«HIGHWAY INFRASTRUCTURE INTRODUCTION Highway infrastructure protection historically has been the primary consideration in determining TS&W limits as ...»
Bridge Overstress Criteria, Michael Ghosn, Charles G. Schilling, Fred Moses, and Gary Runco, Report by the City College of the City University of New York for the FHWA (Washington, D.C., FHWA, 1995).
28.0 70.0 70.1 78.4 45.0 80.0 80.0 80.0 35.0 74.5 77.1 80.0 48.0 80.0 80.0 80.0 40.0 78.0 80.0 80.0 53.0 80.0 80.0 80.0 NOTE: GVWs specific to 22.5-foot tractor wheelbase, 52-inch tractor tandem spread, and trailer 48-inch tandem spread. The distance from the first drive axle (on the tractor to the last trailer axle is the trailer length minus 6 feet.
VI-13 Table VI-4 presents weight values and maximum GVWs for the 6-axle semitrailer combination with the semitrailer supported at the rear by a tridem-axle group. In this case, both the tractor wheelbase and semitrailer length are varied (common descriptive dimensions). The allowable GVW for varying semitrailer lengths is shown in Figure VI-3.
28.0 75.0 70.1 73.4 45.0 85.5 87.1 88.6 35.0 79.5 77.1 84.5 48.0 87.5 90.1 90.0 40.0 82.5 82.1 88.7 53.0 90.5 92.0 94.2
VI-14 Table VI-5 presents the values and maximum GVWs for the RMD combination, a tractorsemitrailer combination with a 3-axle tractor pulling a 2-axle semitrailer and a 2-axle full trailer.
The tractor and semitrailer length of this double are varied, with the trailer remaining constant at 28 feet. The limiting axle loads and maximum GVW for the entire vehicle are easily read from a table. This approach negates the need to compute the many axle group combinations inherent in the use of the existing and proposed formulas (which can amount to as many as 36 different combinations in the case of a 9-axle vehicle). The GVW for varying semitrailer lengths is shown in Table VI-5.
45 109.5 109.5 105.16 107.3 111.4 112 48 111 111 106.6 108.8 112.8 113.4 53 111 111 109.1 111.3 115.2 116 In summary, there is significant variation in the results derived from the three formulaic approaches by vehicle configuration. In general, the TTI formula is better matched than the FBF for bridges, and there is a significant amount of load capacity available before limits are exceeded for the 5- and 6-axle semitrailer and 7-axle RMD configurations. This is not the case, however, for larger vehicles such as the 9-axle turnpike doubles -- FBF allows too much weight for these in terms of the stress criteria. The TTI curve for that vehicle is on the low side of the FBF stress criteria curve. Also, FBF is conservative for multiaxle short straight trucks.
There are benefits to adhering to the criteria on which the FBF is based and incorporating the consideration of continuous beams into the control. Tools such as user-friendly computer software programs can be designed to assess allowable loading configurations for any vehicle, and standard (bridge formula) tables for the more common vehicles can be generated. The use of the FBF stress criteria described in this section addresses the documented drawbacks of FBF and provides a basis for truck weight control that conforms to the criteria upon which both FBF and TTI are based
-- but to which they do not always adhere.
VI-15 It should be noted that the FBF, by design, incorporates a degree of control for pavement damage by explicitly including the number of axles in the formula. The TTI formula and FBF stress criteria indirectly control for pavement damage by adhering to axle weight limits -- the higher GVW limits, such as for LCVs, require more axles to avoid exceeding axle limits.
The condition and performance of highway pavements depend on many factors, including the thickness of the various pavement layers, quality of construction materials and practices, maintenance, properties of the roadbed soil, environmental conditions (most importantly rainfall and temperature), and the number and weights of axle loads to which the pavements are subjected.13
While pavement engineers traditionally have used ESAL factors estimated from the AASHO Road Test (started in 1956 and completed in 1962) as the basis for designing pavements, there is increasing recognition that better relationships between axle load and pavement deterioration are needed. Pavement distress models used in both the 1982 and 1997 Federal HCA Studies (HCAS) abandoned the use of ESALs to relate axle loading to pavement deterioration, and AASHTO will be replacing its ESAL-based pavement design formula with one that more directly relates axle loads to factors that determine pavement life. While ESALs were not used as the basis for estimating pavement costs for this Study, they are widely understood by highway administrators, pavement engineers, and others concerned with the pavement impacts of TS&W scenarios. Therefore, they are used here as a benchmark for comparing relative pavement impacts of various truck configurations with different numbers and types of axles.
Pavement deterioration increases sharply with increases in axle load. On both flexible and rigid pavements, the load equivalence factor for a 20,000-pound single axle is about 1.5. Thus, 100 passes across a pavement by a 20,000-pound axle would have the same effect on pavement life as 150 passes by an 18,000-pound axle.
The number of axles is also important in estimating pavement impact, other things being equal, as a vehicle with more axles has less effect on pavements. For example, a 9-axle combination vehicle carrying 80,000 pounds has less effect on pavements than a 5-axle combination vehicle carrying 80,000 pounds. A significant amount of additional weight can be carried by the 9-axle vehicle without causing greater pavement consumption relative to the 5-axle vehicle. Comparing vehicles in terms of ESALs provides information on load-related pavement impact, but it does not include an offsetting benefit gained by a reduction in the number of trips required to transport TRB Special Report 225, Truck Weight Limits: Issues and Options, 1990.
The increase in pavement costs per added ESAL mile can vary by several orders of magnitude depending upon pavement thickness, quality of construction, and season of the year. Thinner pavements are much more vulnerable to traffic loadings than thicker pavements.14 Additionally, pavements are much more vulnerable to traffic loadings during spring thaw in areas subject to freeze-thaw cycles.
The primary load effect of axle spacing on flexible pavement performance is fatigue.
Axle spacing is a major concern for fatigue. When widely separated loads are brought closer together, the stresses they impart to the pavement structure begin to overlap, and they cease to act as separate entities. While the maximum deflection of the pavement surface continues to increase as axle spacing is reduced, maximum tensile stress at the underside of the surface layer (considered to be a primary cause of fatigue cracking) can actually decrease as axle spacing is reduced. However, effects of the overlapping stress contours also include increasing the duration of the loading period. Thus, the beneficial effects of stress reduction are offset to an unknown degree by an increase in the time or duration of loading. The net effect of changes in axle spacing on pavement deterioration is complex and highly dependent on the nature of the pavement structure.15
In recent years, several studies on the impact of tire characteristics on pavement have raised concern over the possibility of accelerated pavement deterioration, particularly rutting, caused by increasing tire pressures. The tires of the AASHO Road Test trucks of the 1950s were bias-ply construction with inflation pressures between 75 pounds and 80 pounds per square inch (psi). The replacement of bias-ply tires with radial tires and higher inflation pressures, averaging 100 psi, result in a smaller size tire “footprint” on the pavement and, consequently, a concentration of weight over a smaller area.16 These changes hasten the wear of flexible pavements, increasing both the rate of rutting and the rate of cracking.
Results of a study by Hutchinson and Haas compare the average and marginal costs per ESAL on highways with 500,000 ESALs per year and 2 million ESALs per year. The cost per ESAL for highways with 500,00 ESALs is almost four times as great as the cost per ESAL on highways designed for 2 million ESALs. One important implication of this finding is that a policy that encourages heavy trucks to shift from highways with thicker pavements, such as the Interstate or NHS, to highways with thinner pavement can have a significant impact on pavement costs.
TRB Special Report 225.
A study by Bartholomew (1989) summarized surveys of tire pressure conducted in seven States between 1984 and 1986 and found that 70 to 80 percent of the truck tires used were radials and that average tire pressures were about 100 psi.
VI-17 The AASHTO load equivalency factors apply only to axles supported at each end by dual tires.
Recent increases in steering axle loadings and more extensive use of single tires on load-bearing axles have precipitated efforts to examine the effect on pavement deterioration of substituting single for dual tires. Both standard and wide-based tires have been considered. Past investigations of the pavement deterioration effects of single versus dual tires have found that single tires induce more pavement deterioration than dual, but that the differential wear effect diminishes with increases in pavement stiffness, in the width of the single tire, and in tire load.17 A general finding from the studies is that wide-base single tires appear to cause about 1.5 times more rutting than dual tires on flexible pavements (the most common type of pavement) as they do not have good rut resistance. Another finding is that one of the wheels in a dual tire assembly is frequently overloaded due to variability in the roadway cross-section and that the average overload causes an increase in rutting similar to that caused by wide-based single and dual tire assemblies.
Based upon past studies, single tires have more adverse effects on pavements than dual tires,18 it appears likely, however, that past investigations have overstated the adverse effects of single tires by neglecting two potentially important effects: (1) unbalanced loads between the two tires of a dual set, and (2) the effect of randomness in the lateral placement of the truck on the highway.
Unbalanced loads between the tires of a dual set can occur as a result of unequal tire pressures, uneven tire wear, and pavement crown. As with unequal loads on axles within a multiaxle group, pavement deterioration increases as the loads on the two dual tires become more unbalanced.
The second neglected factor, sometimes termed “wander,” is the effect of randomness in the lateral placement of trucks within and sometimes beyond lane boundaries. Less than perfect tracking is beneficial to pavement deterioration, as the fatiguing effect is diminished because the repetitive traffic loads are distributed over wider areas of the pavement surface. The greater overall width of dual tires naturally subjects a greater width of pavement to destructive stresses, therefore, wander is expected to have a smaller beneficial effect for dual than for single tires. Once rutting Gillespie (1993) found that a steering axle carrying 12,000 pounds with conventional single tires is more damaging to flexible pavements than a 20,000-pound axle with conventional dual tires. Gillespie proposed that road damage from an 80,000-pound vehicle combination would be decreased by approximately 10 percent if a mandated load distribution of 10,000 pounds on the steering axle and 35,000 pounds on tandems. Since the operating weight distribution of a 5-axle tractor-semitrailer at 80,000 pounds GVW generally has less than 11,000 pounds on the steering axle, the practical effect of the proposal would be to increase tandem axle weights without a compensating decrease in steering axle weights.
Bauer (1994) summarized several recent studies on the effects of single versus dual tires: “Smith (1989), in a synthesis of several studies... evaluated at 1.5 on average the relationship of the damage caused by wide base single assemblies and that caused by traditional dual tire assemblies with identical loading at the axle. Sebaaly and Tabataee (1992) found rutting damage ratios between wide base and dual tire assemblies varying between
1.4 and 1.6... Bonaquist (1992), reporting on results obtained from a study... on two types of roadway, using a dual tire assembly with 11 R 22.5 and a wide base with 425/65 R 22.5, indicates rutting damage ratios varying from 1.1 to 1.5, depending on the layers of the roadway.” VI-18 begins, however, tires -- especially radial tires -- tend to remain in the rut, thereby greatly reducing the beneficial effects of wander for both single and dual tires.19 Another consideration in evaluating wide-base single versus dual tires is dynamic loadings that arise from the vertical movement of the truck caused by surface roughness. Thus, peak loads are applied to the pavement that are greater than the average static load.20 Signs of pavement damage from dynamic loadings are typically localized, at least initially. Because of the localized nature of the dynamic loading, its severity is much greater than previously thought. 21 A further note on wide-base single tires is that those having only two sidewalls are much more flexible than a pair of dual tires with four sidewalls. This means the tire absorbs more of the dynamic bouncing of the truck, and less of the dynamic load is transmitted to the pavement.