Zero Force Members

The members of truss which support no loading are known as zero force members. These zero force members are used to increase the stability of truss during construction and provide added support if the loading is changed.

Zero force members can generally be found by inspection in two cases.

Case I: If two non collinear members forms a truss joint and no external load or support reaction is applied to joint, then the two members must be zero force members.

For example for the following truss by general inspection, we can say that force in members DC, DE and AF, AB will be zero because both D and A joints are formed by two non collinear members and no external load or support reaction is applied on these joints.

Case II: If three members form a truss joint for which two of the members are collinear, then the third member will be a zero force member provided no external force or support reaction acts along this member.

For example: for the following truss ED and DC are collinear members and no external force or support reaction acts on joint D. So we can say that force in member BD will be zero. Also if we analyse joint B, then we can say that it is also formed by three members as force in member BD   is zero and AB, BC members are collinear so force in member BE will be zero.

Method of Sections

When we have to find force in only a few members of truss, we can analyze the truss using method of sections. It is based on principle that if truss is in equilibrium, then any part of the truss is also in equilibrium.

The steps involved in analyzing a truss using the method of sections are

• The truss is split into two parts by passing an imaginary section.
• The imaginary section has to be such that it does not cut more than three members in which the forces are unknown.
• The conditions of equilibrium are applied for the one part of the truss, and the unknown force in the member is determined.
• Nature of the force in any member is determined similar to method of joints.