If you think about it, there are only a few key factors to predict sea-sickness. As alluded to above, the person is an important factor. Some people are just natural-born barfers!
The nature of the swell is the other critical factor, and that has two components. One is the frequency of the swell – very short-period, choppy waves tend to be okay as do the very, very slow movements of a large ocean liner. It’s those waves that tend to be two to ten seconds apart that are likely to be the most “˜chuck worthy’.
But it’s not just the frequency of the waves that brings on the puke, it’s the peak acceleration of the boat. If the boat drops off the back of steep waves, and leaves you near weightless for a moment, just about everyone is going to be feeling green. But, at a much slower acceleration, everyone will be okay.
Strangely enough, the important factors when it comes to floor bounce are much the same.
The sensitivity of the people, the frequency of the bounce, and the acceleration of the floor joists. Of course the limits are entirely different – you can’t get away with a floor that makes people seasick!
There is nothing much you can do to alter the sensitivity of homeowners to bounciness. However, it is important to note, that clients are – on the whole – becoming more demanding of their floor’s performance. Even the slightest hint of vibration can be an earth-shattering problem for Mr and Mrs Precious! So, this is something that deserves close attention from the designer.
It turns out, in a practical sense, that if the rate of vibration is more than eight cycles a second, floor bounce is too fast to be perceptible. This “˜natural frequency’ of a floor system is a function attributed to a number of variables that interact in very complex ways.
Some of these factors include the weight of the floor finishes and the stiffness of the joists. However, the dominant factor is the span of the floor. With the advent of engineered joists, spans have also increased on average and this means that floor bounce limitation is more and more likely to govern the design.
All modern software and span table solutions for floor joists explicitly take potential bounce into account. These checks are done with varying levels of sophistication – an accurate engineering analysis is potentially very complicated – so most designs adopt a very simple approach that has been proven through more than a decade of use. The shortcoming of this simple approach is that, in some cases, the answer may be a little over or under conservative.
However, there is one great flaw in the majority of our current approaches. The design only considers the floor joists as an isolated system. However, very often some of the floor joists will sit on a beam and the beam itself adds flexibility to the floor system. The increased flexibility will slow down the natural frequency of the floor. And, if the combination of the joist and beam flexibility drops the natural frequency to below the eight cycles per second, you are entering problem territory.
Having said all that, you have to be unlucky to have a floor that exhibits real problems because other factors can come into play. It’s the vibrations that persist that are perceptible. So, the presence of internal non-loadbearing walls, joinery, furniture, and some floor finishes, can all serve to dampen down the vibration from each footfall so quickly that they just aren’t noticeable.
It is very expensive to fix a floor with a significant vibration problem. It would generally mean opening the floor up and installing major stiffening elements. So, take note of the key conditions which are most likely to cause a vibration problem.
Floor joist spans near their limit, others supported on beams that are near their span limit with heavy floor finishes and large open expanses above and below. Keep an eye out for these circumstances and avoid them if you can. If unavoidable, refer them to your favourite structural engineer.
I said before that there is nothing much you can do to alter an owner’s perception of what is acceptable bounce. That is not entirely true.
In my first job at a consulting engineering firm we designed a mezzanine floor with steel floor joists that turned out to be almost “˜trampoline-like’. Our senior engineer told the client that the springy floor was a deliberate feature, and that the latest research from the States reported that it reduced the incidence of workers’ lower back problems.
And they bought it! So, if all else fails and you have a bouncy floor – you can try your luck with that one!
Paul Davis is an independent structural engineer managing his own consulting firm Project X Solutions Pty Ltd. The views in this column are Paul’s and do not reflect the opinions of TimberTrader News.
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