Blast Resistance of FRP-Constrained Steel Tube Concrete Columns Under Axial Compression

Literature Overview

The paper by Liu Lan and colleagues from North University of China, published in Industrial Building (2021, Vol. 51, No. 1, pp. 179-186), addresses an important yet underexplored intersection between structural engineering and protective design: the blast resistance of fibre-reinforced polymer (FRP) confined steel tube concrete (CFST) columns subjected to axial compression. The study employs ANSYS/LS-DYNA to simulate the dynamic response of 16 axially loaded column specimens under blast loading and conducts a parametric analysis of axial compression ratio, slenderness ratio, FRP wrapping configuration, and FRP type. This work is particularly relevant to engineers in petrochemical, military, and critical infrastructure sectors where blast-resistant composite columns are increasingly considered as alternatives to conventional reinforced concrete or steel columns.

Core Technical Findings

The key findings of the study can be summarised as follows:

Axial compression ratio effect: When the axial compression ratio does not exceed 0.5, the presence of axial load enhances the blast resistance of the specimen. However, once the axial compression ratio exceeds 0.5, the axial load significantly degrades blast performance. This threshold behaviour is critical for design practice, as it delineates a safe operating window.
Slenderness ratio effect: Greater slenderness ratios result in larger mid-span displacements and poorer blast resistance. This aligns with classical structural mechanics, where slender members are more susceptible to dynamic buckling and large deformations under impulsive loads.
FRP wrapping configuration: Applying FRP wrapping improves blast resistance, and a reasonable local wrapping scheme can achieve confinement effects approaching those of full-column wrapping. This finding has direct economic implications, as it suggests that full wrapping is not always necessary.
FRP material type: Carbon fibre-reinforced polymer (CFRP) outperforms glass fibre-reinforced polymer (GFRP) in enhancing blast resistance under identical conditions, attributable to CFRP's superior tensile strength and elastic modulus.

Parametric Analysis and Design Implications

The following table summarises the parametric variables and their qualitative influence on blast performance:

Parameter	Range Studied	Influence on Blast Resistance	Design Recommendation
Axial compression ratio	0 to >0.5	Enhances (<0.5); Degrades (>0.5)	Keep ≤ 0.5 for blast-resistant design
Slenderness ratio	Low to High	Higher slenderness → poorer resistance	Minimise slenderness in blast zones
FRP wrapping method	Full vs. Local	Local can approach full performance	Optimise local wrapping for economy
FRP type	CFRP vs. GFRP	CFRP superior to GFRP	Prefer CFRP for critical applications

The finding that an axial compression ratio threshold exists at approximately 0.5 warrants careful engineering interpretation. Below this threshold, the pre-compression state effectively pre-stresses the concrete core and engages the steel tube in a more efficient confinement state, thereby improving energy absorption capacity under blast. Above this threshold, however, the concrete core is already in a highly stressed state, and the additional dynamic blast load pushes the composite section beyond its confinement capacity, leading to premature crushing and loss of load-carrying capacity. This is analogous to the behaviour observed in seismic design of CFST columns, where a ductile failure mode is desired, and excessive axial load compromises ductility.

Finite Element Modelling Considerations

The use of ANSYS/LS-DYNA for blast simulation of composite columns requires careful attention to several modelling aspects:

Material constitutive models: The concrete should be modelled using a dynamic constitutive model such as the Concrete Damaged Plasticity model with appropriate rate-dependent strength enhancement factors. The steel tube should incorporate a strain-rate-sensitive yield surface, typically using Cowper-Symonds or Johnson-Cook models.
FRP modelling: FRP wraps should be modelled as shell elements with appropriate failure criteria, accounting for delamination and matrix cracking.
Contact algorithms: The interface between concrete and steel tube, as well as between FRP and steel tube, requires robust contact formulations to prevent artificial separation or penetration.
Blast loading application: The blast pressure-time history should be applied using the Hopkinson-Cranz or Kingery-Bulmash scaled relationships, with appropriate reflected pressure for near-field scenarios.

Connection with Engineering Practice

In practical blast-resistant design, the findings of this study suggest a multi-layered defence strategy. The combination of steel tube confinement, concrete core, and external FRP wrapping creates a synergistic system where each component contributes to different aspects of blast resistance: the steel tube provides initial confinement and ductility, the concrete absorbs energy through cracking and crushing, and the FRP wrap provides secondary confinement after the steel tube begins to buckle. Engineers should note that the local wrapping strategy, while economically attractive, requires careful placement at critical zones such as mid-span and near supports, where bending moments and shear forces concentrate under blast loading.

Key Questions and Reflections

Several questions arise from this study that merit further investigation:

The study does not address the effect of blast direction (axial vs. lateral blast) on the performance of axially loaded columns. In real-world scenarios, lateral blast is more common, and the interaction between axial pre-load and lateral blast demand could be more complex than the current parametric study suggests.
The long-term durability of FRP wrapping under blast exposure is not considered. Post-blast inspection and potential repair strategies for FRP-confined columns should be developed for practical implementation.
The threshold axial compression ratio of 0.5, while useful as a design guideline, should be validated through physical blast tests before being adopted in formal design codes.

Study Insights and Implications

This research contributes valuable numerical evidence for the progressive enhancement of CFST columns through FRP confinement under blast conditions. The identification of the axial compression ratio threshold at 0.5 provides a clear design boundary, and the demonstration that local FRP wrapping can approximate full wrapping performance offers a practical cost-saving pathway. However, the study remains purely numerical, and experimental validation through scaled blast tests or quasi-static equivalent loading tests would strengthen the reliability of the conclusions. For engineers involved in the design of blast-resistant structures in petrochemical complexes, military installations, or critical infrastructure, this paper offers a starting point for incorporating FRP-confined CFST columns into blast-resistant design frameworks, provided that the identified parametric limits are respected and appropriate safety factors are applied.