Feasibility Verification of Multi-Mode Pushover Analysis for Steel Tube Concrete Arch Bridges

Literature Overview

The paper by Zeng Sen, Gao Longtao, Chen Shaofeng, and Wang Caihua from Harbin Institute of Technology, published in China Civil Engineering Journal (2016, Vol. 49, No. 1, pp. 80-85), investigates the applicability of multi-mode pushover analysis for estimating the inelastic seismic response of concrete-filled steel tube (CFT) arch bridges. The study uses the Chaobai River Bridge as a case example and compares pushover-derived incremental dynamic analysis (IDA) curves against nonlinear time-history analysis results. Funded by the National Natural Science Foundation of China (51308157).

Core Technical Contribution

CFT arch bridges exhibit significant contributions from higher vibration modes to their seismic response, unlike simple frame structures where the fundamental mode often dominates. The authors propose a multi-mode combination pushover method that sequentially activates multiple vibration modes, each with its own capacity curve, and combines the results to estimate total structural response. This approach bridges the gap between the computational efficiency of pushover analysis and the accuracy of nonlinear time-history analysis.

The methodological steps include: determining the participating modes through modal analysis, performing pushover analysis for each mode independently, combining the modal displacements and forces using an appropriate combination rule, and constructing IDA curves from the combined response envelopes.

Comparison of Analytical Methods

The study reports that the multi-mode pushover method yields IDA curves closely matching those from nonlinear time-history analysis, with errors in the single-mode approach being 10% to 15% larger than the multi-mode results.

Relevance to Steel Pipe and Welding Engineering

From the perspective of steel pipe and welding engineering, CFT arch bridges rely heavily on concrete-filled steel tube members whose seismic performance is governed by the interaction between the steel tube and the confined concrete. The steel tubes are typically manufactured from structural steel pipes conforming to GB/T 14975 or ASTM A53, with wall thicknesses selected to resist local buckling under compressive and bending demands. The weld connections between steel tubes and other structural elements—such as flanges, end plates, and gusset plates—are critical load paths whose ductility and strength directly influence the overall seismic capacity.

The multi-mode pushover method has practical implications for weld detailing in CFT arch bridges. When higher modes contribute significantly to response, local members experience complex stress states involving multi-axial bending and torsion that single-mode analysis would underestimate. This means weld details at steel tube connections must be designed for higher demand, potentially requiring full-penetration groove welds with reinforcement welds rather than fillet welds alone, in accordance with AWS D1.1 and GB/T 985.1.

Engineering Practice Considerations

The accuracy improvement of 10% to 15% from multi-mode analysis over single-mode analysis, while modest in absolute terms, is significant for pushover-based design where the method is often used as a screening tool before detailed time-history analysis. For CFT arch bridge projects where steel tube connections are the governing design element, underestimating demands by 10% to 15% could lead to under-designed weld details, insufficient ductile capacity, and potential brittle fracture under severe seismic events.

Study Insights and Reflections

This paper reinforces a principle I have long advocated in structural steel pipe engineering: that the seismic design of steel tube members must account for the full complexity of dynamic response, not merely the fundamental mode. In practice, this translates to more conservative weld design at critical connections. The pushover method, when properly implemented with multi-mode combination, provides a practical tool for engineers to verify that weld details meet the required ductility demands without resorting to the full computational burden of nonlinear time-history analysis for every design iteration. The Chaobai River Bridge case demonstrates that this approach is feasible and reliable enough for engineering practice, which is encouraging for the growing number of CFT arch bridges being constructed in seismic regions.