Digital Image Correlation Application to

Structural Health Monitoring

Erin Santini-Bell, Ph.D., P.E. , University of New Hampshire

Philip A. Brogan, New Hampshire Department of Transportation

Paul J. Lefebvre, Mitchie Corporation

Jason Peddle, University of New Hampshire

Brian Brenner, P.E., Fay, Spofford and Thorndike, INC.

Masoud Sanayei, Ph.D., Tufts University

Presentation Outline

• Background

• Digital Image Correlation

• Verification Tests

• Field Applications

– Vernon Ave Bridge in Barre, Massachusetts

– Belknap Mountain Rd in Gilford, New Hampshire

• Conclusions

2

Motivation

• 1 in 3 Bridges Near End of Design Life (FHWA, 2009)

• Entering Rebuilding Phase

• Increased Public Awareness

• Advances in Digital Imaging Technology

• Opportune Time to Consider Changes to Management Paradigm

Tobin Memorial Bridge

3

Bridge Management

• Bridge management includes

– Visual Inspections

– Load Rating

– Rehabilitation

– Overload permitting

– Maximum-load postings

– Salting

4

Bridge Inspections

• Bridges are inspected every 2 years – Initial inspection – Routine inspection – Special inspection

• Condition Ratings (AASHTO Manual for Bridge Evaluation, 2008)

5

Traditional Bridge Monitoring

• Traditional Instruments

– Linear Varying Differential Transducer (LVDT)

– Strain Gauge

– Accelerometer

– Visual Observations

– Non-Destruction Evaluation Techniques

6

What is Digital Image Correlation?

• Employs Digital Photography

• Measures deformation of surfaces

• Ability to measure stress, strain, and displacement of a field of view

• Results are visual and objective

An Optical Non Contact Method

7

Digital Image Correlation (DIC)

8

How Does it Work - Equipment

DIC Cameras

Displacement Target

9

Digital Image Correlation (DIC)

• Uses multiple cameras to create a 3D image • Calibration process defines camera orientation • Target is covered with a high-contrast speckle pattern • Post-processing software calculates the pixel

movement throughout images • Calculated data includes:

– Displacement – Velocity – Acceleration – Strain – Rotation

10

How Does it Work – Post Processing

Correlated Solutions, Inc.

11

Laboratory Verification

12

• Purpose of experiment was to address accuracy concerns with 3D DIC

• Is 3D analysis as accurate as 2D at this scale?

• Need to be able to trust the data

LVDT

Calibration Card

LOAD

Laboratory Verification

13

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100 120 140 160

Dis

pla

cem

en

t (m

m)

Time (s)

Beam Displacement by Method of Measurement/Calibration Target

4mm

6mm

7mm

9mm

LVDT

Calibration Std Dev Analysis Projection Error

4mm Target .014 .017

6mm Target .017 .019

7mm Target .021 .015

9mm Target .020 .018

DIC Applications

• Deflection measurements were collected using DIC on…

– A 3 span steel girder bridge for model verification

– A single span concrete bridge with FRP strips for retrofit assessment

14

Vernon Avenue over the Ware River Bridge Barre, MA

15

Image Courtesy of Google Maps

WM Barre Landfill

Vernon Avenue Bridge (VAB) • 6 Steel Girders with Reinforce Concrete Deck

• Beams and Deck are

Composite

• 3 Continuous Spans • Field splice in center

span

• 150 Feet Long with 75 ft Center Span

Vernon Ave Bridge, Looking South

16

Instrumentation Plan

17

0 10 11 12

9 8 7 6 5 4 3 2 1

NORTH SECTION GIRDER 2

TEMPERATURE GAUGE

ACCELEROMETER

TILTMETER

STRAIN GAUGE

LEGEND

PRESSURE CELLS

Strain Gauges

Steel Temperature

Concrete Temperature

Girder Section at Station 10

Instrumentation Summary

18

Quantity Instrument Type

100 Strain Gauges

36 Girder Temperature Sensors

30 Concrete Temperature Sensors

16 Accelerometers

16 Tiltmeters

3 Ambient Temperature Sensors

Non-Destructive Load Tests

19

2 Types of Tests • Stop Locations Tests • Crawl Speed Tests 3 Trials of each • One 72 Kips Truck

3 Lanes • West Shoulder • Center • East Shoulder

Load Test Lane and Stop Location Plan

NORTH ABUT

SOUTH ABUT

SOUTH PIER

NORTHPIER

0 4 6 8 10 12 14 2

LANE 1

LANE 2

LANE 3

20

Verification

Vernon Avenue Bridge – Barre, Massachusetts

• Deflection Measurements were collected using DIC and LVDTs simultaneously

21

Verification Results

22

Verification Results

23

Stop on Span 1

Stop on Span 2

Stop on Span 3

Enhanced Designer Model (EDM) Modeling Requirements

24

Girders: Frame Elements Deck: Shell Elements Boundary Conditions: Roller-Roller-Roller-Pin

SAP2000® Bridge Information Modeler (BrIM)

ERM Calibrated Model Stop Location Load Test Strains

25

Bottom Flange of Girder #2 at North Pier (Station 8)

Implementation

26

DIC Data for Independent Evaluation

27

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0 100 200 300

De

fle

ctio

n (

in)

Time (s)

Vertical Deflection of Vernon Ave Bridge near MidspanTruck in West Lane

DIC

Original

Updated

-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0 50 100 150 200 250 300 350

De

fle

ctio

n (

in)

Time (s)

Vertical Deflection of Vernon Ave Bridge at South SpanTruck in West Lane

DIC

Original

Updated

DIC as part of the Load Rating Process

• Similar ratings for Exterior Girders

• Interior Girders gain extra capacity in EDM Rating

• System Behavior • Stiffer Exterior Girder picks

up more load

• Lowers rating of Exterior Girders

• Increases rating of Interior Girders

28

LRFR vs. Baseline EDM Rating by Girder #

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

1 2 3 4 5 6

Loa

d R

ati

ng

Girder Number

EDM Rating

LRFR Rating

Glenn Bridge

29

• Located in Gilford, New Hampshire on Belknap Mountain Road

• 16’ Span – concrete slab on steel beams

• Retrofitted in Fall of 2010 – DIC & LVDT used to verify increase in strength

Glenn Bridge

30

• Steel beams were removed • Non-composite

• FRP strips installed on the underside • Dubois & King

• Sheet metal was glued to the bridge for a speckle pattern

Glenn Bridge

31

• DIC data follows the trend of LVDTs – not as clean

• This test revealed the lower deflection limits of DIC usage on bridges

• A heavier truck would have been beneficial during testing

Glenn Bridge

32

• DIC data follows the trend of LVDTs – not as clean

• This test revealed the lower deflection limits of DIC usage on bridges

• A heavier truck would have been beneficial during testing

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0 20 40 60 80 100 120

Def

lect

ion

(m

m)

Time (s)

Glenn Bridge Midway Measurements

Midway LVDT

Midway DIC

Conclusions

• Detailed 3D FEM of typical highway bridges is feasible to a high degree of accuracy

• Truck load testing performed on a newly constructed bridge can provide highly reliable strain data for calibrating baseline FEM’s.

• DIC data verified the FEM independently

• Calibrated FEM and load test strain data matched closely

• DIC can also be used to validate the use of innovative material.

33

Future Work

• Additional Verification Experiments

• Conduct a third load test at the Vernon Ave Bridge using DIC and applying the lessons learned through the verification experiments

• Load Rating Comparison: AASHTO: LRFR and DIC

34

Thank You for Listening

35

Acknowledgements

NSF CAREER Grant No 0644683, NSF-PFI Grant No. 0650258 MassDOT – Bridge Construction Town of Barre, MA – Owner Fay, Spofford & Thorndike, Inc. – Bridge Design Geocomp Corporation – Instrumentation Bridge Diagnostics, Inc. – Bridge Testing

Erin.Bell@unh.edu for further information, questions or comments

Backup Slides

36

Instrumentation

Strain Gauges

37

Instrumentation

Tiltmeter

Accelerometer

38

Instrumentation Concrete Temp

Pressure Cells

39

Model Updating

1. AS-BUILT MATERIAL PROPERTIES

3. BOUNDARY CONDITIONS

2. CONCRETE SAFETY CURB

4. DECK REINFORCEMENT

40

Material

PropertyOriginal Updated Units

Density, wc 2403 2236 kg/m3

Unconfined

Compressive

Strength, fc'

28 35 MPa

Modulus of

Elasticity, E 24856 26790 MPa

Degree of

FreedomOriginal Updated Units

Axial, Uz Fixed 560.5 kN/mm

Shear, Uy Free 0.992 kN/mm

Shear, Ux Free 0.992 kN/mm

Rotation, Rx Free 1.77E+06 kN-mm/rad

Rotation, Ry Free 1.77E+06 kN-mm/rad

Torsion, Rz Free 1.77E+05 kN-mm/rad

– Elemental Design

Effective width of slab, be

– Composite Behavior

– Distribution Factor, mg, and Impact Factor, IM

Current AASHTO Bridge Design Process

• Typical Design Assumptions

41

(Adapted from AASHTO LRFD,2008) 𝑃𝑢 ,𝑡𝑟𝑢𝑐𝑘 = η𝛾 𝑚𝑔 (1 + 𝐼𝑀)𝑃𝑡𝑟𝑢𝑐𝑘

Modeling for Bridge Management: AASHTO Load and Resistance Factor Rating (LRFR)

42

(Adapted from AASHTO Manual for Bridge Evaluation, 2008)

𝑅𝐹𝑀𝑜𝑑𝑒𝑙 =𝐶 − 𝐷𝐿𝑀𝑜𝑑𝑒𝑙

𝐿𝐿 𝑀𝑜𝑑𝑒𝑙(1 + 𝐼𝑀)

𝑅𝐹𝐿𝑅𝐹𝑅 =𝐶 − 𝛾𝐷𝐶𝐷𝐶

𝛾𝐿𝐿𝐿𝐿(1 + 𝐼𝑀)

Modeling for Bridge Management: AASHTO Load and Resistance Factor Rating (LRFR)

• γDC=1.25; γLL = dependent on rating type

• 2 Ratings:

o Inventory Rating:

– Based on LL that can safely utilize bridge indefinitely.

– RFINV => γLL=1.75

o Operational Rating :

– Based on maximum permissible LL.

– RFOP => γLL=1.25

• Elemental Approach—LL and DL Distribution Factors

43

(Adapted from AASHTO Manual for Bridge Evaluation, 2008)

𝑅𝐹𝐿𝑅𝐹𝑅 =𝐶 − 𝛾𝐷𝐶𝐷𝐶

𝛾𝐿𝐿𝐿𝐿(1 + 𝐼𝑀)

Modeling for Bridge Management: EDM Rating

• γDC=1.25 and

γLL=1.75 factored into applied loads

• DL from self weight of bridge

• LL applied to mimic 2008 LRFD Bridge Design Specs for Worst-Case Traffic Loading Scenario

• Accounts for System Behavior—No Distribution Factors

44

𝑅𝐹𝑀𝑜𝑑𝑒𝑙 =𝐶 − 𝐷𝐿𝑀𝑜𝑑𝑒𝑙

𝐿𝐿 𝑀𝑜𝑑𝑒𝑙(1 + 𝐼𝑀)

Field Displacement Data