The bowing of a strut due to the axial force exceeding the designed load. The safe load depends upon the ratio of the length of the member (L) to the radius of gyration (k) of its section. The value of k is a measure of the hollowness of the section, a tube or channel having a larger k than a solid section of the same area. The effective length L can be reduced by intermediate supports attached to the strut. The value of L/k is large for long slender struts and small for short stubby ones. Instability arises from large values of L/k. When the load increases beyond the critical value, the strut suddenly bows, i.e. adopts a curved shape.