber rigidity. · Determining, in conjunction with the fabricator and the designer, the condition at which the webs are detailed On complex structures, the fabricator should consult with to be vertical and erecting them accordingly; the designer and the erector to determine the load condition · Checking the vertical and horizontal alignment of bear- for which the webs should be vertical. In this manner, all par- ings, falsework, and anchor bolt locations before erect- ticipants will understand the geometric assumptions. ing steel; and · Using appropriate pinning and bolting procedures as Shop-assembly methods were also discussed. The com- detailed here. plexity of the structure must be considered when determining the shop-assembly method. The most common method is the The geometry of the erected structure may be significantly progressive girder assembly, with at least three members in affected by the procedures and sequences used for pinning an assembly, often including one span or bearing to bearing. and bolting the members during the erection process. The Records of the actual shop-assembly blocking dimensions procedures detailed in this report represent a reasonable bal- should be maintained and made available to the erector. ance of the various state requirements. They apply impor- tantly to splices in continuous members and other connec- According to respondents, the use of standard size holes tions where small movements or placement errors can have is encouraged. Oversized holes should generally be avoided, a substantial effect on geometry. The erector should review because the geometry of the structure can easily become the shop-assembly blocking records to determine the effect imperiled. of the camber fabrication tolerances on the final shape of the structure. A recommended summary procedure might con- In regard to fabrication details, particular emphasis should tain the following: be considered on the following: · Initial pinning and bolting should consist of filling the · Holes should be drilled accurately, holes in the connections with 25% pins and 25% bolts · Splice material and main members should be accurately and the bolts at least snug tightened before releasing the match-marked, and crane and having the adjacent girders erected. · Members should be fabricated to appropriate sweep and · The balance of the holes in the connections should be camber tolerances. filled with snug-tight bolts. · Final tightening of the bolts to installation tension should Attention should also be given to shipping stability. The not start until a continuous line or at least adjacent spans fabricator should check the member length-to-flange-width have been erected and the vertical and horizontal align- ratio to ensure shipping stability. Where values exceed 60, ment has been verified. computations should be made to determine if temporary brac- · Pins should not be removed from the connection until ing for shipping is required. after the previous step has been accomplished.
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19 Erectors should also be aware of potential thermal effects, Determining acceptable tolerances of deviation from the such as heating from the sun. Also, only experienced road planned vertical or horizontal alignment of the superstructure crews should be used. based on subsequent performance of the bridge could aid owners in determining whether a true problem exists and lim- An analysis of the questionnaire responses raised two iting or preventing much frustration on the part of fabricators general questions for the bridge community: and erectors. The current frequently cited use of the term plumb without associated tolerances given in specifications 1. Do the reported problems caused by deviations from the can be considered too restrictive and unenforceable. vertical and horizontal alignment of the superstructure have a detrimental effect on the performance of the Finally, many erection problems were reported by owners, constructed bridges? fabricators, and erectors. For the most part, owners related 2. Are the problems described by the respondents endemic specific bridges where problems were encountered during or more isolated? erection, whereas fabricators and erectors provided more general discussions of problems. Although most of the own- There were also comments on the effects of deviation from ers could cite a problem bridge, the problems seemed iso- planned alignment. During construction, when dealing with lated. When asked to discuss the solution to the problem, the any of the erection problems as discussed herein, the impor- owners provided much information on how the problem was tant question to ask is this: Is corrective action needed when solved on the problem bridge. However, few offered a global something does not go as expected? Many owners reported solution to the problem, such as a change in their specifica- problems encountered during erection, such as out-of-plumb tions or practices. This observation suggests that while the girders, which in the end were allowed to remain unaltered problems appear real to the owners, they are not endemic. or that required additional manipulation to complete cross- frame connections. No detrimental effect of such misalign- In many cases, when problems with alignment arose, the ments was subsequently reported. Is this truly a problem, or must the ramifications of the misalignment, or lack thereof, owners chose the do-nothing option with apparently no adverse merely be better understood? One astute fabricator noted that impact on the performance of the bridge. It could be asked the term "plumb" has little meaning and that acceptable tol- that when doing nothing is acceptable, does a problem exist? erances based on subsequent adequate performance need to be developed. For some specific field problems cited, other than those resulting from failure to follow appropriate specifications or The erection of steel bridges, although based on science, acceptable practices, a rigorous incremental analysis of the is an art or craft. The practices and specification requirements erection process could solve problems. Today, such analysis is are based on rules of thumb, experience, and intuition more routine for more complex forms of bridge construction, such so than on rigorous analysis. Rigorous erection analyses, as segmental concrete bridges and cable-supported bridges. including the prediction and reporting of intermediate deflec- However, with such analysis, additional costs are incurred. In tions (deflection before the final erected condition) are not the majority of the reported cases, additional effort in the field made, according to the survey responses. Without such analy- solved the problem. Before more rigorous incremental analy- ses for certain types of structures, problems of fit can be ses are instituted, the question to be answered is whether the expected and, as the responses suggested, they do occur. The potential field costs to solve problems exceed any proposed problems are exacerbated by stage construction. rigorous analysis costs before erection.