2/18/2021
1
Frequently
Misunderstood
Seismic
Provisions
Emily Guglielmo, S.E.
Martin/Martin, Inc.
Importance Factor
• Importance Factor
• Response Modification Coefficient, R
• Overstrength Factor, Ωo
• Drift
• Irregularities
• Redundancy Factor, ρ
• Vertical Seismic Load Effect, Ev
• Orthogonal Effects
• Seismic Weight, W
• Nonstructural Components, Fp
• Foundation Ties
• Modal Analysis
• ASCE 7-16 Site Specific Response Spectrum
Importance Factor
• Table 1.5-2:
Importance Factor
Ie= 1.0, 1.25, 1.5
• Section 13.1.3: Component Importance FactorIp = 1.0, 1.5: Life Safety, Essential, Hazardous
Response Modification Coefficient, R
• Importance Factor
• Response Modification Coefficient, R
• Overstrength Factor, Ωo
• Drift
• Irregularities
• Redundancy Factor, ρ
• Vertical Seismic Load Effect, Ev
• Orthogonal Effects
• Seismic Weight, W
• Nonstructural Components, Fp
• Foundation Ties
• Modal Analysis
• ASCE 7-16 Site Specific Response Spectrum
Response Modification Coefficient, R
• The red line is the force vs. displacement if the structure responded elastically.
• The green line is the actual force vs. displacement of the structure.
• The blue line is the code force per IBC/ ASCE 7.
• Illustrates the significance
of design parameters
contained in ASCE 7.
– Response modification
coefficient, R
– Deflection amplification
factor, Cd
– System overstrength factor,
Ωo.
NEHRP Recommended Seismic Provisions
5
In ASCE 7, seismic design forces are calculated by dividing the force from a
linear response when subjected to the design ground motion by the
response modification coefficient, R.
Response Modification Coefficient, R
6
1 2
3 4
5 6
2/18/2021
2
R = 1
• Like wind (elastic)
• Used by Nuclear and Military Essential
• ASCE 7-16 Proposal… – No ductile detailing required?
– Permitted in all SDCs?
• Permitted for non-building structures, Chapter 15.
7
Response Modification Coefficient, R
If you accept the concept that the R factor reduces elastic seismic design forces because of
system ductility, then by definition using an R of 1.0 should require no ductile detailing.
Logically, this concept should apply to all buildings in all regions. Good chapter headed in
the right direction.
Designers don’t need
another design approach.
The profession wants ASCE
7 to simplify what is already
in the standard.
It is impossible for me to express all of my
concerns with regard to this proposal
adequately. It will introduce into seismic
design category D, E and F territory the
design of building structures without the
appropriate detailing. This is dangerous.
This proposal is an admission that it is too hard to design for
seismic properly, so we will let lazy, uneducated engineers continue
to be lazy and uneducated and design things stupidly. JUST VOTE
NO!
8
Response Modification Coefficient, R
• Composite shear walls
• Coupled special concrete shear walls
• CLT shear walls
ASCE 7-22: New Systems for Table 12.2-1
Response Modification Coefficient, ROverstrength Factor, Ωo
• Importance Factor
• Response Modification Coefficient, R
• Overstrength Factor, Ωo
• Drift
• Irregularities
• Redundancy Factor, ρ
• Vertical Seismic Load Effect, Ev
• Orthogonal Effects
• Seismic Weight, W
• Nonstructural Components, Fp
• Foundation Ties
• Modal Analysis
• ASCE 7-16 Site Specific Response Spectrum
The Ωo coefficient approximates the inherent overstrength and can be broken down into
several components:Ωo = ΩDΩMΩS
11
Overstrength Factor, Ωo
ASCE 7 Section 12.4.3.2
12
Overstrength Factor, Ωo
7 8
9 10
11 12
2/18/2021
3
Question:
When do I need to design with load combinations with overstrength
factors, Ωo?
Answer:
IBC 1605.1 “Buildings shall be designed to resist the load
combinations with overstrength factor specified in Section 12.4.3.1
of ASCE 7 where required by Section 12.2.5.2, 12.3.3.3, or
12.10.2.1…”
13
Overstrength Factor, Ωo
12.2.5.2: Cantilever Column Systems, SDC B-F
Foundations and other elements used to provide overturning
resistance at the base of cantilever column elements shall have
the strength to resist the load combinations with overstrength
factors of Section 12.4.3.2.
14
Overstrength Factor, Ωo
12.3.3.3: Elements Supporting Discontinuous Walls
or Frames, SDC B-F
Columns, beams, trusses, or slabs supporting discontinuous walls or frames shall have the
strength to resist the maximum axial force that can develop in accordance with the load
combinations with overstrength factors of Section 12.4.3.2.
MASONRY
SHEAR WALL
ELEMENTS SUPPORTING
DISCONTINOUS WALL
Overstrength Factor, Ωo12.10.2.1: Collector Elements, SDC C-F
Collector elements, splices, and their connections to resisting elements shall resist
the load combinations of Section 12.4.3.2.
Overstrength Factor, Ωo
• 12.4 Load Combinations with Omega zero
• 12.2.5.2 Cantilever Columns SDC B,C,D,E,F
• 12.10.2.1 Collectors SDC C,D,E,F
• 12.3.3.3 Columns, Beams Supporting
Discontinuous Walls or Frames SDC B,C,D,E,F
• 12.13.6.5 Pile Anchorage SDC D,E,F
• Material Specifications: SDC B,C,D,E,F
• AISC where R>3
• ACI Chapter 21, Appendix D, Etc.
17
Overstrength Factor, Ωo
Where the tabulated value of the overstrength factor, Ωo, is greater than or equal to 2½, Ω0 is
permitted to be reduced by subtracting the value of 1/2 for structures with flexible diaphragms.
Overstrength Factor, Ωo
13 14
15 16
17 18
2/18/2021
4
Increased by Ω0
ASCE 7-16 Change
Requires the use of Ω0 for transfer diaphragms (Horizontal Irregularity Type 4—Out-of-Plane Offset)
Overstrength Factor, Ωo Drift
• Importance Factor
• Response Modification Coefficient, R
• Overstrength Factor, Ωo
• Drift
• Irregularities
• Redundancy Factor, ρ
• Vertical Seismic Load Effect, Ev
• Orthogonal Effects
• Seismic Weight, W
• Nonstructural Components, Fp
• Foundation Ties
• Modal Analysis
• ASCE 7-16 Site Specific Response Spectrum
In ASCE 7, the elastic deformations (ΔS) calculated under reduced
forces are multiplied by Cd to estimate the actual inelastic
deflections.
Deflection amplification factor, Cd
Drift Drift
<Δa
12.12.3 Structural Separation and Property Line
Setback
Drift Irregularities
• Importance Factor
• Response Modification Coefficient, R
• Overstrength Factor, Ωo
• Drift
• Irregularities
• Redundancy Factor, ρ
• Vertical Seismic Load Effect, Ev
• Orthogonal Effects
• Seismic Weight, W
• Nonstructural Components, Fp
• Foundation Ties
• Modal Analysis
• ASCE 7-16 Site Specific Response Spectrum
19 20
21 22
23 24
2/18/2021
5
Irregularities
• Code provisions were developed for buildings with
regular configurations.
• Earthquakes have repeatedly shown that irregular
configurations lead to greater damage.
• Code regulations regarding irregularities
were first introduced in 1988 UBC.
Horizontal Irregularities
Horizontal Irregularities Vertical Irregularities
Vertical Irregularities ATC-123: ASCE 7-22
25 26
27 28
29 30
2/18/2021
6
ATC-123: ASCE 7-22 ATC-123: ASCE 7-22
Redundancy Factor, ρ
• Importance Factor
• Response Modification Coefficient, R
• Overstrength Factor, Ωo
• Drift
• Irregularities
• Redundancy Factor, ρ
• Vertical Seismic Load Effect, Ev
• Orthogonal Effects
• Seismic Weight, W
• Nonstructural Components, Fp
• Foundation Ties
• Modal Analysis
• ASCE 7-16 Site Specific Response Spectrum
Redundancy Factor, ρ
• Damage from the 1994 Northridge earthquake was concentrated in buildings with low redundancy.
• The code was modified to increase redundancy for structures in Seismic Design Categories D, E and F.
• For structures with low inherent redundancy, the required design forces are (arbitrarily?) amplified to increase strength and resistance to damage.
34
ASCE 7-16
12.3.4.1: Conditions Where the Value of ρ is 1.0.
• Several conditions
12.3.4.2: Redundancy Factor, ρ, for SDC D, E, F
• Either ρ = 1.0 or 1.3
35
Redundancy Factor, ρ1. Structures assigned to Seismic Design Category B or C.
2. Drift calculation and P-delta effects.
3. Design of nonstructural components (Chapter 13).
4. Design of non-building structures that are not similar to
buildings (Chapter 15).
Examples: Tanks, amusement structures/ monuments, signs
and billboards, cooling towers.
Examples: Mechanical/ electrical
components, ceilings, cabinets.
36
Redundancy Factor, ρ= 1.0
31 32
33 34
35 36
2/18/2021
7
6. Design of members or connections where the load combinations with
overstrength of 12.4.3.2 are required for design.
7. Diaphragm loads determined using Eq. 12.10-1.
8. Structures with damping systems designed in
accordance with Chapter 18.
5. Design of collector elements, splices and their connections for which the load
combinations with overstrength factor of 12.4.3.2 are used.
9. Out-of-plane wall anchorage (including
connections).
Redundancy Factor, ρ=1.0
ASCE 7-10 12.3.4.2
ρ = 1.0 or 1.3
ρ = 1.3 unless ONE of the following conditions is met:
Condition 1: Can an individual element be removed from
the lateral force resisting system without:
• Causing the remaining structure to suffer a reduction
in story strength > 33%, or
• Creating an extreme torsional irregularity?
Redundancy Factor, ρ
Condition 1: Requires Calculations
Redundancy Factor, ρ
ASCE 7-16 12.3.4.2
ρ = 1.0 or 1.3
ρ = 1.3 unless ONE of the following conditions is met:
Condition 2: If a structure is regular in plan and there are at least 2 bays of
seismic force resisting perimeter framing on each side of the structure in each
orthogonal direction at each story resisting > 35% of the base shear.
Redundancy Factor, ρ
Vertical Seismic Load Effect, Ev
• Importance Factor
• Response Modification Coefficient, R
• Overstrength Factor, Ωo
• Drift
• Irregularities
• Redundancy Factor, ρ
• Vertical Seismic Load Effect, Ev
• Orthogonal Effects
• Seismic Weight, W
• Nonstructural Components, Fp
• Foundation Ties
• Modal Analysis
• ASCE 7-16 Site Specific Response Spectrum
Gravity and Earthquake Effects Additive
1.2D+Ev+Eh+L+0.2S
1.2D+0.2SDSD+ρQE+L+0.2S
=(1.2+0.2SDS)D+ρQE+L+0.2S
Gravity and Earthquake Effects Counteract
0.9D-Ev+Eh
0.9D-(0.2SDSD+ρQE)
=(0.9-0.2SDS)D-ρQE
Vertical Seismic Load Effect - Ev
37 38
39 40
41 42
2/18/2021
8
Orthogonal Effects
• Importance Factor
• Response Modification Coefficient, R
• Overstrength Factor, Ωo
• Drift
• Irregularities
• Redundancy Factor, ρ
• Vertical Seismic Load Effect, Ev
• Orthogonal Effects
• Seismic Weight, W
• Nonstructural Components, Fp
• Foundation Ties
• Modal Analysis
• ASCE 7-16 Site Specific Response Spectrum
Orthogonal Effects
Orthogonal Effects• SDC C, E, E, F for irregular buildings
• SDC D, E, F for corner columns
12.5.3: Two procedures permitted:1) Orthogonal combination procedure with loading applied independently in orthogonal directions:
2) Simultaneous application of orthogonal ground motion.
12.5.2: SDC B forces are permitted to be applied in each orthogonal
directions and interaction effects are permitted to be neglected.
IEEE 693 Equipment applies Orthogonal Effects
to all Conditions, Corner anchor bolts.
Seismic Weight, W
• Importance Factor
• Response Modification Coefficient, R
• Overstrength Factor, Ωo
• Drift
• Irregularities
• Redundancy Factor, ρ
• Vertical Seismic Load Effect, Ev
• Orthogonal Effects
• Seismic Weight, W
• Nonstructural Components, Fp
• Foundation Ties
• Modal Analysis
• ASCE 7-16 Site Specific Response Spectrum
Seismic Weight, W
• Section 12.7.2
• No Live Load except:
• 25% of Storage
• Partitions 10 psf
• Permanent Equipment
• 20% of snow > 30psf
• Roof Gardens
Non-Structural Components, Fp
• Importance Factor
• Response Modification Coefficient, R
• Overstrength Factor, Ωo
• Drift
• Irregularities
• Redundancy Factor, ρ
• Vertical Seismic Load Effect, Ev
• Orthogonal Effects
• Seismic Weight, W
• Nonstructural Components, Fp
• Foundation Ties
• Modal Analysis
• ASCE 7-16 Site Specific Response Spectrum
43 44
45 46
47 48
2/18/2021
9
Nonstructural Components, Fp Nonstructural Components, Fp
• Masses further away from ground experience
higher accelerations
• Higher mode effects cause higher
accelerations than first mode effects at lower
floors
• Forces may be 1.5 to 2.5 times higher at roof
than at-grade
Nonstructural Components, Fp
ASCE 7-16
• New force equation that considers the influence of the structure
characteristics on lateral force based on the ATC-120 project
• New provisions for the design of rooftop structures
• New provisions for component support structures and platforms
Nonstructural Components, Fp
ASCE 7-22?
Foundation Ties
• Importance Factor
• Response Modification Coefficient, R
• Overstrength Factor, Ωo
• Drift
• Irregularities
• Redundancy Factor, ρ
• Vertical Seismic Load Effect, Ev
• Orthogonal Effects
• Seismic Weight, W
• Nonstructural Components, Fp
• Foundation Ties
• Modal Analysis
• ASCE 7-16 Site Specific Response Spectrum
Foundation Ties
• Pile Caps SDC C,D,E,F
• Spread Footings SDC E, F (Liquefiable Sites)
49 50
51 52
53 54
2/18/2021
10
Modal Analysis
• Importance Factor
• Response Modification Coefficient, R
• Overstrength Factor, Ωo
• Drift
• Irregularities
• Redundancy Factor, ρ
• Vertical Seismic Load Effect, Ev
• Orthogonal Effects
• Seismic Weight, W
• Nonstructural Components, Fp
• Foundation Ties
• Modal Analysis
• ASCE 7-16 Site Specific Response Spectrum
56
Modal Analysis
Modal Analysis Modal Analysis
When do you have
to use modal or
time history
analysis?
Tors
ion
Ma
ss
Ge
om
etr
ic
So
ft S
tory
12.9.1 Number of Modes
... The analysis shall include a
sufficient number of modes to obtain
a combined modal mass participation
of at least 90 percent of the actual mass
in each of the orthogonal
horizontal directions of response
considered by the model.
12.9.1 Number of Modes
… The analysis shall include a
sufficient number of modes to obtain
a combined modal mass participation
of 100% of the structure’s mass. For
this purpose, it shall be permitted to
represent all modes with periods less
than 0.05 s in a single rigid body mode
that has a period of 0.05 s.
Exception: Alternatively,... at least 90
percent of the actual mass in each of
the orthogonal horizontal directions...
Modal Analysis
ASCE 7-16: 100% Mass Participation
ASCE 7-10 ASCE 7-16
12.9.4.1 Scaling of Forces
… Where the combined response
for the modal base shear (Vt) is less
than 85 percent of the calculated base
shear (V) using the equivalent lateral
force procedure, the forces shall be
multiplied by 0.85V / Vt
12.9.1.1 Scaling of Forces
… Where the combined response
for the modal base shear (Vt) is less
than 100 percent of the calculated
base shear (V) using the equivalent
lateral force procedure, the forces
shall be multiplied by V/Vt
Modal Analysis
ASCE 7-16: Scaling of Forces
ASCE 7-10 ASCE 7-16
55 56
57 58
59 60
2/18/2021
11
Modal AnalysisASCE 7-22
Modal AnalysisASCE 7-22
ASCE 7-16 Site Specific Response
Spectrum• Importance Factor
• Response Modification Coefficient, R
• Overstrength Factor, Ωo
• Drift
• Irregularities
• Redundancy Factor, ρ
• Vertical Seismic Load Effect, Ev
• Orthogonal Effects
• Seismic Weight, W
• Nonstructural Components, Fp
• Foundation Ties
• Modal Analysis
• ASCE 7-16 Site Specific Response Spectrum
• Summary and background of ASCE 7-16 requirements
• Introduction to Multi-Period Response Spectra (MPRS) ASCE 7-22
• Comparison of SMS and SM1 for ASCE 7-10, ASCE 7-16, ASCE 7-22
• Web-based tool
• Reference Documents
ASCE 7-22 Multi- Period
Response Spectrum Analysis
ASCE 7-16 Site-Specific Ground Motion Procedures
Problem: Use of 2 response periods (0.2s,1.0s) not sufficient to accurately represent response spectral acceleration for all design periods.
• Reasonably Accurate for Stiff Soil Sites, Site Classes A-C
• Generally Non-conservative for Soft Soil Sites, Site Classes D-F, whose seismic hazard is dominated by large magnitude events
ASCE 7-16 Site-Specific
Ground Motion Procedures
• Site Class C Example
61 62
63 64
65 66
2/18/2021
12
• Site Class D Example
ASCE 7-16 Site-Specific
Ground Motion Procedures
• Site Class D Example
ASCE 7-16 Site-Specific
Ground Motion Procedures
• Site Class E Example
ASCE 7-16 Site-Specific
Ground Motion Procedures
• Site Class E Example
ASCE 7-16 Site-Specific
Ground Motion Procedures
ASCE 7-16 Site-Specific
Ground Motion Procedures
• Included updates to the Fa and Fv Tables (11.4-1 and 11.4-2)
• Requirement to perform site-specific ground motion hazard analysis
for:
ASCE 7-16 Site-Specific
Ground Motion Procedures
67 68
69 70
71 72
2/18/2021
13
Exceptions to requiring a site-specific ground motion hazard analysis:• Structures on Site Class E sites with Ss ≥ 1.0, provided Fa taken as from Site Class C
• Structures on Site Class D & E sites with S1 ≥ 0.2, provided:
• =
(Eq. 12.8-2) for T ≤ 1.5Ts
• = 1.5
(Eq. 12.8-3) for TL ≥ T > 1.5Ts
• = 1.5
(Eq. 12.8-4) for T > TL
• Structures on Site Class E Sites with S1 ≥ 0.2, provided T ≤ Ts and ELF is used for the analysis
ASCE 7-16 Site-Specific Ground Motion Procedures
ASCE 7-16 Site-Specific Ground Motion Procedures
Site Class D & E sites with S1 ≥ 0.2, provided Cs is:
• determined by Eq. 12.8-2 for T ≤ 1.5Ts and
• 1.5 times value computed by Eq. 12.8-3 (TL ≥ T > 1.5Ts), Eq. 12.8-4 (T > TL)
1.5Ts
1.5Cs
• Changes to Chapters 11, 20, 21 and 22 (NOT Chapter 12)
• Replaces requirement for site-specific ground motion hazard analysis.• Requirement for site-specific ground motion limited to structures on Site Class F.
• Provides 22-point response spectrum.
• Eliminates the site coefficient (Fa and Fv) tables (and other text).
• Adds 3 site classes (BC, CD, and DE), reduce the step function.
ASCE 7-22 Multi-Period
Response Spectra and
Design ParametersSS S1
ASCE 7-22 Multi-Period Response Spectra and Design Parameters
Relative values of SMS between ASCE 7-10, ASCE 7-16 and ASCE 7-22
ASCE 7-22 Multi-Period Response Spectra and Design Parameters
Relative values of SMS between ASCE 7-10, ASCE 7-16 and ASCE 7-22
ASCE 7-22 Multi-Period Response Spectra and Design
Parameters
73 74
75 76
77 78
2/18/2021
14
Relative values of SM1 between ASCE 7-10, ASCE 7-16 and ASCE 7-22
ASCE 7-22 Multi-Period Response Spectra and Design Parameters
ASCE 7-22 Multi-Period Response Spectra and Design
Parameters
Relative values of SM1 between ASCE 7-10, ASCE 7-16 and ASCE 7-22
•ASCE Hazard (FREE) will provide SMS, SM1, SDS and
SD1 and MPRS.
ASCE 7-22: Online ToolsASCE 7-22 Multi-Period Response
Spectra and Design Parameters:
References
Questions?
Emily Guglielmo, [email protected]
79 80
81 82
83