An approximate method of dynamic amplification factor for alternate load path in redundancy and progressive collapse linear static analysis for steel truss bridges

Linear static analysis with an alternate load path using dynamic amplification factor (DAF) is often used for redundancy and progressive collapse analysis of steel truss bridges to avoid using the more time-consuming dynamic analysis. This study presents an empirical equation to calculate the DAF for this type of analysis against the initial sudden member fracture. Currently, this analysis employs an approximate model with a single degree of freedom to calculate the DAF. With a 5% damping ratio, the constant DAF of 1.854 is used for all types of steel truss bridges. However, this approach is inaccurate because the DAF varies between bridges and with the location of the fractured members as well. Considering some of the approaches developed for building structures but adapting them to steel truss bridges, this paper proposes an empirical equation that allows for the computation of the DAF from the maximum norm stress ${\sigma }_{is}/{\sigma }_{iy}$ in static linear elastic analysis of the damaged model with a member removal. A total of 30 illustrative cases for two typical steel truss bridges are investigated to obtain the data points for the empirical equation. The proposed empirical equation is the enveloped line offset from the best fit line for the data points in illustrative cases.