Behavior of Tubular Gusset-Plate Joints

ABSTRACT The paper evaluates the stress distribution of in-plane gusset plate joints under both static and cyclic fatigue-load conditions. The potential danger of the in-plane stiffness of the gusset plate in restraining the Poisson effect in the web member sections is illustrated. Under fatigue loading, tapered gusset plate joints in which diametrically opposite regions of the web member section are restrained fail at a lower number of load cycles than non-tapered gusset plate joints. Fatigue failure has been defined as the number of alternating load cycles at which a fatigue crack propagates through the thickness of the web member wall. It is proposed to omit a penetration of the web member walls and to use either interior or exterior gusset plates. The improvement of both the stress condition and inherent fatigue life through thick-walled web-member sections near the end of the gusset plate is emphasized. The restraint of diametrically opposite regions of the same web member should be prevented. INTRODUCTION The rational design of welded truss joints built from circular tubes has been complicated by the radial flexibility of the chord wall or the restraining action of stiffening elements. Both effects cause severe stress concentrations. Under increasing static loads certain joints may nevertheless perform satisfactory because of a redistribution of the load transfer after initial yielding occurs. However, in connections subjected to repeatedly fluctuating loads serious stress concentrations invariably cause a premature fatigue failure. In order to improve the life expectancy of these joints, the designer should aim at reducing these stress concentrations. Although modern finite-element computer programs are presently available to assess the design, a basic understanding of the structural behavior of the joint as influenced by the flexibility and restraining effects of the several components is a primary requirement. To reduce stress concentrations thicker walled or ring-stiffened column members, or inter welded branch members have been introduced. Also, gusset plates in the plane of the truss have been incorporated. The latter solution permits a substantial portion of the web member loads to be transferred through the gusset plate and reduces the force transfer through the potentially critical chord member wall. The structural danger in that case is no longer related to the radial flexibility of the chord member, but instead, to the concentrated force flow between the branch-member wall and gusset plate, and the excessive in-plane rigidity of the gusset. This latter effect causes a radial restraint of the branch-member walls and introduces secondary stress concentrations in the walls near the edge of the penetrating gusset plate. While the above direct force transfer and resulting primary stress concentrations are inherent with gusset-plate joints, the secondary stress concentrations can effectively be reduced by design modifications which reduce the in-plane stiffness of the gusset plates.