Please provide the values or equations for (corresponding references if any available)
Thank you
A good equation for ultimate strain of confined concrete is that given by Mander et al. (1988).
eps_cu = 0.004 + 1.4ps x fyh x eps_sm / f'cc
where ps is the volumetric ratio of transverse reinforcement in confined concrete, fyh the yield strength of confining reinforcement, eps_sm is the fracture strain of confining reinforcement, f’cc is the compressive strength of confined concrete
The AASHTO Guide Specification for LRFD Seismic Bridge Design specifies a spalling strain of unconfined concrete equal to 0.005. The equation referenced above by Rafik is also recommended by Priestley, et al. in IUSS Press, 2007, "Displacement Based Seismic Design of Structures". So, while 1988 may seem like a long time ago, the model would seem to remain valid.
Hi Vijay. I would recommend that you use the Karthik and Mander equations. They give values similar to those of the equation written above by Rafik.
A good discussion on this topic was provided in the book of Prof. Moehle (see Seismic Design of RC buildings) , where he compared the proposed equations by several analytical models. It would be prudent to compare the assumptions made in each model and choose the model that has the most relevant assumption for your problem.
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