Can anyone offer any advice on the relative strength of steel and alloy scaffold poles? (same diameter and wall thickness)

More specifically we have been sinking a shaft with steel scaffold.  Now we have been given a pile of alloy scaffold.  Will it be just as strong to use?  Do I remember from CITB days that alloy scaffolding required more supports and shorter spans, or did I dream that?

Thanks in advance.

This doesn't answer your specific question but I think the other issue worth considering is longevity. We've just experienced problems with some steel scaffolding which members of your own club installed in a certain project 37 years ago - it's not done badly (!) but I wonder whether ally scaff would have survived as long. (I'll send you an direct email about this, just for information.)

This doesn't really answer your question either but my vague recollection is that alloy tube has a thicker wall to give it similar "strength". That said, in dig support the load is often far from uniformly distributed so factors other than the normal design rules for scaff probably come into play. Also how do you know what load will be on the scaff?

I seem to recall that the relative merits of alloy vs steel were discussed on here recently - in fact here
http://ukcaving.com/board/index.php?topic=14357.0

Good info there on the preferential corrosion of the two materials but I'm not too concerned about that aspect for now.

Having sunk a number of shafts over the years we feel we have the measure of steel pole.  Experience seems to tell us what spans we can get away with in the various situations we use them in before we need to fix or brace.  If using alloy pole I'm wondering if we can use the same measures or whether we should adjust our 'dead reckoning'.  Mostly the way we use pole is to support timber shoring and mostly the timber shoring distributes the load along the pole's span.

Pitlamp - 37 years sounds pretty good to me too.  If it was members of my club then you are lucky it wasn't supported by old kitchen cupboard doors, wriggly tin, etc. 

I suggest that you stick to your established system, but beware of steel couplings on ally poles, keep an eye on the joint for any corrosion, if your shaft is going anywhere, shutter it and concrete it, then remove the scaffold, talking from experience here!.

Badlad,

An employee of mine whom you know will be able to give you chapter and verse on the difference in strength between steel and ali scaff bars if you ask him.

For the same given diameter and wall thickness, I'd be surprised if the ali was even half as strong as the steel, but that's gut prejudice and is based on no meaningful data.

For scrap, the ali tube is worth about 10 x the value of the steel. Have you considered the possibility of weighing in the ali and using the cash to buy steel?

Nick.

Interesting idea from Nick.

You could check the wall thickness of the Ali against steel. Modulus of Steel is around 200GPa and for Ali it is around 70GPa depending on Alloy type which makes it around a third as strong. But - off the top of my head and not checked these thoughts - it is actually the ratio of second moments - something like pi (Do^4 - Di^4)/64 - so taking the 4th root of 3 (the ratio) would mean the ali should be about 1.3 times as thick as the steel.

I'm sure others will enjoy pointing out the faults in the above...

BS 12811 cites at 4.2.1.2 for steel tubes a minimum nominal yeild strength of 235N/mm^2 for a nominal OD of 48.3mm and minimum minimum wall thickness of 3.2mm and at 4.2.2.1 for ali tubes  a minimum nominal 0.2% proof stress of 195N/mm^2 for a nominal OD of 48.3mm and minimum minimum wall thickness of 4.0mm.  (The OD comes from fitting a standard coupler / scaff clip covered by prEN 74-1.)  So TheBitterEnd's advice fits. 

Appendices D & E of BS 5973 :1993 provide some of the detail you might (not) wish to get into.  PM me if you wish to come and drown in the detail.

And of course once you've met a few scaffolders you'll know they spend a great deal of time checking these calculations...

As Bob says 'drown in the detail'

Nearly all the info online is for 'lifts' or stages to take a compression load. In caves most scaffolding is used for stabilising a crush effect - digging down or along. Digging laterally can be better if the verticals can be settled on solid rock to put the scaff in compression - which is what it is designed for whereas vertical digs are holding back a crush, which is not going to be equal, and unless you brace at as short a vertical interval as possible then bending is possible.

Quite recently a 40' shaft around here collapsed - which had been installed be experienced diggers. 
If you look at trench supports then the bracing is much heavier duty.

The odd bar jammed across to hold back a rock or some shuttering is a different beast to a scaff shaft.
Concrete tubes are always going to be better if they can be fitted (and afforded) but of course are often impractical.

My advice - for what it is worth - is to keep a shaft as small as possible to keep the 'box' small, backfilling against shuttering if neccesary, If it needs to be big then use cross bracing at right angles to form a smaller center box - big enough for your bucket!

Never going to be cheap. If you use old rusty tubes that you got for scrap prices expect them to as rusty inside as outside.

Jopo

Bottlebank said:

And of course once you've met a few scaffolders you'll know they spend a great deal of time checking these calculations...

Click to expand...

In a way that is exactly the point - it is a standardized system so that scaffolders do not need to do the calcs. Hence why ali tube has a thicker wall, same strength so you can use the same rules of thumb. Someone did do the calcs at least once though and yes I have met a structural engineer who worked for big scaff company, he designed the scaffolding for tower block construction and the like...

Jopo - (getting hopelessly off topic but interesting ... at least to some of us ;-) I'm not sure what you mean by "crush effect" AFAIK a typical bay design is for a bricky plus bricks plus mortar, this load is assumed to be uniformly distributed along a bay and is a bending force on the horizontals. The uprights are slender columns that will fail by bucking rather than compression.

Simply that in some cave digs are scaffed to prevent the sides coming to meet in the middle whereas on exterior scaffolding the load is almost always in compression hence the data given may not be too relevant.

Not quite sure why my reply was off topic as the question was related to strength and several answers related to the data and standards online.

Jopo

Re "of topic" I meant my pursuing this further, not your post.

When sinking a shaft using Scaffolding and boards , if necessary back fill with rubble/rocks.
But do not backfill with silt or clay/sludge.The pressure this can produce is immense.
I once saw steel scaffolding bend and collapse just half an hour after backfilling with sludge!
Once seen never forgotten!

Thanks for all the replies.  I have also contacted the person suggested by Nick above and he was very helpful and knowledgeable.

For alloy and steel scaffolding used in line with the British Standard there appears to be no difference in strength.  This may, of course, vary with some of the ways cavers use scaffold, I'm not sure.  Alloy is 195 mega pascals and steel 235 (?), but this difference is balanced by having different wall thicknesses for the two types (4mm for alloy and 3.2mm for steel).  I was also concerned that alloy might 'snap' under load whilst steel would just keep bending.  I am told that with the type of alloy used this should not happen.

So to conclude, I'm reassured that the alloy scaffold I have can be used just as I would use the steel.  However, we also spoke with the boss of a large scaffolding firm in Sheffield.  He recommended only using steel in our digs as he thought it was stronger and less likely to snap!  Take your pick.

It's worth warning any casual readers that this is proper BS alloy scaffold poles and NOT bits of tower scaffold bracing or any other type of alloy tubing.

Cheers