Truss Structures

Abstract

Truss structures constitute a special class of structures in which individual straight members are connected at joints. The members are assumed to be connected to the joints in a manner that permit rotation, and thereby it follows from equilibrium considerations, that the individual structural members act as bars, i.e. structural members that can only carry an axial force in either tension or compression. In a statically determinate truss all the bar forces can be determined by the equilibrium equations, applied to the bars and joints of the truss. There are several strategies for carrying out the corresponding calculations, and three of these will be described in this chapter. The first and conceptually simplest method consists in considering each joint as an isolated body, for which the equilibrium equations must be satisfied. Alternatively, the bar forces can be calculated by using sections to separate larger parts of the structure and then applying suitable equilibrium equations for these larger parts. It is characteristic of the two classic methods of joints and of sections, that they are arranged to determine the bar forces sequentially, and thus are convenient for calculation of the bar forces or a subset of these by hand. Alternatively, a general systematic method can be developed for elastic trusses, irrespective of whether they are statically determinate or indeterminate. The method consists of setting up the equilibrium equations of all joints in a systematic way, using the elastic property of the bars. This method is a special case of the Finite Element Method and is here illustrated by a simple MATLAB program MiniTruss.