National Academies Press: OpenBook

Steel Bridge Erection Practices (2005)

Chapter: Chapter Six - Solutions to Reported Problems

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Suggested Citation:"Chapter Six - Solutions to Reported Problems." National Academies of Sciences, Engineering, and Medicine. 2005. Steel Bridge Erection Practices. Washington, DC: The National Academies Press. doi: 10.17226/13825.
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Page 14
Page 15
Suggested Citation:"Chapter Six - Solutions to Reported Problems." National Academies of Sciences, Engineering, and Medicine. 2005. Steel Bridge Erection Practices. Washington, DC: The National Academies Press. doi: 10.17226/13825.
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Page 15

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15 DISTORTION AT DECK CANTILEVER BRACKETS In regard to owners, three reported that additional analysis of the fascia girder is required to solve the problem of distortion caused by the load of the cantilever brackets. One requires the designer to review the load condition on the fascia girder caused by the cantilever bracket forces and, where necessary, provide additional transverse stiffeners, require the bracket to be supported by the bottom flange of the fascia girder, or allow the contractor to propose an alternate solution. That owner also requires the designer to consider the effect of out- of-plane girder rotation. Two owners have developed software to analyze the fas- cia girder under such conditions. In particular, the Kansas Bridge Office, in conjunction with Kansas State University, developed the program Torsional Analysis of Exterior Gird- ers (TAEG 2.0), to predict the torsional resistance of the exterior girder eccentrically loaded with the screed machine and deck overhang concrete. Still another owner requires the contractor to submit his forming procedures for approval. THERMAL DISTORTION OF SUN HEATING ERECTED MEMBERS Although owners noted distortions of the steel-only super- structure as a result of thermal radiation, no solutions such as general requirements were cited. One owner indicated that the problem was mitigated with the completion of the deck formwork over the girders. Another employed tempo- rary bracing when the problem was encountered on a spe- cific bridge. To avoid reporting a problem that does not really exist, one erector suggested that the erector consider the position of the sun and temperature of the steel when checking the alignment of a structure. One line of girders may be longer than the other because of shading of one by the other. STAGE CONSTRUCTION Owners Several owners have developed strategies for successful stage construction based on past successful practice. Five owners use a closure or construction pour between stages. One spec- ifies at least three lines of girders in the first stage, with a strong preference for six lines where future redecking may be needed. Another owner requires at least three lines in any stage. Two owners use only a top and bottom strut between the girders of adjacent stages, and one of the owners adds cross bracing after the deck pour. Finally, one owner uses slotted holes to facilitate fit-up of adjacent stages. Washington State cited a report by researchers from the University of Washington of particular interest. “Methods of Controlling Stresses and Distortions in Stage-Constructed Steel Bridges” (4) describes six design and construction methods, including a procedure for determining the forces in struts and/or crossframes for several of the methods, a design paradigm for the six methods, and typical strut and/or cross- frame connection details. Erectors Only one erector offered a solution for the problems of stage construction. That solution is field-drilling the holes in one side of the crossframe after the deck is poured in Stage 2. GIRDER STABILITY There were several measures that owners reported in regard to solutions. Two owners address the problem of girder sta- bility by placing more responsibility with the designer. One owner requires checks of the stability of a cantilever girder, adding lateral bracing if required; adequacy of the cross- frames to avoid flange buckling owing to the dead load of the girder and/or concrete; adequacy of the flanges for lateral bending or buckling; erectability of the girders; and effects of the pouring sequence in the positive moment regions. The other owner requires the designer to show lateral bracing at middepth of the girders in either one or two bays (depending on bridge width) on spans more than 45 m (150 ft), to con- trol instability owing to wind loads. Two of the owners place more responsibility with the erector. One requires that the erector show lateral bracing in the erection procedure, if needed. Another recommends that CHAPTER SIX SOLUTIONS TO REPORTED PROBLEMS

the erector initially install enough crossframes to ensure the stability under wind loads of the erected girder, before erect- ing subsequent girders. Other owners reported bridge-specific solutions used when problems arose with no change in their general requirements. 16 UNANTICIPATED DISTORTION One erector believes that the use of undersized bolts will sometimes help in making the initial connections. However, slotted connections do not seem to be the answer when dis- tortion results from loss of geometry control.

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 345: Steel Bridge Erection Practices examines steel bridge erection practices for I-girder, tub-girder, and box-girder bridges; particularly curved, skewed, and staged structures. The report focuses on the impact of design and analysis practices on erection; methods used to predict erection deflections as a function of bridge type and complexity; shop-assembly practices and alternate methods of ensuring properly assembled geometry; stability issues; field connection practices; examples of structures in which erection practices have caused problems; owner requirements for erection procedures, implementation of requirements, and the impact of procedures on the quality of erection; and current and proposed research.

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