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11 This chapter discusses erector practices and views related to steel bridge erection, as reported in the questionnaires and follow-up telephone interviews that were a part of this syn- thesis. Information is based on responses from four erectors. ERECTION PROCEDURE PROVIDED BY OWNER Two of the erectors have not received erection procedures from the owner. The other two have, but they have diametri- cally opposite opinions on the procedureâs worth. One believes that a required erection procedure from the designer has a positive effect because the designer must think through the scheme and the associated forces to erect the structure. The second believes that this practice does not improve the qual- ity of the erected structure. APPROACH TO ERECTION SEQUENCE AND LOCATION OF FALSEWORK The responses to questions regarding approach suggest that a rigorous analysis is not employed. Rather, erectors depend on experience and intuition. One erector indicated that the need to provide temporary support through falsework was based on bridge type, number of spans, and span lengths, indicating that long spans gener- ally require multiple falseworks to control geometry. Another erector pointed to site limitations and the size and strength of the individual girder pieces as discriminators on the need for and location of falsework. Finally, one erector cited the rule of thumb of using falsework near splices and locating them under stiffeners, as well as the need to provide jacking reac- tion capabilities in falsework to adjust as necessary to main- tain proper elevations. An erector suggested that it is often possible to ground assemble two adjacent girders with the lateral bracing and crossframes, and then erect them as a single unit. In that way, temporary bracing is not required. STABILITY WHEN LIFTING CURVED MEMBERS Two of the erectors discussed maintaining individual girder stability through the choice of pick points (crane lifting points) for curved members. Their specific discussion of the calcu- lation of pick points based on certain criteria suggests that this is the extent of their analysis of girder stability. One erector calculates the sum of moments in the transverse direction along the member length when picked to ensure that the girder remains level. The erector also reported that multi- ple cranes or shoring are used. The second erector dis- cussing this topic reported that curved girders can be picked with a single crane using a correctly sized spreader beam or by using two cranes. The location of the pick points can be calculated so that the girder is picked straight without roll. That erector applies the rule of thumb that picking at two points usually eliminates any later stability problems, as long as a line between the pick points runs through the cen- ter of gravity of the girder. FLANGE SIZING REQUIREMENTS FOR STABILITY Two of the erectors reported that an L/b of 60 or less between unbraced points provides stability during transportation and erection. One of them went on to report that an L/b value of 60 to 80 may be adequate, but further stress calculations need to be verified, and values of more than 80 require temporary support (falsework or holding cranes) to provide stability. Another erector indicated that it is desirable that the flanges be sized so that each individual girder piece can laterally sup- port itself when erected in a simple span or cantilever condi- tion, depending on the erection sequence. With longer spans and smaller flanges, temporary lateral support trusses made of angles and wire rope are often required until adjacent gird- ers are erected and permanent crossframes and lateral brac- ing are connected. The final erector simply stated that the sta- bility of single girders needs to be addressed. WHERE DOES RESPONSIBILITY FOR STABILITY LIE? One erector raised the important issue of responsibility for bracing for stability, noting that certain states require the con- tractor to determine if lateral bracing is required for the bridge. The problem is that bracing for wind and steel erection issues may not suffice for deck forces or sequence of pour. The ques- tion remains: Who then is responsible at what stage? CHAPTER FOUR ERECTOR PRACTICES AND VIEWS
FIELD CONNECTION PRACTICES The erectors offered little opinion about field connection prac- tices. One, however, opined that long-span straight bridges can have their splices tightened under the no-load condition. The same erector pointed out that erecting a structure and hav- ing to go over it again to tighten certain members adds to cost. That erector stated that the key is to survey the elevations dur- ing erection before tightening anything. Furthermore, it stated that geometry control is most important; preferring con- centric, not oversized, holes in all members to ensure align- ment, spacing, and cross-slope geometry with the exception of secondary members such as lateral bracing. Another erec- 12 tor stated that the use of drift pins before bolting ensures proper alignment. GENERAL CONSIDERATIONS One astute erector briefly summarized the key aspects for success in this way: Accurate shop fabrication, accurate loca- tion and elevation of supports, maintaining proper elevations at splices, and complete installation of connections before releasing falsework all contribute to a successful steel erec- tion job.
