Design versus Assessment of Concrete Structures Using Stress Fields and Strut-and-Tie Models

Stress fields and strut-and-tie models are widely used for design and assessment of structural concrete members. Although they are often used in the same manner for both purposes, developing suitable stress fields and strut-and-tie models for the design of a new structure or for assessment of the strength of an existing one should not necessarily be performed following the same approach. For design, simple load-carrying models in equilibrium with the external actions can be considered. From the various possibilities, those leading to better behavior at serviceability limit state and to simple reinforcement layouts should be selected (or a combination of them). For the assessment of existing structures, however, avoiding unnecessary strengthening (or minimizing it) should be the objective. Thus, simple stress fields or strut-and-tie models are to be iteratively refined whenever the calculated strength of the member is insufficient with respect to the design actions. This can be done by accounting for kinematic considerations to calculate the higher possible strength of the member accounting for its actual geometry and available reinforcement (allowing to calculate the exact solution according to limit analysis). In this paper, the differences between the two approaches for design and assessment are clarified and explained on the basis of some examples. A number of strategies are comprehensibly presented to obtain suitable stress fields and strut-and-tie models in both cases. The results of exact solutions according to limit analysis (developed using elastic-plastic stress fields) are finally compared to 150 tests of the literature showing the consistency and generality of the presented approaches.