Zero rod
Zero members are members in the framework that experience neither tensile nor compressive forces under a given load according to the theory of the first order, i.e. they are stress-free according to the framework theory. However, they can still have secondary voltages. A member that is a null member in one load case can definitely be a compression or tension member in another load case.
There are several rules for recognizing null bars in a truss.
The three best known follow from the nodal equilibrium: (However, further null members can be contained in the system.)
- If two bars are connected to an unloaded node that are not in the same direction, they are zero bars.
- If two bars and an additional force are connected to a loaded node, and the force (the external load) has the same direction as one of the two bars, the other is a zero bar.
- If three bars are connected to an unloaded node and two of them are in the same direction, the third is a zero bar.