asd vs lrfd

1

General Comparison between

AISC LRFD and ASD

Hamid ZandGT STRUDL Users Group

Las Vegas, Nevada

June 22-25, 2005

2

AISC ASD and LRFD

• AISC = American Institute of Steel

Construction

• ASD = Allowable Stress Design

AISC Ninth Edition

• LRFD = Load and Resistance Factor Design

AISC Third Edition

3

AISC Steel Design Manuals

• 1963 AISC ASD 6th Edition




• 1986 AISC LRFD 1st Edition

• 1993 AISC LRFD 2nd Edition

• 1999 AISC LRFD 3rd Edition

4

ASD and LRFD

Major 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 load

L = live load

W = wind load

E = earthquake load

6

ASD Load Combinations

Or you can use following load combinations with the

parameter ALSTRINC to account for the 1/3 allowable

increase for the wind and seismic load

1. 1.0D + 1.0L

2. 1.0D + 1.0L + 1.0W

3. 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)

D = dead load

L = live load

W = wind load

E = earthquake load

8

Deflection Load Combinations

for ASD and LRFD

• 1.0D + 1.0L

• 1.0D + 1.0L + 1.0W

• 1.0D + 1.0L + 1.0E

D = dead load

L = live load

W = wind load

E = earthquake load

9

Forces and Stresses

• ASD = actual stress values are compared to the AISC

allowable stress values

• LRFD = actual forces and moments

are compared to the AISC

limiting forces and moments

capacity

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

• Compression

KL/r ≤ 200

• Tension

L/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

ASD

ft = 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 + 1.0L

LRFD 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

ASD

W18x46 Area = 13.5 in.2

FX = 400.688 kips Ratio = 0.989

LRFD

W10x49 Area = 14.4 in.2

FX = 640.881 kips Ratio = 0.989

20

Tension Members

Load Factor difference between LRFD and ASD

640.881 / 400.688 = 1.599

Equation Factor difference between LRFD and ASD

LRFD = (1.5) × ASD

Estimate required cross-sectional area for LRFD

LRFD W10x49 Area = 14.4 in.2

Area for LRFD = × × × =135640881

400 688

10

15

0 989

0 98914 395.

.

.

.

.

.

..

21

Tension Members

Code Check based on the ASD9 and using W10x49

FX = 400.734 kips Ratio = 0.928


640.881 / 400.734 = 1.599

LRFD W10x49 Ratio = 0.989

LRFD Ratio computed from ASD = × × =0 928640881

400 734

10

150 989.

.

.

.

..

22

Tension Members

ASD

Example # 1

Live Load = 400 kips


LRFD

Example # 1



Example # 2

Dead Load = 200 kips



Code 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 Ratios

for Compression Elements

ASD

b/t = h/tw =

LRFD

b/t = h/tw =

95 / Fy

056. /E Fy

253 / Fy

149. /E Fy

26



Assume E = 29000 ksi

ASD

b/t = h/tw =

LRFD

b/t = h/tw =

95 / Fy

9536. / Fy

253 / Fy

25374. / Fy

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 r

C

KL r

C

a

c

y

c c

=

−′

+′

−′

12

5

3

3

8 8

2

2

3

3

/

/ /

( )F Q FcrQ

yc= 0 658

2

. λ

Where ′ =CE

QFc

y

2 2π

Where λπ

c

yKL

r

F

E=

λ c Q ≤ 15.

28

Compression Members

ASD KL/r > C′c (ASD E2-2)

LRFD (LRFD A-E3-3)

( )F

E

KL ra =

12

23

2

2

π

/Where ′ =C

E

QFc

y

2 2π

λc Q > 15.

F Fcr

c

y=

08772

.

λWhere λ

πc

yKL

r

F

E=

29

Compression Members

LRFD

F Fcr

c

y=

08772

.

λWhere λ

πc

yKL

r

F

E=

F

KL

r

F

E

Fcr

y

y=

08772

.

π

( )F

E

KL rcr =

0877 2

2

.

/

π( )

FE

KL rcr =

20171

23

2

2

.

/

π

30

Compression Members

ASD LRFD

Fcr / Fa = 1.681

LRFD Fcr = ASD Fa × 1.681

( )F

E

KL ra =

12

23

2

2

π

/ ( )F

E

KL rcr =

20171

23

2

2

.

/

π

31

Compression Members

ASD

(ASD C-E2-2)

LRFD

λc = Maximum of ( λcy , λcz , λe )

KL rK L

r

K L

r

KL

r

y Y

y

z z

z e

/ , ,=

WhereKL

r

E

Fe e

= π

32

Compression Members

LRFD

Where:

λπ

cy

y y

y

yK L

r

F

E=

λπ

czz z

z

yK L

r

F

E=

λ e

y

e

F

F=

33

Compression Members

Flexural-Torsional Buckling

( )F

EC

K LGJ

I Ie

w

x x y z

= +

+

π 2

2

10.

34

Qs Computation

ASD

LRFD

When 95 195/ / / / /F k b t F ky c y c< <

Q b t F ks y c= −1293 0 00309. . ( / ) /

When 056 103. / / . /E F b t E Fy y< <

Q b t F Es y= −1415 0 74. . ( / ) /

( )k

h th t kc c= > =

4 0570 10

0.46

.

// , .if otherwise

35

Qs Computation


ASD

LRFD

When 95 195/ / / / /F k b t F ky c y c< <

Q b t F ks y c= −1293 0 00309. . ( / ) /

When 9536 1754. / / . /F b t Fy y< <

Q b t Fs y= −1415 0 004345. . ( / )

36

Qs Computation

ASD

LRFD

When b t F ky c/ / /≥ 195

( )[ ]Q k F b ts c y= 262002

/ /

When b t E Fy/ . /≥ 103

( )[ ]Q E F b ts y= 0 692

. / /

37

Qs Computation


ASD

LRFD

When b t F ky c/ / /≥ 195

( )[ ]Q k F b ts c y= 262002

/ /

When b t Fy/ . /≥ 1754

( )[ ]Q F b ts y= 200102

/ /

38

Qa Computation

ASD

LRFD

bt

f b t fbe = −

≤253

144 3.

( / )

b tE

f b t

E

fbe = −

≤191 1

0 34.

.

( / )

Assume ksiE bt

f b t fe= = −

2900032526

157 9

,. .

( / )

39

Compression Members

X

Y

Z FIXED JOINT

-100.o

40

Compression Members


• Fixed at the bottom of the column and free at the top

• Loadings are:

• Self weight

• 100 kips compression force at the free end




41

Compression Members

ASD


LRFD


42

Compression Members

ASD

W10x49 Area = 14.4 in.2

FX = 100.734 kips Ratio = 0.941

LRFD

W10x54 Area = 15.8 in.2

FX = 160.967 kips Ratio = 0.944

43

Compression Members


160.967 / 100.734 = 1.598


LRFD Fcr = (1.681) × ASD Fa

Estimate required cross-sectional area for LRFD

LRFD W10x54 Area = 15.8 inch

Area for LRFD = × × × × =14 4160 967

100 734

10

1681

10

085

0 941

0 94416 05.

.

.

.

.

.

.

.

..

44

Compression Members

Code Check based on the ASD9 and use W10x54

FX = 100.806 kips Ratio = 0.845


160.967 / 100.806 = 1.597


LRFD Ratio computed from ASD = × × × =0845160 967

100806

10

1681

10

0850 944.

.

.

.

.

.

..

45

Compression Members

ASD

Example # 1



LRFD

Example # 1



Example # 2

Dead Load = 50 kips

Live 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 MnWhere ϕb = 0.9

48



ASD

LRFD


d t Fw y/ /≤ 640

b t E Fy/ . /≤ 0 38 h t E Fw y/ . /≤ 376

b t Fy/ /≤ 65

b t Fy/ . /≤ 64 7 h t Fw y/ . /≤ 640 3

49

Flexural Members

Compact Section

ASD (ASD F1-1)

Fb = 0.66Fy

LRFD (LRFD A-F1-1)

ϕb Mn = ϕb Mp = ϕb Fy ZZ ≤ 1.5Fy SZ

Where ϕb = 0.9

50

Flexural Members

Compact Section

X

Y

Z

FIXED JOINT

-15.00

-15.00

o

o

FIXED JOINT

Braced at 1/3 Points

51

Flexural Members

Compact Section• Member is 12 feet long

• Fixed at both ends of the member

• Loadings are:

• Self weight

• 15 kips/ft uniform load



• Braced at the 1/3 Points


52

Flexural Members

Compact Section

ASD


LRFD


53

Flexural Members

Compact Section

ASD

W18x40 Sz = 68.4 in.3

MZ = 2165.777 inch-kips Ratio = 0.959

LRFD

W18x40 Zz = 78.4 in.3


54

Flexural Members

Compact Section


3462.933 / 2165.777 = 1.5989


LRFD = (0.66Sz)(1.5989) / (0.9Zz) × ASD

Zz

LRFD W18x40 Zz = 78.4 in.3

for LRFD = × × × =68 43462 933

2165777

0 66

0 9

0 959

0 98278 3.

.

.

.

.

.

..

55

Flexural Members

Compact Section

Code Check based on the ASD9, Profile W18x40



3462.933 / 2165.777 = 1.5989


LRFD Ratio computed from ASD = × × × =0 9593462 933

2165777

0 66

0 9

68 4

78 40 981.

.

.

.

.

.

..

56

Flexural Members

Compact SectionASD

Example # 1

Live Load = 15 kips/ft


LRFD

Example # 1



Example # 2

Dead Load = 7.5 kips/ft

Live Load = 7.5 kips/ft




57

Flexural Members

Noncompact 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

Flexural Members

Noncompact Section

ASD

fb = MZ/SZ ≤ Fb

LRFD

Mu = MZ ≤ ϕb MnWhere ϕb = 0.9

59



ASD

LRFD

65 95F b t Fy y< ≤

d t Fw y> 640

0 38 083. / .E F b t E Fy L< ≤

376 57. .E F h t E Fy w y< ≤

h t Fw b≤ 760

60


for Compression ElementsAssume E = 29000 ksi

ASD

LRFD

65 95F b t Fy y< ≤

d t Fw y> 640

64 7 1413. / / . /F b t Fy L< ≤

640 3 970 7. / . /F h t Fy w y< ≤

h t Fw b≤ 760

61

Flexural Members

Noncompact Section

ASD

(ASD F1-3)

(ASD F1-2)

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

F Fb

tFb y

f

f

y= −

0 79 0 0022

. .

( )If minimum orL L

b

F d A Fb c

f

y f y

> =

76 20000

62

Flexural Members

Noncompact Section

ASD

When

(ASD F1-6)

102 10 510 103 3×≤ ≤

×C

F

l

r

C

F

b

y T

b

y

( )F

F l r

CF F Qb

y T

b

y y s= −×

≤2

3 1530 100 6

2

3

/.

63

Flexural Members

Noncompact Section

ASD

When

(ASD F1-7)

l

r

C

FT

b

y

≥×510 103

( )F

C

l rF Qb

b

T

y s=×

≤170 10

0 63

2/

.

64

Flexural Members

Noncompact Section

ASD

For any value of l/rT

(ASD F1-8)FC

ld AF Qb

b

f

y s=×

≤12 10

0 63

/.

65

Flexural Members

Noncompact Section

LRFD

1. LTB, Lateral-Torsional Buckling

2. FLB, Flange Local Buckling

3. WLB, Web Local Buckling

66

Flexural Members

Noncompact SectionLRFD

– 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 noncompactand 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

Flexural Members

Noncompact Section

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 r

p

r p

p= − −−

−

≤λ λ

λ λ

176. E Fyf

68

Flexural Members

Noncompact Section

LRFD LTB (Table A-F1.1)

Where:

λr =

X1 =

X2 =

X

FX F

L

L1

221 1+ +

π

S

EGJA

z 2

4

2C

I

S

GJ

w

y

z

69

Flexural Members

Noncompact Section

LRFD FLB (Table A-F1.1)


(LRFD A-F1-3)

Where:


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

λ = b/t

λp =

λr =

( )M M M Mn p p r

p

r p

= − −−

−

λ λ

λ λ

0 38. E Fy

083. E FL

70

Flexural Members

Noncompact Section

LRFD WLB (Table A-F1.1)


(LRFD A-F1-3)

Where:


Mr = Re Fy Sz

Re = 1.0 for non-hybrid girder

( )M M M Mn p p r

p

r p

= − −−

−

λ λ

λ λ

71

Flexural Members

Noncompact Section

LRFD WLB (Table A-F1.1)

λ = h/tw

λp =

λr =

376. E Fy

57. E Fy

72

Flexural Members

Noncompact Section

ASD

LRFD

( ) ( )C M M M M

M M

M M M C

b

b

= + + ≤

<

=

175 105 0 3 2 3

10

1 2 1 2

2

1 2

1 2

. . . .

, .maxIf between and

CM

M M M M

M

M

M

b

A B C

A

B

C

=+ + +

=

=

= −

12 5

2 5 3 4 3

.

.

max

max

absolute value of moment at quarter point

absolute value of moment at centerline

absolute value of moment at three quarter point

73

Flexural Members

Noncompact Section

X

Y

Z

Roller

-12.00

-12.00

o

o

Pin

74

Flexural Members

Noncompact Section• 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




• Check code based on the ASD and LRFD codes

75

Flexural Members

Noncompact Section

ASD

W12x65 Cb = 1.0

Actual/Allowable Ratio = 0.988

LRFD

W12x65 Cb = 1.136

Actual/Limiting Ratio = 0.971

Code check is controlled by FLB.

Cb = 1.0 Actual/Limiting Ratio = 0.973

76

Flexural Members

Noncompact SectionASD

Example # 1



LRFD

Example # 1



Example # 2

Dead 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/ ≤ 380

h t E Fw yw/ . /≤ 2 45

78

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/ ≤ 380

h t Fw yw/ . /≤ 417 2

79

Design for Shear

ASD

fv = FY/Ay ≤ (ASD F4-2)

LRFD

Vu = FY ≤ ϕvVn = ϕv (LRFD F2-2)

Where ϕv = 0.9

h t Fw y/ > 380

2 45 307. / / . /E F h t E Fyw w yw< ≤

( )FF

C Fv

y

v y= ≤2 89

0 4.

.

0 62 45

.. /

/F A

E F

h tyw w

yw

w

80

Design for Shear

LRFD

Vu = FY ≤ ϕvVn = ϕv (LRFD F2-3)

Where ϕv = 0.9

307 260. / /E F h tyw w< ≤

( )A

E

h tw

w

4 522

.

/

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


• Fixed at both ends of the member

• Loadings are:

• Self weight




• Braced at the 1/3 Points


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

ASD

W18x40 Ay = 5.638 in.2

FY = 90.241 kips Ratio = 0.8

LRFD

W18x40 Ay = 5.638 in.2

FY = 144.289 kips Ratio = 0.948

85

Design for Shear

Code Check based on the ASD9, Profile W18x40

FY = 90.241 kips Ratio = 0.8


144.289 / 90.241 = 1.5989


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


LRFD Ratio computed from ASD = × × × =08144 289

90 241

0 4

0 6

10

0 90 948.

.

.

.

.

.

..

86

Design for Shear

ASD

Example # 1



LRFD

Example # 1



Example # 2

Dead Load = 7.5 kips/ft

Live 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)

f

F

C f

f

FF

C f

f

F

a

a

my by

a

ey

by

mz bz

a

ez

+

−

+

−

≤

1 1

10.

f

F

f

F

f

F

a

y

by

by

bz

bz0 610

..+ + ≤

P

P

M

M

M

M

u

n

uy

b ny

uz

b nzφ φ φ+ +

≤

8

910.

88

Combined Forces

ASD fa /Fa ≤ 0.15

(ASD H1-1)

LRFD Pu /ϕPn < 0.2

(LRFD H1-1a)

f

F

f

F

f

F

a

a

by

by

bz

bz

+ + ≤ 10.

P

P

M

M

M

M

u

n

uy

b ny

uz

b nz210

φ φ φ+ +

≤ .

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 Forces

ASD<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

< 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 Forces

LRFD<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


< >



< >


< 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 Design

ASD<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


< >



< >


< 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 >

< W14x43 1.4400E+03 1.8144E+04 5.1402E+00 4 >

< W14x48 1.4400E+03 2.0304E+04 5.7521E+00 4 >

< W6x9 3.2400E+03 8.6832E+03 2.4600E+00 18 >

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

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


< >


< >


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

95

Deflection Design

LRFD<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


< >



< >


< W10x33 2.1600E+03 2.0974E+04 5.9418E+00 12 >

< W10x49 1.4400E+03 2.0736E+04 5.8745E+00 8 >

< W10x54 7.2000E+02 1.1376E+04 3.2228E+00 4 >

< W12x40 1.4400E+03 1.6992E+04 4.8138E+00 4 >

< W14x43 2.8800E+03 3.6288E+04 1.0280E+01 8 >

< W14x48 1.4400E+03 2.0304E+04 5.7521E+00 4 >

< W6x9 3.2400E+03 8.6832E+03 2.4600E+00 18 >

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

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


< >


< >


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

96

Deflection Design

ASD


LRFD


97

Compare Design without and with

Deflection Design

ASD

Without Deflection Design WEIGHT = 46.566 kips

With Deflection Design WEIGHT = 46.933 kips

LRFD

Without Deflection Design WEIGHT = 33.874 kips

With Deflection Design WEIGHT = 38.345 kips

98

Design same example based on

Cb = 1.0

Code and deflection design with Cb = 1.0

ASD

Compute Cb WEIGHT = 46.933 kips

Specify Cb = 1.0 WEIGHT = 51.752 kips

LRFD

Compute Cb WEIGHT = 38.345 kips

Specify Cb = 1.0 WEIGHT = 48.421 kips

99

Design Similar example based on

Cb = 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 on

Cb = 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 Analysis

versus

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 Analysis

Commands

• NONLINEAR EFFECT• TENSION ONLY

• COMPRESSION ONLY

• GEOMETRY AXIAL

• MAXIMUM NUMBER OF CYCLES

• CONVERGENCE TOLERANCE

• NONLINEAR ANALYSIS

103

Design using Nonlinear Analysis

Input File # 11. Geometry, Material Type, Properties,

2. Loading ‘SW’, ‘LL’, and ‘WL’

3. FORM LOAD ‘A’ FROM ‘SW’ 1.4

4. FORM LOAD ‘B’ FROM ‘SW’ 1.2 ‘LL’ 1.6

5. FORM LOAD ‘C’ FROM ‘SW’ 1.2 ‘WL’ 1.6 ‘LL’ 0.5

6. FORM LOAD ‘D’ FROM ‘SW’ 0.9 ‘WL’ 1.6

7. DEFINE PHYSICAL MEMBERS

8. PARAMETERS

9. MEMBER CONSTRAINTS

10. LOAD LIST ‘A’ ‘B’ ‘C’ ‘D’ $ Activate only the FORM loads

11. STIFFNESS ANALYSIS

12. SAVE

104


Input File # 21. RESTORE

2. LOAD LIST ‘A’ ‘B’ ‘C’ ‘D’

3. SELECT MEMBERS

4. SMOOTH PHYSICAL MEMBERS

5. DELETE LOADINGS ‘A’ ‘B’ ‘C’ ‘D’

6. SELF WEIGHT LOADING RECOMPUTE






12. STIFFNESS ANALYSIS

13. CHECK MEMBERS

14. STEEL TAKE OFF

15. SAVE

105


Input File # 31. RESTORE


3. SELECT MEMBERS

4. SMOOTH PHYSICAL MEMBERS

5. DELETE LOADINGS ‘A’ ‘B’ ‘C’ ‘D’

6. SELF WEIGHT LOADING RECOMPUTE





106


Input File # 3 (continue)

1. NONLINEAR EFFECT

2. GEOMETRY ALL MEMBERS

3. MAXIMUM NUMBER OF CYCLES

4. CONVERGENCE TOLERANCE DISPLACEMENT


6. NONLINEAR ANALYSIS

7. CHECK MEMBERS

8. STEEL TAKE OFF

9. SAVE

107

General Comparison between AISC

LRFD and ASD

Questions

asd vs lrfd

Documents