AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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AISC Night School – Seismic Design Manual
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AISC Night School – Seismic Design Manual
Copyright © 2015 American Institute of Steel Construction 1
AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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AISC Night School – Seismic Design Manual
AISC is a Registered Provider with The American American Institute of Architects Architects Continuing Education Systems Systems (AIA/CES). Credit(s) earned on completion completion of this program will be reported to AIA/CES for AIA members. members. Certificates of Completion for both AIA members and non-AIA members are available upon request. This program is registered with AIA/CES for continuing professional education. As such, it does not include include content that may be deemed deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questi Questions ons relate related d to specif specific ic materi materials als,, method methods, s, and servic services es will will be addres addressed sed at the conclu conclusio sion n of this this presen presentat tation ion..
AISC Night School – Seismic Design Manual
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AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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© The American American Institute of Steel Steel Construction 2015 2015 The inform informati ation on presen presented ted herein herein is based based on recogn recognize ized d engine engineeri ering ng princi principle pless and is for genera generall info inform rmat atio ion n only only.. Whil Whilee it is beli believ eved ed to be accu accura rate te,, this this info inform rmat atio ion n shou should ld not not be appl applie ied d to any any spec specif ific ic appl applic icat atio ion n with withou outt comp compet eten entt prof profes essi sion onal al exam examin inat atio ion n and and veri verifi fica cati tion on by a lice licens nsed ed prof profes essi sion onal al engi engine neer er.. Anyo Anyone ne maki making ng use use of this this infor informa mati tion on assu assume mess all all liab liabil ilit ityy aris arisin ing g from from such such use. use.
AISC Night School – Seismic Design Manual
Course Description 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 stability bracing of beams and drift requirements. Examples from the Seismic Design Manual will be presented to demonstrate concepts discussed in the session.
AISC Night School – Seismic Design Manual
Copyright © 2015 American Institute of Steel Construction 3
AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Learning Objectives •
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 overstrength factor.
•
Become familiar with the member design requirements of the AISC Seismic Provisions through demonstrated design examples.
AISC Night School – Seismic Design Manual
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Part 2 Presented by Thomas A. Sabol, Ph.D., S.E. Principal at Englekirk Englekirk Institutional Los Angeles, CA
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AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Application of the AISC Seismic Design Manual
Session 2 AISC Night School – Seismic Design Manual
Last Session •
Seismic Performance Goals
•
Seismic Design Categories
•
Seismic Performance Factors (e.g., R ,
•
Organization of AISC 341 Seismic Provisions
•
Steel Material Properties (e.g., yield strength, R y )
•
Welding Filler Metal Properties (e.g., Charpy V-Notch)
AISC Night School – Seismic Design Manual
O )
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AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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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 September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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
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B2 Loads and Load Combinations Applicable 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 + ρQ E + 1.6H
Taking QE with a negative sign is assumed to create the critical case when investigating net tension AISC Night School – Seismic Design Manual
“QE” has both a positive and negative sign
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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 •
(1.2 + 0.2S DS )D +
•
(0.9 - 0.2S DS )D +
oQ E + oQ E +
o
L + 0.2S 1.6H Note: L may be taken as 0.5L for most areas where L o ≤ 100 psf
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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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AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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)
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Example 3.4.2
Moment Frame Column Design (using R = 3 approach)
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AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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.
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Example 3.4.2
The load combinations that include seismic effects are: LRFD
ASD
LRFD Load Combination 5 from ASCE/SEI 7 Section 12.4.2.3 1.2 0.2SDS D
ρQ E
0.5L 0.2S
(including the 0.5 load factor on L permitted in ASCE/SEI 7 Section 12.4.2.3) AISC Night School – Seismic Design Manual
ASD Load Combination 5 from ASCE/SEI 7 Section 12.4.2.3 1.0
F
0.14SDS D H
0.7ρQ E
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AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Example 3.4.2
From ASCE/SEI 7, this structure is assigned to Seismic Design Category C ( = 1.0) and S DS = 0.352.
Given in the problem statement
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Example 3.4.2
The required strengths of Column CL-1 determined by a second-order analysis including the effects of P - and P -Δ with reduced stiffness as required by the direct analysis method are: LRFD P u = 233 kips V u = 35.0 kips M u top = 201 kip-ft M u bot = 320 kip-ft
AISC Night School – Seismic Design Manual
ASD P a = 165 kips V a = 23.4 kips M a top = 131 kip-ft M a bot = 210 kip-ft
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AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Example 3.4.2
There are no transverse loadings between the floors in the plane of bending, and the beams framing into the column weak axis are pinconnected and produce negligible moments.
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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 From AISC Manual Table 1-1, the geometric properties are as follows: W12 87 r x = 5.38 in. AISC Night School – Seismic Design Manual
r y = 3.07 in. 24
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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. KL x r x
1.0 14.0 ft 12.0 in./ft 5.38 in. 31.2
KLy
1.0 14.0 ft 12.0 in./ft
r y
3.07 in. 54.7
governs
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Example 3.4.2
From AISC Manual Table 4-1, the available compressive strength is: LRFD
φc P n
925 kips
AISC Night School – Seismic Design Manual
ASD
P n Ωc
616 kips
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AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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
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Example 3.4.2
Therefore, the member is subject to lateraltorsional buckling. Calculate C b using AISC Specification Equation F1-1.
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
LRFD M utop M ubot
ASD
201 kip-ft
M atop
320 kip-ft
M abot
Mtop
M ( x ) Mtop
M bot L
x
131 kip-ft 210 kip-ft
M ( x ) Mtop
Mtop
M bot L
x
201kip-ft
201kip-ft 320kip-ft x 14.0ft
131 kip-ft
131kip-ft 210kip-ft x 14.0ft
201kip-ft
37.2kips x
131 kip-ft
24.4 kips x
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LRFD
ASD
Quarter point moments are:
Quarter point moments are:
M x
M x
3.50ft
M A
201 kip-ft
24.4kips 3.50ft
70.8 kip-ft 7.00ft
M A
131 kip-ft
37.2kips 3.50ft
M x
3.50ft
M B
201 kip-ft 37.2kips 7.00ft 59.4 kip-ft
AISC Night School – Seismic Design Manual
45.6 kip-ft
M x
7.00ft
M B
131 kip-ft 24.4kips
7.00ft
39.8 kip-ft
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
LRFD 10.5ft
M x
ASD
M C
M x
10.5ft 131 kip-ft
201 kip-ft
24.4kips 10.5ft
37.2kips 10.5ft
125 kip-ft
190 kip-ft M max C b
12.5M max 3M A
210kip-ft
M max
320kip-ft
2.5M max
4M B
3MC
C b
2.5M max
12.5 320 2.5 320
M C
3 70.8
4 59.4
12.5M max 3M A 4 M B
3MC
12.5 210 2.5 210
3 190
2.20
3 45.6
4 39.8
3 125
2.19
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Example 3.4.2
From AISC Manual Table 3-10, with the available flexural strength of a W12 87 is: LRFD φb M n
2.20 477 kip-ft 1,050 kip-ft
Check yielding (plastic moment) limit state, using AISC Manual Table 3-2,
φb M p
495 kip-ft 1,050 kip-ft
ASD M n Ωb
2.19 318 kip-ft 696 kip-ft
Check yielding (plastic moment) limit state, using AISC Manual Table 3-2, M p Ωb
329 kip-ft
696 kip-ft
Therefore, the yielding limit state governs. AISC Night School – Seismic Design Manual
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
3.4.2 Interaction of FlexureExample and Compression in Column CL-1
Using AISC Specification Section H1, check the interaction of compression and flexure in Column CL1, as follows: LRFD Pc
ASD
φc P n , as determined previously
P c
925 kips P r P c
P n , as determined previously Ωc 616 kips
233 kips 925 kips
P r P c
0.252
Because P r /P c > 0.2, use AISC Specification Equation H1-1a.
165 kips 616 kips 0.268
Because P r /P c > 0.2, use AISC Specification Equation H1-1a.
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Example 3.4.2 LRFD Pr Pc
8 M rx 9 M cx
M ry M cy
1.0
ASD Spec. Eq. H1-1a
0.252
8 320 kip-ft 0 9 495 kip-ft
0.827
1.0
0.827 o.k.
AISC Night School – Seismic Design Manual
Pr Pc
8 M rx 9 M cx
M ry M cy
0.268
8 210 kip-ft 9 329 kip-ft
0.835
1.0
1.0 Spec. Eq. H1-1a
0
0.835 o.k.
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AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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: LRFD φvV n
ASD
193 kips 35.0 kips o.k. V n / Ωv 129 kips 23.4 kips o.k.
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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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AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Example 3.4.2
Moment Frame Column Design (using R = 3 approach)
End of Example AISC Night School – Seismic Design Manual
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Example 4.3.1
SMF Story Drift and Stability Check
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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.
AISC Night School – Seismic Design Manual
SMF floor plan
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SMF elevation
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Example 4.3.1
The seismic design story shear acting between the second and third levels, V x , is 140 kips as defined 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: xe =
4e
3e =
0.482 in.
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Story Drift Determination between Levels 3 and 4 xe =
4e
4e
3e =
0.482 in.
Level 4 Undeformed frame 3e
Deformed frame
Level 3
Partial Frame Elevation
AISC Night School – Seismic Design Manual
This is the difference in displacement (drift) between two adjacent floors. The “e” signifies that these displacements were obtained from an elastic analysis.
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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. (
3e
2e)
= 0.365 in.
Solution: From AISC Manual Table 1-1, the geometric properties are as follows: W24x76 bf = 8.99 in. AISC Night School – Seismic Design Manual
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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.
RBS (plan view) AISC Night School – Seismic Design Manual
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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.
AISC Night School – Seismic Design Manual
Some analysis programs allow for direct input of RBS dimensions from which the reduced stiffness can be calculated. This isn’t always practical for preliminary designs because you must know the dimensions of the RBS cut.
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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:
Sum of maximum cuts on both sides of flange
c = 2” Total cut is 2x2” = 4”
(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%). AISC Night School – Seismic Design Manual
This amplification accounts for the fact that the analytical model used gross sections 46
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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: δ xe
δ4 e
δ 3e
δ xe RBS
0.482 in. Amplification of drfit by 9% due to RBS cut
1.09δ xe 1.09 0.482 in. 0.525 in.
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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 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 third level.
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Example 4.3.1 C d amplifies the elastic drift (calculated using reduced forces) into an estimate of the (actual) inelastic drift
Δ
C d δ xe I e
ASCE / SEI 7 Eq. 12.8-15
5.5 0.525 in. 1.0 2.89 in.
C d = 5.5 for SMF per ASCE 7, Table 12.2-1
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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 increased story drifts.)
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Example 4.3.1
ASCE/SEI 7 Section 12.12.1.1 requires for seismic force 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 / ) for any story. Determine the allowable story drift as follows: For ρ = 1.3, this provision has the effect of reducing the allowable drift (i.e., the structure would have to be stiffer than if ρ = 1.0). AISC Night School – Seismic Design Manual
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Example 4.3.1 Story height below Level 3
Δa ρ
0.020 hsx ρ 0.020(12.5 ft)(12 in./ft) 1.0 In this example, ρ = 1.0, because 3.00 in.
Δ
2.89 in.
a
o.k
this provision has not impact on the design
The frame satisfies the drift requirements. AISC Night School – Seismic Design Manual
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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 , which should be checked for each floor. For the purposes of illustration, this example checks the stability coefficient only for the third level.
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The stability coefficient, , is determined as follows: P x ΔI e θ (ASCE/SEI 7 Eq. 12.8-16) V x hsx C d P is total x
Afloor = Aroof ≈ 75 ft(120 ft) = 9,000 ft2
vertical load acting on a given story
Dfloor = 9,000 ft2(85 psf)/1,000 lb/kip
= 765 kips Droof = 9,000 ft2 (68 psf)/1,000 lb/kip)
“D ” and “L” are the dead and live loads, respectively.
= 612 kips Dwall = 175 lb/ft[2(75 ft + 120 ft)]/(1,000 lb/kip)
= 68.3 kips per level AISC Night School – Seismic Design Manual
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Lfloor = 9,000 ft2(50 psf)/(1,000 lb/kip)
= 450 kips Lroof
9,000 ft 2 20 psf / 1,000 lb/kip 180 kips
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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 1.2 factor used in the LRFD load combination, for example. AISC Night School – Seismic Design Manual
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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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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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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Example 4.3.1
The total dead load supported by the columns on the second level, assuming that the columns support the equivalent of two floors worth of curtain wall in addition to other dead loads, is: DRoof
1.0P D
DFloor
DWall
1.0[612 kips 2(765 kips) 2(68.3 kips)] 2,280 kips
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Example 4.3.1
The total live load supported by the columns on the second level is: L L Floor
0.5P L
0.5
2 450 kips
Roof
180 kips
540 kips
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Example 4.3.1
Therefore, the total vertical design load carried by the columns on the second level is:
P x
2,280 kips 540 kips
= 2,820 kips
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Example 4.3.1
The seismic design story drift at the top of the second level, including the 9% amplification on the drift, is: Δ
C d δ xe from ASCE / SEI 7 Eq. 12.8-15 I e 5.5(1.09)(0.365 in.) 1.0 2.19 in.
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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: 2,820 kips 2.19 in. 1.0
θ
140 kips 12.5 ft 12 in./ft 5.5 0.0535
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Example 4.3.1
Because a second-order analysis was used to compute the story drift, is adjusted as follows to verify compliance with max , per ASCE/SEI 7 Section 12.8.7. θ
1
0.0535 θ 1 0.0535 0.0508
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Example 4.3.1
According to ASCE/SEI 7, if 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 effects include P- and for member design. Second-order P-Δ Δ is the first order interstory δ
.
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drift due to lateral loads. δ is the local deformation of the column due to these loads, initial column imperfections, etc. 65
Example 4.3.1
Check the maximum permitted
The stability coefficient may not exceed max . In determining max , is the ratio of shear demand to shear capacity for the level being analyzed, and may be conservatively taken as 1.0. θ
max
0.5 0.25 βC d
ASCE / SEI 7 Eq. 12.8-17
0.5 1.0(5.5) 0.0909
0.25
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Example 4.3.1
The adjusted stability coefficient satisfies the maximum: 0.0508 < 0.0909 o.k. The moment frame meets the allowable story drift and stability requirements for seismic loading.
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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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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Example 4.3.1
SMF Story Drift and Stability Check
End of Example AISC Night School – Seismic Design Manual
AISC Night School – Seismic Design Manual
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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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71
Chapter D
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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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Chapter D
D1 Member Requirements
Seismic Provisions may require certain members to be “moderately ductile,” md , or “highly ductile,” 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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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
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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Chapter D
D1 Member Requirements
Table D1.1 presents md and hd 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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AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Chapter D D1 Member Requirements
Requirements for Width-to-Thickness Ratios
Formerly “seismically compact”
= 7.23 for F y = 50 ksi hd
Formerly “compact”
= 9.15 for F y = 50
md
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Chapter D D1 Member Requirements
Requirements for Width-to-Thickness Ratios Eliminates many rectangular or square HSS sections (e.g., > HSS12x hd =
13.81 for F y = 46 ksi
md =
16.10 for F y = 46
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Chapter D D1 Member Requirements
SDM Table 1-A has summary of width-to-thickness by SFRS compression member type Similar tables for angles and HSS
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Chapter D
D1 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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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
D1 Member Requirements
Similar tables for other shapes
W24x162satisfies widththickness requirements for all
•
SFRS (shown by “ ”)
W24x55 does not satisfy widththickness requirements for OCBF, SCBF and EBF braces
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Chapter D
D1.2 Stability Bracing of Beams
Stability bracing is specified for seismic systems to control lateral-torsional buckling
Lateral-torsional buckling AISC Night School – Seismic Design Manual
82
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
D D1.2 Stability Bracing Chapter of Beams
For moderately and highly ductile members:
Both flanges must be braced or the section torsionally braced
Lateral bracing provided by concrete structural slab and fullheight perpendicular framing
Lateral bracing provided by shallow perpendicular steel framing and stiffener – wood framing was not considered adequate
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D D1.2 Stability Bracing Chapter of Beams
For moderately ductile members:
Unbraced length between lateral braces shall not exceed Lb = 0.17r y E/F y Lateral bracing for top and bottom flanges For F y = 50 ksi, L b ≤ 98.6r y
L b ≤ 0.17r y E/F y AISC Night School – Seismic Design Manual
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Chapter D
D1.2 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
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Chapter D
D1.2 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 Not the same C d in ASCE 7
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)
This is an example of the Seismic Provisions specifying required strength based on expected strength – not code demand AISC Night School – Seismic Design Manual
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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Chapter D
D1.2 Stability Bracing of Beams For moderately and highly ductile members: …and the required stiffness of the brace is
βbr
1 10Mr C d
φ
(Spec . A-6-8 for LRFD)
Lb ho
where C d = 1.0 ho = distance between flange centroids
n e e s w t d i e o b r t e n c e n c a t e g s i n d a l = f o
h
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Chapter D
D1.2 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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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
D D1.2 Stability Bracing Chapter of Beams
At plastic hinges (or directly adjacent thereto):
* *
* *
Lateral bracing for top and bottom flange (not required if there is a concrete structural slab per AISC 358 for SMF and IMF)
Plastic hinge
Bracing adjacent to plastic hinge AISC Night School – Seismic Design Manual
Next Session
89
Chapter D
•
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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Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Chapter D
Questions?
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[email protected]. • Be on the lookout: Check your spam filter! Check your junk folder! • Completely fill out online form. Don’t forget to check the boxes next to each attendee’s name!
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AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
Individual Webinar Registrants CEU/PDH Certificates Within 2 business days… • New reporting site (URL will be provided in the forthcoming email). • Username: Same as AISC website username. • Password: Same as AISC website password.
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8-Session Registrants CEU/PDH Certificates One certificate will be issued at the conclusion of all 8 sessions.
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AISC Night School September 28, 2015
Application of the AISC Seismic Design Manual Session 2: General Design Requirements Pt. 2
8-Session Registrants Quizzes Access to the quiz: Information for accessing the quiz will be emailed to you by Thursday. It will contain a link to access the quiz. EMAIL COMES FROM
[email protected] Quiz and Attendance records: Posted Tuesday mornings. www.aisc.org/nightschool click on Current Course Details. Reasons for quiz: •EEU – must take all quizzes and final to receive EEU •CEUs/PDHS – If you watch a recorded session you must take quiz for CEUs/PDHs. •REINFORCEMENT – Reinforce what you learned tonight. Get more out of the course. NOTE: If you attend the live presentation, you do not have to take the quizzes to receive CEUs/PDHs.
AISC Night School – Seismic Design Manual
8-Session Registrants Recording Access to the recording: Information for accessing the recording will be emailed to you by this Wednesday. The recording will be available for two weeks. For 8-session registrants only. EMAIL COMES FROM
[email protected].
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