Truss is a geometric structure composed of triangular frames composed of straight bars. Connections between components are called nodes. Trusses can be divided into plane trusses and space trusses according to the component axis and external force distribution.
Spatial structures such as roof trusses or bridges are composed of a series of parallel plane trusses. If it mainly bears plane load, it can be simplified as a plane truss to calculate.
The axis of truss member is not on the same plane as the external force. In engineering, some spatial trusses cannot be simplified as plane trusses, such as grid structures. Tower, crane frame, etc. The joints of space truss are smooth spherical hinge joints, and the axes of members all pass through the center of spherical hinge at the connection point and can rotate around any axis of the center of spherical hinge.
Each node has three degrees of freedom in space. The relationship between the number of nodes and the number of members is w = 3j-n, w >；; 0 is a geometrically variable truss, and W=0 is a geometrically invariant spatial truss with no redundant constraints. Space truss, like plane truss, can use partial cutting method and node method to calculate the internal forces of all members in the truss.
The partial cutting rule is to find the internal force of each rod by using six equilibrium conditions of any spatial force system. The node method is to intercept the nodes as isolators, and use the three equilibrium conditions of the spatial force system of each node to find out the internal force of each rod.