Structural Health Monitoring in the United States

In 2024 daily trips across structurally poor U.S. bridges still totalled 168 million, even after a five-year decline. That number is reported by the American Society of Civil Engineers in the 2025 Report Card, which scored the bridge stock a C for the second cycle running, with 46,000 spans flagged as structurally deficient and an average asset age of 47 years against a 50-year design life. According to ASCE, the budget to maintain this system falls $373 billion short of what would be needed. Fortunately, there are economically viable solutions for this problem like structural health monitoring.

The U.S. doesn't have a single aging problem

The US spans across different climates and geological zones, so it's quite clear that a salt-laden concrete column under a Miami high-rise will have different needs than a reinforced concrete pier on the Sacramento River. A concrete column in the heart of Florida loses section to chloride-induced corrosion which isn't even visible to the human eye. And a pier in the Sacramento River has to deal with seismic events that require an instrumented response within seconds.

What do the two structures have in common with a steel girder on a New York State highway? Practically nothing, and yet they all show up under the same FHWA National Bridge Inspection Standards regime under 23 CFR 650, where the parameters that actually predict their failure are very much not the same.

Where a single biennial visual inspection is enough on a stable inland overpass, it is not enough on a 1980s coastal condominium. Where dense fiber-optic instrumentation makes sense on a long-span river crossing, it is wasteful on a 200-foot rural span. That's why even with the migration from inspection to monitoring, the real challenge is creating custom and flexible systems instead of one-size-fits-all solutions.

Now let's look at the different problems that cause infrastructure damage in various US climates.

The Northeast: seventy-year-old bridges and freeze-thaw concrete

The average New York City bridge was built in 1956, so the average age of the local stock is close to 70 years. It's no wonder that inspectors logged 145 red-flag findings on NYC spans across a 28-month window ending in late 2024, hitting every borough other than Staten Island. Two spans drew particular concern for waterline erosion, a portion of the Harlem River Drive just north of the Macombs Dam Bridge at 155th Street and a section of the Marine Parkway-Gil Hodges Memorial Bridge, with the MTA's 2025–2029 capital plan earmarking dedicated funding for Hodges repairs.

In the cold northeast climate, freeze-thaw cycles propagate microcracks in concrete decks every winter. Pore water expands on freezing, generates hydraulic pressure, scales surface mortar, and reduces effective section over decades. Combined with chloride from de-icing salts and rising truck axle loads, the result is a stiffness loss that biennial visual inspections, by design, can only detect once the cracking is visible.

The mechanism is very well understood and monitoring is quite straightforward. Triaxial accelerometers placed at deck mid-span and quarter-spans capture modal frequency drift, which is a global stiffness signature. Tilt at the piers tracks foundation settlement on streambed-affected substructures. Strain gauges on the most loaded members catch the build-up that visual inspection cannot see. Environmental compensation for seasonal thermal effects is non-negotiable.

The DECKAXE-SHM accelerometer and the TLT-STD-LR tiltmeter were designed for exactly this kind of long retrofit deployment, where running cable across an active span is not realistic and battery life over five-plus years is the operational constraint that actually matters.

The South Atlantic: Surfside, SB 4-D and saltwater corrosion

On June 24, 2021, Champlain Towers South in Surfside, Florida collapsed and 98 people died. Photographs taken in the months before showed corroded columns and standing water in the underground garage. A 2018 engineering inspection by Morabito Consultants formally documented major structural damage, cracked concrete, failed waterproofing and corroded reinforcement, and forensic analysis after the collapse estimated that accelerated chloride-driven corrosion had been progressing on the substructure for somewhere between twenty and thirty years before the failure. The Miami-Dade County grand jury investigating the case pointed to saltwater intrusion as the most plausible explanation for the loss of capacity at the foundation level.

The regulatory response was Florida Senate Bill 4-D, enacted in May 2022. Every condominium or cooperative building three stories or taller must now undergo a milestone inspection at 30 years from the original Certificate of Occupancy, or at 25 years for buildings within three miles of the coastline. Re-inspection is required every 10 years. The Structural Integrity Reserve Study (SIRS) was originally due by December 31, 2024 (later extended to December 31, 2025 under HB 913), and as of January 1, 2025 unit owners can no longer waive reserve contributions for the SIRS structural components. Every condo board in Florida is now, by force of law, a buyer of structural information.

The deterioration mechanism in Florida and neighboring states is different from the freeze-thaw of the Northeast. Here, saltwater intrusion accelerates rebar corrosion, expansive rust products spall concrete cover, columns lose effective section silently. This damage is internal, and so it does not read on any visible facade.

Monitoring this type of asset is absolutely possible. Using tiltmeters at columns and slabs, engineers measure differential settlement before crack patterns reach the surface. Environmental sensors track humidity, temperature and salt-air proxies, while vibrometer readings of deck response under normal occupancy load give a modal baseline against which post-storm or post-flood signatures can be compared.

Our position, which the SmartCore Systems partnership in the United States is built around, is that the milestone inspection under SB 4-D is necessary but not sufficient. An inspection is what the engineer sees on the day and it provides one point of data, which is then complemented by the sensors that create a history of data.

The Buildings & Heritage Sites product line, together with the TLT-STD-LR tiltmeter and the ENV-STD-LR environmental sensor, is the configuration we typically recommend for this segment.

The Gulf Coast: hurricane wave loading and ground subsidence

Coastal bridges along the Gulf Coast face a load case much different from mainland infrastructure, namely waves from hurricanes and ground subsidence.

Hurricane storms add height to waves much above the static level of water, which drives lateral pressure on substructures and applies dynamic uplift on bridge decks. Researchers have built Coupled Eulerian-Lagrangian models calibrated against laboratory wave impact tests to simulate this combined load on bridge geometry typical of the Texas and Louisiana coastline. Hurricane Katrina in 2005 wrecked the I-10 Twin Span over Lake Pontchartrain and damaged dozens of bridges, ports and rail assets across Louisiana, Mississippi and Alabama, with Hurricane Harvey in 2017 producing the analogous lesson for the Texas portion of the coast. The replacement and retrofit work that followed those events is now itself 15 to 20 years old.

The hurricane risk is accompanied by a much slower mechanism in the soil. Vertical land motion across Greater Houston combines natural subsidence, groundwater withdrawal and sea-level rise. Satellite-based monitoring programs are being prototyped to track these movements at sub-centimeter accuracy across infrastructure portfolios.

There are two timescales when monitoring these two, separate mechanisms. The fast timescale is for hurricane storms, where dynamic displacement sensors and accelerometers capture the structural response during the storm and let engineers compare the post-event modal signature against baseline. The slow timescale is the multi-year drift, where tiltmeters and chained tilt sensors track pier rotation and foundation movement against the subsiding ground.

The West Coast: seismic monitoring and code-driven instrumentation

California's Strong Motion Instrumentation Program runs close to 1,400 active stations as of late 2024, with around 937 ground-response stations and roughly 273 instrumented buildings, the remainder split across dams, bridges and tunnels. Out of California's 13,214 state-highway bridges, 2,279 have been seismically retrofitted or replaced under updated design codes, at a cumulative Caltrans investment north of $12 billion. The Los Angeles and San Francisco Departments of Building Safety have enforced seismic monitoring code based on building height and aggregate square footage since 2011-2012, with the inventory of buildings requiring instrumentation periodically revisited.

The dominant instrument on these networks is the accelerograph, often paired with sensors measuring relative displacement between two reference points on the structure. The reason is mechanical. After a seismic event, damage shows up as interstory drift, residual rotation and modal frequency shift. All three are accelerometer-readable at the right sampling rate. A wireless network sampling at 40–640 Hz on a DECKAXE-SHM unit covers the full bandwidth of interest for low- and medium-rise seismic response.

We saw this pattern clearly on the Bridge of the Gods, a historic cantilever truss spanning the Columbia River between Oregon and Washington. The continuous wireless deployment there returns modal frequency data on a structure built in 1926 that no biennial visual inspection could resolve at the same temporal density. The result is not a replacement of the inspection regime but a higher-resolution dataset on top of it.

What to monitor on a Pacific Coast asset follows from the dominant risk. Triaxial acceleration at multiple deck or floor levels feeds both modal analysis and drift estimation. The MyMove Modal Analysis Tool computes modal frequencies, mode shapes and damping from continuous wireless data, which is the same set of quantities that a forced-excitation campaign with an eccentric mass shaker produces, at a fraction of the cost and without traffic disruption. At Move our position is that for in-service bridges, operational modal analysis on continuous wireless data is the right default, and shaker campaigns are the auxiliary tool reserved for one-off commissioning or post-event verification.

The Midwest and Mountain regions: tornado, freeze-thaw and the long tail of fatigue

On April 17–18, 2026, over 80 confirmed tornadoes tore through the Upper Midwest in 36 hours. Wisconsin and Illinois took the worst of it, with damage running into Minnesota and Missouri across residential and commercial stock. Tornado loading is intermittent and extreme; building code research designs school archetypes for peak wind speeds of 51.4 to 62.6 m/s, depending on the performance level targeted. But the main and slower mechanism that causes damage is, again, freeze-thaw on concrete decks and steel-deck fatigue under heavy freight loads.

The 2007 collapse of the I-35W bridge in Minneapolis is the regional case study most U.S. engineers study. After 13 people died because of the collapse, the replacement bridge was instrumented by MnDOT and the FHWA with a remote monitoring system providing real-time stress data, which has now been running for nearly two decades and remains one of the more thoroughly documented public SHM deployments on a U.S. interstate span.

Similarly to the Gulf Coast, tornadoes and freeze-thaw require two different timescales of monitoring. Post-tornado and post-overload modal signature needs to be compared against the baseline modal frequencies. In the case of long-term damage, sensors monitor trends on fatigue-prone steel members, thermal compensation across the full annual range, and, where appropriate, modal frequency drift as the global indicator of stiffness loss. Iowa DOT runs continuous SHM on selected bridges through a cooperative program with the Bridge Engineering Center at Iowa State University, monitoring high-performance steel bridge behavior over multi-year horizons.

How Move approaches U.S. infrastructure monitoring

Move Solutions operates in the United States from offices in Mt Pleasant, Pennsylvania, alongside a partnership with SmartCore Systems that handles turnkey design, installation and continuous operation for U.S. asset owners. The product line is built around the constraint that defines almost every U.S. retrofit deployment, which is that you cannot run cable across an active structure economically. Wireless beats wired on retrofits in roughly 95 percent of cases we evaluate, and the architecture decision compounds over the 20-year horizon that an SHM program actually covers.

The sensor selection is defined by the problems that every climate deals with. The DECKAXE-SHM triaxial accelerometer handles modal analysis on bridges and large structures across all five regions. The TLT-STD-LR wireless tiltmeter covers foundation tilt, condo column rotation and pier settlement at sub-milliradian resolution. The DECKVBR-STD vibrometer measures peak particle velocity on adjacent construction sites under DIN 4150-3, BS 7385, and on US blasting work under the RI8507 framework. The ENV-STD-LR captures environmental conditions where corrosion proxies matter. Data lands in MyMove, where the Modal Analysis Tool, the Tiltmeter Chain Tool and the Static Deflection Tool turn raw time-series into deformation profiles, modal parameters and alert thresholds wired into the owner's maintenance workflow.

Frequently Asked Questions

Does FHWA require continuous structural health monitoring on U.S. bridges?

No. The FHWA National Bridge Inspection Standards under 23 CFR 650 require periodic visual inspections at intervals not exceeding 12, 24, 48 or 72 months depending on risk classification, with the new Specifications for the National Bridge Inventory (SNBI) replacing the 1995 Coding Guide through a phased transition that retires the old format at the end of 2025. Continuous monitoring is permitted and increasingly funded under the Bridge Investment Program of the IIJA, but not federally mandated.

When does Move Solutions recommend against continuous monitoring on a U.S. asset?

When the structure has no regulatory obligation, no active deterioration, no adjacent works likely to induce vibration and a visual inspection regime that is genuinely being executed. Continuous monitoring is the right tool for assets where the gap between inspections matters. On a healthy inland overpass with a faithful inspection cycle, the right answer is no sensor network.

What is the minimum sensor density that actually returns useful data on a typical 200-foot span?

Four to eight triaxial accelerometers placed at deck mid-span and quarter-spans, plus one tiltmeter per pier. That density resolves the first three modes on most simply supported and continuous spans in the 200- to 400-foot range, and it leaves room for environmental compensation without producing data the owner cannot interpret.

Can a single SHM strategy work for both a Miami condo and a Sacramento overpass?

No, and trying to standardize the strategy is the most common mistake we see in U.S. portfolios. The Miami condo needs tilt, environmental corrosion proxies and slow-timescale vibration. The Sacramento overpass needs triaxial acceleration sampled at 40 Hz or higher for seismic response and modal analysis. Sensors overlap. Sampling, alarm thresholds and analysis pipelines do not.

How does SHM data interact with the FHWA 23 CFR 650 inspection cycle?

It does not replace the inspection. It complements it. A continuous network flags slow modal drift, transient overloads and post-event signature changes between scheduled inspections. The inspector verifies what the sensor saw, catches visible damage the sensor cannot read, and signs the regulatory paperwork. Both are required, and the phased SNBI transition retiring the 1995 Coding Guide at the end of 2025 carries an element-level data structure that fits naturally alongside continuous sensor data.

Is wireless SHM acceptable under AASHTO LRFD for instrumented bridges?

AASHTO LRFD is a design specification, not an instrumentation mandate. Strain thresholds aligned with LRFD design parameters are routinely used to define warning levels in continuous monitoring systems on LRFD-designed bridges, and several long-term sensor-based monitoring programs on LRFD steel girder bridges are documented in the U.S. peer-reviewed literature. The acceptance question, in our experience, sits at the owner and state DOT level, not at the federal code level.

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