Re_ P-Delta Effect Plus Time-History Analysis or Response Spectrum Analysis

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P-Delta effect plus Time-History analysis or Response

Spectrum analysis.

Posted by dynamics on Thu, Sep 20 2012 11:58 PM

Yes would appreciate if you could post that clause from ASCE 4,

Many thanks

Posted by raycxxx on Thu, Sep 20 2012 11:08 PM

1. Kg member matrices are linear functions of the axial load in the member. These are usually calculatedfrom a static load case solution. I assume that dead load and other permanent loads would be included.Similar to the masses used in the eigensolution. You could include wind or equivalent earthquake loads inthe static case as well. However that would lead to two analysis solutions or more (one for X and one for Z).

2. Yes. Note that if the structure is not symmetric, then a positive side load may have different results than anegative side load. You could have 4 dynamic analysis runs or more.

3. Yes if you are not using the 100-30-30 rule.

4. In ASCE 7-05 there is reference to modal combinations methods from ASCE 4-98 should be used.

We have those rules in our ASCE4 option. Does anyone use or need that rule or any other clause fromASCE4?

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Posted by dynamics on Thu, Sep 20 2012 2:03 PM

Mr. Raycxxx,

With my limited understanding of the new P-delta KG Analysis because of its insufficient documentation andsupporting technical reference let me clarify a couple of salient points in your post below.

1. Load case number 3 is the load case where the program extracts the KG matrices to be used in thesucceeding analysis. I believe this is similar to ETABS and MIdas Gen Programs where they have a specificP-delta load combination from which the KG matrices will be extracted from, am I correct?

2. Having said that, that particular load case should ideally include the lateral load to capture the worst caseeffect of P-delta so that the KG matrices equations are derived from the laterally displaced shaped of thestructure am I correct in my understanding here? Hence, ideally this particular load case should include forexample the static earthquake in the combination if it is defined in the load cases already.

3. What significance is the SRSS combination of the two orthogonal response spectrum load cases is thisjust to get the resultant effect of the response spectrum load?

Your clarification is very much appreciated.

Posted by raycxxx on Thu, Sep 20 2012 1:42 AM

*********************** P D E L T A with Response Spectrum and Load Combinations

*******************************

******************************* Enter static cases

LOAD 1 LOADTYPE DEAD TITLE Dead

SELFWEIGHT Y -1

MEMBER LOAD

2 UNI GY -1.5

************************ Keep entering cases until all entered

*******************

LOAD 2 LOADTYPE LIVE REDUCIBLE TITLE Live











MEMBER LOAD

2 UNI GY -2

2 CON GY -2 3

PERFORM ANALYSIS

CHANGE

***************** Enter static case for computing member axial forces to be used only for member Kg matrices

***************** Alternatively all static loads could be entered into this one case

***************** or use repeat load to replicate the cases here.

LOAD 3 LOADTYPE DEAD+LIVE

repeat load

1 1.0 2 1.0

***************** Case for modes and response spectrum (just before first dynamic case)

LOAD 4 LOADTYPE SEISMIC TITLE Seismic X

***************** Enter all 3D masses to be considered times g here

***************** Solve X direction

SELFWEIGHT X 1

SELFWEIGHT Y 1

SELFWEIGHT Z 1

MEMBER LOAD

2 UNI GX 1.5

2 UNI GY 1.5

2 UNI GZ 1.5

2 UNI GX 2

2 UNI GY 2

2 UNI GZ 2

2 CON GX 2 3

2 CON GY 2 3

2 CON GZ 2 3

SPECTRUM CQC IBC 2006 X 1 ACC DAMP 0.05 LIN

ZIP 92887 SITE CLASS B TL 8.000

*******************************



PDELTA KG ANALYSIS

CHANGE

**************** Solve Z direction

LOAD 5 LOADTYPE SEISMIC TITLE Seismic Z

SPECTRUM CQC IBC 2006 Z 1 ACC DAMP 0.05 LIN

ZIP 92887 SITE CLASS B TL 8.000

******************* Combine first two static cases algebraically

******************* then SRSS the two directions of RSA and add to static

LOAD COMB SRSS 6 LOADTYPE NONE TITLE SRSS Combination Load

-1 1.0 -2 1.0 4 1.0 5 1.0 1.0

******************* Combine first two static cases algebraically

******************* then SRSS the two directions of RSA and subtract from static

LOAD COMB SRSS 7 LOADTYPE NONE TITLE SRSS Combination Load

-1 1.0 -2 1.0 4 1.0 5 1.0 -1.0

PERFORM ANALYSIS

Posted by chaz on Sat, Sep 15 2012 3:36 AM

STAAD.Pro developers please answer the queries of PaulY.

Thank you.

Posted by PaulY on Fri, Jul 3 2009 5:35 AM


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Raycxxx,

According to your reply, it seems that the repeat load command can be replaced by putting theLoad 1 and Load 2 in one load case (if the factors are unity in this case). The commands can bereduced to as follows:

LOAD 1 Dead + Live* * Dead LoadSELFWEIGHT Y -1 MEMBER LOAD 2 UNI GY -1.5 * * Live Load2 UNI GY -2 2 CON GY -2 3

Then Load 4 for repeat load is not necessary. Am I correct?

My another question is that you mentioned "For PDELTA the cases with both horizontal andvertical forces will have more of the Pdelta effect." However, you only use REPEAT LOAD forvertical loads; the horizontal loads (seismic) is inclued later using LOAD COMBINATION. Theveritcal and horizontal loads are not "combined" together as what you mentioned.

Second, when you use response sectrum analysis together with P-delta effect. Does themodal superposition take into account the geometric nonlinearity?

Thanks,Paul

Posted by raycxxx on Tue, Apr 28 2009 11:13 PM

A better choice of input is as follows:

Ensure that you have requested enough modes. The default 6 is often notenough.

CUT OFF MODES 20

LOAD 1 LOADTYPE DEAD TITLE Dead SELFWEIGHT Y -1 MEMBER LOAD 2 UNI GY -1.5

LOAD 2 LOADTYPE LIVE REDUCIBLE TITLE Live MEMBER LOAD 2 UNI GY -2 2 CON GY -2 3

Combine the static cases for PDELTA. For PDELTA the cases with both horizontaland vertical forces will have more of the Pdelta effect.





LOAD 4 LOADTYPE NONE TITLE Repeat Load REPEAT LOAD 1 1.0 2 1.0

To have the PDELTA effect in dynamic cases, place the case just before the PDELTA KGAnalysis command.

The dynamics will include the Kg matrix from the prior case (case 4). | [K+Kg] - ω2 [M] | = 0

LOAD 3 LOADTYPE SEISMIC TITLE Seismic X SELFWEIGHT X 1 SELFWEIGHT Y 1 MEMBER LOAD 2 UNI GX 1.5 2 UNI GY 1.5 2 UNI GX 2 2 UNI GY 2 2 CON GX 2 3 2 CON GY 2 3 SPECTRUM CQC IBC 2006 X 1 ACC DAMP 0.05 LIN ZIP 92887 SITE CLASS B TL 8.000

Combine the static and dynamic by using the LOAD COMBINATION command. Create two cases: static + dynamic and static - dynamic since RSA dynamic resultscan be + or - . If there are 2 or more dynamic cases, you should combine those dynamic cases withSRSS option in LOAD COMBINATION.

LOAD COMBINATION 5 LOADTYPE NONE TITLE LOAD COMB4 1.0 3 1.0

LOAD COMBINATION 6 LOADTYPE NONE TITLE LOAD COMB 4 1.0 3 -1.0

The PDELTA KG command does not need iteration value or smalldelta. They areincluded.

PDELTA KG ANALYSISPRINT MODE SHAPES

Posted by Raymelito on Sun, Apr 26 2009 11:01 PM

Dear Seniors:

When I want to make a Time-History analysis or a Response Spectrum analysis is necessary to make a P-


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Delta analysis to include the second-order effect due the deformation of the structure produced by horizontalforces. STAAD is capable to make a P-Delta effect without any problem, but in its TECHNICALREFERENCE says:

“A PDELTA ANALYSIS will correctly reflect the secondary effects of a combination of load cases only ifthey are defined using the REPEAT LOAD specification... Secondary effects will not be evaluated correctlyfor LOAD COMBINATIONS”

But the REPEAT LOAD specification says:

“RESPONSE SPECTRUM LOAD and TIME HISTORY LOAD should not be used in REPEAT LOAD. It isalso not available for loads generated using the program's load generation facilities such as UBC LOADGeneration, WIND LOAD Generation, MOVING LOAD Generation, etc”

My question is:

How I can to combine several loads case including Time History or Response Spectrum load case, or anyother load derivate from generation load facility, to make an appropriate P Delta analysis?

It’s necessary to model the totality of the mass of the structure in the same directions to avoid that the massbe canceled. That's why is extremely necessary to "combine" it effect in one way (positive) and in the otherway (negative) with the rest of the common load case like dead, live and others. How I can do that and reachan appropriate P Delta effect analysis?

I give you an example where I've defined a seismic load case using the response spectrum analysis, you cansee the error that STAAD give me back because I use response spectrum in a repeat load specification.

25. LOAD 1 LOADTYPE DEAD TITLE Dead

26. SELFWEIGHT Y -1

27. MEMBER LOAD

28. 2 UNI GY -1.5

29. LOAD 2 LOADTYPE LIVE REDUCIBLE TITLE Live

30. MEMBER LOAD

31. 2 UNI GY -2

32. 2 CON GY -2 3

33. LOAD 3 LOADTYPE SEISMIC TITLE Seismic X

34. SELFWEIGHT X 1

35. SELFWEIGHT Y 1

36. MEMBER LOAD

37. 2 UNI GX 1.5

38. 2 UNI GY 1.5

39. 2 UNI GX 2

40. 2 UNI GY 2

41. 2 CON GX 2 3



42. 2 CON GY 2 3

43. SPECTRUM CQC IBC 2006 X 1 ACC DAMP 0.05 LIN

44. ZIP 92887 SITE CLASS B TL 8.000

45. LOAD 4 LOADTYPE NONE TITLE Repeat Load

46. REPEAT LOAD

47. 1 1.0 2 1.0 3 1.0 **ERROR- CASE BEING FACTORED IS DYNAMIC. CASE= 3

48. PDELTA KG 15 ANALYSIS SMALLDELTA PRINT MODE SHAPES

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Re_ P-Delta Effect Plus Time-History Analysis or Response Spectrum Analysis