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Regarding P-D analysis.
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renjithsarma
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Joined: 23 Jun 2016
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PostPosted: Thu Jun 23, 2016 10:36 am    Post subject: Reply with quote

bijay sarkar wrote:
Dear all,

My observation on the subject matter are as follows for your comment :

Analysis which we normally do, is called an elastic linear analysis which does not take care of the deflections of the structures, be it from sway or from slenderness/buckling.
P-Delta Analysis is called a non-linear analysis which incorporates the effects of deflections of the structure.
Now there are two types of deflections : (i) Structural deflection or Global deflection or what we normally call structural “sway”.  (ii) Member deflection or local deflection or slenderness deflection. Additional moments comes from these deflections which are also to be considered for design.
Now, this is pertinent to be confirmed whether the frame is a “sway frame” or a “non-sway frame” for deciding which Fig. (26 or 27) is to be used for effective length factors for columns. For identifying a “sway” or “non-sway” frame, IS 456 has directed to calculate an index Q which depends on the elastic linear analysis of the structure.
If the structure is a “sway frame”, then only we may go for P-Delta analysis, otherwise, elastic linear analysis is enough.
But P-Delta analysis of Staad does not cover the deflection due to slenderness effect of the members which requires a buckling analysis of the frame. Therefore, we may say that after P-Delta analysis, additional moments due to slenderness/buckiling are yet to be added, if any.
For this slenderness/buckling moments, we require to calculate the effective lengths of the members. For “sway frame”, effective lengths shall be in accordance with Fig. 26 of IS 456 and for “non-sway frame”, Fig. 27 shall be used. But as we have already analysed the structure for P-Delta effects, this is my personal opinion that, now, we shall go for Fig.26 only i.e. for non-sway frames to calculate the effective lengths to find out the additional moments for slenderness/buckling. If P-Delta analysis is not carried out and the frame is a sway frame, then only we will go for Fig. 27 to cover up both the deflections as we normally do.
I am submitting my observations stepwise for comments of the sefians :
a)  Conduct elastic linear analysis what we normally do.
(b) Find out Q to decide the type of frame (sway or non-sway).
(c) If non-sway frame, P-Delta analysis is not to be carried out. Effective lengths of members shall be based on Fig. 26 of IS and additional moment due to slenderness/buckling shall be added to the normal elastic linear analysis to find out the design moments.
(d) If sway frame, we may carry out P-Delta analysis or may not.
           (i)If P-Delta analysis is carried out, calculate effective length factors based on Fig. 26 of IS 456 which is for non-sway frame for calculating additional slenderness/buckling moments. Add this moment with the P-Delta analysis. Is it right or Fig. 27 is yet to be used?
          (ii)If P-Delta analysis is not carried out and the frame is a sway frame, calculate the effective length factors based on Fig. 27 of IS 456 which is for sway frame and calculate the additional moment based on the buckling/slenderness equation provided in the IS. Consideration of Fig. 27 of IS will cover both the deflections i.e. from sway of structure as well as slenderness/buckling of the members to find out the additional moments.


bijay sarkar

sir, i was searching for a solution regarding " effective length factor " that is used in ETABS for columns. the value proposed by the software turns to be very high and i was told to edit this value to 1. an another person suggested to do p- delta analysis for design after the effective length factor correction. i would like to understand the theory behind this. is this method you proposed is correct?
Renjith sarma
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rajeshpullat
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Joined: 09 Jun 2017
Posts: 17

PostPosted: Wed Oct 25, 2017 4:21 am    Post subject: Stability index in ETABS Reply with quote

Hi Rajeev
where does ETABS calculate the stability index
I cannot see it anywhere
Could you please tell me?
Rajesh
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syed_A
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Joined: 22 Oct 2011
Posts: 385
Location: Hyderabad

PostPosted: Wed Oct 25, 2017 6:44 am    Post subject: Reply with quote

Dear Engineer
If you are  looking for Q value, Check it under flex details tab ( after design, right click the column, select details ) , Hope it helps

Regards
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