Fundamentals of LRFR and Applications of LRFR Fundamentals of LRFR and Applications of LRFR for Bridge Superstructures Presented by : Sita Ram Pandey & Sreelatha Nandivada AI E i I AI Engineers, Inc. 919 Middle Street, Middletown, CT, 06457 From 4 – day course by National highway Institute
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Fundamentals of LRFR and Applications of LRFRFundamentals of LRFR and Applications of LRFR for Bridge Superstructures
Presented by:Sita Ram Pandey &Sreelatha Nandivada
AI E i IAI Engineers, Inc.919 Middle Street, Middletown, CT, 06457
From4 – day course by National highway Institute
How did we get Here? – Origin & History
• Two Major historical events • Tacoma Narrows bridge Collapse on Nov 7th
1940 • Silver Bridge ; from Point Pleasant WV, and
Gallipolis Ohio over Ohio river collapse inGallipolis Ohio , over Ohio river , collapse in 1967.
• Prompted National concern for Bridge safety conditions and to the establishment of the National Bridge Inspection Standards ( NBIS)Inspection Standards ( NBIS)
Tacoma Narrows Bridge - 1940
Tube - The Tacoma Narrows Bridge Collapse (1940
Purpose for Load Rating
• Ensure Public Safety
• Comply with federal standards / regulations –
• Ensure Public Safety
• Comply with federal standards / regulations –Comply with federal standards / regulations
NBIS
Comply with federal standards / regulations
NBIS
• Rehabilitation and replacement
• Posting needs
• Rehabilitation and replacement
• Posting needs
• Report to the NBI
Processing of Overload permits
• Report to the NBI
Processing of Overload permits• Processing of Overload permits• Processing of Overload permits
When should we Load Rate a bridge
• Design stage.
I iti l i t i ti
• Design stage.
I iti l i t i ti• Initial inventory inspection.
• Change in live loading.
• Initial inventory inspection.
• Change in live loading.
• Change in dead load on structure.
• Physical change in any structural member
• Change in dead load on structure.
• Physical change in any structural member• Physical change in any structural member.
• Change in load rating method.
• Physical change in any structural member.
• Change in load rating method.
Classification of Bridge ConditionsClassification of Bridge Conditions
BridgeCondition
Structurally Deficient
Functionally Obsolete
Not Deficient
id di iBridge Conditions 1991-2009
• 603 254 b id i• 603,254 bridges in the United States as of Decemberas of December 2009
• 71,179 were rated 7 , 79 w dstructurally deficient and 78,468 were rated functionally b lobsolete.
Wh t’ L d R ti ?What’s Load Rating?
• Live Load carrying capacity of the Bridge y g p y g
Using as-built bridge plans
Information gathered from latest field Inspection
• Expressed in Rating Factors or in tonnage for a particularvehicle instead of the design approach of ‘satisfying limitvehicle instead of the design approach of satisfying limitstates’
• Superstructure Spans, primary components of the bridge andconnections shall be load rated until the governing
t i t bli h dcomponent is established
Load Rating Standards and RequirementsStandards
• AASHTO – Manual for Bridge Evaluation (MBE) –single national standards for load rating of highway b idbridges.
• Load rating requirements and reporting are specified in NBISN S
Data
Geometric data
Member and condition data
Loading and traffic data
Load Rating Methods in the MBE
• Allowable Stress Rating ( ASR)
• Load Factor Rating ( LFR)
• Load and Resistance Factor Rating (LRFR)
Load Rating Method: ASR
• Loads are at a working level.
C it i d d b f t f f t• Capacity is reduced by a factor of safety.
• Live Load is the HS20 truck or lane load, whichever governs.
Load Rating Method: LFR
A S h b d l d i h d• A Strength-based load rating method
• Uncalibrated code. Load factors were established based on engineering judgment (Unknown reliability)
• No guidance on adjusting load and resistance factors for changed uncertainty in loadings or member resistance.member resistance.
• Live Load is the HS20 truck or lane load, whichever governswhichever governs.
Load Rating Method: LRFR
li bili b d li i hil h• Reliability- based limit states philosophy.
• Use probabilistic method to derive load & resistance factors
• Uniform reliability in load ratings and load postings.
• Provide guidance on adjusting live load g j gfactors using site-specific traffic data.
Overview Of LRFR
Benefits of LRFR
• Reliability-based, limit states approach consistent
• Rating done at strength limit state and check for serviceability
• More easily adopts site specific i f ti hil i t i i ifinformation while maintaining uniform reliability.
CALIBRATION OF LIMIT STATES
• Only the Strength Limit State was calibratedy gbased upon structural reliability theory. Otherlimit states were calibrated to current practice
• Reliability indices of bridges designed by theStandard Specs ranged from 1.5 to 4.5
T li bili i d f l d f• Target reliability index of 3.5 was selected for
AASHTO Legal LoadsS O ega oads(Routine Commercial Traffic)
l dVDOT Legal Loads
VA Single= 54 kips
VA Truck & Semi Trailer = 80 kips
20’
20k
10’ 33’
17k17k17k17k12k
i kVDOT Permit Trucks
i i ( h)LRFR Limit States (Strength)
i i ( i / i )LRFR Limit States (Service/Fatigue)
dLRFR Load Factors
d ( d )LRFR Load Factors( Contd.)
d ( d )LRFR Load Factors( Contd.)
A l i M th dAnalysis Methods:
• Approximate Method
― Lever Rule.
―Distribution Formulas.
―Special Analysis for Exterior Girder.
• Refined Method
Refined method is used if the girders spacing are out of range of applicability ofLRFD, curved girders, NSG Vehicle and to improve load ratings of low ratedbridges.
T L d Di t ib tiTransverse Load Distribution
Distribution Factor (Contd )Distribution Factor (Contd.)
* The skew correction factor shall be applied for the skewed bridges.
Di t ib ti F tDistribution Factor
Distribution Factor for Exterior Members:Distribution Factor for Exterior Members:
• Distribution Factor for Moment, gm
• Distribution Factor for Shear, gvb , gv
Distribution Factor for Interior Members:
• Distribution Factor for Moment, gm
• Distribution Factor for Shear, gv
Version for each of :
• One lane loaded, and
• Two or more lane loaded• Two or more lane loaded
Distribution Factor (Contd )Distribution Factor (Contd.)
Distribution Factor (Contd )Distribution Factor (Contd.)
Distribution Factor (Contd )Distribution Factor (Contd.)
Distribution Factor (Contd )Distribution Factor (Contd.)
Distribution Factor (Contd )Distribution Factor (Contd.)
Distribution Factor (Contd )Distribution Factor (Contd.)