Paul goes undercover to make the case for timber over steel.
The spy’s mantra is “keep your friends close and your enemies closer “.
In the truss and frame industry you might think your ‘enemies’ are those other timber fabricators that you regularly compete with. But, the real enemy is the light steel framing industry. Over the years the steel industry has made various attempts to make inroads into the timber prefabrication industries’ share of the market. It’s never got far but I have no doubt they will keep trying because there’s so much money at stake.
I’m something of a double agent; as an engineer I need to design in steel and timber. I’ve been out to the dark side, got the enemies’ documents, taken photos with my secret spy camera and smuggled the information out on a microdot. So, here I present various thoughts and comparisons about what my sleuthing has revealed:
Structural safety There’s no doubt steel is significantly stronger for a given section size than timber. However, there is no difference in safety because the timber is sized larger to make up for the difference in strength. And, because timber is so much cheaper per unit volume, our members can be more economical than steel.
Section properties A solid lump of timber is intrinsically very stable. One of the major problems designing and constructing in light gauge steel is that the sections themselves can distort. If you grab a steel C-section stud, you can just twist it. As an engineer, this makes light steel much harder to design than timber.
Eccentric loads Due to the same stability issues noted above, timber is generally better at resisting eccentric loads such as trusses supported on the sides of a bottom chord of a girder.
Transport Those same stability issues mean light steel frames are generally going to be more vulnerable to transport and erection damage than timber frames and trusses.
Redundancy As a whole, timber structures tend to be stronger than the ‘design’ strength as ramification of the grading process. The result is that in an overload situation on average a timber structure will be more likely to stay up than a steel one.
Environmental credentials The timber industry sells itself as an environmentally friendly product. Steel makes a case that it is actually better when you include the potential to recycle at the end of the life of the building. I think that’s somewhat disingenuous because there’s not much salvaging going on in the industry right now.
Connections In steel, these are much harder to make but for a given bolt, screw or rivet size they are much stronger. On the other hand, timber has the wonderful things called nailplates which, if you are religious, are proof that God smiles on timber framing!
Shrinkage Although once a real problem, the shift to seasoned and engineered timbers has largely voided this issue.
On site Steel is so much harder to adjust and/or rectify. Of all the things going for timber, this is the real long-term winner.
Holing Steel is harder to hole on site. I’ve blunted a hole saw or drill in steel far too many times; it’s a real pain in the bum.
Linings Steel can be oily and so glues don’t take so well to it. Any steel greater than about 1.5mm thick becomes quite hard to fix linings to. For instance, a screw can wander around before it bites into the steel and work the hole in the lining too large.
Noise My parents live in a steel house in Queensland and every time the sun warms the frame up and it expands, it sounds like we are in a ship that’s just run aground.
Rot/rust Either material will ultimately degrade in the presence of water. Timber is better in corrosive environments particularly near the sea. Both materials can be protected at a cost so it’s something of a line-ball issue.
Condensation This occurs where the steelwork temperature is lower than the air’s dew point. Repeated condensation can build up in closed building spaces and wreak havoc with linings, insulation etc. Cold steel behind linings can also cause local condensation on the outside of the linings themselves and ultimately discolouration.
Termites The steel framing industry trades heavily on the fact that “termites don’t eat steel”. True as that is, the widespread use of H2 treated timber has, to a large degree, resolved that problem for new timber buildings. Regardless of the frame type, termites are still happy to come into the house and munch on your skirtings and joinery, so the best approach is to keep the critters out with barrier treatments! In that case, steel has no advantage over timber.
Fires I’ve been to many, many house fires and although steel has the obvious advantage of being non-combustible, that doesn’t really matter so much in real life. It’s the contents – the furniture, plasticware, paperwork, carpet, curtains, joinery etc – that burn before the frame. In bushfires, the key points of ignition danger are the windows, decks and eaves, not the frame.
As a spy, subject to the pressure and constant danger of undercover work, I’ve quite enjoyed my brief time out of the shadows. As a reward I’m going to don my tuxedo, take out my pretty co-star and have a chocolate milk: shaken, not stirred.
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. Phone: (02) 4576 1555, email: firstname.lastname@example.org