Bamboo is used out here, but can come down in high winds. For engineers familiar with both, which is safer?
Roger? I find you to be one of the most interesting people on the board.
You been on the Cloudy Bay again? I tried to put you on my Buddy List, only to discover you were already there. That’s enough frippery, mind. I can hear the unmistakable sound of jackboots weighed down by standard-issue two-by-four beam approaching…
My understanding is that good quality bamboo has a higher tensile strength than the steel used in most scaffolding, but that inconsistent bamboo quality can cause problems. Bamboo is also cheaper, which has made it more attractive in places where it is legal to use it.
A new alternative seems to getting some attention, and it is supposed to combine the best of both worlds. It’s called Metal-Bamboo Matrix System Scaffold.
High winds can bring down metal scaffolding, too - this happens in Chicago every couple of years.
I’d almost worry more about how well put together/anchored the scaffolding is than precisely what it is made out of.
Tensile strenth is measured as a ratio of cross section so this is almost certainly untrue but I have no problem with a claim that bamboo may have a higher strength to weight ratio than mild steel.
The problem with bamboo scaffolds is probably the same as other large wood structures, once you stack so much weight on top the compression limits are reached at joints which crush and fail.
I’m not an engineer, so I haven’t a clue whether it’s accurate or not, but the following sites make the claim:
It’s quite possible that these sites are incorrect, and to be fair, all sites claim that different varieties of bamboo differ greatly in tensile strength, but there are nevertheless a ton of sites making the claim.
I gladly stand corrected that bamboo can have better tensile strength than mild steel. Still that is only one measure so it might be that shear and compression loads may be the limiting factor not to mention joints. It would be really interesting to see the ulimate height that a bamboo scaffolding system could be buil to compared to steel. I’m not sure what the highest steel scaffold has been but the statue of liberty comes to mind.
That is one fascinating document!
For reference, a very basic carbon steel might have a tensile strength as low as 40,000 PSI. It’s yield strength would be somewhat lower than that.
The table 10.1 on page 28 gives a parallel-grain tensile strength of 6540 psi. Table 10.2 on the following page gives an ultimate tensile strength of 18000 psi, but it appears this was an assumption on the part of the source, who used an “allowable tensile stress” of 4000 psi. Only in table 10.3 is the claim of 52000 psi made, and I do wonder if someone accidentally slipped an extra zero in there. That said, bamboo is a fibrous material, and many natural fibres (e.g. spider silk) are impressively strong.
After hunting for a solid cite either way, I found this - a chapter from someone’s thesis researching this very subject.
Figures 4.1 and 4.2 show proper, reduced-section tensile test specimens not unlike the steel equivalents used for testing pipe. The results of their testing, given on page 91, state a range of 156-185 N/mm[sup]2[/sup], which converts to 22600-26800 psi. Which is impressive, certainly implies a greater strength weight for weight, and doesn’t rule out a stronger species of bamboo actually beating steel.
Thanks to everyone for their answers and cites. Large Marge, I must confess to my ignorance, which resulted in my attempted joke falling flat. Cloudy Bay is a New Zealand wine, and I thought Waikiki was in New Zealand. My ignorance has had its wings clipped, temporarily at least, by a visit to your blog!
A not totally on-point reply, but I’ve heard it said that they like to use wood supports in mine shafts rather than metal not because it’s stronger but because it has a better failure mode. Metal will just shear and fail, while wood will tend to groan and snap for a while, giving people time to clear the area. So if you’re talking “safer” rather than “stronger”, it’s possible that bamboo may also have a better failure mode, being fibrous. However, that sort of depends on the design of the scaffolding as well.
[related hijack] Wood beams are also said to have a better failure mode than steel in a fire. Steel loses its temper (strength) quickly in high temperatures, while a 12x12" wooden beam loses it more slowly as its substance is burned.
It doesn’t seem likely than bamboo would hold out very long, however, as it does not have the mass that a 12x12 beam does.
An additional factor though, is what happens after the scaffolding comes down. In Hong Kong (the OP’s location), the more or less annual typhoons can whip the bamboo poles at lethal speeds down the street like spears.
When people have brought actual numbers into this discussion they have used tesile strength, but that is inappropriate as the beams are not loaded in tension. What we are really concerned with is their ability to resist buckling. The dominant concern for that is cross section shape, which while fixed in bamboo is adaptable in steel.
Good point! Although to be fair, bamboo has evolved to support its own weight and to tolerate bending loads from wind. Plus in a scaffold, cross-bracing is the usual means used to prevent buckling. All the steel scaffolding I’ve ever seen has been made from tubes, same as bamboo. In fact, for omni-directional resistance to buckling I don’t think you can beat a tube.
The comparison of the tensile strength of bamboo and steel was introduced by DMC and led to a bit of a sidetrack. It’s also a nice example of my bullshit-meter being dead wrong…
Are you suggesting that this makes steel safer?
No worries. I thought Cloudy Bay was a whiskey of some kind, and I thought, “How’d he know I was drinking whiskey?”
Thanks for visiting the old blog; if only Waikiki were in New Zealand. I’ve always wanted to go there.
I don’t think you’re going to get a factual answer to this based on the properties of the materials. Both steel and bamboo are mechanically suitable for scaffolding.
Safety is more likely to relate to how well the scaffolding is assembled. Here steel has a definite advantage in that the the various tubes, collars, T-joints etc. can all be fabricated to fixed sizes. A steel tube can be clamped into the end of a collar by a bolt that bears directly onto it. Bamboo on the other hand has inherent size variability, and a bolt bearing onto it will cause it to splinter and split. It is presumably lashed together to form scaffolding. This requires more care and effort, and might be more prone to having corners cut.
A proper answer might be available in terms of accident statistics, but even then you might have difficulty ensuring that the only variable is bamboo vs. steel.