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Building Seismic Safety Council Provisions Update Committee

Seismic Performance Objectives

James Harris BSSC Colloquium February 11, 2015

Current Performance Objectives

1. “…structures will have a suitably low likelihood of collapse in…the MCE ground motion

2. “…life threatening damage, primarily from failure of nonstructural elements in and on structures, will be unlikely in an unusual but less rare ground motion…the design earthquake ground motion

From the current commentary on Importance factor and occupancy categories

Building Seismic Safety Council Seismic Performance Objectives 2

Current Performance Objectives

Building Seismic Safety Council Seismic Performance Objectives

3

History on Objectives

• Early basis: emulate buildings that succeeded in strong earthquakes; avoid features associated with failures

• SEAOC Blue Book: – Damage, but not collapse in major EQ – Damage, repairable, in moderate EQ – No significant damage in minor EQ

• UBC: zones added to extend applicability elsewhere


History on Objectives

• Nuclear Industry: – Maximum Credible Earthquake – Safe

Shutdown – Operating Basis Earthquake

• San Fernando Earthquake of 1971: performance of hospitals was an issue

• SEAOC/UBC added an Importance Factor, which increased design accelerations by 50%


History on Objectives, cont’d

• ATC 3 (1978): – Provide life safety in ground motion with 10%

chance in 50 years (500 year mri) – Importance Factor on design acceleration

disappeared – Better performance for essential facilities

through smaller limits on drift – Concepts took time to get into standards and

building codes (e.g., ASCE 7 – 1993) Building Seismic Safety Council Seismic Performance Objectives 6


• NEHRP Project ’97: – new maps to fit new criterion: low likelihood of

collapse in MCE .=. 2% chance in 50 year (2500 year mri, assumed margin > 1.5), with a defined deterministic cap

– Importance factor re-introduced, but formatted as a control on damage (i.e. R factor was divided by the I factor to reduce inelastic deformation); result similar to original I factor



• ATC 63: FEMA P695: “low likelihood” defined to be 10% chance of collapse given MCE GM when computed under prescribed methods

• Developed for the purpose of validating seismic design parameters, particularly the response modification, or R, factor



• NEHRP Project ’07: – “low likelihood” of collapse further defined to

mean 1% chance of collapse due to EQ GM in 50 years (but not in areas with deterministic cap on the ground motions)

– 10% chance of collapse given occurrence of MCE maintained, but when coupled with the prior criterion, the result is a “risk-targeted” ground motion. The annual probability of exceedence varies from place to place


Status in 2009 Provisions • We do have a quantitative objective

regarding collapse of ordinary buildings 1%* chance in 50 years

• We have design provisions that deliver a smaller risk for more important structures through the importance factor

I = 1.5 for essential facilities • We have an unquantified functionality

objective for essential facilities Building Seismic Safety Council Seismic Performance Objectives 10

Stated and Implied Quantification

• Risk to Collapse – Category II: 10% given MCER, 1%* in 50 years

– Category III: ~5% given MCER, ~0.5%* in 50 years

– Category IV: ~2.5% given MCER, ~0.25%* in 50 years


• Number of people at risk is the basis for the Risk Category:

Structural Safety for Other Loadings

• Reliability approaches for structural engineering began development in 1950’s

• 1979 report NBS SP 577 led to LRFD – 1982 ANSI A58.1 – 1986 AISC LRFD – …many others

• Calibrated to historical designs, for which there was (general) acceptance for safety


Structural Safety, cont’d

• First Order, Second Moment approach: a simple representation of variability in loads, load effects, and capacities – Safety index, β

• Tied to performance of members within a structure, not exactly the structure as a whole


Structural Safety

• Correlation with other loadings – Seismic 1%* chance of collapse in 50 years – Gravity 0.1% (+) chance of member failure in

50 years (Wind may be higher) • Difference between member and system

limit states – Series versus parallel operation – Differing consequences of the limit state


Safety Performance Objectives

• The difference in risk between high hazard seismic and other loads is not irrational:

COST MATTERS • Typical premium 0.5% to 3% of cost of

construction in moderately high risk area • There is no real basis for a difference in

risk in lower hazard areas where the cost premium is not particularly high


Functional Objectives

• Longstanding qualitative objective for essential facilities: the fundamental question is at what level of ground motion?

• ATC 84 gives the framework for defining a Functional Level Earthquake Ground Motion, on a risk targeted basis (FLER)


Functional Objectives • Example 1, set a 10% chance of loss of function

in 50 years for ordinary buildings and define FLER as the motion giving the conditional probability of 10%, then ground motion mri ~200 years, and current importance factor would imply about 5% chance for loss of function in 50 years for essential facilities.

• Example 2, set 5% chance in 50 years and 10% chance of loss of function given FLER for essential facility results in about 400 for g.m. mri Building Seismic Safety Council Seismic Performance Objectives 17

Functional Objectives

• Basic Issues: – What does society expect for various risk

categories of structures? – What are we getting with current indirect

procedures? • Structural (I factor and drift limits) • Nonstructural (IP and RP factors on 2/3 MCER)

– What are cost increments for changes in target levels of performance?


Economic Loss

• Exceptionally important to individuals and society as a whole

• Not really a stated target for our standards (saving lives in two ways is all we are aiming for at this time – but could eventually offer guidelines – for example linear behavior at a Service Level Earthquake Ground Motion, SLE)


Purpose, 2009 Edition

…The intent of these Provisions is to provide reasonable assurance of seismic performance that will 1. Avoid serious injury and loss of life 2. Avoid loss of function in critical facilities 3. Minimize structural and nonstructural

repair costs where practical to do so These objectives are addressed… Building Seismic Safety Council Seismic Performance Objectives 20

Relevant Efforts for 2015

1. Clarify and quantify performance objectives

2. Rationalize and improve use of Seismic Design Categories

3. Rationalize and improve use of height limits


Clarification, and a Change

1. Avoid serious injury and life loss due to – Structure collapse – Failure of nonstructural components or systems – Release of hazardous materials

2. Preserve means of egress 3. Avoid loss of function in critical facilities 4. Reduce … repair costs where practicable Building Seismic Safety Council Seismic Performance Objectives 22

Clarification and Quantification

• These performance objectives do not all have the same likelihood of being achieved.

• The basic collapse prevention objective is partially quantified; but the others are not.


Quantification of Objectives

• Starting basis implicit in the maps in ASCE 7-10: For Risk Category II – 10% risk of collapse given MCE motion AND – 1% risk of collapse in 50 years, with

deterministic hat • Basic Desire: confirm Risk Category II

objective and develop consistent objectives for other risk categories



Selected Hazard Curves

0.001

0.01

0.1

1

10

1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06

Spec

tral

Acc

eler

atio

n, S

s

Mean reccurence interval

San Francisco Memphis Minneapolis MCE DBESign Wood apartment Big Box Parking garage

25

Spectral Acceleration threshold

Generic Fragilities


26

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.0000 1.0000 2.0000 3.0000 4.0000 5.0000 6.0000 7.0000 8.0000 9.0000 10.0000

Prob

abili

ty o

f fai

lure

Strength normalized at 10% chance of failure

20%

40%

60%

80%

Quantification Is Controversial • Some dissatisfaction with deterministic hat • Some dissatisfaction with risk targeted

ground motions, i.e. some prefer ground motions at a single point on hazard curve

• Some dissatisfaction with 1%/50 year, especially where hazard is low

• Some prefer use of DE in lieu of MCE • Achieved consensus to confirm and state

the basic anchor point Building Seismic Safety Council Seismic Performance Objectives 27

Extension to other Risk Categories: Structural Collapse Safety

• Basic approach, stay with the MCE, and stay with the current importance factors, then define what we get, or define what we want and adjust importance factors

• FEMA P695 provides the basis for R factors

RC Probability Ie factor I 20% 0.8 II 10% 1 III 5% 1.25 IV 2.5% 1.5


• ATC 84 provides the methodology to adjust Ie factors to achieve the objectives

• This is also somewhat controversial

Nonstructural Components/Systems • Prevent failures that endanger life at the

design earthquake (which is two-thirds of the MCER)

• Cannot quantify a probability of success: – Inadequate knowledge of fragilities – Inadequate knowledge of demand given a

ground motion – Design gm not at a consistent probability

• Component Importance factor adjusts level of safety


Release of Hazardous Materials

• Prevent failures of structure or of nonstructural components/systems that would release unacceptable quantities…

• No explicitly quantitative criterion is stated, although importance factors on the structure and on nonstructural components is used as they are for the structural collapse objective


Preservation of Egress

• Stairs…shall be functional following the DE ground motion

• No quantified performance criterion; component importance factor applies for some types of stairs

• Newly stated objective


Functionality of Critical Facilities

• …Avoid earthquake-induced loss of functionality for Risk Category IV strctures and some nonbuilidng Risk Category III nonbuilding structures

• Also don’t have a quantified performance criterion here; recent work casts doubt that we achieve this at the DE ground motion, although many would like to achieve that


Repair Costs

• ASCE 7 aims primarily at nonstructural elements for which seismic anchorage and bracing are both low cost and effective in reducing economic loss

• Some material design standards have provisions that are based upon this idea, but for structural elements (e.g. ACI 318)


Framework from ATC 84


34

Objectives: Bottom Line

• Update to purpose has passed • “Part 3” paper also has passed

– Includes validation or tune-up of the importance factors with appropriate commentary

– Describes possible functionality criteria for RC IV (ground motion, drift most likely)

– Describes how a new ground motion hazard level could be used to check economic loss


Possible Future Simplification to 3 Design Categories

3 Categories would be: 1. Ignore seismic design 2. Transition category: SFRS strength

provides control on area damaged by very rare event; use risk-based ground motion as default. Control worst killers.

3. Basic seismic design: our current Category D criteria, based on risk = 1% in 50 years where MCER SS/S1 > 1.5/0.9


Seismic Design Category 1 of 3 • No seismic requirements • Threshold similar to current (depending on

Risk Category); basis would be that risk is no higher than for other design loads – Approximately 0.1% probability of structural

failure in 50 years is target – Use equivalence to MMI as surrogate for now;

don’t have enough data on performance to be more precise yet


Seismic Design Category 1 of 3

• Threshold of ground motion (for ordinary risk category) could be something like this: MCE about 1/3 of MMI VIII, or roughly MMI VI. This could/should raise the cutoff somewhat from the present

• For higher Risk Categories, could adjust by our Ie factor, provided we accept rationale for Ie factors


Seismic Design Category 2 of 3 • Basic structural frame strength based

upon probabilistic ground motions

• Use R factors validated for SDC 3 (this will likely provide some conservatism)

• Nonstructural very bare bones; really only the proven life safety concerns (this is likely to change at higher Risk Categories)



• Raise the threshold at the high end; consider the high end of current C

• Therefore do a better job of controlling the worst of the proven killers than we currently do in SDC B: weak stories, unreinforced masonry, weak/brittle connections in the main load path, perhaps a few others



• Fundamentally the same as current seismic design category D – Need to decide what to do with the near fault

provisions – are they general or only imposed at extreme motions?

– Life safety for nonstructural similar scope to current, however, will probably need to scale for MCE


Possible Boundaries

Seismic Design Category SMS / I* SM1 / I* 1 – basic structural integrity

upper boundary 0.3-0.4 (0.25) 0.1 (0.1) 2 – similar to current “B”

upper boundary 0.8 (0.5/0.75) 0.3 (0.2/0.3) 3 – similar to current “D”


Notes: • 1 to 2 boundary probably at higher motion than current A to B • 2 to 3 boundary close to current C to D • SM1 to SMS ratio not a constant • Values here are NOT COMMITTEE CONSENSUS!!!

Future Work on Design Categories

• Committee records could provide the starting point for future discussion

• Tabulation of current step functions and how they would be treated under the new proposal will be archived in committee records

• Would need to include adjustment of current exclusions and height limits to fit


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