Hysteretic Performance of Steel Tube Lightweight Aggregate Concrete Columns under Horizontal Reciprocating Loads

Literature Overview

This paper by Fu Zhongqiu, Wu Dongyang, Ji Bohai, and Wang Zhanfei, published in the Journal of Shenyang Jianzhu University (Vol. 35, Issue 2, 2019, pp. 202-210), presents the results of a pseudo-static cyclic loading test on eight steel tube lightweight aggregate concrete (STLAC) columns. Funded by the National Natural Science Foundation of China (Youth Project No. 51208176), this research investigates the failure modes, hysteretic behavior, and confinement effectiveness of STLAC columns under combined axial compression and horizontal reciprocating loads. The study is significant for structural engineers designing seismic-resistant composite columns, as it provides experimental evidence for the ductility and energy dissipation capacity of this composite system.

Core Technical Points

Experimental Configuration and Test Parameters

Eight steel tube lightweight aggregate concrete columns were tested under pseudo-static cyclic loading with varying axial load ratios and outer diameters. The test matrix was designed to isolate the effects of key geometric and loading parameters on structural performance. The following table summarizes the principal test parameters:

The pseudo-static testing method, while not capturing inertial effects inherent in dynamic seismic loading, provides reliable data on the hysteretic behavior, stiffness degradation, and ductility characteristics of the specimens under controlled conditions.

Failure Modes

The experimental results consistently showed that the dominant failure mode was local bulging (outward deformation) of the steel tube at the bottom of the column. This failure mode is characteristic of composite columns where the steel tube provides lateral confinement to the core concrete; as the column undergoes cyclic lateral displacement, the steel tube experiences biaxial stress states that eventually lead to local instability.

The progression of damage typically followed this sequence:

1. Elastic loading with no visible damage.
2. Initial cracking of the lightweight aggregate concrete core at the bottom section.
3. Progressive widening of cracks and onset of steel tube yielding.
4. Local bulging of the steel tube at the bottom, indicating loss of confinement effectiveness.
5. Ultimate failure characterized by extensive bulging and possible concrete spalling.

Hysteretic Performance and Ductility

The hysteretic curves of the tested columns were described as full and well-shaped, indicating good energy dissipation capacity. The skeleton curves were complete, demonstrating that the columns maintained load-carrying capacity through large displacements. Most importantly, all specimens achieved ductility coefficients exceeding 4.0, which is a strong indicator of seismic resilience.

The full hysteretic loops suggest that the STLAC columns can effectively dissipate seismic energy through inelastic deformation, which is a critical requirement for seismic design. The absence of severe pinching in the hysteretic curves indicates that the steel tube confinement maintains a degree of lateral restraint even after significant damage, preventing sudden strength loss.

Confinement Effect Analysis

A central finding of this research is the quantification of the confinement force exerted by the steel tube on the core lightweight aggregate concrete. The steel tube acts as a lateral restraint that prevents the concrete from dilating under compressive loads, thereby enhancing both the compressive strength and the ductility of the composite column.

The confinement force was observed to increase progressively with horizontal displacement, reflecting the increasing lateral pressure exerted by the expanding concrete core on the steel tube. This relationship can be understood through the following mechanism:

• As the column is laterally displaced, the concrete core undergoes compressive deformation and tends to dilate laterally.
• The steel tube resists this dilation, creating a confining pressure that increases with the degree of concrete deformation.
• The confinement force continues to increase until the steel tube reaches its local buckling capacity, at which point it can no longer provide additional restraint.
• Beyond this point, the confinement force plateaus or decreases, and the column enters a post-buckling regime with reduced load-carrying capacity.

This progressive confinement behavior is advantageous for seismic performance because it means that the column continues to gain strength and stiffness as it deforms, providing a self-reinforcing mechanism that delays failure.

Engineering Practice Implications

The findings of this research have direct implications for the design of seismic-resistant composite columns. Several practical considerations emerge:

• Design ductility requirements: The demonstrated ductility coefficients above 4.0 suggest that STLAC columns can meet or exceed the ductility demands of moderate to severe seismic zones. Engineers should verify that the design displacement demands are compatible with the experimentally observed ductility capacity.
• Axial load ratio effects: The study varied axial load ratios, and it is important to note that higher axial loads generally reduce the ductility capacity of composite columns. Designers should ensure that the factored axial load ratio remains within a range that preserves adequate ductility, typically below 0.6-0.7 for seismic applications.
• Steel tube diameter selection: The study varied outer diameters, and the results suggest that larger diameters provide greater confinement effectiveness. However, larger diameters also increase the slenderness ratio, which may reduce the elastic buckling capacity. An optimal diameter must balance these competing effects.
• Lightweight aggregate considerations: The use of lightweight aggregate in the concrete core reduces the self-weight of the structure, which is beneficial for seismic design. However, the lower stiffness and strength of lightweight aggregate concrete compared to normal-weight concrete must be accounted for in the design calculations.

Study Insights and Recommendations

This research provides valuable experimental data on the seismic performance of steel tube lightweight aggregate concrete columns. The consistent demonstration of high ductility and full hysteretic curves supports the use of this composite system in seismic regions. However, several limitations and areas for future investigation should be noted:

• The pseudo-static testing method does not capture rate effects and inertial forces that are present in real seismic events. Dynamic testing or shaking table experiments would provide additional validation.
• The study focused on short columns, and the behavior of slender STLAC columns under combined axial and lateral loads may differ significantly due to P-Δ effects and overall buckling.
• The long-term durability of lightweight aggregate concrete under cyclic loading, particularly in corrosive environments, warrants further investigation.
• The interaction between the steel tube and the lightweight aggregate concrete core could be further characterized through detailed strain measurements and finite element modeling.

In conclusion, this paper provides solid experimental evidence that steel tube lightweight aggregate concrete columns exhibit excellent seismic ductility and energy dissipation capacity, making them a promising option for earthquake-resistant construction, provided that appropriate design considerations are applied.