Experimental Study on Steel Tube Concrete Short Column Seismic Isolation Bearings

Literature Overview

This paper by Wang Wei, Wang Huanding, Geng Shuwei, and Li Xuan from Harbin Architectural University, published in Journal of Vibration Engineering in 1999 (Vol. 12, No. 2, pp. 292-296), presents a pioneering experimental investigation into steel tube concrete (SRC) short columns used as seismic isolation bearings. The research was supported by the Heilongjiang Provincial Natural Science Foundation. The study combines pseudo-static testing of individual SRC short columns with full-scale shake table testing of a four-story brick-masonry building model equipped with SRC isolation bearings.

Experimental Program Design

Pseudo-Static Testing of SRC Short Columns

The pseudo-static (cyclic loading) tests were conducted to establish the restoring force model of SRC short columns used as isolation bearings. Key experimental parameters included:

Shake Table Testing

A four-story brick-masonry building model was tested on an earthquake simulation shake table with SRC short columns installed as isolation bearings. The shake table test simulated various earthquake intensity levels to evaluate the overall seismic isolation performance.

Key Experimental Findings

Restoring Force Model Characteristics

The SRC short column isolation bearing exhibits a distinctive restoring force model with the following characteristics:

• Full hysteresis loops: The curves are "fat" or "full" in shape, indicating substantial energy dissipation capacity.
• Hammer-like shape: The restoring force curve resembles a hammer profile, which is favorable for isolation performance.
• No descending branch: Unlike conventional structural elements, the SRC short column does not exhibit strength degradation (no descending branch), indicating excellent post-peak ductility.
• Hysteresis comparable to steel: The energy dissipation characteristics are similar to those of structural steel, confirming the contribution of the steel tube confinement to ductile behavior.

Seismic Isolation Performance

The shake table test results demonstrate that:

1. The SRC short column isolation system effectively reduces seismic forces transmitted to the superstructure.
2. The isolation bearing maintains stable performance throughout multiple loading cycles without significant degradation.
3. The construction is simple and practical, facilitating field implementation.

Technical Analysis from a Materials and Welding Perspective

Steel Tube-Concrete Interaction

The SRC short column isolation bearing relies on the composite action between the steel tube and the confined concrete core. From a materials science perspective:

• Confinement effect: The steel tube provides lateral confinement to the concrete, preventing premature concrete crushing and enabling the concrete to develop compressive ductility.
• Steel tube behavior: Under cyclic loading, the steel tube experiences alternating tension and compression, requiring adequate ductility and fatigue resistance.
• Interface behavior: The bond between steel tube and concrete is critical for load transfer and composite action.

Welding and Fabrication Considerations

For SRC isolation bearings used in seismic applications, the following fabrication quality aspects are paramount:

• Longitudinal weld quality: The weld connecting the steel tube halves (if applicable) must achieve full fusion and meet seismic-grade requirements per relevant standards.
• Seamless preference: Where possible, seamless steel tubes (per ASTM A53 or GB/T 8163) are preferred to eliminate weld-related vulnerabilities in seismic isolation components.
• End preparation: The end faces of the steel tube must be machined flat and perpendicular to ensure proper bearing contact.
• Concrete placement: The concrete core must be placed with adequate compaction to avoid voids that would compromise the composite action.

Standards and Code Compliance

Study Insights and Reflections

This 1999 study represents an important contribution to the development of seismic isolation technology using steel tube concrete components. The finding that the SRC short column exhibits no descending branch in its restoring force model is particularly significant because it indicates that the bearing can sustain large displacements without strength loss, which is essential for effective seismic isolation.

The hammer-shaped hysteresis loop provides a favorable force-displacement relationship for isolation: the initial stiffness is relatively high (limiting displacements under minor earthquakes), while the energy dissipation capacity is substantial during major seismic events. This dual characteristic makes SRC short columns particularly suitable for retrofitting existing structures where space constraints limit the size of isolation bearings.

The practical advantages highlighted in this study—simple construction, ease of operation, and scalability—make this approach particularly attractive for developing regions and for the seismic retrofitting of existing building stock.