Dear Seniors:
When I want to make a Time-History analysis or a Response Spectrum analysis is necessary to make a P-Delta analysis to include the second-order effect due the deformation of the structure produced by horizontal forces. STAAD is capable to make a P-Delta effect without any problem, but in its TECHNICAL REFERENCE says:
“A PDELTA ANALYSIS will correctly reflect the secondary effects of a combination of load cases only if they are defined using the REPEAT LOAD specification... Secondary effects will not be evaluated correctly for LOAD COMBINATIONS”
But the REPEAT LOAD specification says:
“RESPONSE SPECTRUM LOAD and TIME HISTORY LOAD should not be used in REPEAT LOAD. It is also not available for loads generated using the program's load generation facilities such as UBC LOAD Generation, 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 any other 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 mass be canceled. That's why is extremely necessary to "combine" it effect in one way (positive) and in the other way (negative) with the rest of the common load case like dead, live and others. How I can do that and reach an appropriate P Delta effect analysis?
I give you an example where I've defined a seismic load case using the response spectrum analysis, you can see 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
A better choice of input is as follows:
Ensure that you have requested enough modes. The default 6 is often not enough.
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 horizontal and 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 KG Analysis 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 results can be + or - . If there are 2 or more dynamic cases, you should combine those dynamic cases with SRSS option in LOAD COMBINATION.
LOAD COMBINATION 5 LOADTYPE NONE TITLE LOAD COMB 4 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 are included.
PDELTA KG ANALYSIS PRINT MODE SHAPES
Answer Verified By: Sye
Raycxxx,
According to your reply, it seems that the repeat load command can be replaced by putting the Load 1 and Load 2 in one load case (if the factors are unity in this case). The commands can be reduced to as follows:
LOAD 1 Dead + Live * * Dead Load SELFWEIGHT Y -1 MEMBER LOAD 2 UNI GY -1.5 * * Live Load 2 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 and vertical forces will have more of the Pdelta effect." However, you only use REPEAT LOAD for vertical loads; the horizontal loads (seismic) is inclued later using LOAD COMBINATION. The veritcal and horizontal loads are not "combined" together as what you mentioned.
Second, when you use response sectrum analysis together with P-delta effect. Does the modal superposition take into account the geometric nonlinearity?
Thanks, Paul
STAAD.Pro developers please answer the queries of PaulY.
Thank you.
*********************** 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
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
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
**************** Solve Z direction
LOAD 5 LOADTYPE SEISMIC TITLE Seismic Z
SPECTRUM CQC IBC 2006 Z 1 ACC DAMP 0.05 LIN
******************* 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
******************* 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
The method Ray suggested is only an approximate way of accounting for second order effect in a dynamic load case. If you are asking whether a load combination where static part is combined algebraically and is SRSS-d with the dynamic part, is going to work for BUCKLING or DIRECT ANALYSIS, I would say No.