Floor vibration and serviceability
Why a light floor is more sensitive
Section titled “Why a light floor is more sensitive”Vibration behaviour is set by two things: stiffness and mass.
In a concrete floor the large mass absorbs the energy of a footstep. A light floor has less mass, so the same footstep produces a larger response.
This is not a strength problem: the floor carries the load. But for the user it is one of the most important quality characteristics.
What the user notices
Section titled “What the user notices”- Bounce: the floor springs when someone walks across it.
- The glass effect: objects on the table move when someone walks past.
- Transmission: the movement is felt in the next room.
These are often more annoying in the adjacent room than where the footstep happens.
How it is handled
Section titled “How it is handled”Greater joist depth
Section titled “Greater joist depth”Increasing stiffness is the most effective tool. A deeper joist gives more stiffness for less extra material than thicker sheet does.
Closer spacing
Section titled “Closer spacing”More joists, less load each, a stiffer system.
Blocking
Section titled “Blocking”Blocking between the joists improves not only stability but also load sharing: a footstep is carried by several joists.
For vibration this is one of the cheapest improvements.
Mass in the build-up
Section titled “Mass in the build-up”A wet screed or a thin concrete layer improves vibration behaviour considerably.
For more demanding cases — an apartment-building floor, say — a composite floor on steel decking can be designed — the Lewis system is the established one here: the dovetail sheet counts as the bottom reinforcement, giving a thin floor that carries load and insulates well. Floor build-ups.
Damping
Section titled “Damping”Resilient layers and a decoupled floor improve sound transmission first of all, but they affect the feel of the vibration too.
How it is designed
Section titled “How it is designed”Checking vibration is a serviceability verification: the natural frequency of the floor and its response to excitation are determined and compared against criteria.
The important thing is that it is not left out: a floor sized only for strength can satisfy the standard and still be unpleasant.
The point
Section titled “The point”Vibration is the characteristic by which the occupant judges the floor. Design for it — afterwards it can only be fixed by opening up the build-up.