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Publication Number: FHWA-HRT-13-089
Date: October 2013

 

Long-Term Pavement Performance Pavement Loading User Guide (LTPP PLUG)

PART I - GUIDELINES FOR USING LTPP SPS TPF AXLE LOADING DEFAULTS

WHEN AXLE LOADING DEFAULTS SHOULD AND SHOULD NOT BE USED

Many factors can affect the truck loading patterns on a roadway (e.g., the vehicle types observed, the axle configurations on those vehicles, and the axle weights carried). These different patterns result in the application of very different load spectra on those pavements. Among the most important factors affecting the traffic load applied to any given pavement are:

To account for the wide variation in traffic loading patterns, the best traffic data to use for pavement analysis are data collected with well-calibrated equipment on road segments directly leading to or from the site in question. Unfortunately, accurate truck and axle weight data are difficult and expensive to collect. In addition, rough pavement increases the dynamic motion (bouncing) of trucks, making it nearly impossible for WIM scales to collect accurate data on these roads. Thus, site-specific truck weight data frequently are not available for pavement analysis. When this situation occurs, the pavement analyst must use truck characteristic data (load spectra) collected elsewhere.

As a general guide, a pavement analyst should use data from one specific WIM scale when:

Selecting one specific WIM site from another location generally is not encouraged unless the pavement analyst is confident that the trucking patterns at the data collection site match those of the analysis site, even when the scale is on the same road as the pavement analysis site. Loading patterns frequently change from one segment of road to another. For example, Interstate (I)-405 in the Seattle metropolitan region has a very different loading pattern south of the interchange with I-90 than it has north of that interchange. South of the I-90 interchange, I-405 carries a mix of urban delivery trucks and trucks serving the long haul movements that use I-90 to reach the Port of Seattle and the large warehouse district located in the southern part of the metropolitan area. North of I-90, the vast majority of trucks are urban delivery trucks.

If a single scale is not an obvious choice to provide load spectra data, the next option is to select a default load spectrum that represents the average of the loading patterns observed on a group of similar roads within the same State as the pavement analysis site. Part II of this guide describes how to develop these State-specific defaults. The advantage of selecting data that represent a group of sites is that the effects of any unusual condition that may exist at any one site is minimized by the lack of that unusual condition at the other sites in the group. Group averages are thus likely to be more representative of the loading patterns at other locations around the State. Selecting load spectra data collected only from the same State as the pavement to be analyzed also improves the accuracy of the default load spectra because it ensures that the trucks for which data are being collected follow the same truck size and weight laws.

Unfortunately, some States do not have strong truck weighing programs or have yet to produce default load spectra in formats that are available to users of the MEPDG and DARWin-ME software. For example, some States have had considerable difficulty maintaining WIM scale calibration and do not trust much of the truck load data they have collected.

When a State does not have reliable load spectra data, or the pavement analyst does not have access to a specific State's load spectra data (which may be the case with many LTPP analyses), the best option is to use default load spectra developed at the national level.

Using the high-quality data available from the SPS TPF study, the LTPP program has developed national-level load spectra defaults. These defaults were developed to represent a variety of alternative loading conditions observed in the national data sample. This variety helps pavement analysts select appropriate load spectra for their work, and it allows them to test the sensitivity of their analyses by re-running MEPDG analyses using different (e.g., lighter or heavier) default load spectra to determine the degree to which changes in load spectra affect their specific pavement analysis outcome.

When using national-level defaults, analysts are encouraged to perform at least a few MEPDG sensitivity tests simply because load spectra developed at the national level cannot adequately account for the differences in trucking patterns that result from State-specific size and weight laws. Neither can national defaults account for trucking patterns unique to specific roads or regions within a State. At a minimum, MEPDG sensitivity tests provide pavement analysts with insight into the reliability of their analysis results, given the uncertainties inherent in the traffic inputs used to generate those results.

MEPDG TRAFFIC LOADING DEFAULTS BASED ON LTPP SPS TPF WIM SITES

The LTPP program worked with 22 States to install and operate 26 WIM scales under a set of rigorous quality control (QC) procedures, with frequent equipment calibration, testing, and validation. The result is a wealth of reliable traffic loading data that has been used to develop defaults for use with the MEPDG and DARWin-ME software instead of the initial set of defaults created as part of the NCHRP 1-37A study.

Overview of Traffic Loading Defaults Based on SPS TPF WIM Data

NALS are the primary traffic loading defaults produced with the SPS TPF data. Other defaults produced are axle-per-truck coefficients (APC) and other axle spacing and wheelbase parameters used by the MEPDG software.

Two tiers of NALS defaults were developed based on SPS TPF WIM data, as described in the following sections:

In addition to NALS, this guide contains one set of default APCs based on SPS TPF sites. Representative axle spacing and wheelbase values were also computed based on data from all SPS TPF sites and included in this guide.

All defaults are included in the LTPP PLUG database application and can be viewed in graphical or tabular form. These defaults can be used to develop MEPDG or DARWin-ME files using the LTPP PLUG database application.

Global Axle Loading Defaults Based on LTPP SPS TPF Sites (Tier 1)

Global NALS defaults were computed based on a simple averaging of the representative annual NALS (RANALS) for the 26 SPS TPF sites. This computation was done separately for each vehicle class and axle type.

Compared with the original MEPDG NALS defaults developed under the NCHRP 1-37A project, the SPS TPF NALS defaults have fewer very light and fewer very heavy loads. This is most likely because the new defaults were collected with more consistently calibrated WIM equipment. The better calibration of the WIM scales used to collect the SPS TPF data means that fewer very light loads (caused by under-calibrated scales "observing" very light loads) and fewer very heavy loads (caused by over-calibrated scales "observing" very heavy loads) are contained in the LTPP SPS TPF database. Because the SPS TPF-based NALS have smaller percentages of overloads, pavement life predicted using these global defaults is likely to be longer than predicted using the old global defaults.1 However, these differences are not likely to be significant for many designs because the new global defaults also have a higher percentage of legally loaded heavy axles.

Axle Load Spectra Defaults Representing Alternative Loading Conditions (Tier 2)

Tier 2 defaults are supplemental defaults that represent different loading conditions (e.g., conditions heavier or lighter than the global default) observed in the SPS TPF study. Each of the Tier 2 NALS represents a different loading condition. Part II of this guide describes the methodology to develop these alternative NALS groups so that States can follow similar procedures to develop additional, State-specific, axle loading defaults for the trucks operating on their roads. For analysts who do not have access to State-specific NALS, national defaults are available.

Analysis of the loading conditions observed in the LTPP SPS TPF study revealed that truck loading characteristics (axle weights) can vary considerably from location to location. Therefore, it was determined that default NALS should be developed that allowed users of the MEPDG and DARWin-ME software to select from alternative default loading conditions for their analyses to account for those observed differences in traffic loading per vehicle. For example, alternative loading conditions (Tier 2 NALS) can be required when one set of sites experiences a high percentage of trucks in a specific vehicle class that are fully legally loaded while another set of sites exhibits a high percentage of that same vehicle class that is unloaded. A third NALS might be needed to represent sites that experience a much higher than normal percentage of trucks carrying loads that exceed Federal legal limits.

Alternative NALS also may be needed to account for differences in State truck size and weight laws. Differences in State laws can result in trucks of similar FHWA classifications having very different axle configurations and loading conditions for axles of a similar type. For example, many western States use flexible permitting rules to allow multi-trailer trucks to carry additional gross vehicle weight. These trucks are often found in FHWA Class 13. States that allow these additional weights have trucks with both different axle configurations and different axle weight patterns for FHWA Class 13 than States that do not routinely permit these kinds of loads.

Table 1 summarizes the Tier 2 default NALS that have been developed from the SPS TPF datasets. The first two columns in the table allow the user to select the specific FHWA vehicle class and type of axle of interest. For example, FHWA vehicle Class 4, axle type 1, is the row for single axles on buses. On the far right of table 1 is the number of Tier 2 NALS, or different groups of loading patterns, that have been developed for each class of vehicle and type of axle. If only one NALS is present, it means that, from a pavement design perspective, little difference was found in the axle load spectra observed at the 26 sites in the LTPP SFS TPF data, and all pavement analyses can use a single NALS for that type of axle for that class of vehicles.

The third column of table 1 indicates how frequently axles of each specific type appear on U.S. primary roads. This information is based on the data present in the LTPP database. This column is provided to give users an idea of whether specific kinds of trucks are commonly found on roads for which the MEPDG is likely to be used. (Note that individual roads may have truck volumes that are different than those commonly found in the LTPP database.)

The fourth column in table 1 provides two pieces of information. First, it describes the NALS group that contains the largest number of SPS TPF sites. It is recommended that this group be used as the default Tier 2 NALS if no other information is available about loads carried by that class of vehicle. If the analyst has information about loads, he or she may use that information to choose a different NALS for that type of axle and vehicle classification. Second, the term used in the fourth column of the table (e.g., Moderate or Light) describes the relative weight (or pavement damaging potential) of the average axle in that group relative to other axles. Table 2 provides a quantitative definition of this descriptive loading condition, as well as full and abbreviated names of alternative NALS.

As an example of how to read this fourth column of table 1, the default NALS for Class 5 single axles is Very Light. Thus, the average Class 5 single axle is lighter and causes much less damage than the average default axle for either Class 4 or Class 6 single axles. The default single axles for both Class 4 and Class 6 trucks are considered moderate in weight/damage potential.

The next five columns in table 1, labeled "NALS Clusters Observed in Multiple States (Recommended for National Use)," describe the characteristics of the various NALS groups identified in the review of SPS TPF data. They use the same relative description of axle weight/ pavement damaging potential as is used in column 4 and in table 2. For example, the symbol "M" in the "Moderate" column indicates that a group of SPS TPF sites was found to have NALS similar to each other for that class of vehicle and type of axle, and the default NALS developed based on site-specific NALS included in this group is of moderate weight/damage potential.

The next five columns, labeled "NALS Clusters in a Single State (Recommended for Use in That State on Roads with Similar Truck Traffic)," describe the relative weight/damage potential of the "special case" or outlier conditions identified in the SPS TPF dataset. Also included in these five columns is an abbreviated name of the State where the WIM site was located that produced the outlier described. For example, Class 6 single axle has two light outlier sites, one in Washington and one in Ohio. Possible reasons for the existence of these special-case NALS include:

The following example illustrates how to read table 1. For Class 6 tandem axles, the review of SPS TPF data resulted in the formation of three NALS. Two of these NALS are formed based on groups of sites. One of these two groups has moderate weight tandem axles. This group contained the greatest number of SPS TPF sites and is thus the default NALS initially recommended for Class 6 tandem axles. A second identified group of SPS TPF sites produced NALS for heavy axles. This NALS is also considered reasonably common on roads throughout the nation. In addition to these two fairly common groups, one SPS TPF site was an outlier. That site, observed in Florida, exhibited very heavy Class 6 tandem axles. This result would indicate that it is possible to have very heavy Class 6 tandem axles but that such a condition is considered unusual. This NALS should be used only if the analyst has specific knowledge that such a loading condition is likely to occur at the analysis site.

NALS for each cluster group shown in table 1 are available in the LTPP PLUG database.

Table 1. Summary of NALS cluster groups representing different loading conditions by vehicle class and axle type


FHWA Vehicle Class

Axle Type

Frequency of Vehicle Class (by Volume) on U.S. Primary Road System

Default NALS Category by Loading Condition

NALS Clusters Observed in Multiple States (Recommended for National Use)

NALS Clusters Observed in a Single State (Recommended for Use in That State on Roads with Similar Truck Traffic)

Total NALS Clusters by Weight

Very Light (VL)

Light (L)

Moderate (M)

Heavy (H)

Very Heavy (VH)

Very Light (VL)

Light (L)

Moderate (M)

Heavy (H)

Very Heavy
(VH)

4

1

Moderate

Moderate

 

 

M

 

 

 

 

 

 

 

1

2

Very Heavy

 

 

 

H

VH1, VH2

 

 

 

 

 

3

5

1

Frequent

Very Light

VL

 

 

 

 

 

L (FL)

 

 

 

2

2

Very Light

VL

 

 

 

 

 

 

 

 

 

1

6

1

Low or Moderate

Moderate

 

 

M

 

 

 

L (WA)
L (OH)

 

 

 

3

2

Moderate

 

 

M

H

 

 

 

 

 

VH (FL)

3

7

1

Low

Heavy

 

 

M

H

 

VL (WA)

 

M (OH)

 

 

4

2

Heavy

 

 

M

 

VH

VL (WA)

 

M (OH)

H (OH)

VH (DE), VH (FL1), VH (FL2),
VH (TN)

7

3

Very Heavy

 

 

 

 

VH1, VH2

 

 

 

 

VH (TN)

3

4

Very Heavy

 

 

 

 

VH

 

 

 

 

 

1

8

1

Moderate

Light

 

L

 

 

 

 

 

 

H (FL)

 

2

2

Light

 

L

 

 

 

 

L (AZ)2

 

 

VH (FL)

3

9

1

Most Frequent

Light

 

L

 

 

 

 

 

 

 

 

1

2

Moderate or Heavy

 

 

M

H1, H2

 

 

 

M (FL)

 

VH (AZ)1

5

10

1

Low

Light

 

L

 

 

 

 

 

 

 

 

1

2

Very Heavy

 

 

M

 

VH

 

 

 

 

VH (AZ)

3

3

Heavy

 

 

 

H

VH

 

L (ME)

M (MN)

 

 

4

4

Heavy

 

 

 

H

 

 

 

 

 

 

1

11

1

Low

Moderate

 

L

M

 

 

 

 

 

H (AZ)

 

3

12

1

Low

Light

 

L

 

 

 

VL (NM)
VL(LA)

 

M (ME)

 

 

4

2

Light

 

L

 

 

 

 

 

 

 

 

1

13

1

Low

Moderate

 

 

M

H

 

 

 

M (OH)2

 

VH (OH)

4

2

Very Heavy

 

 

 

H

VH1,VH2

 

 

 

 

 

3

3

Very Heavy

 

 

 

H

VH1,VH2

 

 

 

H (OH)

 

4

4

Very Heavy

 

 

 

H

VH

 

 

 

H (OH)**

 

3

1NALS has very heavy overloads
2NALS identified as outlier based on classification issue (high percentage of very light weight axles)
FL = Florida, WA =Washington (State), OH = Ohio, DE = Delaware, TN = Tennessee. AZ = Arizona, ME = Maine, MN = Minnesota, NM = New Mexico,
LA = Louisiana

Table 2. Summary of axle loading categories by weight for different axle types


Axle Category by Weight

Average RPPIF per Cluster

Percentage of Single Axles
>= 15 kip

Percentage of Tandem Axles >=26 kip

Percentage of Tridem Axles >=39 kip

Percentage of Quad Axles >=54 kip

Very Light (VL)

<0.05

<3

0

N/A

N/A

Light (L)

0.05-0.15

<10

<10

N/A

N/A

Moderate (M)

0.15-0.30

10-30

10-30

N/A

N/A

Heavy (H)

0.30-0.50

>30

30-50

<50

<30

Very Heavy (VH)

>0.50

n/a

>50

>50

>30

RPPIF = Relative Pavement Performance Impact Factor (described in Part II of this guide)
N/A = not applicable

NALS Clusters for Class 9 Tandems

Although for many vehicle classes and axle types there is one clearly defined default loading condition for many vehicle classes and axle types (i.e., the majority of SPS TPF sites have a common axle loading distribution), no one loading condition dominated the Class 9 tandem data. The loading condition of Class 9 tandems varied among the SPS TPF sites, ranging from moderately loaded conditions (less than 30 percent of tandem axles were more than 26 kips) to very heavily loaded (up to 70 percent of tandem axles were more than 26 kips). All of these conditions can be found routinely on U.S. roads.

A sensitivity analysis using the MEPDG procedures indicated that pavement design outcomes are very sensitive to the selection of the Class 9 tandem NALS, primarily because this is the most frequently observed truck carrying heavy loads on U.S. primary roads. Also, the majority of the load on these trucks is carried on their two sets of tandem axles. Hence, the number and weight of Class 9 trucks using a road typically has a major impact on pavement design.

No one loading condition dominated the Class 9 tandem-axle NALS in the SPS TPF dataset; therefore, instead of one default loading condition, all three NALS cluster groups developed for Class 9 tandems (one moderate and two heavy) are recommended as potential defaults.

If no site-specific loading information is available, the moderate (M) loading condition is recommended for roads serving predominately urban delivery trucking movements, where a large percentage of trucks are empty (returning to their distribution centers) or partially full after having made one or more stops. Alternatively, the heavy #1 (H1) loading condition is recommended for roads where long haul trucking or heavy directional hauls overlap with urban delivery movements. The heavy #2 (H2) loading condition is recommended for rural roads where almost all Class 9 truck traffic is fully loaded long haul traffic and few heavy trucks are expected to be operating in a partially full or empty condition.

Characteristics and Recommended Use of Tier 2 NALS Clusters

To help characterize the NALS representing default and alternative axle loading conditions so that pavement analysts can more easily choose between alternative loading conditions, several statistical parameters were computed for each Tier 2 default NALS cluster, including Relative Pavement Performance Impact Factor (RPPIF) values and percentage of heavy axles (i.e., the percentage of axles that are at or above 75 percent of the Federal legal weight limit). In addition, text descriptions were developed of alternative NALS axle loading conditions for each vehicle class and axle type and recommendations for their use. This information is presented in the following subsections.

Vehicle Class 4: Buses (Table 3)

There are multiple styles of buses, ranging from over-the-road coaches (e.g., Greyhound buses), to two- and three-axle urban transit buses, to recreational vehicles (RV) that have been constructed using bus frames and bodies (e.g., tour buses used by many traveling bands). Buses commonly have one steering axle and either a single rear drive axle or a tandem rear axle.

Because none of the SPS TPF WIM scales observed large numbers of urban transit buses, which can have very heavy single axles, a large portion of the observed single axles were steering axles. As a result, all of the observed sites had similar single-axle loading patterns and were grouped into one national NALS, with a moderate loading condition. On the other hand, three different tandem-axle loading conditions were identified. Tandem axles on buses are generally at least moderately loaded because they tend to carry the weight of the bus engine. As the number of passengers carried increases, the axle weight increases further, often approaching or exceeding the Federal axle limits. Lighter tandem axles generally are observed when the buses are lightly loaded with passengers, as is often the case when the "buses" are RVs rather than over-the-road motor coaches. Table 3 describes the alternative Class 4 axle loading conditions.

Table 3. Description of alternative Class 4 axle loading conditions


Vehicle Class

Axle Type

NALS Cluster

RPPIF

% Heavy Axles

Description

Recommended Use

4

1

M(T)

0.20

14

Only one national NALS exists for single axles on buses. It shows a relatively normal distribution of moderately heavily loaded axles with the majority of the load distribution occurring between 10,000 and 16,000 lb.

For use on all roads

4

2

H

0.42

46

This NALS has a large percentage (greater than 65 percent) of tandem axles that weigh less than 28,000 lb and a third of the axles weighing less than 22,000 lb. These lighter axles may be caused by the site observing a mix of RVs built on bus chassis mixed with heavier, more conventional over-the-road motor coaches.

For use on roads where a moderate percentage of buses are "bus style" RVs

VH1(T)

0.56

68

This NALS is recommended as the default. Most of tandem axles are heavy (approximately 70 percent are greater than 26,000 lb), but relatively few (less than 2 percent) exceed the Federal legal limit. Therefore, these sites likely have a lower percentage of RV traffic mixed with the over-the-road coaches.

For use on rural roads where RV traffic is modest

VH2

0.69

85

This NALS contains almost exclusively heavy tandem axles. Less than 16 percent of tandem axles weigh less than 26,000 lb.

For use on roads carrying heavily loaded transit buses and over-the-road coaches

Vehicle Class 5: Two-Axle, Six-Tire Single Unit Vehicles (Table 4)

Class 5 consists of smaller two-axle trucks. This class includes large pick-up trucks (those with dual wheels on their rear axle), as well as urban delivery vehicles, such as panel trucks and smaller tow trucks. By definition, when these vehicles pull trailers, they become Class 8 trucks. (This is unlike conventional Class 3 pick-ups, which remain in Class 3, even when they are pulling small trailers.) However, some classification algorithms do classify Class 5 trucks pulling trailers as Class 5. This results in some TPF sites observing tandem axles in Class 5.

In the SPS TPF dataset, all but one condition for both single and tandem axles are grouped into just one NALS group for each type of axle. There is one special case. In that case, a Florida site was observed at which much heavier single axles were observed than at all other sites. This was because two-axle farm vehicles transport very heavy loads on this road (when compared with normal Class 5 axle loads). Table 4 describes the alternative Class 5 axle loading conditions.

Table 4. Description of alternative Class 5 axle loading conditions

Vehicle Class

Axle Type

NALS Cluster

RPPIF

% Heavy Axles

Description

Recommended Use

5

1

VL(T)

0.04

3

This NALS is recommended as a Tier 2 default. It is appropriate for all roads that are not subject to unusual loading conditions.

For use on all roads

SP L(FL)

0.13

8

This special case is caused by a permitted farm-to-market haul that causes an unusually heavy loading condition for Class 5 trucks. It was found only at one Florida site.

A special case where a large percentage of large pick-ups and other two-axle trucks carry unusually heavy loads for two axle vehicles

5

2

VL(T)

0.00

0

This NALS is recommended as a Tier 2 default. These axles are generally found on small trailers pulled by small trucks and are invariably light.

For use on all roads

Vehicle Class 6: Three-Axle Single Unit Vehicles (Table 5)

Class 6 trucks are frequently smaller natural resource haulers (e.g., sand trucks, small dump trucks) and heavier urban delivery vehicles, such as trucks delivering cases of soft drinks. For these trucks, the weight on the front axle is primarily from the engine, not the load carried, and thus these axles are rarely either unloaded or very heavily loaded. The Class 6 classification also includes tractors operating without trailers. The presence of large numbers of these vehicles in the traffic stream can result in large numbers of lightly loaded tandem axles. Finally, many heavy (Class 7) single unit vehicles are equipped with "lift axles" (i.e., axles which, when not needed, can be lifted off the ground). Thus, trucks that are Class 7 vehicles when full (and all of their axles are on the ground to support the load) are often classified as Class 6 trucks when they are empty because only two of their load axles are touching the pavement. This often increases the number of lightly loaded Class 6 trucks recorded, increasing the percentage of lightly loaded tandem axles at many sites. Table 5 describes the alternative Class 6 axle loading conditions.

Table 5. Description of alternative Class 6 axle loading conditions

Vehicle Class

Axle Type

NALS Cluster

RPPIF

% Heavy Axles

Description

Recommended Use

6

1

M(T)

0.17

10

This NALS is recommended as a Tier 2 default. The vast majority of roads fall into this loading group. Most single axles are steering axles. The weight on the steering axle is primarily from the engine not the load carried. Few axles weighing more than 14,000 lb are observed.

For use on all roads

6

1

SP L(OH)

0.10

3

The vehicle classification scheme used by Ohio can classify some vehicles pulling trailers as Class 6. These light trailers produce much lighter single-axle load spectra than are commonly found elsewhere, with many axles weighing less than 6,000 lb.

For use on some Ohio roads

6

1

SP L(WA)

0.08

3

Washington's WIM data produced load spectra with an unusual percentage (greater than 20 percent) of light axles.

For use on Washington State roads

6

2

M(T)

0.24

20

This NALS is recommended as a Tier 2 default. This NALS represents roadways where up to 50 percent of Class 6 trucks are operating empty or lightly loaded (approximately 40 percent of tandem axles weigh less than 12,000 lb, and approximately 50 percent of tandem axles weigh less than 16,000 lb). The vast majority of roads fall into this default loading group.

For use on most roads, especially where more than 40 percent of Class 6 trucks are operating empty

6

2

H

0.43

30

This NALS represents roadways where less than 30 percent of Class 6 trucks are operating empty. A commensurately larger percentage of trucks are carrying full loads. This is a good NALS to select for roads with a strong directional haul that uses Class 6 trucks (e.g., a farm to market road where this direction is the loaded direction of the haul).

For use on roads with strong directional hauls where Class 6 trucks carry heavy roads

6

2

SP VH(FL)

0.63

36

Data at one Florida site showed a large percentage of very heavily loaded Class 6 tandem axles. This site had 5 percent of its tandem-axle loads greater than 40,000 lb, which is highly unusual for Class 6.

For use on roads experiencing high overloading conditions with Class 6 trucks

Vehicle Class 7: Four-or-More Axle Single Unit Vehicles(Table 6)

Class 7 consists of very large single unit trucks. These trucks are most commonly used for hauling heavy natural resources. They are often very large dump trucks. In the Midwest, they are often used to carry coal. The most common configuration is a single steering axle and a tridem axle that carries the majority of the load. Less common configurations use a quad axle instead of a tridem axle, or a tridem with an extended lift axle (observed as a single), or even a quintuple axle. A larger percentage of these vehicles use lift axles. These axles are usually lowered (used to carry load) when the vehicle is carrying a load, and raised (so that they do not touch the pavement) when the vehicle is empty. As a result, relatively few empty Class 7 trucks are observed at scales. When empty, these trucks are most commonly configured as Class 6 vehicles. The use and configuration of Class 7 trucks varies considerably across the nation based on State-specific size and weight regulations. Both the axle configurations and axle weights are quite variable. Class 7 trucks, along with Class 13 trucks, are possibly the most diverse categories within FHWA's vehicle classification scheme. In addition, the Class 7 truck is uncommon in some States but quite common in others. Tandems are reasonably rare axles on Class 7 trucks. Only 17 of 26 sites observed enough tandems on Class 7 vehicles to construct a site-specific NALS. Table 6 describes the alternative Class 7 axle loading conditions.

Table 6. Description of alternative Class 7 axle loading conditions

Vehicle Class

Axle Type

NALS Cluster

RPPIF

% Heavy Axles

Description

Recommended Use

7

1

M

0.26

26

This group of NALS contains few axles (less than 10 percent) that weigh less than 10,000 lb. The majority of single axles at sites belonging to this group weigh between 11,000 and 18,000 lb, with a mean axle weight of about 13,000 lb.

For use on roads where dirt and other heavy natural resources are typically carried by Class 10 trucks not Class 7 trucks

7

1

H(T)

0.41

52

This NALS is recommended as a Tier 2 default. This NALS contains few lightly loaded axles. The majority of single axles that fall in this group weigh between 13,000 and 22,000 lb, with a mean axle weight of about 18,000 lb. This default group contains axles that are slightly heavier than the alternative moderate (M) group.

For use on roads where five-axle and larger Class 7 trucks are routinely found or where state laws routinely permit very heavy single unit trucks

7

1

SP VL(WA)

0.04

4

The Washington SPS TPF site observed a larger number of light single axles on Class 7 vehicles (approximately 70 percent weighing less than 6,000 lb). This may be the result of a limitation in the Washington vehicle classification algorithm.

For use on roads in Washington State

7

1

SP M(OH)

0.18

18

One of the two Ohio SPS TPF sites observed a larger number of light single axles on Class 7 vehicles. (More than 40 percent weigh less than 7,000 lb.) This may be the result of a limitation in the Ohio vehicle classification algorithm. The large percentage of light axles was somewhat offset by 35 percent of the axles being heavier than 14,000 lb.

For use on roads in Ohio, where large percentages of non-Class 7 vehicles may be classified as Class 7 trucks

7

2

M

0.24

22

Tandems are reasonably rare axles on Class 7 trucks. For this NALS, more than 50 percent of quad axles weigh less than 12,000 lb.

For use on roads serving the "light" direction (backhaul) of a directional natural resource haul

7

2

VH(T)

0.85

64

This NALS is recommended as a Tier 2 default. Tandems are reasonably rare axles on Class 7 trucks. The default has a low percentage of tandem axles supporting little weight but also does not have a concentration of any specific axle weight. Less than 10 percent of these axles carry less than 12,000 lb, and while more than 40 percent of tandem axles weigh more than 30,000 lb, less than 15 percent weigh more than 40,000 lb.

For use on roads where Class 7 trucks use tandem axles and for directional, resource hauls

7

2

SP H(OH)

0.41

28

Ohio has two special-case Class 7 NALS. This specific special case is the heavier of the two special Ohio cases. It has more than 15 percent of axles with little or no load (weighing less than 6,000 lb) and 35 percent of its axles weighing between 20,000 and 36,000 lb.

For use in Ohio, where heavy loads are observed

7

2

SP M(OH)

0.29

16

Ohio has two special case Class 7 NALS. This specific special case has 45 percent of its axles weighing between 16,000 and 26,000 lb. Less than 10 percent of axles weigh more than 32,000 lb. More than 15 percent of axles carry little or no load (weighing less than 6,000 lb).

For use in Ohio, where few very heavy loads are observed

7

2

SP VH(DE)

2.42

81

The Delaware TPF is very heavy. Very few unloaded axles were observed. More than half the tandem axles observed weigh more than 40,000 lb.

For use in Delaware

7

2

SP VH(FL1)

3.48

100

This Florida NALS is very heavy. Less than 10 percent of axles weigh less than 40,000 lb.

For use in Florida, and only where heavy overloads of Class 7 trucks are expected

7

2

SP VH(FL2)

6.11

98

This Florida NALS is very heavy. More than 50 percent of the observed Class 7 tandem axles weigh more than 48,000 lb.

For use in Florida only where extreme overloads of Class 7 trucks are expected

7

2

SP VH(TN)

9.94

91

This Tennessee NALS is very heavy. More than 85 percent of the observed Class 7 tandem axles weigh more than 50,000 lb.

For use on roads in Tennessee where tandem axles are heavily overloaded

7

2

SP VL(WA)

0.03

3

This Washington NALS is very light. More than 60 percent of its tandem axles weigh less than 12,000 lb.

For use only where a high percentages of Class 7 trucks are not loaded

7

3

VH1(T)

0.65

55

This NALS is recommended as a Tier 2 default. The default NALS has most axles weighing between 30,000 and 54,000 lb.

For use on roads where tridem axles are routinely fully loaded

7

3

VH2

1.55

96

This tridem NALS should be used where Class 7 tridem axles are primarily loaded well over the Federal legal limit. The median axle weight for this NALS is roughly 54,000 lb.

For use on roads where tridem axles are heavily overloaded

7

3

SP VH(TN)

2.25

98

This special case NALS has extremely heavy tridem axles. The median axle weight is just under 60,000 lb.

For use on roads in Tennessee where tridem axles are heavily overloaded

7

4

VH(T)

0.78

31

This NALS is recommended as a Tier 2 default. The SPS TPF data showed only a few sites that contain quad axles in Class 7. Thus, only one NALS is provided for quad axles.

For use on all roads

Vehicle Class 8: Four-or-Less Axle, Single Trailer Vehicles (Table 7)

Class 8 consists of smaller two-unit trucks. Common configurations include two-axle tractors pulling a semitrailer with a tandem axle, or a three-axle tractor pulling a semitrailer with a single trailer axle, or a dual axle truck pulling a two-axle trailer. The trailer can consist of two single axles, or it can be a semitrailer with one tandem axle. These trucks tend to be lightly loaded because heavier tractors are needed to pull heavy loads, and the trailers pulled by most dual axle trucks tend to be light-duty configurations. Table 7 describes the alternative Class 8 axle loading conditions.

Table 7. Description of alternative Class 8 axle loading conditions

Vehicle Class

Axle Type

NALS Cluster

RPPIF

% Heavy Axles

Description

Recommended Use

8

1

L (T)

0.11

9

This NALS is recommended as a Tier 2 default. The SPS TPF data found only one group NALS and one outlier for Class 8 single axles. This NALS is dominated by loads associated with the steering axle weight of two- and three-axle tractors, with the vast majority of single-axle weights in the range of 8,000 to 14,000 lb. Generally, less than 20 percent of axles are heavier than 13,000 lb.

For use on all roads

8

1

SP
H (FL)

0.34

31

One Florida site had a very different NALS for Class 8 single axles. It showed more than 25 percent of single axles weighing more than 17,000 lb.

For use on Florida roads serving farms where overloaded axles are common

8

2

L (T)

0.10

5

This NALS is recommended as a Tier 2 default. The default NALS has the vast majority of Class 8 tandem axles weighing less than 24,000 lb, and almost no illegal tandem axles.

For use on all roads

8

2

SP
L (AZ)

0.09

6

One special case was observed in Arizona. This site had more than 40 percent of tandem axles weighing less than 6,000 lb. The rest of the distribution resembled the typical NALS.

For use on roads in Arizona where a high percentage of unloaded axles is common

8

2

SP
VH (FL)

1.09

56

The Florida special case captured a farm-to-market movement where trucks pulling tandem-axle trailers were very heavily loaded. More than 50 percent of tandem axles weighed more than 34,000 lb.

For use on Florida roads serving farms where overloaded axles are common

Vehicle Class 9: Five-Axle, Single Trailer Vehicles (Table 8)

Class 9 is by far the most common truck in use in the United States and, as such, dominates the vast majority of pavement loading computations. Although a number of configurations can be found, the most common Class 9 truck is the "classic" three-axle tractor pulling a tandem-axle equipped semitrailer. Infrequently, the semitrailer can use "split tandems," which, depending on the distance between those axles, can be classified as either two single axles or one tandem. In addition, the common European configuration of a two-axle tractor pulling a tridem-equipped semitrailer can be observed occasionally in the United States. Finally, some full trucks pulling full trailers are correctly classified as Class 9 trucks.

Because so many Class 9 trucks have single-tandem, tandem-axle configurations, the axle weights for single axles are dominated by the steering axle weight of the truck tractor. Because the steering axle on even a fully loaded truck does not approach the legal load limit for single axles, the axle damaging characteristics of the single axles at all of the SPS TPF sites are similar. Thus, only a single-axle NALS group exists.

Unlike the single axles, considerable variety exists in the tandem-axle loads carried by Class 9 trucks. This variety is driven in part by the wide variety of loads carried by these trucks, as well as by their use in both urban delivery modes (where some trailers are full, some are partially full after completing one of several deliveries, and others are empty as the trucks return to their terminals after completing a delivery) and long haul transport (where the vast majority of trucks are fully loaded). But even in long haul travel, a "fully loaded" truck may not approach the legal limit because many commodities (e.g., computer parts) are bulky but not heavy.

Consequently, three different loading conditions are considered "typical" or commonly found. Any one of them can be found in any U.S. State. These three loading conditions differ primarily in the percentage of heavy (fully loaded) axles that were observed. When selecting from among these three conditions, the primary consideration is the percentage of tandem axles that are lightly loaded (weigh less than 20,000 lb) and the percentage that are heavily loaded (weigh more than 30,000 lb). Table 8 describes the alternative Class 9 axle loading conditions.

Table 8. Description of alternative Class 9 axle loading conditions

Vehicle Class

Axle Type

NALS Cluster

RPPIF

% Heavy Axles

Description

Recommended Use

9

1

L(T)

0.14

9

This NALS is recommended as the Tier 2 default. Only one Class 9 single-axle NALS was observed because the majority of Class 9 trucks have only one single axle (the steering axle) and its load is only modestly affected by the load carried by the truck.

All roads

9

2

M

0.30

31

This NALS represents the loading condition frequently observed at SPS TPF sites. This loading condition has more lightly loaded tandem axles (axles between 12,000 and 16,000 lb) than heavily loaded axles (30,000 and 34,000 lb). In this NALS, 40 percent of axles carry loads greater than 20,000 lb.

For use in urban areas and on other roads where 70 percent or more of trucks are not fully loaded

9

2

H1(T)

0.38

39

This loading condition NALS has more loaded axles (between 30,000 and 34,000 lb) than unloaded axles (between 12,000 and 16,000 lb). This is a balanced distribution with similar total percentages of light and heavy loads. In this NALS, 55 percent of axles carry loads greater than 20,000 lb.

For use on highways that serve a mix of urban delivery and long haul truck movements

9

2

H2

0.48

49

This heavy loading NALS is representative of roads where a sizeable majority of tandem axles are heavily loaded. It is commonly found on rural highways that serve significant long haul truck movements.

For use on rural highways that serve significant long haul truck movements

9

2

SP M(FL)

0.27

22

This NALS contains very few loaded axles with no defined loaded peak of distribution. It also does not have a high concentration of axle weights near the conventional unloaded axle weights of 10,000 to 16,000 lb.

For use on roads in Florida with little long haul or "through movement" traffic

9

2

SP VH(AZ)

0.68

70

This NALS is a special very heavy case that contains 70 percent of axles with weights greater than 26,000 lb.

For use on roads with almost all tandem axles fully loaded

Vehicle Class 10: Six-or-More Axle, Single Trailer Vehicles (Table 9)

Class 10 consists of a variety of heavy truck configurations. These trucks have two basic styles: a three-axle tractor pulling a semitrailer with three or more axles, and large trucks pulling full trailers. The first configuration is commonly used by trucks carrying, for example, heavy shipping containers. The second configuration is common in States that allow longer trucking configurations. For example, in many western States, the most common truck for delivering gasoline to gas stations is a four-axle tank truck pulling a three- or four-axle tank trailer. In some States, Class 10 trucks are also the primary vehicles used for hauling heavy natural resources. In these instances, a common Class 10 vehicle is a four-axle dump truck pulling a large three- or four-axle "pup trailer" on a long steel beam. In some States, the primary resource haulers are Class 7 trucks, whereas in other States, this job is mostly performed by Class 10 vehicles. Generally, a State uses one or the other of these truck styles for this task. Thus, a State that commonly uses Class 10 trucks for natural resource hauling will tend to have few very large Class 7 trucks, and vice versa. Table 9 describes the alternative Class 10 axle loading conditions.

Table 9. Description of alternative Class 10 axle loading conditions

Vehicle Class

Axle Type

NALS Cluster

RPPIF

% Heavy Axles

Description

Recommended Use

10

1

L(T)

0.12

4

Only one NALS was observed because single axles are mostly used on Class 10 vehicles as the steering axle of the tractor. Thus, single-axle loads are driven by common steering axle loading patterns.

For use on all roads

10

2

M

0.18

15

This NALS pattern for tandems is dominated by unloaded axles. In this NALS, more than 80 percent of tandem axles weigh less than 20,000 lb.

For use on roads where the Class 10 trucks are primarily unloaded

10

2

VH(T)

0.52

45

This NALS is recommended as a Tier 2 default. It has slightly more loaded axles (weighing 26,000 to 34,000 lb) than unloaded or lightly axles (12,000 to 20,000 lb). It also includes about 5 percent of axles exceeding the Federal legal limit.

For use on roads with an even mix of heavy and light Class 10 trucks

10

2

SP VH(AZ)

1.04

38

This special case NALS did not fit either pattern. Although it has more unloaded axles than the default NALS, it also has more than 12 percent of its axles weighing more than the Federal legal limit, which results in the very heavy loading condition.

For use on roads with high levels of overloaded Class 10 tandem axles

10

3

H(T)

0.35

23

This NALS is recommended as a Tier 2 default. It contains a significant number of both loaded and unloaded tridem axles. More than 40 percent of axles weigh less than 21,000 lb. However, more than 30 percent of the axles weigh between 33,000 and 48,000 lb.

For use on roads with a mix of fully and lightly loaded Class 10 tridem axles

10

3

VH

0.56

43

This alternative to the default NALS has a lower percentage of lightly loaded axles, and a much greater percentage of heavy axles. Less than 22 percent of axles weigh less than 21,000 lb, and more than 50 percent of axles are heavier than 36,000 lb, with more than 10 percent heavier than 48,000 lb.

For use on roads with a mix of loaded and unloaded tridem axles, where a significant portion of the axles exceed the Federal weight limit

10

3

SP L(ME)

0.09

5

This special case is for a Maine site that has a very high percentage of light axles, and a low weight for loaded axles. Almost 75 percent of axles weigh less than 15,000 lb, and less than 5 percent of axles weigh more than 39,000 lb.

For use on Maine roads where almost all tridem axles are unloaded or lightly loaded

10

3

SP M(MN)

0.18

9

This special case in Minnesota also reflects a very high percentage of lightly loaded axles, but with a larger percentage of heavy axles. Fifty-six percent of axles weigh less than 15,000 lb, with another 25 percent of axles weighing between 27,000 and 48,000 lb.

For use on Minnesota roads with a high percentage of unloaded tridem axles

10

4

H(T)

0.46

7

This NALS is recommended as a Tier 2 default. It contains both a significant number of loaded and unloaded quad axles but less than 5 percent of axles weigh more than 57,000 lb.

For use on roads with a mix of heavy and lightly loaded Class 10 quad axles

Vehicle Class 11: Five-or-Less Axle, Multi-Trailer Vehicles (Table 10)

Class 11 trucks are multi-trailer trucks mostly intended for carrying light, bulky cargo. For example, they are good choices for carrying large quantities of mixed packages and other light but high-volume cargo. The vast majority of trucks in this classification consist of a two-axle tractor pulling a smaller (28-ft) single-axle semitrailer and a small two-axle trailer. Because the drive axle of the tractor and the load-bearing axles on the trailers are all single axles, this configuration is one of the heavy truck configurations for which changes in truck loading characteristics (i.e., the commodities being hauled and whether the truck is loaded or not) have a significant effect on the single-axle loading pattern observed for single axles. Consequently, three different single-axle NALS patterns were developed for this class of trucks. Table 10 describes the alternative Class 11 axle loading conditions.

Table 10. Description of alternative Class 11 axle loading conditions

Vehicle Class

Axle Type

NALS Cluster

RPPIF

% Heavy Axles

Description

Recommended Use

11

1

M(T)

0.19

18

This NALS is recommended as a Tier 2 default. The default NALS has relatively few very light axles. Less than 15 percent of axles weigh less than 8,000 lb. It differs from the primary alternative NALS group in that 35 percent of the axles in this group weigh more than 14,000 lb.

For use on all roads

11

1

L

0.08

5

This alternative NALS is lighter than the default. Only 10 percent of axles in this group are heavier than 14,000 lb, while 35 percent are lighter than 8,000 lb.

For use when a high percentage of Class 11 trucks are running empty

11

1

SP H(AZ)

0.38

53

This special case represents a very heavy loading condition observed in Arizona. Two peaks are observed in the load spectra, one represents the steering axles of these trucks with weights near 10,000 lb, and the other represents the loaded single axles with weights ranging between 16,000 and 22,000 lb. More than 45 percent of all axles weigh more than 16,000 lb.

For use when nearly all Class 10 trucks are fully loaded, such as roads used exclusively by long haul trucks, observed in Arizona

Vehicle Class 12: Six-Axle, Multi-Trailer Vehicles (Table 11)

Class 12 trucks are long multi-trailer trucks. They typically have one set of tandem axles and four single axles, one of which is the steering axle. The most common Class 12 truck configuration is a three-axle tractor pulling a single-axle semitrailer and a full trailer with two single axles. Less common configurations include a two-axle tractor pulling either a two-axle semitrailer and two-axle full trailer, or a two-axle tractor pulling a single-axle semitrailer and a three-axle full trailer. These vehicles carry a wide variety of commodities and can exhibit a variety of loading conditions. One NALS group was identified for single axles, but three special-case outliers were also noted in the SPS TPF dataset. Only one tandem-axle NALS group was identified. Table 11 describes the alternative Class 12 axle loading conditions.

Table 11. Description of alternative Class 12 axle loading conditions

Vehicle Class

Axle Type

NALS Cluster

RPPIF

% Heavy Axles

Description

Recommended Use

12

1

M(T)

0.12

7

This NALS is recommended as a Tier 2 default. The default NALS for Class 12 single axles has an almost normal distribution. There are very few very light axles (weights less than 6,000 lb), and very few heavy axles (weights greater than 18,000 lb). The majority of axles weigh between 10,000 and 16,000 lb.

For use on all roads

12

1

SP M(ME)

0.25

25

This special case differs from the default NALS because more than 20 percent of its axles are heavier than 16,000 lb, compared with 5 percent in the default NALS. It represents a site where Class 12 trucks are carrying heavier loads than typically found elsewhere in the country.

For use on all roads where at least 20 percent of Class 12 trucks are carrying heavy commodity loads

12

1

SP VL (LA)

0.04

3

This special case represents a very light loading condition. Two peaks are present in the loading spectra; the larger peak represents unloaded single axles, weighing about 7,000 to 8,000 lb. The smaller peak represents the steering axle of the tractor, weighing about 10,000 to 12,000 lb.

For use on roads where almost all Class 12 trucks are operating empty

12

1

SP VL (NM)

0.03

1

This special case from New Mexico represents a very light axle loading condition. Less than 4 percent of axles weigh more than 12,000 lb. The light single axles in this NALS form a single-peak distribution between 7,000 and 11,000 lb rather than having two peaks.

For use on roads in New Mexico, where almost all Class 12 trucks are observed to be operating empty

12

2

L(T)

0.14

2

This NALS is recommended as a Tier 2 default. Only one NALS was observed for Class 12 tandem axles.

For use on all roads

Vehicle Class 13: Seven-or-More Axle, Multi-Trailer Vehicles (Table 12)

Class 13 is the classification that contains multi-trailer trucks specifically designed for carrying heavy loads. This classification of vehicles contains a wide variety of both truck configurations and axle configurations. They are common in some States but fairly rare in others. In Canada, one specific configuration (the "Canadian B Train") was designed to meet Canada's long haul trucking needs. In return for specific safety improvements (the use of a "B-style" connection between the first and second trailer) and the use of a tridem axle supporting the back of the first trailer and front of the second trailer, the Canadian B Train is permitted to carry additional load and therefore is extremely common in Canada and is also found in many northern States.

In addition, the adjustment of many State-specific truck size and weight laws to meet the needs of local heavy industry has resulted in classification of State-specific truck configurations as Class 13. For example, some western States have allowed the use of triple-trailer trucks. The result is considerable diversity in the number and weight of different types of axles on Class 13 trucks. Table 12 describes the alternative Class 13 axle loading conditions.

Table 12. Description of alternative Class 13 axle loading conditions

Vehicle Class

Axle Type

NALS Cluster

RPPIF

% Heavy Axles

Description

Recommended Use

13

1

M(T)

0.18

11

This NALS is recommended as a Tier 2 default. The default NALS for Class 13 single axles has the rough shape of a triangle with the peak between 12,000 and 13,000 lb. This default NALS has very few unloaded and very few heavily loaded single axles.

For use on all roads, especially where fewer Class 13 trucks use single axles to carry heavy trailer loads

13

1

H

0.32

29

This alternative NALS is similar to the default distribution, but has fewer lightly loaded axles (fewer axles between 9,000 and 12,000 lb) and more axles greater than 15,000 lb. This NALS has 30 percent of axles exceeding 16,000 lb with more than 10 percent exceeding 18,000 lb.

For use on all roads where single axles on Class 13 trailers are routinely carrying heavy loads

13

1

SP M(OH)

0.21

21

This special case comes from Ohio. It differs from the default in three areas. First, it has a very high percentage of very light single axles (more than 35 percent weigh less than 5,000 lb). Second, it has almost 20 percent of loads weighing more than 17,000 lb compared with less than 9 percent for the default, and third, it has more than 7 percent of axles weighing more than the Federal legal limit.

For use in Ohio, where both classification issues place many very light axles in Class 13, and where a high percentage of overloads are observed on Class 13 trucks

13

1

SP VH(OH)

0.55

48

This special case comes from Ohio. This NALS has a high percentage of heavy and overloaded axles. More than 20 percent of axles in this NALS exceed 18,000 lb.

For use in Ohio, where many single axles exceed the Federal legal limit

13

2

VH1(T)

0.87

54

This NALS is recommended as a Tier 2 default. The default NALS has very few light axles. There are two peaks in the axle load distribution. One centers on 22,000 to 24,000 lb and the other centers on 34,000 to 36,000 lb. Roughly 25 percent of tandem axles exceed the Federal legal limit of 34,000 lb.

For use on all roads, with a balance of moderate and heavily loaded axles

13

2

H

0.49

37

This alternative NALS has axles that are not as heavy as found in the default NALS. In this loading pattern, about 17 percent of axles weigh less than 12,000 lb. Only 12 percent of the observed axles exceed the Federal legal limit.

For use on roads where the majority of Class 13 trucks are not fully loaded

13

2

VH2

1.46

58

This alternative NALS differs from the default primarily in that it has a much higher percentage of very heavy axles. More than 40 percent of all axles observed exceed the 34,000 lb Federal limit.

For use on roads where loads Class 13 loads routinely exceed the Federal limits

13

3

VH1(T)

1.12

76

This NALS is recommended as a Tier 2 default. The default NALS contains very few light tridem axles. Fifty percent of the axles in this NALS exceed 48,000 lb.

For use on roads where a large percentage of Class 13 tridem axles are very heavily loaded

13

3

H

0.47

28

This alternative NALS is lighter than the default NALS but it still represents a reasonably heavy loading condition. Twenty percent of the axles are lightly loaded (weigh less than 21,000 lb). The peak of the loaded distribution is between 36,000 and 39,000 lb. Only 5 percent of axles exceed 48,000 lb.

For use on all roads where most tridem axles are fully loaded but where overloads are not routine

13

3

VH2

1.76

86

This alternative NALS is even heavier than the default NALS. It contains very few lightly loaded tridem axles. More than 60 percent of the observed axles exceed 48,000 lb.

For use on roads where overloaded tridem axles are very common

13

3

SP H(OH1)

0.44

24

This special case from Ohio has a very large percentage of very light axles (24 percent weighing less than 15,000 lb). Another 36 percent weigh less than 36,000 lb. However, the NALS also has 11 percent of axles weighing more than 50,000 lb, and 1 percent of axles that weigh more than 70,000 lb.

For use only on roads in Ohio, where some tridem axles exceed 70,000 lb

13

3

SP H(OH2)

0.50

31

This special case is also from Ohio. It has a large percentage of light axles. Only 6 percent are lighter than 15,000 lb, but 20 percent are less than 18,000 lb. Seventy percent of these axles are heavier than 30,000 lb, but only 18 percent exceed 42,000 lb.

For use only in Ohio, on roads where few tridem axles exceed 57,000 lb

13

4

VH(T)

0.83

39

This NALS is recommended as a Tier 2 default. The default NALS for Class 13 quad axles is reasonably flat with no loads in the range between 12,000 and 72,000 lb having less than 2.5 percent of the axles or more than 9 percent of the axles. A small peak exists between 18,000 and 21,000 lb, and a second small peak occurs between 60,000 and 63,000 lb. Thirty-five percent of axles weigh more than 57,000 lb.

For use on all roads where Class 13 quad axles are routinely very heavy

13

4

H

0.43

14

This alternative NALS is lighter than the default NALS, but it is still fairly heavy. It contains a higher percentage of lighter quad axles (a peak value near 12 percent) and many fewer very heavy axles (less than 12 percent of axles weigh more than 57,000 lb).

For use on all roads where only a modest percentage of quad axles are very heavily loaded

13

4

SP H(OH)

0.46

21

The special case site in Ohio has a very large fraction (43 percent) of very light axles. These very light axles are offset by the fact that 16 percent of axles weigh more than 69,000 lb.

For use only in Ohio, where vehicles with very light quad axles are being observed

Description of APC Defaults

APC defaults were computed for each vehicle class and axle type based on data from the 26 SPS TPF sites. Averaging the APC values computed for all SPS TPF sites resulted in one set of global default values, shown in table 13.

Table 13. Global default APC based on 26 SPS TPF sites

Vehicle Class

Single

Tandem

Tridem

Quad

4

1.43

0.57

0.00

0.00

5

2.16

0.02

0.00

0.00

6

1.02

0.99

0.00

0.00

7

1.26

0.20

0.63

0.15

8

2.62

0.49

0.00

0.00

9

1.27

1.86

0.00

0.00

10

1.09

1.15

0.79

0.05

11

4.99

0.00

0.00

0.00

12

3.99

1.00

0.00

0.00

13

1.59

1.26

0.69

0.31

The APC global defaults computed from the LTPP SPS TPF data were compared with the defaults developed in the NCHRP1-37A study.(4) Although some differences were found (especially in classes 11 and 13), these differences were relatively small. The small changes do not result in any significant difference in the required layer thickness or predicted distress for either flexible or rigid pavements. If States have State-specific APC statistics, these should be used in place of the global defaults; otherwise, the global defaults are the best available statistics for use in the MEPDG and DARWin-ME software.

Description of Wheelbase and Axle Spacing Defaults

SPS TPF data were used to investigate the distribution of axle spacing (wheelbase) of the tractor unit of tractor-semitrailer combination trucks for FHWA vehicle classes 8 and above. These statistics can be used to define the categories of "short," "medium," and "long" axle spacing that are used in the analysis of jointed plain concrete pavement (JPCP) slab cracking.

Because 15 ft is the most frequently used joint spacing for JPCP design, the following percentages of axles in the short, medium, and long categories should be used as defaults in the MEPDG and DARWin-ME software for 15-ft joint spacing:

If a different joint spacing is used, the values presented in table 14 can be used to compute the correct percentage of short, medium, and long axles for that desired joint spacing.

Table 14. Distribution of axle spacing on tractor units for FHWA classes 8 through 13


Axle Spacing (ft)

Percentage of Axle Spacings on the Tractor Unit

<=7

0.0

>7 and <=8

0.0

>8 and <=9

0.0

>9 and <=10

0.1

>10 and <=11

0.7

>11 and <=12

3.5

>12 and <=13

7.8

>13 and <=14

5.4

>14 and <=15

3.0

>15 and <=16

8.1

>16 and <=17

12.9

>17 and <=18

32.9

>18 and <=19

9.8

>19 and <=20

7.3

>20 and <=21

6.9

>21 and <=22

0.9

>22 and <=23

0.3

>23 and <=24

0.2

>24

0.2

In addition, the MEPDG states that if other vehicles in the traffic stream have the axle spacings in the range of the short, medium, and long spacings defined above, the frequency of those vehicles could be added to the axle spacing distribution of truck tractors. For example, if 10 percent of truck traffic is multiple trailers (Class 11 and above) that have the trailer-to-trailer axle spacings in the "short" range, 10 percent should be added to the percentage of truck tractors that have "short" axles. Thus, the sum of the percentages of trucks in the short, medium, and long categories will be greater than 100. Table 15 shows the results of the analysis of the axle spacing distribution for units other than tractors for vehicle classes 4 through 13 based on the SPS TPF data. These results provide additional insights regarding the vehicle classes that are likely to have axle spacings that could contribute to the development of top-down cracking in JPCP.

Table 15. Distribution of axle spacing by vehicle class using sample of SPS TPF WIM data


Axle Spacing (ft)

Percentage of Axle Spacing by Class

Class 4

Class 5

Class 6

Class 7

Class 8

Class 9

Class 10

Class 11

Class 12

Class 13

<=8

37

13

49

66

25

47

62

0

20

50

>8 and <=9

0

1

0

0

3

0

0

2

6

4

>9 and <=10

0

1

0

1

1

0

1

17

11

5

>10 and <=11

0

2

0

3

1

3

1

3

3

3

>11 and <=12

0

12

1

11

2

1

1

2

5

3

>12 and <=13

0

7

2

9

8

1

2

12

2

3

>13 and <=14

0

21

3

3

8

1

2

7

0

2

>14 and <=15

0

12

3

2

4

1

1

2

0

2

>15 and <=16

0

4

6

1

2

2

2

2

2

3

>16 and <=17

0

3

6

0

2

4

2

2

3

3

>17 and <=18

0

4

9

0

4

9

3

4

6

3

>18 and <=19

0

3

6

1

4

3

4

2

2

5

>19 and <=20

0

3

5

0

4

2

2

1

5

4

>20 and <=21

0

4

6

0

6

2

1

6

13

2

>21 and <=22

0

5

2

0

5

0

1

24

8

2

>22 and <=23

1

3

1

0

3

0

0

15

12

1

>23 and <=24

20

1

0

0

1

0

0

1

1

1

>24

42

2

0

0

16

25

15

0

0

5

Description of Default Axle Spacings for Multi-Axle Groups

SPS TPF axle spacing data were used to compute the average axle spacing for tandem, tridem, and quad axle groups. These averages were then compared with current MEPDG defaults. The results are presented in table 16.

Table 16. Average axle spacing for multi-axle groups


Source

Axle Spacing (inches)

Tandem

Tridem

Quad

NCHRP 1-37A

51.6

49.2

49.2

LTPP SPS TPF

49.0

50.8

51.8

As table 16 shows, the values are very close. SPS TPF-based averages are slightly lower for tandem axles and higher for tridem and quad axles compared with the current MEPDG default values.

The values based on SPS TPF WIM sites reported in table 16 are recommended for use as the new MEPDG defaults because these values are obtained from accurately calibrated WIM sites.

GUIDELINES FOR SELECTING AXLE LOAD SPECTRA FOR THE MEPDG

Knowledge of Local Traffic Loading Conditions

Before an analyst chooses among the alternative NALS defaults, it is recommended that every effort be made to understand the expected traffic-loading pattern at the site for which default NALS selections are being made. For example, analysts who work at a State department of transportation can talk to the staff in the department's freight office, individuals involved in collecting truck data, and individuals in the department's maintenance office who actively work on that roadway. Another source of commodity information is Metropolitan Planning Organizations (MPO). Personnel from these agencies often can provide a reasonable description of the truck traffic using a specific road that can be used to identify the most appropriate NALS tables to apply. Topics to discuss include the following:

This information should be used to establish a descriptive understanding of the traffic loading conditions for at least the dominant heavy vehicle classes observed at the site ("dominant" is defined at the end of this subsection). The intent is to learn whether the trucks producing the majority of traffic load at this site are loaded, unloaded, or an even mix of these two conditions. This will help determine whether they are of "typical" or usual" weight, or whether they are heavier or lighter than typical, which in turn helps the analyst choose between the Tier 2 default NALS options. Table 3 through table 12 can help the analyst select specific Tier 2 default NALS based on this information. The guidance from these tables is also built into the LTPP PLUG Interactive Traffic Library Database. In addition to defaults created using SPS TPF data, State agencies may choose to put their own default NALS into the PLUG Library Database, including their own information for tables 3 through 12.

Determination of the Dominant Truck Classes

In the selection of NALS defaults, the most care should be applied in selecting the appropriate default for the dominant truck class. The dominant trucks for a given site are those that apply the largest percentage of the total traffic load at that site. (That is, the truck classes that are expected to cause a large amount of the expected pavement damage.) "Dominant" is a function of both the volume of a class of trucks and the expected pavement damage each truck causes. Dominant trucks generally are heavy trucks, and they generally contribute a high percentage of the total truck volume.

The LTPP PLUG software provides several methods to determine dominant truck classes. One such method is described below. Table 17 demonstrates how to estimate the percentage of total load (or load-related damage) carried by each truck class for determining the most dominant truck classes at a site. This table uses RPPIF per truck values as a simplified measure of the expected amount of damage caused by each truck. It is not meant to be used for direct damage or distress prediction but rather to rate the relative importance of loads from the perspective of their potential to cause damage to the pavement. These values are shown in the third column of the table. Analysts may replace the values in this column with their own statistics for "damage per truck." For example, they can insert conventional ESAL/truck values.

Table 17 is an example of how to compute dominant trick classes. To use a table like table 17, determine the percentage of truck traffic for FHWA vehicles in classes 4 through 13 that uses the pavement sections being analyzed. Place these values in the second column of the table. Multiply the value in the second column by the value in the third column and place this value in the fourth column. This is a measure of the relative damage each class of trucks will apply at this site. In the fifth column, convert the value in the fourth column to a percentage of that total traffic load by dividing each value in the fourth column by the total of the fourth column. The "dominant truck class" is the class of trucks that has the highest percentage of the estimated load. In the example shown in table 17, Class 9 is the dominant truck class because no other class of trucks produces more than 12 percent of the total estimated load. All classes that provide more than 25 percent of the traffic load should be considered dominant classes.

To use a table like table 17 effectively, and to perform a good pavement analysis, the analyst should have access to site-specific truck volume data. These data can be collected at almost any pavement analysis section in the United States at a cost that is small relative to their importance in the pavement analysis process. (The vast majority of LTPP test sections have valid, accurate vehicle classification count data.)

Table 17. Example of computing dominant truck classes


Vehicle Class

Percentage of Truck Traffic

RPPIF/Truck

Load Applied by Truck Class
(Percentage of Truck Traffic times RPPIF/Truck)

Percentage of Total Load Applied by Class

4

5

0.60

0.0302

5.9

5

40

0.08

0.0317

6.2

6

6

0.50

0.0303

5.9

7

2

0.53

0.0107

2.1

8

4

0.34

0.0138

2.7

9

36

0.93

0.3339

65.2

10

1

0.53

0.0053

1.0

11

3

0.67

0.0200

3.9

12

1

0.46

0.0046

0.9

13

2

1.58

0.0316

6.2

Applying What the Analyst Has Learned About Truck Loads

The questions that the analyst asks those in the traffic data collection, freight mobility, and maintenance groups are intended to give him or her an understanding of where unloaded, loaded, and over-loaded trucks are operating. Because different kinds of trucks carry different kinds of commodities, loaded, unloaded, and overloaded conditions may vary considerably among types of trucks on a single road. For example, on one road, long haul Class 9 tractor-semitrailers may be operating fully loaded, but rarely overloaded, because they routinely pass port of entry enforcement stations at the State borders. At the same time, natural resource haulers on that same road may frequently be operating in an overloaded condition because their trips are a shorter distance and that distance does not include passing a weight enforcement site.

Insight into the commodities carried on the road helps determine whether loaded trucks are expected to be near the Federal legal limit. Some commodities, such as general household goods or computer parts, tend to be light. These goods "fill up" a truck or container (bulk out), but the "full" truck's axles do not reach the Federal legal limit. (For example, most consumer packages have large amounts of packaging to protect the contents. This makes the packages large but relatively light, filling up the trailer box but not heavily loading the truck's axles.) Other commodities can be quite heavy. For example, frozen food shipped from eastern Washington to the Port of Seattle in containers are very heavy loads. Little packaging is needed and the foods are tightly packed, resulting in very dense containers, and thus heavy axle loads.

If it is known that the pavement analysis section is on a road that carries heavy natural resource loads - and this direction of traffic is the direction that carries the loaded natural resource trucks - then the analyst should select a heavy NALS for the class of trucks used to carry heavy natural resources in that State. If, on the other hand, the side of the road that contains the analysis segment carries the unloaded trucks returning to pick up more natural resources, the analyst should select one of the lighter Class 6 or Class 10 NALS and a moderate Class 7 and/or Class 10 NALS.

Another key question for the analyst to learn about natural resource hauls is what kind of trucks most commonly carry natural resources on that road and what kinds of axle loads are present. In some parts of the country, Class 7 is the primary natural resource hauler. In other parts of the country, Class 10 trucks fulfill this role. In either case, it is also important to understand the common axle weights that these trucks exhibit when loaded. In some parts of the country, these trucks are renowned for having overloaded axles. In other parts of the country, these loads are maintained near the legal limits owing to tight enforcement and specific truck size and weight regulations. The Tier 2 NALS provide a number of different loading conditions for these trucks and discussing likely loading conditions for them with knowledgeable State traffic data personnel increases the accuracy of the NALS selected for these trucks and axle types.

Most trucks hauling natural resources do not travel very long distances. (Natural resources are generally shipped by rail for long distance hauls.) However, many truck classes do operate in long haul, urban delivery, and single-day return modes of operation. These kinds of operating patterns affect how often a truck is observed as being loaded, partially loaded, or empty.

Trucks that operate over very long distances (more than 1 day's travel) tend to operate loaded during the vast majority of time. The longer the distances traveled, the more likely the truck is full (although its axles may still be well below legal Federal axle load limits if it is carrying bulky cargo) because the economics of trucking are such that the cost of finding a return load is lower than the cost of traveling a long distance empty. However, if a roundtrip can be made in 1 day, the likelihood that such the truck is full in one direction and empty in the other increases considerably. This is simply because the labor cost of waiting for a return load can be high relative to the cost of driving back to base. Similarly, in urban areas, delivery trips that start and end at a terminal are commonly full at the start of the trip, partially loaded during the middle of a trip, and empty by the end of the trip.

When considering these operating conditions together, and looking at the loading patterns for truck classes 8, 9, 11, 12, and 13 (but usually not Class 10), it becomes clear why it is important to understand the percentage of through trips versus local delivery trips made. Each of these truck classes is commonly used for both local delivery and long haul trips. The greater the percentage of local delivery (and single-day return) movements occurring on a road, the higher the percentage of light axles should be in the selected NALS. On the other hand, the greater the percentage of through traffic, the smaller the percentage of lightly loaded axles. These conditions reach their extreme on roads like I-10 near the New Mexico/Arizona border. Because there are few local destinations in this area, almost all truck traffic on I-10 is loaded.

Finally, the size and weight laws in place in the State, and the degree to which those laws are enforced on that road, are useful information for the analyst. In some cases, States routinely allow additional axle loads on specific styles of trucks to meet the needs of important State-specific industries. Similarly, there are some commodity movements that are renowned for being overloaded (e.g., some coal movements have this reputation). When these conditions are known to exist, the analyst can select Tier 2 NALS that feature larger percentages of axles that exceed the Federal axle weight limits.

Selection of NALS

Once an analyst understands the basic truck volumes at a site and has obtained the available knowledge on the truck loading conditions, especially for the dominant truck classes, table 3 through table 12 could be used to identify NALS best representing identified loading conditions. To facilitate the selection of Tier 2 NALS, information from these tables has been included in the LTPP PLUG database and software application. Details are provided in the appendices of this guide.

For vehicle classes where no loading conditions are identified because of lack of knowledge or because these classes carry a low percentage of the total volume, it is recommended that analysts use the SPS TPF NALS defaults (either Tier 1 or Tier 2).

MEPDG Sensitivity Test

When NALS defaults are used, and if time and resources permit, the analyst is strongly encouraged to run at least some sensitivity tests of the MEPDG analysis outputs, especially if very limited knowledge of loading conditions exists. The minimum recommended sensitivity tests include two runs. Both are performed by keeping all inputs the same except the NALS. The first test substitutes a heavier Tier 2 NALS than the initial default NALS for the dominant trucks in the design. The second test substitutes a lighter Tier 2 NALS for the dominant trucks classes using the site.

The sensitivity test is intended to simply increase and decrease the total load applied to determine whether the MEPDG outputs for a particular design change significantly based on changing traffic load. If they do, then the analyst can decide how conservative a design to build, given the uncertainty in the traffic loading.

Limitations of Loading Cluster NALS Defaults

The Tier 1 and Tier 2 defaults are useful estimates when site-specific information is not available. However, because they were developed from only 26 WIM sites in 23 States, and all of those sites are LTPP test sites (which means they tend to be on larger roads), they are not truly representative of all the locations at which traffic loading estimates are needed. The small number of SPS TPF WIM sites also limits the geographic scope of the weight data. These estimates may not be representative of specific roads in specific States where loading conditions are dominated by State regulations or specific commodity movements that differ from national norms. When State- or site-specific trucking movements dominate the use of a road, those roadways may experience loading conditions that are different than those observed in the SPS TPF data. Consequently, when analysts believe that these State-specific conditions are present, they are encouraged to (1) work with their State to develop State-specific NALS defaults, (2) talk to their State's traffic data collection personnel to gain more insight into which loading patterns to apply, and (3) perform sensitivity tests of the MEPDG outputs using a variety of different NALS.

MEPDG TRAFFIC INPUTS BEYOND AXLE LOAD SPECTRA

The estimation of traffic load requires a number of factors in addition to the NALS, including:

States are encouraged to develop their own defaults or site-specific values for these statistics.

Although truck volumes will change with the national economy, State economic growth will likely have an even greater impact on truck traffic on most roads. Truck volume time of day distributions are easily collected as part of site-specific truck volume counts. Truck travel distributions are important in States where truck travel varies by season and are especially important where seasonal environmental changes significantly affect material properties. (That is, it is especially important to understand how truck volume patterns change during the year in States with significant freeze-thaw conditions.) All of these data are available to some degree in the LTPP traffic database, but additional information on these factors should also be available through State sources.

1"Overload," as used here, means axles that exceed the legal Federal limit. The axles may or may not exceed the legal limit for the State in question or for the load being moved.

 


The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
FHWA
United States Department of Transportation - Federal Highway Administration