LTBP Program's Literature Review on Weigh-In-Motion Systems

Chapter 1. Introduction

State and Federal highway agencies are responsible for safeguarding the expenditure of billions of dollars invested in highway infrastructure each year. As such, for the purposes of safety and infrastructure preservation, truck size and weight (TS&W) are regulated using Federal and State legislation and policies. However, data on the actual characteristics of the trucks using this infrastructure, including weights, volumes, and configurations, are necessary for many applications, including design, research, maintenance, and preservation. Weigh-in-motion (WIM) systems can capture and record the axle or axle group mass, capturing the gross vehicle weight (GVW) while the vehicle is moving.

As part of the Federal Highway Administration’s (FHWA) Long-Term Bridge Performance (LTBP) Program’s Technical Assistance Contract, a literature review of the state of the practice was performed for WIM systems installed in pavements and on bridges. This literature review focused on the development of WIM systems, concepts for measuring axle loads, the applications of WIM sensors for pavements, and recent advancements in bridge WIM systems. The literature review consists of the following five main topics:

• Regulations on truck weight limits.
  
  
• Permanent WIM systems installed in roadways.
  
  
• Bridge weigh-in-motion (B-WIM) systems.
  
  
• WIM system specifications and accuracy.
  
  
• Long-Term Pavement Performance (LTPP) Program’s WIM experience.

Scope

This literature review outlines important topic areas related to WIM systems and the research performed by various agencies. One goal of the LTBP Program is to investigate the impact of truck loads on the performance and durability of bridges, and this review facilitates selection of suitable WIM technology systems to meet these data collection needs.

The review serves as a reference document for Pooled Fund Project Number TPF-5(283), The Influence of Vehicular Live Loads on Bridge Performance, which targets the impact of vehicle live loads on bridge component durability.⁽¹⁾ Currently, the participating agencies in the pooled fund study are FHWA and State transportation departments in Minnesota, Iowa, Pennsylvania, Georgia, Oregon, Wisconsin, and North Carolina.

The goals for the pooled fund study are highlighted by the following two fundamental questions:

• What are the current truck loads on the Nation’s bridges? There have been changes in truck geometry, axle configurations, suspension, and tire characteristics. In some cases, common loaded truck weights have increased from 72kips (design truck) to more than 110kips.
  
  
• What are the impacts of increased truck loads on the durability of the Nation’s bridges? The freight industry has been requesting increases in allowable truck loads on bridges. The bridge elements that are especially affected by increased truck loads need to be identified. Bridge owners need to address the effects of live loads on bridge component durability by using better tools and strategies to manage and operate bridges, given the constrained financial resources should traffic volumes and weights increase and truck configuration changes.

WIM Overview

WIM is the process of measuring the dynamic tire forces of a moving vehicle and estimating the corresponding tire loads of the static vehicle.⁽²⁾ Efforts to develop and use WIM systems to collect truck weight data in the United States can be traced back to the early 1950s. One of the earliest examples was a WIM system developed in 1951 by Norman and Hopkins at the U.S. Bureau of Public Roads.^(3,4) The WIM system used a floating reinforced concrete platform that was embedded in the roadway and supported at its corners by strain gage load cells, and the measurements were acquired by taking photographs of the traces from an oscilloscope.⁽³⁾ Subsequent developments with the embedded weight sensors included different iterations on platform designs using steel plates and strain gage load cells, steel bending plates instrumented with strain gages, and strip sensors. The utility of the earliest WIM systems was severely limited by the sensing, signal conditioning, and data acquisition technologies available at the time. Modern WIM systems are largely unencumbered by the technology limitations of the past and typically consist of roadway sensors that classify vehicles by type and measure the vehicle weight and the supporting electronic hardware and software needed to process, sort, analyze, and transmit the recorded data. These WIM systems effectively capture and record the axle or axle group weights and the GVW while the vehicle is moving at normal highway speeds.

The operational principle of WIM sensors is based on measuring axle loads through the signals recorded by sensors, such as voltage, strain, and resistance. Typically, WIM sensors are embedded in the pavement surface. The accuracy of WIM systems is affected by the interaction between pavement and vehicle, which is dependent on pavement roughness, vehicle suspension, and speed. Other factors that influence the accuracy of WIM systems are the installation, calibration, and maintenance procedures of the sensor system.

WIM systems are used to determine vehicle characteristics, including GVW, speed, axle weight, and axle spacing. Common WIM sensor technologies used to measure weight include polymeric, ceramic, and quartz piezoelectric systems; bending plates; and load cells.

B-WIM was first used to measure vehicle weight in the 1970s; the data acquisition hardware and software of B-WIM have been continuously developed since then.^(4,6) A B-WIM system uses the measured responses of a bridge (usually strain) to determine the weight and other characteristics of crossing trucks. B-WIM systems typically require more elaborate data analysis and interpretation procedures to determine the truck characteristics than are necessary for traditional WIM systems installed in a single lane of pavement. This is due to factors such as the possibility of multiple presences of trucks and other vehicles on the structure, changes in structural behavior due to environmental effects, geometric and structural complexity of the bridge, and dynamic interactions between trucks and the bridge.