ASD vs LRFD Comparison

1

General Comparison between AISC LRFD and ASD

Hamid ZandGT STRUDL Users Group

Las Vegas, NevadaJune 22-25, 2005

2

AISC ASD and LRFD• AISC = American Institute of Steel

Construction

• ASD = Allowable Stress DesignAISC Ninth Edition

• LRFD = Load and Resistance Factor DesignAISC Third Edition

3

AISC Steel Design Manuals

• 1963 AISC ASD 6th Edition• 1969 AISC ASD 7th Edition• 1978 AISC ASD 8th Edition• 1989 AISC ASD 9th Edition

• 1986 AISC LRFD 1st Edition• 1993 AISC LRFD 2nd Edition• 1999 AISC LRFD 3rd Edition

4

ASD and LRFDMajor Differences

• Load Combinations and load factors• ASD results are based on the stresses and

LRFD results are based on the forces and moments capacity

• Static analysis is acceptable for ASD but nonlinear geometric analysis is required for LRFD

• Beams and flexural members• Cb computation

5

ASD Load Combinations

• 1.0D + 1.0L• 0.75D + 0.75L + 0.75W• 0.75D + 0.75L + 0.75E

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

6

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. 1.0D + 1.0L2. 1.0D + 1.0L + 1.0W3. 1.0D + 1.0L + 1.0E

• PARAMETER $ ALSTRINC based on the % increase

• ALSTRINC 33.333 LOADINGS 2 3

7

LRFD Load Combinations

• 1.4D• 1.2D + 1.6L• 1.2D + 1.6W + 0.5L• 1.2D ± 1.0E + 0.5L• 0.9D ± (1.6W or 1.0E)


8

Deflection Load Combinationsfor ASD and LRFD

• 1.0D + 1.0L• 1.0D + 1.0L + 1.0W• 1.0D + 1.0L + 1.0E


9

Forces and Stresses

• ASD = actual stress values are compared to the AISCallowable stress values

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

10

ASTM Steel Grade• Comparison 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 2-24

• A529 Gr. 42 of ASD, not available in LRFD• A529 Gr. 50 and 55 are new in LRFD• A441 not available in LRFD• A572 Gr. 55 is new in LRFD• A618 Gr. I, II, & III are new in LRFD• A913 Gr. 50, 60, 65, & 70 are new in LRFD• A992 (Fy = 50, Fu = 65) is new in LRFD (new standard)• A847 is new in LRFD

11

Slenderness Ratio

• CompressionKL/r ≤ 200

• TensionL/r ≤ 300

12

Tension Members

• Check L/r ratio• Check Tensile Strength based on the cross-

section’s Gross Area• Check Tensile Strength based on the cross-

section’s Net Area

13

Tension Members

ASDft = FX/Ag ≤ Ft Gross Area

ft = FX/Ae ≤ Ft Net Area

LRFD

Pu = FX ≤ ϕt Pn = ϕt Ag Fy ϕt = 0.9 for Gross Area

Pu = FX ≤ ϕt Pn = ϕt Ae Fu ϕt = 0.75 for Net Area

14

Tension Members

ASD (ASD Section D1)

Gross Area Ft = 0.6Fy

Net Area Ft = 0.5Fu

LRFD (LRFD Section D1)

Gross Area ϕt Pn = ϕt Fy Ag ϕt = 0.9

Net Area ϕt Pn = ϕt Fu Ae ϕt = 0.75

15

Compare ASD to LRFD

ASD 1.0D + 1.0LLRFD 1.2D + 1.6L

0.6Fy (ASD) × (1.5) = 0.9Fy (LRFD)

0.5Fu (ASD) × (1.5) = 0.75Fu (LRFD)

ASD × (1.5) = LRFD

16

Tension Members

X

Y

Z

FIXED JOINT

-400.

o

17

Tension Members• Member is 15 feet long• Fixed at the top of the member and free at the bottom• Loadings are:

• Self weight• 400 kips tension force at the free end• Load combinations based on the ASD and

LRFD codes• Steel grade is A992• Design based on the ASD and LRFD codes

18

Tension Members

ASD

W18x46 Actual/Allowable Ratio = 0.989

LRFD

W10x49 Actual/Limiting Ratio = 0.989

19

Tension Members

ASDW18x46 Area = 13.5 in.2

FX = 400.688 kips Ratio = 0.989

LRFDW10x49 Area = 14.4 in.2

FX = 640.881 kips Ratio = 0.989

20

Tension MembersLoad Factor difference between LRFD and ASD

640.881 / 400.688 = 1.599Equation Factor difference between LRFD and ASD

LRFD = (1.5) × ASD

Estimate required cross-sectional area for LRFD

LRFD W10x49 Area = 14.4 in.2

A rea fo r L R F D 13 5 64 0 88 14 00 6 88

1 01 5

0 9 890 9 89

1 4 3 9 5. ..

.

...

.

21

Tension MembersCode Check based on the ASD9 and using W10x49

FX = 400.734 kips Ratio = 0.928

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

LRFD W10x49 Ratio = 0.989

L R F D R atio co m p uted fro m A S D 0 9 2 8 6 4 0 8 8 14 0 0 7 3 4

1 01 5

0 9 8 9. ..

.

..

22

Tension MembersASD

Example # 1Live Load = 400 kips

W18x46 Actual/Allowable Ratio = 0.989LRFD


W10x49 Actual/Limiting Ratio = 0.989Example # 2

Dead Load = 200 kipsLive Load = 200 kips

W14x43 Actual/Limiting Ratio = 0.989Code check W14x43 based on the ASD9


23

Compression Members

• Check KL/r ratio• Compute Flexural-Torsional Buckling and

Equivalent (KL/r)e

• Find Maximum of KL/r and (KL/r)e

• Compute Qs and Qa based on the b/t and h/tw ratios

• Based on the KL/r ratio, compute allowable stress in ASD or limiting force in LRFD

24

Compression Members

ASD

fa = FX/Ag ≤ Fa

LRFD

Pu = FX ≤ ϕc Pn = ϕc Ag Fcr

Where ϕc = 0.85

25

Limiting Width-Thickness Ratiosfor Compression Elements

ASD

b/t = h/tw =

LRFD

b/t = h/tw =

9 5 / F y

0 5 6. /E F y

2 5 3 / F y

1 4 9. /E F y

26


Assume E = 29000 ksi

ASD

b/t = h/tw =

LRFD

b/t = h/tw =

9 5 / F y

9 5 3 6. / F y

2 5 3 / F y

2 5 3 7 4. / F y

27

Compression Members

ASD KL/r ≤ C′c (ASD E2-1 or A-B5-11)

LRFD (LRFD A-E3-2)

F

QKL r

CF

KL rC

KL r

C

ac

y

c c

12

53

38 8

2

2

3

3

/

/ /

F Q FcrQ

yc 0 6 5 8

2

.

W here C EQ Fc

y

2 2

W here c

yKLr

FE

c Q 1 5.

28

Compression Members

ASD KL/r > C′c (ASD E2-2)

LRFD (LRFD A-E3-3)

F E

KL ra 12

2 3

2

2

/W here C E

Q Fcy

2 2

c Q 1 5.

F Fcrc

y

0 8 7 72

.

W here c

yKLr

FE

29

Compression Members

LRFDF Fcr

cy

0 8 7 72

. W here

cyKL

rFE

FKLr

FE

Fcr

y

y

0 87 72

.

F E

KL rcr

0 8 77 2

2

.

/

FE

K L rcr 20 17 123

2

2

./

30

Compression Members

ASD LRFD

Fcr / Fa = 1.681

LRFD Fcr = ASD Fa × 1.681

F E

KL ra

1 2

2 3

2

2

/ F

EK L rcr

2 0 1 7 123

2

2

./

31

Compression Members

ASD

(ASD C-E2-2)

LRFD

λc = Maximum of ( λcy , λcz , λe )

KL rK L

rK L

rKLr

y Y

y

z z

z e

/ , ,

W here KLr

EFe e

32

Compression Members

LRFDWhere:

cy

y y

y

yK Lr

FE

cz

z z

z

yK Lr

FE

ey

e

FF

33

Compression Members

Flexural-Torsional Buckling

F

E C

K LG J

I Iew

x x y z

2

2

1 0.

34

Qs Computation

ASD

LRFD

W hen 9 5 1 9 5/ / / / /F k b t F ky c y c

Q b t F ks y c 1 2 9 3 0 0 0 3 0 9. . ( / ) /

W hen 0 5 6 1 0 3. / / . /E F b t E Fy y

Q b t F Es y 1 41 5 0 7 4. . ( / ) /

k

h th t kc c

4 0 5 7 0 1 00 .46

.

// , .if o therw ise

35

Qs Computation


ASD

LRFD

W hen 9 5 1 9 5/ / / / /F k b t F ky c y c

Q b t F ks y c 1 2 9 3 0 00 3 0 9. . ( / ) /

W hen 9 5 3 6 1 7 5 4. / / . /F b t Fy y

Q b t Fs y 1 4 1 5 0 0 0 4 3 4 5. . ( / )

36

Qs Computation

ASD

LRFD

W hen b t F ky c/ / / 1 9 5

Q k F b ts c y 2 6 20 0 2/ /

W hen b t E F y/ . / 1 0 3

Q E F b ts y 0 6 9 2. / /

37

Qs Computation


ASD

LRFD

W hen b t F ky c/ / / 1 9 5

Q k F b ts c y 2 6 2 0 0 2/ /

W hen b t F y/ . / 1 7 5 4

Q F b ts y 2 0 0 1 0 2/ /

38

Qa Computation

ASD

LRFD

b tf b t f

be

25 3 1 4 4 3.

( / )

b t Ef b t

Ef

be

1 91 1 0 34. .

( / )

A ssum e ksiE b tf b t f

e

2 9 0 0 0 3 2 5 2 6 1 5 7 9, . .( / )

39

Compression Members

X

Y

Z FIXED JOINT

-100.o

40

Compression Members• Member is 15 feet long• Fixed at the bottom of the column and free at the top• Loadings are:

• Self weight• 100 kips compression force at the free end• Load combinations based on the ASD and

LRFD codes• Steel grade is A992• Design based on the ASD and LRFD codes

41

Compression Members

ASD


LRFD


42

Compression Members

ASDW10x49 Area = 14.4 in.2

FX = 100.734 kips Ratio = 0.941

LRFDW10x54 Area = 15.8 in.2

FX = 160.967 kips Ratio = 0.944

43

Compression MembersLoad Factor difference between LRFD and ASD


LRFD Fcr = (1.681) × ASD Fa

Estimate required cross-sectional area for LRFD

LRFD W10x54 Area = 15.8 inch

A rea fo r L R F D 14 4 16 0 9 6 710 0 7 3 4

1 01 6 8 1

1 00 8 5

0 94 10 94 4

1 6 0 5. ..

..

..

..

.

44

Compression MembersCode Check based on the ASD9 and use W10x54

FX = 100.806 kips Ratio = 0.845


LRFD W10x54 Ratio = 0.944

L R F D R atio co mp uted fro m A S D 0 8 4 5 1 6 0 9 6 710 0 8 0 6

1 01 6 81

1 00 85

0 94 4. ..

..

..

.

45

Compression MembersASD





Dead Load = 50 kipsLive Load = 50 kips



46

Flexural Members• Based on the b/t and h/tw ratios determine the compactness of

the cross-section• Classify flexural members as Compact, Noncompact, or Slender• When 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 computed

• When noncompact or slender section in LRFD, LTB, FLB, and WLB are checked

• LTB for noncompact or slender sections is computed using Lb and Cb. Lb is the laterally unbraced length of the compression flange

47

Flexural Members

ASD

fb = MZ/SZ ≤ Fb

LRFD

Mu = MZ ≤ ϕb Mn

Where ϕb = 0.9

48


ASD

LRFD


d t Fw y/ / 6 4 0

b t E F y/ . / 0 3 8 h t E Fw y/ . / 3 7 6

b t F y/ / 6 5

b t F y/ . / 6 4 7 h t Fw y/ . / 64 0 3

49

Flexural MembersCompact Section

ASD (ASD F1-1)

Fb = 0.66Fy

LRFD (LRFD A-F1-1)

ϕb Mn = ϕb Mp = ϕb Fy ZZ ≤ 1.5Fy SZWhere ϕb = 0.9

50


X

Y

Z

FIXED JOINT

-15.00

-15.00

o

o

FIXED JOINT

Braced at 1/3 Points

51


• Member is 12 feet long• Fixed at both ends of the member• Loadings are:

• Self weight• 15 kips/ft uniform load• Load combinations based on the ASD and

LRFD codes• Steel grade is A992• Braced at the 1/3 Points• Design based on the ASD and LRFD codes

52


ASD


LRFD


53


ASDW18x40 Sz = 68.4 in.3

MZ = 2165.777 inch-kips Ratio = 0.959

LRFDW18x40 Zz = 78.4 in.3


54



Equation Factor difference between LRFD and ASDLRFD = (0.66Sz)(1.5989) / (0.9Zz) × ASD

Zz

LRFDW18x40 Zz = 78.4 in.3

fo r L R F D 6 8 4 3 4 6 2 9 3 32 1 6 5 7 77

0 6 60 9

0 9 590 9 82

7 8 3. ..

..

.

..

55


Code Check based on the ASD9, Profile W18x40



LRFDW18x40 Ratio = 0.982

L R F D R atio co m puted fro m A S D 0 95 9 34 62 93 321 65 7 7 7

0 660 9

68 478 4

0 98 1. ..

..

.

..

56


ASDExample # 1

Live Load = 15 kips/ftW18x40 Actual/Allowable Ratio = 0.959

LRFDExample # 1

Live Load = 15 kips/ftW18x40 Actual/Limiting Ratio = 0.982

Example # 2Dead Load = 7.5 kips/ftLive Load = 7.5 kips/ft



57

Flexural MembersNoncompact Section

ASD• Based on b/t, d/tw and h/tw determine if the section is

noncompact• Compute Cb

• Compute Qs

• Based on the l/rt ratio, compute allowable stress Fb

• Laterally unbraced length of the compression flange (l) has a direct effect on the equations of the noncompact section

58


ASD

fb = MZ/SZ ≤ Fb

LRFD

Mu = MZ ≤ ϕb Mn

Where ϕb = 0.9

59


ASD

LRFD

6 5 9 5F b t Fy y

d t Fw y 6 4 0

0 3 8 0 8 3. / .E F b t E Fy L

3 7 6 5 7. .E F h t E Fy w y

h t Fw b 7 6 0

60



ASD

LRFD

6 5 9 5F b t Fy y

d t Fw y 6 4 0

6 4 7 1 4 1 3. / / . /F b t Fy L

6 4 0 3 9 7 0 7. / . /F h t Fy w y

h t Fw b 7 6 0

61


ASD

(ASD F1-3)

(ASD F1-2)

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

F Fbt

Fb yf

fy

0 7 9 0 0 0 22

. .

I f m inim um o rL L

b

F d A Fb cf

y f y

7 6 2 0 0 0 0

62


ASD

When

(ASD F1-6)

1 0 2 1 0 5 1 0 1 03 3

CF

lr

CF

b

y T

b

y

F

F l r

CF F Qb

y T

by y s

23 1 5 3 0 1 0

0 62

3

/.

63


ASD

When

(ASD F1-7)

lr

CFT

b

y

51 0 10 3

F

C

l rF Qb

b

Ty s

1 7 0 1 00 6

3

2/.

64


ASD

For any value of l/rT

(ASD F1-8)FC

ld AF Qb

b

fy s

1 2 1 00 6

3

/.

65


LRFD

1. LTB, Lateral-Torsional Buckling2. FLB, Flange Local Buckling3. WLB, Web Local Buckling

66


LRFD– LTB

• Compute Cb

• Based on the Lb, compute limiting moment capacity. Lb is the lateral unbraced length of the compression flange,λ = Lb/ry

• Lb has a direct effect on the LTB equations for noncompact and slender sections

– FLB• Compute limiting moment capacity based on the b/t ratio of

the flange, λ = b/t– WLB

• Compute limiting moment capacity based on the h/tw ratio of the web, λ = h/tw

67


LRFD LTB (Table A-F1.1)

For λp < λ ≤ λr

(LRFD A-F1-2)

Where:Mp = Fy Zz ≤ 1.5Fy Sz

Mr = FLSz FL = Smaller of (Fyf − Fr) or Fyw

λ = Lb/ry

λp =

M C M M M Mn b p p rp

r pp

1 76. E F yf

68


LRFD LTB (Table A-F1.1)

Where:

λr =

X1 =

X2 =

XF

X FL

L1

221 1

S

E G JA

z 2

42C

ISG J

w

y

z

69


LRFD FLB (Table A-F1.1)


(LRFD A-F1-3)Where:Mp = Fy Zz ≤ 1.5Fy Sz

Mr = FLSz FL = Smaller of (Fyf − Fr) or Fyw

λ = b/tλp =

λr =

M M M Mn p p rp

r p

0 3 8. E F y

0 8 3. E F L

70


LRFD WLB (Table A-F1.1)


(LRFD A-F1-3)Where:

Mp = Fy Zz ≤ 1.5Fy Sz

Mr = Re Fy Sz

Re = 1.0 for non-hybrid girder

M M M Mn p p rp

r p

71


LRFD WLB (Table A-F1.1)

λ = h/tw

λp =

λr =

3 7 6. E F y

5 7. E F y

72


ASD

LRFD

C M M M M

M MM M M C

b

b

1 7 5 1 0 5 0 3 2 3

1 0

1 2 1 22

1 2

1 2

. . . .

, .maxI f b e tw een and

CM

M M M MMMM

bA B C

A

B

C

1 2 52 5 3 4 3

..

max

max

ab so lute va lue o f m o m ent a t q uarter p o intab so lute value o f m o m ent a t cente rlineab so lute value o f m o m ent a t three q uarte r p o int

73


X

Y

Z

Roller

-12.00

-12.00

o

o

Pin

74


• Member is 12 feet long• Pin at the start of the member• Roller at the end of the member• Cross-section is W12x65• Loadings are:

• Self weight• 12 kips/ft uniform load• Load combinations based on the ASD and LRFD codes

• Steel grade is A992• Check code based on the ASD and LRFD codes

75


ASDW12x65 Cb = 1.0

Actual/Allowable Ratio = 0.988LRFD

W12x65 Cb = 1.136Actual/Limiting Ratio = 0.971

Code check is controlled by FLB.Cb = 1.0 Actual/Limiting Ratio = 0.973

76


ASDExample # 1

Live Load = 12 kips/ftW12x65 Actual/Allowable Ratio = 0.988

LRFDExample # 1

Live Load = 12 kips/ftW12x65 Actual/Limiting Ratio = 0.971

Example # 2Dead Load = 6 kips/ftLive Load = 6 kips/ft



77

Design for Shear

ASD

fv = FY/Aw ≤ Fv = 0.4Fy (ASD F4-1)

LRFD

Vu = FY ≤ ϕvVn = ϕv0.6Fyw Aw (LRFD F2-1)

Where ϕv = 0.9

h t Fw y/ 3 8 0

h t E Fw yw/ . / 2 45

78

Design for ShearAssume E = 29000 ksiASD

fv = FY/Aw ≤ Fv = 0.4Fy (ASD F4-1)

LRFD

Vu = FY ≤ ϕvVn = ϕv0.6Fyw Aw (LRFD F2-1)

Where ϕv = 0.9

h t Fw y/ 3 8 0

h t Fw yw/ . / 4 1 7 2

79

Design for ShearASD

fv = FY/Ay ≤ (ASD F4-2)

LRFD

Vu = FY ≤ ϕvVn = ϕv (LRFD F2-2)

Where ϕv = 0.9

h t Fw y/ 3 8 0

2 4 5 3 0 7. / / . /E F h t E Fyw w yw

FF

C Fvy

v y 2 8 9

0 4.

.

0 62 45

.. /

/F A

E F

h tyw wyw

w

80

Design for Shear

LRFD

Vu = FY ≤ ϕvVn = ϕv (LRFD F2-3)

Where ϕv = 0.9

3 0 7 26 0. / /E F h tyw w

A E

h tw

w

4 5 22

.

/

81

Design for Shear

X

Y

Z

FIXED JOINT

-15.00

-15.00

o

o

FIXED JOINT

Braced at 1/3 Points

82

Design for Shear• Same as example # 3 which is used for design of flexural

member with compact section• Member is 12 feet long• Fixed at both ends of the member• Loadings are:

• Self weight• 15 kips/ft uniform load• Load combinations based on the ASD and LRFD codes

• Steel grade is A992• Braced at the 1/3 Points• Design based on the ASD and LRFD codes

83

Design for Shear

ASD (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”)


84

Design for Shear

ASDW18x40 Ay = 5.638 in.2

FY = 90.241 kips Ratio = 0.8

LRFDW18x40 Ay = 5.638 in.2

FY = 144.289 kips Ratio = 0.948

85

Design for ShearCode Check based on the ASD9, Profile W18x40

FY = 90.241 kips Ratio = 0.8Load Factor difference between LRFD and ASD


LRFD = (0.4)(1.5989) /(0.6)(0.9) × ASD

LRFDW18x40 Ratio = 0.948

L R F D R atio co m puted fro m A S D 0 8 1 4 4 2 8990 2 4 1

0 40 6

1 00 9

0 94 8. ..

.

...

.

86

Design for ShearASD

Example # 1Live Load = 15 kips/ft


Example # 1Live Load = 15 kips/ft


Dead Load = 7.5 kips/ftLive Load = 7.5 kips/ft



87

Combined Forces

ASD fa /Fa > 0.15

(ASD H1-1)

(ASD H1-2)

LRFD Pu /ϕPn ≥ 0.2(LRFD H1-

1a)

fF

C f

fF

F

C f

fF

a

a

m y by

a

eyby

m z bz

a

ez

1 1

1 0.

fF

fF

fF

a

y

by

by

bz

bz0 61 0

..

PP

MM

MM

u

n

uy

b ny

u z

b nz

89

1 0.

88

Combined Forces

ASD fa /Fa ≤ 0.15

(ASD H1-1)

LRFD Pu /ϕPn < 0.2

(LRFD H1-1a)

fF

fF

fF

a

a

by

by

bz

bz

1 0.

PP

MM

MM

u

n

u y

b ny

uz

b nz21 0

.

89

Combined Forces

X

Y

Z

90

Combined Forces• 3D Simple Frame

• 3 Bays in X direction 3 @ 15 ft• 2 Bays in Z direction 2 @ 30 ft• 2 Floors in Y direction 2 @ 15 ft

• Loadings• Self weight of the Steel• Self weight of the Slab 62.5 psf• Other dead loads 15.0 psf• Live load on second floor 50.0 psf• Live load on roof 20.0 psf• Wind load in the X direction 20.0 psf• Wind load in the Z direction 20.0 psf

91

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 ><<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

92

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 >< ><<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


93

Combined Forces

ASD

WEIGHT = 46.566 kips

LRFD


94

Deflection DesignASD



95

Deflection DesignLRFD



96

Deflection Design

ASD


LRFD


97

Compare Design without and with Deflection Design

ASDWithout 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

98

Design same example based onCb = 1.0

Code 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

99

Design Similar example based onCb = 1.0 and LL×5

• Code 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 way

• Also change the 2 bays in the Z direction from 30 ft to 15 ft.

ASD WEIGHT = 25.677 kips

LRFD WEIGHT = 22.636 kips

Difference = 3.041 kips

100

Design Similar example based onCb = 1.0 and LL×10

• Code 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 way

• Also change the 2 bays in the Z direction from 30 ft to 15 ft.

ASD WEIGHT = 31.022 kips

LRFD WEIGHT = 29.051 kips

Difference = 1.971 kips

101

Stiffness Analysisversus

Nonlinear Analysis• Stiffness 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.

102

Nonlinear AnalysisCommands

• NONLINEAR EFFECT• TENSION ONLY• COMPRESSION ONLY• GEOMETRY AXIAL

• MAXIMUM NUMBER OF CYCLES• CONVERGENCE TOLERANCE

• NONLINEAR ANALYSIS

103

Design using Nonlinear AnalysisInput File # 1

1. Geometry, Material Type, Properties, 2. Loading ‘SW’, ‘LL’, and ‘WL’3. FORM LOAD ‘A’ FROM ‘SW’ 1.44. FORM LOAD ‘B’ FROM ‘SW’ 1.2 ‘LL’ 1.65. FORM LOAD ‘C’ FROM ‘SW’ 1.2 ‘WL’ 1.6 ‘LL’ 0.56. FORM LOAD ‘D’ FROM ‘SW’ 0.9 ‘WL’ 1.67. DEFINE PHYSICAL MEMBERS8. PARAMETERS9. MEMBER CONSTRAINTS10. LOAD LIST ‘A’ ‘B’ ‘C’ ‘D’ $ Activate only the FORM loads11. STIFFNESS ANALYSIS12. SAVE

104


1. RESTORE2. LOAD LIST ‘A’ ‘B’ ‘C’ ‘D’3. SELECT MEMBERS4. SMOOTH PHYSICAL MEMBERS5. DELETE LOADINGS ‘A’ ‘B’ ‘C’ ‘D’6. SELF WEIGHT LOADING RECOMPUTE7. FORM LOAD ‘A’ FROM ‘SW’ 1.48. FORM LOAD ‘B’ FROM ‘SW’ 1.2 ‘LL’ 1.69. FORM LOAD ‘C’ FROM ‘SW’ 1.2 ‘WL’ 1.6 ‘LL’ 0.510. FORM LOAD ‘D’ FROM ‘SW’ 0.9 ‘WL’ 1.611. LOAD LIST ‘A’ ‘B’ ‘C’ ‘D’12. STIFFNESS ANALYSIS13. CHECK MEMBERS14. STEEL TAKE OFF15. SAVE

105


1. RESTORE2. LOAD LIST ‘A’ ‘B’ ‘C’ ‘D’3. SELECT MEMBERS4. SMOOTH PHYSICAL MEMBERS5. DELETE LOADINGS ‘A’ ‘B’ ‘C’ ‘D’6. SELF WEIGHT LOADING RECOMPUTE7. FORM LOAD ‘A’ FROM ‘SW’ 1.48. FORM LOAD ‘B’ FROM ‘SW’ 1.2 ‘LL’ 1.69. FORM LOAD ‘C’ FROM ‘SW’ 1.2 ‘WL’ 1.6 ‘LL’ 0.510. FORM LOAD ‘D’ FROM ‘SW’ 0.9 ‘WL’ 1.6

106

Design using Nonlinear AnalysisInput File # 3 (continue)

1. NONLINEAR EFFECT2. GEOMETRY ALL MEMBERS3. MAXIMUM NUMBER OF CYCLES4. CONVERGENCE TOLERANCE DISPLACEMENT5. LOAD LIST ‘A’ ‘B’ ‘C’ ‘D’6. NONLINEAR ANALYSIS7. CHECK MEMBERS8. STEEL TAKE OFF9. SAVE

107

General Comparison between AISC LRFD and ASD

Questions

Documents

ASD vs LRFD Comparison