Transcription
NDDOT Load Rating ManualVersion 2.4March 5, 2021
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Table of Contents1.1Introduction .1Reference Material . 1Definitions . 21.2Qualifications and Responsibilities .3Program Manager . 31.2.1.1Program Manager Qualifications . 31.2.1.2Program Manager Responsibilities (As Related to Load Rating) . 3Load Rating Engineer . 31.2.2.1Load Rating Engineer Qualifications . 31.2.2.2Load Rating Engineer Responsibilities . 3Load Rating Committee . 41.2.3.1Members . 4Consultant Load Rating . 41.31.2.4.1Consultant Load Rating Engineer Qualifications . 41.2.4.2Consultant Load Rating Engineer Responsibilities . 4Load Rating Policy .5Load Rating Methods . 5Load Rating Intervals. 51.4Software .61.5Load Rating Procedures .7Load Factor Rating (LFR) . 7Load and Resistance Factor Rating (LRFR) . 71.5.2.1Limit States. 71.5.2.2Condition Factor. 81.5.2.3Dynamic Load Allowance . 81.5.2.4Live Load Distribution . 81.5.2.5Prestressed Losses . 8Allowable Stress Rating (ASR) . 8Bridge Specific Load Rating Procedures . 81.5.4.1Prestressed Concrete Bridges . 81.5.4.2Timber Bridges . 91.5.4.3Concrete Box Culverts . 91.5.4.4Substructures . 9iii
Bridges Without Plans . 101.5.5.1Steel and Timber Superstructures . 101.5.5.2Concrete Superstructures & Concrete Box Culverts . 111.6Load Rating Vehicles . 131.7Submittal of Load Rating and Transfer of Structure Model . 13Transfer of the BrR Structure Model . 13Load Rating Report Summary Template . 13Detailed Load Rating Report . 131.8Posting Requirements . 14Signing Requirements . 151.9Permit Load Rating. 16Appendix A: ND Defined Trucks . 17iv
1.1IntroductionLoad rating is the determination of the live-load carrying capacity of a bridge. The primary purpose of acomplete and accurate load rating is for the safety of the travelling public and to support the NDDOT’smission to safely move people and goods. Secondary uses of the load rating data are: To determine posting needsTo process overload permit requestsTo provide input to rehabilitation and replacement decisionsTo Comply with Federal RegulationsLoad ratings are required as part of the National Bridge Inspection Standards (NBIS) as part of Title 23,CFR §650.313 part (c). Load ratings and FHWA submittals are also governed by Metric # 13, as part ofthe Metrics for the Oversite the National Bridge Inspection Program.Reference MaterialFHWA and AASHTO have published several publications to implement the NBIS. Use the followingadditional publications in conjunction with this manual:For the Code of Federal Regulations: Code of Federal Regulations 23 CFR 650 Subpart C.For Load Rating Procedures (MBE) The American Association of State Highway and Transportation Officials Manual forBridge Evaluation Current VersionFor FHWA Technical Guidance: The FHWA Load Rating Website - https://www.fhwa.dot.gov/bridge/loadrating/. FHWA 2006 Policy Memo - https://www.fhwa.dot.gov/bridge/nbis/103006.cfmFor NBI Coding guidance: FHWA Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation’sBridges.For NBIS Oversight Program: FHWA Metrics for the Oversight of the National Bridge Inspection ProgramThe North Dakota Department of Transportation has additional requirements governing the load ratingof bridges within the State. For additional information use the following guidance documents inconjunction with this manual.North Dakota Truck Regulations: North Dakota Vehicle Size and Weight ents/MC/Legal.size .weight.Guide .pdf North Dakota Weight Limitations ents/Permits/Weight Limitations Chart .pdfMultiple Vehicle Combinations /documents/Permits/9-1-a%20handout.pdf1
DefinitionsASRAllowable Stress RatingLFRLoad Factor RatingLRFRLoad and Resistance Factor RatingInventory Rating (LFR/ASR)Load ratings based on the inventory level allow comparisons with the capacity for new structures andtherefore, results in a live load, which can safely utilize an existing structure for an indefinite period oftime.Operating Rating (LFR/ASR)Load ratings based on the operating rating level generally describe the maximum permissible live load towhich the structure may be subjected. Allowing unlimited numbers of vehicles to use the bridge atoperating level may shorten the life of the bridge.Design Inventory Level Rating (LRFR)Generally corresponds to the rating at the design level of reliability for new bridges in the AASHTO LRFDBridge Design Specifications.Design Operating Level Rating (LRFR)Maximum load level to which a structure may be subjected. Generally corresponds to the rating at theoperating level of reliability in past load rating practice.Legal Level Load Rating (LRFR)This rating level provides a single safe load capacity (for a given truck configuration) applicable toAASHTO and state legal loads.Permit Level Load Rating (LRFR)Permit load rating checks the safety and serviceability of bridges in the review of permit applications forthe passage of vehicles above the legally established weight limitations.2
1.2Qualifications and ResponsibilitiesThe Bridge Management Section is responsible for the North Dakota Bridge Inspection program incompliance with 23 CFR §650 Subpart C.Program ManagerThe Program Manager oversees compliance of the National Bridge Inspection Standards (NBIS).1.2.1.1 1.2.1.2 Program Manager QualificationsServe as the Bridge Inspection Program Manager for the implementation of the NBIS, ProgramManager as defined by NBIS 23 CFR §650 Subpart CBe a registered professional engineer (PE); or have a minimum of ten years of bridge inspectionexperienceSuccessfully complete NHI Course No. 130055 or 130056 Safety Inspection of In-Service BridgesComplete a cumulative total of 18 hours of FHWA approved bridge inspection refresher trainingover each 60-month periodProgram Manager Responsibilities (As Related to Load Rating)Ensure compliance with federal load rating directives (CFR & MBE)Ensure compliance with North Dakota load rating policies and procedures contained in thismanualCoordinate and conduct load rating trainingPerform quality assurance audits on load rating reportsLoad Rating Engineer1.2.2.1 1.2.2.2 Load Rating Engineer QualificationsRegistered PECompleted the FHWA approved National Highway Institute (NHI) load rating course number130092. (Preferred but not required.)Load Rating Engineer ResponsibilitiesOverall responsibility for load rating of bridgesDevelop and maintain load rating manuals and standardsLoad rate structures in accordance with the MBE and this manualReview load ratings prepared by consultantsPrepare and/or verify load rating reports for in house load ratingsReview load rating reports prepared by consultants3
Load Rating CommitteeStructures in the North Dakota Bridge Inventory have been built across the State during a period of over100 years, with a variety of owners/designers and a series of evolving specifications. Therefore, thereare a wide variety of design methods, construction methods, material options, in-service deterioration,damage, and repair methods in the inventory. This variety creates unique sets of conditions that maymanifest themselves in ways that are not explicitly addressed in this manual or the MBE. At thediscretion of the Load Rating Engineer, the Load Rating Committee will meet to discuss unique orunusual conditions and make decisions on any assumptions made or clarifications needed to complete aload rating. Resolutions to issues encountered will be included in the load rating report, and ifnecessary, revisions or additions to this manual will be created using Errata.1.2.3.1 MembersLoad Rating Engineero The Load Rating Engineer is responsible for setting the agenda, scheduling, and leadingthe meeting.Bridge Management EngineerBridge Design EngineerAsst. Bridge EngineerBridge Engineer (Optional)Consultant Load Rating1.2.4.1 1.2.4.2Consultant Load Rating Engineer QualificationsRegistered PECompleted the FHWA approved National Highway Institute (NHI) load rating course number130092. (Preferred but not required.)Consultant Load Rating Engineer ResponsibilitiesLoad rating shall be completed using an electronic model created with the AASHTOWare Bridge RatingSoftware (BrR) as indicated in Section 1.4 of this manual. For bridge types that are incompatible withBrR, an alternate software may be used with approval by the Load Rating Engineer.Load ratings shall be reported to the Department using the Load Rating Report Summary Template and aLoad Rating Detailed Report as described in Section 1.7.2 and Section 1.7.3.Following completion of the load rating, the electronic model will be submitted to and become theproperty of the Department. The Department reserves the right to use the model for future load rating.All load ratings must be independently reviewed prior to submission to the Department. Provideroutine and consistent checks for data integrity, correctness, and completeness. Identify and addresserrors and omissions.4
1.3Load Rating PolicyThe NDDOT load rating policies are governed by the MBE. The NDDOT Load Rating Manual is intendedto supplement and provide additional guidance and clarification as needed.All structures greater than 20’ and open to public vehicle travel must be load rated. See the NDDOTBridge Inspection Manual, Section 1.3, for guidance on bridge length multiple pipe configurations forload rating purposes.Load Rating MethodsFHWA has defined which load rating methodologies are allowable for structures based on structure typeand year of design as follows:For structures designed using the Load & Resistance Factor Design (LRFD) methodology, rate thestructures using the Load & Resistance Factor Rating (LRFR) method.For structures designed with Allowable Stress Design (ASD), Load Factor Design (LFD), or an unknownmethod, rate the structures using the Load Factor Rating (LFR) method. It is also acceptable to rate usingLRFR, but this shall be approved in advance by the NDDOT Bridge Division.If an existing bridge was designed using Allowable Stress Design (ASD) methodology, it shall be ratedusing LFR. It is also acceptable to rate using LRFR, but this shall be approved in advance by the NDDOTBridge Division. There is an exception for bridges with timber or concrete masonry superstructures. Forthese types only, it is acceptable to utilize Allowable Stress Rating (ASR).For additional guidance see the FHWA 2006 Load Rating Memo.Load Rating IntervalsLoad rating shall be completed at the following intervals:1. Design Phase. All new structures shall be load rated during the design phase. Load ratings inthe design phase shall be submitted to the Load Rating Engineer prior to the submittal of thefinal plan set. For precast items, the load rating must be updated prior to fabrication of theprefabricated elements.For load ratings calculated during the design phase, the Design Level Inventory Rating must begreater than or equal to 1.0. If the Design level Inventory Rating is less than 1.0, the design mustbe revised to increase the capacity until the Design Level Inventory Rating is greater than orequal to 1.0.2. In Service Phase. Load ratings shall be reviewed by the Load Rating Engineer after constructionof the bridge to incorporate any changes in the design details as noted on as-built plans, changeorders, construction inspection, and initial inspection reports.3. Maintenance Phase. The load rating shall also be reviewed when the NBI ratings decline to thevalues listed in the Table 1.3.1 below.5
Load Rating Review TableComponentNBI ratingDeck4Super4Sub4Culvert4Table 1.3-1 NBI Load Rating Review TriggersAfter review, the Load Rating Engineer will determine if the bridge needs to be re-rated.In addition, load ratings shall be updated by the Load Rating Engineer when a meaningful change incondition or service of the structure occurs. A meaningful change is defined as:oooooo1.4Bridge rehabilitation that affects structural componentsA significant change in the live load on the bridge due to a change in the use of thebridgeIncrease in dead load on the bridgeA change in state or federal laws regulating truck weightsStructural damage resulting from a bridge hit, ice damage, flood damage, fire damage,or other causeDeterioration that affects structural capacity of the superstructure including but notlimited to: Rotated or displaced beams Steel section loss or cracking Severed rebar or prestressed strands in concrete beams Splitting, cracking, or rot of timber membersSoftwareAll load ratings shall be calculated using AASHTOWare Bridge Rating (BrR) Software Version 6.8.2, 6.8.3,or 6.8.4. An electronic model of the structure must be developed for future use.For bridge types that are incompatible with BrR, an alternate software may be used with approval by theLoad Rating Engineer.For complex bridges or bridges that require posting, a refined analysis may be required. Approval toproceed with a refined analysis must be obtained in advance by the NDDOT Bridge Division.6
1.5Load Rating ProceduresThe identification information for each structure entered into BrR must match the identificationinformation in the NDDOT Bridge Management System (BMS).In general, primary load-carrying members are required to be load rated. Secondary elements may beload rated if there is significant deterioration or if there is a question regarding the original designcapacity. The load rating engineer is responsible for the decision on load rating secondary elements.Rate both interior and exterior girders. For bridges with a curved edge of deck and chorded girders, setone overhang to the maximum overhang within the span.Other features that must be load rated include: Fracture critical members or components with fatigue prone details Gusset plates and connection elements for nonredundant steel truss bridgesThe NDDOT currently uses ASR, LFR, and LRFR load rating methods. See Section 1.3.1 for guidance forwhen each method can be used.Load Factor Rating (LFR)Check all structures for shear (except for concrete slabs) and bending moment.Use the live load models as indicated in Section 1.6 Load Rating Vehicles.Load and Resistance Factor Rating (LRFR)Check all structures for shear (except for concrete slabs) and bending moment.Use the live load models as indicated in Section 1.6 Load Rating Vehicles.1.5.2.1Limit StatesUse the following limit states as defined by the LRFD specifications and the MBE.Strength I: Load combination used to determine the flexural and shear demands without wind.Strength II: Load combination used to determine the flexural and shear demands of a structuresubject to a permit vehicle or a special design vehicle specified by the owner.Service I: Load combination used for the design of many elements. It is used for service loadstress checks (prestressed concrete), deflection checks, crack control checks in reinforcedconcrete, etc.Service III: Load combination used to check nominal tension in prestressed concrete structures.7
1.5.2.2Condition FactorUse table 1.5-1 to select the condition factor (φc).Condition FactorGeneral Superstructure orCulvert Condition Rating(Item 59 or 62)6 or Higher54 or lowerφc1.00.950.85Table 1.5-1 Condition Factor1.5.2.3Dynamic Load AllowanceUse full AASHTO dynamic load allowance (33%).1.5.2.4Live Load DistributionConcrete Slabs and Timber DecksConcrete slabs and timber decks, the LRFD Specifications provide equations for live load distributionfactors (LLDF) that result in equivalent strip widths, E, that are assumed to carry one lane of traffic. Theequivalent strip width shall be converted to a live load distribution factor for the unit strip by taking thereciprocal of the width. LLDF 1/E.Girder BridgesSome structures may not have been designed with a 1.2 multiple presence factor multiplier for one laneloaded on the exterior beam, which may cause the exterior beam to control. If this should occur on abridge that would require posting, review the configuration of the bridge to determine if the 1.2multiplier should be reconsidered for the load rating.1.5.2.5Prestressed LossesUse the approximate method for estimating time-dependent losses in prestressing steel. Do notconsider any elastic gains in prestressing steel in determining losses. Use an average annual ambientrelative humidity of 70% in calculating prestressing losses due to concrete creep and shrinkage as perAASHTO Figure 5.4.2.3.3-1.Allowable Stress Rating (ASR)The allowable stress rating method can only be used for timber structures.Bridge Specific Load Rating ProceduresThe following section contains details unique to specific bridge types.1.5.4.1Prestressed Concrete BridgesFor prestressed concrete bridges designed continuous, rate the bridge as continuous. If the continuitysteel in the deck over the pier controls the rating and would require the bridge to be posted, then ratethe bridge as simple spans. Provide documentation in the load rating report that the deck steelcontrolled, and a simple span rating was completed.8
1.5.4.2Timber BridgesFor timber members see LFD Table 13.5.1A Tabulated Design Values for Visually Graded Lumber andTimbers or LRFD Table 8.4.1.1.4-1 Reference Design Values for Visually Graded Sawn Lumber. If thetimber material Species and Grade cannot be determined, use the material properties provided in Table1.5-2. Alternatively, physical testing may be used to refine load rating calculations when applicable.ASD Design Values for Timber of Unknown Species & Grade (ksi)ShapeBeamPlankBendingStress mpressionStress(Parallel)Modulus ofElasticity0.6250.5650.9251.45016001700Table 1.5-2 ASD Design Values for Timber of Unknown Species and Grade1.5.4.3Concrete Box CulvertsMultiple-span culverts with a depth of fill that exceeds the distance between faces of end walls do notneed to be load rated for live loads as the live load effects are negligible, per LRFD specifications.Distribution of wheel loads for culverts with less than 2.0 ft of fill shall be taken as specified in LRFDDesign Article 4.6.2.10.Distribution of wheel loads to culverts with 2.0 ft or more of cover shall be as specified in LRFD DesignArticle 3.6.1.2.6.If adequate soil information cannot be obtained, use ‘Standard Soil 1’ that is included in the BrRsoftware.1.5.4.4SubstructuresSubstructure load ratings are not routinely completed. The following guidance should be used indetermining when a substructure load rating is necessary: If a poor substructure is observed (NBI rating of 4 or lower), and it is determined that the defectis detrimental to the substructure’s load carrying capacity. For instance, a high degree of rot orsection loss, changes in timber pile end conditions due to deterioration, and changes in timberpile bracing conditions.Concrete substructures may not require load rating, even if the condition is rated 4 or lower, ifthe engineer, using sound engineering judgement, believes that this rating will not control (belower than the superstructure rating). This determination, along with supporting justification,needs to be documented and included with the load rating reportPiles should be rated if a significant amount of soil has been lost by scour or other meansaround the pile that could cause a buckling issue, if there is significant pile deterioration (decayor brooming of timber piles) that could affect load carrying capacity, or if loss of soil around thepiles would preclude adequate support of friction piles. It is not expected that the CONDITION ofpiling be evaluated below the ground line to determine load carrying capacity.9
Pier caps shall be rated if there is deterioration or other structural issues present that wouldhave an effect on the capacity of the cap. Consideration shall also be given to the structuralgeometry present and its impact on the load rating. For example, load rating of timber bent capsmay govern when the pile spacing is excessive or when there is loss of support by individualtimber piles due to rot or decay that would increase the effective span of the timber bent cap.Only the portion(s) of the substructure causing the poor condition rating (NBI 4 or less) and that wouldresult in the controlling load rating, shall be rated.Bridges Without PlansStructural dimensions and material properties are needed to perform load ratings. However, someexisting bridges may not have all the necessary information to complete a load rating.Before continuing with an alternate load rating process, the load rater must explore all possible sourcesof information about the bridge. These include: NDDOT Bridge Division filesCounty Bridge filesPre-cast fabricator recordsNDDOT microfilm archivesStandard PlansFor structures with no plans or other as-built information, the necessary details may be determined byengineering judgement of comparable structures with known standard plans, or plans of other similarlyconstructed bridges, such as parallel bridges. When the as-built plans cannot be located, the followingprocess may be used to determine similarly constructed bridges:1. Search for plans for any structures that were built of a similar design and time period.2. Search for any common items or reinforcement details.3. Use the collected information to complete the load rating.4. Document the process used in the final report.For structures that have no plans available and no similar bridges could be identified, providedocumentation that all sources have been explored. Load rate the structure according to Section 1.5.6.1or Section 1.5.6.2 based on the material type of the superstructure.1.5.5.1Steel and Timber SuperstructuresSteel and timber structures can be rated using BrR software using field dimensions and the year ofconstruction. Detailed field measurements are required including span length, girder dimension, girderspacing, diaphragm dimensions and locations, deck width, deck overhang, and deck thickness. Materialproperties can be selected using the MBE and LRFD Specifications; for steel members see MBE Table6A.6.2.1-1 Minimum Mechanical Properties of Structural Steel by Year of Construction; For Timberbridges see Section 1.5.4.2For steel super structures with a concrete deck, assume that the girders act non-compositely with thedeck.10
1.5.5.2Concrete Superstructures & Concrete Box CulvertsGiven that North Dakota has many local agency bridges and bridge length culverts without plans, a riskbased criterion must be applied to complete load ratings. The MBE in Section 6.1.4 states “A concretebridge with unknown details need not be posted for restricted loading if it has been carrying normaltraffic for an appreciable period and shows no distress. The bridge shall be inspected regularly to verifysatisfactory performance”. Therefore, concrete bridges with unknown details can be assumed to havesufficient live load capacity for the vehicle for which it was designed if the following criteria can be met: ADT 1000Physical inspection of the bridge by a qualified inspector and evaluation by a qualified loadrating engineer.Superstructure (NBI 59) or culvert (NBI 62) rating of 5 or higher as of the most recent inspection.No available documentation such as plans or shop drawings.Bridge is open without restriction (not posted).If all the above criteria are met the operating and inventory rating shall be given a judgement load rating(NBI 63/65 0) based on the design load.For structures with a design load with an ‘HS’ configuration, the Inventory rating (NBI 66) shall be equalto the design load in tons. The Operating rating (NBI 64) shall be equal to the inventory rating multipliedby the ratio of the inventory load factor (2.17) divided by the operating load factor (1.3) according to theLFD design specifications. See Table 1.5-1 for more information.For structures with a design load with an ‘H’ configuration (Such as H10, H12, H15, & H20), the InventoryRating (NBI 66) shall be equal to 21 tons and the Operating Rating (NBI 64) shall be equal to 36 tons.If this design load is unknown use H15 for bridges built before 1978 and use HS20 for bridges built after1978. See Table 1.5-1 for more information.Design LoadHXXHS15HS20HS25Inventory Rating21 tons27 tons36 tons45 tonsOperating Rating36 tons45 tons60 tons75 tonsTable 1.5-3 Engineering Judgment Ratings for Concrete Bridges with Unknown Details11
If the superstructure (NBI 59) or culvert (NBI 62) rating, as of the most recent inspection, is less than 5;then multiply the inventory and operating ratings by the rating reduction factor as indicated in Table1.5-2.Engineering Judgement Rating Reduction ValuesGeneral Superstructure or CulvertCondition Rating (Item 59 or 62) 543 2Rating Reduction Factor1.00.70.4Recommend closure and immediaterepair or replacementTable 1.5-4 Condition Rating Reduction values for Engineering Judgement Ratings for Concrete Bridges with Unknown DetailsAlternatively, the design vehicle can be used to back-calculate (reverse engineer) the reinforced and/orthe prestressed steel area. Contact the Load Rating Engineer prior to implementing this process.1. Obtain detailed field measurements, including span length, girder dimension, girder spacing,diaphragm dimensions and locations, deck width, deck overhang, and deck thickness.2. Determine the material properties and design vehicle based on year
ASR Allowable Stress Rating LFR Load Factor Rating LRFR Load and Resistance Factor Rating Inventory Rating (LFR/ASR) Load ratings based on the inventory level allow comparisons with the capacity for new structures and therefore, results in a live load, which can safely utiliz
