TECHBRIEF Application of Radiographic

Testing to Multilayered

Gusset Plate Inspection

FHWA Publication No.: FHWA-HRT-12-071

FHWA Contact: Justin Ocel, HRDI-40, (202) 493-3080, 

justin.ocel@dot.gov

Introduction

On August 1, 2007, the I-35W bridge collapsed in Minneapolis, 

MN. The bridge was a three-span (255- by 456- by 255-ft) continuous subdivided Warren deck truss over the Mississippi 

River. The National Transportation Safety Board (NTSB) 

investigated the collapse and leveraged technical assistance 

from the Federal Highway Administration (FHWA). NTSB determined that the cause of the collapse was due to undersized 

gusset plates at a particular node location on the main truss. 

The plates should have been twice as thick or should have 

had twice the yield strength to provide adequate resistance 

to prevent the collapse.(1)

It was noted during the investigation that the gusset plates 

of one particular bottom chord joint (L11 joint) had isolated 

corrosion where the gusset plate intersected the bottom 

chord. While it was determined that this isolated section 

loss did not contribute to the collapse mechanism, it was 

troublesome to NTSB that the corrosion was not noted in the 

Minnesota Department of Transportation inspection records. 

The L11 joint geometry was such that inspectors could not 

see the interior of the connection to document the corrosion 

and could not quantify the remaining section for load rating. 

To minimize this potential widespread problem, NTSB issued 

the following recommendation to FHWA at the conclusion of 

the collapse investigation:

“Require that bridge owners assess the truss bridges 

in their inventories to identify locations where visual 

inspections may not detect gusset plate corrosion and 

where, therefore, appropriate nondestructive evaluation 

technologies should be used to assess gusset plate condition.”(1)(p. 153)

In response to this recommendation, FHWA issued Technical 

Advisory (TA) 5140.31 on January 29, 2010, to provide guidance to bridge owners as to which nondestructive evaluation 

(NDE) technologies can be used to supplement gusset plate 

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.tfhrc.gov/research/
2

inspections when visual techniques are not 

feasible.(2) The TA recommended the use of 

ultrasonic testing (UT) to determine the remaining section thicknesses in gusset plates that 

have areas that cannot be seen. Ultrasonic 

thickness measuring devices (also known as 

D-meters) are attainable, inexpensive, and easy 

to use.

UT relies on imparting high-frequency elastic 

stress waves into a material and using sensors 

to measure the response. Reflections of the 

stress waves from voids (i.e., cracks, corrosion, 

etc.) appear as peaks in the frequency spectrum. 

The D-meter is the simplest of UT devices; it 

measures the reflection of stress waves off the 

back of a steel plate and calculates the thickness 

using equations from the theory of elasticity 

and the known longitudinal stress wave velocity of steel. The primary limitation of UT is that 

the stress wave attenuates in the gap between 

multiple layers of plates (excluding acoustic 

coupling, which can occur in closely mated 

surfaces like pins and hangers). For the inspection of trusses, if the connection uses singlelayer gusset plates, typical UT is sufficient. 

However, when the gusset plate is built from 

multiple layers of individual plates (sometimes 

referred to as “shingle” or “nested” plates), UT 

can only assess section loss in the plate that 

the sensor is coupled to. FHWA acknowledged 

that there was not a suitable technology available for the evaluation of the remaining section 

in multilayered gusset plates.(2) This TechBrief 

explores the use of radiographic testing (RT) 

to identify if it may be suitable for inspecting 

multilayered gusset plates.

Statement of Work

In response to the I-35W collapse, FHWA and 

the American Association of State Highway 

Transportation Officials conducted an experimental program to investigate the structural 

behavior of gusset plates.(3) This research 

program tested 13 gusset plate connections 

to failure, meaning they could no longer support external loads. Four of these experimental 

connections had simulated section loss in the 

gusset plates to investigate the role of deterioration on the overall behavior of the connection. 

This offered a unique opportunity to test NDE 

technologies on real gusset plate connections 

in a controlled environment with a known level 

of section loss. The following sections describe 

the results attained through RT of two multilayer gusset plate specimens.

Description of Test Specimens

Two specimens (307SS3-1 and 307SS3-3) with 

simulated corrosion were used in this study. 

A schematic of the overall connection is shown 

in figure 1. Two gusset plates were used to bolt 

together five built-up I-shapes and boxes. A 

shingle plate was added to each gusset plate 

but did not cover the entire gusset plate. 

Both the primary gusset and shingle plate had 

a nominal thickness of 0.375 inches.

Figure 1. Overall connection geometry used in testing.
3

To simulate section loss from corrosion, a 

fixed pattern was milled out of the back of each 

primary gusset plate. The back of the gusset

plate faces the interior of the connection, 

which is the area that was difficult to inspect 

in the I-35W bridge. Each of the two specimens 

used a different simulated corrosion pattern, as shown in figure 2 and figure 3. The 

remaining thickness in each of the simulated 

corrosion areas was measured with a D-meter. 

The D-meter was also used to verify the 

thicknesses of the gusset plates and shingle 

plate. The thicknesses of specimen 307SS3-1,

specimen 307SS3-3, and the shingle plate 

were 0.371, 0.370, and 0.375 inches, respectively. 

The shingle plate was reused during RT for 

each specimen.

Description of RT

Computed radiography with a gamma ray 

source was used to evaluate the remaining 

thickness of the primary gusset plate that 

was partially hidden behind a shingle plate. 

The advantage of computed radiography is the 

imaging plate is rapidly converted into digital

data for immediate post-processing, which is 

not possible with conventional radiographic 

film that would have to be developed. The 

source was placed on the outside of the connection, and the imaging plate was placed on the 

inside of the connection. 

In this case, the gamma rays were directed 

through the steel plates. The intensity of the 

gamma rays that reached the imaging plate 

was proportional to the density (or thickness) of 

the steel they passed through.(4) A Selenium-75 

radiation source was used, which, at the time of 

testing, had decayed to a strength of 23 Curies. 

All imaging plates had 0.010-inch-thick lead 

screens (front and back) to reduce the effects 

of scattered radiation. The imaging plates were 

14 by 17 inches to cover an area large enough 

to quantify the shape and magnitude of any 

section loss while minimizing the number of 

shots required. The distance from the source 

to the imaging plate was 30 inches, and 

each shot required a 20-min exposure period. 

The Selenium-75 source only required a 50-ft 

standoff radius to establish a safe radiation 

exposure zone around the connection. This 

could mean that for deep deck trusses where 

the lower chord joints are being inspected, 

it may be feasible to perform gamma ray RT 

without lane closures. However, each bridge 

presents its own unique attributes in terms of 

when and how RT would be performed. 

Four images were taken of specimen 307SS3-1 

at different locations of the connection, and 

five images were taken of specimen 307SS3-3. 

Two examples showing the placement of the 

imaging plate and actual RT grayscale image 

are shown in figure 4 and figure 5. In each 

case, the outlines of the gussets, shingles, 

members, and simulated section loss are 

shown. Even the strain gauge cables (used for 

the National Cooperative Highway Research 

Program Project 12-84) affixed to the back of 

the gussets can be seen in figure 4.(3)

Figure 2. Corrosion pattern for specimen 307SS3-1.

Figure 3. Corrosion pattern for specimen 307SS3-3.
4

RT Results

The images created by the radiographic imaging

process provided qualitative results, as the 

milled-out areas and all other areas of different 

thicknesses are clearly visible. However, quantitative results are desired in order to judge the

magnitude of the section loss. To convert the 

grayscale images to an image of remaining section thickness required multiple post-processing

steps. The first step included converting the 

grayscale image into pixels with heights and 

widths of 0.05 inches and assigning each pixel 

a grayscale density value ranging from 0 to 

1,023 (using a 10-bit analog to digital converter) 

based on the grey level in the original image.

It would be expected that regions of equal thickness should have very close grayscale density 

values. However, the raw images showed much 

wider ranges than expected; areas of known 

constant thickness showed a radial gradient 

of density about a point directly in line with 

the radiation source. This occurred for two reasons: (1) the intensity of the gamma ray source 

was inversely proportional to the square of the 

distance between the source and the imaging 

plate and (2) as the angle between the source 

and a point on the imaging plate increased, the 

gamma ray traveled through the plate thickness 

at an angle, thus having an apparent increase 

in density. The imaging plate used for specimen 307SS3-1 was full-sized, while the imaging 

plate for specimen 307SS3-3 was folded in half 

to produce a 7-by-17-inch image. In either case, 

this is a very large area to be covered by the 

Selenimum-75 source, and these radial gradient effects were to be expected. Not correcting 

for these effects can severely impact the ability

to accurately judge the magnitude of the 

remaining section thickness. 

Correcting Grayscale Values

Two corrections had to be applied to the original grayscale density values to correct for the 

effects described above. 

In order to compensate for the error stemming 

from the distance between the source and 

imaging plate, each measured grayscale density 

value was adjusted based on its radial position 

from the source using the equation in figure 6. 

The correction is a simple equation based on 

the Pythagorean theorem in three dimensions 

to account for inverse square decay of the 

gamma ray. Ideally, the exact position of the 

source relative to the film should be documented, which was the case for specimen 

307SS3-3. However, this was not documented 

for specimen 307SS3-1, and the source location was assumed to be in the center of each 

of the four images taken of this specimen.

Figure 4. Scan location and associated grayscale 

image of top northeast quadrant of specimen 

307SS3-1.

Figure 5. Scan location and associated grayscale 

image of bottom northeast quadrant of specimen 

307SS3-3.

densitycor1 = densitymeasured

 (Δx)

2

 + (Δy)

2

 + R2

Figure 6. Densitycor1.
5

Where:

densitycor1 = Modified grayscale density after the 

first correction is applied.

densitymeasured = Raw grayscale density value 

from the analog to digital conversion.

Δx = Horizontal distance between the pixel 

from the theoretical projection of the source 

onto the film.

Δy = Vertical distance between the pixel and 

theoretical projection of the source onto the film.

R = Distance between the film and gamma 

ray source.

To correct for radiation not passing orthogonally 

through the plate, the densitycor1 value had to 

be modified with a second correction. Without 

this correction, the steel would have an apparent 

increase in thickness since the density is proportional to the real gamma ray path length through 

the plate, not the plate thickness. The formulation of the second correction term also relies on 

the Pythagorean theorem, as shown in figure 7. 

Note that the terms used in figure 6 and figure 7 

are shown schematically in figure 8.

Where:

densitycor2 = Final modified grayscale density 

after the second correction is applied.

Converting Grayscale into Physical Values

The total corrected grayscale density values

themselves are not useful for judging the 

amount of section loss present within a plate. 

To be useful, they must be converted into 

physical thickness units. This is done by correlating corrected grayscale density values 

to known thicknesses at multiple points and 

using linear regression to attain a conversion 

between density and thickness. Due to a lack 

of data, higher order fitting algorithms were 

not investigated. Depending on the image, two 

or three calibration points were generally used 

from the following four possibilities depending on the image: (1) air (zero thickness), (2) a 

point on the gusset plate, (3) a point on the 

shingle plate, and (4) the flange of the vertical 

member.

Figure 9 shows the first of two color contour 

plots developed from the grayscale image 

shown in figure 5 based on uncorrected data. 

For this particular image, the source was located 

approximately -0.75 inches on the x-axis and 

-6.90 inches on the y-axis. The plates and the 

simulated section loss should have uniform 

thicknesses; however, on the right side of the 

plot, the thickness appears to be increasing. 

This would be expected in the uncorrected 

image because this part of the image is the 

furthest from the gamma ray source. The same 

effect is not as apparent on the left side of 

the image because the image in mostly air. A 

contour plot using the corrected grayscale 

data is shown in figure 10. With the correction 

applied, the uniformity of the individual element thicknesses is restored on the right side 

of the plot, particularly in the region of simulated corrosion covered by the single plate. 

The accuracy of the RT thickness estimation 

was assessed for each of the nine images collected by determining the remaining thickness 

of the simulated corrosion regions captured in 

each image. Table 1 outlines the results from 

the nine images of the uncorrected RT data, 

corrected RT data, physical measurement 

taken with a D-meter when the connection 

 densitycor2 = densitycor1R

 √(Δx)

2

 + (Δy)

2

 + R2

Figure 7. Densitycor2.

Figure 8. Variables used in grayscale density 

corrections.
6

was disassembled, and the difference between 

the corrected RT and D-meter thickness. In the 

majority of cases, the corrected RT data were in 

better agreement with D-meter measurements, 

especially since many of the uncorrected images 

from specimen 307SS3-3 that predicted the 

simulated section loss had a negative thickness. 

It should also be noted that the source locations are unknown for the four images taken 

of specimen 307SS3-1, and this could weigh 

heavily on the prediction. Except for two outlier 

points, many of the RT thickness predictions 

were within 0.04 inches of the D-meter measurement. FHWA’s Bridge Inspector’s Reference 

Manual does not provide a specific accuracy at 

which section loss must be evaluated, though 

it does indicate that the dimensions of gusset 

plates should be measured within an accuracy 

of 0.0625 inches.(5) This is probably intended to 

be the planar dimensions of the plate, not its 

thickness. The current practice to evaluate the 

remaining section in hidden areas would rely 

on large outside calipers, which have an accuracy in the range of 0.0625 inches. Therefore, 

while it may seem that the RT evaluation has 

a large error based on the plate thickness, 

its accuracy is about the same or better than 

current practices. Additionally, RT evaluation 

can provide much more data on the shape of 

the section loss compared to a caliper.

Results

Overall, RT of multilayered plates works very 

well for identifying the pattern of section loss. 

The quantification of the remaining section 

thickness had an accuracy of ±0.04 inches, 

although there were two outlier points with 

much greater error. It is assumed that with 

more careful documentation of the inspecFigure 9. Thickness of bottom northeast quadrant 

of specimen 307SS3-3 using uncorrected data.

Figure 10. Thickness of bottom northeast quadrant 

of specimen 307SS3-3 using corrected data.

Table 1. Comparisons of section loss estimations from all images.

Specimen Image

Uncorrected 

Thickness from 

RT (inches)

Corrected 

Thickness from 

RT (inches)

D-meter 

Thickness 

(inches)

Difference 

Between 

Corrected RT and 

D-meter (inches)

307SS3-1

Image 1 0.310 0.327 0.245 0.082

Image 2 0.180 0.160 0.176 0.016

Image 3 0.255 0.255 0.236 0.019

Image 4 0.197 0.201 0.236 -0.035

307SS3-3

Image 1 0.233 0.239 0.199 0.040

Image 2 -0.094 0.187 0.199 -0.012

Image 3 -0.208 0.161 0.199 -0.038

Image 4 -0.151 0.173 0.199 -0.026

Image 5 0.028 0.032 0.199 -0.167
7

tion and refinement of inspection procedure, 

RT will be a viable inspection technique for 

multilayered gussets.

It should be noted that the identification of 

all simulated section loss was incomplete in 

this evaluation. When the RT was performed 

on both connections, the imaging plate was set 

on top of the chord member even though the 

simulated section loss was purposely milled 

into the gusset plate below this level. This 

was done because the chord box section had 

chamfered corners that could collect debris 

and be a theoretical area of corrosion. Figure 11 

illustrates the blind spot of the RT encountered 

in this evaluation. 

Recommendations

Until further research can be conducted to identify NDE technologies that can evaluate the 

remaining thickness behind multiple layers of 

plates, RT appears to be a viable tool used 

to determine section loss and its magnitude 

(area and depth) in multilayered gusset plate 

connections. The film sizes used in this project 

were up to 17 inches wide. These large films 

were purposely used to quantify the accuracy 

of RT over large areas. Correction of the images 

must account for the radial degradation of the 

gamma ray strength away from the source as 

well as the angled path of the gamma rays 

through the plates.

The simplest solution to mitigate the remaining

section thickness errors is to use a stronger 

radiation source and larger standoff distance 

between the source and the gusset plate. This 

would lead to a more uniform distribution of 

radiation on the surface and thus eliminate 

the need to correct for it. Conversely, a larger 

number of closely spaced images with a less 

intense source could be used. The tradeoffs 

associated with each mitigation strategy must 

be balanced based on the geometry of the 

truss, available time for inspection, and available funds for inspection, as well as whether 

the bridge can be closed to traffic. 

Future Work

Additional work must be performed to develop 

procedures that can estimate the section loss 

that occurs between the gusset plate and a 

member. Based on the results provided in this 

TechBrief, RT inspection was not able to expose 

the entire area of section loss that extended 

into the faying surface between the chord and 

gusset plate. The only way to mitigate this is to 

take an angled shot through the chord; however, this approach causes shadowing from 

the fasteners and chord plates, which would 

make predicting section loss in these areas 

especially challenging if not impossible. In 

addition, the evaluations were performed 

using cleaned plates with no rust product. It 

would also be worthy to investigate the role 

rust may have in hindering the ability to assess 

the remaining section in RT images.

References

1. National Transportation Safety Board. 

(2008). Collapse of I-35W Highway Bridge: 

Minneapolis, Minnesota, August 1, 2007, 

Highway Accident Report NTSB/HAR-08/

03, Washington, DC.

2. Federal Highway Administration. (2008). 

Load-Carrying Capacity Considerations of 

Gusset Plates in Non-Load-Path-Redundant 

Steel Truss Bridges, Technical Advisory 

5140.29, U.S. Department of Transportation, 

Washington, DC.

Figure 11. Blind spot of image.
8

Researchers—Justin Ocel of FHWA was the principal investigator. The radiographic imagery was 

provided by Todd Kupfer and Dan McLeod of Applied Technical Services. Zachary May and Brian Moore 

assisted in post-processing the images.

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

distribution is being made to the Divisions and Resource Center.

Availability—This TechBrief may be obtained from the FHWA Product Distribution Center by email to 

report.center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at http://www.fhwa.dot.

gov/research.

Key Words—Gusset plate, Bridge inspection, Radiography, Truss bridge, Nondestructive evaluation.

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 the Government, industry, and 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.

october 2012 FHWA-HRT-12-071

HRDI-40/10-12(300)E

3. Transportation Research Board. Guidelines 

for the Load and Resistance Factor Design 

and Rating of Riveted, Bolted, and Welded 

Gusset-Plate Connections for Steel Bridges, 

NCHRP Project 12-84, National Cooperative 

Highway Research Program, Washington, DC. 

Obtained from: http://apps.trb.org/cmsfeed/

TRBNetProjectDisplay.asp?ProjectID=2509. 

4. Kinsella, T.J. (2002). Nondestructive Testing 

Handbook Volume 4: Radiographic Testing, 

Chapter 7: “Principles of Film Radiography,” 

American Society for Nondestructive 

Testing, Columbus, OH. 

5. Federal Highway Administration. (2012). 

Bridge Inspector’s Reference Manual, Vol. 2,

Report No. FHWA-NHI-12-050, Federal Highway Administration, Washington, DC.