lesson 6 asce 7-10: basic requirements for structural design

U.S. Army Corps of Engineers Basic Requirements 1

Lesson 6

ASCE 7-10: Basic Requirements for Structural Design


Outline of Lesson

Basic requirements for all structures• Purpose, exclusions• MCE motion on rock, site amplification, design values• Risk categories, seismic design categories• Geological hazards, geotechnical studies


Outline of Lesson

Basic requirements for building structures• Load path, strength, stiffness• System limits and design parameters• Irregularities and redundancy• Load combinations• Diaphragms, walls, foundations• Drift limits• Simplified alternate


Governing Documents


2012 IBC Chapterfor Seismic Loading

Chapter 16 Structural Design

• Section 1613 Earthquake Loads – Refers to ASCE 7-10, excluding Chapter 14 and Appendix 11A, with one alternative to ASCE 7 provisions.


ASCE 7-10 Section 11.1.1Purpose

“…specified earthquake loads are based upon post-elastic energy dissipation in the structure, and because of this fact, the requirements for design, detailing, and construction shall be satisfied even for structures and members for which load combinations that do not contain earthquake loads indicate larger demands than combinations that include earthquake loads.…”


ASCE 7-10 Section 11.1.2 Scope of Coverage

Every structure, and portion thereof, including nonstructural components

Certain nonbuilding structures, as described in ASCE 7-10 Chapter 15


ASCE 7-10 Section 11.1.2 Exceptions

Detached one- and two-family dwellings located where SS < 0.4 or

where the Seismic Design Category is A, B, or C.

Detached one- and two-family wood-frame dwellings with not more than two stories and that comply with the IRC.

Agricultural storage structures that are intended only for incidental human occupancy.

Structures that require special consideration of their response characteristics and environment that are not addressed in Chapter 15 and for which other regulations provide seismic criteria, such as vehicular bridges, electrical transmission towers, hydraulic structures, buried utility lines and their appurtenances, and nuclear reactors.


Seismic Design Steps

STEP 1: Determine mapped ground motion at building site

STEP 2: Determine Site Class at the building site

STEP 3: Determine design ground motion at building site

STEP 4: Determine building occupancy and associated Risk Category

STEP 5: Determine Seismic Design Category

STEP 6: Perform analytical modeling of the structure

STEP 7: Determine if horizontal and vertical structural irregularities exist


Steps of Seismic Design

STEP 8: Determine redundancy and structural system requirements (Lesson 7)

STEP 9: Determine permitted analysis procedure and seismic force distribution in the structure (Lesson 18)

STEP 10: Perform structural analysis (Not discussed)

STEP 11: Use appropriate load combinations to calculate member forces (Lesson 9)

STEP 12: Design structural members in accordance with the 2012 IBC, ASCE 7-10 and various material standards (Lessons 10 through 17, 20 through 24)


Mapped Ground Motion

As discussed in Lesson 3,

ASCE 7-05: Maximum Considered Earthquake (MCE) - Ground motion that has a 2% probability of exeedance in 50 years.

ASCE 7-10: Risk-Targeted Maximum Considered Earthquake (MCER) - Ground motion associated

with a 1% probability of structural collapse in 50 years.

UNIFORM EQ HAZARD UNIFORM RISK OF COLLAPSE

STEP 1



Determine SS and S1 in accordance with Section 2-

1.6.1 of UFC 3-301-01 for locations within the U.S

• Table E-3 of UFC 3-301-01

• USGS Seismic Design Maps Web Application with the approval of the authority having jurisdiction

http://geohazards.usgs.gov/designmaps/us/application.php

STEP 1



UFC 3-310-01

STEP 1



Select Building Code: 2012 IBC/ASCE 7-10

Select Site Classification

STEP 1



Type in the Latitude and Longitude of the building site

OR

Enter Street Address

STEP 1



Input Information

SS, S1, SMS, SM1, SDS, SD1 Values

MCER and Design Response Spectra

Output is displayed in a separate window. Make sure to disable any pop-up blocker on your web browser

View Detailed Report

STEP 1



Determine SS and S1 in accordance with Section 2-

1.6.2 of UFC 3-301-01 for locations outside of the U.S

• Table F-3 of UFC 3-301-01

• For locations not shown, use best available information with the approval of the authority having jurisdiction or use Appendix G of UFC 3-301-01

https://geohazards.usgs.gov/secure/designmaps/ww/application.php

STEP 1


Mapped Ground MotionSTEP 1



UFC 3-310-04 Section 2-1613.7 requires a site-specific response analysis for structures on sites classified as Site Class F (see ASCE 7-10 Section 20.3.1), unless:

SS ≤ 0.25, and

S1 ≤ 0.10

as determined in accordance with UFC 3-301-01.

STEP 1


Use ASCE 7-10 Chapter 20 to classify the site as below

A Hard Rock B RockC Very Dense Soil or Soft RockD Stiff SoilE Soft clay soilF Soils requiring site response analysis in

accordance with Section 21.1

Site ClassificationSTEP 2


Site Classification

Table 20.3-1 Site ClassificationSite Class

A. Hard Rock > 5000 ft/s NA NA

B. Rock 2500 ft/s to 5000 ft/s NA Na

C. Very dense soil and soft rock 1200 ft/s to 2500 ft/s > 50 > 2000 psf

D. Stiff Soil 600 ft/s to 1200 ft/s 15 to 50 1000 psf to 2000 psf

E. Soft Clay Soil < 600 ft/s <15 < 1000 psf

Any profile with more than 10 ft of soil having the following characteristics:

• Plasticity Index, PI > 20

• Moisture content, w >= 40%

• Undrained shear strength, su < 500 psf

F. Soils requiring site response analysis in accordance with 21.1

Section 20.3.1

N or Nch SUVU

STEP 2


Site Classification

General Site Classification1. Based on the upper 100 ft of the site profile and Table 20.3-1

2. If site-specific data are not available to a depth of 100 ft, soil properties may be estimated based on known geologic conditions.

3. Where properties are not known in sufficient detail, Site Class D shall be used unless it is determined that Site Class E or F is present.

4. Site Class A or B shall not be used if there is more than 10 ft of soil between the rock surface and the bottom of the spread footing or mat foundation.

STEP 2


Site Classification

Conditions

1. Soils vulnerable to potential failure or collapse under seismic loading, such as liquefiable soils, quick and highly sensitive clays, and collapsible weakly cemented soils.

2. Peats and/or highly organic clays (H > 10 ft) of peat and/or highly organic clay where H = thickness of soil.

3. Very highly plasticity clays (H > 25 ft with PI > 75)

4. Very thick soft/medium stiff clays (H>120ft) with Su < 1000psf

Site Class F, Section 20.3.1

STEP 2


Site Classification

ASCE 7-10 Section 20.1

Site Class D must be used when the soil properties are not known in sufficient detail, unless the building official determines that Site Class E or F is likely to be present at the site

STEP 2


Site AmplificationSTEP 2

See Lesson 4


Fa: Site Coefficient for SS

Site Class

Mapped Spectral Response Acc. at Short Periods

SS 0.25 SS = 0.5 SS = 0.75 SS = 1.00 SS 1.25

A 0.8 0.8 0.8 0.8 0.8

B 1.0 1.0 1.0 1.0 1.0

C 1.2 1.2 1.1 1.0 1.0

D 1.6 1.4 1.2 1.1 1.0

E 2.5 1.7 1.2 0.9 0.9

F See ASCE 7-10 Section 11.4.7

ASCE 7-10 Table 11.4-1

STEP 2


Fv: Site Coefficient for S1

ASCE 7-10 Table 11.4-2

Site Class

Mapped Spectral Response Acc. at 1-Second Period

S1 0.1 S1 = 0.2 S1 = 0.3 S1 = 0.4 S1 0.5

A 0.8 0.8 0.8 0.8 0.8

B 1.0 1.0 1.0 1.0 1.0

C 1.7 1.6 1.5 1.4 1.3

D 2.4 2.0 1.8 1.6 1.5

E 3.5 3.2 2.8 2.4 2.4

F See ASCE 7-10 Section 11.4.7

STEP 2


Site-Adjusted Ground Motion

Maps all drawn for one reference site condition: rock (Site Class B)

Determine the MCER motion at a specific site by adjusting for the Site Class at the site:

SMS = Fa SS

SM1 = Fv S1

STEP 2


0.00

0.21

0.42

0.63

0.84

1.05

0 1 2 3 4 5

Period, sec.

Sp

ec

tra

l Ac

ce

lera

tio

n, g

.

SMS = FASS = 1.2(0.75)=0.9g

SM1 = FVS1 = 1.8(0.30) = 0.54gBasic

Site Amplified

MCER Response Spectra Modified for Site Class D

STEP 2

Period, sec

Sp

ectr

al A

ccel

erat

ion

, g


Design Ground Motion

Structures are designed for a Collapse Prevention performance criterion under MCER

Design spectral acceleration values account for expected reserve strength in a structure

SDS = 2/3 Fa SS

SD1 = 2/3 Fv S1

STEP 3


0.00

0.20

0.40

0.60

0.80

1.00

0 1 2 3 4 5

Period, sec.

Sp

ec

tra

l Ac

ele

rati

on

, g.

SDS = (2/3)(0.90) = 0.60g

SD1 = (2/3)(0.54) = 0.36g

Basic

Site Amplified

Scaled

STEP 3

Design Response Spectra

Period, sec

Sp

ectr

al A

ccel

erat

ion

, g


Design Response Spectrum

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 1 2 3 4 5 6 7

TST0 Period, sec

Spe

ctra

l Acc

eler

atio

n, g

0.4SDS

Sa = SD1 / T

Sa = SDS(0.4 + 0.6 T/T0)

Sa = SD1 TL / T2

Drawn for SS = 1.0, Fa = 1.0 S1 = 0.4, Fv = 1.5 TL = 4

STEP 3


TL Map of Contiguous USA(ASCE 7-10 Figure 22.12)

STEP 3


Building Occupancy and Risk Category

Occupancy of a building determines its required performance level

Buildings are assigned Risk Categories based on their occupancies

Higher Risk Category requires more stringent design requirements

STEP 4



UFC 3-310-04 Section 2-202

[Replacement] RISK CATEGORY.

A categorization of buildings and other structures for determination of flood, wind, snow, ice, and earthquake loads based on the risk associated with unacceptable performance as prescribed in UFC 3-301-01 Table 2-2.

STEP 4



OccupancyRisk

Category

Structures with low hazard to human life in the event of failure I

Standard occupancies II

Structures with substantial hazard to human life or economy in the event of failure. III

Designated essential facilities; utilities required for essential facilities; designated aviation-related structures; designated DoD mission-essential facilities not assigned to Risk Category V; and structures containing highly toxic materials

IV

Facilities designed as national strategic military assets. V

See UFC 3-301-01 Table 2-2 for detailed descriptions

STEP 4


Risk Category Ie

I or II 1.0

III 1.25

IV 1.5

Seismic Importance Factor, Ie

(ASCE 7-10 Table 1.5-2)

Buildings assigned to a higher risk category are designed for a higher level of seismic force by using Importance Factor larger than one.

STEP 4


Ie=1.0Ie=1.25Ie=1.5

Roof Displacement

Base ShearElastic

Seismic Importance Factor, Ie

(ASCE 7-10 Table 1.5-2)Systems with Ie =1.5 have lower ductility demands

than systems with Ie =1.0, and will likely have less

damage.

STEP 4


Performance Basis

Emergency Response Ie = 1.5

Opera

tiona

l

Imm

ediat

e

Occup

ancy

Life

Safe

Collap

se

Preve

ntion

Frequent

Design

Maximum Considered

Building Performance

Gro

un

d M

oti

on Ordinary Buildings Ie = 1.0

High Occupancy Ie = 1.25

STEP 4


STEP 5 Seismic Design Category(ASCE 7-10 Section 11.6)

Seismic Design Category (SDC) is a function of the seismic hazard at the location of a structure, the occupancy or the Risk Category of the structure, and the Site Class at the site of the structure.

Most seismic requirements are based on the Seismic Design Category of a structure.


SDC Based on SDS

(Adapted from ASCE 7-10 Table 11.6-1)

Values of SDS

Risk Category

I or II III IV V

SDS < 0.167 A A A

NOT NECESSARY

0.167 < SDS < 0.33 B B C

0.33 < SDS < 0.50 C C D

0.50 < SDS D D D

STEP 5


SDC Based on SD1

(Adapted from ASCE 7-10 Table 11.6-2)

Values of SD1

Risk Category

I or II III IV V

SD1 < 0.067 A A A

NOT NECESSARY

0.067 < SD1 < 0.133 B B C

0.133 < SD1 < 0.20 C C D

0.20 < SD1 D D D

STEP 5


Seismic Design Category ASCE 7-05 Section 11.6

SDC is to be determined from ASCE 7-10 Tables 11.6-1 (based on SDS) and 11.6-2 (based on SD1), and the

more severe one governs.

STEP 5



SDC can be based on SDS alone, provided

• S1 < 0.75g

• Ta < 0.8Ts

• T used to calculate story drift < Ts

• Upper-bound design base shear is used in design

• Diaphragms are rigid, or for diaphragms that are flexible, vertical elements of seismic-force-resisting system are spaced at < 40 ft

STEP 5



SDC can be based on SDS alone for the simplified

design method of ASCE 7-10 Section 12.14 using the SDS value determined in Section 12.14.8.1.

STEP 5


SDC for Low Seismic HazardASCE 7-10 Section 11.6

Values of S1

Risk Category

I or II III IV V

SS ≤ 0.15g and

S1 ≤ 0.04gA A A

NOT NECESSARY

STEP 5


SDC for Low Seismic HazardASCE 7-10 Section 11.4.1

STEP 5

SS 0.15g and S1 0.04g (shown by Green shading)


SDC for High Seismic HazardASCE 7-10 Section 11.6

Values of S1

Risk Category

I or II III IV V

S1 ≥ 0.75g E E FNOT

NECESSARY

STEP 5


SDC for High Seismic HazardASCE 7-10 Section 11.6

STEP 5

S1 ≥ 0.75g (shown by Red shading)


Geologic Hazards andGeotechnical Investigations

SDC C:• Evaluate slope instability, liquefaction, differential

settlement, surface displacement

SDC D, E, F:• More detail than C plus lateral pressures on

basement walls and retaining walls and liqefaction potential

SDC E and F:• Do not locate on active fault

STEP 5


Analytical ModelingASCE Section 12.7

Mathematical model:

To include Strength and stiffness of all members significant to structural response

To represent spatial distribution of the mass and the stiffness

The structure can be considered fixed at the base. Alternatively, where foundation flexibility is to be considered, it needs to be in accordance with Section 12.13.3 or Chapter 19.

STEP 6



In mathematical model,

Cracked sections are to be considered for reinforced concrete and masonry elements

Panel zone deformations are to be considered for steel moment frames

STEP 6



3D modeling requirement:

3D model is required when presence of certain horizonntal structural irregularities can create torsional effects

Not necessary when diaphragms are flexible, except for Horizontal Irregularity Type 5

STEP 6



Diaphragm flexibility:

ASCE 7-10 requires modeling of diaphragm stiffness characteristic when diaphragms can not be classified as “rigid” or “flexible”.

This is NOT NECESSARY according to the 2012 IBC, where any diaphragm that is not flexible is considered rigid.

STEP 6


W = Effective Weight of Building

W = Total Dead Load and Applicable Portions of Other Loads

25% of storage live load (2 exceptions)

Minimum 10 psf partition load

Weight of permanent equipment

20% of uniform design snow load where flat roof snow load exceeds 30 psf

Weight of landscaping and roof gardens

STEP 6


Structural Irregularities

Horizontal (Plan)

Vertical

Photo Credit: EERI Earthquake Damage Slide Set

STEP 7


Horizontal Irregularities

Horizontal Irregularities (Table 12.3-1)

1a. Torsional irregularity

1b. Extreme torsional irregularity

2. Re-entrant corners

3. Diaphragm discontinuity

4. Out-of-plane offsets

5. Nonparallel systems

STEP 7


Vertical Irregularities

Vertical Irregularities (Table 12.3-2)

1a. Stiffness irregularity – soft story,

1b. Stiffness irregularity – extreme soft story

2. Weight (mass) irregularity

3. Vertical geometric irregularity

4. In-plane discontinuity in vertical lateral force- resisting elements

5a. Discontinuity in lateral strength – weak story,

5b. Discontinuity in lateral strength – extreme weak story

STEP 7


Horizontal Irregularity 1a: Torsional Irregularity

Calculation of dA and

dB includes accidental

torsion, with Ax = 1.0.

STEP 7


Horizontal Irregularity 1a: Torsional Irregularity

Referenced in:12.3.3.4 – 25% increase in seismic forces in

connections in diaphragms and collectorsTable 12.6-1 – Permitted analytical procedure12.7.3 – 3-D structural model required12.8.4.3 – Amplification of accidental torsion12.12.1 – Design story drift based on largest

difference in deflection 16.2.2 - 3-D structural model required in nonlinear

response history procedure

STEP 7


Horizontal Irregularity 1b: Extreme Torsional Irregularity

STEP 7



Referenced in:

12.3.3.1 – Prohibited in SDC E and F

12.3.3.4 – 25% increase in seismic forces in connections in diaphragms and collectors

Table 12.6-1 – Permitted analytical procedure

12.7.3 – 3-D structural model required

12.8.4.3 – Amplification of accidental torsion

STEP 7



12.12.1 – Design story drift based on largest difference in deflection

16.2.2 - 3-D structural model required in nonlinear response history procedure

STEP 7


Horizontal Irregularity 2: Reentrant Corner Irregularity

RE-ENTRANT CORNER EXISTS WHEN PROJECTION b > 0.15a, AND PROJECTION d > 0.15c

STEP 7


Horizontal Irregularity 2: Reentrant Corner Irregularity

Referenced in:



STEP 7


Design Force Increase due to Irregularities

Re-entrant corners may form coupled wings, which may respond in an opening and closing fashion. This may give rise to high stresses in the vicinity of re-entrant corners.

STEP 7


Horizontal Irregularity 3: Diaphragm Discontinuity Irregularity

DIAPHRAGM DISCONTINUITY EXISTS WHEN AREA OF OPENING > 0.5ab OREFFECTIVE DIAPHRAGM STIFFNESS CHANGES MORE THAN 50% FROM ONE STORY TO THE NEXT.

STEP 7


Horizontal Irregularity 3: Diaphragm Discontinuity Irregularity

Referenced in:



STEP 7


Horizontal Irregularity 4: Out-of-Plane Offsets Irregularity

ASCE 7-10 clarifies: “….at least one of the vertical elements.”

STEP 7



STEP 7



Referenced in:

12.3.3.3 – Axial force using load combinations with overstrength for discontinuous elements





STEP 7


Horizontal Irregularity 5: Nonparallel Systems Irregularity

STEP 7


Nonparallel system Irregularity exists when the vertical lateral force- resisting elements are not parallel to the major orthogonal axes of the seismic force resisting system.


STEP 7



STEP 7



Referenced in:

12.5.3 – Orthogonal load combinations in SDC C




STEP 7


Vertical Irregularities

Vertical Irregularities (ASCE 7-05 Table 12.3-2)

1a. Stiffness irregularity – soft story,

1b. Stiffness irregularity – extreme soft story

2. Weight (mass) irregularity

3. Vertical geometric irregularity

4. In-plane discontinuity in vertical lateral-force- resisting elements

5a. Discontinuity in lateral strength – weak story,

5b. Discontinuity in lateral strength – extreme weak story

STEP 7


Vertical Irregularity 1a: Stiffness-Soft Story Irregularity

SOFT STORY STIFFNESS < 70% STORY STIFFNESS ABOVE < 80% [AVERAGE STORY STIFFNESS OF 3 STORIES ABOVE]

STEP 7



STEP 7



Referenced in:


STEP 7


Vertical Irregularity 1b: Stiffness-Extreme Soft Story Irregularity

SOFT STORY STIFFNESS < 60% STORY STIFFNESS ABOVE < 70% [AVERAGE STORY STIFFNESS OF 3 STORIES ABOVE]

STEP 7


Vertical Irregularity 1b: Stiffness-Extreme Soft Story Irregularity

Referenced in:



STEP 7


Vertical Irregularity 2: Weight (Mass) Irregularity

STORY MASS > 150% ADJACENT STORY MASS(A ROOF THAT IS LIGHTER THAN THE FLOOR BELOW NEED NOT BE CONSIDERED)

STEP 7


Vertical Irregularity 2: Weight (Mass) Irregularity

Referenced in:


STEP 7


Vertical IrregularitiesException 1 to 12.3.2.2

Vertical structural irregularities of Types 1a, 1b, or 2 in Table 12.3-2 do not apply where no story drift ratio under design lateral seismic force is greater than 130 percent of the story drift ratio of the next story above. Torsional effects need not be considered in the calculation of story drifts. The story drift ratio relationship for the top two stories of the structure are not required to be evaluated.

STEP 7


Vertical IrregularitiesException 2 to 12.3.2.2

Irregularities Types 1a, 1b, and 2 of Table 12.3-2 are not required to be considered for one-story buildings in any seismic design category or for two-story buildings assigned to Seismic Design Categories B, C, or D.

STEP 7


Vertical Irregularity 3: Vertical Geometric Irregularity

HORIZONTAL DIMENSION OF LATERAL FORCE-RESISTING SYSTEM IN STORY > 130% OF THAT IN ADJACENT STORY

STEP 7



STEP 7



Referenced in:


STEP 7


Vertical Irregularity 4: In-Plane Discontinuity

IN-PLANE OFFSET OF LATERAL FORCE-RESISTING ELEMENTS > LENGTH OF THOSEELEMENTS, OR REDUCTION IN STIFFNESS OF RESISTING ELEMENTS IN STORY BELOW

STEP 7


d

offset

Irregularity exists if the offset isgreater than the width (d) or thereexists a reduction in stiffness of thestory below.

Source: FEMA


STEP 7



There is an in-plane offset of a vertical seismic force-resisting element resulting in overturning demands on a supporting beam, column, truss, or slab.

STEP 7



Referenced in:

12.3.3.3 – Axial force using load combinations with overstrength for discontinuous elements



STEP 7


Vertical Irregularity 5a: Weak Story Irregularity

“WEAK STORY” LATERAL STRENGTH < 80% LATERAL STRENGTH ABOVE STORYLATERAL STRENGTH = TOTAL STRENGTH OF SEISMIC FORCE-RESISTING ELEMENTS

STEP 7


Vertical Irregularity 5a: Weak Story Irregularity

Referenced in:



STEP 7


Vertical Irregularity 5b: Extreme Weak Story Irregularity

“WEAK STORY” LATERAL STRENGTH < 65% LATERAL STRENGTH ABOVE STORYLATERAL STRENGTH = TOTAL STRENGTH OF SEISMIC FORCE-RESISTING ELEMENTS

STEP 7



Referenced in:

12.3.3.1 – Prohibited in SDC D, E and F

12.3.3.2 – Not exceed over two stories or 30 ft (9 m) in height (see Exception)


STEP 7



12.3.3.2 Extreme Weak Stories.

EXCEPTION: The limit does not apply where the “weak” story is capable of resisting a total seismic force equal to 0 times the design force prescribed in

Section 12.8.

STEP 7


Steps of Seismic Design

STEP 8: Determine redundancy and structural system requirements (Lesson 7)

STEP 9: Determine permitted analysis procedure and seismic force distribution in the structure (Lesson 18)

STEP 10: Perform structural analysis (Not discussed)

STEP 11: Use appropriate load combinations to calculate member forces (Lesson 9)

STEP 12: Design structural members in accordance with the 2012 IBC, ASCE 7-10 and various material standards (Lessons 10 through 17, 20 through 24)


Thank You!!

Any Remaining Questions?


SUPPLEMENTAL INFORMATION


Structural Irregularities and Seismic Performance

The configuration of a structure can significantly affect its performance during a strong earthquake that produces the ground motion contemplated in the IBC/ASCE 7.


Structural Irregularities and Seismic Performance

IBC/ASCE 7 seismic design provisions were developed basically for regular buildings.

Past earthquakes have repeatedly shown that irregular buildings suffer greater damage than regular buildings.

This happens even with good design and construction.


Earthquake Experience

Magnitude 7.1 Loma Prieta, CA Earthquake of October 17, 1989: Three and four story wood frame, brick veneer buildings in the Marina District of San Francisco sustained damage as a consequence of the ground shaking (10%g at rock sites, 20 to 30%g on sites underlain by bay mud) and liquefaction. The soft first story made the buildings more vulnerable. Buildings at corners of blocks sustained heavier damage than those within the block



Marina District…a stiffness issue (soft story).




Magnitude 6.5 San Fernando, CA earthquake of February 9, 1971: Failure of columns of “soft story” Olive View Hospital. The failure of the canopy pinned the ambulances, rendering them useless. Ground shaking is estimated to have reached approximately 100%g at the site.



1976 Philippines Earthquake: Damage to reinforced concrete building. Torsion (or twisting) of structures is a common cause of failure when the centers of mass and stiffness are different. Buildings at corners of blocks are often more vulnerable than those within the block because two sides are open (e.g. glass windows for advertising) and two sides are solid (e.g. at property lines). The first floor has pancaked.



1985 Mexico City Earthquake: Triangular structures (“flat iron” buildings) created because the streets are not at right angles with each other are even more vulnerable than square buildings at corners of blocks. These buildings have only one solid and two glass walls. Note the torsional distress.


Reasons for Poor Seismic Performance of Irregular Structures

In a regular structure, inelastic demands produced by strong ground shaking tend to be well distributed throughout the structure, resulting in a dispersion of energy dissipation and damage.

In irregular structures, inelastic behavior can concentrate in the zone of irregularity, resulting in rapid failure of structural elements in these areas.



Some irregularities introduce unanticipated stresses into the structure, which designers frequently overlook when detailing the structural system.



Elastic analysis methods typically employed in structural design often cannot predict the distribution of earthquake demands in an irregular structure very well, leading to inadequate design in the zone of irregularity.


Code Regulations Concerning Irregular Structures

Introduced in the 1988 Uniform Building Code (UBC). Evolved since then.

Thrust is to encourage that buildings be designed to have regular configurations.

Important feature is prohibition of gross irregularity in buildings located on sites close to major faults, where very strong ground motion and extreme inelastic demands can be experienced.


Q/A Horizontal Irregularity 3: Diaphragm Discontinuity Irregularity

Q. If the roof diaphragm has an opening in it which results in the stiffness of the 2nd floor diaphragm being 50 percent stiffer than the roof, does that make it irregular? The plan irregularity definition says story to story.

A. Yes, it would be considered irregular...doesn't matter if floor or roof.

STEP 7



Q. I was wondering whether you have heard anything about any exceptions in applying the Type 3 horizontal irregularity. This is the diaphragm stiffness irregularity. One test is if the diaphragm stiffnesses at neighboring stories differ by more than 50%. This has been around since the UBC.

STEP 7



Q (Contd.). It would appear that almost all buildings get hit with this considering metal deck roof diaphragms are always much more flexible than the concrete over metal deck floor systems below. If so, then does the whole building get the penalty for the irregularity regardless of number of stories, or just the offending diaphragm level?

STEP 7



A. “In my heart I want to say that we exclude roof diaphragms from the analysis because it is not a "story". The intent seems related to buildings having floor openings, etc. I admit I am having trouble finding a specific exception or clarification to that regard, however, I am trying to look closely at the definition of the word "story“”.

STEP 7



A. “I agree that the intent is probably to compare the floor diaphragm stiffnesses of adjacent stories. Each story has a floor diaphragm. In a 5 story building, the 5th floor is the diaphragm for the 5th story, the 4th floor is the diaphragm for the 4th story, and so on. Having said that, there does not appear to be an exception that excludes the roof diaphragm nor does the language in the code explicitly exclude the roof diaphragm so it is a little problematic to say it doesn't apply to the roof diaphragm. The requirement dates back to the 1988 UBC so it may have been part of the "Plan Configuration" provisions in ATC 3-06. I don't have a copy of ATC 3-06, so I'm not sure about this.”

STEP 7



A. “Oddly enough, I can’t find any indication that it has been dealt with before in code opinion or examples. I would like to believe that roof diaphragms are not included in this assessment, because so many more buildings would be irregular. I’m guessing it is typically overlooked by default. If you really split hairs on the definition of story it only goes to the top of the rafters on the topmost floor, so the roof diaphragm is not part of the topmost story – there problem solved! Would that work?”

See definition of STORY in 2006, 2009 IBC Section 202.

STEP 7


Typical Plan of Example Building

A

B

C

D

1 2

3 4

5 6

26 - 0

22

N

7 8

26 - 0

26 - 0 26 - 0

26 - 0 26 - 0

26 - 0

22

22

STEP 7


Typical Elevation of Example Building

10

11

12

7

8

9

4

5

6

1

2

3

11@ 12-0 =132-0

16-0

STEP 7Exception 1 to 12.3.2.2


Vertical Structural Irregularity

Thus, per Table 12.3-2, Stiffness-Extreme Soft Story Irregularity (Vertical Irregularity Type 1b) should be considered.

16 ft

12 ft

1st story

2nd story Stiffness ratio =

Stiffness of first to second story =

(1/163)/(1/123) =0.42 < 0.60



Vertical Structural Irregularity Exception 1 to ASCE 7-05 Section 12.3.2.2

Vertical structural irregularities of Type 1a, 1b, or 2 in ASCE Table 12.3-2 do not apply where no story drift ratio under design lateral seismic force is greater than 130 percent of the story drift ratio of the next story above.

2

12

1

1 δ-δ31<

δ

h.

heee



Analysis Results (E-W Direction)Story Force (kips) dxe (in.) dx (in.) D (in.)

12 275 3.51 19.31 0.45

11 268 3.43 18.86 0.75

10 234 3.29 18.11 1.05

9 202 3.10 17.06 1.31

8 171 2.86 15.75 1.53

7 142 2.59 14.22 1.72

6 114 2.27 12.50 1.86

5 89 1.93 10.64 1.98

4 65 1.58 8.66 2.05

3 44 1.20 6.61 2.12

2 26 0.82 4.49 2.12

1 12 0.43 2.37 2.37



Vertical Structural Irregularity

E-W Direction:

Thus, structural irregularity of Type 1b is deemed NOT to exist.

002240=12×16

430=

δ

1

1 ..

he

002240>003520=12×12

430-82031=

δ-δ31

2

12 ....

.h

. ee


lesson 6 asce 7-10: basic requirements for structural design

Documents

seismic design categorystep

basic requirements lesson

seismic design stepsstep

design ground motion

steps of seismic

seismic criteria

building sitestep

design valuesrisk categories