Buildings speak. SKAB listens.
SKAB continuously measures the ambient vibration of a structure, extracts its natural frequencies through operational modal analysis, and tracks how they change over time. An objective, continuous indicator where today, in most buildings, there is no data at all.
Structural monitoring has worked for decades. It costs too much to be used where it matters.
Dynamic identification techniques are well established and successfully applied to bridges, dams and major infrastructure. But a traditional installation is wired, invasive and slow to deploy: viable on a strategic asset, out of reach for a residential block, a school or a small public building.
Too many buildings, too few engineers
After a seismic event, the number of structures to inspect exceeds available survey capacity by orders of magnitude. Priorities are often set without objective data.
No history of the structure
Without a baseline of dynamic behaviour, a post-event survey has nothing to compare against: you observe the current state, not the change.
Degradation is slow and silent
Settlement, bradyseism, excavation and adjacent works act over time. Periodic spot checks rarely catch them in their early stage.
From ambient vibration to a change indicator
No forced excitation: SKAB uses the noise that is already there — traffic, wind, normal building use — as the excitation source, following the classical Operational Modal Analysis approach.
Continuous acquisition
Stations acquire at 200 Hz using very low noise MEMS accelerometers (≈15 µg/√Hz), alongside a wideband channel for impulsive events and a dedicated seismometer computing Housner Spectral Intensity (SI, in kine) and PGA on board. The number of stations depends on the building's geometry, not on a price list.
Event detection and noise rejection
An STA/LTA detector on horizontal energy identifies events. Automatic classification separates known recurring noise (plant, lifts) from the transients that genuinely deserve analysis.
Synchronisation between stations
For measurements from several stations to be comparable — a prerequisite for identifying torsional modes — they must be aligned in time with high precision. SKAB uses a proprietary synchronisation system, with time-quality verification on every acquisition and residual correction during processing.
Modal identification
Cloud pipeline using standard methods from the literature: Welch PSD, Frequency Domain Decomposition, EFDD for damping estimation, and SSI-COV with a stabilisation diagram to separate physical modes from numerical artefacts.
Baseline and comparison
The system builds the habitual dynamic behaviour of the structure over time. After an event it compares the new modal picture against the reference — at equal excitation and comparable environmental conditions, otherwise the comparison is not valid — and derives the Dk index.
Alerting and reporting
Once the configured threshold is exceeded, alerts reach the owner, the manager and the appointed engineer via e-mail, SMS or webhook. Every analysis remains available and exportable from the platform.
Dk — equivalent stiffness reduction
The monitored quantity is not the frequency shift itself, but the equivalent lateral stiffness reduction it implies. In an oscillator frequency scales with the square root of stiffness, so recovering stiffness requires squaring the frequency ratio.
Dk = 1 − ( f / fref )2
This is the final softening damage index introduced by DiPasquale, Ju and Cakmak (1990) and restated as D = 1 − (T₀/T)² in the review by Zhang et al. (2021). It is not a quantity we defined.
The practical consequence is that Dk is not proportional to the frequency shift: a 10% reduction gives Dk 0.19, a 20% reduction gives Dk 0.36. Thinking in percentage terms leads to underestimation, and the thresholds alongside are not frequency percentages.
| Class | Masonry | Reinf. concrete |
|---|---|---|
| Not significant | < 0.07 | < 0.10 |
| Slight damage | 0.07 – 0.15 | 0.10 – 0.25 |
| Moderate damage | 0.15 – 0.30 | 0.25 – 0.50 |
| Severe damage | ≥ 0.30 | 0.50 – 0.65 |
| Very severe damage | — | ≥ 0.65 |
Masonry thresholds from Sivori, Cattari and Lepidi (2022), coinciding with those of the Pozzuoli Operational Model. Reinforced concrete thresholds reconstructed from Navarro et al. (2012) and Vidal et al. (2014) on Lorca, Ditommaso et al. (2013) on L'Aquila and the review by Guéguen et al. (2014).
Why two columns and not one. Thresholds depend on structural typology, and the difference is wide: applying masonry values to a reinforced concrete building produces systematic false alarms. Typology is therefore a mandatory field of the building record, with no default value.
Significant does not mean damaged. Literature thresholds tell you when a building is damaged, not when a measurement is reliable — two different questions. The second is answered by computing, on each building's own baseline, the dispersion of ambient measurements, and treating everything within it as indistinguishable. This matters because the fluctuation is far from negligible: daily variations of up to 6% from temperature alone have been observed on masonry buildings of the Italian OSS network (Cattari et al., 2024).
None of these thresholds has regulatory status. Italy has no official thresholds on natural frequency variation in buildings: those above come from peer-reviewed literature and remain attention indicators. The reinforced concrete calibration is the weakest part, as it largely derives from comparisons across different building populations rather than pre- and post-event measurements on the same building. Operating thresholds on a real case are agreed with the appointed structural engineer.
Three different shifts that look alike in the numbers
Telling them apart is the hard part of the job. Confusing them is the fastest way to declare an undamaged building damaged.
Environmental fluctuation
Temperature, humidity, occupancy. Around ±3% on masonry, with daily peaks up to 6% on some buildings, and a few percent on reinforced concrete. This is not damage, and it is why the baseline is built over time rather than measured once.
Transient softening
During and immediately after strong shaking the frequency can drop by 20–30% with no damage at all, recovering over timescales from seconds to months. It is documented non-linear behaviour, not degradation.
Permanent reduction
What remains once recovery has run its course, measured at rest on ambient vibration. Only this is damage, and it is the only case where applying the thresholds makes sense.
How the platform handles it. A measurement acquired during an event is not classified: it is recorded and flagged as such. In the following days the value remains an attention indicator, not a damage class. Classification applies to at-rest analyses, on ambient vibration and under comparable environmental conditions. The diagnostic figure is not the peak of the reduction, but the value the structure settles at once recovery is complete.
What SKAB does not do
SKAB does not issue structural safety or occupancy judgements. It does not replace damage survey, seismic vulnerability assessment or structural evaluation, which remain the exclusive responsibility of qualified professionals and competent authorities.
SKAB provides measurements, quantitative indicators and a continuous history of the dynamic behaviour of the structure. Engineering interpretation of that data in the specific case is — and remains — the professional's.
Let's talk engineer to engineer
If you are responsible for a building, a portfolio or a construction site and want to know whether SKAB makes sense in your case, we will show you the platform and real data. Technical discussion, no commitment.