108 - Bridge Load Rating

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108.1 Introduction

This section provides guidance in the development of bridge load ratings. Additional information on bridge inspection procedures and load ratings can be found in the DelDOT Bridge Inspection Manual (2017).

108.2 Application

The National Bridge Inspection Standards (NBIS) requires each state transportation department to inspect, prepare reports, and determine load ratings for structures defined as bridges on all public roads. The NBIS is contained in Title 23 of the Code of Federal Regulations (CFR) Part 650, Subpart C.

The Federal definition of a bridge per the NBIS is:

a structure including supports erected over a depression or an obstruction, such as water, highway, or railway, and having a track or passageway for carrying traffic or other moving loads, and having an opening measured along the center of the roadway of more than 20 feet between undercopings of abutments or spring lines of arches, or extreme ends of openings for multiple boxes; it may also include multiple pipes, where the clear distance between openings is less than half of the smaller contiguous opening.

Only bridges that meet the Federal definition are included in the NBI.

In Delaware, Department policy defines all state-owned structures having an opening of greater than 20 square feet, with a minimum vertical clearance of 48 inches, as a bridge. In addition, any structure over a state-maintained roadway shall be considered a bridge, regardless of owner or the mode of transportation being carried by the bridge. These bridges are included in the State Bridge Inventory.

Department policy is to perform load ratings on all vehicular structures meeting either the Federal or state definition of a bridge. This Manual covers the general requirements and procedures for bridge load ratings. For structural types, materials, and analysis methods not dealt with in this Manual, contact the Bridge Management Section.

108.3 Terms

Bridge Management Engineer – The Bridge Management Engineer oversees the Quality Assurance (QA) activities, assists with the Quality Control (QC) process, and is responsible for load-posting bridges.

Load Rating Engineer – The Load Rating Engineer is the individual charged with the overall responsibility for load rating bridges in Delaware for compliance with the NBIS. This includes leading the QC process, assigning load ratings, providing training as needed, and developing load rating procedures and protocol.

Load Rater – The Load Rater is the individual, meeting the qualifications described herein, assigned to perform the load rating of a specific bridge.

108.4 Load Rating Specifications

In general, the Department adopts the load rating procedures in the current AASHTO Manual for Bridge Evaluation. Load Raters should refer to that publication for any items not specifically covered by this Manual. Where there are differences between this Manual and the AASHTO LRFD, this Manual governs.

108.5 Load Rating Process

The load rating review and acceptance process shall be in accordance with Figure 108‑1. In general, the load rating will be prepared by the Bridge Management Section. However, in the case of consultant-designed curved girder bridges, cable-stayed bridges, other complex bridges as designated by the Bridge Management Engineer, and/or projects containing multiple bridges requiring load ratings, the consultant will be required to prepare the load rating, and submit the rating to the Bridge Management Engineer for review.

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Upon the successful completion of the load ratings, the Load Rating Engineer will provide a Load Rating Summary Table for inclusion in the General Plan (see Figure 108‑2).


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In the event that any structural changes occur between the Semi-Final Construction Plan and Final Construction Plan stages, the designer must notify the Load Rating Engineer of these modifications so the bridge load rating can be re-evaluated. Similarly, any structural modification made during construction shall be communicated to the Load Rating Engineer, along with corresponding approved shop or as-built drawings, so the bridge load rating can be re-evaluated.

108.6 Responsibility

Most bridges in Delaware are owned and operated by DelDOT. In Delaware, bridges that are not owned and operated by DelDOT are owned by cities, railroad companies, Delaware River and Bay Authority (DRBA), the DNREC, the USACE, or other private owners.

Inspection and rating of these bridges meeting Federal requirements are the responsibility of the owner, and must be conducted in accordance with the NBIS. Department policy is to perform load ratings for municipally and all state-owned bridges meeting Federal requirements. All other owners are required to load-rate vehicular structures that meet the Federal definition of a bridge.

Title 17, Chapter 5, Section 510 of the Delaware Code allows the Department to conduct investigations of the load-carrying capacity of certain bridges, regardless of ownership or jurisdiction.

Owners are responsible for sending their bridge inspection and load rating results to the Bridge Management Section for inclusion in the NBI.

108.7 Quality Control Procedures

Maintaining a high degree of accuracy and consistency in the load rating program is important. The QC procedures described herein are intended to provide this level of quality.

108.7.1 Quality Control Roles and Responsibilities

All QC activities fall under the responsibility of the Load Rating Engineer. All personnel involved in the load-rating process shall meet the requirements of this Manual. All load-rating calculations shall be accompanied by the qualifications of the Load Rater for consideration and acceptance by the Load Rating Engineer.

108.7.2 Load Rating Engineer Qualifications

The Load Rating Engineer shall be a Delaware Licensed Professional Engineer. In addition, the Load Rating Engineer shall have experience in structural analysis and load-rating procedures of all common bridge structure types.

108.7.3 Load Rater Qualifications

All Load Raters shall possess a Bachelor’s Degree or higher in Civil Engineering. Load Raters shall also have knowledge of structural analysis methods and/or structural design. Load Rater qualifications are checked during the Load Rating Engineer’s review and validation of the load-rating documents.

108.7.4 Load Rating Reviewer Qualifications

The Load Rating Reviewer shall be a Delaware Licensed Professional Engineer. In addition, the Load Rating Reviewer shall have experience in structural analysis and load rating procedures of all common bridge structure types. Load Rating Reviewer Qualifications are checked during the Load Rating Engineer’s review and validation of the load-rating documents.

108.7.5 Review and Validation of Load Rating Reports

The Load Rater shall perform a thorough review of the data and results of each load rating prior to submission to the Load Rating Reviewer, who shall meet the qualifications of this Manual.

Any load rating that results in a recommended bridge load restriction posting shall be reviewed and approved by the Bridge Management Engineer. Refer to Section 108.10.8 – Load Restriction Posting of this Manual.

108.7.6 Resolution of Data, Errors and Changes

The Load Rating Reviewer shall communicate to the Load Rater any errors in the data or issues with the methodology or results. The goal is to ensure that the methodology is appropriate for the structure being rated, and the data are accurate. Any discrepancies that cannot be resolved between the Load Rater and Load Rating Reviewer shall be brought to the attention of the Load Rating Engineer or Bridge Management Engineer for resolution.

108.8 Quality Assurance Procedures

QA procedures consist of reviewing a sample of load-rating reports annually to verify the quality level of the load-rating program. The results of the load rating reviews will be summarized in an annual report.

108.8.1 Quality Assurance Roles and Responsibilities

All QA activities fall under the responsibility of the Bridge Management Engineer. The load-rating reviews will be performed with the assistance of the Load Rating Engineer.

108.8.2 Load Rating Quality Assurance Review Procedures

QA reviews shall be completed by the end of each calendar year. QA reviews shall consist of sampling, reviewing, validating, and reporting on the load ratings that have been performed in the current year.

108.8.2.1 Sampling Parameters

A representative sample of a minimum of 5 percent of the bridges that have been load rated in the current year shall be selected as part of the QA review. Consideration should be given in the selection process to load-posted bridges, deficient bridges, and bridges with unusual changes in the load ratings.

108.8.2.2 Acceptance Criteria

The following criteria shall be used in determining acceptability of a load-rating report:

  1. Is the load-rating report complete?
  2. Does the report format conform to this Manual?
  3. Do the Load Rater and Load Rating Reviewer meet the qualifications of this Manual?
  4. Is the bridge modeled correctly?
  5. Are the input data correct?
  6. Has the bridge record been updated correctly?
  7. Has the load posting restriction/removal been documented, distributed, and signage properly installed?
108.8.2.3 Annual Report

An annual report shall be prepared that contains the results of the Load Rating Quality Assurance process. The report shall include any recommendations for improvement of the load-rating process.

108.9 Load Rating Requirements

Load ratings are required to be completed before a new bridge is entered into the bridge inventory and opened to traffic, or when a bridge is rehabilitated. Load ratings are also required when a bridge inspection reveals deterioration and/or damage that is sufficient to warrant a structural analysis to ascertain the impact to the strength and/or serviceability to an element of the bridge, or the entire bridge. At a minimum, load ratings shall be reviewed and updated at least every 10 years over the life of the bridge.

108.9.1 New Bridges

Department policy is to rate new bridges for current AASHTO and Delaware live load requirements. Department policy is to design all new bridges on state-maintained roads, so that load-rating factors are 1.0 or greater for the AASHTO HL-93 load, as well as all Delaware legal and permit loads. In addition, bridges must be designed to fulfill AASHTO design load requirements for LRFD. If a new bridge does not rate for the AASHTO HL-93 load, the Delaware legal and/or permit loads, the Project Manager shall submit a Design Exception. Design Exceptions are prepared according to Section 102.5.4 – Design Exceptions and Design Variances. A bridge that does not rate for the Delaware legal loads requires a load posting. This should be considered in the evaluation of the Design Exception.

108.9.2 Rehabilitated Bridges

Department policy is to rate bridges that are having structural rehabilitation work activities performed for current AASHTO LRFD design loads, as well as Delaware legal and permit load requirements. The designer shall consider addressing all existing structural deficiencies that will increase the load ratings to AASHTO LRFD design load requirements, strengthen the bridge, remove any load-posting restriction, or bring the bridge back to its original design capacity. On some rehabilitation projects, such as historical or temporary bridges, however, the scope of work may be limited, making it impractical to bring the bridge up to current AASHTO LRFD design-load requirements. and/or cause removal of the load-posting restriction. Similarly, on some bridge rehabilitation projects, it may not be feasible or cost effective to upgrade the capacity of the bridge to meet the AASHTO LRFD design-load requirements, or to result in removal of the load-posting restriction. In either of these situations, the bridge shall be designed to reinstate the bridge back to its original design capacity while trying to achieve rating factors as close to 1.0 as possible for all Delaware legal load vehicles, Specialized Hauling Vehicles (SHVs), and Emergency legal load vehicles. If a rehabilitated bridge does not rate (i.e., rating factor < 1.0) for the AASHTO HL-93 load, the Delaware, SHV, or Emergency legal load vehicle(s), the Project Manager shall submit a Design Exception. Design Exceptions are prepared according to Section 102.5.4 – Design Exceptions and Design Variances. A rehabilitated bridge that does not rate for the Delaware legal loads will require a load posting. This should be considered in the evaluation of the Design Exception.

All inspection reports are filed and available for review by contacting the Bridge Management Section. A special inspection can be scheduled with the Bridge Management Section if the structural deficiencies are not documented in sufficient detail in previous reports.

108.9.3 During Construction

The Standard Specifications require the contractor to submit the proposed loadings (axle spacing, axial loads, stockpiling, and equipment locations), including quantity and type of construction equipment and vehicles it proposes to use, to the Engineer for approval. All primary members, including connections, are to be analyzed for anticipated construction loads. All stresses for existing and proposed members shall be within allowable ranges for strength, service, and fatigue, as directed by the AASHTO LRFD.

108.9.4 Bridge Inspections

By law, all bridges on the NBI are required to be inspected at least every 2 years. All bridges that do not meet the Federal definition of a bridge, but do meet the state definition of a bridge, are required to be inspected at least every 4 years. Inspection frequencies are determined by the structure type, condition, and load-posting restrictions. Bridges in poor structural condition require more frequent inspections. Refer to DelDOT’s Bridge Inspection Manual for inspection frequency requirements. Inspection of bridges is done in conformance with the MBE, FHWA’s Specification for the National Bridge Inventory (2014), DelDOT’s Bridge Inspection Manual (2017), and the DelDOT Bridge Element Inspection Manual (2017). Some structures require more detailed and different types of inspections to determine their actual condition.

Bridges are not typically load rated as a part of their routine inspections. However, load ratings of bridges during inspections are usually prompted by discovery of significant loss of section, continuing deterioration, and suspected loss of capacity. Actual measurements taken by the inspection team that differ from that of the plans shall be used to update the load rating. Areas of deterioration are given special attention during field inspection, because a primary member that is reduced in section may control the capacity of the structure. If deterioration is identified during the field inspection, the inspection team shall produce detailed sketches documenting the deficiencies found, so that the load rating can be re-evaluated to determine if the load-carrying capacity has been compromised.

The Bridge Management Section maintains a file for each bridge, which includes bridge inventory and condition data, sketches, load-rating summaries, maintenance records, and Contract Plans. This information indicates the current condition of the bridge, which can then be used in load-rating calculations of the structural elements.

108.9.5 Load Rating Timeline

All load ratings shall be performed within the timeframes defined below.

108.9.5.1 New Bridge and Rehabilitated Bridges

New bridges and rehabilitated bridges are first rated after distribution of the Semi-Final Construction Plans. If a consultant has been assigned the load rating, then they shall submit the load rating to the Bridge Management Engineer at the Semi-Final Construction Plan Submission for review and approval. Any issues arising from this rating shall be conveyed to the designer, and addressed prior to the distribution of Final Construction Plans.

Final load ratings shall be submitted to the Bridge Management Engineer prior to the PS&E submission for the project. A Load Rating Summary Table, provided by the Load Rating Engineer and indicating the successful completion of the load rating, shall be included in the General Plan in accordance with Section 108.10.6 – Load Rating Report.

Any modification to the bridge, either through shop drawings or field changes, shall be incorporated into the load ratings. Modified load ratings shall be submitted to the Bridge Management Section prior to the completion of the project.

108.9.5.2 Load Ratings Based on Bridge Inspections

Load ratings that are performed as a result of bridge inspection findings shall be completed and finalized within 28 calendar days of the inspection date. Refer to the DelDOT Bridge Inspection Manual for the complete timeline for inspection related activities.

108.9.5.3 Periodic Load Rating Review

Load ratings shall be reviewed and updated at least every 10 years over the life of the bridge.

108.9.6 Rated Members

Department policy is to rate only the primary load-carrying superstructure members in a bridge. This is normally the slabs of slab bridges, girders, trusses, floor beams, stringers, spandrel columns, or arch ring. Concrete box culverts and frames are also rated. The Load Rater must apply engineering judgment to evaluate other elements of a structure, which should be considered primary elements, and included in the load-rating calculation (i.e., cross frames of a curved girder or skewed structure).

Gusseted and/or pinned connections of non-load-path redundant steel-truss bridges shall be evaluated during the bridge load-rating analysis. The evaluation of gusset connections shall include the evaluation of the connecting plates and fasteners.

Not typically included in the load rating are the deck slab, piers, abutments, and foundations. The condition of these elements shall be considered, and they shall be assumed to safely carry the loads transmitted to them, unless there is evidence of serious deterioration. Main elements and components of the substructure (such as fracture-critical steel pier caps, cross beams, or hammerhead piers) whose failure is expected to cause collapse of the bridge shall be identified for special emphasis during inspection. Refer to the MBE for guidance in load-rating substructure elements.

108.10 Load Rating Procedures

This section addresses the standard load-rating procedures adopted by the Department. These procedures include analytical steps, assumptions, methods, tools, loads, factors, and documentation.

108.10.1 Analytical Steps in Load Rating

Analytical steps in load rating are detailed procedures that a Load Rater goes through in performing a load-rating analysis.

The analytical steps required to rate any member is independent of the role played by the member in the overall structure. The analytical steps may vary depending on the choice of method. The following analytical steps are required:

  1. Determine section properties
  2. Determine material properties (e.g., yield strength, compressive strength)
  3. Calculate section capacities
  4. Calculate dead-load effects
  5. Calculate live-load distribution
  6. Calculate live-load effect
  7. Calculate rating factors.

The loads and factors used to analyze critical members and determine the appropriate ratings are outlined in the MBE and the AASHTO LRFD.

108.10.2 Information Gathering

Prior to rating an existing bridge, the engineer must gather all available data pertinent to the structure. This step will aid in the development of member section properties, allowable and/or yield stress, and dead-load effects. For a bridge rehabilitation project, this may require the designer to perform a special inspection of the structure or, at minimum, review results of a recent detailed inspection.

When conditions warrant, reduced sections should be used to obtain a load rating that best reflects the known condition and capacity of the structure. Areas of deterioration must be given special attention during field inspection, because a primary member that is reduced in section may control the capacity of the structure.

The Load Rater also needs a complete description of the bridge, as-built plans, any modifications since it was built, and its present condition. In lieu of plans, a detailed set of measurements and/or sketches from actual field measurements will be needed. The Bridge Management Section maintains a file of past inspection results for each bridge, along with maintenance records, contract plans, and other relevant information. This information indicates the current condition of the bridge, which can then be used in load-rating calculations of the structural elements.

108.10.2.1 Material Properties

Load Raters must make assumptions to efficiently analyze existing bridges. This is due to the wide variety of structural materials available (e.g., steel, concrete, wrought iron, timber, masonry, a combination thereof), assortment of structural types, and variations in quality and strength of the materials. Information concerning material properties may be obtained from contract plans, material testing, or guidelines found in the MBE. For new structures, standard design criteria presented in this Manual shall be used.

For a bridge rehabilitation project, information can be obtained from historical documentation such as as-built contract drawings or historical material properties prescribed in the MBE. In some cases, material testing, as outlined in Section 109.2 – Material Testing, can be performed to determine appropriate material properties.

108.10.3 Load Rating Methods

There are various load-rating methods that are based on the design methods presented in different AASHTO bridge publications. The Department’s standard method for all load ratings is the Load and Resistance Factor Rating (LRFR) Method. Diagnostic Load Testing may be used in conjunction with the LRFR method. In certain situations, other methods of load rating may be considered. These include Load Factor and Allowable Stress Methods. These methods shall only be used with the approval of the Load Rating Engineer.

108.10.3.1 Load and Resistance Factor Rating Method

The LRFR Method provides a methodology for load rating a bridge consistent with the LRFD philosophy of the AASHTO LRFD. This method uses load and resistance factors that have been calibrated based on structural reliability theory to achieve a minimum target reliability for the strength limit state.

Guidance is provided on service limit states that are applicable to bridge load rating. This guidance is not based on reliability theory, but is based on past practice. The LRFR method is preferred because it recognizes a balance between safety and economics. The LRFR method is appropriate regardless of the original criteria and method used in the design of the bridge.

108.10.3.2 Diagnostic Load Testing

Diagnostic load testing may be used in special cases such as the following:

  1. When analytical results provide a rating factor less than 1, but the bridge is otherwise showing no visual signs of distress.
  2. When record construction plans for the bridge are not available or do not have sufficient detailed information.
  3. When calibrating load rating data for such factors as distribution, fixity, and composite action.

The Department typically performs diagnostic load testing by driving a truck of known axle weights over a bridge. Strains are then measured in the load-carrying members with strain gages and specially designed data analysis equipment. Stresses are computed using the measured strains and material properties. These axle weights and computed stresses are used to calibrate the structural analysis model. A more realistic rating of the bridge can then be obtained for all loads. Further guidance for load testing is given in the MBE.

108.10.3.3 Pipe Culverts

LRFR ratings for pipe culverts that are in fair-new condition may be based on the pipe manufacturer’s documentation of the design capacity, provided that the following documentation is available. Sufficient documentation shall be included to confirm that the pipe culvert has been designed for the HL-93 Design Load, and as-built conditions are within the manufacturer’s recommendations (i.e., cover requirements, backfill requirements). If the pipe culvert does not meet all of the above criteria, an LRFR rating shall be performed for the pipe culvert, based on the design requirements of the AASHTO LRFD. The loads and factors shall be computed in accordance with the AASHTO LRFD.

Steel corrugated pipe culverts shall be load-rated using the method described in Design Data Sheet 19 of the National Corrugated Steel Pipe Association (NCSPA). Because of the difficulty in measuring deterioration in corrugated metal pipe culverts, and the resulting strength loss, the Department has created a Corrugated Metal Pipe Inspection Policy. This policy determines when the load rating should be reduced, based on the level of deterioration found during the inspection. The policy can be found in DelDOT’s Bridge Inspection Manual.

108.10.3.4 Other Methods

Other methods of load rating include all design methods contained in previous AASHTO bridge design specifications, such as Allowable Stress Method or Load Factor Method. These methods shall not be used to perform load ratings, regardless of the method used to design the bridge, without the approval of the Load Rating Engineer.

108.10.4 Structural Analysis and Tools

Several computer programs are available for structural analysis of bridges. Wyoming Department of Transportation’s BRASS-GIRDER (LRFD)TM computer program, herein referred to as BRASS, is the standard program used by the Department to rate bridges. BRASS shall be used for steel and concrete girder, rigid-frame, and slab-type bridges. For other material or structure types that cannot be analyzed with BRASS, one of several finite-element programs may be used to perform the structural analysis. Bentley’s STAAD.Pro® is preferred by the Department. Other structural analysis programs may be used with the approval of the Load Rating Engineer. These analysis programs are used in conjunction with hand calculations and/or spreadsheet-based calculations to complete the load rating, in accordance with this Manual. The Load Rater shall clearly understand the basic assumptions of the program and the methodology that is implemented. Sufficient documentation shall be provided to allow verification of the results.

108.10.4.1 Structural Analysis Requirements

Structural analysis and load ratings are prepared for each typical load-carrying member of each structure unit. A structure unit consists of a simple span or a series of continuous spans. At a minimum, an interior and an exterior girder shall be rated for each structure unit. Duplicate load ratings are not required for identical structure units. Engineering judgment may be used to eliminate the need to rate similar structure units.

108.10.4.2 BRASS Data Set Standards

In general, the BRASS commands should follow the order of commands as presented in the BRASS Manual. It is helpful to include spaces in the data file to improve readability.

Comment commands must be included in the data set, and include any and all assumptions or deviations from standard practice made by the Engineer. This will assist reviewers in understanding how the BRASS data were obtained.

Administration commands shall be the first commands in the data set. These commands include the AGENCY, ENGINEER, BRIDGE-NAME, and TITLE commands. They shall contain standard information, including the load-rating agency, Load Rater, bridge number, bridge location information, bridge type, span number, beam designation, and contract numbers.

The Load Rater should pay attention to the default values for each command parameter.

If the default value is acceptable, the value may be omitted from the data file.

File names shall consist of:

For single span: C-Num_girder designation.DAT (.girder for GUI version of BRASS)
For multiple span: C-NUM_#_girder designation.dat (.girder for GUI version of BRASS)
where:
  • C = County Code (1, 2, or 3 for New Castle, Kent, and Sussex Counties, respectively)
  • NUM = Three-digit bridge number (add a fourth digit suffix to bridge number if required)
  • # = Span number(s)
  • Girder designation = description of girder rated

The girder designation may be “int” or “ext” for typical interior or exterior girders. Girder numbers may be used to designate a specific girder. The girder number shall match the construction plans.

Examples:      1-123_s1_int.dat (.girder for GUI version of BRASS)

1-123_s1_g3.dat (.girder for GUI version of BRASS)
1-123_ext.dat (.girder for GUI version of BRASS)
1-123A_s2-s4_int.dat (.girder for GUI version of BRASS)

Do not use spaces or special characters in the data file name.

108.10.5 General Load Rating Equation

The following general expression shall be used in determining the load rating of each component and connection for each force effect (i.e., axial force, flexure, or shear):

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For Strength Limit States:

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Where the following lower limit shall apply:

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For Service Limit States:

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Where:

RF = Rating factor

C = Capacity

fR = Allowable stress specified in the LRFD code

Rn = Nominal member resistance (as inspected)

DC = Dead load effect due to structural components and attachments

DW = Dead load effect due to wearing surface and utilities

LL = Live load effect

IM = Dynamic load allowance

ΥDC = LRFD load factor for structural components and attachments

ΥDW = LRFD load factor for wearing surface and utilities

ΥLL = Evaluation live load factor

φc = Condition factor

φs = System factor

φ = LRFD resistance factor  

The load rating shall be carried out at each applicable limit state and load effect, with the lowest value determining the controlling rating factor. Refer to the MBE for additional guidance concerning loads and load effects.

108.10.5.1 Limit States, Load Effects, and Load Factors

Strength is the primary limit state for load rating. Service limit states shall be checked for design, legal, and permit loads in accordance with the provisions of the MBE, including those listed as optional checks. It is not necessary to check the fatigue limit state for steel bridges, unless prompted by inspection findings. The Load Rating Engineer shall be responsible for determining the need to check the fatigue limit state.

Members shall be evaluated for axial force, flexure, and shear, as appropriate. Shear shall be evaluated for all concrete structures. When using the Modified Compression Field Theory (MCFT) for the evaluation of concrete shear resistance, the longitudinal reinforcement shall be checked for the increased tension caused by shear.

Load factors for design, legal, and permit loads are defined in the MBE. The MBE provides generalized live load factors for load rating that are appropriate for use with DelDOT, SHV, and Emergency standard Legal Loads. Permit load factors for Routine or Annual Permit Types shall be used for DelDOT’s standard Permit Loads.

108.10.5.2 Condition Factor

The condition factor provides a reduction to account for the increased uncertainty in the resistance of deteriorated members and the likely increased future deterioration of these members during the period between inspection cycles. The condition factors provided in the MBE shall be applied, based on the most recent superstructure or culvert condition rating. Improved inspections will reduce, but not totally eliminate, the increased resistance variability in deteriorated members. When section properties are obtained accurately, the condition factor may be increased by 0.05, not to exceed 1.0, in accordance with the MBE.

108.10.5.3 System Factor

System factors are multipliers applied to the nominal resistance to reflect the level of redundancy of the complete superstructure system. Bridges that are less redundant will have their factored member capacity reduced; and accordingly, will have lower ratings. Subsystems that have redundant members shall not be penalized if the overall system is nonredundant. Therefore, closely spaced parallel stringers would be redundant even in a two-girder bridge with closely spaced multiple floor beams and stringers. System factors shall only be applied when checking flexural and axial effects at the strength limit state. It is Department policy to apply the system factors given in the MBE, and not those found in the AASHTO LRFD.

108.10.5.4 Resistance Factors

Resistance factors shall be applied to the strength limit state as specified in the AASHTO LRFD, based on material and load effect.

108.10.5.5 Loads

Only two types of loads are normally considered when load-rating a bridge: dead load and live load. Horizontal and vertical earth loads are also considered when load-rating a slab, rigid frame, or culvert.

108.10.5.5.1 Dead Loads

Dead loads include the weight of anything that is permanently attached to the bridge superstructure. In LRFR, these loads are separated into two types: structural components and attachments (DC); and wearing surface and utilities (DW). All dead loads should be based on dimensions shown in the plans and verified with field measurements, as needed. The presence of utilities and other attachments shall be verified prior to performing a load rating. Allowance for a future wearing surface shall be included in the load rating of dead loads, if so noted on the bridge plans. Refer to Section 106.3.1.2 – Distribution of Dead Loads for guidance regarding distribution of concrete bridge railing.

108.10.5.5.2 Live Loads

As related to trucks, a bridge’s capacity depends not only on the gross weight, but also on the number and spacing of the axles and the distribution of load between the axles. Load ratings shall be performed for Design, Legal, and Permit vehicles.

The Design Loads are the HL-93 Design Load per the AASHTO LRFD, and the HS20-44 per the AASHTO Standard Specifications for Highway Bridges (2002). The design loads shall include all axle configurations, tandems, truck trains, and lane loads associated with the respective vehicle. Design Load Ratings shall be performed at the inventory level and operating level for all design loads, in accordance with the MBE.

Pedestrian live load on sidewalks shall not be considered in the load rating of the bridge.

Because it is not practical to rate a bridge for the numerous legal axle configurations, Delaware’s highway bridges are rated for six standard Legal Loads, which are representative of actual vehicles on the highways. DelDOT’s standard Legal Loads are the S220, S335, S437, T330, T435, and T540 (see Figure 108‑3 (1), (2), (3), (4)). In addition, the AASHTO MBE requires that bridges be load rated for the SHVs and the Emergency legal load vehicles. Critical load effects shall consider the Legal Loads, including truck, truck trains, and lane load combinations found in the MBE, as applicable. Legal Load Ratings shall be performed for all legal loads in accordance with the MBE. The Type 3 Unit, Type 3S2 Unit, and the Type 3-3 Unit AAHTO Legal Loads, as depicted in the MBE, do not need to be considered in the load rating analysis, because the Delaware Legal Loads are more restrictive. Refer to Figure 108‑3 (1), (2), (3), (4) below for the individual legal truck configuration and axle loading.

DelDOT’s Overweight/Oversize Permit Program allows application for operation of vehicles that exceed the legal load limitations. Bridge Management Section reviews permit applications for Superloads, which are vehicles with gross vehicle weight exceeding 120,000 pounds, or with any individual axle weight exceeding 25,000 pounds. A Policy Directive has been implemented that allows for Oversize/Overweight Blanket Permits (Annual Crane Permit). These permits allow unrestricted movement of cranes that exceed the legal load limitations, but are not reviewed as Superloads. Standard Permit Loads have been developed to check the safety and serviceability of bridges as part of the rating process. Because it is not practical to rate a bridge for the countless permit axle configurations, Delaware’s highway bridges are rated for four standard Permit Loads, which are representative of the allowable Blanket Permit vehicles. DelDOT’s standard Permit Loads are AC2, AC3, AC4, and AC5 (see Figure 108‑4). Critical-load effects shall consider the Delaware Standard Permit Loads and the lane-load combinations found in the MBE. Permit Load Ratings shall be performed for all permit loads in accordance with the MBE.


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Additional Legal and/or Permit vehicles may be evaluated if deemed necessary by the Load Rater, the Load Rating Engineer, or the Bridge Management Engineer. These additional evaluations may include, but are not limited to, the Delaware Fire Truck and Bus Vehicles (see Figure 108‑5 and Figure 108‑6).

If the load-rating analysis for a bridge results in a load rating factor <1.0 for one of the legal loads, and the bridge is required to have a load-posting restriction, then the fire truck and bus vehicles shall be checked to determine what affect the posting restriction has on those vehicles. If a particular fire department’s truck or local bus loads and/or axle spacings differ from the trucks identified in Figure 108‑5 and Figure 108‑6, then those axle loads and spacings may be used for analysis upon approval of the Load Rating Engineer.

108.10.5.5.3 Dynamic Load Allowance

The dynamic load allowance specified in the AASHTO LRFD shall be applied to the rating. The dynamic load allowance shall be modified for structures under fill or wood components, as specified in the AASHTO LRFD. The dynamic load allowance shall not be applied to lane loads.

The dynamic load allowance specified for design loads deliberately reflects conservative conditions that may prevail under certain distressed approach and bridge deck conditions with bumps, sags, or other major surface deviations and discontinuities. In longitudinal members having spans greater than 40 feet with less severe approach and deck surface conditions, the dynamic load allowance may be decreased for legal and permit ratings, in accordance with the MBE. The Load Rater shall only apply the reductions after a field review of the approach and deck surface conditions.

In some cases, dynamic load allowance may be modified, based on the results of load-testing performed in accordance with Section 108.10.3.2 – Diagnostic Load Testing.

108.10.5.5.4 Load Distribution

Live load distribution shall be as per the AASHTO LRFD. Bridges for which accurate live load distribution formulas are not readily available may be analyzed by refined methods of analysis, as per the AASHTO LRFD. In some cases, load distribution may be modified based on the results of load testing done in accordance with Section 108.10.3.2 – Diagnostic Load Testing, and the MBE.

108.10.6 Load Rating Report

When ratings are performed, a Load Rating Report shall be submitted to the Load Rating Engineer. The Load Rating Report shall include the following:

  1. Electronic copy of the Load Rating Summary Form, including material properties (documented, assumed, and/or measured), structural analysis and loading assumptions, file names, posting requirements, load rating comments, load rating date, and signatures of rater and reviewer (see Figure 108‑7).
  2. Electronic copy of the Rating Factor Summary Form, including identification of controlling output file and the rating factors for all design, legal, and permit loads (see Figure 108‑8).
  3. Electronic copy of the Posting Weight Summary Form, including the identification of the controlling output file and the Posting Weights for the legal loads (see Figure 108‑9).
  4. Electronic copies of data file(s).
  5. Electronic copies of output summary file(s) for all design, legal, and permit load ratings.
  6. Electronic copy of the Permit Analysis Form (see Figure 108‑10).
  7. Plans or sketches showing all properties and assumptions, as necessary.
  8. Documentation of structural model used in analysis, if other than BRASS, where appropriate.

Upon submittal of the Load Rating Report, the Bridge Management Engineer updates the load rating data in the AASHTOWare Bridge Management System, and stores the Load Rating Report in the hard and electronic bridge files.

When ratings are performed in conjunction with the preparation of design drawings, the Load Rating Engineer will provide a Load Rating Summary Table for inclusion in the General Plan (see Figure 108‑2).

108.10.7 Load Rating Examples

Illustrative LRFR load rating examples are given in the MBE. The Bridge Management Section maintains a collection of BRASS LRFD data files for completed load ratings of various structure types. The rater may contact the Bridge Management Section to obtain a sample data file. A BRASS-GIRDER (LRFD)TM data set is shown in Appendix 108-1.

108.10.8 Load Restriction Posting

Structural capacities and loadings are used to analyze the critical members to determine the appropriate load rating. This may lead to load restrictions of the bridge or identification of components that require rehabilitation or other modification to avoid posting of the bridge.

When a bridge is not able to safely carry the loads allowed by State statute, it is posted for its reduced capacity. It is the Department’s policy to restrict loads on bridges when the legal load rating factor drops below 1 for any of the Delaware Legal Loads, the AASHTO Single-Unit SHVs, and the Emergency legal load vehicles. The minimum posting is 3 tons. A bridge that is not capable of carrying a minimum gross live load of 3 tons shall be closed.

In addition to Department-owned bridges, Title 17, Chapter 5, Section 510 of the Delaware Code allows the Department to conduct investigations of the load-carrying capacity of certain bridges, regardless of ownership or jurisdiction. If DelDOT determines that a load-restriction posting is warranted for a bridge, the owner will be notified of the recommendation to restrict loads on the bridge. If the owner of the bridge fails to implement the recommended restriction, The Department will implement the load restriction in accordance with this Manual and the Delaware Code.

The safe posting load will typically be less than the load determined in the legal load rating. The following formula from the MBE will be used to determine the safe posting load for each vehicle type:

File:Bdm-2021-108-18.png

Where:

RF=Legal load rating factor

W=Weight of rating vehicle

Depending on the range of posting loads for a bridge, and the number of trucks being posted, the Bridge Management Engineer will implement a multi-vehicle posting or a gross vehicle weight posting. The multi-vehicle posting requires the posting signs to have a silhouette of each type of truck being posted, and the safe posting load in tons for each truck type. The gross vehicle weight posting will be signed for a single weight limit in tons, which is applied to all truck types. Refer to Figure 108‑11 (1), (2) for the required posting sign layout for Emergency Vehicles.

Corrugated metal pipe culverts may also require posting based on their inspected condition. The Department has created a Corrugated Metal Pipe Inspection Policy to determine when the safe posting load should be reduced, based on the level of deterioration found during the inspection. Corrugated metal pipe culverts are typically posted for 3 or 15 tons. The policy can be found in DelDOT’s Bridge Inspection Manual.

Other factors such as the character of traffic, the likelihood of overweight vehicles, and enforcement levels may lead to safe posting loads that are higher or lower than those determined above. The safe posting load is recommended by the Bridge Management Engineer, and approved by the Chief Engineer.

The Bridge Management Engineer implements load restrictions by preparing a “Load Restriction Resolution,” which is signed by the Chief Engineer. A sample Load Restriction Resolution is included in Appendix 108-2. The Bridge Management Engineer then distributes the signed resolution to DelDOT’s Chief Traffic Engineer, and copies the proper authorities, including DelDOT Traffic Section, State Police, school transportation directors, DelDOT Public Relations, DelDOT Maintenance, DelDOT Transportation Management Center, and Delaware Motor Transport Association. DelDOT Signs and Markings Section is also copied on the distribution of the Load Restriction Resolution to trigger placement of the load-posting signs. Upon completion of replacement or rehabilitation of a posted structure, the Bridge Management Engineer prepares a “Removal of Load Restriction Resolution,” signed by the Chief Engineer and distributed as above.

Upon distribution of a load-posting resolution, DelDOT Signs and Markings will install regulatory signing in accordance with the FHWA Manual on Uniform Traffic Control Devices (2009). Bridge Management Section will confirm that the proper signs have been installed within 30 days of the distribution of the load-posting documents. Any necessary correction to the signage will be communicated to DelDOT Signs and Markings for correction. Refer to the DelDOT Bridge Inspection Manual for the complete timeline for inspection related activities.

108.11 References

AASHTO, 2002. Standard Specifications for Highway Bridges, 17th Edition.

DelDOT, 2017. Bridge Element Inspection Manual, 2017 Edition.

AASHTO, 2013. Manual for Bridge Evaluation, 2nd Edition with Latest  Interim Revisions.

AASHTO, 2017. AASHTO LRFD Bridge Design Specifications, 8th Edition.

DelDOT, 2017. Bridge Inspection Manual.

DelDOT, 2020. Standard Specifications for Road and Bridge Construction, August.

FHWA, 2009. Manual on Uniform Traffic Control Devices, with Revisions 1 and 2 dated May 2012.

FHWA, 2014. Specification for the National Bridge Inventory Bridge Elements, January 21.


Appendix 108-1 →
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