Vibration & Serviceability

The key to this whole lesson is a distinction that structural engineers make constantly but that non-engineers often miss: a structure has to satisfy two very different kinds of requirement, and passing one does not mean passing the other. The first is strength - technically the ultimate limit states - which asks whether the structure can safely carry its loads without failing, breaking or collapsing. This is the life-safety question, and it is non-negotiable. The second is serviceability - the serviceability limit states - which asks whether the structure performs acceptably in ordinary, everyday use: does it deflect too much, sag visibly, crack finishes, or, crucially here, vibrate in a way people can feel and find unpleasant? A structure can be entirely safe (it will never fail) and still fail serviceability (it sags, or bounces, or feels alive), and that is not a safety failure but it is absolutely a design failure - people will be unhappy in the building.

For timber floors, serviceability - and vibration in particular - is often the governing requirement, meaning it is the one that actually decides how big the floor has to be, more than strength does. This is a genuine mental shift for anyone used to thinking a beam is sized by whether it is 'strong enough'. With a lightweight timber floor, you can very often make it strong enough with a relatively modest section, only to find that at that size it feels unacceptably bouncy - so you have to make it deeper, stiffer or heavier not for safety but for comfort. The floor is sized by feel, by the serviceability check, not by the strength check.

Understanding this reframes how you should think about timber floors from the outset. When you draw a long, clear timber span, the first question in the engineer's mind is often not 'will it hold the load?' (usually yes) but 'will it feel solid enough, or will it need to be deeper or have a topping to control vibration?' - and that answer drives the floor depth, which drives your section, your storey heights and sometimes your whole building. Getting this into your thinking early - that vibration serviceability, not strength, is likely to set the timber floor depth - is exactly the kind of judgement this course exists to build, and it is why an early conversation with the engineer about floor spans and depths is so valuable in timber.

To design vibration out, you first have to understand what causes the sensation, and here a little physics goes a long way without any calculation. Every floor, like any structure, has a natural frequency - the rate at which it naturally wants to vibrate if you disturb it, set by its stiffness and its mass. When something excites the floor at or near that natural frequency, it responds strongly; when the exciting motion is well away from the natural frequency, the floor barely responds. The everyday excitation on a floor is footfall - walking - and human footfall contains rhythms in a fairly low frequency range. The design intuition is this: if a floor's natural frequency is low enough to be near the rhythms of walking, footfall can set it noticeably moving; if the floor's natural frequency is comfortably high, walking cannot easily excite it and it feels solid.

Now the properties come together. A floor's natural frequency rises with stiffness and falls with mass - a stiffer floor has a higher natural frequency, a heavier floor a lower one - and how strongly it keeps moving after being disturbed depends on damping, the floor's ability to absorb vibration energy and settle quickly. Lightweight timber floors are caught in a difficult combination: they are relatively flexible (especially over the long spans timber enables), which lowers stiffness and thus natural frequency, and they are light, which on its own would raise frequency but also means there is little mass to resist being set moving by a footstep and little inertia to damp it, so the response to each footfall is proportionally larger. They also tend to have low inherent damping compared with heavy, wet-jointed concrete construction, so once disturbed they keep vibrating longer. The result is a floor that is easily set moving by footfall and slow to settle - the classic 'lively' or 'bouncy' timber floor.

This is why the problem grows with span and with lightness, precisely the direction mass timber pushes: a long, clear, elegant timber floor - exactly what the material is celebrated for - is the most vibration-prone, because it is the most flexible. It is also why the sensation is about comfort and perception, not damage: the movements involved are tiny and completely safe, but humans are remarkably sensitive to floor vibration and find a lively floor unsettling or annoying, associating it (wrongly) with weakness. So the engineering goal is not to stop a safety problem but to keep the floor's dynamic behaviour - its frequency and its response to footfall - within limits that people find comfortable, which is a real, and often demanding, design task.

The reassuring part is that timber floor vibration is a well-understood, routinely-solved design problem - the world's tall timber buildings have comfortable floors - and it is solved by pulling a set of understandable levers, all aimed at raising the floor's natural frequency and increasing its stiffness, mass and damping so that ordinary footfall cannot set it into perceptible motion. You do not need to calculate them, but you should recognise them, because several are architectural decisions as much as engineering ones.

The most direct lever is stiffness: a deeper, stiffer floor section has a higher natural frequency and deflects less under footfall, so making the floor deeper - a thicker CLT panel, deeper joists or beams, a deeper cassette - is the classic cure, and it is exactly why timber floors are often deeper than a pure strength calculation would require. The second is span: a shorter span is inherently stiffer and less prone to vibration, so reducing the clear span (an extra line of support, a beam, a wall) can transform a floor's feel - which is why the seductive very-long clear span is where vibration bites hardest and where an early engineering conversation matters most. The third is mass: adding mass, typically the acoustic topping the floor probably needs anyway (Module 6.1), increases inertia and damping and helps the floor resist and settle footfall - one of the happy overlaps where the acoustic and vibration fixes point the same way. The fourth is composite action and continuity: making a topping act structurally together with the timber deck (a timber-concrete composite floor, for instance) dramatically increases stiffness and mass at once, and making floors continuous over supports rather than simply spanning between them raises frequency and adds damping.

Notice how many of these are shared with other lessons and with your own design moves: depth affects your storey heights, span is set by your grid and structural layout, and the mass topping is the same one solving acoustics. This is the deeper point of Module 6 - acoustics, moisture, thermal and vibration are not separate add-ons but interacting demands on the same floor and wall, and the good build-up satisfies several at once (a mass topping helps both sound and vibration; keeping the timber warm serves both energy and durability). The engineer chooses and sizes the levers, but you shape the raw material they work with - the spans, the grid, the floor zone available - so the earlier you design generous, sensible spans and enough floor depth, the easier and cheaper it is to deliver floors that feel as solid as they are safe.

For all that you can understand and anticipate vibration, the actual assessment is firmly the structural engineer's, and it is worth being clear why. Whether a floor will feel acceptable is judged by a vibration serviceability check - a genuine dynamic analysis that estimates the floor's natural frequency and its response to footfall and compares them against comfort criteria for the building's use. Those criteria are not universal or intuitive: what is acceptable for a busy office differs from a quiet home differs from a gym or a dance floor, and the limits, the analysis methods and the acceptance thresholds are set by the codes and standards (Eurocode 5 and its associated guidance are widely used for timber floor vibration; national codes and specialist design guides elsewhere) and applied by the engineer. This is emphatically not something to judge by eye or copy from another project, because the perception of vibration is subtle and the analysis genuinely technical.

This makes vibration a near-perfect illustration of the whole course's division of labour, which is why it closes the module. The principle and the anticipation are yours: understand that timber floors are vibration-prone, that serviceability rather than strength usually governs their depth, that long clear spans are the danger zone, and that the levers are stiffness, span, mass, composite action and damping. Bring that judgement to the earliest sketches - do not casually draw a very long clear timber span and assume a shallow floor, keep floor zones generous, discuss ambitious spans with the engineer before they are fixed - because the raw material of spans and depths, which you set, largely determines how hard the vibration problem will be to solve. The binding assessment is the engineer's: the dynamic check, the comfort limit for the use, and the final floor sizing all come from the structural engineer and the code, and a designer who promises a client a particular long-span shallow timber floor without that check is writing a cheque the engineer may not be able to cash.

Held together, Module 6 leaves you with a clear-eyed view of timber's building physics. Timber's great gifts - lightness, flexibility, low conductivity, the beauty of exposed wood - each carry a matching physics challenge - acoustics, vibration, condensation risk, the pull to leave surfaces bare - and none of these is a reason to avoid timber, because all are routinely solved in excellent buildings. But they are reasons to design timber knowingly, to protect the floor and wall zones that solve them at concept stage, to see the acoustic-thermal-moisture-vibration demands as one interacting system on the same elements, and to keep the design judgement while deferring every binding number - the acoustic rating, the moisture class, the U-value and vapour analysis, the vibration limit - to the acoustician, the timber engineer, the building physicist and the code.