When a hinge point without external load is connected by three rods, if the two rods are on the same straight line, the force on the third rod N3 is zero, and this rod is also zero. When a hinge point is connected by three rods, if two of them are on the same straight line and the load is on the same straight line with the above two rods, the third rod is zero.

When the hinge point of the statically determinate truss is on the same vertical line with the bearing and its stressed members, when the vertical force acts in the same direction, the middle members of the vertical members on both sides are not stressed. The zero bar doesn't work under this load condition, but it may bear the load when the load changes, which means that the zero bar is actually ready for work and waiting for the opportunity.

The premise of obtaining zero bar is that the truss is rigid. However, in the actual structure, truss members are all elastic, and the truss can undergo slight elastic deformation. At this time, the zero bar will also bear a certain force, but it is not a real zero bar. Removing the zero column means removing the constraint.

Then the original statically determinate truss becomes insufficient constraint, and the degree of freedom is no longer zero, and it becomes a movable mechanism. Such a small disturbance or load change will make the truss unstable.

The zero bar judgment method mainly includes:

1, l-shaped node. When the nodes of two non-collinear bars are not acted by external force, both bars are zero. If a rod is collinear with the external force, the internal force and external force of the rod are equal, and the one that is not collinear with the external force is zero. ?

2. T-shaped node. If there is no external force to connect the nodes of three bars, if two of them are in a straight line, the non-collinear bars must be zero bars, while the two collinear bars have the same internal force and the same nature (both are tension or pressure). ?

3. "X" shaped node. If two rods are collinear, the internal forces of the two rods on the same line are equal and the properties are the same. ?

4. "K" shaped node. Four steel bars intersect to form a symmetrical K-shaped joint. When there is no load, the axial forces of two diagonal bars are equal under different symbols. Under the symmetrical load, if there is no load at the K-shaped node on the symmetrical axis, the two diagonal bars on the node are zero bars. ?