On primary and secondary stresses in triangulated trusses

The importance of primary and secondary stresses on the strength of statically loaded triangulated trusses having rigid joints is reviewed. Support is given to the contention that accurate modelling of material properties and member end conditions is justified both for predicting overall failure loads and for assessing the sequencing through which member stiffness deteriorates to a consequent mode of failure. In this regard, the results of two large scale tests on welded steel Pratt trusses are presented. A comparison that is made between measured responses up to the ultimate load with that predicted by an elasto-plastic model that accounts for moment-axial force interaction shows excellent agreement. A parametric study on the influence on local and overall strength of a change in diagonal to chord angle and joint eccentricity ratio reveals wide ranging degrees of sensitivity. It was found, for example, that the truss ultimate load exhibited nearly flat response at small and very large values of eccentricity (both positive and negative) whereas significant changes in overall strength emerged at intermediate values. The failure load predicted by the simple pin-ended truss model is shown to be conservative for small eccentricities but would provide an unsafe estimate of resistance if primary moments caused by joint eccentricities were not taken into account.