new mexico department of transportation research bureau expanding the use of weigh-in-motion data

New Mexico Department of Transportation

Research Bureau

Expanding the Use of Weigh-In-Motion Data


Research Bureau

Public-Private Partnerships

Evaluating ESALs for the Long-Term Pavement and Structures Warranties for US550 in Northern New Mexico

Case Study


Research Bureau

Background:

In 1998 the NMDOT contracted with a private sector contractor to reconstruct US550 between San Ysidro and Bloomfield.

The project included numerous innovations, including:

•GARVEE Bonding to Finance Construction

•Contractor –provided design and construction management

•Limited state oversight

•Abbreviated schedule of 3 ½ years to comple 118 miles of major reconstruction

•First-of-its-kind long-term pavement and structures warranties


Research BureauTwo Memos of Understanding between NMDOT and the federal government require a sustained research effort to evaluate the innovations used.

As part of the RoadLIFE initiative, the Research Bureau conducted research to examine accumulation of ESALs along the facility, which is one of three factors in determination of warranty expiration:

1.Money – Total expenditures limited to $114 Million

2.Time – Warranties will expire in 10 years (Str) and 20 years (Pvmt)

3.ESALs – Limited to 2 Million (Str) and 4 Million (Pvmt)


Research Bureau

Equivalent Single Axle Loads (ESALs) are defined by FHWA as:

“The damage per pass to a pavement caused by a specific axle load relative to the damage per pass of a standard 18,000 pound axle load moving on the same pavement.”


Research Bureau

Wx

W18=

L18 L2s Lx L2x

4.7910

G

x

10

G

18

L2x 4.33

ESAL calculations are very sensitive to both axle weight and number of axles in the axle group.

Great care must be taken to ensure that ESALs are calculated correctly.


Research BureauAccording to Lifecycle Cost Analyses, significant maintenance is expected in the latter part of the warranty. Early warranty expiration has significant financial implications for NMDOT.

Accurate ESAL calculation is therefore critical, as are ESAL growth studies to estimate terminal ESAL date.

ESALs are calculated from the data files generated by three weigh-in-motion (WIM) stations installed along the facility.

The critical nature of ESAL counts required the use of special correction factors to account for missing data and equipment failure.

The standard software used by NMDOT to process data from other WIM sites around the state was therefore impractical.


Research BureauNMDOT created project-specific software to calculate ESALs from available WIM data files, and to apply contractually required correction factors.

The software, code-named ALTRIS, calculates ESALs based on weight and axle data, using either the chart-interpolation or ESAL equation method as selected by the user. These methods generally agree to within 1-2%.

The software also evaluates trends in usage of the facility by time of day, day of week, vehicle classification and occurrence of overweight vehicles.

Monthly ESAL data is subjected to linear regression to determine the equation of the growth curve, and this equation is integrated to determine the equation for ESAL accumulation.


Research Bureau

From these mathematical models, the date of accumulation of terminal ESALs may be derived, and the model may be updated as more data becomes available.

PROGRAM OPERATION

1. Open and Read WIM Data Files

2. Parse Data and Populate Database

3. Process Data, Calculate ESALs

4. Apply Correction Factors

5. Create Formatted Reports


Research BureauSample WIM Data

Encoded in this data is information such as station ID, date, hour, direction, number of axles, vehicle classification, axle spacings and axle weights


Research Bureau


Research Bureau

Sample trend studies available through analysis of WIM data

ESAL counts by day of week

ESAL counts by hour of day


Research Bureau

ESALs by Vehicle Classification

Traffic Volume by Vehicle Classification


Research Bureau

Overweight Vehicles by Hour of Day

ESAL Counts by Day and Month


Research Bureau

WIM data is used primarily to calculate ESALs. ESAL reports are sent to FHWA in Washington DC.

As the top tier in traffic data collection, WIM data can also be used to calculate:

•ESAL Counts by Day of Week

•ESAL Counts by Hour of Day

•ESAL Counts by Vehicle Class

•Volume Counts by Vehicle Class

•Overweight Vehicles by Day and Hour

•Annual ESALs by Day and Month

•Traffic Trends by Season and Direction.

SO WHAT???


Research BureauThese studies can be used to:

•Target Law Enforcement for the most likely day, time and direction of illegal loads

•Compare WIM records with oversize permit records to measure compliance with state weight limit laws

•Predict ESAL growth and compare actual ESALs with design assumptions

•Correlate vehicle movements with social and economic activity

•Compare usage, ESAL growth and traffic trends among highways on the transportation network

•Identify anomalous data as an indicator of software or equipment malfunction

•Evaluate stresses imposed by illegal loads on bridges

•Determine Volume and Frequency of Overweight Vehicles


Research Bureau

Overweight vehicle data may be used to evaluate stresses imposed on bridges.

Frequency, time and direction studies related to overweight vehicles may be used to target specific times and locations for law enforcement.


Research BureauWIM data files record axle weights and spacings which can be used in a structural analysis program to evaluate shear and moment.

In this case, the allowable shear and moment envelopes, outlined in red, are developed by evaluating stresses imposed by the hypothetical HS20 vehicle (the bridge is inventory rated at HS20), and the stresses from the actual truck as taken from WIM data are shown in white.

(Note the mathematical relation between shear and moment: shear is the derivative of moment, and moment is the integral of shear.)


Research Bureau

ESALs calculated from WIM data may be used to compare usage of individual highway facilities with national trends.

These graphs show the comparison of truck tonnage carried on the nation’s highways (American Trucking Assn) with ESALs calculated on US550.

Because the greatest contributor of ESALs is Class 9 commercial trucks, ESALs are a good indicator of economic activity.


Research Bureau

Linear regression was performed on monthly ESAL data derived from WIM equipment, and the ESALs per Month equation was obtained.

f(n) = 184n + 7558

ESAL GROWTH MODELING – Estimating Warranty Termination


Research Bureau

Integrating this equation produces a curve that models accumulation of ESALs over time.

This graph shows close correlation between modeled ESAL accumulation and observed accumulation as summed from the ESALs per month data set.

In this case, total ESALs sum to 600,000 over 4 years.


Research Bureau

When compared to the assumed 3.5% annual ESAL growth at the beginning of the project (Blue), we find actual that ESALs per month are far less than assumed, and ESAL growth is far greater.


Research Bureau

If this trend continues, the terminal ESAL count of 4,000,000 will be reached before the 20 year time limit on the pavement warranty, and NMDOT will be responsible for performing required maintenance expected in the latter years of the warranty.


Research BureauAVAILABLE REPORTS

A Detailed Report with Technical Background and Mathematical Rigor

A Brief White Paper Providing a General Overview


Research Bureau

Questions?

new mexico department of transportation research bureau expanding the use of weigh-in-motion data

Documents

axle data

monthly esal data

accumulation of esals

specific axle

axle weight

missing data

axle group

esal growth studies