Section 3.0; Knowledge Gaps and Research Needs

Navigation: Page 1, Cover 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

Comprehensive Truck Size and Weight (TS&W) Study

Phase 1—Synthesis

Working Paper 3—Pavements and TS&W Regulations

3.0 Knowledge Gaps and Research Needs

Research is needed to develop improved load-equivalence factors for use in truck size and weight analyses, highway cost allocation studies, and other policy studies. The AASHTO load-equivalence factors that are currently used in most TS&W studies in the U.S. were developed using data from the AASHTO Road Test conducted in the 1950's. Since the primary purpose behind the development of these factors was to provide measures of total traffic loadings for use in pavement design, relatively little attention was paid to the quantifying the relative impacts of different truck characteristics on pavements.

The development of improved load-equivalence factors should address the following issues:

The relative impacts of single axles, tandem axles, and tridem axles

The effects of tire type, width, and pressure

The effects of different types of suspensions

Axle weight (AASHTO's 4th power relationship vs. the results of recent work by TRI and Brookings).

The research should provide the following:

The best possible set of load-equivalence factors based on available data

Some indication of the level of uncertainty associated with these factors

A plan for how information from ongoing data collection activities (such as SHRP) might be used to update these factors

Identification of new data collection activities that should be initiated.

Research on load-equivalence factors should build upon recent work by Kenis (1990) and Hudson (1992). Kenis used the VESYS 5 computer program to conduct "computer road tests". After verifying that the program could be used for this purpose, Kenis estimated the damage produced by steering axles at the AASHTO Road Test, in order to quantify the error caused by the fact that these axles were neglected when equivalencies were originally developed. Kenis then used VESYS 5 to estimate equivalence factors for conditions not present in AASHTO Road Test, such as tridem axles. Finally, equations relating pavement deflections and strains to load equivalencies based on cracking and rutting were developed.

Hudson (1992) evaluated alternative "primary response equivalency factor methods". These methods use stresses, strains, and deflections to estimate pavement damage. The research effort included a comprehensive review and evaluation to identify equivalency relationships and select several promising methods. Then, field testing of instrumented pavement sections was conducted to evaluate the selected methods. Hudson concluded that primary pavement response based load equivalency factors are a reasonable method to estimate the equivalent damaging effects of various load parameters, as compared to a standard loading condition. Of the methods tested, the deflection method proposed by Hutchinson was found by Hudson to be the most viable of the methods that were analyzed in detail.

In addition to better load-equivalence factors, research is needed to identify and assess the potential merit of alternative approaches to regulating tire pressure and other tire characteristics. For each approach identified, the investigation should

Assess the feasibility and costs of enforcement

Estimate benefits in terms of reduced pavement costs

Estimate costs to the trucking industry of complying with the regulations

Identify and estimate other potentially important benefits and costs.

Consideration should also be given to the development of performance specifications for truck suspension systems to reduce dynamic loading impacts on pavements.

[continued on next page, Page 15, section 4.0 References for Pavements Working Paper ]

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