aisc night school 9 session 2
TRANSCRIPT
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Application of the AISC Seismic Design M
Session 2: General Design Requirement
Copyright © 2015
American Institute of Steel Construction
AISC Night School – Seismic Design Manual
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AISC Night School – Seismic Design Manual
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Session 2: General Design Requirement
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AISC Night School – Seismic Design Manual
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AISC Night School – Seismic Design Manual
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Copyright Materials
This presentation is protected by US and International Copyright laws. Reproduction, distribution,
display and use of the presentation without written permission of AISC is prohibited.
© The American Institute of Steel Construction 2015
The information presented herein is based on recognized engineering principles and is for general
information only. While it is believed to be accurate, this information should not be applied to any
specific application without competent professional examination and verification by a licensed
professional engineer. Anyone making use of this information assumes all liability arising from
such use.
AISC Night School – Seismic Design Manual
Session 2: General Design Requirements Part 2
September 28, 2015
Load combinations for seismic design will be discussed. The session will
present an overview of some of the 2010 Seismic Provisions including
application of the overstrength factor, member requirements, stability bracing
of beams and drift requirements. Examples from the Seismic Design Manual
will be presented to demonstrate concepts discussed in the session.
Course Description
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Application of the AISC Seismic Design M
Session 2: General Design Requirement
Copyright © 2015
American Institute of Steel Construction
AISC Night School – Seismic Design Manual
• Become familiar with load combinations considered for seismic design.
• Gain an understanding of the stability bracing requirements of beams per
the AISC Seismic Provisions.
• Gain an understanding of the application of the overstrength factor.
• Become familiar with the member design requirements of the AISC Seismic
Provisions through demonstrated design examples.
Learning Objectives
AISC Night School – Seismic Design Manual 8
Presented by
Thomas A. Sabol, Ph.D., S.E.
Principal at Englekirk Institutional
Los Angeles, CA
Application of the AISC Seismic Design Manual
Session 2: General Design Requirements Part 2
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Session 2: General Design Requirement
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AISC Night School – Seismic Design Manual
Application of the
AISC Seismic Design Manual
Session 2
AISC Night School – Seismic Design Manual 10
Last Session• Seismic Performance Goals
• Seismic Design Categories
• Seismic Performance Factors (e.g., R , O )
• Organization of AISC 341 Seismic Provisions
• Steel Material Properties (e.g., yield strength,R y )
• Welding Filler Metal Properties (e.g., Charpy V-Notch)
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AISC Night School – Seismic Design Manual 11
AISC Night School – Seismic Design Manual
B1 General Seismic Design Requirements
Seismic Provisions defer to applicable building code
for:
Required seismic strength with some exceptions (e.g.,
where expected strength is used to determine demand
on one member caused by another member )
Determination of Seismic Design Categories
Limitations on height and irregularities
Design story drift limits
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AISC Night School – Seismic Design Manual
B2 Loads and Load Combinations
Applicable Building Code determines:
Loads and load combinations for required strength of
steel seismic systems
Examples in SDM use “First Printing” of ASCE 7-10 and
may be different from your copy of ASCE 7-10
13
AISC Night School – Seismic Design Manual
“QE” has both apositive andnegative sign
B2 Loads and Load CombinationsApplicable Building Code determines:
Loads and load combinations for required strength of
steel seismic systems
Example basic LRFD seismic load combinations from
ASCE 7 (ASD similar)
• (1.2 + 0.2S DS )D + ρQ E +0.5L + 0.2S
•
(0.9 - 0.2S DS )D +ρ
Q E + 1.6H
Taking QE with a negative signis assumed to create the
critical case wheninvestigating net tension
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AISC Night School – Seismic Design Manual
Note: L may be taken as0.5L for most areas whereL o ≤ 100 psf
B2 Loads and Load Combinations
When “amplified seismic load” is required :
Use system overstrength factor, o, from ASCE 7 Table
12.2-1 unless otherwise defined by Seismic Provisions
Example load combinations with o
• (1.2 + 0.2S DS )D + oQ E + L + 0.2S
• (0.9 - 0.2S DS )D + oQ E + 1.6H
15
AISC Night School – Seismic Design Manual
B3 Design Basis
Required strength shall be greater of:
Required strength from application of structural
analysis using loads from the building code
Required strength from Seismic Provisions [e.g.,
expected strength of a member or amplified seismic
load (i.e., seismic load effect with overstrength from
building code)]
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Session 2: General Design Requirement
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AISC Night School – Seismic Design Manual
B3 Design Basis
Available strength (e.g., design strength, R n, or
allowable strength, R n / ) shall be:
Obtained from LRFD or ASD Specification
As modified by the Seismic Provisions (there aren’t too
many)
17
AISC Night School – Seismic Design Manual
Example 3.4.2
Moment Frame Column Design (using R = 3 approach)
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AISC Night School – Seismic Design Manual
Example 3.4.2
Given:
Refer to Column CL-1 in Figure 3-2. Verify that a
W12×87 ASTM A992 W-shape is sufficient to
resist the following required strengths between
the base and second levels. The applicable
building code specifies the use of ASCE/SEI 7
for calculation of loads.
19
AISC Night School – Seismic Design Manual
Example 3.4.2
The load combinations that include seismic
effects are:
20
LRFD ASD
LRFD Load Combination 5
from ASCE/SEI 7 Section
12.4.2.3
(including the 0.5 load factor
on L permitted in ASCE/SEI 7
Section 12.4.2.3)
ASD Load Combination 5
from ASCE/SEI 7 Section
12.4.2.3
1.2 0.2 ρ 0.5 0.2E DS S D Q L S + + + 1.0 0.14
0.7ρ
DS
E
S D H
F Q
+ + +
+
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AISC Night School – Seismic Design Manual
Example 3.4.2
21
From ASCE/SEI 7, this structure is assigned toSeismic Design Category C (ρ = 1.0) and S DS =
0.352.
Given in the problemstatement
AISC Night School – Seismic Design Manual
Example 3.4.2
The required strengths of Column CL-1
determined by a second-order analysis
including the effects of P -δ and P -Δ with
reduced stiffness as required by the direct
analysis method are:
22
LRFD ASD
P u = 233 kips
V u = 35.0 kipsM u top = 201 kip-ft
M u bot = −320 kip-ft
P a = 165 kips
V a = 23.4 kipsM a top = 131 kip-ft
M a bot = −210 kip-ft
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Session 2: General Design Requirement
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AISC Night School – Seismic Design Manual
Example 3.4.2
There are no transverse loadings between thefloors in the plane of bending, and the beams
framing into the column weak axis are pin-
connected and produce negligible moments.
23
AISC Night School – Seismic Design Manual
Example 3.4.2
Solution:From AISC Manual Table 2-4, the material
properties are as follows:
ASTM A992
F y = 50 ksi
F u = 65 ksi
24
From AISC Manual Table 1-1, the geometric
properties are as follows:
W12×87
r x = 5.38 in. r y = 3.07 in.
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AISC Night School – Seismic Design Manual
Example 3.4.2
Available Compressive Strength of Column CL-1
Because the member is being designed using the
direct analysis method, K is taken as 1.0.
25
1.0 14.0 ft 12.0 in./ft
5.38 in.
31.2
x
x
KL
r =
=
1.0 14.0 ft 12.0 in./ft
3.07 in.54.7
y
y
KL
r
=
=
governs
AISC Night School – Seismic Design Manual
Example 3.4.2
From AISC Manual Table 4-1, the available
compressive strength is:
26
LRFD ASD
925 kipsc nP =φ 616 kipsΩ
n
c
P =
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AISC Night School – Seismic Design Manual
Example 3.4.2
Available Flexural Strength of Column CL-1
Check the unbraced length for flexure
From AISC Manual Table 3-2:
L p = 10.8 ft
Lr = 43.1 ft
L p < Lb = 14.0 ft < Lr
27
AISC Night School – Seismic Design Manual
Example 3.4.2
Therefore, the member is subject to lateral-
torsional buckling.
Calculate C b using AISC Specification Equation
F1-1.
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AISC Night School – Seismic Design Manual 29
LRFD ASD
201 kip-ft320 kip-ft
utop
ubot
M M
=
= −
( )
201kip-ft 320kip-ft201kip-ft
14.0ft
201kip-ft 37.2kips
top bot top
M M M x M x
L
x
x
−
= −
= −
= −
131 kip-ft
210 kip-ft
atop
abot
M
M
=
= −
( )
131kip-ft 210kip-ft131kip-ft
14.0ft
131kip-ft 24.4kips
top bot top
M M M x M x
L
x
x
−
= −
= −
= −
AISC Night School – Seismic Design Manual 30
LRFD ASD
Quarter point moments are: Quarter point moments are:
3.50ft
201 kip-ft
37.2kips 3.50ft
70.8 kip-ft
AM x M =
=
−
=
3.50ft
131 kip-ft
24.4kips 3.50ft
45.6 kip-ft
AM x M =
=
−
=
7.00ft
201 kip-ft37.2kips 7.00ft
59.4 kip-ft
BM x M =
=
−
=
7.00ft
131 kip-ft24.4kips 7.00ft
39.8 kip-ft
BM x M =
=
−
=
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AISC Night School – Seismic Design Manual 31
LRFD ASD
10.5ft
201 kip-ft
37.2kips 10.5ft
190 kip-ft
320kip-ft
C
max
M x M
M
= =
=
−
=
=
12.5
2.5 3 4 3
12.5 320
2.5 320 3 70.8 4 59.4 3 190
2.20
max b
max A B C
M C
M M M M =
+ + +
=
+ + +
=
10.5ft131 kip-ft
24.4kips 10.5ft
125 kip-ft
210kip-ft
C
max
M x M
M
= =
=
−
= −
=
12.5
2.5 3 4 3
12.5 210
2.5 210 3 45.6 4 39.8 3 125
2.19
max b
max A B C
M C
M M M M =
+ + +
=
+ + +
=
AISC Night School – Seismic Design Manual
Example 3.4.2
From AISC Manual Table 3-10, with the availableflexural strength of a W12×87 is:
32
LRFD ASD
Check yielding (plastic moment)
limit state, using AISC Manual
Table 3-2,
Check yielding (plastic moment)
limit state, using AISC Manual
Table 3-2,
2.20 477 kip-ft
1,050 kip-ft
b nM =
=
φ
2.19 318 kip-ftΩ
696 kip-ft
n
b
M =
=
495 kip-ft 1,050 kip-ftb pM =
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AISC Night School – Seismic Design Manual
Example 3.4.2Interaction of Flexure and Compression in Column CL-1
Using AISC Specification Section H1, check the
interaction of compression and flexure in Column CL-1, as follows:
33
LRFD ASD
Because P r /P c > 0.2, use AISCSpecification Equation H1-1a.
Because P r /P c > 0.2, use AISCSpecification Equation H1-1a.
, as determined previously
925 kips
233 kips
925 kips
0.252
c c n
r
c
P P
P
P
=
=
=
=
φ , as determined previouslyΩ
616 kips
165 kips
616 kips
0.268
nc
c
r
c
P P
P
P
=
=
=
=
AISC Night School – Seismic Design Manual
Example 3.4.2
34
LRFD ASD
81.0 Eq. H1-1a
9
8 320 kip-ft0.252 0 0.827
9 495 kip-ft
0.827 1.0 o.k.
ry r rx
c cx cy
M P M Spec.
P M M
+ + ≤
+ + =
<
81.0 Eq. H1-1a
9
8 210 kip-ft0.268 0 0.835
9 329 kip-ft
0.835 1.0 o.k.
ry r rx
c cx cy
M P M Spec.
P M M
+ + ≤
+ + =
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Example 3.4.2
Available Shear Strength of Column CL-1
From AISC Manual Table 3-2, the available shear
strength of a W12×87 is:
35
LRFD ASD
193 kips 35.0 kips o.k.v nV = >φ / Ω 129 kips 23.4 kips o.k.n v V = >
AISC Night School – Seismic Design Manual
Example 3.4.2
The W12x87 is adequate to resist the required
strengths given for Column CL-1.
Note: Load combinations that do not include
seismic effects must also be investigated.
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Example 3.4.2
Moment Frame Column Design (using R = 3 approach)
37
End of Example
AISC Night School – Seismic Design Manual
Example 4.3.1
SMF Story Drift and Stability Check
38
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Given:Refer to the floor plan shown in Figure 4-7 and the
SMF elevation shown in Figure 4-8. Determine
if the frame satisfies the ASCE/SEI 7 drift and
stability requirements based on the given
loading.
The applicable building code specifies the use of
ASCE/SEI 7 for calculation of loads.
39
AISC Night School – Seismic Design Manual
SMF floor plan
40
SMF elevation
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Example 4.3.1
The seismic design story shear acting between
the second and third levels, V x , is 140 kips asdefined in ASCE/SEI 7 Section 12.8.4.
From an elastic analysis of the structure that
includes second-order effects and accounts for
panel-zone deformations, the maximum
interstory drift occurs between the third and
fourth levels:
δ xe = δ4e− δ3e = 0.482 in.
41
AISC Night School – Seismic Design Manual
δ xe = δ4e− δ3e = 0.482 in.
42
This is the difference in
displacement (drift)between two adjacent
floors. The “e” signifiesthat these displacementswere obtained from an
elastic analysis.
Story Drift Determination between Levels 3 and 4
δ3e
δ4e
Partial Frame Elevation
Level 4
Level 3
Undeformedframe
Deformedframe
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In this example, the stability check will be made at
the second level. The story drift between the
second and third levels is 0.365 in.
(δ3e− δ2e) = 0.365 in.
43
Solution:
From AISC Manual Table 1-1, the geometric
properties are as follows:
W24x76bf = 8.99 in.
AISC Night School – Seismic Design Manual
Reduced-beam-section (RBS) connections are
used at the frame beam-to-column connections
and the flange cut will reduce the stiffness of
the beam.
Example 4.3.3 illustrates the design of the RBS
geometry and the flange cut on one side of the
web is c = 2 in.
44
RBS (plan view)
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Some analysis programs allow for
direct input of RBS dimensions fromwhich the reduced stiffness can becalculated. This isn’t always practicalfor preliminary designs because you
must know the dimensions of the RBScut.
Section 5.8, Step 1, of ANSI/AISC 358 states that the
calculated elastic drift, based on gross beam
section properties, may be multiplied by 1.1 for
flange reductions up to 50% of the beam flange
width in lieu of specific calculations of effective
stiffness.
Amplification of drift values for cuts less than the
maximum may be linearly interpolated.
45
AISC Night School – Seismic Design Manual
Example 4.3.1
For bf = 8.99 in., the maximum cut is:
0.5(8.99 in.) = 4.50 in.
Thus, the total 4-in. cut is:
(4.00 in./4.50 in.)100 = 88.9% of the maximum cut
The calculated elastic drift needs to be amplified
by 8.89% (say 9%).
46
Sum of maximumcuts on both sides
of flange
c = 2” Total cutis 2x2” = 4”
This amplificationaccounts for the fact
that the analytical modelused gross sections
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Drift Check
From an elastic analysis of the structure that
includes second order effects, the maximum
interstory drift occurs between the third and
fourth levels. The effective elastic drift is:
47
4 3δ δ δ
0.482 in.
xe e e−
=
δ 1.09δ
1.09 0.482 in.
0.525 in.
xe RBS xe
=
=
Amplification ofdrfit by 9% due
to RBS cut
AISC Night School – Seismic Design Manual
Example 4.3.1
Per the AISC Seismic Provisions Section B1, the
design story drift and the story drift limits are
those stipulated by the applicable building code.
ASCE/SEI 7 Section 12.8.6 defines the design
story drift, Δ, computed from δ x , as the
difference in the deflections at the center of
mass at the top and bottom of the story under
consideration, which in this case is the thirdlevel.
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C d = 5.5 for SMFper ASCE 7, Table
12.2-1
Example 4.3.1
49
δΔ ASCE / SEI 7 Eq. 12.8-15
5.5 0.525 in.
1.0
2.89 in.
d xe
e
C
I =
=
=
C d amplifies the elasticdrift (calculated usingreduced forces) into anestimate of the (actual)
inelastic drift
AISC Night School – Seismic Design Manual
Example 4.3.1
From ASCE/SEI 7 Table 12.12-1, the allowable
story drift at level x , Δa, is 0.020hsx , where hsx is the story height below level x .
(Although not assumed in this example, ∆a can be
increased to 0.025hsx if interior walls,
partitions, ceilings and exterior wall systems
are designed to accommodate these increasedstory drifts.)
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AISC Night School – Seismic Design Manual
For ρ = 1.3, this provision
has the effect of reducingthe allowable drift (i.e., thestructure would have to bestiffer than if ρ = 1.0).
Example 4.3.1
ASCE/SEI 7 Section 12.12.1.1 requires for seismicforce resisting systems comprised solely of
moment frames in structures assigned to
Seismic Design Category D, E or F, that the
design story drift shall not exceed (∆a /ρ) for
any story.
Determine the allowable story drift as follows:
51
AISC Night School – Seismic Design Manual
Example 4.3.1
52
Δ 0.020
ρ ρ
0.020(12.5 ft)(12 in./ft)
1.0
3.00 in.
a sx h=
=
=
Δ 2.89 in. a< ∆
o.k
The frame satisfies the drift requirements.
Story heightbelow Level 3
In this example,because ρ = 1.0,this provision hasnot impact on the
design
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Example 4.3.1
Frame Stability Check
ASCE/SEI Section 12.8.7 provides a method for
the evaluation of the P -∆ effects on moment
frames based on a stability coefficient θ, which
should be checked for each floor. For the
purposes of illustration, this example checks
the stability coefficient only for the third level.
53
AISC Night School – Seismic Design Manual
Afloor = Aroof ≈ 75 ft(120 ft) = 9,000 ft2
Dfloor = 9,000 ft2(85 psf)/1,000 lb/kip
= 765 kips
Droof = 9,000 ft2 (68 psf)/1,000 lb/kip)= 612 kips
Dwall = 175 lb/ft[2(75 ft + 120 ft)]/(1,000 lb/kip)
= 68.3 kips per level
The stability coefficient, θ, is determined as follows:
54
“D ” and “L” arethe dead andlive loads,
respectively.
P x is totalvertical loadacting on agiven story
Δθ (ASCE/SEI 7 Eq. 12.8-16)
x e
x sx d
P I
V h C =
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AISC Night School – Seismic Design Manual 55
29,000 ft 20 psf / 1,000 lb/kip
180 kips
roof L =
=
Lfloor = 9,000 ft2(50 psf)/(1,000 lb/kip)
= 450 kips
AISC Night School – Seismic Design Manual
ASCE/SEI 7 does not explicitly specify load
factors to be used on the gravity loads for
determining P x , except that Section 12.8.7
does specify that no individual load factor
need exceed 1.0.
This means that if the combinations of ASCE/SEI 7
Section 2.3 are used, a factor of 1.0 can be
used for dead load rather than the usual 1.2factor used in the LRFD load combination, for
example.
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This also means that the vertical component
0.2S DS
D need not be considered here.
Therefore, for this example, the load combination
used to compute the total vertical load on a
given story, P x , acting simultaneously with the
seismic design story shear, V x , is 1.0D + 0.5L
based on ASCE/SEI 7 Section 2.3 including the
0.5 factor on L permitted by Section 2.3, where L
is the reduced live load.
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AISC Night School – Seismic Design Manual
Note that consistent with this, the same
combination was used in the second order
analysis for this example for the purpose of
computing the fundamental period, base
shear, and design story drift.
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Example 4.3.1
The total dead load supported by the columns onthe second level, assuming that the columns
support the equivalent of two floors worth of
curtain wall in addition to other dead loads, is:
59
1.0 1.0[612 kips 2(765 kips) 2(68.3 kips)]
2,280 kips
DP = + +
=
DFloorDRoof DWall
AISC Night School – Seismic Design Manual
LFloor
Example 4.3.1
The total live load supported by the columns on
the second level is:
60
0.5 0.5 2 450 kips 180 kips
540 kips
LP = +
=
LRoof
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Example 4.3.1
Therefore, the total vertical design load carried bythe columns on the second level is:
61
2,280 kips 540 kips
= 2,820 kips
x P = +
AISC Night School – Seismic Design Manual
Example 4.3.1
The seismic design story drift at the top of the
second level, including the 9% amplification
on the drift, is:
62
δΔ from ASCE / SEI 7 Eq. 12.8-15
5.5(1.09)(0.365 in.)
1.02.19 in.
d xe
e
C
I =
=
=
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Example 4.3.1
From an elastic analysis of the structure, the
seismic design story shear acting at the third
level under the story drift loading using the
equivalent lateral force procedure is V x = 140
kips and the floor-to-floor height is hsx = 12.5 ft.
Therefore, the stability coefficient is:
63
2,820 kips 2.19 in. 1.0θ
140 kips 12.5 ft 12 in./ft 5.5
0.0535
=
=
AISC Night School – Seismic Design Manual
Example 4.3.1
Because a second-order analysis was used to
compute the story drift, θ is adjusted as
follows to verify compliance with θmax , per
ASCE/SEI 7 Section 12.8.7.
64
θ 0.0535
1 θ 1 0.0535
0.0508
=
+ +
=
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Second-order effects include P-δ andP-Δ. Δ is the first order interstorydrift due to lateral loads. δ is thelocal deformation of the column due
to these loads, initial columnimperfections, etc.
Example 4.3.1
According to ASCE/SEI 7, if θ is less than or equal
to 0.10, second-order effects need not be
considered for computing story drift.
Note that whether or not second-order effects on
member forces must be considered per
ASCE/SEI 7 has to be verified, as it was in this
example; however, Chapter C of the AISC
Specification requires second order effects be
considered in all cases in the analysis used
for member design.
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AISC Night School – Seismic Design Manual
Example 4.3.1
Check the maximum permittedθ
The stability coefficient may not exceed θmax . In
determining θmax , β is the ratio of shear demand
to shear capacity for the level being analyzed,
and may be conservatively taken as 1.0.
66
0.5θ 0.25 ASCE / SEI 7 Eq. 12.8-17
β0.5
1.0(5.5)
0.0909 0.25
d C
= ≤
=
= ≤
max
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Example 4.3.1
The adjusted stability coefficient satisfies themaximum:
0.0508 < 0.0909 o.k.
The moment frame meets the allowable story drift
and stability requirements for seismic loading.
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AISC Night School – Seismic Design Manual
Example 4.3.1
Comments:
There are a total of six bays of frames in the SMF
direction in this example. Considering the
relative expense of SMF connections, it is
probably more cost-effective to reduce the
number of bays to four, and increase member
sizes to satisfy the strength and stiffness
requirements.
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Example 4.3.1
SMF Story Drift and Stability Check
69
End of Example
AISC Night School – Seismic Design Manual 70
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Intent of this chapter is to provide analytical requirements
for use in designing structural steel seismic systems.
Currently, there is little prescriptive material in the
provisions section, but there are analytical and modeling
recommendations in the Commentary.
SDM contains discussions and examples illustrating some
of the aspects of seismic system analysis. See Example
5.3.2 as an illustration.
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AISC Night School – Seismic Design Manual
Chapter D
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Chapter D
General Member and Connection Design
Requirements
Contains provisions that apply to multiple systems
(e.g., member requirements for ductility and bracing at
plastic hinges)
General connection requirements (e.g., bolting and
welding requirements and column splices)
Deformation compatibility
H-piles
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AISC Night School – Seismic Design Manual
Chapter D
D1 Member Requirements
Seismic Provisions may require certain members to
be “moderately ductile,”λmd , or “highly ductile,” λ hd
These requirements may be more stringent than
found in Specification Table B4.1
These new designations replace “compact” and
“seismically compact” from earlier editions
These provisions present requirements to limit
(delay) local flange or local web buckling
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Chapter D
D1 Member Requirements
“Compactness” describes a section sufficiently
stocky to develop a fully plastic stress distribution
without buckling
Certain members in seismic systems are expected to
delay onset of buckling beyond initial development of
the plastic distribution, so “compactness” isn’t a
very descriptive term for the behavior sought
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AISC Night School – Seismic Design Manual
Chapter D
D1 Member Requirements
Table D1.1 presents λmd and λhd values…there are no
significant technical changes from “compact” and
“seismically compact” values
Table D1.1 contains helpful graphics to make it easier
to understand which value applies for different parts
of structural sections
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Chapter DD1 Member Requirements
Requirements for Width-to-Thickness Ratios
Formerly“seismicallycompact”
Formerly“compact”
λmd = 9.15for
F y = 50
λhd = 7.23for
F y = 50 ksi
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AISC Night School – Seismic Design Manual
Chapter DD1 Member RequirementsRequirements for Width-to-Thickness Ratios
λmd = 16.10for
F y = 46
λhd = 13.81for
F y = 46 ksi
Eliminates manyrectangular orsquare HSS
sections (e.g.,> HSS12x
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Chapter DD1 Member Requirements
SDM Table 1-A has summary of width-to-thickness by
SFRS compression member type
79
Similartables forangles and
HSS
AISC Night School – Seismic Design Manual
Chapter DD1 Member Requirements
SDM Tables 1-3 to 1-7 identify members that
may be used in different SFRS
Tables cover:
W-shapes
Angles
Rectangular HSS
Square HSS
Round HSS
Pipe
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D1 Member Requirements
W24x162satisfies width-thickness requirements for all
SFRS (shown by “•”)
W24x55 does not satisfy width-thickness requirements for OCBF,SCBF and EBF braces
81
Similar tablesfor other
shapes
AISC Night School – Seismic Design Manual
Chapter DD1.2 Stability Bracing of Beams
Stability bracing is specified for seismic systems to
control lateral-torsional buckling
Lateral-torsional buckling
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Chapter DD1.2 Stability Bracing of Beams
For moderately and highly ductile members:
Both flanges must be braced or the section torsionally
braced
Lateral bracing provided byconcrete structural slab and full-height perpendicular framing
Lateral bracing provided by shallowperpendicular steel framing andstiffener – wood framing was notconsidered adequate
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AISC Night School – Seismic Design Manual
Chapter DD1.2 Stability Bracing of Beams
For moderately ductile members:
Unbraced length between lateral braces shall not
exceed Lb = 0.17r y E/F y
L b ≤0.17r y E/F y
Lateral bracing fortop and bottomflanges
For F y = 50 ksi,L b ≤ 98.6r y
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Chapter DD1.2 Stability Bracing of Beams
For highly ductile members:
Unbraced length between lateral braces shall not
exceed Lb = 0.086r y E/F y
L b ≤0.086r y E/F y
85
AISC Night School – Seismic Design Manual
Not the same C d in ASCE 7
Chapter DD1.2 Stability Bracing of Beams
For moderately and highly ductile members:
Beam bracing shall meet requirements of Specification
Appendix 6 for lateral or torsional bracing where the
required strength of the brace is
P rb = 0.02M r C d /ho (Spec . A-6-7)
and
M r = R y F y Z (Provisions D1-1a for LRFD)
86
This is an example of the SeismicProvisions specifying requiredstrength based on expectedstrength – not code demand
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Chapter DD1.2 Stability Bracing of Beams
For moderately and highly ductile members:
…and the required stiffness of the brace is
(Spec . A-6-8 for LRFD)
where C d = 1.0
ho = distance between flange centroids
87
1 10 r d br
b o
M C
L hβ
φ
=
h o
=
d i s t a n c e
b e t w e e n
f l a n g e
c e n t r o i d s
AISC Night School – Seismic Design Manual
Chapter DD1.2 Stability Bracing of Beams
At plastic hinges (or directly adjacent thereto):
Brace top and bottom flanges or brace against torsional
buckling
Required strength of bracing is P u = 0.06R y F y Z /ho(lateral bracing) or M u = 0.06R y F y Z (torsional bracing)
Bracing stiffness shall satisfy requirements of Appendix
6 of Specification but C d = 1.0 and M r = M u = R y F y Z
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**
**
Lateral bracing for top and bottomflange (not required if there is aconcrete structural slab per AISC358 for SMF and IMF)
Chapter DD1.2 Stability Bracing of Beams
At plastic hinges (or directly adjacent thereto):
Bracing adjacentto plastic hinge
Plastic hinge
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AISC Night School – Seismic Design Manual
Chapter D
90
Next Session• Example: SMF Beam Stability Bracing
• Protected Zones
• Column Requirements
• Example: SMF Column Strength Check
• Bolted and Welded Joints (General)
• Continuity Plates and Stiffeners
• Column Splice
• Example: Column Splice
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Chapter D
91
Questions?
AISC Night School – Seismic Design Manual
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