Seismic Response Analysis of Large Span Upper Bearing Concrete Filled Steel Tube Arch Bridge

Literature Overview

The paper by Zhao Canhui and Zhou Zhixiang (2007), published in the Journal of Chongqing Jiaotong University, investigates the seismic response characteristics of large-span upper-bearing concrete-filled steel tube (CFST) arch bridges. Supported by the National Natural Science Foundation of China (Grant 50578168), the study compares the self-vibration characteristics of mid-bearing and upper-bearing CFST arch bridges, employs time-history analysis under longitudinal, vertical, and transverse excitations, and discusses the influence of multi-point excitation on seismic response.

Core Technical Findings

The study reveals significant differences in seismic behavior between mid-bearing and upper-bearing CFST arch bridges. The upper-bearing configuration, where the deck is supported above the arch ribs, exhibits different dynamic characteristics compared to the mid-bearing configuration, where the deck is supported at the mid-height of the arch.

The multi-point excitation analysis is particularly important because large-span bridges experience differential ground motion across their length, which can induce additional stresses in the arch ribs and deck connections that are not captured by single-point excitation models.

Steel Tube Fabrication and Welding Implications

For large-span CFST arch bridges, the arch ribs are typically constructed from large-diameter steel tubes, often in the range of 1200 mm to 2000 mm in diameter, filled with concrete. The steel tubes for such applications are generally manufactured using LSAW (longitudinal submerged arc welding) or UOE (Upstream-Outside-Inside) processes to achieve the required large diameters and wall thicknesses.

The welding of arch rib segments is a critical construction activity. Field welding of large-diameter CFST tubes typically employs multi-pass SMAW, SAW, or PAW (plasma arc welding) processes. The welding sequence must be carefully planned to minimize residual stresses and distortion, which are particularly critical in arch structures where geometric accuracy directly affects the load path. The HAZ properties of the field welds must be evaluated through hardness surveys and tensile testing to ensure that the weld metal and HAZ meet the minimum requirements specified in standards such as GB 50017 or AISC 360.

Seismic Design Implications for Tube Specifications

The study's findings regarding multi-point excitation sensitivity suggest that the tube material properties and weld quality must be maintained consistently along the entire arch rib length. Any local weakness in the tube material or weld quality can become a critical failure point under multi-point seismic excitation. Engineers should specify uniform chemical composition and mechanical properties for all tube segments, and implement rigorous non-destructive testing (NDT) protocols including phased array ultrasonic testing (PAUT) for weld inspection.

The comparison between mid-bearing and upper-bearing configurations has implications for the selection of tube geometry and material. The upper-bearing configuration's higher sensitivity to multi-point excitation suggests that higher-grade steel tubes with better ductility properties may be warranted, and the welding procedures should emphasize toughness and ductility rather than pure strength. This research provides valuable guidance for the seismic design of large-span CFST arch bridges, but the practical implementation requires careful integration of material selection, fabrication quality, and welding procedures to ensure that the theoretical performance predictions are realized in the actual structure.