Highway Flexible Pavement Design Equations from AASHTO Method
This content delves into the structural design of highway flexible pavements, focusing on the design equations derived from the AASHTO Road Test. It discusses the modification of these equations to account for sub-grade and climatic conditions, providing insights into the principles of pavement design. The AASHTO method simplifies the design process by utilizing specific load applications and terminal serviceability indices for flexible pavements.
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Structural Design of Highway Flexible Pavement Design Highway and Transportation Engineering Al-Mustansiriyah University 2019-2020 Dr. Rana Amir Yousif & Dr. Abeer K. Jameel Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
References Nicholas J. Garber and Lester A. Hoel. Traffic and Highway Engineering , Fourth Edition. Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975. Yaug H. Huang, Pavement Analysis and Design , Prentic Hall Inc., U.S.A., 1993. AASHTO Guide for Design of Pavement Structures 1993 , AASHTO, American Association of State Highway and Transportation Officials, U.S.A., 1993. Oglesby Clarkson H., HighwayEngineering , John Wiley & Sons Inc., U.S.A., 1975. Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations The original equations were based purely on the results of the AASHO Road Test but were modified later by theory and experience to take care of sub-grade and climatic conditions other than those encountered in the Road Test. Original Equations The following are the basic equations developed from the AASHO Road Test for flexible pavements (HRB, 1962) : + 9.36 log (SN + 1) 4.79 log ( Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations serviceability at end of time t ; load on one single axle or a set of tandem axles, in kip ; axle code 1 for single axle, 2 for tandem axle ; structural number of pavement, which was computed as in which : a l , a2, and a3 : are layer coefficients for the surface, base, and subbase, respectively, and D1 , D2 , and D3 : are the thicknesses of the surface , base, and subbase, respectively. Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations The procedure is greatly simplified if an equivalent 18-kip (80-kN) single axle load is used . By combining Eqs. 11.29, 11.30, and 11.31 and setting L1 = 18 and L2 = 1, we obtain the equation Equation 11 .33 is applicable only to the flexible pavements in the AASHO Road Test with an effective sub-grade resilient Modulus of 3000 psi (20 .7 MPa). in which : : is the number of 18-kip (80-kN) single-axle load applications to time t. : is the terminal serviceability index. Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations Modified Equations For other sub-grade and environmental conditions, Eq.11.33 is modified to . in which : : is the effective roadbed soil resilient modulus. When MR = 3000 psi (20 .7 MPa), Eq . 11 .34 is identical to Eq . 11 .33 . Note that Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations To take local precipitation and drainage conditions into account, Eq.11.32 was modified to. in which : : is the drainage coefficient of base course and. :is the drainage coefficient of sub base course . Equation 11.34 is the performance equation that gives the allowable number of 18-kip (80- kN) single-axle load applications Wt18 to cause the reduction of PSI to pt. Note that Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations If the predicted number of applications = the reliability of the design is only 50%, because all variables in Eq.11.34 are based on mean values. To achieve a higher level of reliability, must be smaller than by a normal deviate , Here: : is the normal deviate for a given reliability R, and : is the standard deviation. Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations can be determined from Table 10.1. or, more conveniently, from Table 11.15 . Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations Combining Eqs . 11.34 and 1 .36 and replacing (4.2 ) by yields. Equation 11.37 is the final design equation for flexible pavements . Figure 11.25 is a monograph for solving Eq . 11.37 . Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations Steps to use 11.25 is a monograph for solving Eq . 11.37 . Steps 1 Steps 2 Steps 3 Reliability T(11.14) The number of 18-kip Standard Deviation T(11.15) Steps 4 Steps 6 Steps 5 Design Structure Number, SN Design Serviceability Loss Effective Roadbed Soil (MR) Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations Step 1 Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations Example: (11.10) Given: , , , and Determine: SN from Figure 11 .25 . Solution: As shown by the arrows in Figure 11.25, Step 1: starting from R = 95%, series of lines are drawn through; Step 2 ; Step 3: ; Step 4: ; and Step 5: and finally intersect SN at 5.0, so SN = 5.0. Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations Step 1 Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations Example: (11.11) Given: , , SN = 5 and Determine: by Eq: 11.37. Solution: Given R = 95%, from Table 11.15, ZR = -1 .645. , , Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations From Eq. 11.37, Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
Structural Design of Highway Dr. Rana Amir Yousif & Dr. Abeer K. Jameel Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
