Followings are the basic assumption of Space Truss Analogy-
- The concept of the space truss analogy is based on the assumption that the torsional capacity of rectangular reinforcement concrete section can be derived from the reinforcement and the concrete surrounding the steel only.
- In this case a thin walled section is assumed to act as a space truss as shown in following figure.
- The inclined spric spiral concrete strips between cracks resists the compressive force and longitudinal bars at corners and stirrups resist the tensile forces produced by the torsional moment.
- Before cracking, the concrete resists the torsional stresses and the steel is virtually unstressed.
- After cracking, the beam is no longer elastic and a sudden change of angle of twist occurs.