General Behavior of a Steel Trapezoidal Box-Girder during Construction

During its lifetime, a composite box girder bridge experiences both construction and serviceability stages. The construction stage is the most critical design stage for box girders, since during this period they have lower torsional rigidity compared to the composite section and are very susceptible to cross- section distortion due to torsional loading. Recent failures of box girders, ranging from buckling of bracing members to complete collapse, all happened during construction, indicating that the behavior of box girders during construction is not yet well understood. Currently, design specifications do not provide designers with sufficient guidance regarding the behavior of box girders during construction. This thesis provides additional information about girder behavior during construction by presenting the results of parametric studies using a finite element computer program. Available hand methods are also presented. The required internal diaphragm spacing for limiting cross-section distortion and distortional normal stress is provided. The effect of using different top lateral bracing systems during monolithic and sequenced concrete pours on bracing forces, girder deformations, and normal stresses are presented. The sources of bracing forces, girder deformations, and total normal stresses are explained. Based on the parametric studies, recommendations for the optimal top lateral bracing system and pouring sequence with regard to top lateral bracing forces, girder stresses and deformations are discussed.