Tài liệu General Comparison between AISC LRFD and ASD doc

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... ASD

General Comparison between

AISC LRFD and ASD

Hamid Zand

GT STRUDL Users Group

Las Vegas, Nevada June 22-25, 2005

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

AISC Steel Design Manuals

• 1963 AISC ASD 6 th Edition

• 1969 AISC ASD 7 th Edition

• 1978 AISC ASD 8 th Edition

• 1989 AISC ASD 9 th Edition

• 1986 AISC LRFD 1 st Edition

• 1993 AISC LRFD 2 nd Edition

• 1999 AISC LRFD 3 rd Edition

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

• C b computation

ASD Load Combinations

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

Deflection Load Combinations

for ASD and LRFD

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

ASTM Steel Grade

• Comparison is between Table 1 of the AISC ASD 9 th Edition

on Page 1-7 versus Table 2-1 of the AISC LRFD 3 rd 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 (F y = 50, F u = 65) is new in LRFD (new standard)

• A847 is new in LRFD

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

Tension Members

ASD

f t = FX/A g ≤ F t Gross Area

f t = FX/A e ≤ F t Net Area

LRFD

P u = FX ≤ ϕ t P n = ϕ t A g F y ϕt = 0.9 for Gross Area

P u = FX ≤ ϕ t P n = ϕ t A e F u ϕt = 0.75 for Net Area

Tension Members

X

Y Z

FIXED JOINT

o

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

.

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 Q s and Q a based on the b/t and h/t w

ratios

• Based on the KL/r ratio, compute allowable

stress in ASD or limiting force in LRFD

Limiting Width-Thickness Ratios

for Compression Elements

Limiting Width-Thickness Ratios

for Compression Elements

K L r C

3

3

22

33

F E



F E



F E



F

K L r

F E

/



/



K L r

K L r

F E

F E



 e y

e

F F



Compression Members

Y

-100.

o

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

.

.

.

• When noncompact section in ASD, allowable stress F b is

computed based on the l/r t ratio l is the laterally unbraced

length of the compression flange Also, C b 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 L b and C b L b is the laterally unbraced length of the compression flange

Limiting Width-Thickness Ratios

for Compression Elements

Flexural Members Compact Section

Flexural Members Compact Section

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

• 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

Flexural Members Compact Section

ASD

W18x40 Actual/Allowable Ratio = 0.959

LRFD

W18x40 Actual/Limiting Ratio = 0.982

Flexural Members Compact Section

Flexural Members Compact Section

Load Factor difference between LRFD and ASD

.

Flexural Members Compact Section

Code Check based on the ASD9, Profile W18x40

.

Flexural Members Compact Section

Flexural Members Noncompact Section

• Based on the l/r t ratio, compute allowable stress F b

• Laterally unbraced length of the compression flange (l)

has a direct effect on the equations of the noncompact section

Flexural Members Noncompact Section

Limiting Width-Thickness Ratios

for Compression Elements

Limiting Width-Thickness Ratios

for Compression Elements

Flexural Members Noncompact Section

Flexural Members Noncompact Section

l r

C F

b

b y

Flexural Members Noncompact Section

ASD

When

(ASD F1-7)

l r

C F

T

b y

Flexural Members Noncompact Section

Flexural Members Noncompact Section

LRFD

1 LTB, Lateral-Torsional Buckling

2 FLB, Flange Local Buckling

3 WLB, Web Local Buckling

Flexural Members Noncompact Section

LRFD

– LTB

the lateral unbraced length of the compression flange,

λ = Lb/ry

and slender sections

Flexural Members Noncompact Section

Flexural Members Noncompact Section

Flexural Members Noncompact Section

For λ p < λ ≤ λ r

(LRFD A-F1-3) Where:

Flexural Members Noncompact Section

For λ p < λ ≤ λ r

(LRFD A-F1-3) Where:

Flexural Members Noncompact Section

Flexural Members Noncompact Section

b

A B

m a x

m a x

a b s o l u t e v a l u e o f m o m e n t a t q u a r t e r p o i n t

a b s o l u t e v a l u e o f m o m e n t a t c e n t e r l i n e

Flexural Members Noncompact Section

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

• 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

Flexural Members Noncompact Section

C b = 1.0 Actual/Limiting Ratio = 0.973

Flexural Members Noncompact Section

Design for Shear

Design for Shear

Design for Shear

Design for Shear

Design for Shear

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

Design for Shear

W18x40 Actual/Allowable Ratio = 0.8

W18x40 Actual/Limiting Ratio = 0.948

Design for Shear

Design for Shear

Code Check based on the ASD9, Profile W18x40

.

Design for Shear

a a

m y b y a

e y

b y

m z b z a

f F

f F

a y

M M

M M

f F

f F

a a

M M

M M

u n

Combined Forces

Y

< 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 >

< 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 >

< 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 >

< 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 >

< 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 >

< 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 >

Compare Design without and with

Design same example based on

C b = 1.0

ASD

Specify C b = 1.0 WEIGHT = 51.752 kips

LRFD

Compute C b WEIGHT = 38.345 kips

Specify C = 1.0 WEIGHT = 48.421 kips

Design Similar example based on

• 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

Design Similar example based on

• 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

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.

Design using Nonlinear Analysis

Input File # 1

1 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

Design using Nonlinear Analysis

Input File # 2

Design using Nonlinear Analysis

Input File # 3

1 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

7 FORM LOAD ‘A’ FROM ‘SW’ 1.4

8 FORM LOAD ‘B’ FROM ‘SW’ 1.2 ‘LL’ 1.6

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

10 FORM LOAD ‘D’ FROM ‘SW’ 0.9 ‘WL’ 1.6

Design using Nonlinear Analysis

Input File # 3 (continue)

1 NONLINEAR EFFECT

2 GEOMETRY ALL MEMBERS

3 MAXIMUM NUMBER OF CYCLES

4 CONVERGENCE TOLERANCE DISPLACEMENT

5 LOAD LIST ‘A’ ‘B’ ‘C’ ‘D’

6 NONLINEAR ANALYSIS

7 CHECK MEMBERS

8 STEEL TAKE OFF

9 SAVE

General Comparison between AISC

LRFD and ASD

Questions