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Load rating Segmental - American Segmental Bridge Institute...

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Load Rating of Segmental Concrete Bridges Consistent with LRFR Requirements Corven Engineering Inc. November, 2003 American Segmental Bridge Institute Annual Convention, 2003
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Inventory Rating : for design loads. The design load level that can safely use a bridge for an indefinite period of time. Operating Rating : for design, legal and permit loads . The absolute maximum load level to which a structure may be subjected. Takes advantage of conservatism in design. Definitions
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For Segmental Bridges Inventory and Operating Ratings are performed at both the strength limit state and service limit state . Clearing Up A Misconception FDOT Manual 1995 - Section II.B.8 “The load factor method is the required method for load rating structures, unless circumstances dictate that other methods should be used . …”
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Existing Criteria based on AASHTO Standard Specification (LFD) and Guide Specification for Segmental Bridges. When allowable Service stress is zero, ratings for Inventory and Operating are same – i.e. no benefit for Operating Introduction of AASHTO LRFD & LRFR. Current Issues in Rating Segmental Bridges
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An AASHTO approved guide specification for rating all bridge types consistent with LRFD . Uses target reliability, β , of 2.5 for rating v. 3.5 for design ( via e.g. γ = 1.35 vs. 1.75 ) Load Resistance Factor Rating - LRFR
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R-Q R-Q (R- (R- Q) Q) mean mean β β σ σ Graphical Graphical definition definition of of reliability reliability index index β β
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Reliability Indices 0 1 2 3 4 5 Span Length LFD LRFD β ~ 3.5 β ~ 2.5 LRFR (Inv) LRFR (Op)
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To develop Load Rating Guidelines that: Are consistent with LRFR. Address Segmental Bridges. Goal
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Rating Equation (Capacity) (Permanent Loads) Rating Factor (Vehicle Loads) φ γ γ - = Where, Capacity Factors Live Load Factors φ γ = =
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LRFR - Segmental Bridges at Inventory Level (= LRFD Design) Service Limit State Strength Limit State ok ( 29 3.5 β = γ φ ok ok ok
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? LRFR - Segmental Bridges at Operating Level Service Limit State Strength Limit State ( 29 2.5 β = γ φ ? ? ok
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C S N φ φ φ φ = Capacity Factors – Strength Limit State Condition Factor System Factor Strength Reduction Factor C S N φ φ φ = = =
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LRFR – Condition Factor ranges from 0.85 to 1.00 Structural Condition Good or satisfactory Fair Poor 1.00 0.95 0.85 NBI Rating > 6 5 < 4 Condition Factors - General ( 29 C φ ( 29 C φ
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Examples: PC Balanced Cantilever, to new criteria 1.00 PC Bal Cant, old, no leaks or corrosion 1.00 PC Bal Cant, old, joint leaks, stains 0.85 Span-by-Span, no damage or corrosion 1.00 Condition Factors –Segmental PT ( 29 C φ
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NCHRP 406: many simple and continuous I-beam bridges (conc. and steel) – determined reliability, β, and φ System Factor (φ ) – AASHTO LRFR
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φ = .85 System Factor (φ ) AASHTO LRFR
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φ = 1.00 System Factor (φ ) – AASHTO LRFR
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Longitudinal Continuity (redundancy) Transverse Continuum of Closed Box Number of Tendons per Web (Multiple Tendon Paths) Number of webs System Factor ( 29 s φ for Segmental Bridges Need to account for:
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