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From page 34... ...
33 3.1 Proposed Design Rules Research showed that truck passage over a bridge causes shear strains in the bearing due to axial load effects that are much larger than the shear strains due to rotations. This fact causes difficulties in basing the design procedures directly on test results, because appropriate rotation data are available from the tests conducted for this study, but comparable compression fatigue data are not.
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From page 35... ...
• Introduce a new check for hydrostatic tension stress to guard against internal rupture of the elastomer in bearings that have external plates and are subjected to light axial load and large rotations, and • Eliminate the absolute compressive stress limit (presently 1.6 ksi or 1.75 ksi) to encourage the use of bearings with higher shape factors for high load applications.
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From page 36... ...
Restriction against uplift is necessary if the bearing has external plates bonded to it, because large rotations combined with light axial load could lead to hydrostatic tension and brittle internal rupture of the elastomer. Previously, this provision was unnecessary, because the no uplift provisions prevented such behavior.
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From page 37... ...
3.2 Design Examples A spreadsheet was prepared for designing bearings, based on the proposed design provisions in Appendix G For any set of loadings, the spreadsheet computes the total shear strain and the hydrostatic tension stress.
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From page 38... ...
proposed Method B approaches. It does not satisfy the existing Method A because the axial stress/GS exceeds the limit of 1.0.
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From page 39... ...
To any of these totals must be added the allowance for misalignment of 0.005 rad., which should always be taken in the sense that is most disadvantageous. The misalignment allowance is small (approximately 1⁄8 in.
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From page 40... ...
The bearing may also be evaluated under the proposed Method A Results are as follows: No bonded external play exist OK The total (nonamplified)
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From page 41... ...
The axial strain [Eq.
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From page 42... ...
This is significantly larger than the ±1.0 inches of thermal expansion, so provision should be made for resetting the girder on the bearings during construction. This problem is more likely to occur in steel bridges because they are typically more flexible than concrete ones.
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From page 43... ...
The rotations in the local bearing axes (perp and para) are obtained by using the transformation matrix: The total amplified rotations, including the misalignment allowance, are therefore The shear deformations are obtained using the same transformation matrix The amplified axial stress is For shear strains on the long side (rotation about the weak or para axis)
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From page 44... ...
Trial and error shows that a bearing of 30 in. × 15 in., with 11 layers at 0.5 in.
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From page 45... ...
The shim thickness, based on Fy = 36 ksi, must satisfy This thickness requires 10 gage sheet steel, which is slightly thicker than the 11 gage commonly used for convenience in manufacturing. It illustrates the need for thicker shims when the axial stress on the bearing is high.
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From page 46... ...
Shear strain due to rotation is Shear strain due to shear displacement is The total shear strain is The bearing has no external plates, so hydrostatic tension does not need to be checked. Stability criteria are satisfied.
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From page 47... ...
− Second is to core the center of one of the large bearings. Portions of the bearing core could be tested in shear and compression to provide the information needed through the thickness of the bearing.
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