I have managed to convince my junior engineer that back when I was his age, our engineering calculations had to be written out on blackboards before they were committed to paper (because paper was far more expensive back then!).
If I could convince him of that, then I am hoping that I will have no trouble convincing you of an entirely truthful proposition; the case for tighter deflection limits for floor joists.
As I have discussed before, most structural timber designs are limited by the member’s deflection limits rather than their strength.
Strength design is a non-negotiable area, so a member must be reliably safe and the Australian standards are very clear about where the limits lie.
By way of contrast, deflection limits are rubbery and negotiable. Standards provide deflection limits, but these are generally ‘recommended’ limits.
A designer can apply either loose or more stringent limits, so the essential questions are whether the final user will find them aesthetically acceptable, whether functionality is impaired or whether damage could result if the deflection is too excessive.
Every working day, I make decisions surrounding what is and is not an acceptable deflection level.
A final choice generally depends upon the sensitivity of the end use. A typical decision would be that the lintel that you can see as you come down stairs requires tighter deflection limits than in a single storey house, where you can’t see the lintel deflections as easily.
However, design software and span tables have pre-set deflection limits that are determined by the engineers and programmers. While these limits have been selected based on previous experience, they may be too conservative or too liberal for a specific application.
I would, for example, argue that a stud in a shed wall frame can be allowed to deflect further under wind than a stud in a display home. However, using the span tables, you have no opportunity to push spans further than as published, because you may be compromising the strength.
Some design programs give you the ability to manipulate deflection limits. This may be by way of global settings where you, for example, decide that the market demands ‘cheap and nasty’, and so that is what you provide.
Most programs will also give you access to view the actual design deflections for designs to the standard limit. You might, for instance, look at a truss bottom chord deflection and decide to ‘stiffen’ it up somewhat.
In this article I am making the case for tighter deflection limits in the special case of floor joists with tiled floors. The common limit adopted in most software and span tables for floor joists under permanent loads is a minimum span of 300 or 15 mm.
For a typical carpeted or decorative timber floor in a project home situation, this is generally quite acceptable.
However, when there are floor tiles present, they are much more susceptible to damage on account of their brittleness.
Floor tiles are generally ok when following the even curve of a floor joist. However, when that curve changes abruptly, for instance when the joist is laid parallel to a rigid end wall or where the flooring continues onto a different span or direction joist, there can be rapid changes in the slope of the floor. Consequently, this can crack floor tiles.
This is today an even greater consideration than it once was. For a start, floor tiles have become bigger over the years, so an individual tile is more likely to bridge between areas with significantly different deflection characteristics.
With the advent of engineered joists, spans have also increased on average, and this means more joists are pushing the 15 mm limit.
The cost of fixing cracked floor tiles can be considerable; tile styles change rapidly and colours change from batch to batch, so it is often impossible to get a match, even in a relatively new build.
So what might have been a fairly easy fix of a few cracked tiles before, can easily escalate into the eventual replacement of all of the tiles. This can introduce collateral costs such as removing joinery, accommodation costs for residents, damage to the underfloor heating infrastructure, and more.
So, I am suggesting that if you have a floor that is designed to support tiles, you ensure you design conservatively when it comes to deflection, unless you are sure that there are no abrupt changes of slope in the floor.
Well, hopefully I have convinced you! If you can’t go for that, perhaps, like my young gullible engineer, I can convince you of something more fanciful.
Could you believe in orange-bearing pine trees? The Abbot-Bishop romance? Big Ben going digital or tail lights for horses?
Believe me, it’s way better to be convinced on the point of tighter deflection limits! 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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