Control section of prestressed members without shear reinforcement
Improvements to the next generation of Eurocode 2 around intermediate supports
J.R. Boer (TU Delft - Civil Engineering & Geosciences)
Y. Yang – Mentor (TU Delft - Concrete Structures)
M.S. Ibrahim – Graduation committee member (TU Delft - Concrete Structures)
J.G. Rots – Graduation committee member (TU Delft - Applied Mechanics)
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Abstract
Currently a new Eurocode is in development where the shear capacity will be based on the Critical Shear Crack Theory (CSCT), rather than a purely empirical model. The newly introduced formulae provide good results overall and include the effects of bending moments on the shear capacity. However, the formulae are known to be too conservative for prestressed continuous beams with low amounts of shear reinforcement and severely underestimate the shear capacity. If these formulae are applied, many existing structures would therefore no longer meet the code requirements. New structures with prestressed continuous elements would also require more material and it may become difficult to design efficient concrete members that meet the new code requirements.
To prevent substantial costs, emissions and time investments, it was questioned if the design capacity of prestressed beams near intermediate supports could be increased by changing the location of the control section from 1d away from supports to the critical cross section. The location of the control cross section greatly influences the shear resistance according to the CSCT calculation. However, it is unclear how the critical cross section can be determined accurately.
In this thesis the location of the critical cross section near intermediate supports was investigated for prestressed continuous beams with less than the minimum required shear reinforcement. A small number of models and experiments from literature were compared. Additionally, multiple Finite Element Analyses have been performed with a variety of settings, assuming different shear behaviour. A plasticity approach was also investigated, where the critical cross section is found at the location where the cracking load equals the ultimate load of a crack.
This thesis found that the reinforcement ratios, prestressing stress, shear span and effective depth (as well as the concrete strength in lesser amount) influence the location of the critical cross section. The experiments and models found in literature, as well as the results found using the plasticity approach, indicate that the critical cross section for prestressed beams may be moved from 1d to 1.5d away from intermediate supports. However, due to the limitations and assumptions of the models it would not be safe to apply this change without further validation. It is therefore recommended that experiments are done on prestressed continuous beams with low amounts of shear reinforcement before any changes are made to the location of the control section.