Finite Element Analysis of Lateral Impact Resistance of Square CFST Members with Embedded CFRP Profiles

Literature Overview

The paper by Li Guochang, Liu Lu, and Yang Zhijian from Shenyang Jianzhu University (2022) presents a finite element study on the lateral impact resistance of square concrete-filled steel tube (CFST) members with embedded I-shaped CFRP profiles. Funded by the National Natural Science Foundation of China (Grants 51878419 and 51938009), this research was published in Progress in Steel Building Structures, Volume 24, Issue 1, pages 41–52. The study employs ABAQUS software with strain rate effects considered for dynamic response analysis.

Core Technical Findings

The research investigates a hybrid structural system that combines the compressive strength of concrete, the ductility of steel tubes, and the high specific strength of carbon fiber reinforced polymer (CFRP) profiles. The I-shaped CFRP profile is embedded within the square CFST member to enhance its lateral impact resistance.

Key findings from the parametric analysis include:

• The embedded CFRP I-profile significantly improves the energy absorption and deformation capacity of the member.
• Mid-span deflection increases with both impact energy and shear span ratio.
• The time required to resist impact extends as impact energy and shear span ratio increase.
• Peak impact force is minimally affected by concrete strength but increases markedly with impact energy, steel ratio, and steel tube yield strength.

Technical Interpretation and Strain Rate Effects

The consideration of strain rate effects is a critical aspect of this study. Under dynamic loading conditions, both steel and concrete exhibit strain rate sensitivity, where their strength and stiffness increase with increasing strain rate. For structural steel, the Johnson-Cook constitutive model is commonly used to describe this behavior, where the dynamic yield strength is expressed as a function of strain rate, temperature, and strain hardening.

From a welding and materials perspective, the strain rate effect has important implications for the design of welded joints in impact-resistant structures. The heat-affected zone (HAZ) of a welded joint typically has different strain rate sensitivity compared to the base metal and weld metal. Under high-strain-rate loading, the HAZ may become the weakest link if its dynamic properties are not properly characterized.

The finite element model must accurately capture:

1. The material nonlinearity of steel, concrete, and CFRP under dynamic loading.
2. The strain rate dependency of each material component.
3. The contact interaction between the CFRP profile and the concrete core.
4. The progressive damage and failure modes of each component.
5. The geometric nonlinearity due to large deformations during impact.

Parametric Analysis and Design Implications

The parametric study reveals several important design insights:

• Impact energy is the dominant parameter governing the overall response. Higher impact energy leads to greater deflection, longer response time, and higher peak forces. This is consistent with the fundamental relationship between kinetic energy and structural response.
• Steel ratio has a significant effect on peak force but a less pronounced effect on deflection. This suggests that increasing the steel ratio primarily enhances the stiffness and strength of the member rather than its ductility.
• Shear span ratio affects the failure mode of the member. A lower shear span ratio promotes shear failure, while a higher shear span ratio leads to flexure-dominated behavior. The embedded CFRP profile helps redistribute stresses and delay failure.
• Concrete strength has minimal influence on peak impact force, which is somewhat counterintuitive. This may be because the steel tube and CFRP profile dominate the load-bearing capacity under dynamic loading, while the concrete primarily serves as a confinement and energy absorption medium.

Integration with Engineering Practice

The concept of embedding CFRP profiles within CFST members has potential applications in:

• Bridge columns designed to resist vehicle impact.
• Nuclear facility structures requiring blast resistance.
• Marine structures subjected to wave impact or collision.
• Transportation infrastructure requiring enhanced impact tolerance.

From a fabrication standpoint, the integration of CFRP profiles within CFST members introduces additional challenges:

• The CFRP profile must be precisely positioned within the tube before concrete placement.
• Bonding between CFRP and concrete requires careful surface treatment and possibly mechanical anchoring.
• The thermal expansion mismatch between CFRP and steel/concrete must be considered for temperature-varying environments.
• Welding operations on the steel tube must be planned to avoid damaging the CFRP profile through heat conduction.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation:

• How does the position of the CFRP profile within the tube affect the impact response?
• What is the long-term durability of the CFRP-concrete interface under cyclic or sustained loading?
• Can the findings be extrapolated to other CFRP profile shapes such as channel or box sections?
• How does the strain rate effect vary with temperature, particularly in cold environments?

The study demonstrates that hybrid CFRP-steel-concrete systems offer a promising approach to enhancing impact resistance. However, the practical implementation requires careful consideration of fabrication tolerances, quality control procedures, and long-term performance under service conditions.

Study Insights and Implications

This research contributes valuable insights into the design of impact-resistant hybrid structural members. The combination of steel tubes, concrete, and CFRP profiles creates a synergistic system where each component leverages its unique properties. For engineering practice, the key takeaway is that the embedded CFRP profile primarily enhances energy absorption and deformation capacity rather than peak strength, suggesting that this system is particularly suitable for applications where ductile energy dissipation is more important than maximum load resistance.