Tools for Evaluation of Steel Truss Bridge Gusset Plates

Christopher HigginsQuang NguyenO. Tugrul Turan

School of Civil and Construction EngineeringOregon State University

OUTLINE

Introduction & Background

Digital Imaging Tools

CAD- FE Analysis

Summary and Future Work

INTRODUCTION & BACKGROUND

Introduction & Background

• Gusset plate connection performance of interest.

•Structural evaluations require accurate as-built data.

•Collection of field data is time consuming and expensive. Results likely to be highly variable.

•Rating methods rely on specification-based on design provisions. Need for improved tools.

Collapsed I35W Bridge in Minneapolis, MN

Overall Approach

•Capture images in field with digital cameras (snooper, climbing, from ground level)

•Process images into scaled orthophotos (MatLab)

•Query and extract geometric data (MatLab)

•Directly produce CAD drawings (AutoCAD)

•Directly produce FE models (ABAQUS)

•Run nonlinear finite element analyses to rate connections (ABAQUS)

•No high-level training required (scripted CAD drawing and FE modeling)

•Integration from field inspection to rating and archiving

Collecting Field Data is a Challenge

Inspectors Get Creative

Is there a better way?

Methodology

• Gusset plates lie in plane : 2-D correspondences simplify transformation

• Point correspondences used to map points based on central projection.

• Need to establish control points in the real world image.

X’ is point in photographic image plane (pixel)

X is point in “real world” imageHartley and Zisserman, Multiple View Geometry in Computer

Vision, 2004

Methodology

• Relate image points (pixels) to real world control points via transformation matrix

• Remove image perspective (rectification)

•Direct Linear Transform (DLT) algorithm

•H is 3x3 transformation matrix

• Need minimum of 4 control points (8 DOF and 1 for scale)

• If over constrained then can estimate errors.

X’ and Y’are point coordinates onphotographic image plane (pixles)

'

'

1 1

i i

i i

X X

H Y Y

X and Y are point coordinates inreal world image

Methodology

8” and 24” Reference Targets

• Ultra Corrosion-Resistant Pure Titanium (Grade 2) bars

• Nine aluminum control points

• Safety chain and vise-grip clamp

Reference Targets

Base magnetically attaches and sits 2” off of the face of the plate

Methodology

Methodology

Methodology

Root-Mean-Square Error:

where θ’ is the predicted value, θ is the true value, and n is the sample size.

Standard deviation:

where e is the location error, ē is the average location error, and n is the sample size.

2

1

1( )

n

ii

e en

2

1

1( ' )

n

i ii

RMSEn

Error Estimation

Methodology

Location Uncertainty:

Measurement Error for Length Measurements:

where RMSE is the predicted error value, 3σ provides a 99.7% confidence interval for a normal distribution, and the given pixel size.

Measurements of length require two points thus double the location uncertainty.

2 3measurementE RMSE pixel

int 3poE RMSE pixel

Error Estimation

Methodology

Standoff correction•Targets are offset from plates

•Image dimensions are shorter than true dimensions due to standoff

• Correction may be performed:

actual imageoffset

DL L

D D

Actual dimension onplate

Distance to plate, D

Standoff distance of targets, Doffset

Image dimension

Implementation

Consumer grade camera• Single-lens reflex (SLR)

digital camera (Nikon D200)

• Nikon AF NIKKOR 50mm 1:1.8D lens

• Resolution: 3872x2592 Pixels (10 Megapixel)

Fill the image frame with the gusset plate as much as possible.

Take a well focused image. Take the best available image

with the camera as orthogonal to the gusset plate surface as possible.

Deploy flat-field lenses.Take at least one physical length

measurement on the gusset plate (and mark on the plate so it is visible in the image).

Take a measurement of the plate thickness, as this cannot be determined directly.

Map corrosion damage, if any.

Implementation

MATLAB w/ Image Processing Toolbox - Gusset Plate Rectification Tool

• Loads gusset plate image

• Calculates H matrix

• Calculates the measurement error and standoff correction

• Creates and saves rectified scaled orthographic photograph

• Allows measurements

• Length

• Perimeter

• Fastener locations

• x, y coordinates

• Angles

• Outputs all data into an Microsoft Excel file

Software

Implementation

Microsoft Excel – LSP file creator

• Take data from Gusset Plate Rectification Tool and create a LSP file to be used by AutoCAD to create a scale drawing of the Gusset Plate

• Also creates LSP file for ABAQUS FE analysis

Software

Implementation

Mock Gusset Plate:• Connection U3• Southbound Booth

Ranch Bridge on I-5 Oregon.

• As-constructed drawings from March 1967

Example

Example

Design Drawing Typical View of Connection

Example

Image Rectification Tool

MATLAB program

Example

Example

Camera 3 – 15ft @ 45° Off-center

Example

Camera 2 – 250ft @ 0° Off-center

Typical View of Connection

Compare Design with As-Built

Extra bolts!

Work point shifted due to member alignment!

Plate clipped differently

Remember I35W?

ODOT Field Trials X Y

-17.733230.1962

6

-18.06 27.0375

-18.386823.5519

6

-15.119130.1962

6

-15.554826.9285

7

-15.663723.6608

9

-12.39630.3051

9

-12.7228 27.0375

-12.831723.7698

1

-9.6729230.0873

4

-9.6729226.9285

7

-9.9996823.6608

9

-6.9498430.1962

6

-7.0587626.8196

5

-7.1676923.7698

1

-4.2267730.1962

6

-4.22677 27.0375

-4.3356923.6608

9

-1.6126129.9784

2

-1.61261 27.0375

-1.6126123.6608

9

1.219383 29.8695

1.328306

26.71073

ODOT Field Trials (hidden edges)

ODOT Field Trials

Outside Agencies and Consultants

US Army Corp Unknown Consultant

Close-Range Requirements (climbing)

+

Combine Multiple Images

=Also can get 3D

Mosaic from Multiple Images

Rectified Images Become Models

* FE Model created from panoramic image

Structure, Member, Connection, Fastener

FEA

Nonlinear FE Modeling and Analysis

X,Y coordinates (digital image),

Thickness of plate (field measured), Material

(drawings),and Member forces (stress

sheet)

Automatic Output Generation (Strength)Multiple Members : von Mises Stresses

Validating Methods with Full-Scale Experiments at OSU July 2010

SummaryMethod for rapid and accurate collection of

field geometry of gusset platesImages are rectified and scaled to permit

metrificationData query export via Excel to CAD and FEACompare as-built connections with design

drawingsAnalyze and rate connections directly using

FE methodsQuantitative imaging enriches field

inspection and evaluation

Acknowledgements

Bruce Johnson – Chief Bridge EngineerSteven Soltesz – Research Coordinator

Oregon Transportation Research and Education Consortium

Questions?