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OCR for page 30
30 (a) FL2R (Proposed EZR) (b) FL2L (LRFD EZR) Figure 3.27. Comparison of ends FL2R and FL2L. Figure 3.25. End VA1R after bearing failure. The corresponding image, Figure 3.30, shows the shear the girders to contain only half the amount of shear reinforce- cracks experienced by End FL2R just before the loading was ment requested, leading to the premature failure of the ends stopped. The figure illustrates how the shear cracks form in in shear. One girder was observed to contain half of the spec- the opposite direction as the end zone cracks. ified shear reinforcement when it burst in shear failure. How- End FL1L unexpectedly failed in shear due to the lack of ever, the girder that was tested first did not fail, keeping the adequate shear reinforcement. Images of the shear failure are reinforcement hidden, so it is only assumed that it also con- shown in Figure 3.31. One can see how the prestressing force tained half the specified shear reinforcement. In most cases, pulled the bottom flange in toward the center of the girder the design capacity of the specimens was more than the test once there was no web to resist it. This is the same force that setup could apply. The team decided to continue with test- pulls on the web of precast girders causing end zone cracking. ing and use the first half of the load versus deflection curves Once the reinforcement had been exposed, the team was able to determine when the girders would fail. to calculate the theoretical shear design capacity for the girder Figure 3.29 shows the load-deflection curve for End FL2R. to be around 700 kips. The experimental failure point was still The loading of this end had to be stopped before failure greater than the calculated shear capacity value. because the three hydraulic jacks had reached capacity at 1,200 kips. However, it can be inferred that if more load 3.2.5 Full-Scale Testing Conclusions had been applied, the test values would have risen above the experimental values. The full-scale tests on eight full-scale girders has indicated that end zone cracking due to prestress bursting forces does (a) FL1L (FL Typical EZR) (b) FL1R (FL Modified EZR) Figure 3.26. Support with roller. Figure 3.28. Comparison of ends FL1L and FL1R.
OCR for page 31
31 1200000 1000000 Load (lb) 800000 600000 Capacity Using Capacity Using Estimated Estimated Values Values (1,195 (1,195 k) k) 400000 Capacity Using Measured Values (1,229 k) System Capacity (1,200 k) 200000 Load vs. Deflection 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 1 Deflection (in) Figure 3.29. Load versus deflection curve for End FL2R. not cause a reduction in the structural capacity of prestressed concrete girders. The orientation of the diagonal cracks is nearly perpendicular to the forces caused by diagonal tension (i.e., shear). When external loads are applied, they induce compressive stresses across the bursting force cracks, and therefore the types of cracks are not cumulative to each other. Even when end zone cracks were induced in the testing that were significantly larger than cracks commonly observed in practice, there still was not a measurable reduction in struc- tural capacity. All specimens had capacities at or higher than the expected theoretical capacity. When repairing was performed with epoxy injection in an attempt to restore concrete tensile capacity across the cracks, there was no significant change in capacity between repaired (a) General View (b) Close View Showing Relative Movement of the Bottom Flange Figure 3.30. Shear cracks on End FL2R after loading was stopped. Figure 3.31. End FL1L after failure, east side.