Seismic Performance Comparison of Rectangular Steel Tube Concrete Frames with and without Infill Walls

Overview of the Study

This paper by Wen Yang, Guan Lipai, and Cai Meifeng from Inner Mongolia University of Science and Technology and University of Science and Technology Beijing, published in Journal of Shenyang Jianzhu University (2014, Vol. 30, No. 1, pp. 34-40), presents low-cycle reversed loading tests on rectangular steel tube concrete (RSTC) frame specimens with and without infill walls. The research was supported by the National Natural Science Foundation (Grant 51068021) and Inner Mongolia Natural Science Foundation (Grant 2013MS0715). The study uses the column steel ratio as the comparative analysis parameter.

Test Methodology and Loading Protocol

The specimens were subjected to low-cycle reversed horizontal loading combined with constant vertical loading, simulating seismic loading conditions. The low-cycle loading protocol typically involves displacement-controlled cycling at increasing amplitude levels, with each level repeated twice in opposite directions. This protocol allows measurement of the hysteretic behavior, energy dissipation capacity, and ductility of the frame systems.

The column steel ratio, defined as the ratio of steel cross-sectional area to total column cross-sectional area, serves as the primary variable. This parameter directly influences the composite action between steel tube and concrete, affecting both strength and deformation capacity.

Key Performance Results

The study presents the following comparative results:

The infill wall frames exhibit superior load-bearing capacity but reduced ductility compared to bare frames. Both systems satisfy the seismic ductility requirement of a displacement ductility coefficient greater than 4.0, but the infill wall frames operate closer to this threshold.

Engineering Implications for Steel Tube Fabrication and Welding

The presence of infill walls introduces additional lateral restraint to the frame members, which affects the buckling behavior of the rectangular steel tubes. Rectangular steel tubes are susceptible to local buckling of the flat plates between corner elements, particularly under cyclic loading. The infill wall restraint can delay local buckling but also concentrates stresses at the wall-to-tube interface.

For welded connections in RSTC frames, the increased lateral forces transmitted through infill walls impose additional demands on the beam-column joints. The welds at these joints must accommodate higher shear forces and moment demands. From a welding metallurgy perspective, the HAZ of these critical welds should be designed to maintain adequate toughness at the elevated stress levels expected with infill walls.

The study's conclusion that infill walls should be fully utilized in engineering practice has important implications for seismic retrofitting of existing RSTC structures. Engineers should consider whether existing infill walls contribute beneficially to seismic performance or whether they create unexpected load paths that could lead to brittle failure. The welding quality of connections between infill walls and steel tube members is therefore a critical quality control item.

Reflections on Seismic Design Philosophy

The study demonstrates that the conventional practice of ignoring infill wall effects in seismic design may be overly conservative in terms of strength but potentially non-conservative in terms of ductility. The reduced ductility of infill wall frames (4.07-4.27 versus 5.56-6.80) brings the system closer to the minimum acceptable limit. Engineers should therefore consider the infill wall contribution in both design and assessment, and ensure that the weld connections between infill walls and steel tubes are designed and inspected to maintain their load-transmitting capacity throughout the seismic event.

In my experience with steel tube structure fabrication, the interface between infill walls and steel tubes often receives inadequate attention during welding and connection design. The assumption that infill walls are non-structural elements can lead to simplified connection details that fail under seismic loading. I strongly recommend that all infill wall connections in RSTC frames be designed with full seismic detailing, including continuous welding or high-strength bolted connections with proper shear transfer mechanisms.