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  • 1.  ETABS p-delta analysis

    Posted 06-09-2017 09:35 AM
    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? 


  • 2.  RE: ETABS p-delta analysis

    Posted 06-12-2017 09:34 AM

    Hi Berhan,

    P-Delta analysis should be performed with adequate load combinations for the type of load analyzed. For example, using ASCE load combinations for wind and earthquake design I had used 1.2 D+L. For gravity loads I have used the most critical from 1.4 D and 1.2 D+1.6 L.

    I hope this can help.

    Best, regards

    Sebastian Lopez C.Eng, M.ASCE
    Escuela Colombiana De Ingeniería

  • 3.  RE: ETABS p-delta analysis

    Posted 06-12-2017 11:58 AM
    Hi Berhan,
    Your question relative to p-delta is a very general question. The load ( p- delta) depends to the framing system of beam-column design. The p-- delta is effective value to secondary bending lateral loads and axial compression causes for deflection, in which Gravity is one of the effective factor.
    The deflection spends to moments and secondary moment reaches to maximum value.
    Please, refer to the column behavior, building framing for deflection. Also, columns are subject to simultaneous action of axial loads and moments. How may stories is the steel structure. P-delta accurately shall be calculated for brace and unbraced members.
    I hope it helps.
    It is valued question.

    Soussan Bathaee M. ASCE- SEI

    Sent from my iPad

  • 4.  RE: ETABS p-delta analysis

    Posted 06-12-2017 05:04 PM
    Has been awhile since I last used ETABS. As I know it, it is more suitable for concrete structure.

    Lawrence Wah P.E., M.ASCE, MIstrutE
    Admin. Engineer Manager
    New York NY

  • 5.  RE: ETABS p-delta analysis

    Posted 06-13-2017 09:40 AM
    Berhan, you will be using D+L+S (unfactored) as your load combination of concern for the P-Delta analysis.  You should do the following:
    1. Calculate your initial design story drift Δ in the first story
    2. Check the P-Delta criteria for the subject building (must have basic geometry; R-value; Cd = value; I = value; h1 = ht. of 1st story)
      1. Calculate the Stability Coefficient, Φ = (P1*Δ)/(Vx*hx1*Cd) = ? < 0.10 Ok, if greater than 0.10 P-Delta must be considered.
        1. Determine P1 using Sum(DL) + Sum(LL) + Sum(SL) at hx1 (1st story)
        2. Check Φ against Φmax
      2. If Φ > 0.10 (ratio of secondary moments to primary moments), then P-Delta must be checked
      3. Calculate the incremental augmentation factor, ad, from the P-Delta effects, where ad = (1.0)/(1-Φ)
        1. Final design story drift is Δ'1 = (ad)*(Δ)
        2. Final design story shear is V'1 = (ad)*(V1)
      4. Check for the Allowable Story Drift compliance, Δallow = (0.020)*(h1) > Δ'1
    3. If you are required to consider P-Delta effects because Φ > 0.10, than the structural elements will have to be designed to resist the Final design story shears and moments resulting from C.i/C.ii above.

    Hope this helps you understand the P-Delta concerns.  Take a look at a modern seismic design code such as IBC or IBC Structural/Seismic Design Manual, Volume 1 that deals with Seismic Code Compliance examples.

    Best regards,

    Bill Marczewski, P.E., M.ASCE
    BSM Engineering
    Portland, OR


  • 6.  RE: ETABS p-delta analysis

    Posted 06-16-2017 02:38 PM

    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.

    Brett King P.E., M.ASCE
    Senior Structural Engineer
    GHD Inc.
    Lake Oswego OR

  • 7.  RE: ETABS p-delta analysis

    Posted 06-27-2017 12:31 PM
    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.
    asce7The 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 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.
    aci 318-14


    Napoleon De La Colina A.M.ASCE
    Eng Mgr
    ICM - India
    Bombay MH

  • 8.  RE: ETABS p-delta analysis

    Posted 10-16-2017 03:30 PM
    Check CSI wiki, P-Delta analysis parameters - ETABS - Computers and Structures, Inc. - Technical Knowledge Base
    Csiamerica remove preview
    P-Delta analysis parameters - ETABS - Computers and Structures, Inc. - Technical Knowledge Base
    A P-Delta load combination of 1.2 DL + 0.5 LL is typically conservative when considering P-Delta effect due to the overall sway of a structure. Combinations (3) and (4) will accurately capture this effect, while (5) and (6) should be conservative. Combinations (1) and (2) have no lateral load, therefore P-Delta effect should not be of concern.
    View this on Csiamerica >

    Mohamed Abushady M.ASCE
    Senior Structural Engineer
    ABV Rock Group Co. Ltd