Evaluation of the Segmental Casting Attachments, Wisconsin DOT Structure B-47-40, U.S. Highway 10 Over the St. Croix River, Prescott, Wisconsin, November 8 & 9, 1994

Publications

by D.W. Prine and J. Oleksy
Northwestern University, BIRL Industrial Research Laboratory
Project B570 - Task 1

ITI technical report no. ___

PURPOSE

A series of nondestructive tests were performed on the segmental casting attachments of the rolling bascule bridge (B-47-40) in Prescott, Wisconsin. These tests were performed to determine if the castings remain properly fixed to the girder during bridge open and closure operations. These segmental castings are attached to the bascule girder flange by means of high strength friction bolts. Wisconsin DOT personnel have observed problems on similarly constructed bridges that suspected to result from the inability of the friction bolts to properly fix the castings to the girder. In some cases the bolts are said to have failed.

INTRODUCTION

Wisconsin DOT structure B-47-40 carries east and westbound traffic on U.S. highway 10 over the St. Croix River in the town of Prescott in Pierce County, Wisconsin. The bridge consists of 5 spans and has an overall length of 682.7 feet. The center span (span 3) is a two leaf, rolling bascule lift bridge with an overall length of 205.5 feet. The bridge deck is 66 feet in width and has four lanes of vehicular traffic and a pedestrian sidewalk. The bridge was built in 1991 by Lunda Construction and was designed by Hazelet & Erdall.

The three piece segmental castings are attached to the bascule girders with high strength friction bolts. This is a relatively recent design modification (8 to 10 years ago) and replaces the traditional turned bolts or rivets that were commonly used in this application. We are told that Wisconsin DOT bridge inspectors have observed cases of bolt failure and casting slippage on similarly constructed bridges. These occurrences can lead to dangerous operating conditions. Concerns for safe bridge operation by Wisconsin DOT bridge inspection personnel led them to call upon BIRL engineers to apply advanced NDE technology to the Tayco Street lift bridge (WI-DOT structure B70-97-93) in Menasha, WI in September of 1993. Bridge operation and inspection personnel had observed loud audible impact noises during bridge operation. BIRL engineers applied acoustic emission monitoring techniques to determine that the source of the impact noises was the high strength bolts. Strain gages were applied to the casting/girder interface and large permanent displacements of the casting with respect to the girder were observed. This work was performed under sponsorship of Northwestern University's Infrastructure Technology Institute (ITI).

The loud impact noises that were observed on the Tayco Street bridge were not observed during operation of the Prescott bridge on a preliminary visit in September of 1994. However, continuing concerns on the part of WI-DOT inspection personnel over the performance of the friction bolts led to an agreement with BIRL to perform the type of testing that was previously applied to the Tayco St. bridge on this structure.

On November 7, 1994 BIRL engineers traveled to Prescott Wisconsin. Tests utilizing acoustic emission and strain gage monitoring were performed. Additional experiments were performed using a displacement sensor based on laser triangulation. The tests were completed on November 8 and 9, 1994.

SUMMARY OF RESULTS

Acoustic emission testing showed that the number of AE events and their relative energy were much lower than the activity previously observed on the Tayco Street bridge. The strain gage data taken on the Prescott bridge showed three significant departures from similar tests on the Tayco St. bridge. The peak strain on Prescott was 50 to 75 micro-inches per inch while Tayco St. was 600 to 1200 micro-inches per inch. Secondly, the Prescott strain gage data showed even symmetry and good repeatability on the strain wave forms recorded during raising and lowering while poor odd symmetry and no repeatability was observed in the Tayco St. data. The Prescott strain gages returned to their original zero within the quantization error of the monitoring system (approx. 10 micro-inches per inch) while the Tayco St. strain data was offset by as much as 150 micro-inches per inch following a complete bridge cycle. Finally, the laser displacement gage which had source and target mounted on opposite sides of the casting to girder interface and aligned parallel to this interface allowed us to observe the elastic deformation of the casting under dynamic loading conditions during bridge open and closure and should easily be capable of detecting slippage of the casting. The shape and symmetry of both the laser gage and the strain gages agreed well and the laser gage showed no appreciable slippage between the girder flange and the casting following a complete bridge operating cycle. The laser gage typically showed some offset when the bridge was at the maximum opening point (mid-range). The one exception to this observation was on the S.W. Corner of the structure. The gage was mounted on the center casting in all tests. We believe the offset observed at the bridge up position is the result of load sharing between the castings that is fostered by the wedges applied between castings. The absence of this mid-range offset on the S.W corner may be indicative of improper wedge operation between the center and the third casting. The fit of that wedge should be checked and re-adjusted if necessary since load sharing between the casting segments is beneficial to long term proper operation of the bridge.

These test results indicate that under current operating conditions the segmental casting attachments on the Prescott bridge (B-47-40) exhibit no abnormal behavior as evidenced by AE, strain gage, and displacement testing. However, future re-testing would be prudent based on past experience with this design.

ACOUSTIC EMISSION TESTING

AE testing was performed on each of the four casting assemblies using the field portable AE monitoring system previously developed by BIRL's D. W. Prine. This system has been described in detail in previous reports provided to WI-DOT by BIRL. The test procedure was developed during a series of experiments performed on a similar bridge in Menasha Wisconsin during September of 1993. AE sensors were attached to each of the three casting segments. An additional sensor was attached in the vicinity of the pinion gear on the upper part of the bascule girder. The sensors were 175 KHz resonant piezoelectric devices. Silicone grease was used for acoustic couplant and magnetic hold downs were used to clamp the sensors to the structure. Unity gain line driving pre-amplifiers were used in each signal line to eliminate cable loading effects on the sensors. AE system gain was set at 40 dB on each channel and was checked using an electronic pulser and AE transducer as a simulated AE source. AE data was recorded on disk files using a portable PC attached to the AE monitor via an RS232C serial port. The recorded data was analyzed post test. The casting mounted sensors were used to detect any impact or fretting events related to the casting attachments. The pinion mounted sensor was utilized to intercept drive gear and deck related AE signals and to act as a guard for the casting mounted sensors. The AE parameters of interest for these tests are hits and average relative energy. A hit is defined as the receipt by one sensor of an AE burst. The sensor that first receives the burst is the one most closely located to the AE source. The arrangement of sensors for these tests insures that any signal that hits sensor #1 first (#1 is mounted on casting segment #1) has to originate in that casting segment. The same holds true for the remaining two casting mounted sensors. Bursts that arrive at the sensor mounted near the pinion gear first, originate either in the drive gear, or the upper bridge structure. FIGURE 1 shows the sensor layout superimposed on the bascule girder drawing. Sensors 1, 2, and 3 were mounted on the three casting segments with 1 on the segment that is engaged when the bridge is in the normal down position and the others on the center and upper segment respectively. Sensor 4 is mounted near the pinion gear.

FIGURE 2 shows the total count of AE hits at each of the 4 sensors. FIGURE 3 shows the average relative AE energy for the events received first at each sensor. Both figures show the data for the four tests performed on the Prescott bridge and the test data from the Tayco St. Bridge for comparison. Examination of the three casting mounted sensors hits and energy shows that the Prescott bridge had both lower hit counts and much lower energy. The difference in AE test data between the two bridges is even more apparent if we look at the product of event counts and their average energy which is a relative measure of the total detected AE activity resulting from a complete bridge operating cycle. FIGURE 4 shows this result for the two bridges. Clearly, the Prescott bridge has less overall AE activity.

STRAIN GAGE MONITORING

A Measurements Group quarter bridge type CEA-06-W250A-350 weldable foil strain gage was mounted diagonally across the center casting to bascule girder mating surface on both the north and south corners at the east end of the bridge. This application is a non-standard approach to using strain gages. The purpose is to attempt to observe displacements between the casting segment and the girder flange. The observations are more qualitative than quantitative because the gage is not subjected to a uniform strain field across the gage width. This mounting approach was developed at the suggestion of consulting engineers from Hazlet and Erdal during experiments performed on the Tayco St. Bridge in Menasha, WI.

Strain gage data was recorded using a Somat model S2000 field computer in the time history mode. The S2000's programmable digital low pass filter was set at 15 Hz cut-off to eliminate electrical noise. The strain gage signals were sampled at a 100 Hz rate with 8 bit resolution.

The casting segments on this bridge did not mate flush with the edge of the girder flange. Offsets of between 1/8 and 1/4 inch necessitated the use of shims to allow the gage to be mounted on an acceptably flat surface. This procedure added considerable time to the gage mounting procedure. The results obtained with the laser gage (described below) on the first two test sites led us to discontinue the use of strain gages for this application after discussions with WI-DOT personnel.

The strain gage data are shown in FIGURES 5 and 6 for the south and north corners at the east end of the bridge respectively. Both data sets show even symmetry about the bridge open condition as well as a reversal as the bridge rolls over the gage site on the center casting. The peak strains are around 50 to 75 micro-inches per inch. Contrast these data with data recorded on the Tayco St. bridge shown in FIGURE 7. We no longer see the symmetry or local strain reversals and the peaks range from 600 to 1200 micro-inches per inch. The large step like jumps in the Tayco St. data were accompanied by loud audible impact noises that were not present on the Prescott bridge. Offsets of over 150 micro-inch per inch between start and finish of the test are seen in the Tayco St. data while the offset in the Prescott data between start and finish is within the quantization error of 10 micro-inch per inch. These data indicate that the segmental casting attachments are performing much closer to the desired operation than the Tayco St. Bridge attachments.

LASER DISPLACEMENT SENSOR

In order to obtain a quantitative measurement of the displacement between the center segmented casting and the bascule girder flange it was desired to measure the change in position with a displacement sensor and not a strain sensor as was used previously. Two sensor types were considered. The first was a capacitive sensor that was sensitive enough to make the measurement, but was dependent on target condition and ambient temperature. The second sensor considered was a laser triangulation based system. Temperature does not affect the laser measurements and the laser systems have excellent resolution.

An Aeromat LM300 laser sensor was chosen to make the measurements. This particular model has 0.2µm (0.8 x 10-6 inch) resolution and works best with a white ceramic target, but can work with virtually any color target. The laser device works by detecting the apparent location of a 780nm (near infrared), 2mW laser line. As the target moves toward or away from the array detector, the spot appears to move as seen from an angular view. The laser head was mounted to the girder flange via a magnetic mount and the target was C-clamped to a rib on the center segmental casting (see FIGURE 8). The paint on the girder flange surface was removed to provide a clean surface for the magnet; however, with a more powerful magnet this may not be necessary in the future.

Data was downloaded directly to the laptop computer over an RS232 line. This method was slow and due to constraints of the laser control box is limited to about a 10 Hz data rate. By using the analog output of the device, future measurements may be possible at rates at least 100 times faster.

The laser sensor measures the displacement parallel to the casting/girder mating surface between the target mounting point and the laser head mounting point. The displacement in a system that is operating as intended should be solely due to elastic distortion of the casting as it is acted upon by the combined load of the bridge leaf and counterweight. This action should produce even symmetry about the bridges fully open point. The laser sensor data is shown in FIGURES 9, 10, 11, and 12. All of the plots show excellent even symmetry about the bridge open point as well a the characteristic reversal as the bridge rolls over the sensor's attachment point. Both the S.E and N.E. corners return to zero within 40 micro-inches. The N.W. and S.W. corners show offsets between start and finish of 260 to 280 micro-inches. All of the plots show some offset in the bridge up position with the exception of the S.W. corner (FIGURE 12). These offsets are probably due to load sharing by the casting segments. The lack of center offset on the S.W corner may be due to improper operation of the wedge between the number two and number three casting segments. We suggest that this wedge be examined for improper fit. The positive displacements shown at the start and finish of the N.W. data result from the difficulties experienced by the bridge operator in releasing and re-latching the span locks. The S.E and S.W. data follows a pattern that first swings positive and then negative, while the N.E. and N.W. data is reversed from this convention. This is caused by reversing the direction of the laser head and target that was needed because of physical mounting considerations.

The symmetry and very small start-finish offsets shown in the laser sensor data indicate that the bridge is probably operating normally.

Subject index terms

Bridges, Steel
Bridge inspection
Acoustic emission testing

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