Clarifying Frequently Misunderstood Seismic Provisions · Clarifying Frequently Misunderstood Seismic Provisions ... Collector elements, ... All of the shear walls are relatively

1

Middle Tennessee Region of the Tennessee Structural

Engineers Association Seminar:

Clarifying Frequently

Misunderstood Seismic Provisions

By Emily Guglielmo, SE

Martin/Martin, Inc.

December 13, 2017

“.. Earthquake engineering is a cartoon. . . Earthquakes systematically bring out the

mistakes made in design and construction.”

Fundamentals of Earthquake Engineering, Newmark and Rosenblueth (1971):

“In dealing with earthquakes, we must

contend with appreciable probabilities that failure

will occur... Otherwise, all the wealth of the world

would prove insufficient.. the most modest

structures would be fortresses. We must also

face uncertainty on a large scale, for it is our

task to design engineering systems – about whose pertinent properties we

know little – to resist future earthquakes–

whose characteristics we know even less. . . .”

2

2

Topics

• R, Cd, Ωo

• Redundancy, ρ

• Vertical and Horizontal Combination of

Systems

• Bearing Wall or Building Frame?

• Analysis Procedures

• Structural Irregularities

3

Topics

• R, Cd, Ωo

• Redundancy, ρ


Systems



4

3

Elastic vs. Inelastic Response

• 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.

2009 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

“In dealing with earthquakes, we must contend

with appreciable probabilities that failure will

occur in the near future. Otherwise, all the

wealth of the world would prove insufficient to

fill our needs: the most modest structures

would be fortresses.”

6

4

R = 1

• Like wind (elastic)

• Used by Nuclear and Military Essential

• ASCE 7-16 Proposal… – No ductile detailing required?

– Permitted in all SDCs?

7

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

5

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

9

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

several components:Ωo = ΩDΩMΩS

System Overstrength factor, Ωo

10

6

DESIGN OVERSTRENGTH

ΩD is the overstrength provided by the design engineer and/ or code.

EXAMPLES:

Load and resistance factors.

Design controlled by stiffness.

Architectural requirements.

11

Ωo

MATERIAL OVERSTRENGTH

•ΩM represents material overstrength.

EXAMPLES:

Reinforced masonry, concrete, and steel provisions have historically used a factor

of ~1.25 to account for the ratio of mean to specified strengths.

•A survey of WF steel: Ratios = 1.37 and 1.15 for A36 and A572 Gr. 50.

12

Ωo

7

SYSTEM OVERSTRENGTH

ΩS represents the system overstrength.

EXAMPLES:

Redundancy.

The degree to which non-LFRS elements provide resistance after LFRS has yielded.

13

Ωo

RANGE OF Ωo FOR VARIOUS SYSTEMS:Structural

System

Design

Overstrength

ΩD

Material

Overstrength

ΩM

System

Overstrength

ΩS

Ωo=ΩDΩMΩS ASCE 7

Ωo

Special Moment

Frames (Concrete

and Steel)

1.5-2.5 1.2-1.6 1.0-1.5 2.0-3.5 3.0

Intermediate

Moment Frames

(Concrete and

Steel)

1.0-2.0 1.2-1.6 1.0-2.0 2.0-3.5 3.0

Ordinary Moment

Frames (Concrete

and Steel)

1.0-1.5 1.2-1.6 1.5-2.5 2.0-3.5 3.0

Braced Frames 1.5-2.0 1.2-1.6 1.0-1.5 1.5-2.0 2.0Reinforced

Bearing Wall1.0-1.5 1.2-1.6 1.0-1.5 1.5-2.5 2.5

Unreinforced

Bearing Wall1.0-2.0 0.8-2.0 1.0-2.0 2.0-3.0 2.5

Dual System

(Bracing and

Frame)

1.1-1.75 1.2-1.6 1.0-1.5 1.5-2.5 2.5

8

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.

ASCE 7 Section 12.4.3.2

16

9

Load Combinations with Overstrength

FactorQuestion:

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 12.4.3.2

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.

18

10

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

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12.10.2.1: Collector Elements

SDC C-FCollector elements, splices, and their connections to resisting elements shall resist

the load combinations of Section 12.4.3.2.

“In a way, earthquake engineering is a cartoon . . . Earthquake effects on structures systematically bring out the mistakes made in design and construction, even the minutest mistakes.”

20

11

Ωo Triggers Summary

• 12.4 Load Combinations with Omega zero

• 12.2.5.2 Cantilever Columns SDC B,C,D,E,F

• 12.10.2.1 Collectors – (Light Frame, Wood

excepted) 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.

21

Topics

• R, Cd, Ωo

• Redundancy, ρ


Systems




22

12

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.

23

ASCE 7 SECTION 12.3.4

12.3.4.1: Conditions Where the Value of ρ is 1.0.

• 10 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, ρ

13

1. 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

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).26

REDUNDANCY FACTOR, ρ=1.0

14

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?

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Condition 1: Requires Calculations!

28

15

ASCE 7-10 12.3.4.2

ρ = 1.0 or 1.3


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.

29

ρ = 1.0 or 1.3


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.

30

ASCE 7-10 12.3.4.2

16

Q&A for Redundancy

Question:

Does the redundancy factor apply to the design of foundations?

Answer:

Yes.

31

Question: How many bays are there in shear

wall buildings?

Answer:12.3.4.2-b: The number of bays

for a shear wall is the length of wall

divided by the story height (or two

times the length of shear wall divided by

the story height for light-framed

construction).

32

Q&A for Redundancy

17

Question:

Using Condition 1 to determine ρ for a wood-framed building:

All of the shear walls are relatively long (height of each shear wall is

less than its length). Can I assign ρ=1.0 because there are no shear

walls with an h/lw ratio>1.0?

Answer: Yes33

Q&A for Redundancy

Question:

In Table 12.3-3 does “height-to-length” ratio mean:

• Height-to-length ratio of a story

• Overall height-to-length ratio

Answer:

The h/lw ratio is story height-

to-length ratio.

34

Q&A for Redundancy

18

Question:

Can the value of ρ be different at different levels of the same

building?

Answer:

No.

Question:

Can the value of ρ be different in the two orthogonal directions of

the same building?

Answer:

Yes, ρ can be different for two orthogonal directions when

Condition #1 is being used (not true for Condition 2).

35

Q&A for Redundancy

Question:

If you have a dual system, can you assume ρ=1.0? Table 12.3-3

doesn’t seem to address dual systems?

Answer: No….. “As indicated in the table, braced frame, moment frame,

shear wall, and cantilever column systems must conform to

redundancy requirements. Dual systems also are included

but, in most cases, are inherently redundant. Shear walls

or wall piers with a height-to-length aspect ratio greater than

1.0 within any story have been included; however, the required

design of collector elements and their connections for Ωo

times the design force may address the key issues. In order to satisfy the collector force

requirements, a reasonable number of shear walls usually is required. Regardless, shear wall

systems are addressed in this section so that either an adequate number of wall elements is

included or the proper redundancy factor is applied.”

36

Q&A for Redundancy

19

Question:

Does the redundancy factor need to be determined if dynamic

analysis is used?

Answer:

Yes. The method of analysis doesn’t make the building more or less

redundant.

37

Q&A for Redundancy

Topics

• R, Cd, Ωo

• Redundancy, ρ


Systems




38

20

12.2.2 Horizontal Combinations of Framing Systems

Different Directions

• Different lateral systems may be used to resist seismic forces in each direction.

R, Cd, and Ωo coefficients shall apply to each system.

Special Concrete Shear Walls

R=5.0, Cd=5.0, Ωo=2.5

Sp

eci

al

Ste

el

Mo

me

nt

Fra

me

s

R=

8.0

, C

d=

5.5

, Ω

o=

3.0

Note:

It is possible that one

of the two systems

will limit the overall

system for use and

height. The more

restrictive of the

limitation systems

governs.H

H

HH

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12.2.3 Horizontal Combinations of Framing Systems:

Same Direction

Where different lateral systems are used in combination to resist seismic forces in

the same direction (other than dual systems) the more stringent system

limitation shall apply.

R=5.5

40

21

12.2.3.1 Vertical Combinations of Framing Systems:

Same Direction (ASCE 7-05)

Special Steel Braced Frames above Special Concrete Shear Walls

Exception for rooftop structures, residential

R=6.0

Cd=5.0

Ωo=2.0

R=5.0

Cd=5.0

Ωo=2.5

R: Cannot Increase As You Descend the Building!

Cd, Ωo: Cannot Decrease as You Descend the Building!

R=6.0

Cd=5.0

Ωo=2.0

R=5.0

Cd=5.0

Ωo=2.5

41



Special Concrete Shear Walls Above Special Steel Moment Frames

R=8.0

Cd=5.5

Ωo=3.0

R=5.0

Cd=5.0

Ωo=2.5



R=5.0

Cd=5.0

Ωo=2.5

R=5.0

Cd=5.5

Ωo=3.0

42

22



Special Steel Braced Frames Above Special Concrete Shear Walls Above Special Steel Moment Frames

R=8.0

Cd=5.5

Ωo=3.0

R=5.0

Cd=5.0

Ωo=2.5



R=6.0

Cd=5.0

Ωo=2.0

R=6.0

Cd=5.0

Ωo=2.0

R=5.0

Cd=5.0

Ωo=2.5

R=5.0

Cd=5.5

Ωo=3.0

43



Special Steel Braced Frames Above Special Concrete Shear Walls Above Special Steel Moment Frames

R=8.0

Cd=5.5

Ωo=3.0

R=5.0

Cd=5.0

Ωo=2.5


Cd, Ωo: Correspond to R!

R=6.0

Cd=5.0

Ωo=2.0

R=6.0

Cd=5.0

Ωo=2.0

R=5.0

Cd=5.0

Ωo=2.5

R=5.0

Cd=5.0

Ωo=2.5

44

23

Topics

• R, Cd, Ωo

• Redundancy, ρ


Systems




45

46

Bearing Wall v. Building Frame

24

47

WALL SYSTEM, BEARING: A structural system with bearing walls

providing support for all or major portions of the vertical loads. Shear walls

or braced frames provide seismic force resistance.

BUILDING FRAME SYSTEM: A structural system with an essentially

complete space frame providing support for vertical loads. Seismic force

resistance is provided by shear walls or braced frames.


48

WALL SYSTEM, BEARING: A structural system with bearing walls

providing support for all or major portions of the vertical loads. Shear walls

or braced frames provide seismic force resistance.

BUILDING FRAME SYSTEM: A structural system with an essentially

complete space frame providing support for vertical loads. Seismic force

resistance is provided by shear walls or braced frames.

What about moment frames?


25

49

Question: If some of the gravity loads are resisted by shear walls, is it

possible to classify the system as a building frame system?


50

SEAOC:

Assume all portions of the walls not reinforced as columns or beams are

removed, but the self-weight of the wall is still present.

If wall can support gravity loads and conforms to detailing

requirements for gravity frame members

Building Frame System


26

51

Question: Are walls required to be physically separate

from the building frame system?

Answer: No

Building frame columns can be integral with/

boundary elements.


52

NEHRP:

“A building frame system is when gravity loads are carried primarily by a

frame supported on columns rather than by bearing walls. Some minor

portions of the gravity load may be carried on bearing walls, but the

amount.. should not represent more than a few percent of the building area.”


27

Topics

• R, Cd, Ωo

• Redundancy, ρ


Systems




53

54

28

Permitted Analysis Procedures

55

Height Threshold for Dynamic Analysis

UBC Dynamic for Ht >240’

≈ 24 stories

IBC 2003 T>3.5 Ts

Ts=SD1/SDS≈1/2 sec

T>3.5*1/2=1.75s

≈17-18 stories

IBC 2012 T>3.5Ts AND Ht >160’

≈16 stories

56

29

57

Question: Can I use static (equivalent lateral force procedure) analysis

for the following building:

•SDC F

•Type 1a (torsional) horizontal irregularities

•5-story hotel ballroom (Occupancy III)

•Load bearing metal studs



58

30

59



•SDC F

•Type 1a (torsional) horizontal irregularities

•5-story hotel ballroom (Occupancy III)

•Load bearing metal studs

Answer: ELF Procedure is acceptable. Dynamic analysis is not required!


60



•SDC E

•Type 2 (reentrant corner) vertical irregularities

•2-story office building (Occupancy II)

•Concrete shear walls with steel floor/ roof framing


31


61

62



•SDC E

•Type 2 (reentrant corner) vertical irregularities

•2-story office building (Occupancy II)

•Concrete shear walls with steel floor/ roof framing

Answer: ELF Procedure is acceptable. Dynamic analysis is not

required!


32

63



•SDC D

•175 ft. tall

•No irregularities

•T<3.5Ts



64

33

65



•SDC D

•175 ft. tall

•No irregularities

•T<3.5Ts

Answer: ELF Procedure is acceptable. Dynamic analysis is not

required!


ELF: Equivalent Lateral Force

(Simplified Design Procedure)

12.8 (12.14)

MRS: Modal Response Spectrum 12.9

LTH: Linear Time History 16.1

NTH: Nonlinear Dynamic Time History 16.2

66

34

Topics

• R, Cd, Ωo

• Redundancy, ρ


Systems




67

Definition of horizontal and vertical irregularities in ASCE 7.

Provisions of ASCE 7 triggered by irregularities.

Present changes from ASCE 7-05 to 7-10.

FAQ/ Q&A on structural irregularities.

STRUCTURAL IRREGULARITIES

Photo credit: Eve Fraser-Corp / Foter.com / CC BY-NC

68

35

History of Codes on Irregular Structures:

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.

69

Can you name some irregularities as defined by

ASCE 7?

- Soft story

- Big hole in the diaphragm

- Torsion

70

36

Horizontal Irregularities

71

Vertical Irregularities

72

37

Horizontal Irregularities: Type 1a and 1b

73


Torsional Irregularity

Torsional Irregularity: Maximum story drift (including accidental torsion) at one end of the

structure is more than 1.2 (1.4) times the average of the story drift. Torsional irregularity

requirements apply only to structures in which the diaphragms are rigid or semirigid.

74

δmax

δavg

38

75



δ1=1in.=δ2=2in.

=1.5 in.

1.8<2.0<2.1

1.2xδavg=1.2x1.5=1.8 in.

1.4xδavg=1.4x1.5=2.1 in.

75

76

Horizontal Irregularities: Type 1a


ASCE

Section

Penalty SDC

12.3.3.4 25% increase in seismic forces in connections in

diaphragms and collectors

D, E, F

Table 12.6-1 Permitted Analytical Procedure D, E, F

12.7.3 3D structural model required B, C, D, E, F

12.8.4.3 Amplification of accidental torsion C, D, E, F

12.12.1 Design story drift based on largest difference in deflection C, D, E, F

16.2.2 3D structural model required in non-linear response history

procedure

B, C, D, E, F

39

77



ASCE

Section

Penalty SDC


diaphragms and collectors (unless Ωo already applied)

D, E, F

12.3.3.4

clarified in

ASCE 7-10 but

no substantial

changes.

78



ASCE

Section

Penalty SDC



D, E, F






procedure

B, C, D, E, F

40

79



ASCE

Section

Penalty SDC


80



ASCE

Section

Penalty SDC



D, E, F






procedure

B, C, D, E, F

41

81



ASCE

Section

Penalty SDC


82



ASCE

Section

Penalty SDC



D, E, F






procedure

B, C, D, E, F

42

83



ASCE

Section

Penalty SDC


Torsional Effects

ALL SDC Include inherent and accidental torsion

SDC B Ignore torsional amplification

SDC C, D, E, F Include torsional amplification with Type 1a or Type 1b

irregularities

Change in ASCE 7-10: Exemption for light-framed

construction discontinued.

δmax

δavg

84

43

Why Amplify Accidental Torsion?

85

86



ASCE

Section

Penalty SDC



D, E, F






procedure

B, C, D, E, F

44

87



ASCE

Section

Penalty SDC


88



ASCE

Section

Penalty SDC



D, E, F






procedure

B, C, D, E, F

45

89



ASCE

Section

Penalty SDC


procedure

B, C, D, E, F

90

Horizontal Irregularities: Type 1b

Extreme Torsional Irregularity

ASCE

Section

Penalty SDC

12.3.3.1 Prohibited E, F



D

Table 12.6-1 Permitted Analytical Procedure D

12.7.3 3D structural model required B, C, D

12.8.4.3 Amplification of accidental torsion C, D

12.12.1 Design story drift based on largest difference in deflection C, D


procedure

B, C, D

46

Horizontal Irregularities: Type 2

Reentrant Corner Irregularity



Reentrant Corner Irregularity is defined to exist where both plan projections of the structure

beyond a reentrant corner are greater than 15% of the plan dimension of the structure in the

given direction.

92

47

Question: Do I have a Horizontal Type 2

Irregularity?

Answer:

Regular Structure!



93

94



ASCE

Section

Penalty SDC



D, E, F


48


Diaphragm Discontinuity Irregularity

95



Diaphragm Discontinuity Irregularity: diaphragms with abrupt discontinuities stiffness,

including cutout or open areas greater than 50% of the gross area, or changes in effective

diaphragm stiffness of more than 50% from one story to the next.

96

49

97



ASCE

Section

Penalty SDC



D, E, F


Question: If the roof diaphragm has an opening in

it which results in the stiffness of the 2nd floor

diaphragm being 50% stiffer than the roof, does

that make it irregular? The plan irregularity

definition says story-to-story.

Answer: Yes, it would be considered irregular…

doesn’t matter if floor or roof.

From ICC’s 2006 IBC

Q&A Manual:

98

50

99


Out-of-Plane Offsets Irregularity

99



Out-of-Plane Offsets Irregularity is

where there are discontinuities in a

lateral force-resistance path, such as

out-of-plane offsets of the vertical

elements.

100Image from FEMA Educational Material

51

101



ASCE

Section

Penalty SDC

12.3.3.3 Axial force using load combinations with overstrength for

discontinuous elements.

B, C, D, E, F



D, E, F




procedure

B, C, D, E, F



ASCE

Section

Penalty SDC



B, C, D, E, F

MASONRY

SHEAR WALL

ELEMENTS SUPPORTING

DISCONTINOUS WALL

102

52


Non-Parallel Systems Irregularity



Nonparallel Systems Irregularity

Nonparallel Systems-Irregularity is defined to exist where the vertical lateral force-resisting

elements are not parallel to or symmetric about the major orthogonal axes of the seismic

force–resisting system.

104

53

105



ASCE

Section

Penalty SDC

12.5.3 Orthogonal load combinations C, D, E, F




procedure

B, C, D, E, F

106



ASCE

Section

Penalty SDC

12.5.3 Orthogonal load combinations C, D, E, F

12.5.3: Two procedures permitted:

1) Orthogonal combination procedure with loading applied

independently in orthogonal directions:

100% x-effects + 30% y-effects or 30% x-effects and 100% y-effects

2) Simultaneous application of orthogonal ground motion.

12.5.2: SDC B seismic forces are permitted to be applied in each

orthogonal directions and interaction effects are permitted to be

neglected.

54




Does the plan below have a horizontal irregularity Type 5?

Nonparallel Systems-Irregularity is defined to exist where the vertical lateral force-

resisting elements are not parallel to or symmetric about the major orthogonal axes of

the seismic force–resisting system.

ASCE 7-05:

Irregular!

ASCE 7-10:

??

107




•The ASCE 7-05 text of “parallel to or symmetric about” was sometimes misread to

require that the system be both parallel to and symmetric about the major orthogonal

axes.

•The revised definition of nonparallel systems irregularity clarifies that it only applies

where the vertical elements are not parallel to the major orthogonal axes.

ASCE 7-05:

Irregular!

ASCE 7-10:

Regular!

108

55

Vertical Irregularities: Type 1a and 1b

Stiffness- Soft Story Irregularity

109



Stiffness (Soft Story) Irregularity

Stiffness-Soft Story Irregularity is where there is a story

where the lateral stiffness is less than 70 (60)% of that in

the story above or less than 80 (70)% of the average

stiffness of the three stories above.


56

Question: Why might a soft story exist?

•Architectural constraints/ parking garage at base.

•Increased story height.

•Change in lateral-force-resisting system.

•Connection to base/ foundation.

•Openings in a wall.

•Change in size/ shape of an element.

111

112

Vertical Irregularities: Type 1a


ASCE

Section

Penalty SDC


57

113

Vertical Irregularities: Type 1b

Stiffness- Extreme Soft Story Irregularity

ASCE

Section

Penalty SDC



Vertical Irregularities: Type 2

Weight (Mass) Irregularity

114

58

Weight (Mass) Irregularity is when the mass of any story is more than 150% of the mass of

an adjacent story. A roof that is lighter than the floor below need not be considered.




116

ASCE

Section

Penalty SDC




59

12.3.2.2 Exceptions:

Vertical irregularities Type 1a, 1b, 2 do not apply where:

No story drift ratio is greater than 130% of the drift ratio of

the next story.

1-story buildings in any SDC or 2-story buildings in SDC

B, C, D.

Vertical Irregularities: Type 1a, 1b, and 2

Soft-Story and Weight Irregularities

117

Question:

Does this building have a soft story (Type 1a or 1b)?

h1=16’-0”

htyp=12’-0”

118

60

119

Stiffness ratio, 1st story to 2nd story:

60.042.0

12

1

16

1

3

3

<= Extreme soft story!

Answer: Yes, type 1b, extreme soft story?!?

Exception 1

120

Exception: No story drift ratio is greater than 130% of the drift ratio of the next story.

00352.01212

43.082.03.100224.0

1216

43.0

3.1

2

12

1

1

=−<=

−<

xx

hh

eeeδδδ

Answer: No vertical

irregularity (soft/

extreme soft).

Table from SK

Ghosh

61


Vertical Geometric Irregularity

121

Vertical (Geometric) Irregularity

Vertical Geometric Irregularity is when the horizontal dimension of the seismic force–

resisting system is more than 130% of that in an adjacent story.




62

123

ASCE

Section

Penalty SDC





In-Plane Discontinuity in Vertical Lateral Force-

Resisting Element Irregularity

124

63

In-Plane Discontinuity in Vertical Lateral Force-Resisting Element Irregularity is when an in-

plane offset of the lateral force-resisting elements is greater than the length of those

elements or there exists a reduction in stiffness of the resisting element in the story below.





126

ASCE

Section

Penalty SDC



B, C, D, E, F



D, E, F


Vertical Irregularities: Type 4In-Plane Discontinuity in Vertical Lateral Force-


64




Does not qualify as a Vertical Irregularity 4 in

ASCE 7-05!

127


Discontinuity in Lateral Strength- Weak Story

Irregularity

128

65

Discontinuity in Lateral Strength–Weak Story Irregularity is when

the story lateral strength is less than 80 (65)% of that in the story

above. The story lateral strength is the total lateral strength of all

seismic-resisting elements sharing the story shear for the

direction under consideration.



Irregularity


130

ASCE

Section

Penalty SDC


Table 12.6-1 Permitted Analytical Procedure D

Vertical Irregularities: Type 5a


Irregularity

66

131

ASCE

Section

Penalty SDC

12.3.3.1 Prohibited D, E, F

12.3.3.2 Cannot exceed 2 stories or 30 feet (see exception) B, C

Exception 12.3.3.2:

The limit does not apply where the “weak” story is capable of resisting a total

seismic force equal to Ωo times the design force.

Vertical Irregularities: Type 5b

Discontinuity in Lateral Strength- Extreme Weak

Story Irregularity

Question:What’s the difference between a soft story (1a/1b) and a weak story (5a/ 5b)?

Answer:Soft Stiffness

Weak Strength

A soft story will:

A weak story will:

Question:Is a soft story is always a weak story

or vice versa?

Answer:???

fail under less force than the adjacent stories.drift more than the adjacent stories.

Vertical Irregularities: Soft V. Weak Story

132

67

Summary:

1) Understand the methodology built into the code values for R, Cd, Ωo.

2) The code attempts to steer engineers into redundant, ductile designs with

a linear load path.

3) Irregular structures routinely perform worse in seismic events, even when

properly detailed.

Emily Guglielmo

[email protected]

415-814-0030133

Documents

Clarifying Frequently Misunderstood Seismic Provisions · Clarifying Frequently Misunderstood Seismic Provisions ... Collector elements, ... All of the shear walls are relatively