I have the attached load-deflection curve for an RC beam tested under a three-point loading test up to failure. What is the appropriate way/ most acceptable method to determine the cracking load, stiffness, and rotational ductility?
For measuring stiffness in elastic range consider initial straight line of load deflection curve .
Cracking load means load at first structural crack ( 0.2 mm width)
Rotational ductility seems to be difficult to explain . It may correspond to ratio of yield load to maximum elastic load. check from research article.
Dear Mr. Ahmad Abu-abah,
If you are testing RC beams, then you must be familiar with the non-linear behavior of RC structures regarding determination of the Cracking (C), Yielding (Y) and Ultimate (U) points. You should read papers and books in this field. Particularly, you can read my book "Non-linear behavior of reinforced concrete structures under the seismic loading", or other literature. I can tell you based on my huge experience that displacement equal to about 5.5mm and corresponding to it load will be coordinates of the Cracking Point on your curve. There are formulas suggested by me to calculate coordinates of the characteristic points (C, Y and U). However, there are many other scientists who also did similar research. So, get a help from your supervisor or try to find appropriate papers to learn about this problem.
Best regards,
Mikayel Melkumyan
Doctor of Sciences (Engineering), Professor
Academician of the Saint-Petersburg Arctic Academy of Sciences
Academician of the Athens Institute for Education and Research
President of the Armenian Association for Earthquake Engineering
Vice-President of the International Association of CIS Countries on Base Isolation
Member of the American Association for Science and Technology
Foreign member of the Research Center of Seismic Resistant Structures of the Institute of Industrial Science, University of Tokyo
Eminent Expert of the Committee of Eminent Experts in International Research Base of Seismic Mitigation and Isolation of Gansu Province in China
Founder of the "Save the Yerevan Schools From Earthquakes" foundation
Director of the "Melkumyan Seismic Technologies" LLC
+374 (91) 94-54-02
First, get the yield force Fy= 0.85Fmax and then determine the corresponding yield displacement using interpolation.
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