«HIGHWAY INFRASTRUCTURE INTRODUCTION Highway infrastructure protection historically has been the primary consideration in determining TS&W limits as ...»
The subject of road-friendly suspensions -- within the context of the broader subject of vehiclepavement interaction -- was researched as an Organization for Economic Cooperation and Development (OECD) Project -- the Dynamic Interaction between Vehicles and Infrastructure Experiment (DIVINE) Project -- involving the United States and 16 other countries.22 The work focused on (1) how well different suspension systems distribute load among axles in a group (the more evenly, the better); (2) how well different suspension systems dampen vertical dynamic The TRB Special Report 225 examined the importance of loading imbalance and wander. The TRB Study examined two types of pavement deterioration: surface cracking due to fatigue and permanent deformation or rutting in the wheel tracks. Fatigue was found to be more sensitive to the differences between single and dual tires than rutting. Both balanced and unbalanced dual-tire loads were considered in analyzing the affect on wander. The analysis indicated that the adverse effects of single tires on pavement deterioration were reduced when wander was taken into account, although the effects were still significant.
From research summarized by the Midwest Research Institute (MRI) that suggests dynamic loadings are a consideration in assessing the relative merits of wide base single versus dual tires. Gyenes and Mitchell report that the magnitude of the added dynamic components was earlier thought to increase road damage over that of the static loading alone between 13 and 38 percent, according to research reported by Eisenmann. The MRI research noted that many recent studies have pointed out the fallacy in the earlier work, which assumed that the dynamic component of loading was distributed uniformly over the pavement in the direction of travel. The research found, however that the dynamic component is very localized, arising out of pavement surface irregularities and therefore is spatially correlated with these irregularities.
Gillespie, et. al. estimate that damage due to the combination of static and dynamic loading can be two to four times that due to static loading locally. Von Becker estimates the combined loading produces a “shock factor” between 1.3 and 1.55, depending upon suspension characteristics. Applying the fourth power law would translate these figures into relative damage estimates ranging from 2.8 to 4.8 times the static loading damage.
Gyenes and Mitchell suggest impact factors in the range of 1.3 to 1.5 for relative damage estimates of 2.8 to 5.1.
TRB Special Report 225 noted that a heavy truck travels along the highway, axle loads applied to the pavement surface fluctuate above and below their average values. The degree of fluctuation depends on factors such as pavement roughness, speed, radial stiffness of the tires, mechanical properties of the suspension system, and overall configuration of the vehicle. On the assumption that the pavement deterioration effects of dynamic loads are similar to those of static loads and follow a fourth-power relationship, increases in the degrees of fluctuation increase pavement deterioration.
The findings of the DIVINE research primarily relate to the physical interaction between heavy vehicles and the highway infrastructure -- pavements and bridges. The research breaks new ground, providing scientific evidence of the effects of heavy vehicles. Conclusions that relate to vehicle and pavement interaction are summarized from the final report.
Pavement wear -- the gradual loss of functional condition -- is expressed in permanent deformations to the longitudinal profile of the pavement surface. Whereas, pavement damage results from an accumulation of rutting and cracking distress from repeated applications of vehicle loads. "Road research... has historically tended to over-emphasize pavement damage, and the true importance and nature of pavement wear has not yet been recognized."23 The DIVINE research focused primarily on examining pavement wear rather than damage.
Two scientific breakthroughs resulted from the DIVINE accelerated pavement tests: "the effects of dynamic loading were measured for the first time, and a detailed statistical analysis of both the
pavement and vehicle variables was undertaken."24 Conclusions reached are:
• Changes in pavement profile under dynamically-active steel suspensions relate to: local structural compliance (the opposite of strength), and local dynamic wheel load.
• Changes in pavement profile under dynamically-quiet air suspensions are mainly related to the local structural compliance of the pavement.
• The relationship between tensile strain at the bottom of the pavement surfacing layer and dynamic wheel loading appears to depend on the pavement thickness. For thick pavement, strain is directly related to dynamic wheel loading. For thin pavement, strain directly related to dynamic wheel loading is weaker. This difference in pavement behavior is believed to be related to changes in tire contact conditions occurring from variances in the dynamic wheel load.
• Air suspension would increase pavement life by 60 percent for thick pavement and 15 percent for thin pavement (based on two types of implied assumptions: selected pavement response parameter measured and analyzed, and the "damage law" applied).
• Spatial repeatability on a relatively smooth road would increase total wheel loading at certain locations by approximately 10 percent, reducing pavement life at those locations by approximately 35 percent to 50 percent.
OECD DIVINE Programme, Final Report "Dynamic Interaction of Heavy Vehicles with Roads and Bridges," May 1997, p. 145.
Additionally, recent research outside the DIVINE Program evaluated the role of suspension damping in enhancing the road friendliness of a heavy vehicle. The findings indicated an increase in linear suspension damping tends to reduce the dynamic load coefficient and the dynamic tire forces -- factors related to road wear. The research concluded that linear and air spring suspensions with light linear damping offer significant potentials to enhance the road friendliness of the vehicle with a slight deterioration in ride quality. 26 It is worth noting that approximately 90 percent of all truck-tractors and 70 percent of all van trailers sold in the United States are equipped with air suspensions. Additional studies on various types of axle suspension systems include studies on: torsion suspensions, four-leaf suspensions, and walking-beam suspensions.27 The research has yet to produce any compelling argument to incorporate a suspension system determinant into U.S. regulations, although some countries have done so. Mexico is in the final stages of preparing regulations that will allow up to 2,200 pounds of additional weight for each trailer axle equipped with air suspension or its equivalent. For a drive axle, Mexico may allow up to an additional 3,300 pounds. The impacts of different suspension systems on pavement deterioration are of secondary importance compared to the static axle load levels themselves. Use of road-friendly suspensions is beneficial, particularly for large trucking operations with wellcontrolled axle loadings.
The widespread use of lift axles in Canada and the United States raises concern for resulting pavement deterioration when a driver, attempting to improve fuel consumption, fails to lower the axle when loaded. A 1988 and 1989 survey conducted in Ontario and Quebec found that approximately 17 percent and 21 percent, respectively, of trucks on highways in those Provinces had lift axles.28 Lift axles have been adopted in response to GVW limits governed by the number Ibid, p. 147.
In the Rakheja and Woodroofe model suspension effects are represented using a sprung mass, an unsprung mass, and restoring and dissipative effects due to suspension and tire. The tire is modeled assuming linear spring rate, viscous damping, and point contact with the road.
Sousa, Lysmer and Monismith investigated the influence of dynamic effects on pavement life for different types of axle suspension systems. They calculated a Reduction of Pavement Life (RPL) index of 19 percent for torsion suspensions (an ideal suspension would have RPL of 0). Similar results were found by Peterson in a study for RTAC: under rough roads at 50 mph, air bag suspensions exhibited dynamic loading coefficients (DLC) of 16 percent, spring suspensions had a DLC of 24 percent, and rubber spring walking beam suspensions had a DLC of 39 percent. Problems with walking-beam suspensions were also noted by Gillespie, et. al. who state that on rough and moderately rough roads, walking-beam suspensions without shock absorbers are typically 50 percent more damaging than other suspension types.
Billing, et. al.
VI-21 of axles (such as the FBF), and because trucks with multiple widely spaced axles have difficulty turning on dry roads and the lift axles can be raised by the driver prior to turns.
Lift axles make compliance with and enforcement of axle weight limits difficult. Improperly adjusted lift axles can damage pavements. The lift axle can be adjusted to any level by the driver.
If the lift axle load is too high, the lift axle is overloaded. If it is too low, other axles may be overloaded. For example, under current Federal limits, a 4-axle single unit truck with a wheelbase of 30 feet can carry 62,000 pounds: 20,000 pounds on the steering axle and 42,000 pounds on the rear tridem. This vehicle would produce approximately 2.1 ESALs on flexible pavements. However, if the first axle of the tridem is a lift axle carrying little or no weight, this vehicle would produce approximately 4.0 ESALs.
Unit pavement costs and pavement costs per unit of payload-mile by configuration are shown in Tables VI-6 and VI-7. They illustrate how the addition of axles allows for increased payloads and at the same time reduces pavement deterioration. Particularly striking, are comparisons between the 3- and 4-axle single unit trucks, the 5- and 6-axle semitrailer combinations, and the 5and 8-axle doubles. As shown in Table VI-7, the 4-axle truck has costs per payload ton-mile about 75 percent of that for the 3-axle truck even though its gross weight is 10,000 pounds more than the 3-axle truck. The comparison of the 6-axle semitrailer with the 5-axle is very similar on non-Interstate highways. The costs for the 8-axle double-trailer are less than half those for the 5-axle double-trailer. Triples do not compare well with doubles. Generally, truck owners would be opposed to adding axles because this increases the tare weight of the vehicle and reduces payload capacity.
TS&W REGULATION RELATED TO PAVEMENT PRESERVATION
TIRE REGULATIONSFederal law and most State laws, do not address truck tire pressure. Tire pressure may have a large effect on fatigue of flexible pavements as discussed earlier (albeit a small to moderate effect on rigid pavements), and today's tire pressures are higher than in the 1950s -- primarily the consequence of a change from bias to radial ply tires. Concern has been raised about accelerated pavement rutting as a result of increased tire pressures. Recent research gives conflicting views as to whether or not pressures should be regulated.29 Federal, and most State, laws do not discourage or prohibit the use of wide-base tires. The consensus of United States and international research is that these tires have substantially more TRB Special Report 225 (1990) suggested regulation could be warranted if the more pessimistic analyses proved to be correct. NCHRP Study (1993) suggested limiting tire pressure to the recommended cold setting plus 15-psi; AASHTO (1993) suggested more research is required to answer all questions regarding the relationship of tire size, contact pressure, and contact area to pavement damage.
VI-22 adverse effects on pavements than dual tires because current designs employ smaller, overall tire-road contact patch sizes than equivalent dual tire sizes. Future tire designs could address this issue. Wide-base tires -- which are widely used in Europe -- are being increasingly adopted by U.S. trucking operations. The benefits of wide-base tires are reduced energy use, emissions, tire weights, and truck operating costs. The trade off between changes in Federal pavement costs and operating benefits that would result from permitting or prohibiting extensive adoption of widebase tires in the United States has not been analyzed.
Historically, many States specified some form of tire load regulation for safety. In recent years, additional States have adopted tire load regulations to control the damage effect of wide-base tires. They restrict the weight that can be carried on a tire based on its width. The limits range from 550 pounds per inch (in Alaska, Mississippi, and North Dakota) to 800 pounds per inch (in Indiana, Massachusetts, New Jersey, New York, and Pennsylvania). Such restrictions result in lower pavement costs; however, the size of the pavement cost savings (either in absolute terms or in relation to the increase in goods movement costs also resulting from these restrictions) have not been estimated.