v Orthogonal Effects Redundancy Factor, ρ Seismic Weight, W ......of design parameters contained in ASCE 7. – Response modification coefficient, R – Deflection amplification factor,

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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




• Drift

• Irregularities






• Foundation Ties

• Modal Analysis



• 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

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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.


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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.

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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!

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• 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




• Drift

• Irregularities






• Foundation Ties

• Modal Analysis


The Ωo coefficient approximates the inherent overstrength and can be broken down into

several components:Ωo = ΩDΩMΩS

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Overstrength Factor, Ωo

ASCE 7 Section 12.4.3.2

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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…”

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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.

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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.


• 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.

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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.


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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




• Drift

• Irregularities






• Foundation Ties

• Modal Analysis


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




• Drift

• Irregularities






• Foundation Ties

• Modal Analysis


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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

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ATC-123: ASCE 7-22 ATC-123: ASCE 7-22

Redundancy Factor, ρ




• Drift

• Irregularities






• Foundation Ties

• Modal Analysis



• 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.

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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

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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.

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Redundancy Factor, ρ= 1.0

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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?


Condition 1: Requires Calculations


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.


Vertical Seismic Load Effect, Ev




• Drift

• Irregularities






• Foundation Ties

• Modal Analysis


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

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Orthogonal Effects




• Drift

• Irregularities






• Foundation Ties

• Modal Analysis


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




• Drift

• Irregularities






• Foundation Ties

• Modal Analysis


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




• Drift

• Irregularities






• Foundation Ties

• Modal Analysis


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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




• Drift

• Irregularities






• Foundation Ties

• Modal Analysis


Foundation Ties

• Pile Caps SDC C,D,E,F

• Spread Footings SDC E, F (Liquefiable Sites)

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Modal Analysis




• Drift

• Irregularities






• Foundation Ties

• Modal Analysis


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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

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Modal AnalysisASCE 7-22

Modal AnalysisASCE 7-22

ASCE 7-16 Site Specific Response

Spectrum• Importance Factor



• Drift

• Irregularities






• Foundation Ties

• Modal Analysis


• 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

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• Site Class D Example



• Site Class D Example



• Site Class E Example



• Site Class E Example





• 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:



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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



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


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

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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

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]

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