What is stronger?

[BrokenBiker]

Some 50x50x2.5 box section, or 2 bits of 50x25x2 rectangular section, back to back, stitch welded together every 150mm with a 150mm weld to basically make a bit of 50x50?

Will be approximately 3 meters long.

I assume the 2 bits of 50x25 will have less flex due to the middle being 2 bits of steel together to form a wall, but then what orientation would be best? Wall upright or horizontal? Resistance to bending is the goal here

 

[tigdlo]

How will it be loaded? Either option is either strong enough or not, deflection could be an issue as 3m is a long beam for both options. Search for moment load instead of bending, it will bring up more relavant results. These calculators appeared to work for me, https://www.mig-welding.co.uk/forum...section-bending-strength.126231/#post-2179933

Wall upright or horizontal? Resistance to bending is the goal here

Upright assuming the load is vertical, if the double wall were horizantal they would be on the neutral axis so won't be taking any load.

 

[BrokenBiker]

It will be bolted directly to a garage door, and then anchored at either end to brick work via slotting into some steel receivers and pins down through the top to prevent the door opening should the lock be bust off. It would be loaded either by pulling against the door, or pushing into it, both from the outside.

So the load would be pushing/pulling parallel to the floor, or perpendicular to the door, whichever what you wish to look at it

 

[wyn]

The 50x25.

 

[DaveB]

How will it be loaded?

If your bending it vertically then place the 'wall' between the boxes vertically.
If it's taking any other loads the 50x50 is likely stronger.

 

[Maker]

You need to calculate the "second moment of intertia" for each cross section to compare the rigidity.

Given the application, I dont think it will really matter. If you're buying new material, go with the square and if you're using up old stock, use whichever you feel would be less useful in a future project.

 

[BrokenBiker]

You need to calculate the "second moment of intertia" for each cross section to compare the rigidity.

Given the application, I dont think it will really matter. If you're buying new material, go with the square and if you're using up old stock, use whichever you feel would be less useful in a future project.

Using current stock, I have full 7.5 meter lengths of both, was only ever destined for making little tables and stands for the shed, the 50x25 would match other tables, but the 50x50 would do just fine. Might just use the 50x50 as less to do, just cut to length, weld 2 end caps on, drill some holes

 

[wyn]

If you have the two 50x25 with the join horizontal it will give you a wall thickness in the horizontal direction of 8mm. The 50x50 will only be 5mm.

Sort of like using a bit of flat bar of 8mm thick or 5mm thick.

 

[BrokenBiker]

If you have the two 50x25 with the join horizontal it will give you a wall thickness in the horizontal direction of 8mm. The 50x50 will only be 5mm.

Sort of like using a bit of flat bar of 8mm thick or 5mm thick.

So the 50x25, welded back to back and the centre walk horizontal?

 

[wyn]

So the 50x25, welded back to back and the centre walk horizontal?

Yeah, that's how i see it in my head without using calculations which i have long forgotten.

 

[BrokenBiker]

Yeah, that's how i see it in my head without using calculations which i have long forgotten.

That was kind of the way I thought would be best as well, plus the fact then the holes at the end are going through 4 layers of steel rather than 2

 

[Agroshield]

I think the 50 x 50 x 2.5mm wall wins.

With the joint between the double 50 x 25 in one direction, the difference is insignificant.

With the joint between the double 50 x 25 in the other direction, the 50 x 50 is 19% stronger.

Please let someone who knows what they are doing verify attached.

 

[Kram]

For your application the brick work or surrounding features will be the weak point.
If its relevant, 50x50 will let you use a larger pin into the ground tho well over the top...

 

[BrokenBiker]

Cheers all. Think the 50x50 is the way forward, I only bought that to have it if needed anyway so no loss using some of it.

He is paying the going rate for materials plus labour anyway but means I don't have to go and buy more steel to do stuff for me

 

[DaveB]

So the 50x25, welded back to back and the centre walk horizontal?

No.

If you try to bend a 50x2 flat bar by hand can you do it on both axis ?

 

[BrokenBiker]

No.

If you try to bend a 50x2 flat bar by hand can you do it on both axis ?

It's not flat bar, it's box...and you would have a harder time bending it along the 50mm axis than the 2mm

 

[dan treg]

if you were to treat as a joist (albeit) metal the 50mm needs to be vertical the 25mm horizontal ie like having 2 joists side by side . but from my calcs the 50 x50 wins on the bending moment calcs

 

[DaveB]

It's not flat bar, it's box...and you would have a harder time bending it along the 50mm axis than the 2mm

I've just realised that I've misread your planned usage of the box or boxes.
It is being bent horizontally so if you go with the double 50x25 put the inner walls horizontally.

Dan Treg has done the maths and has the 50x50x2.5 winning on bending performace but I suspect local buckling at the loaded locations is the limiting factor and with holes drilled in it the pair of 50x25 probably does better.