Ultimate Seismic Performance Analysis of Steel Tube Concrete Piers under Dynamic Loading

Literature Overview

This paper by Sun Hao and colleagues from Central South University, published in the Journal of the China Railway Society (2023, Vol. 45, No. 3, pp. 97-108), presents an advanced numerical investigation into the ultimate seismic performance of circular steel tube concrete (STC) bridge piers under dynamic loading conditions. Supported by the National Natural Science Foundation of China (Grants 51978664, 51978673) and the Central South University Basic Research Fund, the study employs refined finite element modelling in Abaqus with crack and ductile damage models to capture the complex behaviour of STC piers under various loading scenarios.

Core Technical Content

The research develops a three-dimensional solid-shell finite element model of circular STC bridge piers, incorporating two key constitutive enhancements: a crack model for concrete to capture cracking and crack propagation under cyclic loading, and a ductile damage model for the steel tube to represent progressive material degradation. The model was validated against four types of experimental data: unidirectional pseudo-static, unidirectional pseudo-dynamic, bidirectional pseudo-dynamic, and shake-table loading tests.

Constitutive Model Components

Key Experimental Validation Results

The refined model successfully reproduced:

• The "pinched" hysteresis curves characteristic of STC piers under cyclic loading, reflecting the progressive degradation of composite action between steel and concrete.
• The bearing capacity degradation during the large plastic deformation stage, caused by concrete crushing and steel tube local buckling.
• The post-earthquake residual displacement under both unidirectional and bidirectional dynamic loading.
• The interface slip and confinement behaviour between the steel tube and concrete core.

The study also found that fully-filled STC piers exhibited the smallest seismic response among all configurations tested, confirming that concrete infill significantly enhances the ultimate seismic capacity of the pier.

Technical Points Relevant to Steel Pipe and Welding Practice

The research has several implications for steel pipe manufacturing and welding in seismic structural applications:

• Steel tube local buckling is identified as a critical failure mode. The ductile damage model captures the progressive strain accumulation that leads to buckling initiation. This implies that the steel tube material must have adequate elongation and strain-hardening capacity to accommodate large inelastic deformations without premature fracture. For seismic applications, steel grades with high ductility (elongation > 25%) and controlled strain-hardening behaviour are preferred.
• Interface slip between the steel tube and concrete is a significant degradation mechanism. The quality of the steel tube inner surface, the concrete placement method, and the presence of construction joints all affect the bond and mechanical interlock at the interface. From a pipe manufacturing perspective, the inner surface finish of the steel tube should be smooth and free from scale or coating to maximise concrete-steel bonding.
• Residual displacement after earthquake loading is a critical performance metric for bridge piers. The study proposes a calculation formula relating the residual displacement at the pier top to the maximum displacement response under bidirectional seismic excitation. This formula can be used for post-earthquake assessment and repair decision-making.
• Welding at pier-base joints is a critical structural detail. The steel tube-to-foundation connection must accommodate the large plastic rotations demanded during seismic events. Full-penetration butt welds with generous weld reinforcement are typically required, and the weld metal must have ductility matching or exceeding that of the base steel.

Finite Element Modelling Methodology

The modelling approach represents a significant advancement over conventional equivalent beam-column modelling for STC piers. Key methodological aspects include:

1. Shell elements for the steel tube accurately capture the membrane and bending behaviour of the thin-walled tube, including local buckling modes that beam elements cannot represent.
2. Solid elements for the concrete core with the crack model enable simulation of crack initiation, propagation, and coalescence, which governs the concrete's contribution to lateral resistance and energy dissipation.
3. Cohesive zone elements at the steel-concrete interface provide a realistic representation of the bond-slip behaviour, which is critical for capturing the composite action degradation under cyclic loading.
4. Bidirectional loading simulation accounts for the combined effects of two orthogonal horizontal components of earthquake motion, which can significantly amplify the structural response compared to unidirectional loading.

Engineering Practice Implications

For bridge engineers and steel pipe suppliers working on seismic structural applications:

1. Steel tube specifications for seismic piers should include requirements for elongation, reduction of area, and strain-hardening ratio in addition to conventional yield and tensile strength.
2. The steel tube thickness should be designed to prevent local buckling before the pier reaches its full plastic hinge capacity. The slenderness ratio (D/t) should be limited according to seismic design codes.
3. Weld joints at pier bases should be designed with consideration for the large plastic rotations expected during major earthquakes. Weld procedure qualification should include cyclic loading tests to verify fatigue and ductility performance.
4. The proposed residual displacement formula can be incorporated into post-earthquake rapid assessment protocols to determine whether a damaged pier requires repair or replacement.

Study Insights and Reflections

This research demonstrates the power of refined finite element modelling with appropriate constitutive models for predicting the ultimate seismic performance of STC piers. The validation against multiple experimental loading scenarios provides confidence in the model's predictive capability. The proposed residual displacement formula offers a practical tool for engineers who need to assess pier damage after an earthquake without conducting full-scale testing. However, the study's focus on circular cross-sections limits its direct applicability to elliptical or rectangular STC piers, which exhibit different buckling and confinement behaviour. Future work should extend the modelling framework to other cross-section geometries and investigate the effects of steel tube material properties (including the influence of welding-induced residual stresses and heat-affected zone properties) on the seismic performance predictions. The integration of welding metallurgy considerations into structural seismic analysis remains an under-explored but important research direction.