|
Fiberglass Bridge Testing Report August 1, 1997 The No Name Creek fiberglass bridge in Russell Co. was first structurally
tested on 11-19-96. It was retested on 5-8-97. These tests consisted
of parking the 40,000 pound rear axles of two fully loaded dump
trucks on the bridge and measuring midspan deflections. The two
lane 27 foot wide bridge spans 22 feet and is constructed of three
fiberglass sandwich panels each measuring 23 feet long and 9 feet
wide. Traffic flows east and west on the bridge. Deflection data
was taken at five points on the midspan with mechanical dial indicators.
In addition to these structural tests the bridge has been monitored
for any indications of creep. The results and reports of these
tests follow. Creep Testing. The long term creep deflection data shows no trend at this time in the test program. There are differences in the data taken on the different days but they can be explained by differences in the weather and temperature when the data was taken. The first data was taken on a sunny 50 degree day and the rest were taken on overcast/rain or partly cloudy days. When analyzing the data one notices that the bridge has moved up and down considerably. It is believed that a logical explanation for this motion can be found in the weather and the short term temperature history of the deck before test time. In the past it has been noticed that the bridge bows upward when the upper surface is warmed by the sun. It is expected that this same behavior will be seen in the data the next time we measure the bridge on a sunny day. The following is the raw creep data measured on the test days.
The locations are equally spaced along the centerline of the bridge
from east to west. The measurements are inches from the test jig
beams under the bridge to the bottom of the bridge deck, measured
with an inside micrometer. The pad measurements are done with
the depth gauge end of a dial caliper. Test Dates, Weather/Temp
On the 5-22-97 date the bridge was monitored for 5 hours. The
weather was 100% overcast with on and off rain for the entire
test. The air temperature fell and then rose from 61.0 to 59.7
to 61.3 degrees during the test period. The upper deck temperature
followed from 61.0 to 58.8 to 60.4 degrees. The deck temperature
lagged behind the air temperature as expected. The center of the
bridge, location 3, dropped .012 inches then recovered to .009
inches down. The motion of the bridge also followed and lagged
the deck temperature. This would be expected due to the thermal
mass of the bridge. The deck temperature was measured at the upper
surface. Structural Testing. The first structural test's, 11-19-97, weather was sunny, overnight low of 30, afternoon high 60, temperature at the 2:00 pm test was 50 degrees. The second structural test's, 5-8-97, weather was partly cloudy with overnight low of 45, afternoon high 75, test time air temperature of 71.1 degrees. The deck temperature was 71.4 degrees. The dial indicators were placed on the mid-span of the bridge at the north and south edges, at the center line, at the joint between the center and south panels, and at the center of the south panel. The dial indicators were zeroed when installed. Zero load deflection readings were recorded just before the first truck was parked on the bridge. The first truck was parked in the south lane facing east with the back axles centered over the mid-span. The outside edge of the tires was 50 inches from the south edge of the bridge. Deflection measurements were recorded. The second truck was parked in the north lane facing west. Deflections were recorded with both trucks on the bridge. The first truck was removed from the bridge and the deflection data for the second truck was recorded. The second truck was removed and zero load deflections were again recorded. The deflections reported here for the first test from north to
south across the midspan have been corrected for the initial non-zero
readings of the dial indicators. As the bridge's upper surface
warms in the sun the mid-span bows upward. During the time between
noon and 1:30 PM the bridge mid-span moved upward by as much as
.090 inches.
The larger deflections on the north edge can partially be explained by the placement of the second truck, it being closer to the edge of the bridge. It also had a slightly larger load. The south panel does seem to be slightly more stiff. This is not an indication of a problem, just an observation. The maximum deflection of .181 inches with an applied load of 85,000 pounds yields a very respectable span/deflection ratio of 1450. The data from the second structural test follows. Note: The S
edge deflection was measured at 6" from the edge instead of 2"
from the edge as in the earlier test. All of the other test points
are identical to the first test. The loads are similar but not
identical. Each truck's outside wheel was 50 inches from the edge
of the bridge.
The second structural test data is very similar to the original,
no significant difference can be detected. The larger value for
the 2 truck reading South edge can be explained by the different
placement of that dial indicator. Visual Observations. It has been observed that people have been shooting at the bridge. There is very little damage from this. There are three spots that show effects of bullets. The first is on the guard rail, a slight scratch of the resin and paint from a bullet that bounced from one slope of the W rail to the other slope on the same rail. The intended target was one of the reflectors which show numerous bullet holes. This first damage was noted in May. In July more damage was found as follows. The second spot is on the bridge structure. The lower face of the sandwich protrudes slightly (1/4 inch) on the north side of the bridge, A bullet was fired at close range straight down at the edge of this protrusion. It left a 1/8 inch deep trace with some exposed fibers. The third spot is on a north side center guard rail post. A bullet was probably fired at close range at the center of the post. A slight (3/16 inch Dia) delamination of the top layer of fibers shows. It appears that the bridge is very damage resistant, none of these
three spots should affect the life of the bridge. And I believe
that the damage is minor enough that the vandals will not continue
to "test" the bridge since they get very little reward (damage)
for their actions. |