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OCR for page 4
2.0 Weight Data
The Pavement Design Guide procedure, developed under NCHRP Project 1-37A, performs a
detailed analysis of pavement deterioration caused by single, tandem, tridem, and quad axles
of varying weights. Accordingly, instead of estimates of 18,000-pound equivalent single-axle
loads (ESALs), the Pavement Design Guide software requires estimates of the numbers and
weights of four types of axles: single, tandem, tridem, and quad.
For each VC and axle type, it is necessary to estimate the percentages of axles falling into each
of several specified load ranges. These load ranges are listed in Table 2.1. For single axles, there
are thirty-eight 1,000-pound load ranges covering the 3,000- to 41,000-pound weight range
plus one range for lighter axles. For tandem axles, there are thirty-eight 2,000-pound ranges
covering the 6,000- to 82,000-pound weight range, etc.
For each VC, the Pavement Design Guide software requires (and the Project 139 software,
TrafLoad, produces) between one and four of these load distributions, also known as load
spectra. The Pavement Design Guide software treats the load spectra for each VC as being con-
stant over time. The load spectra are obtained by aggregating vehicle weight data collected at
one or more weigh-in-motion (WIM) sites.
The WIM data used for developing the load spectra also produce the average number of axles
of each type associated with each class of vehicle. For example, a standard five-axle tractor
semi-trailer has one single axle and two tandem axles. Thus, to compute the total load applied
by 1,000 of these trucks, it is necessary to multiply the single-axle load spectrum by 1,000 and
the tandem-axle spectrum by 2,000. The expected number of axles of each type for a given
number of vehicles in a particular VC is developed from WIM data in conjunction with the
development of the load spectra.
For any pavement project, the required load spectra are developed by TrafLoad using data col-
lected from WIM equipment either on the same road at a site reasonably near the pavement proj-
ect (Pavement Design Guide Level 1 data), or from WIM data collected elsewhere (Levels 2 and
3). From these same data, TrafLoad develops the number of axles of each type associated with
each class of vehicle.
The first section of this chapter discusses potential sources of error in the load spectra so that
readers can understand what is important to ensure accurate load estimates, why some data-
collection plans are better than others, and why specific data are requested for developing
accurate load estimates. Section 2.2 provides brief descriptions of the three levels of WIM data
collection that TrafLoad supports. Sections 2.32.5 discuss in more detail the three levels of
data collection and provide guidelines for collecting the required data. And the final section
of this chapter presents additional information on the collection of weight data.
2-4
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Table 2.1 Load Ranges Used for Load Spectra
a
Upper Limit of Load Range (kips )
by Type of Axle Group
Load Range Single Tandem Tridem Quad
1 3 6 12 12
2 4 8 15 15
3 5 10 18 18
4 6 12 21 21
5 7 14 24 24
6 8 16 27 27
7 9 18 30 30
8 10 20 33 33
9 11 22 36 36
10 12 24 39 39
11 13 26 42 42
12 14 28 45 45
13 15 30 48 48
14 16 32 51 51
15 17 34 54 54
16 18 36 57 57
17 19 38 60 60
18 20 40 63 63
19 21 42 66 66
20 22 44 69 69
21 23 46 72 72
22 24 48 75 75
23 25 50 78 78
24 26 52 81 81
25 27 54 84 84
26 28 56 87 87
27 29 58 90 90
28 30 60 93 93
29 31 62 96 96
30 32 64 99 99
31 33 66 102 102
32 34 68
33 35 70
34 36 72
35 37 74
36 38 76
37 39 78
38 40 80
39 41 82
a One kip = 1,000 pounds = 4.448 kN.
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