Compressive Properties of Foam Aluminum Filled Galvanized Steel Tubes After High Temperature Exposure

Literature Overview

This paper by Wang Zhanguang, Dong Zhiwei, and Li Jie, published in Building Structures (2020, Vol. 50, No. 15, pp. 126-130), investigates the post-fire compressive behavior of circular foam aluminum filled galvanized steel tube columns. The research was supported by the Guizhou Provincial Science and Technology Plan Project (Qiankehe LH Zi [2017] 7169) and the National Natural Science Foundation (11462007). The study addresses an important gap in understanding how hybrid composite columns with aluminum foam infill perform after exposure to elevated temperatures, which is critical for fire-resistant structural design.

Research Background and Significance

Foam aluminum filled steel tube columns represent an emerging composite structural system that combines the energy absorption capability of cellular metal materials with the load-bearing capacity of steel tubes. Aluminum foam, with its open or closed cell structure, provides excellent crashworthiness and energy dissipation characteristics. When used as infill material in steel tube columns, it can enhance the overall structural performance under both service and extreme loading conditions.

However, the behavior of such hybrid columns after fire exposure remains poorly understood. Fire can significantly degrade the mechanical properties of both steel and aluminum foam, potentially compromising the structural integrity of columns that must remain functional during and after fire events. This research provides essential data for the development of fire-resistant design guidelines for foam-filled composite columns.

Experimental Program and Test Results

The researchers designed and fabricated a series of circular foam aluminum filled galvanized steel tube specimens with varying parameters including foam aluminum relative density, steel tube wall thickness, and specimen slenderness ratio. The specimens were heated according to a standard fire temperature-time curve, cooled to ambient temperature, and then subjected to axial compression tests.

The compression tests revealed that the foam aluminum filled galvanized steel tubes exhibit two distinct stages under axial loading: an elastic stage followed by a yield plateau stage. The average crushing load and energy absorption capacity were found to increase with increasing foam aluminum relative density and steel tube wall thickness. Conversely, these performance indicators decrease with increasing exposure temperature. The slenderness ratio of the members was found to have minimal influence on compressive performance and energy absorption capacity.

A notable finding is that the experimental results consistently exceeded the predictions from theoretical formulas, suggesting that the theoretical models may not fully capture the synergistic interaction between the foam aluminum infill and the galvanized steel tube shell.

Material Property Degradation Under Fire

The degradation of mechanical properties at elevated temperatures is a critical consideration. Galvanized steel tubes experience reduced yield strength and elastic modulus as temperature increases, with significant property loss occurring above approximately 550 degrees Celsius. The zinc coating may also undergo phase changes or spalling at high temperatures, affecting the corrosion protection of the steel substrate.

Aluminum foam properties are also temperature-sensitive. The aluminum matrix softens at elevated temperatures, reducing the crushing strength of the foam cells. The degree of property degradation depends on the foam microstructure, with closed-cell foams generally retaining more strength than open-cell foams at moderate temperatures.

Engineering Implications for Fire-Resistant Design

The findings have direct implications for the design of foam-filled composite columns in fire-prone environments:

1. Fire protection requirements: The temperature-dependent degradation of compressive capacity necessitates adequate fire protection measures to maintain structural integrity during fire events.
2. Post-fire assessment: Columns exposed to fire should be evaluated for residual capacity using the relationships established in this study before deciding on repair or replacement.
3. Design conservatism: The consistent overperformance relative to theoretical predictions suggests that current design approaches may be overly conservative, presenting opportunities for optimization.
4. Material selection: Higher relative density foams and thicker steel tubes provide better post-fire performance but increase material costs and weight.

Study Insights and Reflections

This research makes a meaningful contribution to the understanding of hybrid composite column behavior under fire conditions. The systematic investigation of multiple parameters provides a comprehensive picture of the factors influencing post-fire performance. The observation that experimental results exceed theoretical predictions is particularly interesting, as it suggests that the interaction between the foam and steel tube creates a synergistic effect not captured by simple superposition models. For structural engineers, this work provides essential data for the rational design of fire-resistant foam-filled composite columns, supporting the development of performance-based fire engineering approaches for innovative structural systems.