Witzenburg Municipality Tender No: 8/2/5/18

8.7 INFORMATION ON WIM (WEIGH IN MOTION)

8.7.1High Speed Weigh-in-Motion Apparatus (Piezo Quartz Sensor/Type Lineas 9195E)

The axle weighing senor is to be firmly installed at the locations shown in the drawings. The equipment must be capable of recording dynamic gross vehicle weights and vehicle axle loads. Dynamic recording must be capable at passing speeds of up to 120 km/h.

The purpose of this apparatus is to record the characteristics of all vehicles that travel along the road not travelling through the weigh station (i.e. vehicles other than trucks). Particular the apparatus is to record (using a digital video camera with plate recognition) an image of offending vehicles that have chosen not to enter the weigh station as instructed.

8.7.2 Capacity

The sensors must be capable of weighting the following loads:

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  Nominal axle load 25,000 kg
  
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  Working load 30,000 kg
  
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  Load limit 37,500 kg
  
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  Breaking load (load at rupture)> 40, 000 kg
  

8.7.3Accuracy

The accuracy of the WIM system is to be better than ±10% of axle load and ±7% of gross weight of the static weight in dynamic mode when weighing vehicles at a speed of 80 km/h. The weighing system shall also achieve the following.

Error on GVW: ≤ ± 7% with a statistical probability of  = 1

Error on Axle load: ≤ ± 10% with a statistical probability of  = 1

Error on Axle group load: ≤ ± 10% with a statistical probability of  = 1

Error on Axle spacing: ≤ ± 0.5 m (2) between 2.axles at 1 to 80 km/h

Weighing speed range: 1 to 120 km/h

Weighing speed accuracy: ≤ ± 20 km/h for range 1 to 80 km/h

Overload capacity per axle: ≤ ± 50% with uniform load distribution.

8.7.4Capacity

The system must be capable of conducing accurate dynamic vehicle weighing in compliance with accuracies indicated above for dynamic weighing and recording the following information:

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  Determination of axle loads;
  
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  Automatic vehicle classification/ weighing;
  
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  Weight comparison with stored limit tables;
  
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  Axle load recording;
  
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  Gross weight calculation;
  

The apparatus in association with the auxiliary devices such as loops and axle detectors should be capable of recording:

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  axle loads;
  
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  total loads of multiple axles;
  
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  axle spacing of the vehicle;
  
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  GVW – gross vehicle weight - total weight of the vehicle;
  
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  total vehicle length;
  
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  overload of the vehicle with respect to axles, multiple axle, and total weight;
  
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  maximum overload of the vehicle in accordance with a stored vehicle table;
  
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  variation of the vehicle;
  
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  speed during passage over the weighing system (axle detection);
  
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  classification of the vehicle with respect to number and spacing of axle;
  
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  reliability of the weighing operation by evaluation of the speed variation;
  
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  a consecutive number;
  
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  the date and time of passage.
  

8.7.5Installation and Mounting

The sensors are to be mounted level with the surface of the road.

The system must be earthed properly and protected against electrical surges and lightning.

8.7.6Exact Position

The exact position of all the items shall be finalized after the axle load stations have been constructed and following a site inspection.

Design Life of the Equipment

The minimum design life for the electronic equipment before replacement is to be ten years.

Figure (1) Staggered WIM Layout

Figure (2) Standard WIM System – (I)

all mass in cm

All dimensions in cm

8.7.7Video Camera with violators recording capability

This equipment shall have the capability of capturing and recording the number plate (alpha-numeric characters) of all vehicles, including offending vehicle on signal from HSWIM equipment or when triggered by manual switching. A digital video camera capable of capturing a clear image of a vehicle (2/3^rd of front and 1/3^rd of the site) is to be securely mounted in a weatherproof and dustproof secure housing as shown in the drawings.

The weighing platform is to be connected to the control computer network in the Control Office on LS WIM by service able secure ducted cables. Equipment mounted along the roadside will suitably by mounted on plinths and housed in enclosures to protect the equipment against the environmental conditions of the area. Feeder cables to individual road sensors shall be installed in such a way in the road that aides the maintenance thereof.

The system must be earthed properly and protected against electrical surges and lightning.

The image is to be relayed to the control office with the following data shown ion the image as collected by the relevant apparatus:

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  the axle loads;
  
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  the vehicle speed;
  
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  the total loads of multiple axles;
  
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  the total weigh of the vehicle;
  
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  the maximum overload of the vehicle in accordance with a stored vehicle table;
  
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  variation of the vehicle;
  
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  classification of the vehicle with respect to number and spacing of axle;
  
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  a consecutive number;
  
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  the date and time of passage
  
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  picture (2/3^rd of front and 1/3^rd of the site) of overloaded vehicles
  

The images are to be stored on the system’s network hard drives in the most economical format that provides a clear image on a suitable computer screen. The verification of the vehicle characteristics (overload) an operator will delete all images of non-offending vehicles.

Exact Position

The exact position of all the items shall be finalised after the axle load stations have been constructed and followings a site inspection or recommendation.

8.7.8Permanent WIM Operations

The majority of WIM data collection is now done with permanently installed weight sensors, although many sites do not collect data continuously at these sites. Instead, they attach data collection electronics to previously mounted sensors when data collection is desired. The scale sensors are then calibrated (or should be calibrated), and data are collected for the desired time period.

Permanently mounting WIM sensors allows them to be installed flush with the roadway surface. When done properly, this eliminates the bump that vehicles experience when crossing surface-mounted sensors. The removal of impact loads on sensors and the elimination of extra vertical motion caused by bumps result in improved system accuracy. Permanently mounting sensors flush with the pavement surface also decreases the impact loads on sensors themselves, which in turn increases sensor life. One common cause of sensor failure is when sensors become directly exposed to horizontal forces from tire contact. This exposure often leads to early fatigue failure for both sensors and the bonds between sensors and pavement.

A variety of other factors play important roles in both the output accuracy and the life of permanently mounted sensors. Pavement at permanent sensor locations should be:

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  Flat (no horizontal or vertical curves)
  
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  Smooth (no bumps or other surface conditions that increase dynamic vehicle motion)
  
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  Strong (to reduce pavement flex underneath the WIM sensor), and
  
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  In good condition.
  

Flat, smooth pavement reduces vehicle dynamic motion and increases the accuracy of all WIM sensors. Strong pavement results in longer-lived pavement, which in turn increases sensor life. Strong pavement is especially important for strip sensors that are embedded directly into the pavement. The output from these sensors depends on the performance of the pavement itself. If pavement strength varies significantly over time (e.g.,with environmental conditions), sensor output will also vary, and this greatly decreases the likelihood of accurate sensor calibration.

Good condition pavement reduces vehicle dynamics and makes the bond between sensors and pavement more likely to last. A common cause of sensor failure is the failure of sensor/ pavement bonds, which is often traced to poor pavement condition.

Poor installation is another common cause of this failure.

Poor cleaning or drying of pavement cuts results in a weak bond that allows moisture intrusion and further deterioration of the bond.

Moisture is also a common cause of equipment failure because of intrusion into either the sensor itself or the communication lines connecting the sensor to the data collection electronics.

Each vendor and each state highway agency has its own procedures for fighting moisture intrusion. Similarly, agencies and vendors have equipment and procedures for protecting permanent equipment from lightning strikes, other environmental effects (extreme temperatures, humidity, dust), insects, power surges, and various other causes of equipment or communications failure. No single document exists that lists best practices for protecting equipment from these common problems.

8.7.9SITE SELECTION

The site selected for a WIM system should be based on meeting the required "site design life" and accuracy necessary to support the user. In order to meet these requirements, the geometric design, pavement condition, and general characteristics of a potential site should be considered. Selecting an adequate site for the WIM location is a very important part of meeting the established WIM system requirements.