Civil Engineering/stability index


QUESTION: i want calculate the stability index for control braced RC frame. the equation for this work is : / V .H
what is the true load combination for calculate "P" . is 1.2DL+LL ?
thank you for your help

ANSWER: Dear Behzad

1.2 DL + LL should be considered as an extreme value for pdelta calculation. I think you are obligated to follow this way because you must be able to defend your calculations. But I believe it is better to follow the Eurocode method in which you must apply a portion of live load ( like 2 times of live load when  earthquake occurs, i.e. 2 x 0.25 ( total live load)).

After all as per UBC or Iranian national code , you should apply 1.2 DL + LL


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

QUESTION: dear babak thank you for this guidance
yes this formula is for control that is need P-delta effect or no. but load combination for this action is DL+LL.
i have a 3 story RC frame and i want to assign the ACI318-11, 10-10-5-2 says that if stability index less than 0.05 frame is nonsway so for beam=0.5 and for col=1 . but if stability index greater than 0.05 this coefficient for beam=0.35 and for col=0.7
i want know that what is the correct load combination for "P" in this formula?
thank you.

Dear Behzad

As for amplification factor for taking the p-delta into consideration , you should use  factored axial loads according to load combination considered for design of the column. This is what ACI needs to be satisfied. Please dont hesitate to let me know in the case of further explanation you would need.


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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.

Sinam Consulting Engineers Co.

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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