Wired vs. Wireless Sensors for Bridge Monitoring: A Structural Engineer's Decision

Bridge monitoring is quite different from any other infrastructure monitoring project. The cost figures, the installation assumptions, the failure modes used for buildings or construction sites don't transfer to a 200-metre multi-span viaduct carrying 40,000 vehicles per day. So how does cabled monitoring differ from the newer wireless monitoring and which is more secure for bridges?

Why bridges are a different monitoring problem

In most monitoring projects like buildings or construction site, a technician can walk to most sensor locations in minutes. On a bridge, reaching a sensor might require lane closures, scaffolding, rope access teams, or marine equipment. This access problem shapes every technology decision downstream.

Add structural dynamics to the picture. Bridges experience live loads from traffic, wind, and thermal cycles. Monitoring them requires accelerometers for modal analysis, tiltmeters for angular deviation, and vibrometers for vibration velocity.  

The cable-routing problem on live structures

Picture running sensor cables on an existing multi-span bridge. We would need conduit fixed along the deck soffit, routed through or around each pier, with junction boxes at every connection point. At expansion joints, the cable path is interrupted by structural movement, so you need flexible couplings or breakout boxes. Every penetration through concrete requires waterproofing. Every exposed cable run requires UV-resistant conduit and corrosion-resistant fixings.

On a new bridge, this infrastructure can be designed in from the start: pre-installed cable ducts, junction boxes cast into pier heads, integrated circuits. The marginal cost is low, but any technological advancement or scope adjustment might still require a lot of rewiring in the future.

On an existing bridge, the calculation becomes much more difficult. Running cables requires access to the deck underside and each pier: scaffolding or under-bridge inspection units, lane closures, traffic management plans approved by the road authority. In the UK, a lane closure on a motorway bridge costs upward of GBP 1,500 per day in traffic management alone, before any engineering work begins. In Italy, ANAS technical circulars for national road bridge works require formal closure plans that can take weeks to approve.

Every additional sensor location means extending the cable network. Adding a tiltmeter to a pier base that wasn't in the original scope means re-mobilising access equipment, running new cable back to the data logger, and re-commissioning the system. The marginal cost of expanding a wired network on a live bridge is high.

What LoRaWAN sensors mean for bridge geometry

Bridges are perfect examples of why wireless sensors are becoming the standard in monitoring. They use LoRaWAN (Long Range Wide Area Network), which operates in the sub-GHz unlicensed spectrum, designed specifically for low-data-rate sensor telemetry over long distances. In open or semi-open environments, a single gateway can receive data from sensors several kilometres away.

Wired monitoring requires infrastructure along every metre between sensor and logger. LoRaWAN requires a sensor, a battery, and a clear radio path to a gateway that can sit hundreds of metres away.

Bridges are ideal for LoRaWAN. Their geometry is linear and open, with minimal obstruction between sensors and gateway. A gateway mounted on the parapet or at road level near one abutment can typically cover an entire multi-span structure without intermediate repeaters. Sub-GHz radio frequencies also penetrate concrete and diffract around structural steel more effectively than Wi-Fi or Zigbee, which operate at higher frequencies with shorter wavelengths.

On the Scrivia river railway bridge, a 7-span structure approximately 160 metres long, Move Solutions deployed wireless sensors across the full length of the bridge with gateway coverage from a single point. This was possible without cable runs between spans or junction boxes at piers. The sensors were mounted directly on the structural elements where measurement was needed.

Sensor placement follows structural logic instead of cable paths

The most structurally important measurement points on a bridge are often the hardest to reach physically. Consider these critical monitoring zones:

Pier bases require settlement and rotation monitoring. Many piers sit in watercourses or on slopes, making cable routing to their foundations impractical without significant civil works.

Deck soffits are where deflection and dynamic response are measured. These locations are 10 to 30 metres above ground with no permanent access. Running and maintaining cables along a deck soffit requires repeated access equipment deployments.

Bearing zones transfer load between deck and substructure. They're recessed inside abutments or pier heads, often in confined spaces where cable routing adds complexity without adding value.

Expansion joints present a specific challenge for wired systems: the joint moves. Any cable crossing an expansion joint needs a flexible coupling that tolerates the movement range without fatigue failure. This is a known failure mode on wired bridge monitoring systems.

Wireless sensors eliminate the constraint that every measurement point must have a physical cable path back to a data logger. The only requirements are a stable mounting surface and adequate radio line-of-sight to the gateway. On the Vespucci Bridge in Florence (162 metres overall length), sensors were placed at structurally critical positions including pier elements and deck zones that would have required disproportionate cable infrastructure to reach with a wired system.

Battery life

Replacing a battery on a bridge pier accessible only by scaffolding costs roughly the same as a day of lane closures. That reframes the entire battery life question.

Modern structural monitoring sensors are not consumer IoT devices. They operate on aggressive duty cycles: acquire data at defined intervals, transmit a compact packet via LoRaWAN, then sleep. Move Solutions sensors are designed for multi-year autonomous operation. At typical monitoring intervals for structural health assessment, battery life extends well beyond five years.

That means one access event per sensor over the typical duration of a monitoring campaign. Compare the cost of that single battery replacement to the cost of permanent cable infrastructure, including ongoing inspection and repair of cables, connectors, and conduit exposed to weather, UV, and structural vibration.

The tradeoff is real, though. A wired sensor with permanent power never needs battery access. For sensors in permanently accessible locations (a weather station on the parapet, a load cell on an accessible bearing shelf), wired power eliminates the battery variable entirely. The decision should be driven by access difficulty at each specific sensor location, not by a blanket preference for one technology.

Static monitoring, dynamic monitoring, and what each requires

Bridge structural health monitoring splits into two fundamentally different data categories.

Static monitoring captures slow changes: tilt accumulation at piers (measured in degrees or fractions of degrees over weeks and months), differential settlement between foundations, slow drift in alignment. Data rates are low. A reading every 15 minutes or every hour is sufficient. This is well within LoRaWAN's bandwidth, and wireless tiltmeters handle it without compromise.

Dynamic monitoring captures fast events: vibration response under traffic, modal frequencies of the structure (the natural frequencies at which the bridge resonates), peak particle velocity during construction or seismic events. These measurements happen in milliseconds. The sensor needs an onboard accelerometer or vibrometer sampling at hundreds of hertz. Move Solution's sensors come fully prepared for both static and dynamic acquisitions.

Data acquisition happens locally on the sensor. It captures a vibration event at high speed, processes it onboard (extracting peak values, RMS, or spectral data), and transmits a compact summary via LoRaWAN. Raw waveforms can be stored locally and downloaded during periodic access. This edge-processing approach means wireless sensors support both static and dynamic monitoring without exceeding the bandwidth constraints of long-range radio.

Our MyMove platform ingests both data types from the same sensor network. Its Modal Analysis Tool processes dynamic data to identify shifts in natural frequencies, an early indicator of structural degradation. Its Tiltmeter Chains Tool tracks cumulative angular changes across multiple sensors on connected structural elements. Both tools work with data from wireless sensors deployed across the structure.

The portfolio question

An infrastructure owner managing dozens of bridges, tunnels, and retaining walls faces a problem the single-structure case doesn't: consistency across a mixed asset portfolio.

Wireless monitoring with a standardised protocol like LoRaWAN allows the same sensor types and data platform to cover structurally different assets without redesigning cable infrastructure for each one. A tiltmeter on a bridge pier and a tiltmeter on a retaining wall transmit the same data format to the same gateway type, visible in the same platform.

COST Action TU1406, a European research initiative on quality specifications for roadway bridges, found that roughly half of Europe's bridge stock was built before 1980 and is approaching or exceeding its original design life. For a portfolio manager, that means most of the structures needing monitoring were never designed with sensor infrastructure in mind. Retrofit monitoring at that scale requires a technology that adapts to whatever geometry and access conditions each structure presents.

What comes next

One of the shifts that's happening in the monitoring sector is what you can do once the data is obtained. Tools like MyMove let you monitor in real time and set up automatic analysis. You can also get notified whenever the reported data starts becoming alarming.

Continuous structural data from wireless sensor networks creates the foundation for condition-based maintenance: repairing structures based on measured performance rather than fixed inspection schedules. The sensor technology is ready. The data platforms exist. The remaining challenge is institutional: getting bridge owners, regulators, and inspection frameworks to trust continuous monitoring data as a complement to visual inspection.

That institutional shift is already underway in Italy, the UK, and across the EU. The structures that are instrumented now will generate the track record that makes it standard practice for the next generation of infrastructure.