Impact Resistance of Concrete-Filled Steel Tube Members Under Long-Term Sustained Loads

Literature Overview

Published in Explosion and Shock Waves (Vol. 41, No. 8, 2021, pp. 89-101), this paper investigates the impact resistance of concrete-filled steel tube (CFST) members subjected to long-term sustained axial loads followed by lateral impact loading. The research was supported by the National Natural Science Foundation of China (Project No. 51778274), the Gansu Provincial University Collaborative Innovation Team Project (2018C-08), and the Lanzhou Science and Technology Plan (2019-1-61). The study employs ABAQUS finite element software to establish coupled analysis models for long-term load and lateral impact load, as well as a model for calculating residual compressive bearing capacity after impact.

Technical Background and Engineering Relevance

CFST members are widely used in bridges, buildings, and industrial structures where high strength and ductility are required. However, many such structures are subjected to long-term sustained loads (dead load, superimposed dead load) before experiencing impact events such as vehicle collisions, projectile impacts, or blast loading. The interaction between long-term sustained loading and impact loading is a critical but under-investigated aspect of structural engineering.

The study compares two loading scenarios:

1. Single loading mode: Impact applied directly to an unloaded member
2. Long-term load mode: Sustained axial load applied first, followed by lateral impact

Finite Element Modeling Approach

The authors developed two coupled analysis models in ABAQUS:

The residual compressive bearing capacity coefficient is used as a quantitative metric to compare the two loading modes. This coefficient represents the ratio of the post-impact compressive capacity to the original compressive capacity.

Key Findings on Dynamic Response

The study revealed several important differences between the two loading modes:

The observation that the impact force peak and plateau values decrease under long-term sustained loading is counterintuitive but physically explainable: the pre-existing axial deformation and stress state under sustained load reduces the effective stiffness of the member, leading to lower impact forces but larger deformations.

Energy Dissipation Mechanism Analysis

A particularly important finding is the difference in energy dissipation mechanisms between the two loading modes:

• Under long-term load conditions, the axial load does additional work during the impact process compared to the single loading mode.
• The additional work is primarily dissipated through plastic deformation of the outer steel tube.
• The core concrete contributes relatively little to the additional energy dissipation.

This finding has significant implications for understanding the damage mechanism: the steel tube bears the brunt of the additional deformation under long-term load conditions, while the concrete core, already under sustained compression, has limited additional capacity to contribute to energy absorption.

Residual Compressive Capacity Assessment

The most critical finding for structural safety is that the residual compressive bearing capacity coefficient under long-term load conditions is consistently lower than that under single loading mode conditions. This means:

1. A member that can continue to bear load after impact under single loading mode may lose its load-bearing capacity under long-term load conditions with the same impact parameters.
2. The long-term sustained load significantly reduces the member's post-impact structural integrity.
3. The margin of safety against collapse after impact is substantially reduced when long-term loads are present.

Mitigation Strategies

The study identifies several effective strategies to reduce the adverse effects of long-term loads on impact resistance:

These strategies are practical and can be implemented through design choices during the structural design phase.

Engineering Practice Implications

From a steel pipe and welding engineering perspective:

1. Pipe material selection: Higher-grade steel pipes (e.g., API 5L X70, X80, or X100) provide better impact resistance under long-term load conditions, but cost considerations must be balanced.
2. Weld quality for CFST: The longitudinal weld in LSAW or HFW pipes used for CFST construction must be of high quality, as the weld region is a potential weak point under combined sustained and impact loading.
3. Residual stress management: Welding residual stresses in the steel tube can interact with long-term sustained loads and impact loads, potentially accelerating damage. Stress-relief welding or post-weld heat treatment may be beneficial.
4. Inspection and monitoring: For critical CFST structures, post-impact inspection should assess both the steel tube and the concrete core for damage, with particular attention to the weld regions.

Study Insights and Reflections

This study addresses a critical gap in the understanding of CFST member behavior under realistic loading scenarios. The finding that long-term sustained loads can cause a member to lose its post-impact load-bearing capacity even when the same impact would not cause such loss under single loading conditions is a significant safety concern. This has direct implications for the design of bridges, offshore platforms, and industrial structures where CFST members are subjected to both long-term service loads and potential impact events. The emphasis on the steel tube as the primary energy dissipation component under long-term load conditions reinforces the importance of pipe material quality, weld integrity, and residual stress control in CFST construction. Future research should investigate the effect of long-term load duration (creep effects) and the combined effect of corrosion degradation on impact resistance under sustained loading.