Pseudo-Static Testing of Steel Tube-Confined Concrete Columns Under Ambient Temperature Variation

Literature Overview

This study investigates the influence of ambient temperature on the mechanical behavior and seismic performance of steel tube-confined concrete (STC) columns through pseudo-static cyclic loading tests. Temperature effects on structural materials are increasingly significant in the context of climate change, extreme weather events, and the growing number of structures located in regions with severe temperature variations. The research provides valuable data on how temperature-induced changes in material properties and residual stresses affect the load-carrying capacity, ductility, and energy dissipation of STC columns.

Core Technical Framework

Test Variables

Variable	Levels Tested	Rationale
Ambient temperature	-20°C, 0°C, 20°C, 40°C, 60°C	Covers typical service temperature range plus extreme conditions
Concrete strength grade	C40, C60	Represents common structural concrete grades
Steel tube grade	Q345, Q390	Common structural steel grades for STC
Slenderness ratio	6, 8, 10	Covers short to medium-length columns
Axial load ratio	0.2, 0.4, 0.6	Typical axial load levels for bridge and building columns

Material Property Temperature Dependence

The study systematically characterizes how temperature affects the constituent materials:

Material Property	20°C (Reference)	-20°C	60°C	Change at 60°C
Concrete compressive strength	1.00	1.05	0.85	-15%
Steel yield strength	1.00	1.08	0.90	-10%
Steel elastic modulus	1.00	1.03	0.92	-8%
Concrete elastic modulus	1.00	1.06	0.88	-12%
Steel-concrete bond strength	1.00	1.04	0.82	-18%

Key Experimental Results

Temperature Effects on Peak Strength

The peak lateral load capacity of STC columns shows a non-linear temperature dependence. At low temperatures (-20°C), the strength increases by 5–8% due to enhanced material properties and reduced thermal expansion stresses. At moderate elevated temperatures (40°C), the strength remains within 5% of the room temperature value. At high temperatures (60°C), the strength decreases by 10–15%, primarily driven by concrete strength degradation and reduced interfacial bond strength.

Ductility and Energy Dissipation

The ductility performance is more sensitive to temperature than the strength performance. At 60°C, the displacement ductility factor decreases by 15–25% compared to room temperature conditions, and the energy dissipation capacity decreases by 20–30%. This is attributed to the accelerated damage accumulation in the concrete core and the reduced confinement effectiveness of the steel tube at elevated temperatures.

Hysteretic Behavior at Different Temperatures

Temperature	Loop Shape	Pinch Effect	Peak Drift	Strength Degradation at 3% Drift
-20°C	Full, stable	Minimal	5.5–6.5%	< 10%
20°C	Full, stable	Slight	4.5–5.5%	10–15%
40°C	Moderately full	Moderate	4.0–5.0%	15–20%
60°C	Pinched	Significant	3.0–4.0%	20–30%

Engineering Practice Implications

Design Considerations for Temperature-Exposed Structures

The findings have direct implications for the seismic design of STC columns in bridges and buildings located in regions with significant temperature variations:

Design temperature effects: Codes should incorporate temperature reduction factors for concrete and steel properties when assessing seismic capacity, particularly for structures in hot climates or those exposed to fire conditions
Thermal residual stress effects: The differential thermal expansion between steel and concrete creates residual stresses that can either enhance or reduce the effective confinement, depending on the temperature history
Construction sequence effects: The temperature at the time of concrete placement and curing significantly affects the initial residual stress state and subsequent mechanical behavior

Inspection and Assessment Guidelines

For existing STC structures exposed to temperature variations, the following assessment considerations apply:

Non-destructive evaluation: Ultrasonic pulse velocity measurements should be temperature-compensated, as concrete acoustic properties vary significantly with temperature
Material sampling: Core samples extracted for material testing should be conditioned to standard temperature before testing to avoid misleading results
Structural monitoring: Temperature-sensitive structures should be equipped with temperature sensors to enable real-time correction of measured structural responses

FMEA Analysis for Temperature-Induced Failure Modes

Failure Mode	Cause	Effect	Detection Method	Mitigation
Concrete strength degradation	Sustained high temperature	Reduced load capacity	Core testing, UT	Thermal insulation, cooling systems
Steel tube yielding	Combined thermal and mechanical stress	Loss of confinement	Strain gauges, visual inspection	Temperature monitoring, material upgrade
Interface debonding	Differential thermal expansion	Loss of composite action	MT, UT at interface	Mechanical anchoring, flexible grout
Concrete spalling	Thermal cracking + cyclic loading	Exposed steel tube	Visual inspection, PT	Proper confinement design, cover protection

Study Insights and Conclusions

This research fills an important gap in the understanding of temperature effects on steel tube-confined concrete structures. The results demonstrate that while STC columns maintain good seismic performance across a wide temperature range (-20°C to 40°C), significant degradation occurs at temperatures exceeding 50°C. The non-linear interaction between temperature-induced material property changes and the composite action between steel and concrete creates a complex behavior that cannot be accurately predicted by simple superposition of individual material temperature effects. Engineers designing or assessing STC structures in temperature-variable environments should adopt a systematic approach that considers the full temperature history, the interaction between thermal and mechanical loads, and the progressive nature of temperature-induced damage. Future research should extend to long-term cyclic loading at elevated temperatures to capture the combined effects of fatigue and thermal aging.