Weigh In Motion (WIM) systems by Kistler are helping to protect a bridge structure in Mexico

Bridges are sensitive components of the traffic infrastructure, and their lifetimes are limited. The accident at Genoa has made it clearer than ever before that they require closer attention. Weigh In Motion systems by Kistler offer an efficient way of monitoring bridge use and imposing restrictions when necessary – as illustrated by an application that is now operational in western Mexico.

A serious accident occurred on the El Carrizo highway bridge in January 2018: a tanker full of diesel fuel crashed and caught fire. Fortunately, nobody was injured – but the road had to be closed for weeks to investigate possible damage to the bridge. With a central tower that is 226 meters tall, El Carrizo is currently the second highest bridge in North America. It is a link in the spectacular route that was opened in fall 2013 through the Sierra Madre Occidental, part of the North American Cordillera.

The route from Mazatlán to Durango is important in terms of tourism as well as commercial transport, and the new road from the coast to the interior of Mexico has slashed the journey time from six or more hours to only three. But after the bridge was closed, trucks had to take a long detour through the mountains: this extended the driving time to eight hours until the route was fully re-opened to traffic at the end of August 2018. To prevent serious accidents in the future, two checkpoints with Weigh In Motion (WIM) technology have now been installed by SEMIC, who developed a customized hardware-software solution. Vehicles that exceed the permitted total weight are automatically preselected so they can be turned back at the toll plazas that were already in place. Kistler supplied the local contractors responsible for this project with the necessary technology: the Type 9835A WIM system, with four Lineas Type 9195GC41 WIM sensors and the WIM data logger (5204AC04).

Precision combined with simple integration

Carlos López is the CEO of SEMIC (Servicios Mexicanos de Ingeniería Civil), the company responsible for installing the technology. He comments: "The high accuracy of the Lineas sensors, and ease of integration into our systems: those were the factors that persuaded us to choose Kistler. We're highly satisfied with the solution because it meets our client's expectations perfectly." CAPUFE (Caminos y Puentes Federales), the government-owned agency that operates the highway, commissioned SEMIC to set up an automated dynamic system to identify overloaded vehicles. The technical specifications were drawn up by Francisco J. Moreno and Andrés Araújo of CAPUFE's engineering department. López continues: "We found Kistler's data logger exceptionally convincing in terms of flexibility and user-friendliness. That made it relatively easy for us to meet CAPUFE's requirements."

Both measuring stations were set up near existing toll plazas. Coscomate, in the west-east direction, is located immediately before the toll plaza, while Mesillas is eight kilometers ahead of the corresponding toll plaza in the east-west direction. "Alongside logistical considerations, the condition of the road plays a major part in determining the optimum position for the sensors. Kistler assisted SEMIC with positioning the sensors so as to ensure maximum measurement accuracy," according to Jeffrey Rice, Kistler's Sales Manager Road & Traffic Americas. "Since 2018, we've been offering this service throughout the world as a way of maximizing our systems' performance for our customers." SEMIC, which assigned 20 of its total workforce of 100 to this project, also carried out extensive preliminary investigations to guarantee the performance of the Weigh In Motion equipment. Parameters measured included suitable geometric characteristics and variance of traffic speeds, surface and structural pavement conditions to ensure the lifecycle of the investment.

Reliably detecting overloaded vehicles

There are only single lanes on most of the route from the coast into the interior of the country, and traffic largely consists of heavy commercial vehicles such as tankers and trucks – both with and without trailers – traveling at average speeds of 90 km/h. Two Kistler Lineas sensors at each measuring station ensure that the axle load and total weight of every vehicle are determined accurately. The new Weigh In Motion system delivers a precise analysis of route utilization by vehicle type, and it registers the axle load and total weight of all vehicles (see the graphic). In the period from August to September 2018 alone, the system identified 135 vehicles that exceeded the maximum weight of 59.4 tonnes.

Installation took place in summer 2018. "Once Kistler had trained our engineers while the first sensors were being installed, SEMIC was able to set up the second station entirely on its own. The setup of the data logger and the software-based evaluation unit went very smoothly," López recalls happily. These steps were followed by calibration at the end of August, with the help of a three-axle 30-tonne truck. Official certification by a Mexican laboratory working in collaboration with Kistler's engineers was completed at the end of the year.

The final word from López: "Thanks to Kistler's systems, we successfully installed a fast, practical WIM solution that optimally meets the operator's requirements. In fact, the results were so convincing that we've already commissioned a second station at Cadereyta in the north-east of the country. More projects can be expected to follow in 2019 – and to implement them, we shall be opting for Kistler products again."

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