Analytical Solution of Structural Number Equation

The design of flexible pavements attempts at bridging the gap between the axle loading intensity and the strength of roadbed material by providing a layer of the strongest material at the top, while gradually reducing the strengths of base and sub-base materials towards bottom, so as to match with that of the roadbed or sub-grade soil. A skillful distribution of the road structure materials not only provides a durable pavement but also brings economy. Structural Number (SN) is used for determination of pavement thickness in accordance with AASHTO GUIDE FOR THE DESIGN OF PAVEMENT STRUCTURES -1986. An attempt, therefore, at having a precise value of SN may bring slight to some economy in pavement design in addition to higher level of confidence. This leads to a conclusion that a numerical solution of SN equation is always preferable. AASHTO has designed state-of-the-art software for complete pavement design, known as “DARWin”. Of course, DARWin also determines the design SN value. Therefore, one may pose a question that in presence of such software, why a separate module for determination of precise SN value is required. The answer is straightforward: Small organizations and students cannot afford such professional software, as their cost is prohibitively high. The authors have observed the use of spreadsheet such as Microsoft Excel for determination of SN value by equating LHS and RHS of the SN equation. Trial values of SN are input in a cell cross-referenced to the cell/formula for RHS, till the time the two sides are equal. This value is taken as design SN for further calculations. In addition to lacking full automation, such mechanism may produce grossly inaccurate results in case cell links are broken. The computer program presented in this paper addresses the need of academic institutions, small engineering organizations and students.

کلیدواژه ها:

Pavments ، axle loading ، sub-grade soil ، spreadsheet ، automation