| 摘要 |
A manhole cover of substantially monolithic construction is claimed. The manhole cover comprises a central cover portion having a center, and having a thickness that varies radially from the center in accordance with an exponential or parabolic function. An intermediate cover portion surrounds the central cover portion and has a substantially uniform thickness. An outer bearing portion surrounds the inter-mediate cover portion and has a thickness greater than the intermediate cover portion. The thickness of the central portion may vary radially from the center in accordance with the exponential function, t r = T ~e -cB, where t r is the thickness at a given radial point r from the center, c = (r/R)2, where R is the radius of the cover, B is a dispersion constant, and T is a constant that determines the thickness of the central portion at the center.
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