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31.1 Bridge Bearings Elastomeric bearings have been used in bridges since the late 1950s, and have grown in popularity so they are now the most common type of bridge bearing in all regions of the United States. They are capable of resisting typical bridge loads and accommodating deformations without the use of machined or moving parts, which largely eliminates any need for maintenance.
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without losing its shear flexibility. This is achieved by adding internal horizontal reinforcing plates known as shims in the bearing.
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tion always is preceded by the tensile debonding, so the one acts as a precursor of the other. The lateral expansion of the rubber layers causes tension in the steel plates.
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Common bearings have S in the range 3 < S < 8. The shape factor also provides a useful basis for normalizing the compressive stress, since the shear strain caused by compression is, according to small displacement theory, directly proportional to σ/GS.
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the bearing. If large rotations occur, one side of the sole plate experiences net upward movement, the bearing necessarily follows and the elastomer experiences direct tension.
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where θx = the rotation applied to the bearing about the x-axis, hri = the thickness of one rubber layer, and n = number of internal rubber layers Lastly, the length and width of the bearing must each be greater than three times the total thickness to prevent instability. 1.4.2 Method B Method B specifies that shear modulus of the elastomer should be between 0.080 ksi and 0.175 ksi, and nominal hardness should be between 50 and 60 on the Shore A scale.
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years, AASHTO determined that rotation of those bearings warranted further study, with the intent of proposing updated design provisions that would address the problems outlined above. 1.6 Previous Studies Elastomeric bearings were developed for commercial use in the 1950s, although references exist to earlier developments, and a good summary of general research and practice up to 1980 is provided in NCHRP Report 248.
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