EVALUACIÓN DEL A.V.A

Another consequence of the complex interaction between vehicles and roads is pavement deterioration. The occurrence of pavement deterioration has long been a significant issue in many developed countries where road networks are used to transport increasingly large volumes of freight for a single vehicle. The predicted increases in the volume of freight inevitably leads to an increase in the dynamic loads exerted onto pavements. There exists a great amount of literature concerned with understanding the cause and mechanisms of pavement damage.

Road-friendliness is the ability of a vehicle, or suspension system, to reduce the dynamic loads induced onto pavements during transportation as much as possible. A road-friendly vehicle is one that will, “to the greatest extent possible, isolate the vehicle from road unevenness so that dynamic wheel loading is minimised for given static loads” (OECD 1998). It is also important that a road-friendly suspension system is able to perform well on both smooth and rough pavements (OECD 1998). This requires knowledge of the entire interaction, including pavement composition, vehicle dynamic characteristics and the dynamic loads induced onto pavements (Cebon 1999, p. 508). For a long period of time, engineers have sought for a single relationship between the dynamic forces induced by a vehicle and the deterioration of pavements.

A large-scale study conducted during 1958-60 by the American Association of State Highway Officials (AASHO), who enlisted the US Army Corps to drive two-hundred vehicles over six custom test tracks in Ottawa, Illinois (AASHO 1962a; AASHO 1962b). The total mileage completed by AASHO during this two-year period equated to 17 million vehicle-miles. The analysis of the data collected from these experiments found the decrease in pavement condition caused by a single axle of a heavy vehicle was proportional to the fourth-power of its static load (Cebon 1999, pp. 9-10). These results finally yielded the relationship that pavement engineers had been seeking, known as the fourth-power law. The fourth-power law, shown in Equation 2- 1, enabled the dramatic simplification in estimating the dynamic tyre forces induced by various vehicles in traffic, where all vehicles could now be defined in terms of their Equivalent Standard Axle Loads (ESALs) (Cebon 1999, p. 10).

4 0

.

P

No ESALs

P





=  





where 𝑃𝑃 is the static load and 𝑃𝑃₀ is generally 80 kN.

The validity of the fourth-power law has been questioned by numerous authors, particularly the variation in the value of the exponent due to a number of factors including diverse pavement types, suspensions, axle configurations, traffic volume and tyre sizes, among others (Cebon 1999, p. 10). Despite the criticism, the use of ESALs and the fourth-power law still hold significant influence for economic and political decision-making (Cebon 1999, p. 10). The CEU (1996) outlined the requirements for road-friendly suspensions (or equivalent to air suspension) under directive 92/7/EEC, later amended by 96/53/EC, as:

1) The sprung mass must have a natural frequency no higher than 2.0 Hz.

2) The damping ratio of the suspension system must be more than 0.20 during operation. 3) The damping ratio of the suspension system, with all shock absorbers removed or

ineffective, must be no more than 0.50.

4) For multiple axle groupings, the static load-sharing between each axle must be within 5 %.

The Department of Transport and Regional Services (DoTaRS) in Australia use the same guidelines and test methods to determine road-friendliness as the CEU, outlined under the Vehicle Standards Bulletin 11 (VSB11) (2004). Heavy vehicles equipped with a road-friendly suspension system are permitted to operate with increased mass limits. The next major study, overseen by the Organisation for Economic Co-operation and Development (OECD), was the Dynamic Interaction between Vehicles and Infrastructure Experiment (DIVINE) (1998). This worldwide study, with active participation from seventeen member countries of the OECD, was separated into six main elements outlined in Table 2-A.

Table 2-A: The six research elements of DIVINE and the leading organisations of each element (OECD 1998).

No. Research Element Research Element Leader Country

1 Accelerated dynamic pavements tests

Technical Research Centre of Finland, Federal Highway Administration

Finland, USA 2 Pavement primary

response testing

Federal Highway Administration USA

3 Road simulator testing National Research Council Canada 4 Computer simulation of

heavy vehicle dynamics

Netherlands Organisation for Applied Scientific Research

Netherlands

5 Spatial repeatability of dynamic loads

French Institute of Science and Technology for Transport

France

6 Bridge dynamic loads Swiss Federal Laboratories for Materials Science and Technology

Switzerland

Road-friendliness was further investigated and examined in DIVINE, with emphasis on the comparison between steel and air suspension systems fitted to heavy vehicles (namely trucks). One of the key findings of this study was that heavy vehicles with air suspension generally exhibit lower dynamic loads, expressed as the rms dynamic tyre force divided by the static force, commonly known as the Dynamic Load Coefficient (DLC), compared to common steel- leaf spring suspension systems. Previous studies discovered the most important factors for road- friendly suspensions are low sprung mass natural frequency, by means of low spring stiffness and reduced coulomb damping (friction), along with adequate viscous damping of the suspension system (OECD 1998).

The study also examined the relationship between the dynamic characteristics of a vehicle (namely the sprung mass natural frequency and the damping ratio of the suspension system) and the DLC. The study observed that a “strong relationship exists between the sprung mass frequency and DLC” (OECD 1998). The viscous damping was also found to exhibit significant influence on the DLC, i.e. as the viscous damping decreases, the DLC increases rapidly (OECD 1998). Some key recommendations from the DIVINE study were that the sprung mass natural frequency, outlined under CEU directive 96/53/EC, be reduced from 2.0 Hz to 1.5 Hz and interestingly noted that suspension damping ratios above 0.20 yield “diminishing benefits” (OECD 1998). Ideally, the dampers should be replaced once they fall below 0.20, however

significant reductions in the road-friendliness of heavy vehicles may not occur until the dampers fall below 0.10 – 0.15 (OECD 1998). One shortcoming of the DIVINE study was the lack of attention given to the payload conditions of the various heavy vehicles. As heavy vehicles are often equipped with a trailer (which may be loaded with a wide variety of products), it is difficult to control and account for the variation in the payload being transported. Considering that the sprung mass natural frequency of a vehicle is also a function of the payload mass, variations in the dynamic characteristics are to be expected and may alter the system from its original configuration.

In order to effectively apply these recommendations, the dynamic characteristics of the sprung mass (or the FRF) of heavy vehicles must be established. Numerous techniques exist to estimate these dynamic characteristics; however they have not been evaluated to determine their accuracy and repeatability. A method to accurately estimate the FRF of a vehicle during normal operation is important to establish whether a heavy vehicle may be considered road-friendly.