NSCP 2010 Seismic Provisions

NSCP 2010 6th Edition C101-10

Seismic Provisions

Bartolome I. Luceña Jr.

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

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

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)

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”

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 Framesdensity = 8 pcfPeriod T = 1.0 secDamping = 5%

4.3

Wind:

120’90’

62.5’100 mph Fastest mileExposure C

Velocity pressure qs= 25.6 psfGust/Exposure factor Ce = 1.25Pressure coefficient Cq = 1.3Load 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

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

4.5

This example uses old version of NEHRP. It is used for illustrative purposes only.

Comparison: Earthquake vs. Wind

V

VEQ

W120

2952

4067 3 .

V

VEQ

W90

2952

3049 7 .

• ELASTIC Earthquake forces 7 to 10 times wind!

• Virtually impossible to obtain economical design

4.6

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

Interim Conclusion (The Good News)

The frame, designed for a wind force which is 15% of theELASTIC earthquake force, can survive the earthquake if:

It has the capability to undergo numerous cycles ofINELASIC deformation

It suffers no appreciable loss of strength

It has the capability to deform at least 5 to 6 timesthe yield deformation

REQUIRES ADEQUATE DETAILING4.12

Interim Conclusion (The Bad News)

As a result of the large displacements associated with theinelastic deformations, the structure will suffer considerablestructural and nonstructural damage.

This damage must be controlled byadequate detailing and by limiting structural deformations (drift)

4.13

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

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.


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.


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.


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


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

Building Seismic Safety Councilhttp://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

ASCE 7-05 Seismic Provisions

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Structure Configuration(regular or irregular)

Plan Configuration ASCE 7-05 12.3.2.1

Vertical Configuration ASCE 7-05 12.3.2.2

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

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.

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

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.


Type 4: Out-of-Plane Offsets

Discontinuities in a lateral force resistance path, such as out-of-plane offsets of the vertical elements.


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.

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

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

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.

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.

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.


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.

Equivalent Force Method(ASCE 7-05 12.8)

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

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

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

0.01 for S1 < 0.6g

0.5S1/(R/I) for S1 > 0.6g

See ASCE 7-05 12.8.1.1

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

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

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

Building Period Coefficient, CT

See ASCE 7-05 12.8.2

Base Shear Summary

V = CsW

W = Building Seismic Weight

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 = CThn

x or Ta = 0.1N

CT = 0.028, 0.016, 0.030, or 0.020

hn = building heightN = 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

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

=

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.

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

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

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.”

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

ASCE 7 Load Combinations that 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

Definition of E

When Seismic effects and Dead Load effects are additive:

E = Eh + Ev = QE + 0.2SDSD

When Seismic effects and Dead Load effects counteract:

E = Eh - Ev = QE - 0.2SDSD

QE = Effect of horizontal seismic forces

= the reliability factor

See ASCE 7-05 12.4

The Reliability Factor,

The reliability factor is intended to account for redundancy in the structure.

The factor, , 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:

= 1.3

With listed exceptions (ASCE 7-05 12.3.4.2)

See ASCE 7-05 12.3.4

Documents

NSCP 2010 Seismic Provisions