Zero-Force Members

Learning Goal:

To be able to identify zero-force members and understand their importance in truss analysis.

Zero-force members occur frequently in trusses and are typically used either to stabilize a truss during construction or to carry transient loads. Truss analysis can be greatly simplified by identifying and, subsequently, eliminating zero-force members.

Zero-force members

Zero-force members are truss members that carry no load under a given load configuration. Because they carry no load and their weight is negligible, compared with the loads carried by the truss, zero-force members experience no force. These members can be identified by drawing a free-body diagram of each joint and applying the conditions

? F x =0 ? F y =0

where x and y refer to the coordinate system applied at each joint.

Application of the equilibrium conditions leads to a simple method for identifying the zero-force members in planar trusses:

If the only members of an unloaded joint are two noncollinear members, both members are zero-force members.

If an unloaded joint has two collinear members and one noncollinear member only, the noncollinear member is a zero-force member.

Part A - Identifying zero-force members

Consider the truss shown below. Identify the zero-force members.

Check all that apply.

(1)	A B
(2)	D E
(3)	C E
(4)	B E
(5)	A E
(6)	B C
(7)	C D

D PRE A B C