Vehicle Damage Factor (VDF)
Vehicle damage factor (VDF) is the number of standard axles per truck. So determination of VDF can be made through the determination of the load equivalency factor (LEF) for each axle of the truck and then taking the sum total of the equivalent standard axles for all the axles in the truck. The concept of LEF can be approached in a different (different from AASHTO/structural number procedure) way way also (as (as mentioned mentioned in Yoder Yoder and Witczak, Witczak, 1975). Note that the ratio of damage damage by a single pass of the axle in question to a standard axle is the LEF. If Na passes of an axle cause failure (note that the “failure” has to be defined) of a pavement, pavement, as do N s passes of a standard axle, then the damage due to one pass of the axle and the standard axle can be denoted by 1/N a and 1/Ns, respectively. According to the definition of LEF, LEF LEF
1 Na 1 Ns
Ns Na
Tests with fatigue cracking and analysis of cracking data with respect to tensile strain data show that the number of repetitions to failure of a pavement by fatigue cracking, due to repeated tensile strain, can be expressed as follows:
1 N f k 1 εt
k 2
where Nf is the repetitions to failure, εt is the tensile strain in the asphalt mix layer, and k 1 and k 2 are constants obtained by plotting the experimental data (log εt versus log Nf ). It follows that LEF can also be then expressed as follows:
ε LEF LEF ta ε ts
k 2
where εta and εts correspond to the strains for the axle in question and the standard axle. The value of k 2 has been reported to be between 3 and 6, with most common values being 4 – 5. 5. The most simple approach (and proven to be a good approximation) is to use the fourth power law to to convert any axle load to an an LEF as follows:
W LEF a Ws
4
For example, 4
40,000 LEF for a 40,000-lb load 24.4 18,000 (In comparison, the AASHTO LEF for a 40-kip single-axle load for a flexible pavement with SN = 5 and p t = 2.5 is 21.1, and that for SN = 6 and p t = 2.5 is 23.0.)
Calculation of VDF The calculation of VDF is similar to that of truck load factor, in AASHTO Guide, as follows. This example is for five axle (two tandem and one single) tractor trailers for a specific weight station.
TABLE 5.7 Example of Calculation of Load Equivalency Factor (LEF) for Structural Number = 5, pt = 2.5a Single-Axle Load (lb) LEF Number of Axles 18-Kip EquivalentAxle Load (EAL) Under 3000 0.0002 0 0 3000 – 6999 0.0050 1 0.005 7000 – 7999 0.0320 6 0.192 8000 – 11,999 0.0870 144 12.528 12,000 – 15,999 0.3600 16 5.760 26,000 – 29,999 5.3890 1 5.3890 Tandem-Axle Load LEF Number of Axles 18-Kip EAL (lb) Under 6000 0.0100 0 0 6000 – 11,993 0.0100 14 0.140 12,000 – 17,999 0.0400 21 0.924 18,000 – 23,999 0.1480 44 6.512 24,000 – 29,999 0.4260 42 17.892 30,000 – 32,000 0.7530 44 33.132 32,001 – 32,500 0.8850 21 18.585 32,501 – 33,999 1.0020 101 101.202 34,000 – 35,999 1.2300 43 52.890 18-kip EALs for all trucks 255.151 Therefore, for five-axle tracks, as weighed in this specific weigh station, 18 kip EALs for all trucks 255.151 Truck load factor 1.5464 Number of trucks 165 a pt = terminal serviceability. Note: The numbers in this table represent data obtained from a weigh station (W-4 f orm) for 165 trucks of five-axle, tractor semitrailer type. Source: From AASHTO Guide for Design of Pavement Structure © 1986, by the American Association of State Highway and Transportation Officials, Washington D.C. Used by permission.
