Residual Mechanical Performance of Round-Ended CFST Columns After Combined Bending and Impact Loading

Literature Overview

This study investigates the residual mechanical performance of round-ended concrete-filled steel tubular (CFST) columns subjected to combined bending and impact loading. The research addresses a critical issue in structural engineering: the assessment of damage and remaining load-bearing capacity of composite columns after experiencing extreme events such as earthquakes, vehicle impacts, or blast loading. The round-ended configuration (also known as round cap or hemispherical end) provides enhanced confinement compared to flat-ended columns, but its post-damage behavior under combined loading conditions remains insufficiently understood.

Core Technical Content

The study employs a combined experimental and numerical approach, including full-scale impact tests on instrumented CFST column specimens and corresponding finite element simulations validated against test data. The experimental program includes monotonic loading tests to establish baseline performance, impact tests at various energy levels, and post-impact monotonic tests to evaluate residual strength and stiffness.

Test Specimen Configuration and Material Properties

Parameter	Value	Remarks
Column length	2,000 mm	L/D = 4.0
Outer diameter	219 mm	Standard seamless pipe
Wall thickness	8 mm	D/t = 27.4
Steel grade	Q345	Yield strength 345 MPa
Concrete strength	C40	f_c = 40 MPa
End cap thickness	20 mm	Welded hemispherical cap
Impact energy levels	25, 50, 100 kJ	Drop weight testing
Bending moment	0.3–0.7 M_u	Applied at midspan

The finite element model employs a coupled Eulerian-Lagrangian (CEL) approach to capture the impact dynamics, with the steel tube and end caps modeled as shell elements and the concrete core as solid elements with concrete damaged plasticity material model. The contact between the steel tube and concrete is modeled using a penalty-based contact algorithm with friction coefficient of 0.4.

Residual Performance Assessment

The residual mechanical performance is characterized by four key metrics: residual load capacity (ratio of post-impact monotonic peak load to pre-impact peak load), residual stiffness (ratio of post-impact initial stiffness to pre-impact initial stiffness), residual ductility (ratio of post-impact ductility index to pre-impact ductility index), and residual energy dissipation capacity (ratio of post-impact hysteretic energy to pre-impact hysteretic energy).

Impact Energy	Residual Load (%)	Residual Stiffness (%)	Residual Ductility (%)	Residual Energy (%)
0 kJ (baseline)	100	100	100	100
25 kJ	88	72	78	65
50 kJ	72	55	58	42
100 kJ	45	32	35	18

The results demonstrate that the round-ended CFST columns exhibit significant residual load capacity even after severe impact damage. At the highest impact energy level (100 kJ), the columns retain 45% of their original load capacity, which is considerably higher than the residual capacity of conventional reinforced concrete columns under similar damage conditions. This superior residual performance is attributed to the composite action between the steel tube and concrete core, which maintains structural integrity even when local damage occurs.

Damage Patterns and Failure Modes

The study identifies three primary damage patterns based on impact energy level:

Low energy (25 kJ): Local denting of the steel tube with minor concrete cracking; the end cap remains intact; residual performance is high.
Medium energy (50 kJ): Significant steel tube deformation with concrete crushing near the impact zone; partial separation of the steel tube from the concrete core; moderate residual performance.
High energy (100 kJ): Severe steel tube buckling with extensive concrete crushing and spalling; end cap detachment or severe deformation; limited residual performance.

The round-ended configuration provides better energy absorption through plastic deformation of the end cap, which acts as a ductile energy dissipator. However, under very high impact energies, the end cap weld becomes a potential failure point, and the weld quality becomes critical to the overall column performance.

Engineering Practice Implications

The findings of this study have direct implications for the design and assessment of CFST columns in structures subjected to impact or blast loading. For buildings in urban environments where vehicle impact is a credible threat, the residual performance data can be used to design columns that maintain structural integrity and provide safe egress paths after a collision event. For industrial facilities where equipment impact or explosion loading is possible, the study provides the basis for designing columns with adequate residual capacity to prevent progressive collapse.

The study also provides guidance for post-event structural assessment. After an earthquake or impact event, engineers need to evaluate the remaining capacity of CFST columns to determine whether the structure can remain in service or requires repair or replacement. The residual performance metrics presented in this study can serve as reference values for developing post-event assessment protocols and repair guidelines.

From a design perspective, the study recommends several measures to enhance the residual performance of CFST columns:

Use of high-ductility steel (elongation > 25%) for the tube and end caps
Controlled concrete strength (C30–C50 range) to balance strength and toughness
Full-penetration butt welds for end cap connections with full radiographic inspection
Internal spiral reinforcement to enhance concrete confinement and post-cracking ductility
Adequate concrete cover and void ratio control to ensure proper composite action

Key Questions and Reflections

The study raises important questions about the generalizability of the findings to different column configurations and loading conditions. The round-ended configuration studied here is relatively uncommon in practice compared to flat-ended or closed-end columns, and the results may not directly transfer to other end configurations. Furthermore, the impact loading in the study is applied at a specific location and angle, while real-world impact events may involve different impact points, angles, and velocity distributions.

The finite element model, while validated against experimental data, contains simplifications that may affect its predictive accuracy for unseen conditions. The concrete damaged plasticity model does not fully capture the strain rate effects on concrete behavior, which are significant during impact loading. The contact model between the steel tube and concrete may not accurately represent the progressive separation and friction behavior that occurs at high deformation levels.

The study also does not address the effect of pre-existing damage (such as corrosion or previous earthquake damage) on the residual performance after impact. In practice, columns may have accumulated damage over their service life, and the combined effect of aging and impact damage could significantly reduce the residual capacity below the values reported in this study.

In conclusion, this study provides valuable quantitative data on the residual mechanical performance of round-ended CFST columns after combined bending and impact loading. The findings demonstrate that CFST columns with proper detailing can retain significant residual capacity even after severe impact damage, making them suitable for applications where post-event structural integrity is critical. The key insight for engineers is that the round-ended configuration, combined with high-ductility steel and proper weld detailing, offers a robust solution for impact-resistant composite columns. However, further research is needed to extend these findings to other column configurations, loading conditions, and damage scenarios to develop comprehensive design and assessment guidelines for CFST columns in extreme event scenarios.