Civil Engineering/Staad



My question is related to Staad software. I want to ask that when ever I model a structure and assign member releases at any joint (Mx, My and Mz), The analysis say that structure is unstable at that joint. So I want to know how to assign member releases?

Thanks in advance.

ANSWER: Dear Nouman

I dont have special information about Staad but in matrix and finite elment analysis you should keep one end member in a simple joint being moment resisting , if not the system will get unstable. For instance if you va e2 members in a jiont with moment release you should define only one member as released. The other will be released due to moment equilibrium.


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

QUESTION: I have one more question sir.

While computing seismic loads, we normally get Ss and S1 values like 0.085g and 0.031g. While g is valuse of gravitation. Do we need to consider Ss value 0.085g or 0.85 and S1 value as it is or 0.31 while computing Fa and Fv values? And what is the reason for using g?

Thanks in advance.

Dear Nouman

It depends on the units you are using. If you use kgf as unit for force you dont need a g=9.81 m/s^2 and you should use g=1 . But if you use N ( newton) or KN you should use g=0.81 . Because the factor S1 is the acceleration which is multiplied by the mass ( with kg as unit) to give you a force according to second law of Newton. Please tell me if you have any further problem.


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Babak Esmailzadeh Hakimi


I can answer questions in Structural Engineering, Earthquake Engineering, Project Management and Project Control topics!


I am experienced in the area of earthquake engineering in electrical power industry, i.e. Lifeline Earthquake Engineering. My focus has been on structural seismic design of power installations. Recently I have got involved in project management in the category of constructional engineering.

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Bs degree in Civil Engineering from Ferdowsi University ,1988, Mashhad Iran, Ms degree in Earthquake Engineering, Sharif University of Technology, 1993, Tehran, Iran

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