lesson 6 asce 7-10: basic requirements for structural design
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
U.S. Army Corps of Engineers Basic Requirements 1
Lesson 6
ASCE 7-10: Basic Requirements for Structural Design
U.S. Army Corps of Engineers Basic Requirements 2
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
U.S. Army Corps of Engineers Basic Requirements 3
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
U.S. Army Corps of Engineers Basic Requirements 4
Governing Documents
U.S. Army Corps of Engineers Basic Requirements 5
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.
U.S. Army Corps of Engineers Basic Requirements 6
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.…”
U.S. Army Corps of Engineers Basic Requirements 7
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
U.S. Army Corps of Engineers Basic Requirements 8
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.
U.S. Army Corps of Engineers Basic Requirements 9
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
U.S. Army Corps of Engineers Basic Requirements 10
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)
U.S. Army Corps of Engineers Basic Requirements 11
U.S. Army Corps of Engineers Basic Requirements 12
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
U.S. Army Corps of Engineers Basic Requirements 13
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
U.S. Army Corps of Engineers Basic Requirements 14
Mapped Ground Motion
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
U.S. Army Corps of Engineers Basic Requirements 15
Mapped Ground Motion
UFC 3-310-01
STEP 1
U.S. Army Corps of Engineers Basic Requirements 16
Mapped Ground Motion
Select Building Code: 2012 IBC/ASCE 7-10
Select Site Classification
STEP 1
U.S. Army Corps of Engineers Basic Requirements 17
Mapped Ground Motion
Type in the Latitude and Longitude of the building site
OR
Enter Street Address
STEP 1
U.S. Army Corps of Engineers Basic Requirements 18
Mapped Ground Motion
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
U.S. Army Corps of Engineers Basic Requirements 19
Mapped Ground Motion
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
U.S. Army Corps of Engineers Basic Requirements 20
Mapped Ground MotionSTEP 1
U.S. Army Corps of Engineers Basic Requirements 21
Mapped Ground Motion
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
U.S. Army Corps of Engineers Basic Requirements 22
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
U.S. Army Corps of Engineers Basic Requirements 23
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
U.S. Army Corps of Engineers Basic Requirements 24
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
U.S. Army Corps of Engineers Basic Requirements 25
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
U.S. Army Corps of Engineers Basic Requirements 26
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
U.S. Army Corps of Engineers Basic Requirements 27
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
U.S. Army Corps of Engineers Basic Requirements 28
Site AmplificationSTEP 2
See Lesson 4
U.S. Army Corps of Engineers Basic Requirements 29
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
U.S. Army Corps of Engineers Basic Requirements 30
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
U.S. Army Corps of Engineers Basic Requirements 31
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
U.S. Army Corps of Engineers Basic Requirements 32
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
U.S. Army Corps of Engineers Basic Requirements 33
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
U.S. Army Corps of Engineers Basic Requirements 34
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
U.S. Army Corps of Engineers Basic Requirements 35
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
U.S. Army Corps of Engineers Basic Requirements 36
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
U.S. Army Corps of Engineers Basic Requirements 37
TL Map of Contiguous USA(ASCE 7-10 Figure 22.12)
STEP 3
U.S. Army Corps of Engineers Basic Requirements 38
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
U.S. Army Corps of Engineers Basic Requirements 39
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
U.S. Army Corps of Engineers Basic Requirements 40
Building Occupancy and Risk Category
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
U.S. Army Corps of Engineers Basic Requirements 41
Building Occupancy and Risk Category
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
U.S. Army Corps of Engineers Basic Requirements 42
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
U.S. Army Corps of Engineers Basic Requirements 43
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
U.S. Army Corps of Engineers Basic Requirements 44
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
U.S. Army Corps of Engineers Basic Requirements 45
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
U.S. Army Corps of Engineers Basic Requirements 46
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.
U.S. Army Corps of Engineers Basic Requirements 47
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
U.S. Army Corps of Engineers Basic Requirements 48
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
U.S. Army Corps of Engineers Basic Requirements 49
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
U.S. Army Corps of Engineers Basic Requirements 50
Seismic Design Category ASCE 7-05 Section 11.6
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
U.S. Army Corps of Engineers Basic Requirements 51
Seismic Design Category ASCE 7-05 Section 11.6
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
U.S. Army Corps of Engineers Basic Requirements 52
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
U.S. Army Corps of Engineers Basic Requirements 53
SDC for Low Seismic HazardASCE 7-10 Section 11.4.1
STEP 5
SS 0.15g and S1 0.04g (shown by Green shading)
U.S. Army Corps of Engineers Basic Requirements 54
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
U.S. Army Corps of Engineers Basic Requirements 55
SDC for High Seismic HazardASCE 7-10 Section 11.6
STEP 5
S1 ≥ 0.75g (shown by Red shading)
U.S. Army Corps of Engineers Basic Requirements 56
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
U.S. Army Corps of Engineers Basic Requirements 57
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
U.S. Army Corps of Engineers Basic Requirements 58
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
U.S. Army Corps of Engineers Basic Requirements 59
Analytical ModelingASCE Section 12.7
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
U.S. Army Corps of Engineers Basic Requirements 60
Analytical ModelingASCE Section 12.7
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
U.S. Army Corps of Engineers Basic Requirements 61
Analytical ModelingASCE Section 12.7
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
U.S. Army Corps of Engineers Basic Requirements 62
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
U.S. Army Corps of Engineers Basic Requirements 63
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
U.S. Army Corps of Engineers Basic Requirements 64
Structural Irregularities
Horizontal (Plan)
Vertical
Photo Credit: EERI Earthquake Damage Slide Set
STEP 7
U.S. Army Corps of Engineers Basic Requirements 65
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
U.S. Army Corps of Engineers Basic Requirements 66
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
U.S. Army Corps of Engineers Basic Requirements 67
Horizontal Irregularity 1a: Torsional Irregularity
Calculation of dA and
dB includes accidental
torsion, with Ax = 1.0.
STEP 7
U.S. Army Corps of Engineers Basic Requirements 68
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
U.S. Army Corps of Engineers Basic Requirements 69
Horizontal Irregularity 1b: Extreme Torsional Irregularity
STEP 7
U.S. Army Corps of Engineers Basic Requirements 70
Horizontal Irregularity 1b: Extreme Torsional Irregularity
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
U.S. Army Corps of Engineers Basic Requirements 71
Horizontal Irregularity 1b: Extreme Torsional Irregularity
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
U.S. Army Corps of Engineers Basic Requirements 72
Horizontal Irregularity 2: Reentrant Corner Irregularity
RE-ENTRANT CORNER EXISTS WHEN PROJECTION b > 0.15a, AND PROJECTION d > 0.15c
STEP 7
U.S. Army Corps of Engineers Basic Requirements 73
Horizontal Irregularity 2: Reentrant Corner Irregularity
Referenced in:
12.3.3.4 – 25% increase in seismic forces in connections in diaphragms and collectors
Table 12.6-1 – Permitted analytical procedure
STEP 7
U.S. Army Corps of Engineers Basic Requirements 74
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
U.S. Army Corps of Engineers Basic Requirements 75
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
U.S. Army Corps of Engineers Basic Requirements 76
Horizontal Irregularity 3: Diaphragm Discontinuity Irregularity
Referenced in:
12.3.3.4 – 25% increase in seismic forces in connections in diaphragms and collectors
Table 12.6-1 – Permitted analytical procedure
STEP 7
U.S. Army Corps of Engineers Basic Requirements 77
Horizontal Irregularity 4: Out-of-Plane Offsets Irregularity
ASCE 7-10 clarifies: “….at least one of the vertical elements.”
STEP 7
U.S. Army Corps of Engineers Basic Requirements 78
Horizontal Irregularity 4: Out-of-Plane Offsets Irregularity
STEP 7
U.S. Army Corps of Engineers Basic Requirements 79
Horizontal Irregularity 4: Out-of-Plane Offsets Irregularity
STEP 7
U.S. Army Corps of Engineers Basic Requirements 80
Horizontal Irregularity 4: Out-of-Plane Offsets Irregularity
Referenced in:
12.3.3.3 – Axial force using load combinations with overstrength for discontinuous elements
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
16.2.2 - 3-D structural model required in nonlinear response history procedure
STEP 7
U.S. Army Corps of Engineers Basic Requirements 81
Horizontal Irregularity 5: Nonparallel Systems Irregularity
STEP 7
U.S. Army Corps of Engineers Basic Requirements 82
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.
Horizontal Irregularity 5: Nonparallel Systems Irregularity
STEP 7
U.S. Army Corps of Engineers Basic Requirements 83
Horizontal Irregularity 5: Nonparallel Systems Irregularity
STEP 7
U.S. Army Corps of Engineers Basic Requirements 84
Horizontal Irregularity 5: Nonparallel Systems Irregularity
Referenced in:
12.5.3 – Orthogonal load combinations in SDC C
Table 12.6-1 – Permitted analytical procedure
12.7.3 – 3-D structural model required
16.2.2 - 3-D structural model required in nonlinear response history procedure
STEP 7
U.S. Army Corps of Engineers Basic Requirements 85
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
U.S. Army Corps of Engineers Basic Requirements 86
Vertical Irregularity 1a: Stiffness-Soft Story Irregularity
SOFT STORY STIFFNESS < 70% STORY STIFFNESS ABOVE < 80% [AVERAGE STORY STIFFNESS OF 3 STORIES ABOVE]
STEP 7
U.S. Army Corps of Engineers Basic Requirements 87
Vertical Irregularity 1a: Stiffness-Soft Story Irregularity
STEP 7
U.S. Army Corps of Engineers Basic Requirements 88
Vertical Irregularity 1a: Stiffness-Soft Story Irregularity
Referenced in:
Table 12.6-1 – Permitted analytical procedure
STEP 7
U.S. Army Corps of Engineers Basic Requirements 89
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
U.S. Army Corps of Engineers Basic Requirements 90
Vertical Irregularity 1b: Stiffness-Extreme Soft Story Irregularity
Referenced in:
12.3.3.1 – Prohibited in SDC E and F
Table 12.6-1 – Permitted analytical procedure
STEP 7
U.S. Army Corps of Engineers Basic Requirements 91
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
U.S. Army Corps of Engineers Basic Requirements 92
Vertical Irregularity 2: Weight (Mass) Irregularity
Referenced in:
Table 12.6-1 – Permitted analytical procedure
STEP 7
U.S. Army Corps of Engineers Basic Requirements 93
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
U.S. Army Corps of Engineers Basic Requirements 94
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
U.S. Army Corps of Engineers Basic Requirements 95
Vertical Irregularity 3: Vertical Geometric Irregularity
HORIZONTAL DIMENSION OF LATERAL FORCE-RESISTING SYSTEM IN STORY > 130% OF THAT IN ADJACENT STORY
STEP 7
U.S. Army Corps of Engineers Basic Requirements 96
Vertical Irregularity 3: Vertical Geometric Irregularity
STEP 7
U.S. Army Corps of Engineers Basic Requirements 97
Vertical Irregularity 3: Vertical Geometric Irregularity
Referenced in:
Table 12.6-1 – Permitted analytical procedure
STEP 7
U.S. Army Corps of Engineers Basic Requirements 98
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
U.S. Army Corps of Engineers Basic Requirements 99
d
offset
Irregularity exists if the offset isgreater than the width (d) or thereexists a reduction in stiffness of thestory below.
Source: FEMA
Vertical Irregularity 4: In-Plane Discontinuity
STEP 7
U.S. Army Corps of Engineers Basic Requirements 100
Vertical Irregularity 4: In-Plane Discontinuity
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
U.S. Army Corps of Engineers Basic Requirements 101
Vertical Irregularity 4: In-Plane Discontinuity
Referenced in:
12.3.3.3 – Axial force using load combinations with overstrength for discontinuous elements
12.3.3.4 – 25% increase in seismic forces in connections in diaphragms and collectors
Table 12.6-1 – Permitted analytical procedure
STEP 7
U.S. Army Corps of Engineers Basic Requirements 102
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
U.S. Army Corps of Engineers Basic Requirements 103
Vertical Irregularity 5a: Weak Story Irregularity
Referenced in:
12.3.3.1 – Prohibited in SDC E and F
Table 12.6-1 – Permitted analytical procedure
STEP 7
U.S. Army Corps of Engineers Basic Requirements 104
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
U.S. Army Corps of Engineers Basic Requirements 105
Vertical Irregularity 5b: Extreme Weak Story Irregularity
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)
Table 12.6-1 – Permitted analytical procedure
STEP 7
U.S. Army Corps of Engineers Basic Requirements 106
Vertical Irregularity 5b: Extreme Weak Story Irregularity
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
U.S. Army Corps of Engineers Basic Requirements 107
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)
U.S. Army Corps of Engineers Basic Requirements 108
Thank You!!
Any Remaining Questions?
U.S. Army Corps of Engineers Basic Requirements 109
SUPPLEMENTAL INFORMATION
U.S. Army Corps of Engineers Basic Requirements 110
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.
U.S. Army Corps of Engineers Basic Requirements 111
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.
U.S. Army Corps of Engineers Basic Requirements 112
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
U.S. Army Corps of Engineers Basic Requirements 113
Earthquake Experience
Photo Credit: EERI Earthquake Damage Slide Set
U.S. Army Corps of Engineers Basic Requirements 114
Earthquake Experience
Marina District…a stiffness issue (soft story).
Photo Credit: EERI Earthquake Damage Slide Set
U.S. Army Corps of Engineers Basic Requirements 115
Earthquake Experience
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.
U.S. Army Corps of Engineers Basic Requirements 116
Earthquake Experience
Photo Credit: EERI Earthquake Damage Slide Set
U.S. Army Corps of Engineers Basic Requirements 117
Earthquake Experience
U.S. Army Corps of Engineers Basic Requirements 118
Earthquake Experience
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.
U.S. Army Corps of Engineers Basic Requirements 119
Earthquake Experience
Photo Credit: EERI Earthquake Damage Slide Set
U.S. Army Corps of Engineers Basic Requirements 120
Earthquake Experience
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.
U.S. Army Corps of Engineers Basic Requirements 121
Earthquake Experience
Photo Credit: EERI Earthquake Damage Slide Set
U.S. Army Corps of Engineers Basic Requirements 122
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.
U.S. Army Corps of Engineers Basic Requirements 123
Reasons for Poor Seismic Performance of Irregular Structures
Some irregularities introduce unanticipated stresses into the structure, which designers frequently overlook when detailing the structural system.
U.S. Army Corps of Engineers Basic Requirements 124
Reasons for Poor Seismic Performance of Irregular Structures
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.
U.S. Army Corps of Engineers Basic Requirements 125
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.
U.S. Army Corps of Engineers Basic Requirements 126
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
U.S. Army Corps of Engineers Basic Requirements 127
Q/A Horizontal Irregularity 3: Diaphragm Discontinuity Irregularity
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
U.S. Army Corps of Engineers Basic Requirements 128
Q/A Horizontal Irregularity 3: Diaphragm Discontinuity Irregularity
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
U.S. Army Corps of Engineers Basic Requirements 129
Q/A Horizontal Irregularity 3: Diaphragm Discontinuity Irregularity
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
U.S. Army Corps of Engineers Basic Requirements 130
Q/A Horizontal Irregularity 3: Diaphragm Discontinuity Irregularity
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
U.S. Army Corps of Engineers Basic Requirements 131
Q/A Horizontal Irregularity 3: Diaphragm Discontinuity Irregularity
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
U.S. Army Corps of Engineers Basic Requirements 132
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
U.S. Army Corps of Engineers Basic Requirements 133
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
U.S. Army Corps of Engineers Basic Requirements 134
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
STEP 7Exception 1 to 12.3.2.2
U.S. Army Corps of Engineers Basic Requirements 135
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
STEP 7Exception 1 to 12.3.2.2
U.S. Army Corps of Engineers Basic Requirements 136
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
STEP 7Exception 1 to 12.3.2.2
U.S. Army Corps of Engineers Basic Requirements 137
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
STEP 7Exception 1 to 12.3.2.2