Effects of Cross-Frame on Stability of Double I-Girder System under Erection

Double-girder systems investigated here consist of two steel I-shaped straight girders connected by intermediate cross-frames. In the erection stage, loading on the girder system, consisting mainly of girder self-weight, may not be significant. However, the concrete deck typically is not present to provide continuous lateral support to girder flanges, and there may be fewer cross-frames than in the service condition, resulting in fewer discrete lateral constraints to girder flanges and torsional constraints to girder sections. Such conditions could contribute to significant lowering of lateral torsional buckling capacity of the girder system compared with the service condition, and the girder system may become unstable during construction if the girder span is too long or if cross-frames do not provide enough discrete lateral support. Thus, it is important to investigate the effects of cross-frames on the stability of double-girder systems during construction. The AASHTO load and resistance factor design code has no specific requirements for maximum allowable spacing of cross-frames. To identify the dominant parameters and quantify the effects of cross-frames on double-girder system stability, finite element analyses were conducted to investigate the effects of the area of the cross-frame braces and cross-frame spacing on lateral torsional buckling capacity of double-girder systems under self-weight. The area of cross-frame members had negligible effects, but cross-frame spacing above a certain value affects system capacity. A method is presented to determine maximum cross-frame spacing (i.e., minimum number of cross-frames) for double-girder systems during erection. The goal is to achieve construction safety and labor reduction at the same time.