Chapter 6 - Indeterminate Truss

Abstract

An indeterminate truss is obtained by adding extra bars to a determinate truss, by increasing the number of support restraints, or both. The extra bars and restraints are referred to as redundant members and redundant support conditions, respectively. An indeterminate truss remains stable even when some or all of the redundant bars or restraints are removed. In contrast, a determinate truss will collapse when a single bar or a support restraint is eliminated. An indeterminate truss is preferred because of its stability and the increased strength that emerge from the redundant members. Temperature and support settling induce stress in an indeterminate truss, but not in a determinate truss. The analysis of an indeterminate truss requires three sets of equations of determinate analysis, along with an additional set of constraints called the compatibility conditions: equilibrium equations, deformation displacement relations, force deformation relations, and compatibility conditions. Any indeterminate truss can be analyzed through an application of the four types of equations. The types of response variables are not increased between determinate and indeterminate analysis. They remain the same—namely, bar forces, reactions, nodal displacements, bar deformations, stress, and strain. Their individual numbers can increase; for example, the number of bar forces increases between determinate and indeterminate trusses, and this is specified through the term referred to as “the degree of indeterminacy.”