ASCE705 Seismic

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

1 ASCE 7-05 Seismic Provisions - A Beginner's

Guide to ASCE 7-05

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 2 ASCE 7-05 Seismic Provisions - A Beginner's

Guide to ASCE 7-05

Hazard Levels

Incipient Collapse

Life Safety

Immediate

Reoccupancy

Fully Operational

Occasional

50% in 50 years

Rare

10% in 50 years

Very Rare

5% in 50 years

Max Considered

2% in 50 years

Performance Levels


Guide to ASCE 7-05

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)


Guide to ASCE 7-05

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.”


Guide to ASCE 7-05

Compare Wind and Seismic Design of Simple Building

120’

90’

62.5’

Earthquake:

Assume 0.4g NEHRP

Wind:

100 MPH Exposure C

Building Properties:

Moment Resisting Frames

density r = 8 pcf

Period T = 1.0 sec

Damping x = 5%

4.3


Guide to ASCE 7-05

Wind:

120’

90’

62.5’100 mph Fastest mile

Exposure C

Velocity pressure qs= 25.6 psf

Gust/Exposure factor Ce = 1.25

Pressure coefficient Cq = 1.3

Load Factor for Wind = 1.3

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


Guide to ASCE 7-05

Earthquake:

120’

90’

62.5’Building Weight W=

120*90*62.5*8/1000 = 5400 kips

Total ELASTIC earthquake force (in each direction):

VEQ = 0.480*5400 = 2592 kips

CA S

TS

V

12 12 0 4 10

100 480

2 3 2 3

. . . .

..

/ /

V C WEQ S

This example uses an old version of both the NEHRP and the ASCE 7

Wind Load Criteria. It is used for illustrative purposes only.


Guide to ASCE 7-05

Comparison: Earthquake vs. Wind

V

V

EQ

W120

2952

4067 3 .

V

V

EQ

W 90

2952

3049 7 .

• ELASTIC Earthquake forces are 7 to 10 times wind!

• Virtually impossible to obtain economical design

4.6


Guide to ASCE 7-05

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


Guide to ASCE 7-05

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 suffers no appreciable loss of strength

It has the capability to deform at least 5 to 6 times

the yield deformation

REQUIRES ADEQUATE DETAILING

4.12


Guide to ASCE 7-05

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


Guide to ASCE 7-05

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


Guide to ASCE 7-05

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.


Guide to ASCE 7-05


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.


Guide to ASCE 7-05


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.


Guide to ASCE 7-05


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


Guide to ASCE 7-05


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


Guide to ASCE 7-05

Building Seismic Safety Council http://www.bssconline.org/

Private

Voluntary

National Forum

Issues:

Technical

Social

Economical

Members are

organizations

(ASCE, ACI, AISC,

AIA, ICBO, BOCA,

EERI, SEAOC,

etc…)

Consensus Process


Guide to ASCE 7-05

http://www.bssconline.org/

ASCE 7-05 Seismic Provisions


Guide to ASCE 7-05

Seismic Ground Motion Values

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.

See ASCE 7-05 11.4


Guide to ASCE 7-05

http://earthquake.usgs.gov/research/hazmaps/

SS

Use Map to find the

maximum

considered ground

motion for short

periods.

The contours are

irregularly spaced

Values are in % of g

See ASCE 7-05 22


Guide to ASCE 7-05

S1

Use Map to find the

maximum

considered ground

motion for short

periods.

The contours are

irregularly spaced

Values are in % of g

See ASCE 7-05 22


Guide to ASCE 7-05

Site Classes

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.

See ASCE 7-05 11.4.2, 20


Guide to ASCE 7-05

Compute SMS and SM1

SMS = FaSS

Fa from Table

11.4-1

SM1= FvS1

Fv from Table

11.4-2

See ASCE 7-05 11.4.3


Guide to ASCE 7-05

Spectral Response Accelerations

SDS and SD1

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.

See ASCE 7-05 11.4.4

SDS = 2*SMS/3 SD1 = 2*SM1/3


Guide to ASCE 7-05

Design Response Spectrum

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.

See ASCE 7-05 11.4.5


Guide to ASCE 7-05

Importance Factor, I

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.

See ASCE 7-05 11.5


Guide to ASCE 7-05

Seismic Design Categories

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

See ASCE 7-05 11.6


Guide to ASCE 7-05

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


Guide to ASCE 7-05

Seismic Design Category A

Very limited seismic exposure and risk

Lateral forces taken to equal 1% of structure

weight.

A complete load path must be in place.

See ASCE 7-05 11.7


Guide to ASCE 7-05

Soil Report Requirements

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.

See ASCE 7-05 11.8


Guide to ASCE 7-05

Seismic Load Analysis Procedures

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

See ASCE 7-05 12.6


Guide to ASCE 7-05

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


Guide to ASCE 7-05

Structure Configuration

(regular or irregular)

Plan Configuration

ASCE 7-05 12.3.2.1

Vertical Configuration

ASCE 7-05 12.3.2.2


Guide to ASCE 7-05

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


Guide to ASCE 7-05

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.


Guide to ASCE 7-05

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


Guide to ASCE 7-05

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.


Guide to ASCE 7-05

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.


Guide to ASCE 7-05

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.


Guide to ASCE 7-05

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


Guide to ASCE 7-05

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 43 ASCE 7-05 Seismic Provisions - A Beginner's

Guide to ASCE 7-05

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.


Guide to ASCE 7-05

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.


Guide to ASCE 7-05

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.


Guide to ASCE 7-05

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.


Guide to ASCE 7-05

Equivalent Force Method (ASCE 7-05 12.8)


Guide to ASCE 7-05

Base Shear Determination

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)

See ASCE 7-05 12.8.1


Guide to ASCE 7-05

Seismic Weight, W

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.

See ASCE 7-05 12.7.2


Guide to ASCE 7-05

Seismic Response Coefficient, Cs

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 ASCE 7-05 12.8.1.1

See also ASCE 7-05 Supplement No. 2 51 ASCE 7-05 Seismic Provisions - A Beginner's

Guide to ASCE 7-05

http://content.seinstitute.org/files/pdf/SupplementNo2ofthe2005EditionofASCE7.pdf



Response Modification

Coefficient, R

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.

See ASCE 7-05 12.2


Guide to ASCE 7-05

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


Guide to ASCE 7-05

Approximate Fundamental

Period, Ta

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.

See ASCE 7-05 12.8.2


Guide to ASCE 7-05

Building Period Coefficient, CT

See ASCE 7-05 12.8.2


Guide to ASCE 7-05

Base Shear Summary

V = CsW

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 Design Spectrum

From map

R from Table 12.2-1 based

on the Basic Seismic-Force-

Resisting System

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

I from Table 11.5-1 based on

Occupancy Category

Vertical Distribution of Base Shear

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

See ASCE 7-05 12.8.3


Guide to ASCE 7-05

Story Force, Fx

Fx = CvxV

Where Cvx = Vertical Distribution Factor

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

Cvx

Wx

hx

k

1

n

i

Wi

hi

k

=


Guide to ASCE 7-05

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.


Guide to ASCE 7-05

Horizontal Distribution

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.

See ASCE 7-05 12.8.4


Guide to ASCE 7-05

Torsion

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.

See ASCE 7-05 12.8.4.1-3


Guide to ASCE 7-05

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.”


Guide to ASCE 7-05

Overturning

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.

See ASCE 7-05 12.8.5


Guide to ASCE 7-05

ASCE 7 Load Combinations which

include Seismic Effects

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

See ASCE 7-05 2.3 & 2.4


Guide to ASCE 7-05

Definition of E

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

See ASCE 7-05 12.4


Guide to ASCE 7-05

The Reliability Factor, D

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)

See ASCE 7-05 12.3.4


Guide to ASCE 7-05

Documents

ASCE705 Seismic