Parabolic cover for manhole

A framework for covering an opening to an installation, comprising:

• a central cover portion having a point of origin and a depth that varies from the point of origin along an axis in accordance with an exponential function;
• an intermediate cover portion, surrounding said central cover potion and having a substantially uniform thickness along said axis;
• and
• an outer bearing portion, surrounding said intermediate cover portion and having a depth that is substantially uniform, said outer bearing portion having a thickness greater than said intermediate cover portion.

A manhole cover of substantially monolithic construction, comprising:

• a central cover portion having a center, and having a thickness that varies radially from the center in accordance with a parabolic function;
• an intermediate cover portion, surrounding said central cover portion and having a substantially uniform thickness;
• and
• an outer bearing portion, surrounding said intermediate cover portion and having a thickness greater than said intermediate cover portion.

A framework for covering an opening to an installation, comprising:

• a central cover portion;
• and
• an outer bearing portion surrounding said central cover portion,
• said central cover portion having a defined point of origin, and having a depth that varies from the point of origin along an axis in accordance with a parabolic function, and
• said outer bearing portion having a depth that is substantially uniform.

A method of configuring a framework for covering an opening to an installation, comprising the steps of:

• (a) specifying the outside radius of the framework for a circular configuration, or the length and width of the framework for a rectangular or square configuration;
• (b) specifying a maximum thickness parameter for the framework;
• (c) specifying a maximum allowable stress for the framework;
• (d) selecting a particular function for calculating a variable thickness of the framework;
• (e) calculating the variable thickness of the framework using said function and at least the parameters specified in steps (a) and (b);
• (f) composing a complete design of the framework using the results obtained in step (e),;
• the complete design including a central cover portion having a variable thickness, an intermediate cover portion surrounding said central cover portion and having a substantially uniform thickness, and an outer bearing portion surrounding said intermediate cover portion and having a thickness greater than said intermediate cover portion.
• (g) calculating the maximum stress level for the framework design based on a particular load condition, and comparing the calculated level with the maximum allowable stress specified in step (c);
• (h) adjusting, if necessary, at least one of the parameters used in the calculation of step (e) and repeating steps (e), (f) and (g), until the weight of the framework is minimized for a particular stress;
• and
• (i) producing a framework in accordance with the design composed in step (f) and adjusted, if necessary, in step (h).