chekre
Structural
- May 8, 2013
- 173
Good day,
For computing the second order effects, ACI 318-11 recommends 3 methods:
1-Non linear second order analysis.
2- linear second order analysis.
3- moment magnification factor.
For the moment magnification factor, we must uncheck the p delta option in etabs and run the analysis based on a cracked model (EI are reduced). Then, we must calculate the Q factor ( section 10.10.5) to verify if the stories are sway or non sway.
If found to be non sway (Q<0.05), then 1st order moments are multiplied by a factor deltA ns to take into account the member curvature assuming that k equal to 1. (section 10.10.6.)
If found to be sway, then we have to calculate the effective length factor k which varies between 1 and ∞ and magnify our moments according to section 10.10.7.
For the linear second order analysis, P delta option is activated, effective length factor is equal to 1.
2 types of p delta exist : big delta which is the lateral deformation of the building due to horizontal loads and the small delta which is the deformation of the vertical member between chord members.
Big delta is automatically calculated by activating the pdelta option while for the calculation of the small delta, we need to subdivide our columns into many segments or we can keep it as it is and etabs , while designing the member, will multiply by delta ns (used in the moment magnification factor) to take into account the member curvature (simpler than dividing the columns into many segments).
Non linear second order analysis differs by the fact that we should create a non linear case including p delta. One more difference is the fact that Φk is 0.8 while in the linear second order analysis, this factor is 0.875. Therefore, reduction factors are to be changed while performing a non linear second order analysis to become 0.63 for walls and columns,0.3 for beams and 0.23 for slabs.
linear second order analysis is used when dealing with response spectrum and non linear second order analysis is recommended to be used in time history or pushover analysis.
any comments on the above ?
For computing the second order effects, ACI 318-11 recommends 3 methods:
1-Non linear second order analysis.
2- linear second order analysis.
3- moment magnification factor.
For the moment magnification factor, we must uncheck the p delta option in etabs and run the analysis based on a cracked model (EI are reduced). Then, we must calculate the Q factor ( section 10.10.5) to verify if the stories are sway or non sway.
If found to be non sway (Q<0.05), then 1st order moments are multiplied by a factor deltA ns to take into account the member curvature assuming that k equal to 1. (section 10.10.6.)
If found to be sway, then we have to calculate the effective length factor k which varies between 1 and ∞ and magnify our moments according to section 10.10.7.
For the linear second order analysis, P delta option is activated, effective length factor is equal to 1.
2 types of p delta exist : big delta which is the lateral deformation of the building due to horizontal loads and the small delta which is the deformation of the vertical member between chord members.
Big delta is automatically calculated by activating the pdelta option while for the calculation of the small delta, we need to subdivide our columns into many segments or we can keep it as it is and etabs , while designing the member, will multiply by delta ns (used in the moment magnification factor) to take into account the member curvature (simpler than dividing the columns into many segments).
Non linear second order analysis differs by the fact that we should create a non linear case including p delta. One more difference is the fact that Φk is 0.8 while in the linear second order analysis, this factor is 0.875. Therefore, reduction factors are to be changed while performing a non linear second order analysis to become 0.63 for walls and columns,0.3 for beams and 0.23 for slabs.
linear second order analysis is used when dealing with response spectrum and non linear second order analysis is recommended to be used in time history or pushover analysis.
any comments on the above ?
