Blast Resistance of Steel Tube RPC Columns After Fire Exposure

Literature Overview

This paper, published in Vibration and Shock (2016, Vol. 35, No. 13), presents field-scale blast tests on four large-scale steel tube reinforced polymer concrete (RPC) column specimens exposed to varying durations of fire prior to detonation. The study was conducted at the State Key Laboratory of Explosion Science and Technology Disaster Mitigation, PLA University of Science and Technology, and was supported by the National Natural Science Foundation of China (Grants 51378498, 51578541, 51321064) and the Jiangsu Provincial Natural Science Foundation (Grant BK20141066). The authors—Zou Huihui, Chen Wanxiang, Guo Zhikun, Jiang Meng, and Xiang Hengbo—investigated how fire exposure time and scaled distance influence the post-fire blast response of composite steel tube RPC columns, a topic of significant practical relevance for industrial facilities, military installations, and energy infrastructure where both fire and blast hazards coexist.

Core Technical Findings

The experimental programme involved four full-scale steel tube RPC column specimens subjected to outdoor blast testing after exposure to controlled fire conditions. The key test parameters were fire exposure duration (0 min, 60 min, and 105 min) and scaled distance (ranging from 0.48 to 0.58 m/kg^(1/3)). The measured responses included shock wave reflection pressure, displacement time histories, and strain time histories at critical sections of the columns.

The principal conclusions drawn from the tests are summarised in the table below.

Fire Exposure	Scaled Distance (m/kg^(1/3))	Observed Behaviour	Failure Mode
0 min (unexposed)	0.58	Essentially elastic response	No visible damage
60 min	0.58	Good deformation capacity and shear resistance	Bending-dominated
60 min	0.48	Transition from bending to combined bending-shear	Bending-shear transition
105 min	0.58	Significant degradation; mid-span plastic hinge formed	Plastic hinge at mid-span

A particularly important quantitative finding is that for the 105-minute fire-exposed specimen at a scaled distance of 0.58 m/kg^(1/3), the maximum mid-span displacement increased by approximately 40% and the residual displacement increased by approximately 160% compared to the 60-minute fire-exposed specimen under the same blast loading conditions. This demonstrates that fire exposure time has a disproportionately large effect on residual damage accumulation, which is critical for assessing the usability of structures after combined fire-blast events.

Interpretation of Technical Points

The Confinement Effect of Steel Tubes After Fire

One of the most noteworthy observations in this study is that even after significant fire exposure, the steel tube continues to provide effective lateral confinement to the RPC core. This is consistent with the fundamental mechanics of composite steel tube concrete (CFST) or steel tube RPC columns: the steel tube acts as a continuous external confining jacket that prevents lateral expansion of the core material under compressive loading. However, fire exposure degrades the mechanical properties of the steel tube material. At temperatures above approximately 500 °C, the yield strength of structural steel begins to decrease significantly, and by 600 °C, the strength retention ratio can drop to approximately 60–70% of the room-temperature value, depending on the steel grade. The RPC material, while more thermally stable than ordinary concrete due to its low water-cement ratio and dense microstructure, also experiences some degree of property degradation at elevated temperatures.

The study confirms that the confinement effect is not eliminated by fire but is progressively weakened. This has important implications for the design of composite columns in fire-blast coupled scenarios: the steel tube provides a degree of residual capacity that can be leveraged in post-fire structural assessment, but the degradation is non-linear with respect to fire exposure duration.

Transition from Bending to Bending-Shear Failure

The transition from bending-dominated failure to combined bending-shear failure observed when the scaled distance decreased from 0.58 to 0.48 m/kg^(1/3) for the 60-minute fire-exposed specimen is a classic phenomenon in blast engineering. At larger scaled distances, the blast pressure is more uniformly distributed over the structural element, producing predominantly flexural demand. At smaller scaled distances, the pressure gradient across the element increases, and the shorter pulse duration concentrates energy into a more localised region, increasing shear demand. For fire-exposed specimens, this transition occurs at a larger scaled distance than would be expected for an unexposed specimen, indicating that fire exposure effectively reduces the shear capacity of the column more severely than its flexural capacity.

Residual Displacement as a Key Performance Indicator

The 160% increase in residual displacement for the 105-minute fire-exposed specimen compared to the 60-minute specimen at the same scaled distance is particularly significant from a post-disaster assessment perspective. Residual displacement is often a more critical indicator of structural usability than peak displacement, because it directly reflects permanent damage that cannot be recovered. In engineering practice, a residual displacement exceeding 0.5% of the structural height is often considered indicative of severe damage requiring repair or replacement. The study's findings suggest that fire exposure time is a primary driver of residual damage accumulation in composite steel tube RPC columns under blast loading.

Process and Standards Analysis

Relevance to Design Standards

The current design standards for steel tube concrete and steel tube RPC columns—such as GB 50017-2017 (Design Standard for Steel Structures), GB/T 22515-2018 (Steel tube concrete structures), and EN 1993-1-1 (Eurocode 3)—primarily address fire resistance through thermal property degradation curves and fire exposure duration limits. However, the coupling of fire and blast loading is not adequately addressed in most existing codes. The findings of this study provide valuable empirical data for developing coupled fire-blast design methodologies.

From a steel tube manufacturing perspective, the study highlights the importance of material selection for composite columns intended for dual-hazard environments. The steel tube material should possess good high-temperature strength retention and ductility. Grades such as Q345GJ (per GB/T 1591) or structural steel grades with favourable high-temperature properties are preferred. The welding quality of the steel tube fabrication joints is also critical, as welding heat-affected zones (HAZ) may have different thermal expansion characteristics and high-temperature strength behaviour compared to the base metal.

Welding Considerations for Steel Tube RPC Columns

In the fabrication of steel tube RPC columns, the steel tube is typically formed from rolled steel plate through longitudinal welding (LSAW or HFW processes) and circumferential welding (SMAW or SAW processes). The welding processes introduce residual stresses and microstructural variations in the HAZ that can affect the fire-blast coupled response. Key welding considerations include:

Preheating and interpass temperature control to minimise residual stress levels in the steel tube
Post-weld heat treatment (PWHT) to relieve residual stresses, particularly for thick-walled tubes
Welding procedure qualification that accounts for high-temperature mechanical properties, not just room-temperature properties
Non-destructive testing (NDT) protocols that include examination of welds for indications that could propagate under combined thermal and blast loading

Integration with Engineering Practice

Case Study Insight: Industrial Storage Facilities

In the design of storage tanks and containment structures for chemical and petrochemical facilities, the coexistence of fire and explosion hazards is a well-recognised design challenge. The findings of this study directly inform the design of support columns and structural frames in such facilities. The recommendation is that for columns expected to be exposed to fire for more than 60 minutes before a potential blast event, additional structural reinforcement or alternative material systems should be considered. The 105-minute fire exposure scenario in the study represents a severe but realistic condition for hydrocarbon pool fires, where flame temperatures can exceed 1000 °C.

Practical Design Recommendations

Based on the study findings, the following practical recommendations can be made for engineers designing steel tube RPC columns in dual-hazard environments:

Limit fire exposure duration to below 60 minutes where possible through fire protection measures (fireproof coatings, fire barriers, or water spray systems)
Design for a minimum scaled distance of 0.58 m/kg^(1/3) or greater to maintain bending-dominated behaviour, which is more ductile and predictable
Incorporate residual displacement limits into performance-based design criteria, targeting residual displacements below 0.3% of column height for acceptable post-event usability
Select steel tube grades with demonstrated high-temperature strength retention, and verify welding procedures for high-temperature performance
Conduct coupled fire-blast analysis in the design phase rather than relying solely on separate fire and blast analyses

Key Questions and Reflections

The study raises several important questions that warrant further investigation. First, the specimens were tested outdoors, which means that wind effects, ambient temperature variations, and ground reflection effects were not fully controlled. Indoor or semi-indoor testing facilities would provide more controlled experimental conditions but would be significantly more expensive. Second, the study did not investigate the effect of fire exposure temperature gradient on the column, which in practice can be highly non-uniform depending on the fire scenario and column orientation. Third, the RPC material used in the study may not represent the full range of RPC formulations available in practice, and the fire-blast coupled response could vary significantly with RPC mix design.

From a personal perspective, I find the most valuable aspect of this study to be the quantitative relationship between fire exposure time and residual displacement increase. The 160% increase in residual displacement between the 60-minute and 105-minute fire exposure cases at the same scaled distance is a powerful demonstration of the non-linear degradation of structural performance with increasing fire exposure. This finding should be communicated clearly to structural engineers who may not be aware of the disproportionate impact of extended fire exposure on blast resistance.

Study Insights and Implications

This study makes a valuable contribution to the understanding of coupled fire-blast structural response of composite steel tube RPC columns. The full-scale outdoor blast testing methodology, while less controlled than laboratory testing, provides more realistic data for engineering design. The key insight is that fire exposure time is a critical parameter that must be considered in the design of composite columns for dual-hazard environments. The confinement effect of the steel tube provides residual capacity even after fire exposure, but this capacity degrades significantly with extended exposure. For engineering practice, this means that fire protection measures that limit exposure duration to below 60 minutes can substantially preserve the blast resistance of composite steel tube RPC columns. The study also highlights the need for more comprehensive design standards that address coupled fire-blast loading scenarios, which remain a significant gap in current structural design codes.