CHAPTER 4 TYPE II BEARING EXPERIMENTAL RESULTS
4.5 LARGE DISPLACEMENT RESPONSE WITH UNSEATING AT PTFE
Four tests were performed to unseating of Type II bearings at the sliding interface between the top and middle plates. Three of the four tests were performed on a longitudinal
orientation. If a Type II bearing unseats in the longitudinal direction, the consequences may not be catastrophic for the bridge because the bottom flange can fall on the middle plate (although impact and concentrated loads would need to be considered for a bridge girder, especially if the design had required web stiffeners at the bearing support location). Each of the longitudinal unseating tests was also tested to ultimate strength with reversal post- unseating, to investigate the behavior and capacity of the bearing in the unseated configuration.
The force versus displacement results obtained from the Type II 7c longitudinal unseating test (7c [1] (6)) are shown in Figure 4.13, together with the results from the last large displacement longitudinal test (see Appendix B) for comparison. The test consisted of a single ramp to unseating in the (−X) direction at approximately −400% ESS, followed by reversal in the (+X) direction. The peak load during reversal was limited by rupture of one of the concrete anchors when the top plate had returned to approximately −100% ESS. When one of the anchors ruptured, the bottom plate rotated on the concrete surface. The
remaining anchor restrained the bottom plate so that it could not slide freely on the concrete. Consequently, the rotated configuration of the elastomeric block was forced to resist a disproportionate load at the side nearer to the remaining anchor. The elastomer began to shear internally, resulting in shear capacity degradation starting around −20% ESS.
Figure 4.13. Force versus displacement for unseating and reversal with Type II 7c bearing, longitudinal orientation.
4.5.2 Type II 9a, Longitudinal Unseating
The complete force versus displacement results obtained from the Type II 9a longitudinal unseating test (9a (4)) are shown in Figure 4.14. A portion of this test data has already been noted in a previous section (see also Appendix B), but the plotted data had been truncated so that the initial sliding and evolution of PTFE damage and removal could be seen more clearly. This test was driven to unseating after the PTFE had been fully removed with cycles up to 450% ESS. Unseating occurred at about +575% ESS, and the top plate was driven back against the elastomer block. The initial stiffness of the response from about +575% to about +375% represents the shear stiffness of the full height of the elastomer block. Initially, the edge of the steel fixture to which the top plate was anchored pressed against the middle plate, but at about +375% ESS, the steel fixture slid forward and over the middle plate until the side of the middle plate and the upper portion of the elastomer block were constrained against the underside of the steel fixture and the side of the top plate. The top plate then
drove against the elastomer directly, but the effective height of the elastomer was reduced by the height of the top plate, causing an apparent increase in stiffness. The peak load during reversal was limited by rupture of one of the concrete anchors when the top plate had returned to approximately −200% ESS. Similarly to the 7c unseating test, the resulting displacement of one side of the bottom plate on the concrete created a stress concentration, and the elastomer began to shear internally, resulting in shear capacity degradation starting around −100% ESS.
Figure 4.14. Force versus displacement for unseating and reversal with Type II 9a bearing, longitudinal orientation.
4.5.3 Type II 13a, Longitudinal Unseating
This longitudinal unseating test was run separately from the cyclic test to investigate PTFE damage and failure, using a +8 in. offset for the initial position. The data for the unseating test (13a (5)) are shown in Figure 4.15, together with the data obtained from the previous cyclic longitudinal test (13a (4); also see Appendix B). Test 13a (5) was driven to unseating with a monotonic ramp in the (+X) direction after the PTFE had been fully removed with cycles up to 600% ESS in the previous 13a (4) test. Unseating occurred at about +725% ESS, and the top plate was driven back against the elastomer block. The top plate thickness was 2-1/2 in. compared with the combined height of 3 in. for the reinforced elastomer block and middle plate, so the shear transmitted from the top plate passed through only about 1/2 in. of elastomer to reach the bottom plate. For this reason, the response is stiffer
immediately after unseating when compared with the shear stiffness observed during 13a (4) at reversals between sliding segments. The peak load occurred at about +585% ESS, when one of the anchors ruptured. This ESS value is based on the full elastomer height, but with only 1/2 in. effective, the adjusted estimated shear strain in the elastomer between the top and bottom plates is approximately 525% to displace from +725% to +585% ESS
relative to the initial zero position centered on the bearing. Unique to this test, the resulting stress concentration was not sufficient to initiate rupture in the elastomer, and the second anchor failed at approximately +550% ESS relative to the initial zero position. The bearing was then free to slide on the concrete, and the slip segment with a resistance of about 32 kips (approximate sliding friction coefficient of 0.32 with vertical load of 100 kips) is representative of the subsequent sliding mechanism.
Figure 4.15. Force versus displacement for unseating and reversal with Type II 13a bearing, longitudinal orientation.
4.5.4 Summary of Unseating Test Characteristics
Summary values for the Type II bearing unseating tests are provided in Table 4.4. The first fields list the displacements at which instability and unseating occur. Instability is the first instance at which the calculated horizontal load crosses zero (i.e., where an external horizontal force is required to act in the opposite direction from the current displacement direction to maintain equilibrium with simulated vertical load and mechanical forces developed in the deformed elastomer). Values are provided in terms of both absolute displacement and ESS. For Type II bearings, the nominal displacement capacity is 100% ESS, according to the IDOT Bridge Manual (IDOT 2012a). All limits for instability are far in excess of the nominal displacement capacity for the respective bearings. Plate overlap is determined according to the measured relative positions of the top and middle plates. For 7c and 9a bearings, the values represent the combination of the PTFE overlap and the tapered edge of the middle plate (1 in. for 7c, 1-1/2 in. for 9a). The shaded cell for the 13a is an approximate value, determined visually from video capture during the experiment and examination of the specimen after the test. The top plate experienced a slight slip from the etched region where the PTFE had been applied, and it caught on the tapered edge of the middle plate at the instability limit. The value shown represents the overlap of the top plate
on the tapered edge of the middle plate during the deformation from the instability limit to unseating. The peak loads are the recorded maximum values at incipient failure of the first anchor for each test, and the maximum average shear stress is determined by dividing the peak load by the nominal elastomer area.
Table 4.4. Summary Data for Type II Unseating Tests
Dx (in.) ESS (%) Dx (in.) ESS (%)
7c[1] (6) 6.2 330 7.4 395 1.7 40.3 0.48 9a (4) 10.8 575 10.8 578 1.9 64.4 0.60 11a (5) 15.4 772 16.2 812 0.28 13a (4) 12.7 680 13.6 724 0.875 122.2 0.47 Max. Avg. Shear Stress (ksi) Displacement to: Instability Unseating Test Plate Overlap at Unseating (in.) Peak Load (k)