response spectrum analysis in STAAD

Hi

Three questions regarding the response spectrum analysis. 

Q1:
I am not quite sure why I got zero on the Y and Z directions?  Please advise.

LOAD 4 LOADTYPE
Seismic  TITLE RESP. SPECTRUM LOADS X

MEMBER LOAD

15 TO 20 35 TO 40 UNI
GX 10

15 TO 20 35 TO 40 UNI
GY 10

15 TO 20 35 TO 40 UNI
GX 200

15 TO 20 35 TO 40 UNI
GY 200

5 6 25 26 UNI GX 150

5 6 25 26 UNI GY 150

SPECTRUM SRSS IBC 2006
X 1 ACC DAMP 0.05

ZIP 2908 SITE CLASS D
FA 1.600 FV 2.400 TL 6.000

SELFWEIGHT X 1 LIST 5 6
15 TO 20 25 26 35 TO 40

SELFWEIGHT Y 1 LIST 1
TO 4 7 TO 14 21 TO 24 27 TO 34

SELFWEIGHT Z 1 LIST 41
TO 52

*

PERFORM ANALYSIS

CHANGE

*

LOAD 5 LOADTYPE
Seismic  TITLE RESP. SPECTRUM LOADS Y

MEMBER LOAD

15 TO 20 35 TO 40 UNI
GX 10

15 TO 20 35 TO 40 UNI
GY 10

15 TO 20 35 TO 40 UNI
GX 200

15 TO 20 35 TO 40 UNI
GY 200

5 6 25 26 UNI GX 150

5 6 25 26 UNI GY 150

SPECTRUM SRSS IBC 2006
Y 1 ACC DAMP 0.05

ZIP 2908 SITE CLASS D
FA 1.600 FV 2.400 TL 6.000

SELFWEIGHT X 1 LIST 5 6
15 TO 20 25 26 35 TO 40

SELFWEIGHT Y 1 LIST 1
TO 4 7 TO 14 21 TO 24 27 TO 34

SELFWEIGHT Z 1 LIST 41
TO 52

*

PERFORM ANALYSIS

CHANGE

*

LOAD 6 LOADTYPE
Seismic  TITLE RESP. SPECTRUM LOADS Z

MEMBER LOAD

15 TO 20 35 TO 40 UNI
GX 10

15 TO 20 35 TO 40 UNI
GY 10

15 TO 20 35 TO 40 UNI
GX 200

15 TO 20 35 TO 40 UNI
GY 200

5 6 25 26 UNI GX 150

5 6 25 26 UNI GY 150

SPECTRUM SRSS IBC 2006
Z 1 ACC DAMP 0.05

ZIP 2908 SITE CLASS D
FA 1.600 FV 2.400 TL 6.000

SELFWEIGHT X 1 LIST 5 6
15 TO 20 25 26 35 TO 40

SELFWEIGHT Y 1 LIST 1
TO 4 7 TO 14 21 TO 24 27 TO 34

SELFWEIGHT Z 1 LIST 41 TO
52

*

PERFORM ANALYSIS

CHANGE

 

Q2:
 The base shear results are less on the load case #1 (Seismic X) compared to load #4 (Response Spectrum in X).  Why?  I thought that the static analysis (V=Cs*W) shall be conservative compared to the dynamic analysis, right?  Please advise. 

 

Q3:
 Why I got same results on the repeat load (load case #7) and load combination (load case #8)?  Please advise.

Thanks again for the help.

 

Response Spectrum Verifications-3D.std
  • On my question #1, I meant load case #5 (Resp. Spectrum Y direction) and #6 (Resp. Spectrum Z direction).

    Thanks,

  • The zero response in Y and Z were due to insufficient number of modes considered. If you take out the CUT OFF MODE SHAPE command, you would see participation in Y and Z. Also you need to include the masses in the Z direction. Provide all seismic masses as part of the first seismic case (case 4 ). Mass definitions in subsequent dynamic cases are not read by STAAD.Pro

    From the dimensions of the structure and the cross sectional properties of the members, it is evident that it is not a building structure. IBC Equivalent Lateral Load analysis that STAAD carries out, is applicable for building structures and hence is not appropriate in this case. A generic response spectrum analysis would be more appropriate.  

    The analysis you have carried out is a linear one and hence the REPEAT load would yield the same result as a LOAD COMBINATION.



  • Thanks so much, Sye

    I remove the CUT OFF MODE SHAPE command and it shows all the results.  I still have some confusing.  Please advise.

    Q1: I still do not understand what you meant by “Also you need to include the masses in the Z direction”.  I thought I did (please see attachment for screen-shot)

    Q2: I have the “PDELTA 35 ANALYSIS" after the last load case (#6) and before concrete design but it still carries “a linear analysis” (i.e. same as load combinations and Repeat load) – why?  

    LOAD 7 LOADTYPE None  TITLE LOAD CASE 5

    REPEAT LOAD

    1 1.0 2 0.3

    *

    LOAD COMB 8 COMBINATION LOAD CASE 6

    1 1.0 2 0.3

    *

    PERFORM ANALYSIS

    CHANGE

    *

    PDELTA 35 ANALYSIS

    *

    START CONCRETE DESIGN

    Q3: I got the warning as “   *WARNING- APPLIED SELFWEIGHT IS LESS THAN TOTAL WEIGHT OF ALL STRUCTURAL ELEMENTS IN LOAD CASE      4 ALONG X.”  I really do not know what it means.  Please advise.

    Thanks again for all your help.

  • Hi Sye

    Just think of the selfweight last night - why do we have selfweight X and Z?  Does selfweight only apply on Y direction (gravity)?  Please advise.  Thanks,

  • Hi Sye,

    Sorry to bother you again.  I have gone through your comments again – now I understand what you meant by adding masses in the Z direction.  Please ignore my previous question and I am going to rearrange them.    I did add that and update the model just like the building type.  Now I have some findings/questions.

    Q1: The seismic load X (load case #1) is closed to Resp spectrum X (load case #5) now.  But the seismic load Z (load case #2) is far different than Resp spectrum Z (load case #7).  Please advise (see attachment).

    Q2:  Now my model is like a real building but the base shear results are still less on the load case #1 (Seismic X) compared to load #4 (Response Spectrum in X).  Why?  I thought that the static analysis (V=Cs*W) shall be conservative compared to the dynamic analysis, right?  Please advise.  

    Q3: I have the “PDELTA 35 ANALYSIS" after the last load case (#6) and before concrete design but it still carries “a linear analysis” (i.e. same as load combinations and Repeat load) – why?  

    Q4: I got the warning as “   *WARNING- APPLIED SELFWEIGHT IS LESS THAN TOTAL WEIGHT OF ALL STRUCTURAL ELEMENTS IN LOAD CASE      4 ALONG X.”  I really do not know what it means.  Please advise.

    Q5: Why do we have selfweight X and Z for Response Spectrum? Does selfweight only apply on Y direction (gravity)?  Please advise.

    Thanks again for all your help

    Response Spectrum Verifications Example.std