Author Topic: Steel vs aluminium scaff tubes  (Read 678 times)

Offline Pitlamp

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Steel vs aluminium scaff tubes
« on: October 18, 2019, 04:42:13 pm »
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?

Offline Benfool

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Re: Steel vs aluminium scaff tubes
« Reply #1 on: October 18, 2019, 04:59:33 pm »
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

Offline mikem

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Re: Steel vs aluminium scaff tubes
« Reply #2 on: October 18, 2019, 07:04:03 pm »
& if the ali is dented or deformed it will give way even sooner...

Offline Steve Clark

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Re: Steel vs aluminium scaff tubes
« Reply #3 on: October 18, 2019, 07:22:08 pm »
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

Offline nickwilliams

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Re: Steel vs aluminium scaff tubes
« Reply #4 on: October 18, 2019, 07:30:58 pm »
(Largely superseded by Steve's post which was made while I was looking this up, but here goes anyway):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
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Offline Graigwen

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Re: Steel vs aluminium scaff tubes
« Reply #5 on: October 18, 2019, 07:32:51 pm »
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.

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Offline Pitlamp

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Re: Steel vs aluminium scaff tubes
« Reply #6 on: October 18, 2019, 07:39:00 pm »
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.

Offline Pitlamp

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Re: Steel vs aluminium scaff tubes
« Reply #7 on: October 18, 2019, 07:40:03 pm »
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.

Offline Steve Clark

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Re: Steel vs aluminium scaff tubes
« Reply #8 on: October 18, 2019, 07:57:20 pm »
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

Offline Steve Clark

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Re: Steel vs aluminium scaff tubes
« Reply #9 on: October 18, 2019, 08:05:23 pm »
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.


Offline mikem

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Re: Steel vs aluminium scaff tubes
« Reply #10 on: October 18, 2019, 08:46:37 pm »

Offline Pitlamp

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Re: Steel vs aluminium scaff tubes
« Reply #11 on: October 19, 2019, 06:09:11 pm »
Thanks everyone - already there's valuable information in this topic for all diggers to be aware of.