P-Delta effect plus Time-History analysis or Response Spectrum analysis.

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