Considerations in the Design of Large-Size Welded Tubular Truss Joints

The rational design of welded truss joints built from tubes has been complicated by the radial flexibility of the chord tube wall. For certain types of joints this flexibility might be efficient, under static axial loads, since it provides, under increasing loads, a load-distributing action between branch members and chord tube. However, for most joints the flexibility is a source of severe stress concentrations. In connections subjected to dynamic loads, as for example offshore well drilling structures, stress concentrations can be most hazardous. Although even an approximate analysis of tubular joints of different configuration is extremely difficult, several designs are presently used and aimed at reducing the influence of the wall flexibility. This objective can be achieved by using thicker-walled tubes, or by inter-welding incoming branch members. In the latter instance the force acting normal to the chord tube can be reduced considerably. A second group of joints incorporates gusset plates or stiffening rings, which are intended to either contain the load transfer primarily within the gusset plate or to provide an element—the stiffening ring—to stiffen the chord wall and to distribute the forces over a larger part of the chord member wall.