Experimental study on K-joints of concrete-filled steel tubular truss structures

• Failure modes of CFST K-joints were studied and compared with those of CHS K-joints.
• K-joint specimens with size on the same scale as that of joints of real bridges were tested.
• Punching shear failure was shown to be a typical failure mode of CFST K-joints.
• Design resistance of CFST K-joints can be predicted by the EC-3 formula on CHS K-joints.
• The effect of studs welded to the chord interior surface of CFST K-joints was investigated.

The failure modes of K-joints of Concrete Filled Steel Tubular (CFST) truss structures were investigated through laboratory tests. CFST K-joint specimens consisted of a tubular chord filled with concrete and two braces inclined at the same angle with respect to the chord. Their size was chosen of the same scale as that of joints of real tubular structures currently used in bridges. The results were compared with those obtained by testing K-joint specimens of Circular Hollow Section (CHS) truss structures with the same size. Contrary to CHS K-joints, where a chord face failure mode occurred, in CFST K-joints no failure mode with inward deformation of the chord section can occur. Hence, punching shear failure was shown to be the typical failure mode of CFST K-joints, if no brace failure mode occurred before. Resistance of CFST K-joints to punching shear was then compared with the resistance values calculated through the Eurocode 3 and the AWS code formulae on CHS K-joints. The values provided by the former formula were quite close to (although always lower than) the actual resistance to punching shear of CFST K-joints. Finally, the favourable effect on joint resistance due to additional studs welded to the interior surface of the chord tube of CFST K-joint was also investigated.