Choosing the Right Method for Bridge & Structure Sensors

Structural health monitoring (SHM) has moved from a research novelty to a standard part of managing aging infrastructure. Accelerometers, strain gauges, tilt sensors, and vibration monitors are now routinely deployed on bridges, towers, parking structures, and industrial steel frames — often on assets that can't afford downtime, permitting delays, or coating damage.

Which raises a practical question every project engineer eventually faces: how do you actually attach the sensor?

The two dominant options — drilled/bolted anchors and rare-earth magnetic mounts — solve the same problem in very different ways. Here's how to choose.

The Case for Drilled Anchors

Drilling and bolting remain the standard for permanent installations. The benefits are well understood:

  • Maximum long-term holding strength, largely independent of surface condition or vibration load
  • Predictable engineering values — anchor manufacturers publish rated shear and pullout strengths for specific substrates
  • No dependency on surface finish — works through paint, rust, or coatings without a meaningful strength penalty

The tradeoffs are just as well known. Drilling into a bridge girder, tower leg, or parking structure beam typically means:

  • Coating penetration, which introduces a corrosion risk at the fastener site
  • Permitting or engineering sign-off, since any penetration of a structural member can require review
  • Irreversibility — once a hole is drilled, it's there, whether or not the monitoring campaign continues
  • Installation time and access equipment, especially at height or over water

For permanent, decades-long installations where the sensor stays put for the life of the structure, this tradeoff is usually worth it.

The Case for Magnetic Mounts

Neodymium magnetic mounts have become the preferred option for a growing share of SHM deployments — particularly temporary, campaign-based, or exploratory monitoring work. The appeal is straightforward:

  • Non-invasive. No penetration of the structural coating or substrate, which sidesteps corrosion risk and often simplifies the permitting conversation.
  • Fast installation and removal. A technician can place, reposition, or remove a magnetically mounted sensor in minutes — useful when you're chasing a vibration signature across a span or reacting to an inspection finding.
  • Reversible. For seismic-event response, construction-phase monitoring, or short-duration studies, magnetic mounts leave the structure exactly as they found it.
  • Repositionable without waste. If initial sensor placement doesn't capture the data you need, moving a magnetic mount costs nothing. Moving a drilled anchor means a new hole.

The limitation engineers need to plan around is holding force, which isn't a fixed number — it depends on:

  • Surface condition. Paint, rust, and coatings all reduce the effective pull force compared to bare, machined steel. A magnet rated for 100 lbs of pull on clean steel may hold meaningfully less on a painted girder.
  • Air gap. Any standoff between a magnet and steel — from a mounting bracket, gasket, or coating thickness — sharply reduces holding force, since magnetic pull falls off quickly with distance.
  • Vibration and dynamic load. Structures under live traffic or wind loading impose cyclic forces that static pull-force ratings don't fully capture. Sizing with margin matters more here than in a static application.
  • Temperature. Outdoor structural applications can see wide temperature swings. Standard N42 neodymium magnets are typically rated to around 176°F (80°C) before performance degrades; higher-temperature grades are available for exposed, sun-loaded steel in hot climates.

Sizing a Magnetic Mount: The Basic Rule

A common starting point is to size the magnet's rated pull force to 5–10x the static weight of the sensor and housing, then derate further for surface conditions, air gap, and expected vibration. For a 2 lb sensor package on painted structural steel with light vibration exposure, that might mean specifying a magnet rated well above 20 lbs of pull on bare steel — with the final number confirmed against the actual mounting surface, not a catalog number alone.

This is also where the choice of coating matters. NiCuNi (nickel-copper-nickel)- plated neodymium magnets are the standard for general outdoor use and offer good corrosion resistance in most bridge and structural environments. For prolonged wet exposure, marine environments, or long-duration deployments, an epoxy coating provides an extra layer of protection against corrosion-driven strength loss.

Which Should You Choose?

Drilled Anchors Magnetic Mounts
Best for Permanent, decades-long installations Temporary, campaign-based, or exploratory monitoring
Installation speed Slow — requires drilling, access, often sign-off Fast — minutes per sensor
Reversibility None Full
Coating/corrosion risk Penetrates coating at fastener site None
Holding force predictability High, standardized Variable — depends on surface, gap, temperature
Typical use case Permanent SHM networks, critical safety monitoring Post-event response, short studies, construction-phase monitoring, sensor placement testing

Many SHM programs end up using both magnetic mounts during an initial data-gathering or sensor-placement-optimization phase and drilled, permanent mounts once optimal sensor locations are confirmed.

Specifying the Right Magnet

Because pull-force requirements vary widely with surface condition and sensor load, generic hardware-store magnets rarely meet the demands of structural monitoring work. Engineers speccing a magnetic mounting solution should look at:

If you're speccing magnets for a monitoring deployment and need pull-force guidance for your specific sensor weight, surface material, or environmental conditions, our team can help you find the right fit — or request a custom quote for project-specific requirements.

Safety Warning: Children should not be allowed to play with neodymium magnets as they can be dangerous. Small magnets pose a choking hazard and should never be swallowed or inserted into any part of the body.

Magnets can be dangerous. Neodymium magnets, especially, must be handled with care to avoid personal injury and damage to the magnets. Fingers and other body parts can get severely pinched between two attracting magnets. Bones can be broken by larger magnets. Visit our Magnet Safety page to learn more.