Testing Procedure 51

The first step in testing a girder was positioning it in the reaction frame. In an effort to reduce the time and cost of testing each girder a method was devised in cooperation with the Civil Engineering Machine Shop to position the girder without disassembling the reaction frame. This was completed by using a series of carts in combination with the overhead crane to position each girder. Figure 4.19 shows a photograph of a girder being positioned for testing. After the girder was positioned under the reaction frame formwork and reinforcement were placed for the casting of the concrete deck. Concrete for the deck was placed using a concrete bucket with a chute because the bucket could not be placed directly over the deck due to interference with the reaction frame. After the deck had cured under moist burlap for several days, the steel bearing

Figure 4.19 Girder Positioning for Testing

plates were placed on the surface of the deck with hydrocal so that the hydraulic cylinders could be positioned. A load cell was attached to the piston of each hydraulic cylinder and would bear against the reaction beam. Load cells with a capacity of 60 kips were attached to the double acting cylinders while 100 kip load cells were attached to the single acting jacks.

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The surface of each girder was painted with a mixture of white latex paint and water to increase the visibility of cracks on its surface. A 10-inch square grid was also drawn on both sides of the girder web. The grid points were numbered the same on both sides of the girder so that photographs could be correlated after testing. Next all instrumentation devices and targets were attached to the girder, including LVDTs, Krypton LED targets, Zurich Gage targets, and concrete surface gages. After all devices were attached the instruments and strain gage wires were attached to the data acquisition equipment.

Up to four different data acquisition systems were utilized to record the data for an individual girder. The first system consisted of two National Instruments (NI) SCXI-1001 12- slot chassises that were connected in series and connected to a PC running a data acquisition program that was written in LabVIEW. This system recorded all of the information from the strain gages that were attached to the reinforcing steel, LVDTs, and load cells. The strain gages were connected to directly to NI SCXI-1314 terminal blocks which provided quarter bridge completion resitors. The SCXI-1314 terminal blocks were then attached to NI SCXI-1520 8- channel universal strain modules. External signal conditioning was provided for the LVDTs and the load cells such that only voltages were supplied to the NI system. Two NI SCXI-1102 32- channel 10 volt analog input modules connected to NI BNC-2095 rack mountable terminal blocks were used to measure the signals from the load cells and LVDTs. Data were recorded with this system at a rate of 1 Hz for the entire duration of each test. The LabVIEW data acquisition program enabled the person controlling the test to monitor data in real time. Monitoring the data during a test was critical to insure the safety of researchers, enable the applied load to be properly distributed between hydraulic systems, and assist in determining when failure would occur.

A second NI data acquisition system consisting of an SCXI-1001 12 slot chassis with SCXI-1520 modules and SCXI-1314 terminal blocks was used to record the data from the concrete surface gages. This system was utilized in addition to the first system so that the

sampling rate of the system could be increased easily as failure approached. Strain data from the concrete surface gages was taken at a rate of 1 hertz for most testing and was increased to a rate as high as 100 hertz as failure approached. A third NI system that consisted of an SCXI-1000DC chassis along with an SCXI-1540 8-channel LVDT amplifier module and a SCXI-1315

connector block was used in conjunction with a program written in Visual C++ to record readings from the Zurich Gages. The fourth and final system that was used to acquire data during testing included the controller and computer that were supplied with the Krypton System. The software and data acquisition programs were prepared by the manufacturer and used to record position data for each of the LED targets at a rate of 1 Hz for the duration of a test. Figure 4.20 shows a photograph of the load controller and the primary NI data acquisition system.

In the days leading up to a scheduled test a short pretest exercise was carried out to ensure that both the loading system and data acquisition systems were performing as expected. During the pretest a small amount of load (1 to 2 kips / foot) was applied to each girder. This allowed researchers to check the hydraulic system for leaks, a critical step given that there were more than 150 connection points where leaks were possible. Immediately prior to the first day of testing initial readings were taken on all of the LVDTs and the load cells and strain gages were nulled and shunt-calibrated. Two full sets of Zurich Gage readings, each consisting of 1058 individual readings, were also taken to establish the initial distances between the targets.

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Figure 4.20 Primary Data Acquisition System and Loading System Controller When a test was started the applied load was increased until the first diagonal cracks formed. The formation of the first web shear cracks was easily identified by an audible cracking sound, visual inspection, and sudden changes in the values of LVDTs and strain gages. First cracking was defined as “load stage 1” at which time the mid-span displacement was held constant, cracks were marked, crack widths were measured, and photographs were taken. Additional “load stages” were held when significant additional cracking had occurred, local damage was observed, or after a significant increase in applied load or mid-span displacement had taken place. Zurich Gage readings were taken at the additional “load stages” in addition to the tasks completed for “load stage 1.” The Zurich Gage readings were omitted from “load stage 1” since little information could be gathered due to the small deformations that were associated with first cracking. Testing continued until one end of the girder failed, at which time the applied loading was removed.

The repair procedure for each girder differed slightly depending on the level of damage the girder sustained. The first step in repairing a girder was removing all of the loose and damaged concrete from the failure region; a roto-hammer with a chisel bit was used to remove the damaged concrete and to roughen the surface of undamaged concrete in the repair area. Then a 6-inch square grid of #3 bars was added to each side of the web, the grid was spaced a distance of 5 inches from the surface of the web. If the bottom bulb of the girder was heavily damaged the flexural capacity would be enhanced by placing #9 bars along the top surface of the bulb. If the bottom bulb was primarily intact the prestressing strands were externally anchored to prevent slip. Self consolidating concrete was then used to cast a repair that ranged in length from 10 to 16 feet. The repair region was then vertically post-tensioned by using HSS sections and Dywidag bars located on 2 foot centers. Figure 4.21 shows various photos of girder repairs. Once the girder was repaired the girder could be reloaded until a failure occurred at the end opposite the repair.

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(a) Grid of Reinforcement (b) Formwork

(c) External Strand Anchorage (d) External Shear Reinforcement Figure 4.21 Girder Repair Photographs