Seismic behaviour of multi cavity concrete filled steel tube branching columns

Structural concept and test programme

The paper studies a branching short column used in a super high-rise mega frame, where the upper part is a pentagonal four cavity concrete filled steel tube double limb and the lower part is an octagonal thirteen cavity concrete filled steel tube single limb.

The specimens were tested under vertical axial load and horizontal reciprocating load to evaluate how local cross section reinforcement affects seismic capacity, deformation, and energy dissipation.

Failure mechanism and reinforcement effects

Reinforcement scheme	Main test effect	Practical lesson
Basic construction	Failure occurred near the upper root of the branching section	The fork zone is the critical fatigue and tearing region.
Overall plate thickening	Capacity increased slightly but deformation dropped markedly	Uniform thickening is not always the most efficient strengthening method.
Local plate thickening with angle steel on the far corner	Capacity increased but deformation decreased	Local reinforcement can improve strength while reducing ductility if not detailed carefully.
Inner circular steel tube in the cavity	Capacity increased and deformation improved	Internal restraint is a balanced solution for strength and ductility.

The most important failure mode was tearing of the steel plate far from the neutral axis, often initiated by cracking at welds and developing into tearing of adjoining plate areas.

This indicates that the branch transition is not only a geometric discontinuity but also a weld stress concentration zone, where the combined effect of bending, shear, and local buckling must be controlled.

Welding and fabrication lessons

For thick plate fabricated boxes, the weld toe, corner weld, and heat affected zone should receive special attention because brittle cracking can reduce the apparent ductility of the whole column.

Good practice includes qualified welding procedure specifications, adequate preheat control, low hydrogen consumables, sound root and cap fusion, and nondestructive examination of high stress welds, especially near the branch and angle reinforcements.

The paper also shows that simplified numerical modelling can be acceptable for engineering design if the failure mechanism is well captured, but the model must still be checked against physical test behaviour.

In summary, the study teaches that branch columns should be strengthened by improving ductility and weld integrity together, not by simply adding metal thickness at the cost of deformation capacity.