Mechanical Properties of Square Steel Tube Concrete Frame with Cross-Stiffened Thin Steel Plate Shear Wall

Literature Overview

This paper by Wang Xiantie, Bai Lianping, Wang Liankun, Ma Yousuf, and Liu Tianlong, published in "Earthquake Engineering and Engineering Dynamics" (2013, Vol. 33, No. 2, pp. 103-109), presents a numerical analysis study on the mechanical behavior of square steel tube concrete (SRC) frames combined with cross-stiffened thin steel plate shear walls. Funded by the National Natural Science Foundation of China (Grants 51108369, 51208385), the Ministry of Education Postdoctoral Fund for New Teachers (20116120120008), and the Shaanxi Provincial Department of Education Special Research Fund Project (11JKY0942), this research was conducted at Xi'an University of Architecture and Technology and Wuyi University. The study addresses an important structural system for high-rise buildings and long-span structures where both gravity load resistance and lateral load resistance are required. The use of SRC columns combined with thin steel plate shear walls offers an efficient solution that leverages the high strength of the composite columns and the high shear capacity of the steel plate walls.

Numerical Analysis Methodology

The study employed the general-purpose nonlinear finite element software ABAQUS 6.10 to analyze both cross-stiffened thin steel plate shear wall systems and unstiffened thin steel plate shear wall systems. The numerical models were validated against experimental data to ensure reliability. The analysis focused on several key aspects of structural behavior.

Shear Force Distribution and Load Path Analysis

One of the most practically significant findings of this paper is the detailed analysis of shear force distribution between the steel plate shear wall and the SRC frame. The results show that during the initial loading stage, when the story drift angle is less than 0.2 percent, the steel plate shear wall carries the majority of the shear force. As the lateral displacement increases, the proportion of shear carried by the steel plate wall gradually decreases while the frame proportion increases. When the story drift angle reaches 1 percent, the shear force distribution ratio between the frame and the steel plate wall stabilizes and remains constant thereafter.

This progressive shift in shear distribution has important implications for the design of the SRC columns. The columns experience increasing axial load from gravity loads and additional shear from the frame action as the lateral displacement increases. The interaction between the axial compression from gravity loads and the shear from lateral loads must be carefully considered in the column design to prevent premature failure.

Critical Design Concern: Concrete-Steel Separation

The paper identifies a critical design concern that deserves particular attention: due to the tension field action in the thin steel plate shear wall, the steel tube wall of the SRC columns is prone to separation from the concrete core. This separation would compromise the composite action between the steel tube and the concrete, reducing the effective load-bearing capacity and stiffness of the columns. The tension field action develops when the steel plate wall yields in shear and forms a diagonal tension band, which transfers forces to the boundary elements (columns) through the connection details. These forces can be large enough to cause the steel tube to pull away from the concrete core, particularly at the column ends where the moment and shear are highest.

Welding and Fabrication Considerations

The fabrication and welding quality of this structural system is critical for its seismic performance. The following considerations are important:

1. The SRC columns are fabricated from square steel tubes, typically produced by welding flat plates together. The longitudinal and transverse welds must be inspected using ultrasonic testing to ensure full penetration and absence of defects. Inadequate weld quality could lead to premature tube splitting under the high stresses developed during seismic loading.
2. The connection details between the steel plate shear wall and the SRC columns are critical load transfer points. These connections typically involve bolted or welded end plates and should be designed to accommodate the large inelastic deformations expected during seismic events. Welded connections should be designed with adequate ductility, and the weld details should avoid stress concentrations that could initiate fracture.
3. The cross stiffeners on the steel plate shear wall should be welded to the plate with full-penetration fillet welds or groove welds to ensure effective load transfer from the stiffener to the plate. The weld quality should be verified through ultrasonic testing.
4. The concrete filling of the SRC columns must be complete and uniform to ensure effective composite action. Voids or honeycombing in the concrete would reduce the confinement effectiveness and increase the risk of concrete-steel separation.

Engineering Practice Recommendations

1. The rib-to-plate stiffness ratio of 30 identified as effective for cross stiffeners should be used as a starting point for the design of stiffened steel plate shear walls, with adjustments based on specific project requirements.
2. The shear force distribution ratio between the frame and the steel plate wall should be incorporated into the design of both the columns and the wall-to-column connections. The columns should be designed for the maximum combined effect of axial load and shear that occurs at large drift angles.
3. Measures to prevent concrete-steel separation in SRC columns should be incorporated into the design, such as the use of headed studs or shear connectors between the steel tube and the concrete core, or the use of high-strength bonding agents at the interface.
4. The hysteretic performance of the system, which was found to be full and stable, supports its suitability for seismic applications, provided that the connection details are designed with adequate ductility and the fabrication quality is rigorously controlled.

Study Insights and Reflections

This paper provides valuable insights into the behavior of a hybrid structural system that combines the advantages of SRC columns and thin steel plate shear walls. The identification of the concrete-steel separation phenomenon as a critical design concern is particularly important, as it highlights a failure mode that may not be captured by conventional design methods. From a welding and materials engineering perspective, the seismic performance of this system is fundamentally dependent on the ductility and integrity of all welded connections. The steel tube fabrication welds, the stiffener-to-plate welds, and the wall-to-column connection welds must all be designed and executed to withstand the large inelastic deformations expected during seismic events. The progressive shear redistribution from the wall to the frame, as the structure enters the inelastic range, means that the columns are subjected to increasing demands as the lateral displacement grows, which must be explicitly accounted for in the design. The full and stable hysteretic loops observed in the numerical analysis are encouraging for seismic design, but it is essential to verify these predictions through cyclic testing of full-scale specimens, particularly focusing on the connection details and the concrete-steel interface behavior.