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4/22/15Topics to cover:HS20-44 vs HL-93(Standard Specifications vs LRFD Code)Fouad Jaber, P.E.Assistant State Bridge EngineerNDOR BRIDGE CONFERECEKEARNEY,NEBRASKAApril 14th, 2015Topic 1Comparison of ASD, LFD andLRFDØ Topic 1:Comparison of ASD, LFD and LRFDØ Topic 2:LRFD Objective and calibrationØ Topic 3:Comparison of HS20-44 VS HL-93Ø Topic 4:FHWA SHV’s new memoØ Topic 5:Husbandry Vehicles and NE Legal LoadsØ Conclusion and things to considerGolden Rule of Engineering§ A. Load Resistance§ B. Load Resistance§ C. Load ResistanceDesigns must be safe,therefore?2015NebraskaBridgeConference1
4/22/15Uncertainty§ Material dimensions and location§ Material strength§ Failure mode and prediction method§ Long term material performance§ Material weights§ Prediction of potential Live loads§ Load analysis and distribution methods§ General uncertainty associated with structural functionAllowable Stress Design(aka Working Stress Design)FS: Factor of Safety.Ru: RESISTANCEΣDL ΣLL Ru/FSLoad Factor Design(aka Strength Design)Allowable Stress DesignLFD§ Advantages§ Simplistic§ Limitations§ Inadequate account of variability§ Stress not a good measure of resistance§ Factor of Safety is subjective§ No risk assessment based on reliability theory2015NebraskaBridgeConference§ γ : Coefficients Load Factor§ φ : Resistance Factor§ ΣβDL, ΣβLL :Loads combinations Coefficientsγ(ΣβDL x DL ΣβLL x LL) φRuEX:1.3 (1.0 x DL 1.67x LL)1.3DL 2.17LL2
4/22/15Load and Resistance Factor Design(aka Reliability Based Design or Limit State Design)Load Factor DesignLRFD§ Advantages§ Load factor applied to each load combination§ Types of loads have different levels of uncertainty§ § § § η : Factorφ : resistance FactorγDL, γLL : Load Factorη ηD ηR ηI 0.95 to 1.050§ Limitations§ More complex than ASD§ No risk assessment based on reliability theoryη(ΣγDL (DL) ΣγLL (LL) )EX: φRuη (1.25DL 1.75LL)1.25DL 1.75LLLoad and Resistance FactorDesign§ Advantages§ Accounts for variability§ Uniform levels of safety§ Risk assessment based on reliability theory§ Limitations§ Requires availability of statistical data§ Resistance factors vary§ Old habits2015NebraskaBridgeConferenceTopic Wrap Up1. State the difference between ASD, LFD and LRFDThe three design methods are distinguished byhow uncertainty is accounted for.3
4/22/15Topic 2Fundamentals of LRFDObjective of LRFDDevelop a comprehensive and consistent Load andResistance Factor Design (LRFD) specification that iscalibrated to obtain uniform reliability (a measure ofsafety) at the strength limit state for all materials.Objective and calibrationCALIBRATIONSelection of a set of γ’s and φ’s to approximate a targetlevel of reliability in an LRFD-format specification.AASHTO chose this reliability to be 3.5 @ inventory levelAnd 2.5 @ operating levelCalibration continuedOnly the strength limit states of the LRFD Specificationsare calibrated based upon the theory of structuralreliability, wherein statistical load and resistance data arerequired. The other limit states are based upon the designcriteria of the Standard Specifications.Calibration for service limit state is done andimplementation is underway.2015NebraskaBridgeConference4
4/22/15Calibration cont.§ Calibration consists of up to three steps:1.2.3.Reliability-based calibration,Calibration or comparison to past practice, and,Liberal doses of engineering e reliability-based LRFD design methodology is notperfect, but it represents an improvement over the ASDand LFD methodologies.LRFD utilizes structural reliability to help us selectimproved load and resistance factors, and it provides aframework for future improvement.5
4/22/15FHWA MEMOCONCLUSIONS(continued)Most of the features which designers dislike about theLRFD Specifications have little,if anything, to do with the LRFD design methodology.Topic 3H/HS20 TRUCKS USED INSTANDARD SPECS.2015NebraskaBridgeConference6
4/22/15HS-20 Live LoadTRUCK LOAD OR LANE LOADTandem and Lane LoadLRFD– HL-93 Loading2015NebraskaBridgeConferenceMultiple Lanes7
4/22/15EXEMPTION§ Culverts shall be design only to axle loads of atruck or Tandem (no lane loads)1.The design truck x 1.2 factored force effectis equivalent to the old Standard Spec. HS-202.The tandem x 1.2 factored force effect isequivalent to the old standard Spec. MilitaryloadsLOADS REDUCTION FATORS§ FACTORS WERE DEVELOPED ON THE BASISOF AN ADTT 5000BRIDGE OWNERS MAY REDUCE THE LOADSBY:§ IF 100 ADTT ,1000,REDUCE THE LOADSBY 5%§ IF ADTT 100, REDUCE THE LOADS BY 10%§ THE REDUCTUON IS BASED ON THEREDUCED PROBABILITY OF ATTAINING THEDESIGN EVENTS DURING A 75-YEAR DESIGNLIFE WITH REDUCED TRUCK VOLUME.2015NebraskaBridgeConferenceSYSTEM PRESERVATIONStandardSpecifications’50 to 60-year designlifev.LRFD Specifications’75-year design life8
4/22/15Topic 4 : FHWA Single Unit PostingVehicle§ SHV orSpecializedHaulingVehicles areLegal LoadsIn NE2015NebraskaBridgeConferenceTriple axle configurations of single unit vehicles (legal loads in Nebraska) havebeen observed to have load effects greater than HL-93 tandem axle load.9
4/22/15Pooled Fund Study of the Impactsof Implements of Husbandry onBridgesTopic 5Vehicle and Bridge Data§ 121 Husbandry Vehicles§ § § § § 2 Two Axle Vehicles37 Three Axle Vehicles46 Four Axle Vehicles29 Five Axle Vehicles7 Six Axle Vehicles§ 174 Bridges§ § § § 32 County steel girder-concrete deck43 State steel girder-concrete deck52 Country timber girder-timber deck47 County steel girder-timber deckGeneric VehiclesGeneric Vehicles Cont’d Generic three axle vehicles§ Generic four axle vehicle2015NebraskaBridgeConference10
4/22/15Generic Vehicles Cont’d§ Generic five axle vehicleComparison of Generic Vehiclesand current AASHTOspecifications§ Included in comparison§ HS20§ HL93§ SHV§ Closely-spaced multi-axle single unittrucksSingle Span Moment RatiosComparison§ Comparison done by Moment Ratios at critical locations§ John Kulicki study and development of HL93§ Example Moment Ratio CalculationMoment Ratio Maximum moment value of AV3 vehicles at ------------------Maximum moment value of the three axle husbandry vehicles at 0.4L§ Ideal Moment Ratio is greater than 1.02015NebraskaBridgeConference11
4/22/15Two Span Moment RatiosThree Span Moment RatiosDistribution of OR ratios for AV3Distribution of OR ratio AV380Distribution of OR ratio AV4)Distribution of OR ratios for AV412010070976860628050606140403028202010000OR ratio 0.750.75 OR ratio 12015NebraskaBridgeConference1 OR ratio 1.2500OR ratio 0.750.75 OR ratio 11 OR ratio 1.25OR ratio 1.25OR ratio 1.2512
4/22/15NEBRASKA’s Legal Weight LimitsDistribution of OR ratios for AV5Distribution of OR ratio AV51601401381201008060402001730OR ratio 0.750.75 OR ratio 11 OR ratio 1.25OR ratio 1.25Wheeled grain cart, tank wagons, and fence-linefeeder§ Single axle – 20,000 pounds maximum§ Gross weight – 20,000 pounds multiplied by thenumber of axles§ Maximum gross weight – up to 80,000 poundson Interstate and Defense highway§ 15% load increase during harvest season and.§ 25% increase for sugar beets will be allowed§ The weight of the farm tractor towing the implement is not includedin the gross weight limit§ Taken from Nebraska DOR Truck Information Guide and NebraskaLawWHAT YOU NEED TO CONSIDERUsing HS20-44 Loading§ Difficult to be used without the Standard spec’s.§ Standard Specs was ceased to be updated byAASHTO more than 10 years ago§ Can’t be used with New LRFD specs withoutadditional calibration. We can’t mix codes.§ New and young designers don’t even know whatLFD ,ASD and HS20-44 are. They don’t teach it incollege anymore.§ Moving forward§ Net Load effect difference between HL-93 andHS20-44 loading is very small. Reliability hasimproved§ Trucks are getting heavier and heavier§ We are designing for 75 years service life2015NebraskaBridgeConferenceQUESTIONS?13
" Taken from Nebraska DOR Truck Information Guide and Nebraska Law NEBRASKA’s Legal Weight Limits Using HS20-44 Loading " Difficult to be used without the Standard spec’s. " Standard Specs was ceased to be updated by AASHTO more than 10 years ago " Can’t be used with New LRFD specs without additional calibration. We can’t mix codes.
