Civil Engineering/theories

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QUESTION: Sir,
wanted to know on which theory is limit state design based on for RC and structural steel,
i think rc is based on elastic theory, steel is based on plastic theory. correct me if im wrong

ANSWER: Dear Arsalan

Limit state theory is based on elastic theory but some aspects of plastic behaviour to develop ultimate limits  ( especially in Rc )are in concern. This is not related to plastic theory which is a branch in structural analysis.

Regards
BEH

---------- FOLLOW-UP ----------

QUESTION: Sir,
then why do we find out plastic section modulus when designing a structural steel member, instead of elastic section modulus in limit state method in steel structure . can you also clarify what you meant by
"Limit state theory is based on elastic theory but some aspects of plastic behaviour to develop ultimate limits  ( especially in Rc )are in concern."

ANSWER: Dear Arsalan

The Mu in RC is based on plastic behaviour of concrete and steel. But this is an ultimate state for a member and no plastic hinge develops along the member. Plastic design is based on plastic hinge development which is not the case with limit state design. After all we assume that after loading removal the structure return back to its at rest position. THis is based on elastic theory.

Regards
BEH

---------- FOLLOW-UP ----------

QUESTION: Sir,
agreed with your statements for rc structures. but for structural steel (steel structures) its still not clear what you meant , can you briefly explain in context to steel structures. i know that limit state design is used for steel structures also but have a doubt why using plastic section modulus to estimate the strength of a member.

Answer
Dear Arsalan

Why you agree with statement about Rc? Because you think that RC is treated elastically but as I said the Mu in Rc is calculated based on plastic behaviour of concrete and steel. In steel we treat the same. Because an ultimate force must be base on ultimate behaviour of the element . In old version of design the elastic limit was in concern. In limit state this is changed to ultimate limit.

Regards
BEH

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