Field testing and finite element analysis of retrofit methods for distortion-induced fatigue in steel bridges

Crack formation due to out-of-plane distortion in the web-gap region has been a common occurrence in multi-girder steel bridges. These cracks result from the fatigue stresses that are induced in the web-gap due to cyclic diaphragm forces resulting from differential deflections between girders. The study presented herein investigated the different repair methods that can be used to control formation of these cracks. The study involved field testing and analytical modeling of a skewed multi-girder steel bridge designated as Design No. 1283, which is built on county road D-180 that crosses over I-380 in the state of Iowa. Different repair methods were suggested to reduce the induced stresses and strains in the web-gap under truck loads. These methods included loosening of the bolts connecting the cross-bracing to the stiffener, connecting the stiffener to the girder top flange or adding another stiffener on the opposite side of the girder web. The results indicated that the first two of these repair alternatives were effective in reducing induced stresses and strains in the web-gap region. The impact of web-gap height on the distortion induced in the web-gap was also studied. Furthermore, influence surfaces for different responses such as, web-gap strains, stresses, out-of-plane displacements at critical locations, and forces in the adjacent diaphragm were developed. Moreover, relationships between the relative out-of-plane displacements and vertical stresses induced within the web-gap region were also provided. These developed relationships and surfaces serve as a quick estimate of induced stresses at critical locations in other web-gap regions of the bridge

Inelastic Bending Capacity in Cold-Formed Steel Members

The enclosed research report has been prepared for the AISI as supporting material for proposed additions to the AISI Specification (AISI-S100-07) with respect to inelastic bending of cold- formed steel flexural members. In particular, an extension to the Direct Strength Method of Appendix 1 of AISI-S100-07 is proposed which allows for design capacities to exceed My (and approach Mp) as a function of the slenderness in the local-global or distortional modes

Inelastic Bending Capacity in Cold-Formed Steel Members

The enclosed research report has been prepared for the AISI as supporting material for proposed additions to the AISI Specification (AISI-S100-07) with respect to inelastic bending of cold- formed steel flexural members. In particular, an extension to the Direct Strength Method of Appendix 1 of AISI-S100-07 is proposed which allows for design capacities to exceed My (and approach Mp) as a function of the slenderness in the local-global or distortional modes

Direct Strength Prediction of Cold-Formed Steel Beam-Columns

The Direct Strength Method (DSM) of cold-formed steel member design employs local, distortional, and global cross-section elastic buckling analysis with empirically derived “direct” expressions to predict member strength. DSM is an accepted design method in national design specifications (e.g., AISI-S100-16) and enables a unified, robust, and flexible design approach. However, for beam-columns DSM in current design specifications employs simplified linear interaction expressions based on combining the isolated axial and bending elastic buckling and strength response. Today, local, distortional, and global elastic buckling under any combination of axial load and bending moments may be found using elastic buckling analysis tools such as the finite strip method (e.g., CUFSM). Thus, stability may be assessed under the combined actions, but new DSM expressions are needed to utilize this explicit stability information in determining beam-column strength. In this report, new strength expressions for each limit state are developed. In addition, the results of beam-column tests performed by the authors and those available in the literature are used to validate the performance of the new proposed DSM for beam-columns. The development of DSM for beam-columns has the potential to provide a more mechanically sound solution to the strength of cold-formed steel beam-columns, eliminate excessive conservativeness, and at the same time encourage the next generation of optimized, high strength, cold-formed steel shapes. This report covers: a new formulation for DSM that can account for stability and strength under multiple actions; targeted testing under P-M-M loadings to explore the beam-column stability space explicitly and find capacities; nonlinear FEA analysis to expand the studies and flesh out issues in the final design methods; and technology transfer to ease the use of the develop method and its related tools.American Iron and Steel Institut

Direct Strength Prediction of Cold-Formed Steel Beam-Columns

The Direct Strength Method (DSM) of cold-formed steel member design employs local, distortional, and global cross-section elastic buckling analysis with empirically derived “direct” expressions to predict member strength. DSM is an accepted design method in national design specifications (e.g., AISI-S100-16) and enables a unified, robust, and flexible design approach. However, for beam-columns DSM in current design specifications employs simplified linear interaction expressions based on combining the isolated axial and bending elastic buckling and strength response. Today, local, distortional, and global elastic buckling under any combination of axial load and bending moments may be found using elastic buckling analysis tools such as the finite strip method (e.g., CUFSM). Thus, stability may be assessed under the combined actions, but new DSM expressions are needed to utilize this explicit stability information in determining beam-column strength. In this report, new strength expressions for each limit state are developed. In addition, the results of beam-column tests performed by the authors and those available in the literature are used to validate the performance of the new proposed DSM for beam-columns. The development of DSM for beam-columns has the potential to provide a more mechanically sound solution to the strength of cold-formed steel beam-columns, eliminate excessive conservativeness, and at the same time encourage the next generation of optimized, high strength, cold-formed steel shapes. This report covers: a new formulation for DSM that can account for stability and strength under multiple actions; targeted testing under P-M-M loadings to explore the beam-column stability space explicitly and find capacities; nonlinear FEA analysis to expand the studies and flesh out issues in the final design methods; and technology transfer to ease the use of the develop method and its related tools.American Iron and Steel Institut