Experimental Test Set-up 40

A steel reaction structure, shown in Figures 4.7 and 4.8, was designed and built to resist a uniformly distributed load of 60 kips / foot applied by a series of hydraulic jacks placed along

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the top of a girder. The frame consisted of twelve W12 x 65 steel columns post-tensioned to the strong floor at 9-foot spacing along the length of a girder and 4.5 feet on either side of the center- line of a girder. This provided six pairs of columns on either side of a girder. Twelve W18 x 119 steel sections (transverse beams) were used in pairs to span across a girder and connect each pair of columns using 7/8” A325 structural bolts. The transverse beams were designed to take a load of 540 kips and resist a bending moment of 1200 kip-ft. A 50-foot long W27 x 146 steel section (longitudinal beam) was placed underneath the transverse beams and provided the surface against which the hydraulic jacks reacted. The longitudinal beam was held in place by saddle clamps when a loading was not applied and had web stiffeners welded into place at each location where it was intersected by the transverse beams to prevent local buckling. The bottom flange of the longitudinal beam was also connected to the columns by tension members to decrease the unbraced length and provide stability against lateral torsional buckling.

Abutments were also designed and built to support the ends of the girders. Researchers expected that an end reaction in excess of 1000 kips could be obtained during testing. In an effort to prevent cracking of the strong floor the abutments were designed to span between the web walls of the strong floor. This allowed the load to be distributed directly to the web walls without introducing excessive flexural stresses into the top flange of the box girder that makes up the strong floor. Each abutment was fabricated from two 14-foot long W27 x 146 steel wide flange sections that were welded together along the top and bottom flanges to make a box

section. Additional 1” thick steel plates were welded to the top and bottom flanges of the section near midspan to increase the moment capacity of the abutments. The inside of the box sections also included deformed bar reinforcement to increase flexure and shear capacity and were filled with concrete. Filling the sections with concrete ensured that there would be no problems with local buckling or instability. The abutments were designed to support a 1500 kip load applied at the midspan of a 12-foot simply supported clear span.

Figure 4.9 Hydraulic Loading System

The distributed load was applied to a girder using a combination of 22 single acting 100- ton hydraulic cylinders and 22 double acting 60 ton hydraulic cylinders. Each of the girders was

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tested by controlling the midspan displacement. One of the double acting cylinders at midspan was outfitted with an LVDT (displacement transducer) to measure the position of the piston. The signal from the LVDT was fed to an Instron controller which implemented a PID control algorithm to command a servo valve that controlled the flow of oil to the ports of the control jack. The other 21 double acting jacks were supplied the same top port and bottom port pressures as the control jack through a series of 12 hydraulic manifolds. The 22 single acting cylinders were connected through a series of 6 manifolds and were controlled by a single pressure regulating valve. By using the force controlled single acting cylinders in combination with the displacement controlled double acting cylinders researchers were able to insure that the test was being conducted in a displacement controlled manner. This was accomplished by adjusting the load carried by the force controlled component of the system to assure that the load carried by the displacement controlled component of the system was always near 30 percent of the total applied load. Figure 4.9 shows the typical layout of the hydraulic cylinders and graphically presents the contribution of the different loading systems to the total load.

Figure 4.10 Hydraulic Cylinders Prior to Testing

The hydraulic cylinders were placed on 12-inch wide 1-inch thick steel bearing plates along the center line of the cast-in-place deck. The cylinders were arranged in pairs consisting of a single acting cylinder and a double acting cylinder. Figure 4.10 is a photograph of the hydraulic

cylinders placed on top of the cast-in-place deck prior to testing. On average the center to center spacing between cylinders was one foot with 22 cylinders on either side of midspan. This provided a uniformly distributed loading that was applied over the central 44 feet of the 50 foot span. This configuration for loading was typical and only altered to guard against a failure in a repaired region or to limit the possibility of a flexural failure. Figure 4.11 details the loading configuration details for each girder test.

Hydraulic oil at a supply pressure of 2900 psi was provided to the cylinders from a hydraulic power supply located in the service level of the Newmark Laboratory. This limitation on supply pressure was primarily due to the maximum pressure for which the servo valve was rated. Since the 2900 psi supply pressure was less than the 10,000 psi rated capacity of the cylinders the double acting cylinders were able to apply a load of 35 kips and the single acting cylinders could apply a load of 58 kips. This resulted in each pair of cylinders applying a load of 93 kips or a distributed load of 46.5 kips / foot.

43 Span Length = 50 ft Total Length = 52 ft a b c w Specimen

Loading Pattern Load

a (ft) b (ft) c (ft) w (kips/ft) G1E 3.0 44.0 3.0 26.03 G1W 3.0 44.0 3.0 30.09 G2E 3.0 44.0 3.0 33.79 G2W 3.0 44.0 3.0 38.73 G3E 3.0 44.0 3.0 35.68 G3W 3.0 44.0 3.0 38.82 G4E 3.0 44.0 3.0 42.73 G4W 3.0 44.0 3.0 42.73 G5E 3.0 44.0 3.0 23.70 G5W 3.0 44.0 3.0 19.91 G6E 15.0 32.0 3.0 38.32 G6W 3.0 44.0 3.0 27.85 G7E 3.0 44.0 3.0 33.47 G7W 11.0 28.0 11.0 44.75 G8E 13.0 28.0 9.0 43.72 G8W 3.0 34.0 13.0 32.70 G9E 3.0 44.0 3.0 32.80 G9W 3.0 4.0 26.0 10.0 21.0 36.0 37.20 22.32 G10E 3.0 44.0 3.0 33.93 G10W 3.0 43.0 4.0 42.85

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