Transcript NEHRP 1997 Provisions

ASCE 7-05 Seismic
Provisions
A Beginner’s Guide to ASCE 7-05
Dr. T. Bart Quimby, P.E., F.ASCE
Quimby & Associates
www.bgstructuralengineering.com
ASCE 7-05 Seismic Provisions - A Beginner's
Guide to ASCE 7-05
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Earthquake Protective Design
Philosophical Issues
 High probability
of “Failure”
 “Failure”
redefined to
permit behavior
(yielding) that
would be
considered
failure under
other loads.
 High Uncertainty
 Importance of
Details
“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…
We must also face uncertainty on a
large scale… In a way, earthquake
engineering is a cartoon…
Earthquakes systematically bring out
the mistakes made in design and
construction, even the minutest
mistakes.” Newmark & Rosenblueth
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Performance Levels Hazard Levels
 Incipient Collapse
 Occasional

50% in 50 years
 Life Safety
 Rare
 Immediate

Reoccupancy
 Fully Operational
10% in 50 years
 Very Rare

5% in 50 years
 Max Considered

2% in 50 years
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Design Objective Defined
 A specific performance level given a specific
earthquake hazard level
 Stated basis of current codes:

Life safety (+some damage control) at 10% in
50 year event (nominally)
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Purpose of the Provisions
FEMA 302 Section 1.1
“The design earthquake ground motion levels specified herein
could result in both structural and nonstructural damage.
For most structures designed and constructed according to
these Provisions, structural damage from the design earthquake
ground motion would be repairable although perhaps not
economically so. For essential facilities, it is expected that the
damage from the design earthquake ground motion would not
be so severe as to preclude continued occupancy and function
of the facility.”
“For ground motions larger than the design levels, the intent of
these Provisions is that there be a low likelihood of
structural collapse.”
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Compare Wind and Seismic Design of Simple Building
Building Properties:
Moment Resisting Frames
density r = 8 pcf
Period T = 1.0 sec
Damping x = 5%
62.5’
90’
120’
Wind:
100 MPH Exposure C
Earthquake:
Assume 0.4g NEHRP
4.3
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Wind:
100 mph Fastest mile
Exposure C
62.5’
Velocity pressure qs = 25.6 psf
Gust/Exposure factor Ce = 1.25
Pressure coefficient Cq = 1.3
Load Factor for Wind = 1.3
90’
120’
Total wind force on 120’ face:
VW120= 62.5*120*25.6*1.25*1.3*1.3/1000 = 406 kips
Total wind force on 90’ face:
VW90 = 62.5*90*25.6*1.25*1.3*1.3/1000 = 304 kips
4.4
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Earthquake:
Building Weight W=
120*90*62.5*8/1000 = 5400 kips
62.5’
VEQ  CSW
90’
120’
12
. AV S 12
.  0.4  10
.
CS 

 0.480
2/ 3
2/ 3
T
10
.
Total ELASTIC earthquake force (in each direction):
VEQ = 0.480*5400 = 2592 kips
This example uses an old version of both the NEHRP and the ASCE 7
Wind Load Criteria. It is used for illustrative purposes only.
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Comparison: Earthquake vs. Wind
VEQ
VW120
2952

 7.3
406
VEQ
VW 90

2952
 9.7
304
• ELASTIC Earthquake forces are 7 to 10 times wind!
• Virtually impossible to obtain economical design
4.6
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How to Deal with Huge Earthquake Force?
• Isolate structure from ground (Base Isolation)
• Increase Damping (Passive Energy Dissipation)
• Allow Inelastic Response
Historically, Building Codes use Inelastic Response Procedure.
Inelastic response occurs though structural damage (yielding).
We must control the damage for the method to be successful.
4.7
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Interim Conclusion (The Good News)
The frame, designed for a wind force which is 15% of the
ELASTIC earthquake force, can survive the earthquake if:
It has the capability to undergo numerous cycles of
INELASIC deformation
It has the capability to deform at least 5 to 6 times
the yield deformation
It suffers no appreciable loss of strength
REQUIRES ADEQUATE DETAILING
4.12
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Interim Conclusion (The Bad News)
As a result of the large displacements associated with the
inelastic deformations, the structure will suffer considerable
structural and nonstructural damage.
This damage must be controlled by
adequate detailing and by limiting
structural deformations (drift)
4.13
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Elastic vs. Inelastic Response





The red line shows
the force and
displacement that
would be reached if
the structure
responded elastically.
The green line shows
the actual force vs.
displacement
response of the
structure
The pink line indicates
the minimum strength
required to hold
everything together
during inelastic
behavior
The blue line is the
force level that we
design for.
We rely on the
ductility of the system
to prevent collapse.
From 1997 NEHRP Provisions
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Historical Development of Seismic Codes
 1755 - Lisbon: ground shaking waves
 1906 - San Francisco: Fire, lateral force from wind
 1911 - Messina, Italy: Static inertial force (10%), First




recognition of F=ma
1923 - Tokyo: Prediction by seismic gap
1925 - Santa Barbara: USCGS instructed to develop
strong motion seismographs.
1927 - U.B.C.: Inertial forces and soil effects in the
U.S. (7.5% or 10% of D+L)
1933 - Long Beach: First instrumental records
(flawed): reinforcement required for masonry; quality
assurance; design review & construction inspection.
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Historical Development of Seismic Codes
 1940 - El Centro: Earthquake ground motion record. Makes possible
the computation of structural response. Became the most used record.
 1943 - City of Los Angles Building Code: Dynamic property of building
used in addition to mass (Number of stories relates to period and to
distribution of force)
 1952 - San Francisco Joint Committee:





Modal analysis used as a basis for static forces and distribution.
Difference between design force and computed forces not resolved.
Distinction for soils types dropped
Overturning reductions
Torsion
 1956 - World Conference on Earthquake Engineering
 1957 - Mexico City: Success with design using dynamic analysis.
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Historical Development of Seismic Codes
 1960 - SEAOC blue book
Design accel. Similar to 1943 LA and 1952 SF
 Factor for performance of structural systems (K)
 Effect of higher modes on vertical distribution
1961 - “Design of Multi-Story Reinforced Concrete Buildings for
Earthquake Motions”, Blume, Newmark, and Corning
 Inelastic response
 Ductility in concrete
1964 Alaska Earthquake: Lack of instrumental data.
Observations influenced thinking on torsional response,
anchorage of cladding, and overall load path concepts.
1964 - Niigata, Japan: Liquefaction
1967 - Caracas Earthquake: Non structural infill and overturning.





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Historical Development of Seismic Codes
 1974 Applied Technology Council Report ATC 2
Continued to use single design spectrum for buildings
 1976 ATC 3







Probabilistic ground accelerations
Realistic response accelerations and explicit factors for inelastic action
Strength design
Ground motion attenuation
Nationwide applicability
Existing buildings
 1977 National Earthquake Hazards Reduction Act: Federal support
and direction
 1979 Building Seismic Safety Council: response to ATC 3 - extensive
review and trial designs
 1985 - BSSC/NEHRP Recommended provisions: Son of ATC 3
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Historical Development of Seismic Codes
 1985 - Mexico City Earthquake: Extreme site effects
 1988 - New SEAOC (1987) and UBC requirements:
Allowable stress design and a single map.
 1988 Armenia Earthquake: Structural details and site
effects
 1989 Loma Prieta Earthquake: A performance test
for buildings & bridges.
 1991 NEHRP Provisions into Model Codes
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Building Seismic Safety Council
http://www.bssconline.org/
 Private
 Members are
 Voluntary
organizations
(ASCE, ACI, AISC,
AIA, ICBO, BOCA,
EERI, SEAOC,
etc…)
 Consensus Process
 National Forum
 Issues:



Technical
Social
Economical
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ASCE 7-05 Seismic Provisions
ASCE 7-05 Seismic Provisions - A Beginner's
Guide to ASCE 7-05
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Seismic Ground Motion Values
See ASCE 7-05 11.4
 Mapped Acceleration Parameters
Ss = Mapped 5% damped, spectral response
acceleration parameter at short periods
 S1 = Mapped 5% damped spectral response
acceleration parameter at a period of 1 sec.
 Can be found online at
http://earthquake.usgs.gov/research/hazmaps/
 You need Java to run the downloadable
application.

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SS
See ASCE 7-05 22
 Use Map to find the
maximum
considered ground
motion for short
periods.
 The contours are
irregularly spaced
 Values are in % of g
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S1
See ASCE 7-05 22
 Use Map to find the
maximum
considered ground
motion for short
periods.
 The contours are
irregularly spaced
 Values are in % of g
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Site Classes
See ASCE 7-05 11.4.2, 20
 Site Classes are also labeled A-F
A is for hard rock, F for very soft soils
 See definitions in ASCE 7-05 20
 Choice of site class is based on soil stiffness which is measured in
different ways for different types of soil.
 See ASCE 7-05 20 for procedure
 If insufficient data is available, assume Site Class D unless there is a
probability of a Site Class F.

ASCE 7-05 Seismic Provisions - A Beginner's
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Compute SMS and SM1
See ASCE 7-05 11.4.3
 SMS = FaSS

Fa from Table
11.4-1
 SM1= FvS1

Fv from Table
11.4-2
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Spectral Response Accelerations
SDS and SD1
See ASCE 7-05 11.4.4
SDS = 2*SMS/3
SD1 = 2*SM1/3
 SDS is the design, 5% damped, spectral
response acceleration for short periods.
 SD1 is the design, 5% damped, spectral
response acceleration at a period of 1 sec.
 SDS and SD1 are used in selecting the Seismic
Design Category and in the analysis
methods.
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Design Response Spectrum
See ASCE 7-05 11.4.5
 Period Limiting Values
 T0 = .2 SD1/SDS
 TS = SD1/SDS
 TL from ASCE 7-05 22
 Sa, design spectral
response acceleration


Sa is a function of
structure period, T
Four regions, four
equations.
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Importance Factor, I
See ASCE 7-05 11.5
 See ASCE 7-05 Table 11.5-1

Function of Occupancy Category
 Requirement for structures adjacent to
occupancy category IV structures where
access is needed to get to the category IV
structure.
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Seismic Design Categories
See ASCE 7-05 11.6
 To be determined for every structure
 function of:


Occupancy Category
Spectral Response Accelerations SDS and SD1.
 Used to determine analysis options, detailed
requirements, height limitations, and other
limits on usage.
 Seismic Design Categories labeled A-F
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Seismic Design Categories
 The most restrictive
value controls
 SDC E:
 OC I, II, III where
S1 > 0.75
 SDC F:
 OC IV where S1
> 0.75
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Seismic Design Category A
See ASCE 7-05 11.7
 Very limited seismic exposure and risk
 Lateral forces taken to equal 1% of structure
weight.
 A complete load path must be in place.
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Soil Report Requirements
See ASCE 7-05 11.8
 Limits on where you can place a structure
(SDC E or F)
 SDC C – F:

specific evaluation of listed hazards.
 SDC D-F:

Even more evaluation requirements.
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Seismic Load Analysis Procedures
See ASCE 7-05 12.6
 Equivalent Lateral Force (ELF)
 Static approximation.
 May not be used on structures of Seismic Design
Categories E or F with particular irregularities. (ASCE
7-05 Table 12.6-1)
 Modal Analysis
 2D and 3D dynamic analysis
 Required for buildings with particular irregularities
 Site Specific Response Spectrum
 Permitted for all structures
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Analysis Procedures
 Category A: regular and irregular structures designed
for a minimum lateral force
 Category B & C: regular and irregular structures
using any of the three methods
 Category D, E, & F: Table 12.6-1 with some limits on
SDS and SD1



ELF for regular and some irregular
Modal for some irregular
Site specific required in Site Classes E or F
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Structure Configuration
(regular or irregular)
 Plan Configuration

ASCE 7-05 12.3.2.1
 Vertical Configuration

ASCE 7-05 12.3.2.2
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Plan Structural Irregularities
 1a - Torsional Irregularity
 1b - Extreme Torsional Irregularity
 2 - Re-entrant Corners
 3 - Diaphragm Discontinuity
 4 - Out-of-plane Offsets
 5 - Nonparallel Systems
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Type 1: Torsional Irregularities
 1a - Torsional Irregularity
larger story drift more than 1.2
times average story drift
 1b - Extreme Torsional Irregularity
 larger story drift more than 1.4
times average story drift
 Not permitted in Design
Categories E & F
 Design forces for lateral force
connections to be increased 25% in
Design Categories D, E, & F.

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Type 2: Re-entrant Corners
 Both projections
beyond the corner are
more than 15% of the
plan dimension of the
structure in the same
direction
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Type 3: Diaphragm
Discontinuities
 Diaphragms with abrupt discontinuities or variations
in stiffness, including those having cutout or open
areas greater than 50% of the gross enclosed
diaphragm area, or changes in effective diaphragm
stiffness of more than 50% from one story to the next.
 Design forces for lateral force connections to be
increased 25% in Design Categories D, E, & F.
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Type 4: Out-of-Plane Offsets
 Discontinuities in a lateral
force resistance path, such
as out-of-plane offsets of
the vertical elements.
 Design forces for lateral
force connections to be
increased 25% in Design
Categories D, E, & F.
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Type 5: Nonparallel Systems
 The vertical lateral force-
resisting elements are not
parallel to or symmetric about
the major orthogonal axes of
the lateral force resisting
system.
 Analyze for forces applied in
the direction that causes the
most critical load effect for
Design Categories C - F.
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Vertical Irregularities
 1a - Stiffness Irregularity -Soft Story
 1b - Stiffness Irregularity - Extreme Soft Story
 2 - Weight (Mass) Irregularity
 3 - Vertical Geometry Irregularity
 4 - In-plane Discontinuity in Vertical Lateral Force
Resisting Elements
 5 - Discontinuity in Capacity - Weak Story
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Type 1: Stiffness Irregularities
 1a - Soft Story
the lateral stiffness is less than
70% of that in the story above
or less than 80% of the average
stiffness of the three stories
above.
 1b - Extreme Soft Story
 the lateral stiffness is less than
60% of that in the story above
or less than 70% of the average
stiffness of the three stories
above.
 Not permitted in Design
Categories E & F

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Type 2: Weight (Mass) Irregularity
 Mass irregularity shall
be considered to exist
where the effective
mass of any story is
more than 150% of
the effective mass of
an adjacent story. A
roof that is lighter
than the floor below
need not be
considered.
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Type 3: Vertical Geometry
Irregularity
 Vertical geometry
irregularity shall be
considered to exist where
the horizontal dimension of
the lateral force-resisting
system in any story is
more than 130% of that in
an adjacent story.
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Type 4: In-Plane Discontinuity in Vertical
Lateral Force Resisting Elements
 An in-plane offset of the lateral force-resisting elements greater
than the length of those elements or a reduction in stiffness in
the resisting element in the story below.
 Design forces for lateral force connections to be increased 25%
in Design Categories D, E, & F.
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Type 5: Discontinuity in
Capacity - Soft Story
 A weak story is one in which the
story lateral strength is less than
80% of that in the story above. The
story strength is the total strength of
all seismic-resisting elements
sharing the story shear for the
direction under consideration.
 Do not confuse STIFFNESS with
STRENGTH.
 Not permitted in Design Categories
E & F.
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Equivalent Force Method
(ASCE 7-05 12.8)
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Base Shear Determination
See ASCE 7-05 12.8.1
Base Shear, V = CsW
Where:
Cs = seismic response coefficient
W = the effective seismic weight, including
applicable portions of other storage and snow
loads (See ASCE 7-05 12.7.2)
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Seismic Weight, W
See ASCE 7-05 12.7.2
 W is to include:
 all dead load (all permanent components of the
building, including permanent equipment)
 25% of any design storage floor live loads except
for floor live load in public garages and open
parking structures.
 If partition loads are considered in floor design, at
least 10 psf is to be included.
 A portion of the snow load (20% pf minimum) in
regions where the flat roof snow load exceeds 30
psf.
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Seismic Response Coefficient, Cs
See ASCE 7-05 12.8.1.1
Cs = SDS /(R/I)
Cs need not exceed
SD1/(T(R/I)) for T < TL
SD1TL/(T2(R/I)) for T > TL
Cs shall not be taken less than
Max[0.044SDSI, 0.01] for S1 < 0.6g
0.5S1/(R/I) for S1 > 0.6g
See also ASCE 7-05 Supplement No. 2
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Response Modification
Coefficient, R
See ASCE 7-05 12.2
 The response modification factor, R, accounts for the dynamic
characteristics, lateral force resistance, and energy dissipation capacity
of the structural system.
 Can be different for different directions.
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Fundamental Period, T
 May be computed by analytical means
 May be computed by approximate means, Ta
 Where analysis is used to compute T:
T < Cu Ta
 May also use Ta in place of actual T
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Approximate Fundamental
Period, Ta
See ASCE 7-05 12.8.2
 An approximate means may be used.
Ta = CThnx
Where:
CT = Building period coefficient.
hn = height above the base to the highest level of the
building
 for moment frames not exceeding 12 stories and having a
minimum story height of 10 ft, Ta may be taken as 0.1N, where
N = number of stories.
 For masonry or concrete shear wall buildings use eq 12.8-9
 Ta may be different in each direction.
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Building Period Coefficient, CT
See ASCE 7-05 12.8.2
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Base Shear Summary
V = CsW
From Design Spectrum
W = Building Seismic Weight
Max[0.044SDSI,0.01] or 0.5S1/(R/I) < SDS/(R/I) < SD1/(T(R/I)) or TLSD1/(T2(R/I))
From map
R from Table 12.2-1 based
on the Basic Seismic-ForceResisting System
I from Table 11.5-1 based on
Occupancy Category
Numerical Analysis or Ta
= CThnx or Ta = 0.1N
CT = 0.028, 0.016, 0.030, or
0.020
hn = building height
N = number of storys
Vertical Distribution of Base Shear
See ASCE 7-05 12.8.3
 For short period buildings the vertical
distribution follows generally follows the
first mode of vibration in which the force
increases linearly with height for evenly
distributed mass.
 For long period buildings the force is
shifted upwards to account for the
whipping action associated with
increased flexibility
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Story Force, Fx
Fx = CvxV
Where Cvx = Vertical Distribution Factor
Cvx
W x hx
k
n
W i hi
k
i= 1
Wx = Weight at level x
hx = elevation of level x above the base
k = exponent related to structure period
When T < 0.5 s, k =1, When T > 2.5 s, k =2,
Linearly interpolate when 0.5 < T < 2.5 s
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Story Shear, Vx
 Story shear, Vx, is the shear force at a given story
level
 Vx is the sum of all the forces above that level.
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Horizontal Distribution
See ASCE 7-05 12.8.4
 Being an inertial force, the Story Force, Fx, is
distributed in accordance with the distribution
of the mass at each level.
 The Story Shear, Vx, is distributed to the
vertical lateral force resisting elements based
on the relative lateral stiffnesses of the
vertical resisting elements and the
diaphragm.
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Torsion
See ASCE 7-05 12.8.4.1-3
 The analysis must take into account any torsional effects
resulting from the location of the masses relative to the
centers of resistance.
 In addition to the predicted torsion, accidental torsion must
be applied for structures with rigid diaphragms by assuming
the center of mass at each level is moved from its actual
location a distance equal to 5% the building dimension
perpendicular to the direction of motion.
 Buildings of Seismic Design Categories C, D, E, and F with
torsional irregularities are to have torsional moments
magnified.
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Using the results of the Seismic
Analysis
“The effects on the structure and its
components due to gravity loads and seismic
forces shall be combined in accordance with
the factored load combinations as presented
in ASCE 7 except that the effect of seismic
loads, E, shall be as defined herein.”
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Overturning
See ASCE 7-05 12.8.5
 The effects of overturning must be considered.
 The overturning moment at any level is the sum of the
moments at that level created by the Story Forces at each
level above it.
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ASCE 7 Load Combinations which
include Seismic Effects
See ASCE 7-05 2.3 & 2.4
LRFD
5: 1.2D + 1.0E + L + 0.2S
7: 0.9D + 1.0E
ASD
5: D + (W or 0.7E)
6: D + 0.75(W or 0.7E) + 0.75L + 0.75(Lr or S or R)
8: 0.6D + 0.7E
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Definition of E
See ASCE 7-05 12.4
 When Seismic effects and Dead Load effects
are additive:
E = Eh + Ev = DQE + 0.2SDSD
 When Seismic effects and Dead Load effects
counteract:
E = Eh - Ev = DQE - 0.2SDSD
 QE = Effect of horizontal seismic forces
 D = the reliability factor
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The Reliability Factor, D
See ASCE 7-05 12.3.4
 The reliability factor is intended to account for redundancy in the
structure.
 The factor, D, may be taken as 1.0 for eight cases listed in
ASCE 7-05 12.3.4.1, including Seismic Design Categories A-C.
 For structures of Seismic Design Categories D-F:
D = 1.3
With listed exceptions (ASCE 7-05 12.3.4.2)
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