Codes for existing structures - cvut.cz
Transcript of Codes for existing structures - cvut.cz
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Lucca, Dicembre 20131
Codes for existing structuresPietro CROCE
Dipartimento di Ingegneria Civile e IndustrialeUniversità di Pisa
Project number: CZ/011/LLP-LdV/TOI/134005
Seminar: Assessment of existing structures
• Need and criteria for codes and recommendations• Example codes• Example contents with illustrations• Safety acceptance – performance criteria• Applicability to case studies• Future tendencies
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Why reassess an existing structure?
• Deviations from original design• Doubts about safety• Adverse inspection results• Change of use• Lifetime prolongation• Inadequate serviceability
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Structural failures experience
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Typical questions
• What type of inspections are necessary?• What type of measurements shall be
taken?• What analyses shall be performed?• What is the future
risk in usingthe structure?
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How to find the Answers
• No classical code approach• New information becomes available• New techniques can be implemented• New material technologies can be used• New decision criteria under new
uncertainties
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Questions related to codes
• Are existing structures covered by codes for new structures?
• Is there a separate code and to which type of buildings does it apply?
• Do codes allow for relaxation or lower performance?
• What aspects are covered (inspections etc.)?
• What are the governmental regulatory bodies behind?
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Possible requirements for a code on existing structures
•Applicability : the code should be applicable to typicalassessment cases.
• Compatibility to codes for new structures: the codeshould use the same philosophy as current codes fornewstructures.
• Flexibility : the code should be flexible to includeadditional information gained by inspection.
• Ease of use: the code should be understandable toengineers and easy to use in practice.
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Example: Building Code
• 1997 UBC: 2 pages• 2000 IBC: 14 pages• 2003 International Existing
Building Code: 67 pages +214 pages Annexes
• 2012 newversion290 pages
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Prenormative and regulatory tools
• ISO 13822, 2003
• ICC Existing Buildings Code, 2009
• SIA 462 (Switzerland), 1994
• Danish Technical Research Council
• ASCE Seismic Evaluation, 2003
• ACI 437R -03, 2003
• JCSS Recommendations, 2001
• Eurocode 8 – Part 3
• NTC2008
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ISO 13822
• General Framework of Assessment• Data for assessment• Structural Analysis• Verification (Limit State)• Assessment based on satisfactory past performance• Interventions• Report• Judgement and Decisions
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ISO 13822
General flow of assessment
Procedures
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Phase: Preliminary Assessment
• Visual inspection
• Review of documentation
• Code compatibility
• Scoring system:1. age of the structure2. general condition3. loading (modifications)4. structural system5. residual working life
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Phase: Detailed assessment
• Additional inspections
• More detailed analyses1. progressive collapse2. full probabilistic3. sensitivity analyses4. risk analyses
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Phase: Detailed Assessment
• Quantitative inspections
• Updating of information
• Structural reanalysis
• Reliability analysis
• Acceptance criteria
Histogram
0
5
10
15
20
25
30
35
0 30 42 54 66 78 90 102
Freq
uenc
y
Frequency
Normal
Lognorm "0"
Gumbel
Lognormal
Gamma
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New Information (Updating)
A) Proof Load
B) Variables (concrete strength)
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A) Example: Proof Loading (Survival of a load)> Updating of resistance
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B) Example: Concrete strength data
Histogram
0
5
10
15
20
25
30
35
0 30 42 54 66 78 90 102
Fre
quen
cy
Frequency
Normal
Lognorm "0"
Gumbel
Lognormal
Gamma
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Decision Criteria
• Target reliability• Economical considerations• Time constraints• Sociopolotical aspects• Codes and standards• Complexity of analysis• Experience in other fields
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Safety Acceptance Criteria
- European Experience (limit state verification)
- New practice in the US (performance based design)
- Optimisation based on LQI- Judgement
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Conclusions regarding reliability acceptance
• A lower safety level compared to a new structure is acceptable
• Various criteria have been proposed in the literature
• Acceptance criteria depend on cost of safety, consequences of failure, desired residual lifetime
• A decrease of the acceptable reliability index ß by 0.5 can be recommended
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Example: Updated earthquake acceleration
ID 39892
Latitudine 39,30°Longitudine 16,27°
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Seismic hazard curve and updated value due to relaxed acceptance criteria
475 YEARS
10 % in 50
YEARS0,27
25 % in
50 YEARS 170 YEARS
0,16 0,18
20 % in
50
YEARS
225 YEARS
JCSS
CBC
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Railway Bridges
• 100 years old
• Scoring system verification
(foundation, corrosion, joints, supports)
• R (steel resistance) from code on old bridges
• S (train load) from DB
(German Railways)
• Durability problems
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Example: Concrete floor structure(Detailed Procedure)
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Reassessment of r.c. floor structure
flexural limit state function
g = Mu - Ma
Mu: Ultimate Bending MomentMa: Acting Bending Moment
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Variable Distribution c.o.v.
Steel strength
Lognormal 0.06
Concrete Strength
Lognormal 0.14
Cover thickness
Lognormal 0.25
Updating of random variables(due to destructive tests)
Reliability index ß is increased from 3.70(prior information) to 3.80, due to
reduced variability of the parameters
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Typical limit states
- extreme load
- Fatigue
Which measures are necessary in order to meet acceptance criteria (residual life time 20 years)?
Steel road bridges
(Phase 3 Procedure)
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Fatigue models
• Fracture Mechanics approach• Crack growth propagation• Influence of inspections (measurement of
cracks)
2ca
Wf
A
Bb
2c
Sr
Sr
Wf
b
WcpL
θ
25m 30m 25m
5m
Detail locationCover plate detail
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0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.5 1 1.5 2 2.5 3 3.5
Crack size (mm)
DetectionProbability
ECT
DPI
CWMPI
ACFM
Variable Distribution Type
ad POD* Inspection
ag Uniform Repair
afail Derived Mixed
Sr RayleighLoad
Smax Gumbel
Fatigue assessment: Random Variables (examples)
* POD for MPI used in case study
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1,00E-05
1,00E-04
1,00E-03
1,00E-02
1,00E-01
1,00E+00
30 60 90 120T (Years)
Pf
GIADI =∩=
LTIADI =∩=
GLTIADI +=∩=
DI =
Prior
I: Inspection, D=DetectionIA: Invasive Action, LT=Load Truncation, G=Weld Toe Grinding
Fatigue assessment: typical results
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• Inspection and crack detection at T=30y• Alternatives considered:
1. Load truncation (LT)2. Weld toe grinding (G)3. Load truncation + weld toe grinding (LT+G)
Fatigue assessment: scenarios
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Future tendencies
• No classical code approach• Quantification of new information • Updated design values• Uncertainties (climate change etc.)• Relaxed acceptance criteria• Robustness aspects
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Vita nominale di progettoTabella 2.4.I – Vita nominale VN per diversi tipi di opere
TIPO DESCRIZIONE Vita Nominale
VN
(in anni) 1 Opere provvisorie – Opere provvisionali – Strutture in fase costruttiva(1)
≤ 10
2 Opere ordinarie, ponti, opere infrastrutturali e dighe, di dimensioni contenute, o di importanza normale
≥ 50
3 Opere, ponti, opere infrastrutturali e dighe, di grandi dimensioni, o di rilevante importanza
≥ 100
1] Le verifiche sismiche di opere provvisorie o strutture in fase costruttiva possono omettersi quando le relative
durate previste in progetto siano inferiori a 2 anni.
Tab. 2.4.I – Valori minimi della Vita nominale VN di progetto per i diversi tipi di costruzioni
TIPI DI COSTRUZIONI DI NUOVA REALIZZAZIONE
Valori minimi
di VN (anni)
1 Costruzioni temporanee(1) (provvisorie) 10
2 Costruzioni con livelli di prestazioni ordinari 50
3 Costruzioni con livelli di prestazioni elevati 100
(1) Costruzioni o parti di esse che possono essere smantellate con l’intento di essere riutilizzate non sono da
considerarsi temporanee
NTC 2008
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Vita nominale di progettoLa vita nominale di progetto VN di un’opera è convenzionalmente definita come il numero di anni nel quale l’opera, purché soggetta alla manutenzione ordinaria così come prevista in sede di progetto, è previsto che mantenga i livelli
prestazionali per i quali è stata progettata.Non è una vita intesa in senso biologico
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Thank you for your attention