Berhan,
ETABS requires you to define one load combination (one at a time for each analysis) that best represents the conditions of your analysis. Since you are interested in analysing the second order effects due to lateral loading of the seven Load combinations offered by ASCE7 (LRDF), those containing lateral load cases are the eligible ones, namely 4, 5, 6 & 7 below.
The most commonly used cases are 5 or 4 from the above list as Brett rightly suggested. Normally this is sufficient. However, you may want to explore the impact of large moments and drifts with reduced axial loads for which you would have to conduct a second run changing the P-Delta load combination definition and compare the results.
Similar thing can be affirmed using equivalent load combinations of Eurocodes or any other code.
Notice that the user manual makes it clear that conducting the P-delta analysis in ETABS is not an option, it is a must, irrespective if the column is slender or not.
In addition to the P-delta analysis it is advisable to apply stiffness modifiers (following ACI 318-2014, table 6.6.3.1.1.a) in that way there would be an additional lateral drift which would likely increase the second order effect in vertical elements and help represent a most realistic behaviour of the structure.
Napoleon
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Napoleon De La Colina A.M.ASCE
Eng Mgr
ICM - India
Bombay MH
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Original Message:
Sent: 06-16-2017 14:08
From: Brett King
Subject: ETABS p-delta analysis
Berhan, There are a couple of ways to perform P-Delta analysis in Etabs or Sap2000. To learn the intricacies and make sure you are doing it right will probably require digging into the manuals and help section at the CSI web site.
The load combinations you use will depend on the code you use to some extent although as a general philosophy they should all be at a factored level. My comments are based on AISC Specifications Part C.
One way. You can create factored Load Cases that include your entire load combination such as 1.2D +1.6L + Lateral. My underline emphasis is intentional because I'm not talking about defining load combinations. Once the load case is defined, you can select non-linear under analysis type and then select P-Delta. Do this for each factored load case. Of course, you have to consider notional loads and all that too according to your code.
Second way. I often prefer this method. Create one factored Load Case for factored dead load and a factored lateral load such as 1.2D + Wind. I believe just about any lateral load to push the structure laterally will work as long as it has the full dead load you want to consider on it. Run this one as a non-linear P-Delta load case. The software calculates a softened stiffness matrix based on the P-Delta effects.
Then for the remainder of your load cases, the way you would normally define them, select the Stiffness at End of Nonlinear Case option under Stiffness to Use section. Pick the load case you defined above as the starting load case for all the rest of the load lases you want to consider and they run linearly including the P-Delta effect. Define all your load combination as you usually would in the Define:Load Combinations section.
Hope this is helpful. Sorry if I covered stuff you already know about.
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Brett King P.E., M.ASCE
Senior Structural Engineer
GHD Inc.
Lake Oswego OR
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Original Message:
Sent: 06-08-2017 23:12
From: Berhan Gebregziabiher
Subject: ETABS p-delta analysis
I wanted to perform the p-delta analysis of a steel framed building. Is there a specific load combination that need to be used in the p-delta analysis-(iterative based on load)? Can anybody share what procedure you have been using for that analysis in ETABS?
Thanks