Program for Load-Zoning Analysis (PLZA): User's Guide

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E.G. Fernando, W. Liu

Publication Date:

December 1999


Texas has approximately 17,500 miles of load-zoned pavements, comprising over 20 percent of the number of centerline miles on the state-maintained system. These pavements are primarily low-volume farm-to-market roads constructed in the 1950s at a time when legal load limits were lower than they are now. About 98 percent of the load-zoned pavements in Texas are posted with a gross vehicle weight (GVW) restriction of 58,420 lbs. While load limits based on gross vehicle weight are simple to implement, the procedure is fundamentally flawed since the gross load from a vehicle is transmitted through the axle tires. Recognizing the need for a better methodology of determining load limits, the Texas Department of Transportation (TxDOT) sponsored a project with the Texas Transportation Institute (TTI) to develop a procedure for evaluating load restrictions on the basis of axle load and axle configuration. The product of this project is the Program for Load-Zoning Analysis, PLZA, which is described in this report. PLZA may be used to evaluate the need for load restrictions and to determine, as appropriate, the single and tandem axle load limits based on a user-prescribed reliability level for the route under investigation. The program incorporates a layered elastic pavement model for predicting the induced response under surface wheel loads. Both linear and nonlinear pavement materials may be modeled. The predicted horizontal strain at the bottom of the asphalt layer and the vertical strain at the top of the subgrade are used to determine the service life for a given pavement and loading condition. To combine the effects of different axle loads and axle configurations, Miner's hypothesis of cumulative damage is used to predict pavement service life. Instructions in the operation of the PLZA software are provided in this guide. The program is now implemented in TxDOT for evaluating load zoning needs.

Report Number:



Load-Zoning, Axle Load Limits, Nondestructive Testing, Pavement Evaluation, Pavement Performance Prediction

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