Box section or angle?

[blackrose]

Which would be stronger if bolted to a steel bulkhead to be used as supports (in the orientation shown below) for steps?

 

For the box section support, the bolts would only go through one vertical wall up against the bulkhead, not through the whole thing.

 

I'm thinking the angle is stronger simply because of the wall thickness. Or does the box section shape give it a lot of extra strength?

 

Edited January 29, 2021 by blackrose

[IanM]

If I'd read your post whilst I was still at work, I would have looked it up for you but I've just got home and haven't got my all knowing book here ?

 

I wouldn't have thought there was much in it to be honest.  Also remember with the box section you actually have to get the bolts inside the thing!

[blackrose]

Just now, IanM said:

If I'd read your post whilst I was still at work, I would have looked it up for you but I've just got home and haven't got my all knowing book here ?

 

I wouldn't have thought there was much in it to be honest.  Also remember with the box section you actually have to get the bolts inside the thing!

 

Yes, the box section would be a bit more fiddly to get the bolts inside, but not a major drama.

[GUMPY]

My laymans view is that there wont be much in it but the angle will be easier to fit.

 

[BEngo]

How long is a piece of string?

 

What size bolts are you going to use?

What failure mode are you expecting?

You could shear the bolts, cause collapse of the box section, bend the  angle or have failure in shear of the box or angle at the bolts.

 

What sizes  are the two items?  The box and angle dimensions including thickness and exact material would be needed to calculate accurately. 

 

From a practical perspective I would use the angle.  Easier to fit, no box to trap dirt and then corrosion.  Even 25 x 25 x 3  mm would be strong enough, with 2 or more 6 mm bolts, for a step support. Even one 6mm bolt will support a big bloke but it does not prevent the angle rotating.  I would prefer 38 for x 38 ease of attaching stuff to it though, but you snap has nothing to scale the items by.

 If attaching to steel, to avoid encouraging dissimilar metal corrosion if is best to seal the face betwixt angle or box and the steel.  Butyl sealant is good, silicone is unsuitable because the gap is too small, polyurethane may be OK.

 

N

 

[GRLMK38]

Are they both steel?

[blackrose]

31 minutes ago, BEngo said:

How long is a piece of string?

 

What size bolts are you going to use?

What failure mode are you expecting?

You could shear the bolts, cause collapse of the box section, bend the  angle or have failure in shear of the box or angle at the bolts.

 

What sizes  are the two items?  The box and angle dimensions including thickness and exact material would be needed to calculate accurately. 

 

From a practical perspective I would use the angle.  Easier to fit, no box to trap dirt and then corrosion.  Even 25 x 25 x 3  mm would be strong enough, with 2 or more 6 mm bolts, for a step support. Even one 6mm bolt will support a big bloke but it does not prevent the angle rotating.  I would prefer 38 for x 38 ease of attaching stuff to it though, but you snap has nothing to scale the items by.

 If attaching to steel, to avoid encouraging dissimilar metal corrosion if is best to seal the face betwixt angle or box and the steel.  Butyl sealant is good, silicone is unsuitable because the gap is too small, polyurethane may be OK.

 

N

 

 

In this case the piece of string is as follows:

 

Angle: 50mm x 50mm x 5mm.

Box section: 75mm x 50mm x 3mm

Both aluminum so corrosion is not a factor.

Bolts will be 8mm A2 stainless but I'm not sure if that's relevant as I was asking about the ally rather than the bolts.

Yes I'll be sealing between the two metals with PU sealant and the steel is painted, but dissimilar metal corrosion is only generally an issue when the dissimilar metals are immersed in an electrolyte. Most of us have aluminum windows, and brass mushroom vents, stainless or brass skin fittings. I've had an aluminum generator locker bolted to my stern deck for the last 8 years without issue.

 

In terms of a failure mode I guess it would be tension on the outside of the bolted face (the face against the bulkhead) and compression on the inside.

 

I think both supports would be strong enough, but since I have the choice I'd like to use the stronger option.

43 minutes ago, GRLMK38 said:

Are they both steel?

 

No aluminum.

[system 4-50]

I'd go for the box, with no technical knowlege to back it up.  I think loads on the very edge of the angle will cause it to bend slightly with thyme.  Bolting a box is a doddle after drilling a couple of holes to push the bolts right thru.  I might push a bit of wood into the box for extra rigidity.

[Kieron G]

Without sleeving the bolts going through the aluminium box I think it will tend to crush when you tighten them. I would use angle myself

[John Lewis]

Angle would be stronger. the stress would be up in the top left corner.

[ditchcrawler]

Why not fix it end on if you are worried about it

[Troyboy]

1 hour ago, Loddon said:

My laymans view is that there wont be much in it but the angle will be easier to fit.

 

And easier to paint in the future.

[GUMPY]

10 minutes ago, Troyboy said:

And easier to paint in the future.

Only if you have the right primer 

[manxmike]

1 hour ago, system 4-50 said:

 I think loads on the very edge of the angle will cause it to bend slightly with thyme.  

Purely as an interested spectator, do you think using herbs will serve any useful purpose? To be honest thyme isn't that heavy unless you use an enormous plant ?

[Captain Pegg]

The technically correct answer to the question is the box section. It has a moment of inertia over 4 times higher than the angle and hence has over four times the bending strength.

 

Realistically though this isn’t an engineering problem - unless the whole thing is subject to a structural design - and the fixings will probably be the point of weakness in any case.

 

JP

Edited January 29, 2021 by Captain Pegg

[system 4-50]

15 minutes ago, manxmike said:

Purely as an interested spectator, do you think using herbs will serve any useful purpose? To be honest thyme isn't that heavy unless you use an enormous plant ?

Thyme is admittedly not as heavy as Rosemary - but don't tell her I said that.

[Tacet]

It depends, as has been said, the manner in which it might fail.

 

The box section will be stronger when acting as a beam (more meat in the lowest part).  The angle will  better resist the the bolts being torn through or the holes elongated (more meat where the screws passes through).  There will not be much in it (aka I don't know) which better resist being pushed out of square by the overhanging load = 2 @ 3mm v 1 @ 5mm.  D squared seems to come into calculations fairly frequently and this might favour the angle?

Edited January 29, 2021 by Tacet

[blackrose]

1 hour ago, Kieron G said:

Without sleeving the bolts going through the aluminium box I think it will tend to crush when you tighten them. I would use angle myself

 

As I've already said, the bolts aren't going right through the box. The length of these supports (either angle or box) is only 10cm so I can easily get bolts into the box section that sit on the inside face against the bulkhead

1 hour ago, ditchcrawler said:

Why not fix it end on if you are worried about it

 

I don't know what you mean by end on? And I'm not worried about it. As I said I think either option will be strong enough.

1 hour ago, Troyboy said:

And easier to paint in the future.

 

I wasn't intending to paint either the angle or the box section

[blackrose]

34 minutes ago, Captain Pegg said:

The technically correct answer to the question is the box section. It has a moment of inertia over 4 times higher than the angle and hence has over four times the bending strength.

 

 

Isn't that 4x higher moment of inertia along it's length rather than with the force pushing down across it's cross section?

18 minutes ago, Tacet said:

 

The box section will be stronger when acting as a beam (more meat in the lowest part).  

 

Yes that's what I meant.

19 minutes ago, Tacet said:

which better resist being pushed out of square by the overhanging load = 2 @ 3mm v 1 @ 5mm.  D squared seems to come into calculations fairly frequently and this might favour the angle?

 

The angle it is then.

 

Thanks All.

[ditchcrawler]

10 minutes ago, blackrose said:

 

 

I don't know what you mean by end on? And I'm not worried about it. As I said I think either option will be strong enough.

 

Screw the box section vertically so the weight is supported on the end of the box and not the side. But then I don't know what you are trying to do 

[Captain Pegg]

29 minutes ago, blackrose said:

 

Isn't that 4x higher moment of inertia along it's length rather than with the force pushing down across it's cross section?

 

Yes that's what I meant.

 

The angle it is then.

 

Thanks All.

Moment of inertia (I) is a physical property of the section, a measure of the dispersal of material around its centre of mass. You’re thinking of stiffness (EI/L) but in this case E and L are the same. So the answer to your question is the one with the highest I value and that’s the box.

 

I see you’ve plumped for the angle, a completely illogical choice based on your original question and intention, however it’s not going to fail.

 

JP

 

[Momac]

If I get my calculator or section tables  I could tell you exactly the structural difference between the two sections . Too much wine in me at the moment. Sounds like others beat me too it.

I agree the box is liable to win on the basis of the section properties . A box sections is also many times stronger in torsion compared to an open section. 

 

Bolts / connections are another matter . Greater vertical distance between bolts increases leverage and reduces bolt load . But the bolt capacity will depend on all connected parts .

 

A picture (sketch reasonably to scale with dimensions) of the proposals would be worth a thousand words .

 

 

[Drayke]

4 hours ago, blackrose said:

 

In this case the piece of string is as follows:

 

Angle: 50mm x 50mm x 5mm.

Box section: 75mm x 50mm x 3m

Both aluminum so corrosion is not a factor.

Bolts will be 8mm A2 stainless but I'm not sure if that's relevant as I was asking about the ally rather than the bolts. Don't forget to use some large penny washer as well.

Yes I'll be sealing between the two metals with PU sealant and the steel is painted, but dissimilar metal corrosion is only generally an issue when the dissimilar metals are immersed in an electrolyte. Most of us have aluminum windows, and brass mushroom vents, stainless or brass skin fittings. I've had an aluminum generator locker bolted to my stern deck for the last 8 years without issue.

 

In terms of a failure mode I guess it would be tension on the outside of the bolted face (the face against the bulkhead) and compression on the inside.

 

I think both supports would be strong enough, but since I have the choice I'd like to use the stronger option.

 

No aluminum.

 

[blackrose]

2 hours ago, ditchcrawler said:

Screw the box section vertically so the weight is supported on the end of the box and not the side. But then I don't know what you are trying to do 

 

Ah yes I see what you mean. That would be stronger. That would work

2 hours ago, Captain Pegg said:

Moment of inertia (I) is a physical property of the section, a measure of the dispersal of material around its centre of mass. You’re thinking of stiffness (EI/L) but in this case E and L are the same. So the answer to your question is the one with the highest I value and that’s the box.

 

I see you’ve plumped for the angle, a completely illogical choice based on your original question and intention, however it’s not going to fail.

 

JP

 

 

Well if you look back over the thread you'll see it wasn't actually my logic that said the angle was stronger.

[blackrose]

5 minutes ago, F DRAYKE said:

 

 

Penny washers? You think 20mm washers will pull through 5mm steel plate?