Heat-curved Girders: Deflections and Camber Loss During and Subsequent to Bridge Construction

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The present AASHTO bridge design specifications require that additional camber be built into steel girders that are to be heat curved. The additional camber is provided to allow for subsequent losses due to the dissipation of residual stresses imposed by the heat-curving process during fabrication of the girders. Many bridge engineers and steel fabricators, however, question whether the additional camber is necessary. To check the camber loss in a heat-curved girder bridge, a 140-ft., simply supported span was instrumented during construction of the bridge. The span was composed of four steel plate girders having radii of curvature varying from 802.51 ft. on the inside to 834.51 ft. on the outside of the alignment curvature. Girder deflection and camber loss were measured prior to and subsequent to the construction of the bridge deck. The AASHTO specifications for highway bridges indicate that losses of camber in heat-curved girders will occur both during construction of the bridge and subsequently under service loading. It is suggested that 50 of the camber loss will occur during construction and an additional 50% after the bridge has been subjected to several months of service loading. For the bridge investigated, some camber loss due to construction loading occurred shortly after placement of the concrete deck. The amount of loss, however, was only one-fourth of that determined from the AASHTO equation. In addition, there were no significant camber losses due to service loading over 6% months. The total loss under both construction and service loading was only 13% of that predicted by the AASHTO equation. Therefore, the results of the study suggest that the relationship given in the specifications for the calculation of the potential camber loss in a heat-curved girder is not applicable to girders having radii of curvature greater than 800 ft. The order of magnitude of the camber losses further suggests that the specifications may not be completely applicable to girders having radii of curvature less than 800 ft.

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