Steel vs aluminium scaff tubes

Pitlamp

Pitlamp

Well-known member
We've discussed the pros & cons of steel and ally scaff tubes on here (with the emphasis on longevity) before but I have a question about their relative strength.

If you had two identical length scaffold tubes, one steel and one ally, supported horizontally an equal distance apart at each end - then you applied a downwards increasing force in the centre of each tube - which would start to bend first?

Can anyone advise please?
 
https://www.engineeringtoolbox.com/young-modulus-d_417.html

Tensile and yield strength of Steel (of all types) is substantially higher than that of aluminium.  Therefore, assuming the two scaff bars have the same wall thickness and diameter, the aluminium bar will deform first.

B
 
Ali scaffold tube has a thicker wall. (4.5 vs 4mm)

Yield strength of the actual alloy used and steel are very similar. The alloy tube will carry slightly more load before it permanently deforms.

The steel is a lot stiffer (Youngs modulus) and will deflect less under load, effectively feeling stronger.

Steel is obviously far heavier, even with the thinner wall.

https://www.yourspreadsheets.co.uk/properties-of-scaffold-members.html
 
(Largely superseded by Steve's post which was made while I was looking this up, but here goes anyway):

According to EN 39:2001, steel scaffold tubes have a nominal OD of 48.3 mm and a wall thickness of 3.2 or 4.0 mm. According to BS 1139-1.2:1990, aluminimum scaffold tubes have a nominal OD of 48.3 mm and a wall thickness of 4.47 mm
 
Benfool said:
Therefore, assuming the two scaff bars have the same wall thickness and diameter, the aluminium bar will deform first.
Click to expand...

Don't assume the bars will all have the same wall thickness. We were pleased recently to acquire some new, free, shiny steel bars. Only when stacked alongside our old secondhand bars was the much thinner walls noticed.

.
 
Aha - Steve's strut loads table is very useful! I'm assuming that "axial load" is what I was asking about?

So very short tubes are stronger if made of ali - but for over 0.8 m lengths, steel is more resistant to bending than ali. Have I understood that right?

I note that the first section on properties gives a higher "allowable bending moment" for aluminium than steel.
 
Ah - no, I guess what I'm on about is "radial" load?
I think the axial load probably means end to end compression, which isn't what I was asking about.
 
Pitlamp said:
Aha - Steve's strut loads table is very useful! I'm assuming that "axial load" is what I was asking about?

So very short tubes are stronger if made of ali - but for over 0.8 m lengths, steel is more resistant to bending than ali. Have I understood that right?

I note that the first section on properties gives a higher "allowable bending moment" for aluminium than steel.
Click to expand...

WHEN BEING USED AS A PROP :

For short lengths, any tube in compression will fail by crushing. It is termed a 'compact' section. Load capacity directly releated to the yeild stress of the material.

As the tube gets longer (termed more 'slender') there is a limit where the tube will start to fail by buckling sideways. The length where this starts to occur is determined by the size & shape of the section, but also the stiffness of the material (Young modulus). Steel is 3 times stiffer than aluminium so the critical length, where buckling occurs rather than crushing, is longer.

google : Euler's critical load
 
Your original example is BEING USED AS A BEAM :

In this case, the load capacity in the centre of the span is related to the allowable bending moment. For the simple example :

Moment = Load x Length / 4

So your allowable Load = 4 x Moment / Length

For a 1m ali bar, Load = 4 x 1.33 / 1 = 5.32kN =~500kgs.

 
Thanks everyone - already there's valuable information in this topic for all diggers to be aware of.
 
Does the age make a significant difference? I'd assume that second-hand scarf is like second-hand rope: good if you're lucky but quite possible dodgy...
 
Usually as solid as it looks - same principle as karabiners don't particularly have a shelf life whilst plastics and fabrics (harnesses and ropes) do. Ball park figure I heard is good for 30 years underground, whilst treated railway sleepers nearer 100
 
I've been reliably informed by John Barnatt that the scaffold rig at the top of the 125' pitch in Odin Mine was already in-situ on his first visit as a teenager in 1972. It's been used quite a bit recently as a top belay and proved to be solid, but there are plans for DCA (me) to resin-bolt this pitch and then remove the scaff, but it's proving difficult to get assistance, presumably as it will be quite a gnarly (and certainly dirty) job - though not all on the same trip I should add! Once all the bolts have been tested (and passed), it can then be cut up in small pieces and hauled out - conveniently there's now a large hole up to the entrance level nearby!

So if anyone would like to help later this summer, let us know. We even bought extra-long P-anchors from BP to ensure solidity, as they may be going into mineral rather than limestone - if we can see any wall detail at all! Actually I'm exaggerating - it's fine.
 

Attachments

  • IMG_20230103_114044_HDR.jpg
    IMG_20230103_114044_HDR.jpg
    176.1 KB · Views: 54
  • IMG_20230103_114222_HDR.jpg
    IMG_20230103_114222_HDR.jpg
    174 KB · Views: 52
  • IMG_20230103_114235_HDR.jpg
    IMG_20230103_114235_HDR.jpg
    194.4 KB · Views: 52
mikem said:
Usually as solid as it looks - same principle as karabiners don't particularly have a shelf life whilst plastics and fabrics (harnesses and ropes) do. Ball park figure I heard is good for 30 years underground, whilst treated railway sleepers nearer 100
Click to expand...
agreed, aluminium is more prone to fatigue cracking which could instantly fail, steel is normally fine if it dosent look as rotten as a pair
 
You must log in or register to reply here.
Back
Top