About LTBP

This research was conducted 

as part of the Federal Highway 

Administration’s Long-Term 

Bridge Performance (LTBP) 

Program. The LTBP Program is 

a minimum 20-year research 

effort to collect scientific performance field data, from 

a representative sample of 

bridges nationwide, that will 

help the bridge community 

better understand bridge 

deterioration and performance. 

The products from this program will be a collection of 

data-driven tools including 

predictive and forecasting 

models that will enhance the 

abilities of bridge owners to 

optimize their management 

of bridges.

FHWA LTBP Bridge Performance Primer

TECHBRIEF

Objective

This TechBrief defines bridge performance and discusses 

the importance of measuring performance and current 

practices for doing so. It identifies key issues to improving 

performance measures and the role the Long-Term Bridge 

Performance (LTBP) Program plays in this effort. 

Introduction

Under the LTBP Program, bridge performance encompasses how bridges function and behave when subjected 

to the complex and interrelated factors they face day in 

and day out—traffic volumes, loads, deicing chemicals, 

freeze-thaw cycles, rains, or high winds. Bridge design, 

construction, materials, age, and maintenance history also 

play roles in performance. It is the combination of these 

factors—unique for each individual bridge—that governs 

performance of that bridge.

Bridges are critical nodes in the highway infrastructure. 

A bridge with one or more elements in poor condition 

(exhibiting poor performance) has the potential to reduce 

the operating capacity of the highway system of which it is 

a part. Under typical service conditions, many bridges will 

eventually reach a state where some work is necessary 

to return one or more of its components to a satisfactory 

level of condition and/or operational capacity. Bridge work 

FHWA Publication No.: FHWA-HRT-14-032

FHWA Contact: Susan Lane, HRDI-60, 202-493-3151, susan.lane@dot.gov

This document is a technical summary of the published Federal Highway Administration (FHWA) 

report FHWA-HRT-13-051, LTBP Bridge Performance Primer, which is available at http://www.fhwa.

dot.gov/research/tfhrc/programs/infrastructure/structures/ltbp/pubs.cfm.

Research, Development, and Technology | Turner-Fairbank Highway Research Center | 6300 Georgetown Pike, McLean, VA 22101-2296

http://www.fhwa.dot.gov/research/
2

zones usually involve one or more conditions 

that result in disruption to efficient and economical traffic flow. These conditions include 

narrowed or closed lanes, live load restrictions, speed reductions, and lengthy detours. 

Consequently, poor bridge performance often 

leads to significant negative impacts on local 

and regional economies and environments.

Bridge managers have access to a vast knowledge base of inventory and condition data on 

bridges to support the tools they use in their 

decision-making activities. One such activity, 

periodically assessing bridge performance, 

is typically done by employing generally 

accepted formulas and criteria for evaluating 

bridge condition and health at some given 

point in time. In the bridge performance 

primer, some of these commonly used 

formulas and criteria are identified and discussed. Many of these performance measures 

were created and have served well when 

used for condition evaluation and distribution 

of funds. However, when used for research 

purposes, these measures do not address all 

the variables researchers need to evaluate. 

For research the shortcomings of these performance measures include a lack of ability 

to correlate changes in these measures over 

time with the underlying forces that govern 

performance, and to correlate performance 

measures with effective mitigation actions. 

Examples of additional data researchers are 

seeking are a detailed history of preservation and maintenance and repair activities 

accomplished on the bridge, all of which could 

potentially alter the path of performance 

over time. The primer examines the current 

knowledge base of bridge measures and data 

and addresses the need for improved depth 

and quality of data for research on bridge 

performance. The level of understanding of 

how and why bridges perform the way they 

do and how to improve bridge performance 

can be enhanced. The bridge performance 

primer is intended to provide a perspective 

on bridge performance that will point the way 

to approaches to improve the understanding 

of bridge performance. One point is of 

paramount importance—the need to develop 

performance measures that help separate 

and assess all of the impacts of the various 

forces affecting bridge performance. These 

performance measures can lead to improved 

deterioration models, more accurate life-cycle 

costs analysis, and other tools to identify the 

most effective ways (including materials and 

methods) to improve bridge performance.

The Highway Bridge 

Infrastructure

The 2008 National Bridge Inventory (NBI) 

database contains records for 601,411 bridges, 

of which 127,052 are classified as tunnels or 

culverts.(1) The remaining 474,359 are single or 

multispan bridges separating highway traffic 

from other traffic and/or some topographical 

feature, usually a stream or river. The diversity 

of the bridge infrastructure in terms of age 

and design parameters (including structural 

type, materials of construction, width, length, 

etc.) is broad. The breadth of the diversity is 

reflected in the many different combinations 

of attributes or parameters that describe those 

bridges. Table 1 provides an abbreviated list of 

these characteristics and indicates how many 

different types of characteristics there are in 

the NBI for each.

Table 1. Diversity of bridge characteristics.(2)

NBI Item # of Types

Kind of material, main span, and/or 

approach span 10

Structure type, main span, and/or 

approach span 23

Design load 10

Bridge posting 6

Deck structure type 9

Wearing surface 9

Membrane 5

Protective system 9
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The appendix of the primer provides additional 

detail on the diversity in the condition and status 

of bridges. These features are important factors 

that govern performance of each bridge. Other 

factors that can significantly impact performance 

and can vary considerably from bridge to bridge 

include the following:

• Service conditions—traffic volumes carried; 

truck loadings (including overweight 

permit loads); level of vulnerability to the 

forces of natural events such as floods, 

ice, waterborne debris, wind, and seismic 

loadings; and susceptibility to longer-term 

effects of climate and the service 

environment (e.g., material deterioration 

that increases over time).

• The types, frequency, and effectiveness 

of bridge preservation, preventive 

maintenance, rehabilitation, and/or replacement actions.1,2

This broad diversity in the nature and the individual service conditions of different bridges presents a challenge to any research effort to more 

clearly understand bridge performance. It can be 

difficult to obtain and/or effectively analyze all of 

the necessary types and details of data on service 

conditions, conditions of bridge materials and 

elements, and the actions taken on bridges by the 

owners. In order to create a better understanding of bridge performance, FHWA has initiated 

the LTBP Program. The overall objective of the 

LTBP Program is to inspect, evaluate, and periodically monitor representative samples of bridges 

nationwide in order to collect, document, maintain, and manage high-quality, quantitative performance data over an extended period of time. 

These data then become the basis for research 

tools to better manage assets, understand bridge 

performance, and create data-driven approaches 

to improving performance. 

Measuring Bridge Performance

There are different ways to measure bridge performance, and these usually involve comparison 

of performance against some set of established 

standards. Some measures of performance are 

in the form of an index value calculated for each 

bridge from a defined formula using input data 

such as NBI condition ratings, traffic volumes, etc. 

The Federal sufficiency rating (SR) is one example 

of this type; several State transportation departments use a “health index” to measure bridge 

condition. Similar indices have been developed 

by transportation agencies in other countries. 

Another type of measurement is based on 

whether a bridge meets some defined criteria. 

This type of measurement includes approaches 

that classify bridges as “structurally deficient” 

or as “functionally obsolete.” Still others reflect a 

compliance with a desirable level of operational 

capacity or service. These include posted load, 

load rating, limitations on deflections under live 

load, rideability of the bridge-wearing surface, 

traffic congestion near or on the bridge, number 

of accidents on the bridge, percentage of rain 

events and floods that overtop the bridge, etc. 

The bridge performance primer provides some 

insight into some of the measures commonly 

used today. It is important to note these measures usually reflect a “snapshot” of performance 

at a given point in time. The primer discusses 

what additional research data and information is 

needed to better understand and develop solutions to improving bridge performance.

Experience has shown the performance of any 

specific bridge is dependent on complex interactions of multiple factors, many of which are 

closely linked and include the following: 

• Original design parameters and specifications, such as bridge type, materials of 

construction, geometry, and load capacity. 

 1

 Replacement actions may or may not be performed by the owner; they may be performed by others on behalf of the 

owner.

 2 The definition of bridge preservation is actions or strategies that prevent, delay or reduce deterioration of bridges 

or bridge elements, restore the function of existing bridges, keep bridges in good condition and extend their 

life. Preservation actions may be preventive or condition-driven. The definition of preventive maintenance is 

a planned strategy of cost-effective treatments to an existing roadway system and its appurtenances that preserves the system, retards future deterioration, and maintains or improves the function condition of the system 

(without substantially increasing structural capacity).(3)
4

• Initial quality of materials and of the as-built 

construction. 

• Varying environmental conditions of climate 

and air quality, marine environment, or even 

surrounding soil. 

• Incidence of corrosion or other deterioration 

processes. 

• Traffic volumes and frequency and weight of 

truck traffic carried by the structure.

• Types, frequency, and effectiveness of bridge 

preservation, preventive maintenance, 

rehabilitation, and/or replacement actions.

All of these factors combine to affect the condition and operational capacities of the bridge 

and its various structural elements at any given 

point in the life of the bridge. Measures such as 

those mentioned above can be used to evaluate the overall performance of a bridge or a 

group of bridges under different service conditions. Researchers hope to show the qualitative 

or quantitative impact of a parameter or set of 

parameters on some specific aspect of bridge 

performance.

Why Measure Bridge 

Performance?

Bridge performance measures have different uses 

depending on the perspective and the responsibilities of those persons using the performance 

measures. Bridge performance measures are 

useful for the following reasons: 

• Identifying clear links between specific 

policies (such as the type and quantity 

of anti-icing materials), actions, and the 

resulting change in performance level of a 

bridge element.

• Improving knowledge of how and why 

bridges deteriorate.

• Gaining a better understanding of 

the effectiveness of various design, 

construction, inspection, and preservation 

strategies, as well as management practices. 

• Gaining a better understanding of the 

effectiveness of durability strategies for 

new bridge construction, including material 

selection.

• Improving bridge management practice 

using qualitative and quantitative data.

• Evaluating serviceability and durability.

• Setting priorities for resource allocations 

and evaluating organization-wide policies 

and programs such as the split between 

maintenance and capital funds.

• Establishing risk-based evaluations of 

bridges that are vulnerable to failure.

Current Approaches to 

Measuring Bridge Performance

Understanding bridge performance is a challenging task. The primer describes some current 

approaches to measuring bridge performance 

and provides some discussion of the data upon 

which the calculations of these measures are 

based. The value of performance measures 

commonly used today lies in their relative simplicity, their familiar nature, and the long-standing 

and broad-based acceptance of them. The data 

used as a basis for many of the current performance measures come from NBI, which includes 

condition and appraisal ratings on a scale of 

0 (failed condition—out of service—beyond 

corrective action) through 9 (excellent condition). 

In practice, ratings in the range of 4 to 7 are most 

common for bridges in service. Condition ratings 

are assessed for the deck, superstructure, and 

substructure of the bridge, and appraisal ratings 

assess key functional characteristics of the bridge. 

One type of commonly used bridge performance 

measure uses a single criterion as the basis for 

evaluation of performance. An example of this 

type is the determination if a bridge is structurally 

deficient (SD). A bridge is classified as SD if items 

58 (deck), 59 (superstructure), and 60 (substructure) or 62 (culvert) are rated in “poor” condition 

or worse (4, 3, 2, 1, or 0 on the NBI rating scale). 

A bridge can also be classified as structurally 
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deficient if its load-carrying capacity is 

significantly below current design standards 

(i.e., if item 67—structural evaluation appraisal—

is coded 2, 1, or 0) or if the waterway adequacy 

for the feature below the bridge is coded 2 

or below, meaning basically intolerable and 

requiring high priority for replacement. In a 

similar manner, bridges can be classified as 

functionally obsolete because their design is 

outdated—they may have lower load-carrying 

capacity, narrower shoulders, or less clearance 

than bridges built to the current standard. These 

types of measures provide bridge managers 

with a consistent basis for identifying bridges as 

candidates for future improvement actions.

A more complex type of performance measure is represented by the Federal SR, an index 

devised and used in the past by FHWA to evaluate the eligibility of bridges for Federal highway 

bridge rehabilitation and replacement funds. 

The SR formula is a method of evaluating 

highway bridge data by calculating and summing 

four separate factors to obtain a numeric value 

that is indicative of bridge sufficiency to remain 

in service. On the resulting rating scale, 100 

would represent an entirely sufficient bridge, and 

0 would represent an entirely insufficient or 

deficient bridge. No bridge is given an SR of 

below 0.

The SR is calculated using a complex formula 

wherein weighting factors are assigned to several different bridge parameters and attributes in 

order to arrive at the numerical index for each 

bridge. The basic formula follows: 

SR = S1 + S2 + S3 – S4

 Figure 1. Equation. Federal SR.(2)

These four factors provide consideration and 

weight as follows:

• S1: structural adequacy and safety 

(condition ratings for deck, superstructure, 

and substructure plus the inventory (load) 

rating). Maximum value = 55 percent.

• S2: serviceability and functional 

obsolescence (traffic lanes, average daily 

traffic, structure type, structural evaluation, 

waterway adequacy, Strategic Highway 

Network (STRAHNET) designation, and 

several key geometric parameters). 

Maximum value = 30 percent.

• S3: essentiality for public use (detour 

length, average daily traffic, and STRAHNET 

designation). Maximum value = 15 percent.

• S4: special reductions (detour length, 

traffic safety features, and structure type). 

Maximum value = 6 percent.

While these performance measures have served 

well when used for condition evaluation and 

apportionment of funds, these measures do not 

cover all of the variables researchers are seeking. 

Additional data needed for research come in 

three forms. First, condition information for not 

only bridge components such as the deck, superstructure, substructure, but also for the condition 

of the individual elements of a bridge, such as 

individual beams, pier columns, and abutments. 

Second, additional condition information to 

accurately distinguish what is happening to the 

bridge components over time. Third, measures 

that relate directly to the causes of symptoms of 

deterioration (such as rates of corrosion of the 

reinforcing bars). 

The implementation of a more detailed “element 

level” inspection system by many states has 

provided more bridge inspection data to use in 

evaluating bridge performance. Condition data 

is recorded on individual elements of the bridge 

rather than on the general elements of deck, 

superstructure, and substructure. Thus, severity 

of any deterioration is better defined and the

extent is estimated and recorded. 
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The Keys to Improving Bridge 

Performance

The need for additional, useful, reliable bridge 

performance measures is clear. Those studying 

bridge performance will be able to evaluate the 

impact of different maintenance practices and 

priorities, design methodologies, and new technologies on future bridge performance. The keys 

to improving bridge performance measures are 

as follows:

1. Establishing clear, objective research measures targeted to the causes of bridge 

deterioration.

2. Identifying the elements and characteristics 

that most seriously impact bridge performance.

3. Identifying critically needed data for experimental studies to improve the knowledge 

of the multivariable cause-and-effect 

relationships that govern performance.

4. Collecting data to fill the gaps identified in 

the previous three points and to create valid 

models that describe deterioration mechanisms, address the effectiveness of mitigation actions, predict future deterioration, 

and support more realistic life-cycle cost 

calculations.

The large amount of “legacy data” currently available for bridges provides a solid foundation upon 

which to build a better knowledge base of data. 

The NBI and the State transportation department 

bridge element level databases can be used as 

important and fundamental resources in furthering the understanding of bridge performance. The 

past, current, and future data contained in these 

two resources is very helpful in identifying trends 

in bridge performance and in identifying general 

parameters that govern performance. Bridge 

owners also possess other useful data such as the 

following:

• Design drawings and specifications.

• Analytical models.

• Construction records.

• Inspection reports.

• Photographic documentation.

• History of maintenance and preservation 

actions and timing, including costs.

Beyond these current resources, the additional 

research data needed to properly evaluate bridge 

performance can be quite extensive. Table 2 illustrates the breadth of data that may be necessary 

to better understand bridge performance.

The primer describes a recommended breakdown 

of bridge performance issues into four categories 

of performance: structural condition (for durability and serviceability), functionality (for safety and 

traffic capacity), structural integrity (for safety and 

stability), and risk and costs (to the user and to 

the agency). This definition helps isolate the most 

critical aspects of bridge performance and provides the basis for long-term research studies to 

improve the understanding of these issues.
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Conclusions

Nearly everyone in the United States, from bridge 

maintenance engineers to the everyday road user, 

has a stake in ensuring the performance of bridges 

in the nation is good or even excellent in terms of 

durability, operational capacity, roadway safety, 

resistance against failure, and life-cycle costs. 

For example, for commercial interests, shippers, 

drivers, etc., a simple measure of bridges with 

posted weight limits or geometrical dimensions 

may suffice. 

On the other hand, members of the bridge community—designers, construction engineers, 

inspectors, maintenance engineers, and bridge 

management personnel—responsible for maintaining performance must be able to properly 

and effectively evaluate bridge performance in 

precise and targeted manners. Toward this end, 

we must better understand bridge performance, 

which must be broken down into very specific 

issues that can be evaluated in terms of cause and 

effect. This will allow actions or programs to be 

identified to ensure a high level of performance at 

a reasonable cost. Understanding bridge performance can be a formidable task given the many 

factors that can govern performance under different circumstances. The LTBP Program is being 

implemented to identify the most critical aspects 

of bridge performance and conduct studies 

to provide the high-quality data necessary to 

better understand how multiple, variable factors 

affect aspects of performance. This should 

ultimately improve performance and extend the 

life of bridges at a minimum cost.

QA/QC = Quality Assurance/Quality Control

NDE = Nondestructive Evaluation

Category Data

Design and 

construction

Design plans and specifications

Critical design details

Change orders

Inspection notes

Construction QA/QC

Corrosion protection measures

Operating 

conditions

Local climate

Snow and ice removal practices

Freeze-thaw cycles

Rainfall and runoff; drainage control

Marine environment

Industrial pollutants

Dynamic 

loadings

Traffic volume

Truck volumes and weights

Weigh-in-motion data

Overload permits

Debris, ice

Impact loads

Flexibility, vibrations

Corrosion 

protection 

measures

Concrete cover over reinforcement

Corrosion resistant reinforcement

Deck overlays, membranes, 

and sealers

Other concrete sealers

Steel coatings—including 

weathering steel

Concrete characteristics—including 

high-performance concrete

Material 

conditions

Concrete

Steel

Reinforcing bars

Prestressing steel

Deck

Concrete superstructure

Steel superstructure

Concrete substructure 

Geometric 

data

Deflections

Rotations

Settlements

Loss of camber

Horizontal alignment and skew

Condition of 

components

Bearings

Joints

 Approach slabs

Details requiring NDE evaluation

Table 2. Possible durability and serviceability

performance data.
Researchers—This research was conducted by FHWA Office of Infrastructure Research and 

Development and Rutgers, the State University of New Jersey. For additional information, 

contact Susan Lane in the Office of Infrastructure Research and Development, located at 6300 

Georgetown Pike, McLean, VA 22101-2296. 

Distribution—This TechBrief is being distributed according to a standard distribution. Direct 

distribution is being made to the Divisions and Resource Center.

Key Words—LTBP Program, bridge performance, performance, long-term bridge performance, 

performance measures.

Notice—This document is disseminated under the sponsorship of the U.S. Department of 

Transportation in the interest of information exchange. The U.S. Government assumes no 

liability for the use of the information contained in this document. The U.S. Government does 

not endorse products or manufacturers. Trademarks or manufacturers’ names appear in this 

report only because they are considered essential to the objective of the document.

Quality Assurance Statement—The Federal Highway Administration (FHWA) provides highquality information to serve Government, industry, and the public in a manner that promotes 

public understanding. Standards and policies are used to ensure and maximize the quality, 

objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and 

adjusts its programs and processes to ensure continuous quality improvement.

FEBRUARY 2014 FHWA-HRT-14-032

HRDI-60/02-14(750)E

8

References

1. National Bridge Inventory Data, FHWA, 2008 

Data, http://www.fhwa.dot.gov/bridge/nbi.

htm.

2. FHWA Recording and Coding Guide for the 

Structure Inventory and Appraisal of the 

Nation’s Bridges, FHWA, U.S. Department of 

Transportation, Report No. FHWA-PD-96-001, 

McLean, VA, 2005.

3. Bridge Preservation Guide, FHWA, U.S. 

Department of Transportation, Report No. 

FHWA-HIF-11042, McLean, VA, August 2011.