Zhuojin Pipe Fitting Co., LtdResidual Bearing Capacity of Hollow Sandwich Square Steel Tube Recycled Concrete Columns After Fire Exposure
Literature Overview
This paper by Zhang Yuzhuo, Lü Xuetao, and Li Xiaoting, published in the Journal of Architecture and Civil Engineering (2019, Vol. 36, No. 1, pp. 76-84), investigates the post-fire residual mechanical performance of hollow sandwich square steel tube columns filled with recycled aggregate concrete. The research was supported by the National Natural Science Foundation of China (Grants 51208246 and 51808352) and the Liaoning Provincial Department of Education (Grant LJYL033). The study addresses two critical concerns in modern construction: the use of recycled concrete for sustainability and the fire resistance of composite steel-concrete members.
Theoretical Framework and Finite Element Modeling
The authors employed ABAQUS finite element software to establish a detailed numerical model of the hollow sandwich square steel tube recycled concrete column subjected to ISO 834 standard fire exposure. The finite element model captures the temperature-dependent material behavior of both the steel tube and the recycled concrete, as well as the complex thermal gradient distribution within the hollow sandwich cross-section.
The hollow sandwich configuration consists of an outer square steel tube, an inner hollow core, and a recycled concrete layer between the outer tube and the inner void. This geometry creates a unique thermal and mechanical environment where the concrete layer is thermally insulated from the hollow core side, leading to asymmetric temperature distribution and stress development.
| Modeling Parameter | Description | Significance |
|---|---|---|
| Fire standard | ISO 834 | Standard fire curve for structural fire design |
| Steel material model | Temperature-dependent constitutive law | Captures strength degradation and thermal expansion |
| Concrete material model | Damage plasticity with thermal coupling | Reflects cracking and spalling behavior |
| Contact interface | Frictional contact with thermal coupling | Models steel-concrete bond and heat transfer |
| Hollow core | Adiabatic boundary condition | Represents internal air insulation |
Parametric Analysis Results
The parametric study examined seven key parameters affecting the post-fire residual bearing capacity. The results reveal distinct trends for each parameter:
Steel and concrete strength effects: The residual bearing capacity increases monotonically with both steel grade and concrete strength. Higher-grade steel retains more of its yield strength at elevated temperatures, while higher-strength concrete provides greater confinement and compressive resistance after fire exposure.
Slenderness ratio effect: As the slenderness ratio (calculated length divided by the least cross-sectional dimension) increases, the residual bearing capacity decreases. This is consistent with the well-established Euler buckling behavior of columns, which is exacerbated by the reduced material stiffness after fire exposure.
Fire duration effect: The residual bearing capacity decreases with increasing fire duration, but the relationship is non-linear. A critical threshold exists at approximately 60 minutes of fire exposure, beyond which the residual capacity drops sharply. This threshold corresponds to the temperature at which steel undergoes significant strength degradation and concrete experiences severe spalling and cracking.
Hollow ratio effect: The hollow ratio (the ratio of hollow core area to total cross-sectional area) has a relatively minor influence on residual bearing capacity, with a slight decreasing trend as the hollow ratio increases. The reduced concrete volume slightly diminishes the compressive capacity but also reduces the thermal mass and heat absorption.
Recycled concrete replacement rate effect: The recycled aggregate replacement rate exhibits a non-monotonic relationship with residual bearing capacity. Initially, increasing the replacement rate slightly improves performance, likely due to better workability and denser packing, but beyond a certain threshold, the inferior bond strength and higher permeability of recycled aggregate lead to capacity reduction.
Eccentricity effect: The eccentricity ratio (the ratio of eccentric load to cross-sectional dimension) causes a monotonic decrease in residual bearing capacity. Higher eccentricity introduces greater bending moments, which combined with the fire-induced material degradation, accelerates failure.
Simplified Calculation Formula
Based on the extensive numerical results, the authors proposed a simplified calculation formula for the post-fire residual bearing capacity of hollow sandwich square steel tube recycled concrete columns. The formula incorporates the key parameters identified in the parametric study and provides a practical tool for preliminary design and assessment. The accuracy of the proposed formula was validated against the finite element results, demonstrating good agreement across the range of parameters studied.
Engineering Practice and Design Considerations
For structural engineers designing hollow sandwich steel tube recycled concrete columns for fire-prone applications, this research provides several important guidelines. The 60-minute fire duration threshold should be treated as a critical design boundary. Columns exposed to fire beyond this duration should be assumed to have severely compromised capacity and should be designed with adequate safety margins or protected with fire-resistant coatings and insulation.
From a steel pipe manufacturing standpoint, the study highlights the importance of selecting appropriate steel grades for fire-exposed composite members. Higher-grade steels (such as Q355 or Q390 per GB/T standards) offer better post-fire performance but may be more susceptible to temper embrittlement during fire exposure. The thermal expansion mismatch between steel and recycled concrete during fire exposure creates significant interface stresses that can lead to debonding and spalling. Manufacturers should consider surface treatments and connection details that maintain the steel-concrete bond under thermal cycling.
The recycled concrete aspect introduces additional quality control challenges. The variability in recycled aggregate properties, including moisture content, dust content, and particle shape, affects both the fresh and hardened concrete properties. Tighter quality control measures are needed for recycled aggregate concrete compared to virgin concrete, particularly regarding water-to-binder ratio control and air content management.
Study Insights and Reflections
The research effectively bridges two important domains — sustainable construction materials and fire engineering — in a manner that is both scientifically rigorous and practically useful. The hollow sandwich configuration is an innovative structural concept that combines the benefits of hollow sections (reduced weight, improved thermal insulation) with the composite action of steel and concrete. However, the study relies entirely on numerical simulation without experimental validation, which limits the confidence in the results.
The proposed simplified formula is a valuable engineering tool, but its applicability should be verified through experimental testing before widespread adoption in design codes. Future research should include full-scale fire tests on hollow sandwich columns, investigation of the effect of fire exposure patterns (such as one-sided or corner fire), and studies on the repair and rehabilitation of fire-damaged hollow sandwich columns. The interaction between fire exposure and the recycled concrete's higher permeability and lower interfacial transition zone strength warrants particular attention.