An experimental and finite element (FE) research program was conducted to investigate the â fit-for-purposeâ design of welds in hollow structural section (HSS) connections using weld effective lengths, and the applicability of the North American directional strength-increase factor for fillet welds to HSS. Weld effective lengths for circular hollow section (CHS) connections are studied for the first time.

Experimental testing of a 10-m span, simply-supported, rectangular hollow section (RHS) Warren truss, with fillet- and groove-welded overlapped K-connections, was performed by applying a single quasi-static point load to truss panel points to cause sequential rupture of nine test welds to overlapping branches. The weld in-situ strength, effect of key connection parameters, and safety (reliability) index of AISC formulae for weld design in RHS overlapped K-connections were determined. Modifications to the AISC formulae and a simplified method for modelling overlap-jointed RHS trusses under service load conditions are proposed.

The North American directional strength-increase factor for fillet welds to HSS (covering both RHS and CHS) was then studied through 33 experimental tests and 73 non-linear FE analyses of weld-critical HSS-to-rigid plate connections. The safety index was determined for prominent North American design codes, and a recommendation to prohibit the use of the directional strength-increase factor for the design of fillet welds to all HSS was made. More accurate design formulae for fillet welds to HSS are proposed.

Twelve large-scale experiments and 256 non-linear FE tests were then conducted to determine weld effective lengths for CHS-to-CHS X- (or Cross-) connections, which are shown to decrease as branch-to-chord width ratio, chord wall slenderness, and branch-to-chord thickness ratio increase. North American fillet weld design formulae were found to already provide adequate reliability provided that the directional strength-increase factor is not used. A new fillet weld design approach for CHS X-connections, using weld effective lengths, is proposed.