Impact Resistance of Steel Tube Concrete Structural Members

Literature Overview

The study by Li Lijun and Wang Rui, published in the Journal of Beijing Institute of Technology (2012, Vol. 32, No. 10, pp. 1018-1021), investigates the impact resistance of steel tube concrete (STC) structural members using drop hammer impact testing. Funded by the National Natural Science Foundation of China (Youth Science Fund Project No. 51108296), this research addresses an important aspect of structural safety, particularly for structures exposed to accidental loading such as vehicle impacts, blast loads, or falling objects. The drop hammer test is a well-established method for evaluating the dynamic response and energy absorption capacity of structural members.

Experimental Methodology and Test Setup

The drop hammer impact test involves releasing a hammer of known mass from a controlled height to impart a known impact energy to the test specimen. The impact energy is calculated as E = mgh, where m is the hammer mass, g is gravitational acceleration, and h is the drop height. The test specimens were STC columns or beams with different steel tube wall thicknesses and concrete strengths, subjected to lateral impact at various energy levels.

The test setup typically includes:

A rigid impactor (drop hammer) with instrumented load cells to measure the impact force time history.
Displacement sensors (LVDTs) to measure the lateral deflection of the specimen.
High-speed data acquisition system to capture the dynamic response with sufficient temporal resolution.
Reaction frame to support the specimen and measure reaction forces.

The key measured outputs include the impact force time-history curve, the maximum lateral displacement, and the post-impact damage assessment.

Impact Force Time-History Characteristics

The study identified three distinct phases in the impact force time-history curve:

Phase	Description	Duration	Force Level
Rapid loading phase	Initial contact and rapid force increase	Short	Rising to peak
Plateau phase	Sustained force at near-peak level	Moderate	Near peak value
Unloading phase	Force decrease as specimen rebounds	Variable	Decreasing to zero

This three-phase behavior is characteristic of the dynamic response of STC members under impact loading. The rapid loading phase corresponds to the initial elastic response and the onset of plastic deformation. The plateau phase reflects the sustained plastic deformation and energy absorption by the member. The unloading phase occurs as the kinetic energy of the hammer is fully absorbed and the member begins to rebound.

Influence of Impact Energy on Structural Response

The study examined the effect of impact energy on the lateral displacement and damage characteristics:

As impact energy increases, the maximum lateral deflection of the specimen increases proportionally.
The STC members exhibit a clear yielding phenomenon under lateral impact, indicating significant plastic deformation capacity.
The dynamic response becomes more pronounced with increasing impact energy, as evidenced by the increased amplitude and duration of the impact force time-history curve.
The post-impact damage, including local buckling of the steel tube and concrete crushing, becomes more severe with higher impact energy.

The energy absorption capacity of STC members is attributed to the combined effect of steel tube plastic deformation, concrete crushing, and the interaction between the steel tube and concrete core. The steel tube provides confinement to the concrete, preventing premature spalling and enhancing the concrete's compressive strength, while the concrete core provides a rigid backing that delays the onset of steel tube buckling.

Comparison with Reinforced Concrete Members

STC members generally exhibit superior impact resistance compared to conventional reinforced concrete (RC) members for several reasons:

Property	STC Member	RC Member
Energy absorption capacity	Higher due to steel tube plastic deformation	Lower due to brittle concrete failure
Post-impact residual strength	Higher due to intact steel tube	Lower due to concrete crushing and reinforcement buckling
Ductility	Higher due to steel tube yielding	Lower due to concrete brittleness
Local damage	Controlled by steel tube confinement	Uncontrolled spalling and fragmentation
Impact force duration	Longer due to greater deformation capacity	Shorter due to rapid failure

These advantages make STC members particularly suitable for applications where impact resistance is critical, such as blast walls, protective barriers, and structures in seismically active regions where accidental impact loading may occur.

Engineering Practice Implications

The findings of this study have several practical implications for the steel pipe industry and structural engineering:

Pipe specification selection: The wall thickness of the steel tube is a critical parameter for impact resistance. Thicker tubes provide greater energy absorption but at the cost of increased weight and material usage. The optimal wall thickness should be determined based on the expected impact energy level.
Concrete grade selection: Higher-strength concrete improves the compressive resistance of the STC member but may reduce the overall ductility. A balance between strength and ductility should be achieved through appropriate concrete grade selection.
Post-impact assessment: The impact force time-history curve and maximum lateral displacement can be used as indicators of the severity of impact damage. Members with lateral displacements exceeding a specified threshold should be inspected for structural integrity.
Design considerations: For structures designed to resist impact loading, the STC members should be designed with sufficient ductility to absorb the expected impact energy without catastrophic failure. The design should account for the dynamic amplification factor, which is typically 1.2-1.5 for impact loading.

Key Questions and Reflections

One important question concerns the effect of impact velocity on the response of STC members. The drop hammer test typically involves impact velocities in the range of 5-15 m/s, but real-world impact events may involve a wider range of velocities. The dynamic response of STC members may be velocity-dependent, with higher velocities leading to more brittle behavior and reduced energy absorption capacity.

Another reflection concerns the cumulative damage from repeated impacts. The study focuses on single-impact events, but in practice, structures may be subjected to multiple impacts over their service life. The residual strength and stiffness after each impact, and the degradation of these properties with repeated loading, are important considerations for long-term structural safety.

Additionally, the study considers STC members in isolation, but in practice, impact-loaded members are part of a structural system. The boundary conditions, including the degree of restraint at the supports, can significantly affect the impact response. More restrained members may exhibit higher impact forces but lower lateral displacements, while less restrained members may exhibit lower forces but greater displacements.

Summary and Study Insights

This literature provides valuable experimental evidence of the excellent impact resistance of STC structural members, demonstrated through systematic drop hammer testing. The identification of the three-phase impact force time-history behavior and the clear yielding phenomenon under lateral impact offers insight into the dynamic response mechanisms of STC members. The comparison with conventional RC members highlights the advantages of STC in energy absorption and damage control. For engineers in the steel pipe industry, this work underscores the importance of providing high-quality steel tubes with consistent mechanical properties for STC applications where impact resistance is critical, and it provides a basis for developing pipe specifications tailored to impact-resistant structural applications.