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Scale Information
Weighbridge Structure
Comparing truck scale application capacities can be confusing. Different manufacturers highlight differ-
ent values in their sales, engineering, and service documents, which can be perceived as the same thing -
even when they are not. For example, the Concentrated Load Capacity (CLC) and "r" factor ratings
(from Federal Bridge Standards) are often perceived as a method of determining the strength of a weigh-
bridge. This is a common misunderstanding that leads to confusion about judging weighbridge strength.
Concentrated Load Capacity (CLC)
The CLC rating resulting from an NTEP test is only a measurement of how well the load cells (and load
cell suspensions) react to large loads placed on the scale deck in a 4' x 10' area. The 4' x 10' loading area
is generally in the middle of an unsupported span to simulate an actual live load bearing on tandem truck
axles. When a CLC load is applied to the weighbridge during an NTEP test, the NTEP tester records the
displayed weight. If the scale falls within acceptance testing tolerances, the scale has that CLC weight
value recorded as the CLC on the Certificate of Conformance.
The CLC rating is not a measure of weighbridge strength or rigidity, because weighbridge deflection is
not measured in the NTEP test. It is irrelevant if the loaded weighbridge sags 1/10th of an inch, or 10
inches, as long as the scale weighs within the acceptance tolerance. The scale's CLC weight rating passes
in either case. A high CLC rating could be given to an extremely flexible deck sitting on load cell mounts
which were capable of accurately handling the side loading resulting from a severely sagging weigh-
bridge.
Federal Bridge Standards "r" factor
The "r" factor is a better method of determining a weighbridge's carrying capacity. Again there are
variables that will modify the rating of the scale, such as the number of axles and axle spacing. The "r'
factor is a method of determining acceptability of a roadway bridge to carry expected loads. Since a truck
scale is similar to the unsupported span of a bridge structure, this type of rating has validity as an indica-
tion of weighbridge strength in a vehicle scale.
Table B of the Federal Bridge Gross Weight Formula in NIST Handbook 44 is referenced in determining
"r" factors. This table assigns the maximum legal highway load (34,000 lbs) bearing on a dual tandem
axle (axles 4' apart) as a reference value of 1. This value of 1 for a bridge span assumes a large safety
factor is built into the bridge as a safeguard against catastrophic failure. Bridges designed for loads other
than 34,000 lbs on 4' axle spacing are assigned an "r" factor by simple division. A bridge designed for a
17,000 lbs load on 4' axle spacing would be assigned a 0.5 "r" rating, and one designed for 68,000 lbs on
that same spacing would be assigned a 2.0 rating.
Some scale manufacturers have advocated adopting this concept for rating the unsupported spans of
vehicle scales. If a truck scale manufacturer designs a scale for a 60,000 lb load on 4' tandem axle spac-
ing, the "r" factor for that scale would then be 1.76.
®
The SURVIVOR
SR series scale decks, designed for a 90,000 lb load on a 4' tandem axle spacing, rate a
2.65 "r" factor, one of the highest in the truck scale industry.
It is important to remember that the manufacturers set the amount of weights used in NTEP testing for
their scales, and the total is directly related to the CLC assigned to the scale. Because the CLC does not
measure weighbridge deflection, neither then does the "r" factor calculated from the CLC. While a large
"r" factor may indicate the ability of a structure to handle a certain load without catastrophic failure, it
does not address the effects of repeated flexing on ultimate weighbridge life. While a significant deck
deflection might not collapse a scale, it will certainly shorten the useful life cycle of a weighbridge if
repeated frequently enough.
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