national bridge inspection program - university of...

National Bridge Inspection Program

National Bridge Inspection Program

Why?1967 – Silver Bridge collapse (fracture of an eyebar at a pin connection)

1968 – National bridge inspection (NBI) program initiated (requiring regular and periodic inspections)

1971 – National bridge inspection standards (NBIS) adopted (prescribe how, with what frequency, and by whom bridge inspections must be completed)

1987 – Schoharie Creek collapse (scour)

2007 – Minnesota I-35W collapse (undersized gusset plate design)

National Bridge Inspection Standards

How?Visual assessmentCollection of 116 standards NBI fields of data collected75 supplemental state data fields

When?Every 24 months unless otherwise warranted by conditionsFor non-NBI structures, inspected at least every 60 months

Who?Qualified and trained inspectors each having a minimum of 5 years bridge inspection experience

Comprehensive two-week inspector training courseFracture critical inspection course

VTrans’ Inspection Program

2,688 bridges (span length greater than 20 feet)313 interstate highway764 state highway1,606 town highway5 private or railroad

1,294 bridges (span length greater than or equal to 6 feet and less than or equal to 20 feet)

223 interstate highway1,071 state highway

VTrans’ Inspection Program

219 rail bridges (state owned railroads)

253 retaining wall (along state highways having a height greater than 3 feet)

108 recreational path structures (span length greater than 6feet)

156 overhead sign support structures

Inspection TermsBridges are considered STRUCTURALLY DEFICIENT if significant load-carrying elements are found to be in poor or worse condition due to deterioration and/or damage, or the adequacy of the waterway opening provided is determined to be extremely insufficient. The fact that a bridge is structurally deficient does not immediate imply that it is likely to collapse or that it is unsafe.

Bridges are considered FUNCTIONALLY OBSOLETE when the geometry of the roadway no longer meets today’s minimum design standards for width or vertical clearance for that roadway classifications, or the adequacy of the waterway opening provided is determined to be insufficient. The fact that a bridge is functionally obsolete does not imply that it is unsafe.

Inspection TermsThe SUFFICIENCY RATING of a bridge is a single number from 0 (low) to 100 (high) taking into account, bridge condition, geometry, traffic, and how well the waterway passes underneath the bridge. Sufficiency rating is used to determine funding eligibility. A low sufficiency rating does not necessarily meanthe bridge is unsafe or in need or immediate repair.

A FRACTURE CRITICAL MEMBER is a steel member in tension that does not have enough additional, structural members to sufficiently redistribute load in the bridge if one member loses capacity, thus resulting in a portion of or entire bridge collapse.

FATIGUE is a material response that describes the tendency of a material to break when subjected to repeated loading.

Structural Deficiencies in Vermont

Bridge Age and Structural Deficient Trends

147187

236

392

437

200

266

343 349

34

99

0.00 2.67 1.696.38

15.5620.00 23.31

28.8634.67

47.06

55.56

0

50

100

150

200

250

300

350

400

450

500

< 10

≥ 10

and

< 2

0

≥ 20

and

< 3

0

≥ 30

and

< 4

0

≥ 40

and

< 5

0

≥ 50

and

< 6

0

≥ 60

and

< 7

0

≥ 70

and

< 8

0

≥ 80

and

< 9

0

≥ 90

and

< 1

00

≥ 10

0

Age

Num

ber o

f Brid

ges

0

10

20

30

40

50

60

70

80

90

100

Perc

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Gro

up CountSD

Types of Inspections

InitialThe first inspection of a bridge to determine baseline structural conditions

RoutineRegularly scheduled inspection consisting of observations and/ormeasurements needed to determine the physical and function condition of the bridge

In-DepthA close-up inspection of one or more members to identify deficiencies

SpecialAn inspection scheduled at the discretion of the inspector used to monitor a known or suspected deficiency

Types of Inspections

DamageUnscheduled inspection to assess structural damage resulting from environmental factors or human actions

Fracture criticalA hands-on inspection of a fracture critical member

UnderwaterInspection of the underwater portion of the bridge substructure

National Bridge Inspection Ratings

Condition ratingsDeck

Directly carries traffic

SuperstructureSupports the deck and connects one substructure element to another

SubstructureSupports the superstructure and distributes all bridge loads to below-ground bridge footings

CulvertA pipe or small structure used for drainage under a highway or railroad

National Bridge Inspection RatingsAppraisal ratings

Waterway adequacy (frequency of overtopping)

Structural evaluation (load rating)

Approach roadway alignment

Deck geometry (roadway width)

Under clearances (vertical and lateral)

NBI Condition Ratings (scale 9 to 0)

9 – Excellent condition

8 – Very good conditionno problems noted

7 – Good conditionsome minor problems


6 – Satisfactory conditionstructural elements show some minor deterioration.

5 – Fair conditionall primary structural elements are sound but may have minor section loss, cracking, spalling or scour.


4 – Poor conditionadvanced section loss, deterioration or scour

Serious conditionloss of section, deterioration, spalling or scour have seriously affected primary structural components. Local failures are possible.


2 – Critical conditionadvanced deterioration of primary structural elements. Unless closely monitored it may be necessary to close the bridge until corrective action is taken


1 – “Imminent” failure conditionmajor deterioration or section loss present in critical structural components or obvious vertical or horizontal movement affecting structure stability. Bridge is closed to traffic but corrective action may out back in light service.

0 – Failed conditionout or service – beyond corrective action.

NBI Appraisal Ratings (scale 9 to 0)

9 – Superior to present desirable criteria

8 – Equal to present desirable criteria

7 – Better than present minimum criteria

6 – Equal to present minimum criteria

5 – Somewhat better than minimum adequacy to tolerate being left in place as is

NBI Appraisal Ratings (scale 9 to 0)

4 – Meets minimum tolerable limits to be left in place as is

3 – Basically intolerable requiring high priority of corrective action

2 – Basically intolerable requiring high priority of replacement

1 – Not used

0 – Bridge closed

VTrans’ Bridge Inspection Charge

Four teams consisting of at least one individual meeting the minimum qualifications specified with the National Bridge Inspection Standards

http://www.fhwa.dot.gov/bridge/nbis.htm

Inspection records and bridge inventories are prepared and maintained in accordance with the standards

http://www.fhwa.dot.gov/bridge/nbi.htm

Each structure required to be inspected shall be rated as to its safe load carrying capacity

Why aren’t all structures rated?Missing information or lack of record plans

VTrans’ Bridge Inspection Charge

Provide technical assistance to bridge owners

Provide recommendations to DMV for oversize / overweight movements and routing on interstate and state highways

Prepare inspection results and summaries for bridge owners

Conduct and develop, with a multidisciplinary team, scour determination and plans of action (POA)

Inspection Results and Summaries

For each bridge inspected, a structure inspection, inventory and appraisal sheet will be created and sent to the respective bridge owner (DTA or Selectboard chair)

To get a sense of the immediate and future needs, the inspection summary and needs section is key – these are the inspectors comments.

Don’t wait for a special letter.

Preventative Maintenance – Why? 1987 Inspection

Preventative Maintenance – Why?1991 Inspection

Rate of Deterioration? 2001 Inspection

Preventative Maintenance – What?

Smooth approaches and wearing surface to reduce impact damages

Plug wind slots in concrete curbs and drain off ends of the deck

Maintain runner planks on timber decks

Maintain a crash worthy rail system on and off the bridge

Wash bridges in the spring of every year

Apply water repellent every other year on all exposed concrete surfaces

Preventative Maintenance – What?Spot paint steel members or grease to protect the steel

Keep the bearing areas clean and take measures to reduce debris buildup or moisture on and around the bearings

Maintain protective rip-rap countermeasures

Monitor for scour after high water events and place concrete sub-footings as needed

Maintain leak free roofs and sideboards on covered bridges

Maintain deck joints and ensure that they do not leak

Remove debris from steel or timber members

Questions?

Pamela Maza Thurber, P.E.Bridge Management and Inspection Engineer1 National Life Drive – Structures SectionMontpelier, Vermont 05633

Telephone: 828-0041Fax number: 828-3566Email: [email protected]

Thank you

national bridge inspection program - university of...

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