Time-dependent effects in the analysis and design of slender concrete compression members

Abstract: The report deals with the effect of time-dependent concrete properties in the analysis and design of slender compression members. The main focus is on how to take these effects into account in nonlinear analysis, not on the properties as such in a materials science perspective.Simplified methods for practical design have earlier been calibrated against accurate calculations based on nonlinear analysis. Creep was then taken into account in a simplified way, using an effective creep ratio and an extended concrete stress-strain curve; shrinkage and strength increase were disregarded. The significance of these simplifications is studied here by comparisons with a more rigorous analysis, including a complete creep function plus the effects of shrinkage and strength increase.A good reason for not taking into account strength increase in normal design is that high loads can occur early in the service life. For slender compression members, however, this means that strength at the beginning of the service life is combined with second order effects at the end of it (including the full effect of creep). This is conservative but in principle not logical. Therefore, the effect of strength increase has been studied here. (Whether it should be allowed to take it into account in design is another question, to be considered by code writers.) The reduction of concrete strength due to high sustained stress is studied from different angles. The conclusion is that there is no need to take this into account in design. There are several independent reasons for this, each sufficient on its own: load factors, lower stress levels in case of second order effects, strength increase.The realism of the models for creep, shrinkage and strength increase given in Eurocode 2 (2004), when used in an accurate nonlinear analysis, has been examined by comparisons with tests of slender columns reported in the literature. Good agreement is found in most cases. The comparisons also confirm that high sustained stress has no effect in slender columns.