When the Ground Thaws: Managing the Hidden Hand of Thermal Stress in Bridge Structural Health Monitoring
Ali Siamaki
For geotechnical and structural engineers monitoring the pulse of vital infrastructure like bridges, the data is everything. Yet, a silent, powerful force frequently corrupts these critical readings: seasonal thermal stress. As cold regions transition from the grip of frost to the spring thaw, instrumentation designed to measure structural load—our anchor load cells, embedded strain gauges, and tiebacks—begin to report significant, non-load-related anomalies. These aren't signs of structural failure; they are simply the physics of expansion and contraction, often amplified by the relentless cycle of freeze-thaw and large-scale structural movements.
In this article, we explained how to distinguish true structural load from the effects of temperature, ensuring that our monitoring programs maintain their integrity and reliability.
The Macro-Scale Driver: How Bridges Move
Before we can correct a sensor, we must understand how temperature moves the bridge structure itself. In Structural Health Monitoring (SHM), thermal effects don't just affect the components; they redefine the entire structural boundary condition.
1- Global Movement and Abutment Constraint
Long-span bridges, particularly those constructed from steel or post-tensioned concrete, are subject to significant dimensional changes based on the material's Coefficient of Thermal Expansion (α). A bridge hundreds of meters long can easily expand or contract by tens of millimeters over a season.
The Problem: When a bridge deck expands and presses against a rigid abutment or pier, or when it contracts and pulls away, the resulting force is transferred directly to the foundation elements. Load cells under bridge bearings or those monitoring tiebacks near abutments will register a false load change that stems purely from the structure pushing against its constraint. This masks the real geotechnical load changes.
The Imperative: The SHM program must include Finite Element Analysis (FEA) to model this structural movement and predict the magnitude of the thermally induced load. This allows engineers to isolate the load component due to true operational factors (e.g., traffic or foundation settlement).
2- Diurnal Gradients and "False Bending"
Solar radiation creates extreme temperature differentials—or gradients—between an exposed bridge deck surface and the shaded soffit. These gradients induce significant, non-load-related bending moments that cause the girder or deck to slightly arch or sag (hogging/sagging) independent of traffic.
The Impact: Strain gauges bonded to the underside of a steel girder will read this thermal bending strain alongside the strain from vehicle traffic. If unaccounted for, this thermal signal can easily be misinterpreted as long-term creep, early-stage cracking, or foundation rotation.
Surgical Correction: Strategies for Key Instruments
Effective thermal management requires a two-pronged approach: mitigating the thermal influence during installation and correcting the data with mathematical precision.
1- Vibrating Wire/Hydraulic Load Cells: Isolating the Unwanted Force
Load cells are vulnerable to thermal drift due to the expansion and contraction of the steel tendon or pile being monitored, often compounded by the load cell body itself.
The Correction Standard: Compensating for Tendon Expansion
The core solution involves calculating and removing the parasitic force (Pthermal) generated by the tendon material's expansion:
Ptrue=Pmeasured−(Ematerial⋅Amaterial⋅αmaterial⋅ΔT)
Where ΔT is the difference between the current temperature (Ttendon) and the reference temperature (Tref) at the time of initial lock-off or seating. Mandatory co-logging of temperature and load is the foundation of this process.
Proactive Mitigation:
Thermal Armor: For exposed load cells, applying thermal insulation jackets or rigid, non-conductive enclosures is crucial to dampen rapid diurnal temperature swings and maintain a stable internal temperature (Tinst).
Deep Placement: For anchor systems, seating the load-bearing plate below the local frost line ensures the assembly operates within a more stable geothermal environment, reducing surface-effect coupling.
2- Strain Gauges: The Power of the Dummy and Full Bridge
Strain gauges measure total strain (ϵtotal), which is the mechanical strain we want, plus the thermal strain we don't.
The Correction Standard: The Dummy Gauge Technique
While many modern gauges are self-temperature-compensated (STC), the most robust field-validated technique is the dummy gauge for non-STC gauges and precise applications.
An identical, unstressed gauge is mounted on a scrap piece of the exact same structural material and placed in thermal contact with the structure. When wired into a Wheatstone bridge on an adjacent arm, the thermal resistance changes cancel each other out, isolating ϵmech.
Proactive Mitigation:
Multi-Gauge Arrays: Deploying gauges on opposing faces of a bending member allows for the mathematical separation of axial strain (average) from bending strain (difference), significantly simplifying the isolation of the thermal bending moment.
Protective Cloaking: Apply robust, multi-layer protection—epoxies followed by metal shields—to prevent rapid thermal shock from sudden solar exposure.
3- Ground Anchors/Tiebacks: Decoupling Frost Action
Tiebacks that pass through shallow, frost-susceptible bridge approach embankments or foundation overburden are subject to adfreezing and thaw weakening.
The Challenge of Adfreezing: As the ground freezes, the soil expands and bonds (adfreezing) to the anchor tendon in the shallow, unbonded zone. This creates a powerful tensile surcharge load—a false increase in the anchor reading—which is entirely non-structural. The spring thaw releases this bond, causing an abrupt load drop.
The Design Imperative: Geothermal Decoupling
Oversized Casing and Inert Fill: The unbonded length must be physically separated from the freezing soil. This is best achieved by installing an oversized permanent casing and filling the annulus with a compressible, thermally inert material (e.g., low-density foam or grout with a high deformation modulus).
The Thermistor Verdict: This requires instrumentation beyond the load cell. Thermistor cables installed adjacent to the anchor are crucial for monitoring the movement of the 0∘C isotherm, allowing engineers to correlate load spikes directly with the frost front depth, confirming the reading is parasitic.
Case Study Example: Thermal Surcharge on Bridge Struts
Context: Steel compression struts supporting a cofferdam for a major bridge pier in a high-latitude region were monitored with VW load cells.
Problem: Diurnal load fluctuations of up to ±20% of the initial 900 kN design load were observed, correlating perfectly with ambient temperature swings.
Correction & Results: By applying the thermal load compensation formula (Section 3.1) using the strut's steel properties, the calculated Pthermal component accounted for 98.5% of the observed diurnal variation.
Conclusion: The corrected data showed the strut load was stable. This case proved that the large-magnitude fluctuations were entirely parasitic, non-structural stress, validating the design and eliminating the need for unnecessary remediation. The ability to isolate the thermal signal was critical for project safety and cost control.
Conclusion: Mastering the Invisible Forces
The management of seasonal thermal stress is not a peripheral task but a fundamental discipline in reliable structural health monitoring. The magnitude of forces generated by temperature gradients and freeze-thaw cycles can easily exceed the forces generated by operational loads, rendering raw data inconclusive without rigorous, technically grounded correction.
For the modern geotechnical and structural engineer, mastering the physics of thermal compensation is paramount. By applying the principles of physical decoupling (mitigation) and rigorous mathematical correction (compensation), we ensure that the data flowing from our bridges truly speaks of their structural health, free from the noise of the changing seasons. The integrity of our infrastructure depends on it.
