section 1.2 Truck Characteristics Affecting Pavements; (f) Tridem Axles

In a paper prepared for The Association of American Railroads, Hudson and Buttler summarized available information about the effect of tridem axles on pavement damage. They note that no tridem axles were used or observed in the AASHTO Road Test and that

"to provide an equivalence value for tridem axles, the developers of the AASHTO [Pavement Design] Guide substituted a dummy variable level of three for 'number of axles' in the AASHTO equation. This methodology is incorrect. Note that the AASHTO equation uses a dummy variable for number of axles, 1 for single, 2 for tandem. This was merely a convenience to permit a regression analysis to be made for variables for which there is no quantitative value, such as axle type. Nothing about the original equation suggests that it is possible to create a third level of the dummy variable for tridem axles. Considering the error it is no surprise that many researchers suggest that the true effects of tridem axles is worse than that listed in the AASHTO Design Guide."

In summarizing the literature and results of their own analyses, Hudson and Buttler conclude that, on flexible pavements, a tridem axle set of 38 to 39,000 pounds equally distributed on three axles has the same damaging effect as one 18,000-pound single axle. In sharp contrast, the AASHTO load equivalence factor for a 38 to 39,000-pound tridem on flexible pavements is roughly 0.4. Hudson and Buttler also conclude that, on flexible pavements, the AASHTO load equivalence for tandems also understated, although by much less than the understatement for tridems. Specifically, they conclude that a tandem axle carrying 30 to 32,000 pounds has the damaging effect of one 18,000-pound single axle. The AASHTO load equivalence for a 30 to 32,000-pound tandem is roughly 0.8. On rigid pavements, Hudson and Buttler conclude that a tridem-axle set carrying 36 to 37,000 pounds evenly distributed on three axles has the same damaging effect as one 18,000-pound single axle. The AASHTO load equivalence factor for a 36 to 37,000 pound tridem on rigid pavements is roughly 0.8.

[continued on next page, Page 9, Section 2.1 Axle Weight Limits ]

« Previous Page | Page 2.. Phase 1.1, Background. | 1.2 Truck Characteristics Affecting Pavements. (a) Axle Weights | Page 4, Section 1.2 [b] Tire Characteristics | Page 5, Section 1.2 (c) Suspension Systems | Page 6, Section 1.2; (d) Axle Spacing | Page 7, Section 1.2; (e) Liftable Axles | Page 8, Section 1.2; (f) Tridem Axles | Page 9, Section 2.1 Axle Weight Limits | Page 10, Section 2.2 Bridge Formula | Page 11, Section 2.3 - 80,000 Pound GVW Cap | Page 11, Section 2.4 Policies to Encourage Tridems | Page 11, section 2.5 Weight Limits Per Unit of Tire Width | Page 12, section 2.6 Turner Trucks | Page 13, section 2.7-New Approach; TRB Truck Weight Study | Page 14, section Section 3.0; Knowledge Gaps and Research Needs | Page 15, section 4.0 References for Pavements Working Paper | Next Page »

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