Stiffness of reinforced concrete slabs subjected to torsion

The study of torsion in reinforced concrete members began in the early twentieth century but it has become more widespread in the past 60 years. Even today, this topic has not been deeply researched with respect to slabs. This work reports a study on the behavior of reinforced concrete slabs subjected to torsion, with particular attention to the stiffness in state I and state II (uncracked and cracked phase) and to the relation T-theta (torsional moment-twist). It is known that torsional forces lead to a much larger loss of stiffness in state II, due to the strong cracking, than the loss of flexural stiffness. When the member is predominantly subjected to bending moments the K (I)/K (II) a parts per thousand 3-5 is usually admitted, but when torsion has a preponderant role this relationship is not valid. This study may alert the scientific community to a seemingly unknown relation: torsional stiffness of reinforced concrete slabs, in the cracked phase, is about 1/17-1/15 of the stiffness in the elastic phase. For the analysis of slab deformation, particularly under service conditions, accurate knowledge of the slab stiffness in state II is important. The knowledge of T-theta relation is also important to perform a numerical nonlinear analysis. An experimental program was designed and carried out in which 9 slabs were tested until failure. To submit the slabs predominantly to torsional moments, vertical movement at 3 corners was restricted and the action was applied to the fourth corner. Based on the results obtained, the following relations were defined: load-displacement (P-d), torsional moment-twist (T-theta) and K (T,I)/K (T,II).