6. Testing of Bridge Model Specimen
6.1 Measurement Method
Optical measurements were used instead of electronic string potentiometers or LVDT's because of the large number of measurement points located along the measured chord. These optical measurements were made using a Leica NA2 auto-level and self-fabricated precision scales placed on the bridge and on stationary reference locations.
The Leica NA2 auto-level has a 32x magnification and an accuracy of 0.3 mm, with a minimum focal length of 1.6 m. This level was used together with a Leica GMP3 optical micrometer, which attaches to the NA2 auto-level and doubles the magnification and increases the measurement accuracy. The auto-level and micrometer setup was supported by a standard surveying tripod. Figure 6.1 shows the Leica equipment used.
Eighty scales with 0.254 mm (0.01 in.) graduations were used to measure the deflection of the specimen chord. The scales were suspended from lightweight steel T-shaped towers in pairs at 40 locations along the chord; 36 along the stringer plies and 4 on the ends of each pile cap. One scale was attached to each end of the T shape; the scales were allowed to rotate freely relative to the T tower. These towers were attached to the specimen with small nails. Each ply of the chord had nine towers attached. The towers were oriented the same distance along the specimen as neighboring plies, resulting in nine lines of four towers along the specimen. To prevent the layered scales from blocking the optical measurement, the width of the T towers increased for each ply from 51 mm, 76 mm, 102 mm and 127 mm (2 in. to 3 in., 4 in. and 5 in.) for the outer ply inward. Each group of four towers was referred to as a "scale tree" because of their appearance. Figure 6.2 illustrates a "scale tree" and an enlarged single scale.
Each span of each stringer had three measurement positions, two near the ends of the span and one at mid-span. Thus, three measurement positions for three spans resulted in nine measurement locations for each stringer. Figure 6.3 illustrates the nine locations along the bridge specimen where the "scale trees" and pile cap scales were located.
Position 1 of each span was located 25 mm (1 in.) north of the southern pile cap of that span, position 2 was at mid span, and position 3 was 25 mm (1 in.) south of the northern pile cap. The spacing is the same for all three spans and all four stringer plies.
Each scale is labeled according to which span, span position, and ply they are on and which side of the tower they are on. An example label for the south side, mid-span scale on the middle span of the outer most ply is: BD8-2S because it is located in span BD on stringer ply 8, at position 2, and attached to the south side of the T tower. The scales suspended on the pile caps are designated by the gridline the cap is on and which side they are suspended on. Thus Cap A-N corresponds to the scale on pile cap A attached to the north side. Although the labeling system is intricate it was necessary to identify all 80 scales. Hanging the scales in pairs from the T towers allowed the deflection of the specimen to be calculated as the average change in scale readings at each location with a negative value corresponding to downward deflection.
Because of the large number of scales and measurement points it was necessary to have multiple locations to set up the auto-level to allow clear viewing of every scale. Figure 6.4 illustrates the positions of the auto-level locations that were used relative to the model bridge specimen. To minimize the duration of each load test, three locations of the auto-level were used. One location was near the south end of the specimen, one was near the north end of the specimen; and one was near the center of the specimen. The locations were identified as locations 1, 2, and 3 respectively. Location 3 was approximately 8 in. lower to the ground than the others because of the drastic elevation change between the stringer scales and the pile cap scales.
Location 1 allowed for reading all hanging scales at the south end, location 2 allowed for reading all hanging scales at the north end and location 3 allowed for reading all of the pile cap scales.
Load was applied to the specimen using the hydraulic bottle jack and proving ring as described in section 5.1. Also a load-distributing apparatus was between the bottle jack and the test specimen. The loading setup that was used is illustrated in Figure 6.5. The load distributor consisted of a 76 mm (3 in.) diameter steel pipe with a 102 mm (4 in.) square by 6 mm (0.25 in.) flat plate welded on at mid length of the pipe to allow the bottle jack to set vertically. The steel pipe was supported by two solid steel bars that were 19 mm (3/4 in.) square and 203 mm (8 in.) long. These steel bars were centered above each chord and rested on the three crossties nearest to the load location. This was done to distribute the load in a similar manner as the steel train rails of field bridges. It was also assumed that the load distributor evenly distributed the load between the two bridge chords. Figure 6.6 illustrates how the load is distributed between the crossties of the specimen.
The loading system allowed for the natural distribution of load to occur through the crossties and stringers as well as for the possible uneven rotation of each chord.
Load testing of the completed specimen consisted of a sequence of four load tests. The first load test was used to develop the pattern in which the optical readings were taken. The other three load tests were used to measure the specimen's behavior from the loads applied at different locations. Three load locations were used for the four load tests. Figures 6.7, 6.8 and 6.9 illustrate the three load locations.
Load test 1 consisted of applying a 2224 N (500 lb) load at load location 1, at mid-span of the center span. Elevation readings were recorded from the 80 scales described in section 6.3. Readings were taken before and after the constant load was applied, it was assumed that 1112 N (250 lb) would act on each chord of the bridge model. This loading served as a trial run for the testing setup and the optical data acquisition technique. Applying only 2224 N (500 lb). did not impose high stresses on the specimen. As a result of this test the pattern in which measurements were made in the following three load tests was modified. The three auto level positions were established during this load test.
Load tests 2 and 3 consisted of applying a 13344 N (3000 lb) load level to the specimen at different locations. Load test 2 was conducted at load location 1; load test 3 was at load location 2, at mid-span of the northern span. In each load test, the load was applied in three 4448 N (1000 lb) increments. A complete set of elevation readings was recorded before and between each load increment, as well as after the load was removed. Four hundred elevation readings were taken for each load test. The incremental load allowed for the investigation of any non-linear deflection and support movements that might occur.
Load test 4 consisted of applying 6672 N (1500 lb) at load location 4, directly above pile cap B, in 2224 N (500 lb) increments. This load location is directly above pile cap B as shown in Figure 6.9. The lower load level was used because of the load location. Most of the load was resisted by one bent directly, rather then distributing the load between the nearest bents. As in load tests 2 and 3, four hundred elevation readings were taken before, during and after the incremental loading.