Frequently asked questions¶
Why is the "statistical distribution parameter" (Z_R) negative?¶
It is correct. Z_R is the standard normal deviate related to the reliability R in the AASHTO equation: it is negative for any R greater than 50% (with R = 90% it equals −1.282). The higher the reliability, the more negative Z_R is and the more conservative the design. See Empirical method AASHTO.
Are the methods (AASHTO 1993, Ivanov, Westergaard) outdated?¶
They are classical and well-established methods, still the reference standard in professional practice and in the Italian Catalogue of Road Pavements (CNR). They represent the most widely used regulatory basis for pavement sizing and verification. The most recent evolution is the mechanistic-empirical approach, more sophisticated but more demanding in terms of data and calibration.
Why is there no "Calculate" button?¶
The calculation starts automatically when you open the Results tab — but only if you have changed something since the last calculation. If the data has not changed, RPD NX shows the already-computed result without re-running the calculation.
The check is not satisfied: how do I pass it?¶
With FS < 1 the How to pass the check card appears, proposing, as alternatives, to increase one layer (each proposal is clickable and applies the thickness while recomputing) or to insert a geogrid with a given TBR. After applying, you can undo and try another solution.
What is the geogrid TBR for?¶
The Traffic Benefit Ratio is the multiplier of the allowable axles due to the bituminous geogrid reinforcement. In the AASHTO method the allowable axles are multiplied by the TBR (default 1 = no effect). The actual value must be taken from the manufacturer's data sheet.
The package is over-designed: can I reduce it?¶
Yes. When the check is satisfied with a wide margin, the Package optimization card appears: it proposes to reduce the thickness of one layer while keeping the check satisfied, to save material.
In what units is the Structural Number?¶
The Structural Number is expressed in inches (with the cm equivalent shown next to it), as per the AASHTO method. Layer thicknesses, on the other hand, are entered in cm.
In how many languages is RPD NX available?¶
In 7 languages: Italian, English, French, Spanish, German, Romanian and Danish. You switch from the flag menu in the top-right corner.
Where do I find the theoretical documents of the methods?¶
From the app's ? menu → Resources tab → "Method documentation" section: you will find the PDFs "Empirical method" and "Rational method — Ivanov".
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