A roof truss is a coplanar system of structural members joined together at their ends to form a stable framework. If small changes in the lengths of the members due to loads are neglected, the relative positions of the joints cannot change.
Characteristics of Trusses
Three bars pinned together to form a triangle represents the simplest type of roof truss .
The top members are called the upper chord; the bottom members, the lower chord; and the verticals and diagonals web members.
The purpose of roof trusses is to act like big beams, to support the roof covering over long spans. They not only have to carry their own weight and the weight of the roofing and roof beams, or purlins, but cranes, wind loads, snow loads, suspended ceilings, and equipment, and a live load to take care of construction, maintenance, and repair loading. These loads are applied at the intersection of the members, or panel points, so that the members will be subjected principally to axial stresses—tension or compression.
Bow’s Notation
For simple designation of loads and stresses, capital letters are placed in the spaces between truss members and between forces. Each member and load is then designated by the letters on opposite sides of it. For example, the upper chord members areAF,BH,CJ, andDL. The loads areAB,BC, andCD, and the reactions areEA andDE. Stresses in the members generally are designated by the same letters but in lowercase.
Method of Joints
A useful method for determining the stresses in roof truss members is to select sections
that isolate the joints one at a time and then apply the laws of equilibrium to each.
Considering the stresses in the cut members as external forces, the sum of the horizontal components of the forces acting at a joint must be zero, and so must be the sum of the vertical components. Since the lines of action of all the forces are known, we can therefore compute two unknown magnitudes at each joint by this method. The procedure is to start at a joint that has only two unknowns (generally at the support) and then, as stresses in members are determined, analyze successive joints.