Steel vs aluminium scaff tubes

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?
 

Benfool

Member
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
 

Steve Clark

Well-known member
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
 

nickwilliams

Well-known member
(Largely superseded by Steve's post which was made while I was looking this up, but here goes anyway):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
 

Graigwen

Active member
Benfool said:
Therefore, assuming the two scaff bars have the same wall thickness and diameter, the aluminium bar will deform first.

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.

.
 

Pitlamp

Well-known member
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.
 

Pitlamp

Well-known member
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.
 

Steve Clark

Well-known member
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.

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
 

Steve Clark

Well-known member
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.

 

Pitlamp

Well-known member
Thanks everyone - already there's valuable information in this topic for all diggers to be aware of.
 
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