Using a constant value for the ultimate concrete strain regardless of the axial load level does not accurately represent the state of strains in concrete sections. Although this assumption does not have a major influence on the section capacity, it violates basic mechanics. This becomes clear for a section supporting a concentric axial load. The ultimate resistance for such a section develops when the concrete stress reaches its maximum value, which corresponds to a concrete strain value of about 0.002. Proposed values for the ultimate concrete strain and the stress-block parameters (alpha1 and beta1) can be calculated using the Stress-Block Parameters Tool given below for concrete sections reinforced with steel bars or superelastic shape memory alloy bars. These tools have been developed based on the assumptions mentioned in the paper below and cannot be used without through understanding of implications on other design parameters.
For more information, please refer to:
Elbahy Y.I., Youssef M.A., Nehdi M., 2009, “Stress Block Parameters for Concrete Flexural Members Reinforced with Shape Memory Alloys”, Materials and Structures, 42(10): 1335-1351.
The unique properties of superelastic Shape Memory Alloys (SMAs) have motivated researchers to explore their use as reinforcing bars. The capacity of a steel Reinforced Concrete (RC) section is calculated by assuming a maximum concrete strain εc-max and utilizing stress block parameters, α1 and β1, to simplify the nonlinear stress-strain curve of concrete. Recommended values for εcmax, α1, and β1 are given in different design standards. However, these values are expected to be different for SMA RC sections. In this paper, the suitability of using sectional analysis to evaluate the monotonic moment-curvature relationship for SMA RC sections is confirmed. A parametric study is then conducted to identify the characteristics of this relationship for steel and SMA RC sections. Specific mechanical properties are assumed for both steel and SMA. Results were used to judge on εc-max, α1, and β1 values given in the Canadian standard and to propose equations to estimate their recommended values for steel and SMA RC sections.
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