Robustness Assessment Framework for Through Tied-Arch Bridge Considering Tie-Bar Failure

Compared with those of other bridge types, the structural system of through tied-arch bridges is particularly susceptible to failure owing to the deterioration and fracturing of tie-bars as well as a lack of structural robustness, which are the main internal and external causes of progressive collapse accidents, respectively. In particular, low structural robustness strongly influences the severity of accidents induced by tie-bar breakage. To objectively reflect the possible failure mode induced by tie-bar failure and to provide improved structural robustness in through tied-arch bridges, within which the suspended deck system is relatively independent, a robustness assessment framework considering tie-bar failure is proposed in this study, by considering the design and maintenance essentials. First, the main analysis parameters for the extreme-event limit state considering tie-bar failure are put forward, including the critical tie-bar failure rate, partial factor for load effects, transverse reduction factor of multilane loading, dynamic magnification factor of material strength, and reduction factors of residual strength. Second, the quantitative assessment index of structural robustness I_T,rob is proposed based on the safety redundancy of the remaining structural members, and robustness assessment rules are put forward in accordance with the different calculation results of I_T,rob and horizontal displacement. A typical rigid-framed through tied-arch bridges is assessed using the proposed framework. The assessment results reveal that multiple fortifications against extreme events of tie-bar failure can be established by designing the tie-bars and the piers with sufficient safety features to form the first and final line of defense against the tie-bar breakage effect. Moreover, proper maintenance should be conducted to guarantee the structural robustness of bridges in use. The methodology is well suited for the design of new through tied-arch bridges and to access the robustness of existing bridges to ensure the safety of transportation services.