Tài liệu General Comparison between AISC LRFD and ASD doc

1
General Comparison between
AISC LRFD and ASD
Hamid Zand
GT 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 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
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
•
C
b
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 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
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
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
ϕ
t

= 0.9 for Gross Area
P
u
= FX ≤ ϕ
t
P
n
= ϕ
t
A
e
F
u
ϕ
t
= 0.75 for Net Area
14
Tension Members
ASD (ASD Section D1)
Gross Area F
t
= 0.6F
y
Net Area F
t
= 0.5F
u
LRFD (LRFD Section D1)
Gross Area ϕ
t

P
n
= ϕ
t
F
y
A
g
ϕ
t
= 0.9
Net Area ϕ
t
P
n
= ϕ
t
F
u
A
e
ϕ
t
= 0.75
15
Compare ASD to LRFD
ASD 1.0D + 1.0L
LRFD 1.2D + 1.6L
0.6F
y

(ASD) × (1.5) = 0.9F
y
(LRFD)
0.5F
u
(ASD) × (1.5) = 0.75F
u
(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
A r e a f o r L R F D
= × × × =
1 3 5
6 4 0 8 8 1
4 0 0 6 8 8
1 0
1 5
0 9 8 9
0 9 8 9
1 4 3 9 5.
.
.
.
.
.
.
.
21
Tension Members
Code Check based on the ASD9 and using W10x49
FX = 400.734 kips Ratio = 0.928
Load Factor difference between LRFD and ASD
640.881 / 400.734 = 1.599
LRFD W10x49 Ratio = 0.989
L R F D R a t i o c o m p u t e d f r o m A S D
= × × =
0 9 2 8
6 4 0 8 8 1
4 0 0 7 3 4

1 0
1 5
0 9 8 9.
.
.
.
.
.
22
Tension Members
ASD
Example # 1
Live Load = 400 kips
W18x46 Actual/Allowable Ratio = 0.989
LRFD
Example # 1
Live Load = 400 kips
W10x49 Actual/Limiting Ratio = 0.989
Example # 2
Dead Load = 200 kips
Live Load = 200 kips
W14x43 Actual/Limiting Ratio = 0.989
Code check W14x43 based on the ASD9
W14x43 Actual/Allowable Ratio = 1.06
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 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
24
Compression Members
ASD
f
a
= FX/A
g
≤ F
a
LRFD
P
u
= FX ≤ ϕ

c
P
n
= ϕ
c
A
g
F
cr
Where ϕ
c
= 0.85
25
Limiting Width-Thickness Ratios
for Compression Elements
ASD
b/t = h/t
w
=
LRFD
b/t = h/t
w
=
9 5 / F
y
0 5 6. /E F
y
2 5 3 / F
y
1 4 9. /E F

y