asd vs lrfd_forwebsite

General Comparison between AISC LRFD and ASDHAMID ZANDGT STRUDL USERS GROUPLAS VEGAS, NEVADAJUNE 22-25, 2005

EDITED BY: CIVILAX.COM*

AISC ASD and LRFDAISC=American Institute of SteelConstruction

ASD=Allowable Stress DesignAISC Ninth Edition

LRFD=Load and Resistance Factor DesignAISC Third Edition*

AISC Steel Design Manuals1963 AISC ASD 6th Edition1969 AISC ASD 7th Edition1978 AISC ASD 8th Edition1989 AISC ASD 9th Edition

1986 AISC LRFD 1st Edition1993 AISC LRFD 2nd Edition1999 AISC LRFD 3rd Edition*

ASD and LRFDMajor DifferencesLoad Combinations and load factorsASD results are based on the stresses and LRFD results are based on the forces and moments capacityStatic analysis is acceptable for ASD but nonlinear geometric analysis is required for LRFDBeams and flexural membersCb computation*

ASD Load Combinations1.0D + 1.0L0.75D + 0.75L + 0.75W0.75D + 0.75L + 0.75E

D=dead loadL=live loadW=wind loadE=earthquake load*

ASD Load CombinationsOr you can use following load combinations with theparameter ALSTRINC to account for the 1/3 allowableincrease for the wind and seismic load

1.0D + 1.0L1.0D + 1.0L + 1.0W1.0D + 1.0L + 1.0E

PARAMETER$ ALSTRINC based on the % increaseALSTRINC 33.333 LOADINGS 2 3*

LRFD Load Combinations1.4D1.2D + 1.6L1.2D + 1.6W + 0.5L1.2D 1.0E + 0.5L0.9D (1.6W or 1.0E)


Deflection Load Combinationsfor ASD and LRFD1.0D + 1.0L1.0D + 1.0L + 1.0W1.0D + 1.0L + 1.0E


Forces and StressesASD=actual stress values are compared to the AISCallowable stress values

LRFD=actual forces and momentsare compared to the AISClimiting forces and momentscapacity*

ASTM Steel GradeComparison is between Table 1 of the AISC ASD 9th Edition on Page 1-7 versus Table 2-1 of the AISC LRFD 3rd Edition on Page 2-24A529 Gr. 42 of ASD, not available in LRFDA529 Gr. 50 and 55 are new in LRFDA441 not available in LRFDA572 Gr. 55 is new in LRFDA618 Gr. I, II, & III are new in LRFDA913 Gr. 50, 60, 65, & 70 are new in LRFDA992 (Fy = 50, Fu = 65) is new in LRFD (new standard)A847 is new in LRFD*

Slenderness RatioCompressionKL/r 200

TensionL/r 300*

Tension MembersCheck L/r ratioCheck Tensile Strength based on the cross-sections Gross AreaCheck Tensile Strength based on the cross-sections Net Area*

Tension MembersASDft = FX/Ag FtGross Areaft = FX/Ae FtNet Area

LRFD

Pu = FX t Pn = t Ag Fyt = 0.9 for Gross AreaPu = FX t Pn = t Ae Fut = 0.75 for Net Area*

Tension MembersASD(ASD Section D1)Gross AreaFt = 0.6FyNet AreaFt = 0.5Fu

LRFD(LRFD Section D1)Gross Areat Pn = t Fy Ag t = 0.9Net Areat Pn = t Fu Ae t = 0.75*

Compare ASD to LRFDASD1.0D + 1.0LLRFD1.2D + 1.6L

0.6Fy (ASD) (1.5) = 0.9Fy (LRFD)

0.5Fu (ASD) (1.5) = 0.75Fu (LRFD)

ASD (1.5) = LRFD*

Tension Members*

Tension MembersMember is 15 feet longFixed at the top of the member and free at the bottomLoadings are:Self weight400 kips tension force at the free endLoad combinations based on the ASD and LRFD codesSteel grade is A992Design based on the ASD and LRFD codes*

Tension MembersASD

W18x46Actual/Allowable Ratio = 0.989

LRFD

W10x49Actual/Limiting Ratio = 0.989*

Tension MembersASDW18x46Area = 13.5 in.2FX = 400.688 kipsRatio = 0.989

LRFDW10x49Area = 14.4 in.2FX = 640.881 kipsRatio = 0.989*

Tension MembersLoad Factor difference between LRFD and ASD640.881 / 400.688 = 1.599Equation Factor difference between LRFD and ASDLRFD = (1.5) ASD

Estimate required cross-sectional area for LRFD

LRFDW10x49Area = 14.4 in.2*

Tension MembersCode Check based on the ASD9 and using W10x49FX = 400.734 kipsRatio = 0.928

Load Factor difference between LRFD and ASD640.881 / 400.734 = 1.599

LRFDW10x49 Ratio = 0.989*

Tension MembersASDExample # 1Live Load = 400 kipsW18x46Actual/Allowable Ratio = 0.989LRFDExample # 1Live Load = 400 kipsW10x49Actual/Limiting Ratio = 0.989Example # 2Dead Load = 200 kipsLive Load = 200 kipsW14x43Actual/Limiting Ratio = 0.989Code check W14x43 based on the ASD9W14x43Actual/Allowable Ratio = 1.06*

Compression MembersCheck KL/r ratioCompute Flexural-Torsional Buckling and Equivalent (KL/r)eFind Maximum of KL/r and (KL/r)eCompute Qs and Qa based on the b/t and h/tw ratiosBased on the KL/r ratio, compute allowable stress in ASD or limiting force in LRFD*

Compression MembersASD

fa = FX/Ag Fa

LRFD

Pu = FX c Pn = c Ag FcrWhere c = 0.85*

Limiting Width-Thickness Ratiosfor Compression ElementsASD

b/t =h/tw =

LRFD

b/t =h/tw =*

Limiting Width-Thickness Ratiosfor Compression ElementsAssume E = 29000 ksiASD

b/t =h/tw =

LRFD

b/t =h/tw =*

Compression MembersASD KL/r Cc(ASD E2-1 or A-B5-11)

LRFD(LRFD A-E3-2)*

Compression MembersASD KL/r > Cc(ASD E2-2)

LRFD(LRFD A-E3-3)*

Compression MembersLRFD

*

Compression MembersASDLRFD

Fcr / Fa = 1.681

LRFD Fcr = ASD Fa 1.681

*


(ASD C-E2-2)

LRFD

c = Maximum of ( cy , cz , e )*

Compression MembersLRFDWhere:*

Compression MembersFlexural-Torsional Buckling*

Qs ComputationASD

LRFD

*

Qs ComputationAssume E = 29000 ksiASD

LRFD

*

Qs ComputationASD

LRFD

*

Qs ComputationAssume E = 29000 ksiASD

LRFD

*

Qa ComputationASD

LRFD

*

Compression Members*

Compression MembersMember is 15 feet longFixed at the bottom of the column and free at the topLoadings are:Self weight100 kips compression force at the free endLoad combinations based on the ASD and LRFD codesSteel grade is A992Design based on the ASD and LRFD codes*



LRFD


Compression MembersASDW10x49Area = 14.4 in.2FX = 100.734 kipsRatio = 0.941

LRFDW10x54Area = 15.8 in.2FX = 160.967 kipsRatio = 0.944*

Compression MembersLoad Factor difference between LRFD and ASD160.967 / 100.734 = 1.598Equation Factor difference between LRFD and ASDLRFD Fcr = (1.681) ASD Fa

Estimate required cross-sectional area for LRFD

LRFDW10x54Area = 15.8 inch*

Compression MembersCode Check based on the ASD9 and use W10x54FX = 100.806 kipsRatio = 0.845



Compression MembersASDExample # 1Live Load = 100 kipsW10x49Actual/Allowable Ratio = 0.941LRFDExample # 1Live Load = 100 kipsW10x54Actual/Limiting Ratio = 0.944Example # 2Dead Load = 50 kipsLive Load = 50 kipsW10x49Actual/Limiting Ratio = 0.921Code check W10x49 based on the ASD9W10x49Actual/Allowable Ratio = 0.941*

Flexural MembersBased on the b/t and h/tw ratios determine the compactness of the cross-sectionClassify flexural members as Compact, Noncompact, or SlenderWhen noncompact section in ASD, allowable stress Fb is computed based on the l/rt ratio. l is the laterally unbraced length of the compression flange. Also, Cb has to be computedWhen noncompact or slender section in LRFD, LTB, FLB, and WLB are checkedLTB for noncompact or slender sections is computed using Lb and Cb. Lb is the laterally unbraced length of the compression flange*

Flexural MembersASD

fb = MZ/SZ Fb

LRFD

Mu = MZ b MnWhere b = 0.9*


LRFD

Assume E = 29000 ksi*

Flexural MembersCompact SectionASD(ASD F1-1)

Fb = 0.66Fy

LRFD(LRFD A-F1-1)

b Mn = b Mp = b Fy ZZ 1.5Fy SZWhere b = 0.9*

Flexural MembersCompact Section*Braced at 1/3 Points

Flexural MembersCompact SectionMember is 12 feet longFixed at both ends of the memberLoadings are:Self weight15 kips/ft uniform loadLoad combinations based on the ASD and LRFD codesSteel grade is A992Braced at the 1/3 PointsDesign based on the ASD and LRFD codes*

Flexural MembersCompact SectionASD


LRFD


Flexural MembersCompact SectionASDW18x40Sz = 68.4 in.3MZ = 2165.777 inch-kipsRatio = 0.959

LRFDW18x40Zz = 78.4 in.3MZ = 3462.933 inch-kipsRatio = 0.982*

Flexural MembersCompact SectionLoad Factor difference between LRFD and ASD3462.933 / 2165.777 = 1.5989Equation Factor difference between LRFD and ASDLRFD = (0.66Sz)(1.5989) / (0.9Zz) ASD

Zz

LRFDW18x40Zz = 78.4 in.3*

Flexural MembersCompact SectionCode Check based on the ASD9, Profile W18x40MZ = 2165.777 inch-kipsRatio = 0.959



Flexural MembersCompact SectionASDExample # 1Live Load = 15 kips/ftW18x40Actual/Allowable Ratio = 0.959LRFDExample # 1Live Load = 15 kips/ftW18x40Actual/Limiting Ratio = 0.982Example # 2Dead Load = 7.5 kips/ftLive Load = 7.5 kips/ftW18x40Actual/Limiting Ratio = 0.859Code check W18x40 based on the ASD9W18x40Actual/Allowable Ratio = 0.959*

Flexural MembersNoncompact SectionASDBased on b/t, d/tw and h/tw determine if the section is noncompactCompute CbCompute QsBased on the l/rt ratio, compute allowable stress FbLaterally unbraced length of the compression flange (l) has a direct effect on the equations of the noncompact section*

Flexural MembersNoncompact SectionASD

fb = MZ/SZ Fb

LRFD

Mu = MZ b MnWhere b = 0.9*


LRFD*

Limiting Width-Thickness Ratiosfor Compression ElementsAssume E = 29000 ksiASD

LRFD*


(ASD F1-3)

(ASD F1-2)

ASD Equations F1-6, F1-7, and F1-8 must to be checked.*


When

(ASD F1-6)*


When

(ASD F1-7)*


For any value of l/rT

(ASD F1-8)*

Flexural MembersNoncompact SectionLRFD

LTB, Lateral-Torsional BucklingFLB, Flange Local BucklingWLB, Web Local Buckling*

Flexural MembersNoncompact SectionLRFDLTBCompute CbBased on the Lb, compute limiting moment capacity. Lb is the lateral unbraced length of the compression flange, = Lb/ryLb has a direct effect on the LTB equations for noncompact and slender sectionsFLBCompute limiting moment capacity based on the b/t ratio of the flange, = b/tWLBCompute limiting moment capacity based on the h/tw ratio of the web, = h/tw*

Flexural MembersNoncompact SectionLRFDLTB(Table A-F1.1)For p < r

(LRFD A-F1-2)

Where:Mp = Fy Zz 1.5Fy SzMr = FLSzFL = Smaller of (Fyf Fr) or Fyw = Lb/ry

p =*

Flexural MembersNoncompact SectionLRFDLTB(Table A-F1.1)

Where:

r =

X1 =

X2 =*

Flexural MembersNoncompact SectionLRFDFLB(Table A-F1.1)For p < r

(LRFD A-F1-3)Where:Mp = Fy Zz 1.5Fy SzMr = FLSzFL = Smaller of (Fyf Fr) or Fyw = b/tp =r =*

Flexural MembersNoncompact SectionLRFDWLB(Table A-F1.1)For p < r

(LRFD A-F1-3)Where:Mp = Fy Zz 1.5Fy SzMr = Re Fy SzRe = 1.0for non-hybrid girder*

Flexural MembersNoncompact SectionLRFDWLB(Table A-F1.1)

= h/tw

p =

r =*


LRFD*

Flexural MembersNoncompact Section*

Flexural MembersNoncompact SectionMember is 12 feet longPin at the start of the memberRoller at the end of the memberCross-section is W12x65Loadings are:Self weight12 kips/ft uniform loadLoad combinations based on the ASD and LRFD codesSteel grade is A992Check code based on the ASD and LRFD codes*

Flexural MembersNoncompact SectionASDW12x65Cb = 1.0Actual/Allowable Ratio = 0.988LRFDW12x65Cb = 1.136Actual/Limiting Ratio = 0.971Code check is controlled by FLB.Cb = 1.0Actual/Limiting Ratio = 0.973*

Flexural MembersNoncompact SectionASDExample # 1Live Load = 12 kips/ftW12x65Actual/Allowable Ratio = 0.988LRFDExample # 1Live Load = 12 kips/ftW12x65Actual/Limiting Ratio = 0.971Example # 2Dead Load = 6 kips/ftLive Load = 6 kips/ftW12x65Actual/Limiting Ratio = 0.85Code check W12x65 based on the ASD9W12x65Actual/Allowable Ratio = 0.988*

Design for ShearASD

fv = FY/Aw Fv = 0.4Fy(ASD F4-1)

LRFD

Vu = FY vVn = v0.6Fyw Aw(LRFD F2-1)Where v = 0.9*

Design for ShearAssume E = 29000 ksiASD

fv = FY/Aw Fv = 0.4Fy(ASD F4-1)

LRFD

Vu = FY vVn = v0.6Fyw Aw(LRFD F2-1)Where v = 0.9*

Design for ShearASD

fv = FY/Ay (ASD F4-2)

LRFD

Vu = FY vVn = v(LRFD F2-2)

Where v = 0.9*

Design for ShearLRFD

Vu = FY vVn = v(LRFD F2-3)

Where v = 0.9*

Design for Shear*Braced at 1/3 Points

Design for ShearSame as example # 3 which is used for design of flexural member with compact sectionMember is 12 feet longFixed at both ends of the memberLoadings are:Self weight15 kips/ft uniform loadLoad combinations based on the ASD and LRFD codesSteel grade is A992Braced at the 1/3 PointsDesign based on the ASD and LRFD codes*

Design for ShearASD(Check shear at the end of the member, equation F4-1 Y)


LRFD(Check shear at the end of the member, equation A-F2-1 Y)


Design for ShearASDW18x40Ay = 5.638 in.2FY = 90.241 kipsRatio = 0.8

LRFDW18x40Ay = 5.638 in.2FY = 144.289 kipsRatio = 0.948*

Design for ShearCode Check based on the ASD9, Profile W18x40FY = 90.241 kipsRatio = 0.8Load Factor difference between LRFD and ASD144.289 / 90.241 = 1.5989Equation Factor difference between LRFD and ASDLRFD = (0.4)(1.5989) /(0.6)(0.9) ASD


Design for ShearASDExample # 1Live Load = 15 kips/ftW18x40Actual/Allowable Ratio = 0.8LRFDExample # 1Live Load = 15 kips/ftW18x40Actual/Limiting Ratio = 0.948Example # 2Dead Load = 7.5 kips/ftLive Load = 7.5 kips/ftW18x40Actual/Limiting Ratio = 0.83Code check W18x40 based on the ASD9W18x40Actual/Allowable Ratio = 0.8*

Combined ForcesASD fa /Fa > 0.15

(ASD H1-1)

(ASD H1-2)

LRFD Pu /Pn 0.2(LRFD H1-1a)*

Combined ForcesASD fa /Fa 0.15

(ASD H1-1)

LRFD Pu /Pn < 0.2

(LRFD H1-1a)*

Combined Forces*

Combined Forces3D Simple Frame3 Bays in X direction3 @ 15 ft2 Bays in Z direction2 @ 30 ft2 Floors in Y direction2 @ 15 ftLoadingsSelf weight of the SteelSelf weight of the Slab62.5 psfOther dead loads15.0 psfLive load on second floor50.0 psfLive load on roof20.0 psfWind load in the X direction20.0 psfWind load in the Z direction20.0 psf*

Combined ForcesASD

< Active Units Weight Unit = KIP Length Unit = INCH >< >< Steel Take Off Itemize Based on the PROFILE >< Total Length, Volume, Weight, and Number of Members >< >< Profile Names Total Length Total Volume Total Weight # of Members >< W10x33 2.1600E+03 2.0974E+04 5.9418E+00 12 >< W12x58 1.4400E+03 2.4480E+04 6.9352E+00 4 >< W12x65 1.4400E+03 2.7504E+04 7.7919E+00 4 >< W12x72 2.1600E+03 4.5576E+04 1.2912E+01 12 >< W6x9 3.2400E+03 8.6832E+03 2.4600E+00 18 >< W8x40 1.4400E+03 1.6848E+04 4.7730E+00 4 >< W8x48 1.4400E+03 2.0304E+04 5.7521E+00 4 >

>< ACTIVE UNITS WEIGHT KIP LENGTH INCH >< >< TOTAL LENGTH, WEIGHT AND VOLUME FOR SPECIFIED MEMBERS >< >< LENGTH = 1.3320E+04 WEIGHT = 4.6566E+01 VOLUME = 1.6437E+05 >>*

Combined ForcesLRFD

< Active Units Weight Unit = KIP Length Unit = INCH >< >< Steel Take Off Itemize Based on the PROFILE >< Total Length, Volume, Weight, and Number of Members >< >< Profile Names Total Length Total Volume Total Weight # of Members >< W10x33 3.6000E+03 3.4956E+04 9.9030E+00 16 >< W10x39 1.4400E+03 1.6560E+04 4.6914E+00 4 >< W10x49 7.2000E+02 1.0368E+04 2.9373E+00 4 >< W12x45 1.4400E+03 1.9008E+04 5.3850E+00 4 >< W6x9 3.2400E+03 8.6832E+03 2.4600E+00 18 >< W8x31 1.4400E+03 1.3147E+04 3.7246E+00 4 >< W8x40 1.4400E+03 1.6848E+04 4.7730E+00 8 >< >


Combined ForcesASD

WEIGHT = 46.566 kips

LRFD

WEIGHT = 33.874 kips*

Deflection DesignASD



Deflection DesignLRFD



Deflection DesignASD

WEIGHT = 46.933 kips

LRFD

WEIGHT = 38.345 kips*

Compare Design without and with Deflection DesignASDWithout Deflection Design WEIGHT = 46.566 kipsWith Deflection Design WEIGHT = 46.933 kips

LRFDWithout Deflection Design WEIGHT = 33.874 kipsWith Deflection Design WEIGHT = 38.345 kips*

Design same example based onCb = 1.0Code and deflection design with Cb = 1.0

ASDCompute Cb WEIGHT = 46.933 kipsSpecify Cb = 1.0 WEIGHT = 51.752 kips

LRFDCompute Cb WEIGHT = 38.345 kipsSpecify Cb = 1.0 WEIGHT = 48.421 kips*

Design Similar example based onCb = 1.0 and LL5Code and deflection design with Cb = 1.0 and increase the live load by a factor of 5.Area loads are distributed using two way option instead of one wayAlso change the 2 bays in the Z direction from 30 ft to 15 ft.

ASDWEIGHT = 25.677 kips

LRFDWEIGHT = 22.636 kips

Difference = 3.041 kips*

Design Similar example based onCb = 1.0 and LL10Code and deflection design with Cb = 1.0 and increase the live load by a factor of 10.Area loads are distributed using two way option instead of one wayAlso change the 2 bays in the Z direction from 30 ft to 15 ft.

ASDWEIGHT = 31.022 kips

LRFDWEIGHT = 29.051 kips

Difference = 1.971 kips*

Stiffness AnalysisversusNonlinear AnalysisStiffness Analysis Load Combinations or Form Loads can be used.Nonlinear Analysis Form Loads must be used. Load Combinations are not valid.Nonlinear Analysis Specify type of Nonlinearity.Nonlinear Analysis Specify Maximum Number of Cycles.Nonlinear Analysis Specify Convergence Tolerance.*

Nonlinear AnalysisCommandsNONLINEAR EFFECTTENSION ONLYCOMPRESSION ONLYGEOMETRY AXIALMAXIMUM NUMBER OF CYCLESCONVERGENCE TOLERANCE

NONLINEAR ANALYSIS*

Design using Nonlinear AnalysisInput File # 1Geometry, Material Type, Properties, Loading SW, LL, and WLFORM LOAD A FROM SW 1.4FORM LOAD B FROM SW 1.2 LL 1.6FORM LOAD C FROM SW 1.2 WL 1.6 LL 0.5FORM LOAD D FROM SW 0.9 WL 1.6DEFINE PHYSICAL MEMBERSPARAMETERSMEMBER CONSTRAINTSLOAD LIST A B C D$ Activate only the FORM loadsSTIFFNESS ANALYSISSAVE*

Design using Nonlinear AnalysisInput File # 2RESTORELOAD LIST A B C DSELECT MEMBERSSMOOTH PHYSICAL MEMBERSDELETE LOADINGS A B C DSELF WEIGHT LOADING RECOMPUTEFORM LOAD A FROM SW 1.4FORM LOAD B FROM SW 1.2 LL 1.6FORM LOAD C FROM SW 1.2 WL 1.6 LL 0.5FORM LOAD D FROM SW 0.9 WL 1.6LOAD LIST A B C DSTIFFNESS ANALYSISCHECK MEMBERSSTEEL TAKE OFFSAVE*

Design using Nonlinear AnalysisInput File # 3RESTORELOAD LIST A B C DSELECT MEMBERSSMOOTH PHYSICAL MEMBERSDELETE LOADINGS A B C DSELF WEIGHT LOADING RECOMPUTEFORM LOAD A FROM SW 1.4FORM LOAD B FROM SW 1.2 LL 1.6FORM LOAD C FROM SW 1.2 WL 1.6 LL 0.5FORM LOAD D FROM SW 0.9 WL 1.6*

Design using Nonlinear AnalysisInput File # 3 (continue)NONLINEAR EFFECTGEOMETRY ALL MEMBERSMAXIMUM NUMBER OF CYCLESCONVERGENCE TOLERANCE DISPLACEMENTLOAD LIST A B C DNONLINEAR ANALYSISCHECK MEMBERSSTEEL TAKE OFFSAVE*

General Comparison between AISC LRFD and ASD

Questions*

General Comparison between AISC LRFD and ASDGeneral Comparison between AISC LRFD and ASD5/25/2005GT STRUDL, CASE Center*GT STRUDL, CASE CenterGeneral Comparison between AISC LRFD and ASDGeneral Comparison between AISC LRFD and ASD5/25/2005GT STRUDL, CASE Center*GT STRUDL, CASE CenterGeneral Comparison between AISC LRFD and ASDGeneral Comparison between AISC LRFD and ASD5/25/2005GT STRUDL, CASE Center*GT STRUDL, CASE CenterGeneral Comparison between AISC LRFD and ASDGeneral Comparison between AISC LRFD and ASD5/25/2005GT STRUDL, CASE Center*GT STRUDL, CASE Center

asd vs lrfd_forwebsite

Documents