Safety Acceptance criteria in structural codes: a critical review

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Safety Acceptance criteria in structural codes a
critical review
D. Diamantidis, University
of Applied Sciences,
Regensburg, Germany
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OUTLINE

• I Introduction
• II General risk acceptance criteria
• III Developments in current standards
• IV Reflection of experience
• V Conclusions

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The oldest rule Hammourabi code, Babylone,
1728-1686 BC
Rule 229 If a builder build a house for some
one, and does not construct it properly, and the
house which he built fall in and kill its owner,
then that builder shall be put to death.
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Failure Causes

Design

Execution

Use

Other

Origin
2
0

5
0

15

15


Errors due to human activity

Actions

Causes
80

20

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II General Risk Acceptance Criteria

• Human Safety (Societal Risk - ALARP)
• Calibration
• Optimization (including human life - LQI)

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ALARP Risk Acceptance Criteria(societal risk)
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Hazard probability levels
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Hazard severity levels
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Risk Acceptability Matrixfor risk
verificationAL Allowable NAL Not
AllowableALARP As Low As Reasonably Practicable
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Example tunnel structure
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II General Risk Acceptance Criteria

• Human Safety (Societal Risk - ALARP)
• Calibration
• Optimization (including human life - LQI)

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Domains of experienced fatalities
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Relation between Failure Probability pf and
Reliability Index ?
Calibration through computation of ? values for
various member types (piles, columns, beams etc.
inherent in the codes)
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Example dikes in Netherlands (Vrouwenvelder)
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II General Risk Acceptance Criteria

• Human Safety (Societal Risk - ALARP)
• Calibration
• Optimization (including human life - LQI)

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Cost evaluation including loss of human life
Life Quality Index (LQI)

• LQI gw e (1-w)
• g the gross domestic product per person per
  year
• e the life expectancy at birth
• w the proportion of life spent in economic
  activity.

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ICAF Implied cost of averting a fatality

• Dgmax g/2 (1-w)/(w)
• g gross domestic product
• per year per person
• e life expectancy at birth
• w proportion of life spent in
• economic activity

• ICAF ge/4 (1-w)/(w)
• ICAF 2 5 Mio.

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Optimization criterion

• Costs
• Annualised investment costs
• Annual maintenance/operation costs
• Benefits
• Human risk reduction
• Direct/Indirect financial loss reduction

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III Developments in current Standards

• Limit State Design
• Partial safety factor format (Eurocodes)
• LRFD format (ACI)
• Checks at member level
• Performance Based Design
• FEMA, ASCE, ATC, NZBC
• Overall check of the structure under
• extreme loads (earthquake, blast, fire)

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Limit State Design
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Target Reliability (1 year ref. Period)new
structures, ULS, component level
Consequences
Cost of safety
Background Eurocodes, JCSS, 2001
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JCSS (2001) proposal for existing structures
ßE ßN ?ß ßE
target reliability index for an existing
structure ßN target reliability index for
a new structure ?ß reduction factor
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Performance Based Design PBD

• Loma Prieta earthquake,
• October 17, 1989
• Oakland, California
• Magnitude 6.9

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Performance objectives
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EQ Probability levels
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Performance Based Design
Hazard Levels
Performance
Levels
Commonly selected performance objectives
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PBD criteria

• pE . pNPE lt pT
• pE propability of event
• pNPEconditional probability of no
• performance given event
• pT acceptable probability

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PBD criteria (new structure)

• pE . pNPE lt pT
• pE 2 in 50 years
• pNPE 10
• pT 4x10-5 per year

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PBD criteria (old structure)

• pE . pNPE lt pT
• pE 4 in 50 years
• pNPE25
• pT 2x10-4 per year (5 times larger)

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IV Reflections (experience)

• Code committee work on safety targets (FIB, JCSS,
  ISO) for normal structures
• Safety acceptance criteria for special structures
  (tunnels, offshore, etc.)
• Safety acceptance criteria for various existing
  structures (buildings, bridges, tunnels, offshore
  structures)

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Sydney Cross City tunnel Resistance
considerations

• Fibers improve
• local resistance
• fire resistance

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Resistance safety factor ?R for fiber reinforced
shotcrete
?R

• Large consequences of failure(ß4,3 ) ?R 1,50

Moderate consequences of failure(ß3.8 ) ?R 1,35
ß
ß
ß
Minor consequences of failure(ß3.3 ) ?R 1,20
Vr
0,18
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Messina straits, submerged floating
tunnel(Preliminary design, 1987 1993)
Buoyant concrete structure 30m under water Cross
section alternative 42.5mx24m Anchorage system
four inclined steel anchors Progressive collapse
due to failure of tethers (similar to TLP
platforms) Acceptable system failure 10-6 per year
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Existing buidings in Germany

• Office building
• Concrete construction
• 70 years old
• Safety standards not satisfied
• Safety acceptance under reduced load

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V Conclusions
1) Failure records indicate in general
satisfactory safety level in standards 2) Risk
acceptance in codes are usually specified for
component failures (ULS) 3) Performance Based
Design can be used to investigate global failure
in case of extreme loads (earthquake, blast) 4)
Lower safety levels for existing structures are
acceptable compared to new ones 5) Need for a)
harmonization b) safety class
differentiation c) robustness
criteria gt Risk based rules