Protocol to Evaluate and Load Rate Existing Bridges Using...

Maria M. Szerszen, Ph.D.Associate ProfessorDepartment of Civil EngineeringUniversity of Nebraska-Lincoln

“This report was funded in part through grant[s] from the Federal Highway Administration [and Federal Transit Administration], U.S. Department of Transportation. The views and opinions of the authors [or agency] expressed herein do not necessarily state or reflect those of the U.S. Department of Transportation.”

Nebraska Transportation Center262 Prem S. Paul Research Center at Whittier School2200 Vine StreetLincoln, NE 68583-0851(402) 472-1993

Daniel G. Linzell, Ph.D.Associate Dean for Graduate and International Programs ProfessorSaeed Eftekhar Azam, Ph.D.Postdoctoral Research Associate

Ali Al-HajamiGraduate StudentJoshua Steelman, Ph.D.Assistant ProfessorRichard L. Wood, Ph.D.Assistant Professor

Protocol to Evaluate and Load Rate Existing Bridges Using Field Testing

2019

Final Report26-1121-4032-001NTRC Project SC-11

Protocol to Evaluate and Load Rate Existing Bridges using Field Testing

Final Report December 2018

University of Nebraska–Lincoln Sponsored by

Nebraska Transportation Center Nebraska Department of Transportation

Project MO44

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Disclaimer Notice

The contents of this report reflect the views of the authors, who are responsible for the facts and the accuracy of the information presented herein. The opinions, findings and conclusions expressed in this publication are those of the authors and not necessarily those of the sponsors. This report does not constitute a standard, specification, or regulation.

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Acknowledgments

The authors would like to acknowledge the Members of the Technical Advisory Board and Nebraska Department of Transportation bridge engineers and technical staff for assistance

provided in association with all aspects of the project. The authors would also like to acknowledge the Lancaster County Engineering Office assistance with testing of Bridge J-143.

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In Memoriam

This report is dedicated to the memory of Dr. Maria Szerszen, Associate Professor of Civil Engineering at the University of Nebraska-Lincoln and original PI on the project. Dr. Szerszen

was a talented researcher, dedicated teacher and a valued friend, mentor and colleague to everyone who collaborated with her to complete this research effort.

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Technical Report Documentation Page

1. Report No

NTRC Project SC-11, 26-1121-4032-001

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle

Protocol to Evaluate and Load Rate Existing Bridges Using Field Testing

5. Report Date

December 2018

6. Performing Organization Code

7. Author/s

Maria Szerszen, Daniel Linzell, Saeed Eftekhar Azam, Ali Alhajami, Joshua Steelman, Richard L. Wood

8. Performing Organization Report No.

NDOT Report M044, 26-1121-4032-001

9. Performing Organization Name and Address

Nebraska Transportation Center

College of Engineering, UNL

362Q Whittier Research Center

2200 Vine Street

Lincoln, NE 68583

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

12. Sponsoring Organization Name and Address

Nebraska Department of Roads

Research Section

1400 Hwy 2

Lincoln, NE 68509

13. Type of Report and Period Covered

Technical Report

July 1, 2015 – December 21, 2018

14. Sponsoring Agency CodeNTC RiP No. 39676

15. Supplementary Notes

16. Abstract

UNL researchers developed a protocol that would help entities tasked with performing a load rating to determine when a field test would be beneficial in lieu of a traditional, analytical approach or engineering judgment. Decision trees and checklists facilitate selection of an analytical or field-testing approach to produce the ratings. Three trees are provided; one that helps the user choose between analytical or field testing ratings and trees that guide the user through the process of completing a load rating using analytical means or field testing. Information obtained from in-situ tests of select bridges help demonstrate the efficacy of using field data to produce more accurate and robust load ratings.

17. Key Words

bridge field tests – bridge rating

18. Distribution Statement

No restrictions

19. Security Classification (of this report)

Unclassified

20. Security Classification (of this page)

Unclassified

21. No. Of Pages

71

22. Price

N/A

Form DOT F 1700.7 (8-72) Reproduction of form and completed page is authorized

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PROTOCOL TO EVALUATE AND LOAD RATE EXISTING BRIDGES USING FIELD TESTING

Final Report

December 2018

Principal Investigators Maria M. Szerszen, Daniel G. Linzell

Co-Principal Investigators Saeed Eftekhar Azam, Joshua Steelman, Richard L. Wood

Post-Doctoral Scholar Saeed Eftekhar Azam

Research Assistant Ali Alhajami

Authors Maria Szerszen, Daniel G. Linzell, Saeed Eftekhar Azam, Ali Alhajami, Joshua Steelman,

Richard L. Wood

Sponsored by Nebraska Department of Transportation

(NTRC Project SC-11)

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TABLE OF CONTENTS

Contents 1 Introduction ...................................................................................................................................... 11

1.1 Governing Agencies and Documents ......................................................................................... 12

1.2 Definitions ................................................................................................................................. 15

2 Background ....................................................................................................................................... 16

2.1 NDOT Br465 – Load Rating Summary Sheet (see Figure 1) .................................................... 16

2.2 Load Rating ............................................................................................................................... 16

2.2.1 Rating Factor: LRFR versus LFR ......................................................................................... 18 2.2.1.1 Differences in Unfactored Dead Loads ...................................................................................... 18 2.2.1.2 Differences in Factored Live Loads ........................................................................................... 18

2.2.1.3 Atypical Bridge Characteristics .................................................................................................. 18

2.2.1.4 LRFR Criteria ............................................................................................................................. 18

2.2.1.5 Effect of Average Daily Truck Traffic on Ratings ..................................................................... 19

2.2.2 Analytical Load Rating ......................................................................................................... 19

2.2.3 Experimental Load Rating .................................................................................................... 19

2.3 Bridge Analysis .......................................................................................................................... 20

2.4 Field Testing .............................................................................................................................. 21

3 Objectives and Scope ....................................................................................................................... 21

3.1 Objectives .................................................................................................................................. 21

3.2 Scope.......................................................................................................................................... 21

4 Global Decision Tree (Figure 3) ...................................................................................................... 24

4.1 Changes to the Bridge (Cell 4.1) ............................................................................................... 25

4.2 No Changes to the Bridge, Current Posting? (Cell 4.2) ............................................................ 25

4.2.1 Not Posted (Cell 4.2.1) .......................................................................................................... 25

4.2.2 Posted .................................................................................................................................... 26

4.2.3 Is Existing Load Rating Based on Quantitative Methods? .................................................... 26

4.3 Is It Possible to Construct an Accurate Numerical Model? (Cell 4.3) ..................................... 26

4.3.1 Design/Rehabilitation Information ........................................................................................ 26

4.3.2 Field Inspection Information ................................................................................................. 26

4.3.3 Are Plans Available? ............................................................................................................. 27

4.4 Perform Analytical Load Rating (Cell 4.4) ............................................................................... 27

4.5 Adequacy of Analytical Results and Need for Supplemental Field Testing (Cell 4.5) .............. 27

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4.6 Perform Experimental Load Rating (Cell 4.6) .......................................................................... 27

5 Analytical Load Rating Decision Tree (Figure 4) .......................................................................... 27

5.1 Analytical Load Rating of New and Existing Bridges ............................................................... 28

5.2 Analytical Load Rating Process ................................................................................................ 28

5.2.1 Gathering Data through Inspection (Cell 5.2.1) .................................................................... 28

5.2.2 Constructing a Numerical Model and Selecting Rating Loads (Cell 5.2.2) .......................... 28

5.2.3 Choosing Load Rating Method (Cell 5.2.3) .......................................................................... 28

5.2.4 Performing Load Rating, Analysis of the Results, and Decision Making (Cell 5.2.4) ......... 29

5.2.5 Performing Experiments for Obtaining Material Properties (Cell 5.2.5) .............................. 29

6 Experimental Load Rating Decision Tree (Figure 5) .................................................................... 29

6.1 Benefits of Using Load Testing for Load Rating ....................................................................... 30

6.1.1 Unknown or Low-Rated Components ................................................................................... 30

6.1.2 Accurate Load Distribution Factor ........................................................................................ 30

6.1.3 Deteriorated or Damaged Members ...................................................................................... 30

6.1.4 Dynamic Load Allowance ..................................................................................................... 30

6.2 Bridge Inspection and Analytical Load Rating (Cell 6.2) ......................................................... 30

6.3 Proof Test or Diagnostic Test? (Cell 6.3) ................................................................................. 31

6.3.1 Perform a Proof Load Test (Cell 6.3.1) ................................................................................. 31 6.3.1.1 Dynamic Test or Static Test? (Cell 6.3.1.1) ............................................................................... 31

6.3.1.2 Calculate Final Load Rating Using the Maximum Safe Load of the Structure (Cell 6.3.1.2) .... 31

6.3.2 Perform a Diagnostic Load Test (Cell 6.3.2) ........................................................................ 31 6.3.2.1 Dynamic Test or Static Test? (Cell 6.3.2.1) ............................................................................... 32

6.3.2.2 Load Rating through Diagnostic Load Testing (Cell 6.3.2.2) .................................................... 32

6.4 Factors That May Improve Bridge Load Rating Using Nondestructive Load Tests ................. 33

6.4.1 Unintended Composite Action .............................................................................................. 33

6.4.2 Unintended Continuity or Fixity ........................................................................................... 33

6.4.3 Participation of Secondary Members .................................................................................... 33

6.4.4 Participation of Nonstructural Members ............................................................................... 33

6.4.5 Portion of Load Carried by Deck .......................................................................................... 34

7 Conclusions ....................................................................................................................................... 34

8 Appendix ........................................................................................................................................... 36

8.1 Load Testing Procedure ............................................................................................................ 36

8.1.1 Review of Available Information, Bridge Inspection (If Feasible), Analytical Load Rating (If Feasible), Test Load Selection (Cell 6.2) ........................................................................................... 36

8.1.1.1 Preliminary Analysis .................................................................................................................. 36

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8.1.1.2 Perform Analytical Load Rating ................................................................................................ 36

8.1.2 Diagnostic or Proof Load Test (Cell 6.3.2) ........................................................................... 37

8.1.3 Plan Diagnostic Load Testing (Cell 6.3.2.1) ......................................................................... 37 8.1.3.1 Plan Dynamic and Static Tests ................................................................................................... 37 8.1.3.2 Bridge Instrumentation and Response Measurement ................................................................. 37

8.1.4 Calculate Final Load Rating (Cell 6.3.2.2) ........................................................................... 37 8.1.4.1 Analysis of Test Results and Model Calibration ........................................................................ 37

8.1.4.2 Experimental Load Rating – Update Analytical Load Rating Using Test Data ......................... 37

8.2 Nebraska Bridge Tests and Experimental Ratings .................................................................... 38

8.3 Single-Span, Steel Bridge Located in Saunders County (ID #C007803635)............................. 38

8.3.1 Bridge Description ................................................................................................................ 38

8.3.2 Field Testing .......................................................................................................................... 39 8.3.2.1 Instrumentation Plan .................................................................................................................. 39

8.3.2.2 Bridge Loading ........................................................................................................................... 41

8.3.3 Load Test Results .................................................................................................................. 41

8.3.4 Model Calibration and Validation ......................................................................................... 42

8.3.1 Dynamic Amplification Factor.............................................................................................. 44

8.3.2 Experimental Load Rating Results ........................................................................................ 45

8.4 Single-Span, Timber Bridge Located in Platte County (ID #C007101805) .............................. 46



8.4.2.2 Bridge Loading ........................................................................................................................... 49





8.5 Single-Span, Steel Bridge Located in Lancaster County (ID # J-143) ...................................... 53



8.5.2.2 Bridge Loading ........................................................................................................................... 56




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8.6 Three Span, Hybrid Plate Girder and Truss Bridge Located in Thayer County (ID # S00500446)62



8.6.2.2 Bridge Loading ........................................................................................................................... 65





References .................................................................................................................................................. 70

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1 Introduction According to the National Bridge Inventory (NBI) in 2012, 10% of bridges and culverts in the US are posted and, subsequently, have reduced load carrying capacity. The following methods were the basis for these postings [1]:

1. analytical (93% of bridges and culverts);2. field evaluation and engineering judgment without analyses being performed (7%); and3. in-situ load testing (less than 1%).

Current and former faculty members at the University of Nebraska-Lincoln (UNL) have contributed to research focusing on completion of load rating procedures using field data, with the intent of facilitating broader application of field testing for load rating where appropriate. Puckett et al. developed a framework for simplified live load distribution-factor computations [2]. Linzell et al. conducted a numerical and experimental study of live load distribution and dynamic amplification on a curved, prestressed concrete bridge [3]. Nowak et al. developed diagnostic and proof load testing strategies for highway bridges [4, 5]. Given the above information and known successes associated with load rating bridges using field testing, the project summarized herein was completed by UNL researchers for the Nebraska Department of Transportation (NDOT) with the intent of developing decision-making tools to assist owners with choosing between analytical or experimental load ratings. The project also included examples that demonstrated the potential benefits of using field testing to load rate existing bridges. Developed procedures would facilitate safe and cost-effective management of a wide range of bridges, especially those that are generally observed to be in good condition but are restricted for live load for various reasons, such as: the bridge type and original design procedures and loads; a lack of design and/or rehabilitation plans; the chosen load rating analysis techniques; utilized visual inspection techniques; or unusual details and structural configurations [6]. UNL researchers developed a protocol that would help entities tasked with performing a load rating to determine when a field test would be beneficial in lieu of a traditional, analytical approach or engineering judgment. The resulting tool facilitates selecting load rating via analysis or field-testing as a function of:

1. Available information (e.g. plans, design documentation, inspection reports, load ratings,etc.);

2. Accessibility;3. Site conditions;4. Bridge geometry (e.g., length, width, span number, under- and over-clearance, type of

crossing, etc.); and5. Bridge super- and substructure type (concrete, steel, timber).

The tool utilizes decision trees and checklists to facilitate selection of an analytical or field-testing approach to produce the ratings. Three trees are provided; one that helps the user choose between analytical or field testing ratings and trees that guide the user through the process of completing a load rating using analytical means or field testing. The first tree is used to:

1. Present the overarching process to select analysis or field testing to produce the ratings;2. Determine if an analytical approach is preferred as a function of items listed above (bridge

type and geometry, etc.); and

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3. Determine if a field testing approach is preferred as a function of items listed above(available information, accessibility and site conditions, bridge geometry and type).

The other trees provide information that helps the reader: 1. Understand the benefits and shortcomings associated with either approach for various

bridge super- and substructure combinations;2. Determine if adequate information is available to select an approach;3. Understand what information is needed to complete a load rating based on an analysis or

field testing for each super- and substructure combination;4. Adequately prepare for completion of the load rating; and5. Understand the steps needed to complete the load rating and, if necessary, where to find

resources that facilitate completing each step in the analytical or field testing process.Information used to develop the overarching and analysis decision trees and checklists was largely obtained from existing literature. Information used to develop field testing decision trees and checklists leveraged existing literature along with information collected from field testing of select bridges in Nebraska. In the sections that follow, brief background related to bridge load rating is provided along with an abbreviated literature search that focuses on important and pertinent information sources. Methodologies used to develop and use the decision trees are then summarized, with field testing sections utilizing information obtained from in-situ tests of select different bridges completed by UNL to demonstrate the efficacy of using field data to produce more accurate and robust load ratings. A brief conclusions section, one that summarizes the development process and resulting tools, is then presented.

1.1 Governing Agencies and Documents As a result of the collapse of Silver Bridge over the Ohio River in Point Pleasant, VA, the U.S. Congress passed legislation FHWA - FAPG 23 CFR 650C, National Bridge Inspection Standards. This act established the national standard for the appropriate safety inspection and assessment of highway bridges constructed on all public roads [7]. While FHWA has the responsibility for overseeing the implementation of the NBIS for all states in the United States, the following responsibilities are delegated to State Departments of Transportation [7]:

• Bridge Inspection Organization (23 CFR § 650.307)• Qualifications of Personnel (23 CFR § 650.309)• Inspection Frequency (23 CFR § 650.311)• Inspection Procedures (23 CFR § 650.313) – includes inspection, load rating, scour data,

recordkeeping and necessary follow-up)• Inventory (23 CFR § 650.315)

NDOT established the Bridge Inspection Program Manual (BIPM) [7] to meet NBIS requirements. Items presented in this report are based on the NBIS and the BIPM and supplemental information and documents produced by the following federal agencies:

• the American Association of State Highway and Transportation Officials (AASHTO);• the U.S. Department of Transportation, Federal Highway Administration (FHWA); and• the National Cooperative Highway Research Program (NCHRP).

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In addition to referring to the BIPM, the AASHTO Manual for Bridge Evaluation (MBE) [8] was also used extensively for developing the decision-making trees in this report. Persons involved with load rating bridges in Nebraska must have a working knowledge of the BIPM. It is especially important that those individuals understand forms, checklists and other essential documents provided in the BIPM Appendix, which can be found online at https://dot.nebraska.gov/business-center/bridge/inspection/ and at https://dot.nebraska.gov/business-center/bridge/forms/. Of particular interest is Br465, the Load Rating Summary Sheet (LRSS), which provides a systematic fashion for summarizing and reporting load rating information to NDOT and is reproduced in Figure 1.

https://dot.nebraska.gov/business-center/bridge/inspection/https://dot.nebraska.gov/business-center/bridge/forms/

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Figure 1: Load Rating Summary Sheet (LRSS).

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In addition to above-referenced documents, the following references were also used to assist with the development of this report:

• the NCHRP Research Results Digest 234, “Manual for Bridge Rating Through LoadTesting;” [9] and

• the Iowa DOT Bridge Rating Manual [10].The user is also encouraged to refer to Experimental Load Rating case studies performed by Institute for Transportation at the Iowa State University:

• Demonstration of Load Rating Capabilities through Physical Load Testing: JohnsonCounty Bridge Case Study [11];

• Demonstration of Load Rating Capabilities through Physical Load Testing: Ida CountyBridge Case Study [12]; and

• Demonstration of Load Rating Capabilities through Physical Load Testing: Sioux CountyBridge Case Study [13].

A complete list of references can be found in the Reference section.

1.2 Definitions Frequently used terms in this document include:

• Load Rating – Procedure of estimating live load capacity of a bridge or a culvert based onexamination of its existing condition.

• Inventory Level – Corresponds to the rating at the design level of reliability for new bridgesin the AASHTO Specifications.

• Inventory Rating – Load ratings based on the Inventory Level, which determines a live loadthat can safely utilize an existing structure for an indefinite period of time.

• Operating Level – Maximum load level that could be applied to a structure; generally,corresponds to the rating at the Operating Level of reliability in past load rating practice.

• Operating Rating – Load rating based on the Operating Level, which defines the maximumlive load that could traverse the structure. If unlimited numbers of vehicles traverse a bridgeat its Operating Level bridge life could be adversely affected.

• Condition Ratings – Used to describe the current condition of a bridge as compared to itsas-built condition. The rating encompasses materials used to construct the bridge andphysical condition of its deck, superstructure and substructure components. Conditionevaluation of channels and channel protection devices are also included. Culverts are alsoincorporated. Condition ratings are documented using codes that provide an overallcharacterization of the general condition of the item (e.g., deck, superstructure) being rated.See BIPM Chapter 3 for details of condition codes and their significance.

• Bridge – a structure, including supports, erected over a depression or an obstruction suchas water, a highway, or a railway; having an opening measured along the centerline of theroadway of more than 20 feet between under copings of abutments, arch spring lines orextreme ends of openings for multiple box sections. Bridges feature a track or passagewayfor carrying traffic or other moving loads.

• Bridge Posting – load restriction when Inventory Rating Factor of a bridge is less than one,appropriate signs must indicate load limits for that bridge.

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2 Background The previous section provided a short synopsis of documents available for completing bridge load ratings analytically or via field testing. Before presenting and discussing developed decision trees, additional background on the load rating process using analytical means or via field testing is provided. This section also briefly summarizes relevant research focused on developing and improving tools and resources that could benefit readers of this report.

2.1 NDOT Br465 – Load Rating Summary Sheet (see Figure 1) According to NBIP, all Nebraska bridges in the Bridge Inventory that are reported to FHWA must have a LRSS. In the LRSS rating results for AASHTO rating loads and Nebraska legal trucks are summarized. In addition, the LRSS recaps key information related to the load rating and the life of the bridge and the Load Rating Engineer provides general comments procedures that were used to complete the rating. Instructions for completing the LRSS are provided in Section 5.11.2 of the NBIP. It is the responsibility of the Bridge Owner to retain the original sealed, signed and dated LRSS by the Load Rating Engineer in the Bridge Owner’s Bridge Record. The LRSS is comprised of the following sections:

• Heading Information Section: includes Structure ID, location, Load Rating Engineeridentity, etc.;

• Structure Identification Section: includes information regarding the structure such as themain construction material, the owner, the maintainer, the National Highway SystemIndicator, and the structure name, if applicable;

• Description Section: provides a brief description of the structure type and basic geometriclayout, span lengths, continuous or simple spans, deck material and thickness, overlaymaterial and thickness, and skew;

• Ratings and Loads Section: includes the condition ratings for bridge deck, superstructure,and substructure, design load for operating or inventory type, information regardingoverlay and wearing surface, and a rating table;

• Documentation Section: includes the source of rating such as analytical or experimentalload rating, BrR computations and software version; and

• Additional Comments Section: provides a comprehensive summary of the bridgecondition, and conveys crucial information needed by Highway Superintendents, theirmaintenance staff, and engineers who might be rerating the bridge in the future.

For more detail related to each section of LRSS see Section 5.11.2 of NBIP.

2.2 Load Rating As defined by the BIPM and the MBE, load rating involves the determination of the safe load carrying capacity of a bridge in terms of a rating factor when the bridge is subjected to a specific vehicle, typically truck load models. The resulting information is used to make decisions regarding the posting of bridges and, ultimately, bridge maintenance and management. When a rating is determined, results are typically displayed as a ratio of the capacity of the most critical member to the effect on that member caused by the rating load. As bridge design philosophies have changed over time so have rating design philosophies, with both Allowable

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Stress Design (ASD), and Load Factor Rating (LFR) approaches being used. LFR ratings are determined using the following equation:

𝑅𝑅𝑅𝑅 = 𝐶𝐶 − 𝛾𝛾𝐷𝐷𝐷𝐷 𝐷𝐷𝐷𝐷𝛾𝛾𝐷𝐷𝐷𝐷 𝐷𝐷𝐷𝐷 (1 + 𝐼𝐼𝐼𝐼)

. 2.2-1

Where: 𝐶𝐶 = ∅ 𝑅𝑅𝑛𝑛 denotes reduced member resistance; 𝑅𝑅𝑛𝑛 denotes nominal member resistance; ∅ is resistance factor of the structural member; 𝐼𝐼𝐼𝐼 is the dynamic load allowance; 𝐷𝐷𝐷𝐷 denotes unfactored dead loads; 𝛾𝛾𝐷𝐷𝐷𝐷 stands for dead load factor; 𝐷𝐷𝐷𝐷 are unfactored live loads; and 𝛾𝛾𝐷𝐷𝐷𝐷 is the live load factor. The MBE provides additional information related to calculating and/or selecting factors and the dynamic load allowance. More recently, the MBE incorporated a Load and Resistance Factor Rating (LRFR) approach bridge rating. The general LRFR load rating equation is:

𝑅𝑅𝑅𝑅 = 𝐶𝐶− 𝛾𝛾𝐷𝐷𝐷𝐷 𝐷𝐷𝐶𝐶 − 𝛾𝛾𝐷𝐷𝐷𝐷 𝐷𝐷𝐷𝐷 ± 𝛾𝛾𝑃𝑃 𝑃𝑃𝛾𝛾𝐷𝐷𝐷𝐷 𝐷𝐷𝐷𝐷 (1 + 𝐼𝐼𝐼𝐼)

. 2.2-2

Where certain variables are as defined above and: 𝐷𝐷𝐶𝐶 and 𝛾𝛾𝐷𝐷𝐶𝐶 are the structural component dead load effects their corresponding LRFD load factors; 𝐷𝐷𝐷𝐷 denotes dead load effects due to the wearing surface and utilities and 𝛾𝛾𝐷𝐷𝐷𝐷 is the corresponding load factor; and 𝑃𝑃 stands for permanent loads other than dead loads with 𝛾𝛾𝑃𝑃 being the corresponding LRFD load factor. A key difference between LRFR and LFR is that, at the Strength Limit State, member capacity is 𝐶𝐶 = ∅𝑐𝑐∅𝑠𝑠∅ 𝑅𝑅𝑛𝑛, where ∅𝑐𝑐 and ∅𝑠𝑠 denote Condition and System Factors, respectively. Condition Factors are included in LRFR load rating processes to provide a means for reduction of member capacity based on increased uncertainty due to member deterioration and section loss, with values being provided in Table 6A.4.2.3-1 of the MBE. System Factors account for a bridge’s level of redundancy and values are provided in Table 6A.4.2.4-1 of the MBE. The following lower limit applies to the condition and system factors:

∅𝑐𝑐 ∅𝑠𝑠 ≥ 0.85. 2.2-3

and, at the Service Limit State, 𝐶𝐶 = 𝑅𝑅𝑛𝑛. Rating factors greater than one indicate that the bridge can safely resist considered loads. Depending on the level of complexity of the approach used to determine capacity, these values could be determined based on assumptions associated with material properties, geometric properties, connection and boundary conditions, load types and application, and overall structural response [7]. It is understood by the bridge engineering community that actual capacity is generally higher than what is calculated using routine simplified design methods and assumptions [8, 9]. When a bridge’s computed load capacity is below the imposed demand as determined using the load rating equations discussed above, it may be beneficial to take advantage of inherent redundancies that might been not accounted for using conventional analyses and/or computations. This may necessitate developing more complicated models or, in many cases, basing the rating off of field testing data.

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2.2.1 Rating Factor: LRFR versus LFR According to Chapter 3 of the BIPM (Item 64 – Operating Rating), of the three rating methods LFR is the most suitable to use as a national standard and, therefore, the FHWA has chosen it as the standard for computing Inventory and Operating Ratings reported to the NBI. However, highway agencies can elect to use either of the three methods to establish posting limits. Mlynarski et al. [14] conducted a comprehensive comparison of AASHTO bridge load rating methods. The comparison was based on extensive analysis of 1,500 bridges of varying material types and structural configurations. The study centered on determining their moment and shear rating factors using LRFR and LFR. For some bridges, substantial differences were observed between the rating factors calculated using LFR and LRFR. For most of the bridges a reduction in the rating factors was observed when LRFR was used. Mlynarski et al. [14] presented the primary sources for these differences in report Section 3.3.2 and those sources are briefly discussed in the following subsections.

2.2.1.1 Differences in Unfactored Dead Loads Despite the expectation that distributed dead loads would be the same for straight main supporting members (beams or girders) using either LRFR or LFR, it was observed that composite dead load distributions differed for a small percentage of simple span steel girders involved in the study. When this occurred, LRFR produced marginally higher dead loads.

2.2.1.2 Differences in Factored Live Loads Factored, distributed LRFR live loads were observed to be greater than factored LFR live loads for the same, straight girder for both moment and shear. Given that girder resistances and dead loads were generally similar, numerators in the LFR and LRFR rating factor equations were also similar. As a result, factored live loads were identified as likely causing most straight girders to have lower ratings for LRFR than LFR.

2.2.1.3 Atypical Bridge Characteristics Substantial changes were observed for loads calculated using the LFR and LRFR for bridges with atypical geometries. For instance, in the case of a skewed bridge load distribution factors are calculated differently using LFR verses LRFR. For LFR, load distribution factors are solely a function of girder spacing. When using LRFR, they are a function of span length, girder spacing, and relative deck/beam stiffness. Similarly, for bridges having sidewalks and bridges having small girder spacing and thick decks, load ratings varied significantly between the two methods.

2.2.1.4 LRFR Criteria LRFR requires checking vastly more criteria than LFR. In most cases when LRFR lower rating factors were calculated, it was perceived that they were largely controlled by a criterion not included or rarely applied using LFR. For instance, when rating concrete structures using LFR shear could be neglected. However, for LRFR shear must be considered for permit vehicles and for design and legal loads if signs of distress exist. For simple span steel girder bridges rated using LRFR, in almost one-third of the cases capacity was controlled by shear capacity at the bearing stiffeners. However, this item was neglected when completing LFR ratings. Additional new rating criteria examples when using LRFR method include: examining the serviceability limit state for

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prestressed concrete girders; including a shear friction rating for composite concrete girders; and considering flexure in prestressed I- and box-girders.

2.2.1.5 Effect of Average Daily Truck Traffic on Ratings Mlynarski et al. [14] calculated legal and routine permit ratings for Average Daily Truck Traffic (ADTT) categories using the LRFR method and compared them to LFR values. In all cases the rating factors decreased with increasing ADTT due to an increase in the corresponding live load factor.

2.2.2 Analytical Load Rating Traditionally, completing a load rating encompassed constructing a numerical model that conservatively idealizes actual loads, member stiffness and material properties and support conditions. The model that is selected can range in complexity from a basic, one-dimensional model analyzed by hand to a sophisticated, three-dimensional finite element model that can automatically position and move specified trucks across the “bridge” [15]. This type of approach is termed an “analytical load rating” in this report. See Section 5 for additional details.

2.2.3 Experimental Load Rating More recently, AASHTO permitted basing ratings off of controlled load testing [8], which traditionally involved updating an analytical load rating using test data. Load tests can be used to verify component and system performance under a known live load. They can also provide an alternative evaluation methodology. This type of approach is termed an “experimental load rating” in this report. The main notion behind completing an experimental load rating is reducing uncertainties in the load rating process by relying on field measured data. Experimental load ratings should be completed by well-qualified field testing groups using appropriate equipment, such as by UNL Department of Civil Engineering researchers using their Mobile Infrastructure Testing Laboratory (MISL) as shown in Figure 2. See Section 6 for additional experimental load rating details.

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Figure 2: UNL Department of Civil Engineering’s Mobile Infrastructure Testing Laboratory (M.I.S.L)

2.3 Bridge Analysis As discussed in the AASHTO G13.1 Guidelines for Steel Girder Bridge Analysis [16], analysis methods can be generally classified as being conducted by hand or with the aid of a computer. While hand calculations are sufficiently accurate for analysis of system response featuring simple geometry and loading and boundary conditions, computer programs can be used to complete more detailed structural analyses that could provide more accurate, and enhanced, load ratings for a wider variety of situations. When dealing with the analysis of an existing bridge, economic considerations associated with possible posting and closure may dictate more accurate assessment of its material and geometric properties and loading and boundary conditions. This may necessitate improving on, or replacing, models used during the design phase with models that are calibrated against information obtained from the bridge site, which could include material tests, nondestructive evaluations or controlled testing under known vehicle loads. Analytical bridge models are typically constructed using commercial software, such as AASHTOWare and SAP2000 [17]. In many cases, users of these software packages must be well versed in their operation and in techniques and assumptions used to produce results. UNL are well-versed in completing these analyses, with an example being analytically load rating a tied-arch bridge [18]. Most existing software dedicated to bridge rating support conventional bridge systems, such as straight slab, I-girder, box girder and truss bridges. Load rating of innovative bridge systems or unconventional bridge types typically requires higher fidelity, detailed numerical models and experienced personnel [19].

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2.4 Field Testing As stated in the BIPM and the MBE, field testing can be performed in lieu of or complementary to analytical load ratings. A number of studies have been completed that focus on the use of field testing to assess in-situ bridge behavior under a number of demands, including: live traffic; soil and hydrostatic loads; seismic actions; and simulated dead loads [3, 6, 8, 20-24]. A subset of these studies focused on developing accurate load ratings from field test data [3, 15, 22], with certain studies leading to the development of various guidelines that provide information to help bridge owners better understand how a bridge is field tested. However, limited guidelines are available to help owners determine when field testing is necessary in comparison to, or in conjunction with, load rating analyses. In this regard NCHRP Research Digest 234 Manual for Bridge Rating Through Load Testing was the first reference that discussed incorporating test data to update analytical load ratings. The publication includes comparisons between experimental and analytical load rating for representative bridges having simple geometries. Findings from this effort were the basis for developing Chapter 8 of the MBE, which currently is the main reference for the experimental load rating of bridges.

3 Objectives and Scope

3.1 Objectives The objectives of this project were to: (i) develop tools that assist bridge owners with necessary decision making for load rating common bridges in Nebraska using analytical or experimental testing means; and (ii) demonstrate the efficacy of load rating determined with field testing by comparing to ratings obtained analytically for representative, rural bridges.

3.2 Scope Given the objectives, the project scope included:

1. Development of decision trees and supporting text and references that help guide usersthrough the decision making process;

2. Demonstrating how bridges are rated using field testing via (i) completion of four tests ofrepresentative rural, Nebraska bridges and (ii) comparisons between analytical andexperimental load ratings. Bridges that were tested included:

• a single span, steel bridge located in Saunders County;• a single span, timber bridge located in Platte County;• a single span, hybrid steel stringer bridge with cast in place concrete box culvert

extensions located in Lancaster County; and• a three-span, truss bridge located in Thayer County.

Three decision trees were developed and are shown in Figure 3, Figure 4 and Figure 5. The first decision tree helps the user determine an appropriate load rating approach (analytical vs. experimental). The second assists with completing an analytical load rating and the third with completing an experimental rating.

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Figure 3: Global Decision Tree.

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Figure 4: Analytical Load Rating Decision Tree.

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Figure 5: Experimental Load Rating Decision Tree.

Section 4 of this report will further describe Figure 3, Section 5 will provide additional details on Figure 4, and Section 6 will discuss Figure 5. Representative experimental loads ratings that supplement information presented in Section 6 can be found in the Appendix (Section 8). Section 7 summarizes the study and presents conclusions.

4 Global Decision Tree (Figure 3) As presented in Figure 3, prior to initiating an analytical or experimental load rating, bridge owners must determine which approach best suits their needs and resources. The decision to perform an analytical load rating ultimately centers on determining if enough resources and information are available about the bridge and its current condition to produce an analytical model whose results will provide accurate, but conservative, estimates of current capacity. Conversely, the need for field testing is, in part, based on establishing if analytically determined load rating results accurately indicate that the bridge should/should not be posted. Specific reasons for selecting

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experimental load testing include rehabilitation or the presence of deficiencies that cannot be modeled accurately, a lack of design or rehabilitation plans and other items as discussed below. Sections that follow provide more detail on Figure 3, with cell identification numbers listed when necessary and help owners determine if enough information is available to complete an analytical load rating by presenting common questions that need to be answered. Available resources that help to answer those questions are also provided. Following steps presented in the Global Decision Tree helps an owner quickly determine which method is best for load rating bridge(s) under consideration.

4.1 Changes to the Bridge (Cell 4.1) As shown in Cell 4.1 in Figure 3, the decision to load rate a bridge involves determining if the answer to at least one of the following questions is yes:

• Is the bridge reconstructed?• Is the bridge repaired?• Is there any bridge component whose condition factor has dropped?• Is there any appreciable change to dead or live loads?

According to the BIPM, repair encompasses bringing the bridge back to its prior condition and reconstruction is any work that changes the bridge roadway width, its load carrying capacity, its structure or geometry or anything requiring a PE to design, seal and sign resulting plans and specifications. Information regarding a bridge’s repair or reconstruction history can be accessed from individual bridge records. If the answer to all the questions in Cell 4.1 is no, one next needs to determine if the bridge is posted.

4.2 No Changes to the Bridge, Current Posting? (Cell 4.2) As shown in Cell 4.2 in Figure 3, users of the Global Decision Tree should establish if the bridge is posted. This information can be obtained from available records, a site visit, or, for bridges that are part of the NBI database, from item 70, Bridge Posting [7]. As stated in Section 1, the NBIS was established to determine current bridge condition. For bridges that are on public roads, the NBI requires bridge posting if maximum legal load configurations in the State where the bridge is located exceed loads used to establish the structure’s Operating Rating. For bridges in the NBI, which comprises all bridges on public roads, NDOT mandates that bridge owners have a valid, current LRSS, reproduced in Figure 1, on file. This sheet provides documentation of calculations for that bridge’s load rating and recommendations for posting, if necessary. NDOT also requires that the load rating be: prepared by a Professional Engineer licensed in Nebraska; based on documented, current condition codes; and supported by calculations. Owners must also install posting signs for load restrictions indicated by the LRSS.

4.2.1 Not Posted (Cell 4.2.1) If information discussed in the previous section cannot be located, then the bridge is assumed to not be posted. If the answer to questions in Cell 4.1 is negative and the bridge is not posted, then no further action is required.

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4.2.2 Posted If the bridge is posted, or if the answer to at least one of the questions in Cell 4.2.2 is positive, one needs to determine if it is possible to construct an accurate numerical model (see Cell 4.3). The owner also needs to determine whether analytical or experimental methods were used to post the bridge or if the posting was purely based on qualitative assessment and engineering judgment.

4.2.3 Is Existing Load Rating Based on Quantitative Methods? NDOT requires that all bridges in Nebraska (spans over 20 ft. long) must have a current load rating. As stated previously, up to 7% of bridges or culverts in the US were posted based on a qualitative load rating, which includes ratings based on engineering judgment and visual inspection. It is strongly recommended that analytical or experimental procedures be used to load rate those structures. To update the load rating for such bridges or culverts, additional information is required to determine if the rating can be completed analytically. This information can include: archival data and plans; bridge rehabilitation and inspection; additional or new field data; or information from other sources. Necessary information is discussed in more detail below.

4.3 Is It Possible to Construct an Accurate Numerical Model? (Cell 4.3) If the bridge was posted using engineering judgment or other qualitative methods, or if the answer to any of the questions in Cell 4.1 is yes (i.e. visible signs of damage leading to change in condition rating as defined by the BIPM, a change in loads, the completion of repair or reconstruction), a load rating must be performed using analytical or experimental means. The owner must ensure that the most accurate numerical model possible is used to complete the analytical load rating. For instance, if reinforcement details are not available, the capacity of a reinforced concrete section might be underestimated. The same applies to its material properties. That being said, prior to conducting load tests on posted bridges accuracy of the numerical model should be ascertained. In some cases, conducting low-cost tests that establish important bridge properties could be used to improve numerical model accuracy. See MBE Section 6 for an overview of available tests for determining material properties. Based on the likelihood of developing an accurate numerical model, a decision regarding the type of load rating approach can be made. This decision centers on:

• design or rehabilitation information;• availability of plans;• accuracy of material and geometric property information; and• accuracy of understanding of support or connection (i.e., boundary) conditions.

4.3.1 Design/Rehabilitation InformationKnowledge of design codes and potential rehabilitation procedures help load rating personnel construct a more accurate numerical model. NDOT mandates that bridge owners must keep information needed for load rating and analysis of bridges in their possession.

4.3.2 Field Inspection Information Field inspection data required for analytical or experimental load rating is documented in the BIPM Chapter 4 and Section 4 of the MBE. NDOT has developed an archive of data and plans, the Bridge Document Management System (BDMS), to assist owners with acquiring necessary field inspection

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information. The BDMS contains plans, measurements, shop drawings, inspection reports, inspection photos and load rating information.

4.3.3 Are Plans Available? The NBIS requires that all bridge records be retained. BIPM Section 2.3.2 states that it is the responsibility of a bridge owner to keep individual bridge records. Mandatory bridge record items are specified in Section 2.5 of the BIPM. Bridge owners are responsible for keeping an electronic back-up of their bridge records either on their websites or on NDOT’s website (https://dot.nebraska.gov/business-center/bridge). In the absence of bridge plans or if significant changes have occurred, a bridge owner can also utilize lidar or point cloud data collection to provide high-density and high-fidelity geometric data.

4.4 Perform Analytical Load Rating (Cell 4.4) Once the necessary information is compiled and the owner believes an accurate, but reasonably conservative, load rating of the bridge can be obtained analytically, the analysis begins. Analytical load ratings can be performed at varying levels of complexity using various tools. Many resources are available to determine which complexity level is appropriate and how to complete the process. More detail on the analytical load rating decision-making process can be found in Figure 5.

4.5 Adequacy of Analytical Results and Need for Supplemental Field Testing (Cell 4.5)

Once the analytical load rating is completed and after consulting a Professional Engineer licensed in Nebraska, the owner must determine if results require field testing to obtain a better understanding of the bridge’s current load carrying capacity. When making this determination, current structural condition and any assumptions made in association with determining that condition must be considered. It is well understood that structural systems often have additional capacity beyond that predicted by routine analytical methods due to simplifying and conservative assumptions. See Section 5.1.1 for further details.

4.6 Perform Experimental Load Rating (Cell 4.6) If the bridge needs to be posted based on an analytical load rating, prior to making a decision to improve bridge capacity via rehabilitation or replacement, it is recommended that owners strive to update the analytical load rating using field testing. However, prior to the load test, the engineer of record should ensure that the most accurate information regarding material and geometric properties, boundary conditions and loads was used for constructing numerical model analysis.

5 Analytical Load Rating Decision Tree (Figure 4) As discussed in the previous section, when it is determined that an analytical approach to load rate the bridge of interest is feasible, specific information is needed to assist with making analysis decisions. Analytical load rating decisions are primarily influenced by:

• potential changes in condition factors since last load rating (if applicable) and their causes;• the age of the bridge or culvert and materials used in its construction; and• the way material properties were determined.

https://dot.nebraska.gov/business-center/bridge

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The procedures for the analytical load rating of bridges and culverts are well-documented in existing literature. The reader is referred to the following key sources:

• BIPM, Chapter 5; and,• MBE, Section 6.

Key analytical load rating decision-making steps are outlined in Figure 4.

5.1 Analytical Load Rating of New and Existing Bridges NDOT requires all bridges have a current load rating (see Section 5.1 of the BIPM), one that is performed by qualified personnel. Section 5.8 of the BIPM outlines situations requiring new or updated load ratings or assessments. Necessary steps in Figure 4 for load rating new or existing bridges are summarized below with cell identification numbers listed when necessary.

5.2 Analytical Load Rating Process

5.2.1 Gathering Data through Inspection (Cell 5.2.1) In general, the analytical load rating of structures requires an inspection. The goal of such inspections is to gather data about:

• current condition;• geometric and material properties; and• boundary conditions.

A complete list of data to be gathered from an inspection cab be found in Section 6 of the MBE.

5.2.2 Constructing a Numerical Model and Selecting Rating Loads (Cell 5.2.2) Once geometric and material properties of the bridge are determined, an analytical model should be developed. See Section 2.3 for more details about analytical modeling. Once the model is constructed, appropriate rating live loads must be determined and applied to the model. In general, load rating vehicles should be representative of trucks in the United States. Load ratings are normally completed for numerous types of trucks and design loadings as required by the MBE. Each state determines by statue the maximum legal axle weight and spacing for applicable vehicles. Nebraska uses the following Legal Trucks for rating: Type 3; Type 3S2; Type 3-3; and Specialized Hauling Vehicles (SHVs). The Type 3 Nebraska Legal Truck is the same as the Type 3 AASHTO Legal Truck while the other two Nebraska Legal Trucks have the same configuration of axles but slightly larger front axle weights when compared to AASHTO Legal Trucks [7]. For more information regarding Nebraska Legal Trucks see Section 5.6 of the BIPM.

5.2.3 Choosing Load Rating Method (Cell 5.2.3) The MBE permits completing loads ratings using three methods: Allowable Stress Rating (ASR), LFR, and LRFR. Sections 6B and 6A in the 2nd Edition of the MBE discuss each method in detail [25]. The choice of a load rating method is a function of many factors, such as the year the bridge was built, rebuilt, or repaired or the type of bridge and materials used. MBE guidance related to method selection as a function of bridge type and material is listed below [7, 10]:

1. Truss bridges: follow recommendations for steel, timber, or concrete members as required.2. Timber bridges and members:

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• built before October 2010 use ASR or LRFR;• built after October 2010 use LRFR.

3. Steel bridges and members:• designed prior to October 2010 or built or rehabilitated before 1994 use LFR or the

LRFR;• otherwise use LRFR.

4. Concrete bridges and members:• Designed prior to October 2010 or built or rehabilitated before 1994 use LFR or

LRFR;• otherwise use LRFR.

If load rating methods are updated, an existing load rating performed using ASR or LFR does not need to be recalculated using the new methods. However, an existing bridge or culvert originally load rated using ASR and in need of a revised rating should be evaluated using LFR or LRFR methods.

5.2.4 Performing Load Rating, Analysis of the Results, and Decision Making (Cell 5.2.4) Once live loads are applied to the structure and appropriate response is calculated using the selected analytical model, results will be used to load rate critical members. If the load rating of the bridge under consideration at the Operating Level is greater than or equal to one, no further action will be required. Conversely, if the Operating Rating is less than one posting is mandated.

5.2.5 Performing Experiments for Obtaining Material Properties (Cell 5.2.5) Before posting a bridge based on an analytical load rating, owners are initially encouraged to investigate determining in-situ material properties using destructive or nondestructive testing. Standard procedures for obtaining material properties using nondestructive techniques are outlined in MBE Section 5. If the owners deem material testing appropriate, the load rating should be repeated using measured values. If the analytical load rating mandates posting using either nominal or measured material properties, owners are encouraged to consider performing an experimental load rating. The procedures for experimental load rating are summarized in Figure 5 and discussed in the following section.

6 Experimental Load Rating Decision Tree (Figure 5) Once it is determined that a test would be an appropriate load rating option, certain general decisions need to be made. Those decisions are summarized in Figure 5 with cell identification numbers listed when necessary. To reiterate, factors that could influence the load-carrying capacity of a bridge include unintended composite action, continuity, and participation of secondary or nonstructural members in the live load resisting system. For a detailed discussion of these factors, see MBE Section 8.2. Load tests are divided into two categories based on applied load magnitudes:

1- diagnostic tests, where the gross load could be a fraction of the legal design load; and2- proof tests in which load is incrementally increased to determine maximum capacity.

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Diagnostic load tests are performed to determine certain structural response features to external loads, such as load distribution factors, or to validate numerical models. Proof load tests are performed to establish a bridge’s maximum, safe, linearly-elastic capacity. Both diagnostic and proof tests could be performed using static or quasi-static (i.e., loads traversing the bridge at slow/idle speeds) loadings. For a discussion of types of load tests see MBE 8.4.

6.1 Benefits of Using Load Testing for Load Rating Load tests provide a physically verified understanding of structural behavior. The inherent conservatism in design bases can often be safely reduced by relying on measured rather than assumed structural response. Some benefits of load tests for typical bridges are articulated in the following sections. See MBE Section 8.3 and NCHRP Research Results Digest No. 234 [9] for more details.

6.1.1 Unknown or Low-Rated Components In some cases, as-built information does not exist for a bridge component or member and an accurate numerical model cannot be developed. In other cases, an analytical load rating may be overly conservative due to simplifications, inaccuracies or unknowns associated with the selected numerical model. In both cases, a load test may provide improved data for establishing an accurate load carrying capacity.

6.1.2 Accurate Load Distribution Factor Distribution of live loads is an important factor for obtaining accurate bridge load ratings. It is known that load distribution factors required by design guides are conservative [3]. Load tests provide accurate load distribution factors that can lead to improved ratings.

6.1.3 Deteriorated or Damaged Members In most cases, commonly used numerical models do not accurately reflect member deterioration or damage levels and locations observed during an inspection [16]. These inaccuracies are caused by the level of sophistication of available modeling techniques. In such cases, load testing more accurately represents deteriorated or damaged member effects on the entire bridge and, as a result, provides more accurate and, more than likely, improved ratings.

6.1.4 Dynamic Load Allowance Dynamic effects are largely caused by road surface roughness, deck deterioration, expansion joint openings and misalignments and approach slab settlement or deterioration. A secondary factor influencing dynamic effects is an interaction between the moving vehicle and the bridge. Design impact factors are intentionally conservative for most bridges. Load tests at posted speeds provide a quantitative assessment of dynamic effects and often lead to an improvement in the rating.

6.2 Bridge Inspection and Analytical Load Rating (Cell 6.2) Before planning a load test, a comprehensive assessment of the physical condition of the bridge should be carried out using a field inspection. If feasible, a preliminary, analytical load rating using procedures described in earlier sections in this report and in MBE Section 6 should be carried out. The analytical load rating and field inspection should be used as a basis for planning and

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conducting the load test to ensure the safety of the bridge and individuals completing the test under prescribed loads. The analytical model will also be used to determine the target test loadings. An improved version of the model, one that is modified based on test results, will also be used to determine revised load rating factors at the completion of the test.

6.3 Proof Test or Diagnostic Test? (Cell 6.3) While clear benefits exist from performing either a proof test or diagnostic test, in general diagnostic tests adequately characterize bridge behavior so that an accurate load rating can be completed. When deciding between a proof or diagnostic load test, it should be recognized that proof tests typically require enhanced planning, extensively experienced personnel and extremely careful monitoring of the test in real time to ensure personnel safety and prevent permanent damage to the bridge.

6.3.1 Perform a Proof Load Test (Cell 6.3.1) Proof tests are not recommended unless (i) it is completely unfeasible to construct an accurate numerical model of the structure or (ii) if the rating performed using a diagnostic load test is deemed to be too restrictive (see MBE 8.4.1.2).

6.3.1.1 Dynamic Test or Static Test? (Cell 6.3.1.1) According to Section 8.4.1.2 of MBE, proof load tests commonly involve static loads. However, at the discretion of the owners and the load rating engineer, dynamic tests could be performed to remove speed restrictions for heavy vehicles.

6.3.1.2 Calculate Final Load Rating Using the Maximum Safe Load of the Structure (Cell 6.3.1.2)

Proof load testing provides a substitute to analytically computing the load rating of a bridge. A proof test "proves" the capability of the bridge to carry its full dead load in addition to a "magnified" live load. The target test load is factored to provide a margin of safety in the event of an occasional overload during normal operation of the bridge. When completing a proof load test, loads must be incrementally increased and response measured and monitored until the desired load is reached or until the test is terminated when either of the following occurs.

1- the desired live load plus the appropriate margin of safety is reached; or2- bridge response reveals the onset of nonlinear behavior.

For details concerning proof load testing and determining target test load the reader is referred to the MBE Section 8.8.3.3.

6.3.2 Perform a Diagnostic Load Test (Cell 6.3.2) If a model has been developed and the following parameters need to be calibrated:

1- material properties;2- the level of composite action;3- continuity of connections or members;4- boundary conditions;

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5- deficiencies; and6- modifications and rehabilitation;

a diagnostic load test is recommended. See MBE Section 8.8.2 for further details.

6.3.2.1 Dynamic Test or Static Test? (Cell 6.3.2.1) Diagnostic tests typically involve the application of static and dynamic loads. In the context of completing a load rating, static or quasi-static tests are typically performed and data used to calibrate bridge numerical model parameters prior to completion of a more accurate analytical load rating. Dynamic load tests are commonly carried out to determine levels of live load dynamic amplification for a particular bridge. Vibration tests also could be carried out to calculate bridge modal properties. The current vibration test state-of-the-art is limited to operational modal analysis of structures subjected to broadband ambient excitations.

6.3.2.2 Load Rating through Diagnostic Load Testing (Cell 6.3.2.2) Prior to load testing a bridge, the structure should be analytically load rated. The diagnostic load test will be used to adjust the model so that it better reflects the bridge’s actual behavior. A summary of key aspects associated with successfully load rating a bridge using a diagnostic load test are presented. 6.3.2.2.1 Range of Validity of Diagnostic Load Test Diagnostic load tests are commonly used to load rate bridges featuring linear behavior. If it is determined that a bridge will behave linearly it is possible to extrapolate results to the design load level. 6.3.2.2.2 Diagnostic Load Rating Process A critical stage in diagnostic load testing and rating is the interpretation and analysis of discrepancies between measured response during the test and that predicted by the model. According to MBE Section 8.8.2.3, the following relationship is used to modify an analytical load rating using load test data:

𝑅𝑅𝑅𝑅𝑇𝑇 = 𝑅𝑅𝑅𝑅𝐶𝐶 𝐾𝐾. MBE 6.3.2.2.1-1

Where: 𝑅𝑅𝑅𝑅𝑇𝑇 is load rate based on load test, 𝑅𝑅𝑅𝑅𝐶𝐶 is analytical load rate, and 𝐾𝐾 is adjustment factor calculated using:

𝐾𝐾 = 1 + 𝐾𝐾𝑎𝑎𝐾𝐾𝑏𝑏 . MBE 6.3.2.2.1-2

Where 𝐾𝐾𝑎𝑎 accounts for potential benefits derived from the load test and calibration of model parameters and 𝐾𝐾𝑏𝑏 accounts for load test results and the extent to which one can extrapolate bridge behavior under diagnostic load test results to anticipated design load levels. As explained in MBE Section 8.8.2.3, 𝐾𝐾𝑎𝑎 =

𝜀𝜀𝐶𝐶 𝜀𝜀𝑇𝑇� − 1 where 𝜀𝜀𝐶𝐶 and 𝜀𝜀𝑇𝑇 are the maximum strain obtained from the calibrated numerical model and that from the load test, respectively. 𝐾𝐾𝑏𝑏 needs to be determined from MBE Table 8.8.2.3.1-1 based on the magnitude of the test load relative to the unfactored rating load and also on the judgement of testing team with respect to how readily observed discrepancies from expectations can be explained.

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It must be highlighted that the analytical rating factor should be calculated using a model that is calibrated against test results. In doing so, 𝑅𝑅𝑅𝑅𝐶𝐶 is determined after model geometric and material properties and support and continuity conditions are modified based on evaluation of load test results, including observations made during placement of the test vehicle on the bridge. For a detailed discussion of this approach the reader is referred to MBE Section 8.8.2 and NCHRP Research Results Digest No. 234.

6.4 Factors That May Improve Bridge Load Rating Using Nondestructive Load Tests

Routine numerical models constructed for bridge design and load rating are inherently conservative. Therefore, it is expected that actual bridge performance is more robust than conventional design or rating methods indicate. A list of potential drivers behind a more robust rating based on testing is provided in the following sections. See BIPM Section 5.11.12 and MBE Section 8.2 for more extended discussions on these items.

6.4.1 Unintended Composite Action Field tests have shown that even a noncomposite deck can demonstrate a certain level of composite action, which would lead to improved ratings. It should be highlighted, however, that extrapolation of composite action for arbitrary, service loads to loads having higher intensity is not recommended. Horizontal shear forces between the superstructure and concrete deck that produce this composite action and enhance bridge capacity may exceed the static coefficient of friction and revert to a lower, dynamic coefficient of friction that could result in reduced capacity.

6.4.2 Unintended Continuity or Fixity Stringer-to-floor-beam connections and bearings are commonly idealized as pinned. Tests have shown that there can be significant end fixity at stringer-to-floor-beam connections and from frozen bearings. In similar fashion to unintended composite action, extrapolating unintended continuity or fixity observed at lower load levels to higher levels should not be completed arbitrarily.

6.4.3 Participation of Secondary Members Members that are interpreted to not be primarily involved in bridge load resistance and are designed based on this assumption are called secondary members. These members can include bracing, diaphragms, and cross frames. The members do contribute to structural stiffness but are not commonly designed to resist the live load. A load test can reveal their actual levels of participation in live load resistance and subsequently improve the load rating.

6.4.4 Participation of Nonstructural Members Nonstructural members, which can include bridge railing, parapets, and safety barriers, are also not commonly included in live load resistance during design. A test can reveal similar effects on live load carrying capacity to that discussed for secondary members.

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6.4.5 Portion of Load Carried by Deck For relatively short span bridges, a portion of the live load may be transmitted directly by the deck to the substructure via two-way action. Taking advantage of this behavior for loads greater than those applied during the field test must be supported by well-documented calculations completed by qualified individuals.

7 Conclusions This report details the results of a study conducted to help bridge owners safely and cost-effectively manage their inventory. Decision-making tools were developed and described herein to provide guidance when owners are considering alternatives between basing load ratings solely on analytical techniques or on a combination of experimental and analytical methods. While the information is applicable to any bridge, the tools will be especially beneficial for bridges observed to be in good condition, yet live load restricted based on routine analytical rating methods. Examples demonstrating potential benefits associated with using field testing to load rate existing bridges are also provided in an accompanying appendix. In this report the following decision trees were developed to guide bridge load rating decisions:

• A Global Decision Tree that helps owners choose between analytical or experimentalmethods;

• An Analytical Load Rating Decision Tree that succinctly outlines the analytical load ratingprocedure and provides references for more information; and

• An Experimental Load Rating Decision Tree that also succinctly outlines the experimentalprocedure along with providing relevant references.

Supporting sections were created that help the user navigate each decision tree with, again, relevant references being provided. To demonstrate the efficacy of using experimental techniques to improve bridge load ratings, developed decision trees were used to load rate selected case study bridges in Nebraska. Bridges that were tested and load rated were:

• a single span, steel bridge located in Saunders County;• a single span, timber bridge located in Platte County;• a single span, steel stringer bridge having cast in place, concrete, box culvert extensions

located in Lancaster County; and• a three span, truss bridge located in Thayer County.

All bridges were either posted or closed to traffic based on analytical or qualitative decision making processes. The Operating Rating Factor determined using an experimental load rating was calculated to be greater than 1 for all of the bridges that were tested, thereby removing the need for load restrictions or bridge closure should the owner wish to do so. The following factors were interpreted to be drivers behind the improved load ratings based on experimental testing:

• for the Saunders and Plate County Bridges the experimentally obtained girder live loaddistribution factor;

• for the Lancaster County Bridge, the experimentally obtained girder live load distributionfactor coupled with unintended end fixity of the stringers, and

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• for the Thayer County Bridge, unintended end fixity at truss member connections.

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8 Appendix Summaries of the Nebraska bridge tests and corresponding load ratings using test data are provided. Bridges that were tested included a two, single span steel bridges, one of which included adjacent box culverts, a single span timber bridge and a three-span, truss bridge. Information presented in Figure 5 was used to guide completion of the field tests. Details on the procedure followed for each test is presented in the following section. Remaining Appendix sections summarize each test, resulting load ratings obtained using test data and comparisons between that load rating and ratings produced using analytical models developed prior to testing.

8.1 Load Testing Procedure Figure 5 indicates that, prior to field testing, essential steps need to be performed. These steps are described in more detail in the following sections in association with the four Nebraska bridge tests. Corresponding decision tree cell reference identifiers are also provided.

8.1.1 Review of Available Information, Bridge Inspection (If Feasible), Analytical Load Rating (If Feasible), Test Load Selection (Cell 6.2)

Available bridge documentation (e.g. design and rating calculations, drawings, inspection reports) was obtained and reviewed. A site visit and inspection was performed to gather important data. This included information on bridge type, age, dimensions, materials along with completion of visual inspections and subsequent condition assessments. Collected geometric information included: span length and width; member length, spacing and orientation; connection types; slab and wearing surface thickness; dimensions of primary and secondary supporting and connecting elements; and dimensions of any superimposed dead loads (e.g. parapets, railings). Bridge age and construction materials types and properties were also verified, if possible. Material properties can be established using destructive or, preferably, nondestructive testing. Visual inspections and condition assessments focused on signs of deterioration of steel (e.g. corrosion, sections loss, cracks), concrete (e.g. cracking, spalling, efflorescence) and timber (e.g. rot, decay, splitting) super- and substructure elements. Important site information, such as superstructure under- and over-clearances and accessibility was also documented.

8.1.1.1 Preliminary Analysis Each bridge had a preliminary analysis completed using available information. CSI Bridge was used to model all tested bridges, with model output being used to complete preliminary ratings. These analyses also informed instrumentation selection and placement and helped identify appropriate test load positions and magnitudes. Given their span lengths, bridges that were tested were loaded and analyzed using the shortest wheelbase Type 3 Legal Truck so that critical bending and shear effects were created.

8.1.1.2 Perform Analytical Load Rating Once a numerical model was developed and critical test load determined, an analytical load rating was completed to determine each bridge’s theoretical load carrying capacity. The procedure presented in Figure 4, as described in Section 5, was followed to complete the analytical load ratings. Additional detail on that procedure is not provided here.

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8.1.2 Diagnostic or Proof Load Test (Cell 6.3.2) As stated in Section 6.3, proof tests are completed only if it is not feasible to construct an accurate numerical model or if results from a diagnostic load test are deemed too restrictive. Since analytical models could be constructed for each bridge that was tested, those tests were diagnostic in nature. In addition, none of the tested bridges were subjected to more restrictive live loads after diagnostic load tests were completed, facts that again, did not substantiate completion of a proof load test.

8.1.3 Plan Diagnostic Load Testing (Cell 6.3.2.1)

8.1.3.1 Plan Dynamic and Static Tests Testing plans were developed with tests performed using trucks positioned statically at various transverse and longitudinal locations along the bridge and with the same trucks traversing the structure at two speeds. Two-lane bridges had trucks positioned 1 to 2 ft. from the curb and bridges with more than two lanes had the truck centrally placed in each lane. Regardless of the number of lanes, the truck was also positioned at the center of the slab. Crawl (3 mph) and posted speeds (30 mph or more) were selected for the dynamic tests. Each dynamic test was repeated at least twice to ensure reproducibility.

8.1.3.2 Bridge Instrumentation and Response Measurement A data acquisition system and strain transducers were used to measure bridge response during testing. Transducers were placed onto critical members at sections where maximum effects were anticipated to occur.

8.1.4 Calculate Final Load Rating (Cell 6.3.2.2)

8.1.4.1 Analysis of Test Results and Model Calibration When possible, strains were measured at the bottom and upper flange longitudinal supporting members (e.g. girders, stringers) to compute the neutral axis location in the cross-section and identify the existence of unintended composite action. For bridges where upper flange strains could not be measured, FE model calibration was used to determine if composite action was evident. FE model calibration encompassed reducing discrepancies between measured and predicted strains. Parameters selected to assist with model calibration included levels of composite action, section moments of inertia and section moduli, member end restraint and material properties. Live Load Distribution Factors (LDFs) were determined using longitudinal strains measured at mid-span of main, longitudinal supporting members. LDFs were calculated for three truck placements on each bridge. In general, these factors were lower than those provided using AASHTO Standard Specification LDF distribution factors.

8.1.4.2 Experimental Load Rating – Update Analytical Load Rating Using Test Data As stated earlier, diagnostic load ratings were completed for each of the tested bridges using the calibrated models. Ratings were performed using LFR.

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8.2 Nebraska Bridge Tests and Experimental Ratings Several posted bridges were reviewed as potential candidates for testing and experimental rating and, as stated in the report, four bridges were selected. Selected bridges were of different type and, subsequently, each required a specific approach to complete its analysis, instrumentation, testing and rating. Each of the tests and resulting ratings are summarized in the following sections.

8.3 Single-Span, Steel Bridge Located in Saunders County (ID #C007803635)

8.3.1 Bridge Description The Saunders County Bridge is a two-lane, single-span, simply supported bridge consisting of a 5 in. thick concrete deck having a gravel wearing surface and supported by 10 steel rolled sections. Its span length is 24 feet and width is 20 feet. The thickness of top layer of gravel is 1.5 in. Interior and exterior girders were of differing in the cross-section. Spacing between interior girders is 2.4 feet and is 2.2 feet between the exterior and first interior girder. Relevant photos of the bridge are shown in Figure 6 (a-c).

(a) (b) (c) Figure 6: Two-lane, single span, Saunders County Bridge: (a) Top view, (b) posting information, and (c) bearings.

Posting information is shown in Figure 6 (b). The bridge substructure consists of concrete abutment caps and wing walls cast integral with the deck, with the caps supported by steel piles. Backwalls were timber as shown in Figure 6 (c). A rail was affixed to fascia girders as shown in Figure 6 (a). Section and plan views of the superstructure are shown in Figure 7 and Figure 8.

Figure 7: Saunders County Bridge section.

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Figure 8: Saunders County Bridge framing plan.

8.3.2 Field Testing Basic bridge information, including general geometry and photographs, was obtained from NDOT and an instrumentation scheme and testing plan were developed.

8.3.2.1 Instrumentation Plan The instrumentation plan was established based on (i) critical sections suggested by NDOT and (ii) information needed to construct and calibrate an accurate analytical model of the bridge. Straintransducers were installed on the bottom of the girders, as shown in Figure 9 to Figure 12. A totalof 30 strain transducers were installed on the bottom flange at mid-span and the ends all girders,with placed parallel to girder, longitudinal axes. Due to a poor bond between certain transducersand the steel girders, twenty-two provided reproducible measurements.

Figure 9: Saunders County Bridge transducer locations – section.

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Figure 10: Saunders County Bridge transducer locations – plan.

(a) (b) Figure 11: Saunders County Bridge strain transducers and wireless data acquisition system.

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8.3.2.2 Bridge Loading Loading of the structure was completed using a loaded Type 3 truck provided by NDOT. The loaded truck weighed 50.56 kips, with th

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