Square Hollow Sandwich Steel Tube Concrete Compression Torsion Tests

Test Program and Failure Behavior

This study investigates square hollow sandwich concrete filled steel tube members under combined compression and torsion. The experimental program includes twelve specimens with variations in cross section and axial compression ratio. The results are important because tubular members in pipe structures, towers, and frames may experience twisting loads due to eccentric loading, misalignment, or seismic action.

The tests show that a small axial compression ratio can significantly reduce torsional capacity. When the axial compression ratio reaches a relatively high level, the member may enter the plastic stage and lose load carrying capacity rapidly. The failure mode changes with the compression level, indicating that the interaction between axial force and torsion is not a simple linear superposition.

Condition	Observed effect	Fabrication implication
low axial compression ratio	torsional capacity decreases noticeably as ratio increases	design should consider accidental torsion
high axial compression ratio	rapid post peak drop and poor ductility	avoid thin walls and poor confinement
higher outer steel strength	improved torsional capacity	control material grade and weld strength
higher concrete strength	limited effect on capacity	avoid relying only on concrete upgrade
larger slenderness ratio	lower torsional stiffness	check lateral support and buckling restraint

The table highlights that the outer steel tube plays a dominant role in torsional resistance, while concrete strength has a secondary effect. This finding is consistent with thin walled tube mechanics, where shear flow is primarily carried by the steel tube wall.

Torsional Behavior Under Compression

The torsional stiffness is strongly affected by slenderness ratio, although the ultimate capacity is less sensitive. This distinction matters in serviceability design. A slender member may remain strong but become too flexible under torsion, leading to excessive twist or vibration. In pipe structures, such deformation can affect flange alignment, valve operation, and support loads.

The axial compression ratio changes the stress state in the tube wall. Compression increases the principal compressive stress, which reduces the available shear capacity and may promote local buckling. When torsion is combined with compression, the tube wall may yield on one side while buckling on another. The study shows that this interaction can produce a brittle post peak response if the member lacks sufficient confinement and wall stability.

The outer steel strength is more influential than concrete strength for torsional capacity. This has a direct implication for material selection. Increasing concrete grade may improve compressive strength, but it will not substantially improve torsional behavior unless the steel tube confinement is also enhanced. Therefore, wall thickness, yield strength, and cross section geometry should be optimized first.

Quality Control Lessons

From a fabrication viewpoint, the most critical issue is the integrity of the outer steel tube. Torsional resistance depends on a continuous, uniform wall section. If the tube has weld defects, seam cracking, uneven wall thickness, or poor corner geometry, the torsional capacity may be lower than predicted. For square hollow sections, the corners are especially sensitive because they concentrate strain during torsion and local buckling.

The study also reminds engineers that concrete filling quality matters even if concrete strength is not the dominant factor. Voids, honeycombing, or incomplete filling can reduce composite action and allow the steel tube to buckle prematurely. In sandwich members, the inner tube and concrete layer must be assembled with adequate positioning to maintain symmetry. Asymmetry can introduce secondary bending under torsion and reduce ductility.

The test results suggest that combined compression and torsion should be treated as a stability problem, not merely a strength problem. The engineer should verify local buckling, weld accessibility, and deformation compatibility. This paper is useful for pipe structure designers because it shows that torsional performance can be dominated by the steel tube and its fabrication quality, while concrete strength alone is not a reliable solution.