Tension stiffening
Three basic phenomena of concrete behaviour lead to deflections increasing with time: creep, shrinkage and loss of tension stiffening. Creep and shrinkage have been studied extensively and reasonable methods exist to predict their magnitude and development with time. This is not so for tension stiffening, which is related to the interaction of the reinforcement and the concrete and has a significant effect on the deflection of reinforced concrete members. No formulae have been developed to predict how tension stiffening changes over time. In the long term, it reduces to about half its initial value. How rapidly this reduction occurs has not been studied. This makes it difficult to calculate the development of the deflections of slab structures with time. The research programme described in Technical Report 59 was set up to solve this problem.
The methods of predicting deformations given in various design codes are considered in the Report and simple modifications to BS 8110 (now withdrawn and superceded) and Eurocode 2 are proposed to take account of the improved understanding of the behaviour of the concrete tension zone.
The chapters cover current methods of predicting deformations, deformation of tension zones, practical considerations and recommendations, and the application in nonlinear analysis programs for the serviceability limit state. An Appendix covers the development of a theory of the deformation of tension zones.
