A PROPOSAL OF SIMPLIFIED SEISMIC DESIGN METHOD FOR STEEL PIER WITH POTENTIAL FAILURE MODES OF LOCAL INSTABILITY AND ULTRA-LOW CYCLE FATIGUE

Local instability (LI) and ultralow-cycle fatigue (ULCF) are two potential failure modes of steel piers subjected to strong earthquakes. Steel bridges constructed in highly seismic regions must account for both failure modes in their seismic design calculations. While the issue of LI has largely been addressed, with preventive measures incorporated into various bridge design codes, the ULCF assessment remains challenging due to its high computational cost. Therefore, investigating the conditions under which the ULCF assessment can be avoided is of great significance for simplifying the seismic design of steel bridges. To this end, this study focuses on the damage occurrence priority of the two failure modes of steel piers. The proposed simplified seismic design method is the avoidance of ULCF check by constructing piers with the failure occurrence priority of LI. Taking the widely used box section single-column steel pier as an example, 90 piers with different structural parameters, representative of practical engineering applications, were designed in this paper. Based on a hypothetical cyclic loading protocol, the parameter ranges of steel piers with different damage priorities were determined. Discussion on the seismic performance of piers showed that the order of failure occurrence sequence is controlled by the mechanical properties of pier structure, rather than the load conditions it is subjected to. Additionally, the credibility of the derived damage occurrence sequence and the simplified seismic design method was verified through dynamic analyses using real seismic wave inputs. The work shown in this paper can serve as a reference to simplify the seismic design process of steel piers. Copyright (c) 2025 by The Hong Kong Institute of Steel Construction. All rights reserved.