I am working on a project which uses ASCE 7-10 for seismic loads and AISC 360-10 for steel design. The project is in Seismic Design Category D and has irregularities which require use of a modal response spectrum analysis for seismic loading.
I have previously contacted Bentley about how to set up the model to use the Direct Analysis Method with the response spectrum analysis. I was referred to the webpage http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/21018.response-spectrum-loading-in-direct-analysis for Bentley's recommendation and set my model up based on the recommendations found on that webpage -- DAM for the gravity and gravity + wind load cases and direct analysis with LOAD COMBINATION cases for the gravity + seismic load cases.
The third party review engineer is not wanting to accept the proposed solution above as complying with the ASCE requirement of a modal response spectrum seismic analysis and the AISC requirement for a second order analysis. It is my understanding that ASCE and AISC have acknowledged that the use of both methods at the same time is not mathematically possible, but that they have not provided guidance about the conflict. The review engineer is asking for a new analysis using AISC Appendix 7. A sample loading is given below:
DEFINEREFERENCELOADS
LOADR1LOADTYPEDeadTITLEDEADLOAD
SELFWEIGHTY-1
FLOORLOAD
YRANGE911FLOAD-0.1GY
YRANGE1921FLOAD-0.1GY
YRANGE2931FLOAD-0.05GY
LOADR2LOADTYPELiveTITLELIVELOAD
FLOORLOAD
YRANGE911FLOAD-0.05GY
YRANGE1921FLOAD-0.05GY
YRANGE2931FLOAD-0.05GY
LOADR3LOADTYPEMassTITLEMASSFORDYNAMICLOAD
SELFWEIGHTX1
SELFWEIGHTY1
SELFWEIGHTZ1
FLOORLOAD
YRANGE911FLOAD0.15GX
YRANGE1921FLOAD0.15GX
YRANGE2931FLOAD0.1GX
*
YRANGE911FLOAD0.15GY
YRANGE1921FLOAD0.15GY
YRANGE2931FLOAD0.1GY
*
YRANGE911FLOAD0.15GZ
YRANGE1921FLOAD0.15GZ
YRANGE2931FLOAD0.1GZ
ENDDEFINEREFERENCELOADS
DEFINEDIRECTANALYSIS
FLEX1ALL
FYLD7200ALL
AXIALALL
NOTIONALLOADFACTOR0.002
END
*******************************************************
*DIRECT ANALYSIS CASES
LOAD1:1.4XDL + NOTIONALLOADINX
REFERENCELOAD
R11.4
NOTIONALLOAD
R1X0.0028
LOAD2:1.4XDL - NOTIONALLOADINX
REFERENCELOAD
R11.4
NOTIONALLOAD
R1X-0.0028
LOAD3:1.4XDL + NOTIONALLOADINZ
REFERENCELOAD
R11.4
NOTIONALLOAD
R1Z0.0028
LOAD4:1.4XDL - NOTIONALLOADINZ
REFERENCELOAD
R11.4
NOTIONALLOAD
R1Z-0.0028
*
LOAD5:1.2XDL + 1.6XLL + NOTIONALLOADINX
REFERENCELOAD
R11.2R21.6
NOTIONALLOAD
R1X0.0024R2X0.0032
LOAD6:1.2XDL + 1.6XLL - NOTIONALLOADINX
REFERENCELOAD
R11.2R21.6
NOTIONALLOAD
R1X-0.0024R2X-0.0032
LOAD7:1.2XDL + 1.6XLL + NOTIONALLOADINZ
REFERENCELOAD
R11.2R21.6
NOTIONALLOAD
R1Z0.0024R2Z0.0032
LOAD8:1.2XDL + 1.6XLL - NOTIONALLOADINZ
REFERENCELOAD
R11.2R21.6
NOTIONALLOAD
R1Z-0.0024R2Z-0.0032
*
LOAD9:1.2XDL + 1.0XLL + NOTIONALLOADINX
REFERENCELOAD
R11.2R21.0
NOTIONALLOAD
R1X0.0024R2X0.002
LOAD10:1.2XDL + 1.0XLL - NOTIONALLOADINX
REFERENCELOAD
R11.2R21.0
NOTIONALLOAD
R1X-0.0024R2X-0.002
LOAD11:1.2XDL + 1.0XLL + NOTIONALLOADINZ
REFERENCELOAD
R11.2R21.0
NOTIONALLOAD
R1Z0.0024R2Z0.002
LOAD12:1.2XDL + 1.0XLL - NOTIONALLOADINZ
REFERENCELOAD
R11.2R21.0
NOTIONALLOAD
R1Z-0.0024R2Z-0.002
PERFORMDIRECTANALYSISLRFD
CHANGE
*******************************************************
*RESPONSE SPECTRUM CASES WITH PDELTA ACCOUNTED FOR BY
*USING PDELTA KG
LOAD100LOADTYPENoneTITLE:1.2XDEAD + 1.0XLLFORKGMATRIX
REFERENCELOAD
R11.2R21.0
LOAD101LOADTYPENoneTITLERESPONSESPECTRUMLOADINX
SPECTRUMSRSSIBC2012X1ACCDAMP0.05LIN
ZIP92887SITECLASSAFA0.800FV0.800TL8.000
PDELTAKGANALYSISSMALLDELTA
CHANGE
LOAD102LOADTYPENoneTITLERESPONSESPECTRUMLOADINZ
SPECTRUMSRSSIBC2012Z1ACCDAMP0.05LIN
ZIP92887SITECLASSAFA0.800FV0.800TL8.000
PDELTAKGANALYSISSMALLDELTA
CHANGE
********************************************************
*LOAD COMBINATIONS COMBINING GRAVITY + RESPONSE SPECTRUM
LOADCOMB200:1.2XDEAD + 1.0XLL + SLINX
91.01011.0
LOADCOMB201:1.2XDEAD + 1.0XLL - SLINX
101.0101-1.0
LOADCOMB202:1.2XDEAD + 1.0XLL + SLINZ
111.01021.0
LOADCOMB203:1.2XDEAD + 1.0XLL - SLINZ
121.0102-1.0
PERFORMANALYSIS
CHANGE
FINISH
At this point, the project schedule doesn't have time to totally redo the design load combinations again. Does Bentley have another recommendation about how to address the conflict between DAM and 2nd order analysis in AISC and ASCE? What are the implications of using the DAM solution previously provided by Bentley and not having a specific DAM solution for the seismic load cases? Has ASCE and AISC provided a work-around?