7e.bridge.kosnik.pdf

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ICAE-6 Advances in Acoustic Emission - 2007

179 2007 AEWG, AE Group

ACOUSTIC EMISSION EVALUATION OF RETROFITS ON THE

I-80 BRYTE BEND BRIDGE, SACRAMENTO, CALIFORNIA

DAVID E. KOSNIK and DANIEL R. MARRON

Infrastructure Technology Institute, Northwestern University, Evanston, Illinois 60208, USA

Abstract

Fatigue-prone details on highway bridges present a challenge to engineers as structures ageand face increased traffic loads. The Bryte Bend Bridge (Caltrans Bridge B-22-26 R/L), a steel

trapezoidal box girder structure, which carries I-80 over the Sacramento River at Sacramento,California, contained fatigue-prone connections between the unusually thin girder walls and the

internal diaphragms. These details were slated for retrofit when very active cracking was dis-covered nearby. Engineers at Northwestern Universitys Infrastructure Technology Institute per-

formed acoustic emission (AE) tests before the retrofit, during evaluation of several retrofit de-signs, and after the final retrofit. Evaluation of the retrofit designs was particularly useful as it

was discovered that some of the proposed retrofit designs actually exacerbated crack activity.AE testing of the final design revealed that the retrofit was effective in reducing crack activity.

Keywords:Steel box girder, fatigue-prone detail, retrofit evaluation

Introduction

The Bryte Bend Bridge (Caltrans Bridge B-22-26 R/L), opened in 1971, carries Interstate 80

over the Sacramento River at Sacramento, California. I-80 is a truck route of national impor-tance; regionally, it is particularly important to agricultural trucking. Many of these agricultural

trucks are especially heavy, as produce such as tomatoes are shipped in water. According to a

2005 California Department of Transportation (Caltrans) report, the average annual daily trafficfor I-80 near the bridge is 84,000 vehicles per day, 9.57% of which is trucks; 45.89% of truckson the route consist of five or more axles [1]. An overall view of the bridge is shown in Fig. 1.

The bridge consists of twin 1235-m (4,050) welded trapezoidal steel boxes with compositeconcrete decks. Inside the box girder are K-shaped vertical stiffener cross frames. The box

girder walls are 9.5-mm (3/8) thick [2]. Fig. 2 shows the cross frames in the interior of the boxgirder. The twin bridges carry three lanes of traffic each.

An in-depth visual inspection by Caltrans personnel revealed cracking in the webs of the

trapezoidal box at the cross frame connections. Cracks typically initiated at the toe of the weldconnecting cross frame to the web. The cracking was attributed to out-of-plane bending at the

connection. Several cracks, which had turned into the flange, were later drilled [2].Crack sites on the bridge are identified by span number, cross frame (XF) number, and girder

number, where girder G1 is the left sloped web, G2 is the center vertical web, and G3 is the rightsloped web. Work described in this paper was carried out on spans 18 and 19.

Crack Characterization

Engineers from the Infrastructure Technology Institute (ITI) at Northwestern University first

performed AE tests at crack sites selected by Caltrans in 1993 and 1994. Data were acquired us-ing a Vallen Systeme AMS-3 acoustic emission (AE) monitoring system and 375-kHz piezoelec-

tric transducers. An AE transducer was placed at the visible crack tip and guard transducers


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ICAE-6 Advances in Acoustic Emiss ion - 2007

180

were placed in an array around the crack tip. AE first-hit channel analysis showed that thecracks were actively driven by live stresses in the structure [2].

Fig. 1: I-80 Bryte Bend Bridge overall view.

Fig. 2: Box girder (north half) interior showing cross frames.

AE Evaluation of Retrofit Prototypes

Because of the complex nature of the structure, Caltrans engineers opted to test prototypes of

two proposed retrofits on selected cross frames in span 19 of the Bryte Bend Bridge. Shoe platesand knee braces were added to the outside corners and both sides of the center web of cross

frame 3. Shoe plates and knee braces were also added to cross frame 4; additionally, the diago-nal braces on frame 4 were cut from their attachment points at the deck and reattached to a new

horizontal cross member.In 1996, ITI engineers assisted in the evaluation of these retrofit designs through AE and

strain gauge testing. First-hit channel (FHC) analysis was employed to distinguish crack-relatedevents from events originating elsewhere on the structure. The activity level for each crack is

given as the ratio (expressed as a percentage) of crack-related AE hits to total recorded AE hits[2]. The activity level at each crack is shown in Table 1 and Fig. 3.


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Table 1: Crack activity at prototype retrofit sites on span 19 (modified from [2]).

Crack Site % Crack Activity Be-

fore Modification

% Crack Activity After

Modification

XF3-G1 17.8 4.8

XF3-G2 29.6 3.0

XF3-G3 25.5 4.3

XF4-G1 4.8 3.4XF4-G2 5.3 6.4

XF4-G3 26.0 25.3

XF3-G1XF3-G2

XF3-G3XF4-G1

XF4-G2XF4-G3

After

Before0

5

10

15

20

25

30

%T

otalhits

Crack Site

Crack Activity

Fig. 3: Crack activity at prototype retrofit sites (modified from [2]).

The retrofit prototype installed at cross frame 3 was clearly shown to be superior to the proto-type installed at cross frame 4. In fact, the retrofit at cross frame 4 seemed to make the crack ac-

tivity level slightly worse on the center web connection.The value of AE testing is clearly illustrated in this prototype evaluation experience. On a

complex structure, it is often difficult to accurately predict the performance of a retrofit. AEanalysis, however, provides repeatable quantitative data on crack activity levels before and after

a fatigue-mitigation retrofit.

AE Evaluation of Full Retrofit

In 2004, Caltrans let contracts to retrofit all active and potential crack sites with the designtested on cross frame 3 as noted above. The retrofit is shown in Fig. 4.

ITI engineers were engaged to perform AE tests on five selected sites along span 18 of thebridge in September 2004, before the retrofit, and again in September 2005, after the retrofit. A

six-channel Vallen Systeme AMSY5 AE monitoring system was used for both tests. The six375-kHz piezoelectric transducers (with internal preamplifiers) were deployed as in an array for

FHC analysis as shown in Fig. 5, with transducer 1 as close as possible to the crack tip, transduc


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ers 2-5 forming a guard array around the crack tip, and transducer 6 mounted on the diagonalbrace to intercept any noise transmitted through the cross frame itself.

Fig. 4: Final retrofit design (modified from [3]).

Fig. 5: AE transducer array for first-hit channel analysis.

The transducers were acoustically coupled to the structure with silicone grease and were heldin place with magnetic clamps. At each site, system integrity was verified using internal calibra-tion and pencil-lead breaks; then, AE data were acquired for 30-60 minutes using a laptop com-

puter. Data were recorded in both statistical and digitized waveform formats.FHC analysis of the data taken before and after the full retrofit suggests that the retrofit was

quite effective in reducing fatigue crack growth. Each site showed a dramatic decrease in crackactivity as measured in number of AE hits per hour from the crack with peak amplitude above 55

dB, as shown in Fig. 6.


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