Vibration & Natural Frequency Check
Avoiding Resonance: Vibration & Natural Frequency
A long, light FRP panel can vibrate under footfall or passing machinery. Resonance occurs when an excitation frequency (a walker step, a pump or fan speed) matches the panel natural frequency. FRP actually has higher material damping than steel, so it settles faster - but its lower stiffness makes longer spans more prone to perceptible motion. The check is quick and worth doing.
Fundamental Natural Frequency (simply supported)
f1 = (pi / 2L^2) * sqrt(EI / m)
where L is the span, EI the bending stiffness and m the mass per unit length. The rule of thumb for comfort is to keep the fundamental frequency above roughly 3 Hz for footfall-sensitive areas and well clear of any running-machinery frequency (avoid the 1 to 1.3x band around it). Increasing panel depth or reducing span raises f1 sharply (it scales with 1/L^2).
Damping and Perception
FRP resin matrix damps vibration far more than steel, so even when a panel is tapped it does not "ring" like a steel plate. This reduces footfall rumble around sensitive areas and lowers transmitted noise to rooms below. The design problem is not FRP damping - it is keeping long, light spans from entering the perceptible range in the first place. For very long or machine-adjacent spans, choose a deeper pultruded section or an intermediate support.
Pumps, Fans and Compressors
Identify the operating speed of any rotating equipment nearby (e.g. a 1480 rpm motor ~24.7 Hz). Size the supporting grating so its natural frequency sits at least 20-30% above or below that value. If the panel passes the load and deflection check but vibrates under a running machine, the fix is stiffness (deeper section / shorter span), not more clips. Combine with the fatigue & dynamic loading derating for sustained vibration cycles.
When to Worry
Long, light spans
Span > ~1.5 m on a thin panel: estimate f1; stiffen if near 3 Hz.
Near running machinery
Compare natural frequency to equipment rpm; keep clear of resonance band.
Perceptible bounce
A panel that bounces under a walker is over-spanned - shorten or deepen.
Frequency Worked Example
The fundamental natural frequency of a simply supported grating span is approximated by f ≈ (π/2)·√(EI/(m·L⁴)) for a uniform member. In practical terms: a long, lightly loaded FRP span is a soft spring — its natural frequency drops as span⁴ rises, so doubling the span cuts frequency by a factor of ~16 in the ideal beam model. If that frequency lands on a forcing frequency (a pump at 50 Hz, a walker at 2–3 Hz), the span can amplify perceptible bounce. The design response is stiffness and damping: shorten spans, add stiffness, or accept the deflection limit that keeps perceptible motion inside comfort.
When to Treat Vibration Seriously
- Long, light spans — high L/D ratios amplify both deflection and resonance risk.
- Near running machinery — pumps, fans and compressors inject steady forcing frequencies.
- Perceptible bounce — if a deck bounces under footfall, it is near resonance or under-stiff; verify the frequency against the forcing source.
- Vibration-sensitive service — cleanroom subfloors and inspection platforms where micro-motion matters.
- Long, light spans — high L/D ratios amplify both deflection and resonance risk.
- Near running machinery — pumps, fans and compressors inject steady forcing frequencies.
- Perceptible bounce — if a deck bounces under footfall, it is near resonance or under-stiff; verify the frequency against the forcing source.
- Vibration-sensitive service — cleanroom subfloors and inspection platforms where micro-motion matters.
Need a Vibration Check?
Send span, panel depth and nearby equipment speed - ZeAllgrate returns the natural-frequency check and a stiffened selection if required.
Source: ZeAllgrate (Fiberglass Grating Manufacturers Council / ACMA) design guidelines; ASTM International; ISO 14122; OSHA 29 CFR 1910. Indicative data for specification guidance only — final design verified by ZeAllgrate engineering against certified load/span charts.