How Weigh In Motion (WIM) and Structural Health Monitoring (SHM) Prevent Collapses

What is Weigh In Motion (WIM) and how does it work?
Weigh In Motion (WIM) is a technology that automatically monitors the weight of commercial vehicles in real time. Kistler's WIM systems use piezoelectric sensors embedded in the pavement to measure axle loads of trucks traveling up to 120 km/h. These robust quartz sensors provide highly accurate, maintenance-free data that is unaffected by temperature, allowing for seamless and non-intrusive installation.

Why is WIM important for bridge safety in the U.S.?
In the United States, over 250,000 bridges are more than 50 years old, with around 150,000 classified as deficient or obsolete. Overweight vehicles accelerate structural deterioration, sometimes forcing costly or disruptive bridge closures. WIM technology helps monitor traffic loads, detect overweight trucks, and prevent structural damage, protecting both infrastructure and commerce.

What is Structural Health Monitoring (SHM) and how does it work with WIM?
Structural Health Monitoring (SHM) is a technology that tracks the condition of bridges in real time using sensors such as accelerometers, strain gauges, and displacement sensors. When combined with Weigh In Motion (WIM) systems, SHM provides a comprehensive bridge management solution, allowing engineers to monitor both structural integrity and live traffic loads simultaneously.

What do SHM sensors measure on a bridge?
SHM sensors are strategically placed at critical points on a bridge to measure:

• Vibrations and oscillations
• Deflection and movement
• Structural changes or damage

For instance, accelerometers detect shifts in a bridge’s natural frequency, which can indicate potential structural damage before it becomes critical.

How does Kistler support bridge owners and Departments of Transportation (DOTs)?
Kistler’s WIM solutions assist DOTs and bridge owners in managing risks related to overweight traffic. For example, the New York City DOT launched the nation’s first automated WIM direct enforcement program on the Brooklyn Queens Expressway (BQE), enabling authorities to enforce weight limits using WIM data alone—the only system of its kind in the U.S.

What makes Kistler’s WIM sensors reliable and effective?
Kistler’s WIM sensors are built with quartz crystals capable of withstanding axle loads of 30 tons or more. They offer excellent linearity and high natural frequency, ensuring precise measurement of each vehicle. This technology allows real-time traffic monitoring, structural health assessment, and the implementation of weight-based tolling or maintenance strategies to prevent overload damage.

What benefits does WIM provide for bridge management?
By integrating WIM systems, bridge owners can:

• Assess structural health in real time
• Plan maintenance proactively
• Implement weight-based tolling or enforcement strategies
• Extend the service life of aging bridges and improve overall bridge safety

What makes Kistler’s SHM sensors reliable?
Kistler’s high-precision accelerometers offer a wide frequency range (0–1500 Hz) and operate in extreme temperatures from –55°C to +125°C. These specifications ensure accurate, stable, and long-term monitoring even in harsh environmental conditions.

How does combining SHM with WIM improve bridge management?
By integrating SHM data with live traffic and weight measurements from WIM systems, engineers can:

• Accurately assess a bridge’s structural health
• Predict remaining life expectancy under real traffic conditions
• Identify areas of concern before they require costly repairs
• Optimize maintenance scheduling and safety planning

What are the benefits of using SHM and WIM together?
The combination of SHM and WIM allows bridge owners to:

• Monitor heavy traffic and overloaded vehicles in real time
• Track structural performance under varying loads
• Enhance bridge safety and longevity
• Make data-driven decisions for maintenance, traffic management, and enforcement