Highway design in most states is based on the ESAL traffic input anticipated over a future 10 to 50 year period.ДЄlong with load type and repetitions, the load distributions across a particular pavement must be estimated. This information then helps determine the structural design. Typically, designers must not only calculate ESALs or load spectra for various vehicles but also must forecast the expected number of ESALs or load spectra a pavement will encounter over its entire design life. Example load spectra input screen from NCHRP 1-37A. Often, load spectra data can be obtained from weigh-in-motion stations.ДЇigure 7. single axle, tandem axle, tridem axle, quad axle) over a given time period (Figure 7). typical load spectrum input would be in the form of a table that shows the relative axle weight frequencies for each common axle combination (e.g. This is a more precise characterization of traffic but relies on the same input data used to calculate ESALs.

Load spectra.The 2002 G uide for the Design of New and Rehabilitated Pavement Structures (NCHRP 1-37A) essentially does away with the ESAL and determines loading directly from axle configurations and weights.

is the 80 kN (18,000 lbs) equivalent single axle load (normally designated ESAL). The most commonly used equivalent load in the U.S. Based on AASHO Road Test results, the most common approach is to convert wheel loads of various magnitudes and repetitions (вЂњmixed trafficвЂќ) to an equivalent number of вЂњstandardвЂќ or вЂњequivalentвЂќ loads. Furthermore, it is not the wheel load but rather the damage to the pavement caused by the wheel load that is of primary concern.There are currently two basic methods for characterizing wheel load repetitions: Repetition of Wheel LoadsДЄlthough it is not too difficult to determine the wheel and axle loads for an individual vehicle, it becomes quite complicated to determine the number and types of wheel/axle loads that a particular pavement will be subject to over its entire design life. One of the most common is the FHWA bridge formula (sometimes called the Federal Bridge Formula B). (based on data from USDOT, 2000).ДЄlthough each state and the FHWA have established maximum axle-tire load combinations, there are other restrictions as well.

Range of allowable axle and truck weights in the U.S. Figure 6 shows the range of maximum limits for single axle, tandem axle and gross vehicle weight (GVW) established by the states and the FHWA.ДЇigure 6. vary a bit based on various Federal and State laws. Tandem drive axle on a tractor frame during manufacturing.ДЇederal and State laws establish maximum axle and gross vehicle weights to limit pavement damage. Tire-axle combinations (from Mahoney, 1984).ДЇigure 5.

Single axle single tire(truck steering axles, etc.)ДЇigure 4.

Tire-axle combinations are typically described as (Figure 4): Therefore, axle and tire arrangements are quite important. As tire loads get closer together their influence areas on the pavement begin to overlap, at which point the design characteristic of concern is no longer the single isolated tire load but rather the combined effect of all the interacting tire loads. While the tire contact pressure and area is of vital concern in pavement performance, the number of contact points per vehicle and their spacing is also critical. Structural design calculations using load spectra are generally more complex than those using ESALs.Д«oth approaches use the same type and quality of data but the load spectra approach has the potential to be more accurate in its load characterization.

It does not involve conversion to equivalent values. This approach characterizes loads directly by number of axles, configuration and weight. This approach converts wheel loads of various magnitudes and repetitions (вЂњmixed trafficвЂќ) to an equivalent number of вЂњstandardвЂќ or вЂњequivalentвЂќ loads. This quantification is usually done in one of two ways: Therefore, pavement structural design requires a quantification of all expected loads a pavement will encounter over its design life. This damage is cumulative over the life of the pavement and when it reaches some maximum value the pavement is considered to have reached the end of its useful service life.

The simplest pavement structural model asserts that each individual load inflicts a certain amount of unrecoverable damage. Loads, along with the environment, damage pavement over time.

Distribution of traffic across the pavement.

Loads, the vehicle forces exerted on the pavement (e.g., by trucks, heavy machinery, airplanes), can be characterized by the following parameters: Therefore, in order to adequately design a pavement something must be known about the expected loads it will encounter. One of the primary functions of a pavement is load distribution.

Tandem axle weight distribution