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Arno Hefer Administrator Posts: 18
2/24/2017
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The LET Standard Axle Design Tool allows you to analyse pavement response and expected structural capacity under a user-defined axle load using layered elastic theory. We recommend using the LET Standard Axle Design Tool for routine pavement design and analyses. This may include designing new flexible pavements or flexible pavement rehabilitation design.
Getting Started
The LET Standard Axle Design Tool is accessed through the Design Tools icon on the Rubicon Toolbox Online Tools.
If this is the first time you are using the Online Tools, you will need to set up a project by clicking on Manage Projects and create a new project. Alternatively, you can use an existing project if one has already been created.
Before using the LET Standard Axle Design Tool, you may need to check that the correct default Loads and Materials have been selected for your analysis. This can be done by clicking on Settings. The default Loads and Materials can be set as per project requirements. These settings are not fixed and can be changed as and when required. Note that you can add your own customized loads by clicking on Manage Load Setups. The various failure criteria can be viewed by clicking on Criteria.
Once you have created a new project or selected an existing project, click on the Design Tools icon and select LET Standard Axle Design from the list of design tools. A LET Standard Axle Analysis page will open. Proceed to complete the analysis, working from left to right.
Start by clicking on Load Setup. Note, if loads are not available for selection it means that the default loads have not been setup or selected under Settings.
Defining the Pavement Structure
The LET Standard Axle Design tool makes provision for handling different pavement behavioral phases. This simplified approach is typically used to model the deterioration of bound (or cohesive) layers; traditionally cement stabilized layers. As an example, consider the following pavement structure with a typical setup based on the South African Mechanistic Pavement Design Method and associated material classes.
To define the pavement structure click on Pavement: Phase 1. A default table with five layers will display on-screen, as well as text boxes to input the name of the analysis, a description and notes.
The selection of a predefined or default material automatically populates the Thickness, Stiffness, Poisson’s ratio and Transfer Functions (or Failure Criterion) fields from the default database. These default values and Transfer Functions are merely indicative, to provide guidance, and should be assessed carefully and adjusted if needed to reflect the specific project or design conditions. The actual inputs should be relevant to the project under consideration and should ideally be derived from processing of data collected for the project; e.g. stiffness values can be backcalculated from deflection data and substantiated or validated using other data sources such as dynamic cone penetrometer data, density measurements, moisture measurements, and laboratory test data. Note, if the Material Class is changed after finalization of the thickness, stiffness values etc., these values will change to the default values associated with the new material. Also note, if materials are not available for selection it means that the default materials have not been selected under Settings.
A Phase 1 Pavement Structure table, containing the basic predefined materials, is shown below. The default Transfer Function (or Failure Criterion) is associated with the selected material, and normally a good start. In many cases, one function up or down is selected to meet the required design reliability. In the screen shot below, Cat B represents Category B according to the South African design method associated with a 90% design reliability. It is also possible to select None, implying that the calculated response is not considered in the structural capacity analysis. This does not, however, mean that the entire layer is ignored. The stiffness contribution to the pavement system is still considered.
Once you have changed the default settings to the required project inputs, in Phase 1, proceed with defining Phase 2 of the Pavement Structure or Calculate if the Phase 2 analysis is not applicable. If Pavement: Phase 2 is selected, the input table, for Phase 2, automatically appears with prepopulated data from Phase 1 and you only need to change the properties that will change for Phase 2. This ascertains accuracy and compatibility between the two phases with only minor changes needed to reflect the change(s) in the relevant layer(s). The screen shots below show the Phase 2 pavement structure, with prepopulated values from the Phase 1 example structure inputs, and highlights the changes required for the Phase 2 setup.
Analysis and Reporting
Once you have completed the Phase 2 setup, click on Calculate to generate the analysis report. An example of the Phase 1 analysis report is shown below. The layer highlighted in red represents the critical layer that dictates the pavement life for the phase under consideration.
The screen shot below shows the Phase 2 output. As demonstrated, the analysis for this example indicates failure (cracking) of the asphalt surfacing. In such a case a re-run is typically done, changing the asphalt transfer function for both phases to None, which allows deformation of the structural layers to dictate structural capacity. Note, in this example cracking of the surface only takes place after about 10 years, which is the typical life expectancy of asphalt surfaces due to environmental influences in the region considered. Since the surface can be treated by a mill and replace action, the approach of omitting the asphalt layer from the analysis is valid.
Note, for a new analysis, the Save option is initially inactive. Once an analysis is saved for the first time, the save option becomes active. For overwriting an existing saved analysis, click on Save. To save as an alternative analysis, click on Save As. Alternatively you can clone your analysis, from the Online Tools page, and rename it as needed.
The report for each phase can be copied to the clipboard by right clicking on the report and selecting Copy image or Save image as. The image can then be inserted into a document of your choice (Word, Excel etc.).
edited by on 8/23/2024
edited by on 8/23/2024
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