My new bow well steps/benches

[blackrose]

I've made the frames for my benches out of 50 x 50 x 5mm aluminium angle and was going to use some 2ft lengths of 18mm wooden decking I've got to go across the frames. That was until a neighbour mentioned that if I used a solid hexagrip phenolic ply it would spread the load across the frame whereas my decking won't.

 

The longest and therefore weakest length of aluminum frame is 1.4m (bottom picture). Does anyone have any idea of the point loading strength of the aluminium across that sort of span? I don't really want to have to buy more materials if I don't need to.

 

 

 

Edited March 6, 2021 by blackrose

[LadyG]

I [as in Thorne Boat Services) used angle iron. Solid as a rock, and the phenol board has epoxy to seal edges, takes a lot of stick, there are also brackets welded to hull, it will not shift. 

Edited March 6, 2021 by LadyG

[BEngo]

What sort of load will it get? Call that W

The bending moment M is W x length between supports and the stress is M divided by Z.  Called section modulus.

 

You will need to look up both Z for the angle and the safe working stress for aluminium somewhere like engineers toolbox.  Be careful about the units.  W in Kg will need to be multiplied by 9.81 to turn it into Newtons. Lengths in metres.

 

Alternatively for Z ignore the top flange and look only at the vertical.  Then Z is B  x Dsquared divided by 6.   B is 0.005 mm D is 0.05 m.

 

The actual stress will then be lower than calculated.

We ought to calculate shear stress but that will be tiny compared to the bending. It is load divided by the cross sectio   area of the angle.  It  adds to the bending stress

 

If you are going to jump onto it all bets are off.

 

From gut feel I would expect 18mm decking strips  to work fine, provided you keep the fastening  holes small and away from the vertical web, which is doing most of the work . Glue would be good.  There will only be any real benefit in hexaply if it is bonded to the angle all along, front and rear so it works as part of the angle.

 

N

 

PS.  To pick a safe working stress it would help to know what sort of Al alloy the angle is.  It is not likely to be pure Al, where a working load 0f 60 MN per sq m would be OK, but more fancy stuff could take up to double that, and more for exotica ( which it won't be unless you acquired it from an aircraft factory scrap bin.)

N

Edited March 6, 2021 by BEngo

[blackrose]

42 minutes ago, BEngo said:

What sort of load will it get? Call that W

The bending moment M is W x length between supports and the stress is M divided by Z.  Called section modulus.

 

You will need to look up both Z for the angle and the safe working stress for aluminium somewhere like engineers toolbox.  Be careful about the units.  W in Kg will need to be multiplied by 9.81 to turn it into Newtons. Lengths in metres.

 

Alternatively for Z ignore the top flange and look only at the vertical.  Then Z is B  x Dsquared divided by 6.   B is 0.005 mm D is 0.05 m.

 

The actual stress will then be lower than calculated.

We ought to calculate shear stress but that will be tiny compared to the bending. It is load divided by the cross sectio   area of the angle.  It  adds to the bending stress

 

If you are going to jump onto it all bets are off.

 

From gut feel I would expect 18mm decking strips  to work fine, provided you keep the fastening  holes small and away from the vertical web, which is doing most of the work . Glue would be good.  There will only be any real benefit in hexaply if it is bonded to the angle all along, front and rear so it works as part of the angle.

 

N

 

PS.  To pick a safe working stress it would help to know what sort of Al alloy the angle is.  It is not likely to be pure Al, where a working load 0f 60 MN per sq m would be OK, but more fancy stuff could take up to double that, and more for exotica ( which it won't be unless you acquired it from an aircraft factory scrap bin.)

N

 

Thanks. It's just standard aluminum angle. I think they call it architectural grade rather than the fancier construction grade. I'm not sure if construction grade is even available in such narrow cross-sections.

 

Anyway, rather than the calculation perhaps I'll just put something underneath it to protect the deck and jump up and down on the longest beam to see if it goes!

 

I think I disagree that hexagrip would need to be bonded to the aluminum. If it's laying flat on the beam the load will be spread. I don't think bonding it would make much difference. Perhaps that's actually the best option rather than use my short lengths of decking.

 

1 hour ago, LadyG said:

I [as in Thorne Boat Services) used angle iron. Solid as a rock, and the phenol board has epoxy to seal edges, takes a lot of stick, there are also brackets welded to hull, it will not shift. 

 

Thanks, but I think you're answering a different question than the one I asked.

Edited March 6, 2021 by blackrose

[Sea Dog]

I don't remember how long the benches in my well deck are, possibly a little shorter than yours, but they are 18mm phenolic ply and they sit on a transverse 25 mm angle aluminium bracket at either end. They're not bonded - they sit loose so I can lift them out easily. They seem perfectly happy to me without the fore and aft stretchers you have. Certainly I don't really notice any flex whether using them as steps or seats. Of course, using several 2' lengths robs you of the strength a single piece would provide, but then your angle aluminium is twice mine. Have you not just popped a bit of wood on and tried it?

[blackrose]

13 minutes ago, Sea Dog said:

I don't remember how long the benches in my well deck are, possibly a little shorter than yours, but they are 18mm phenolic ply and they sit on a transverse 25 mm angle aluminium bracket at either end. They're not bonded - they sit loose so I can lift them out easily. They seem perfectly happy to me without the fore and aft stretchers you have. Certainly I don't really notice any flex whether using them as steps or seats. Of course, using several 2' lengths robs you of the strength a single piece would provide, but then your angle aluminium is twice mine. Have you not just popped a bit of wood on and tried it?

 

No I haven't tried it yet. Those pictures are just the frames assembled without doing the nuts & bolts up. I need to put some undercoat and topcoats around those gas locker brackets tomorrow & Monday and then I'll bolt it all together. 

 

Your post is reassuring and reminds me that my larger phenolic ply cruiser stern deckboard is 1.5m x just over 1m and only supported around the edges, so it's pretty strong stuff in its own right. I hadn't included the intrinsic strength of the phenolic ply which on top on the aluminium frames should be fine. I think the decking might be a mistake anyway as the grooves would be running in the transverse direction across the width of the boat and that stuff can get pretty slippery when wet. It's not easy to paint tanalised wood with a non-slip paint either.

[Sea Dog]

11 minutes ago, blackrose said:

 

 I think the decking might be a mistake anyway as the grooves would be running in the transverse direction across the width of the boat and that stuff can get pretty slippery when wet. 

Not to mention the "stripey bum" effect...

 

[ditchcrawler]

1 hour ago, BEngo said:

What sort of load will it get? Call that W

The bending moment M is W x length between supports and the stress is M divided by Z.  Called section modulus.

 

You will need to look up both Z for the angle and the safe working stress for aluminium somewhere like engineers toolbox.  Be careful about the units.  W in Kg will need to be multiplied by 9.81 to turn it into Newtons. Lengths in metres.

 

Alternatively for Z ignore the top flange and look only at the vertical.  Then Z is B  x Dsquared divided by 6.   B is 0.005 mm D is 0.05 m.

 

The actual stress will then be lower than calculated.

We ought to calculate shear stress but that will be tiny compared to the bending. It is load divided by the cross sectio   area of the angle.  It  adds to the bending stress

 

If you are going to jump onto it all bets are off.

 

From gut feel I would expect 18mm decking strips  to work fine, provided you keep the fastening  holes small and away from the vertical web, which is doing most of the work . Glue would be good.  There will only be any real benefit in hexaply if it is bonded to the angle all along, front and rear so it works as part of the angle.

 

N

 

PS.  To pick a safe working stress it would help to know what sort of Al alloy the angle is.  It is not likely to be pure Al, where a working load 0f 60 MN per sq m would be OK, but more fancy stuff could take up to double that, and more for exotica ( which it won't be unless you acquired it from an aircraft factory scrap bin.)

N

Or just get a couple of big blokes to stand on it and see if it holds OK

[blackrose]

8 minutes ago, Sea Dog said:

Not to mention the "stripey bum" effect...

 

 

Another thing I hadn't considered! ?

[David Mack]

51 minutes ago, blackrose said:

 

I think I disagree that hexagrip would need to be bonded to the aluminum. If it's laying flat on the beam the load will be spread. I don't think bonding it would make much difference. Perhaps that's actually the best option rather than use my short lengths of decking.

If you bond the hexagrip to the aluminium angle you form a composite section - the hexagrip becomes a much larger top flange to the angle. The strength and stiffness of that composite section is much greater than the combined  strength and stiffness of the hexagrip and angle separately. But in order to mobilise that greater strength and stiffness you impose a significant shear stress in the glue joint, so the joint needs to be well made using a suitable adhesive.

[blackrose]

14 minutes ago, ditchcrawler said:

Or just get a couple of big blokes to stand on it and see if it holds OK

 

As BEngo said, that won't really let you know what will happen if it gets jumped on, which is bound to happen in use. I'm sure with the ply it will be fine.

[blackrose]

11 minutes ago, David Mack said:

If you bond the hexagrip to the aluminium angle you form a composite section - the hexagrip becomes a much larger top flange to the angle. The strength and stiffness of that composite section is much greater than the combined  strength and stiffness of the hexagrip and angle separately. But in order to mobilise that greater strength and stiffness you impose a significant shear stress in the glue joint, so the joint needs to be well made using a suitable adhesive.

 

As I suggested earlier, a section of the two materials will be formed just be laying the hexagrip on top of the angle without any adhesive. If a load is applied to 18mm hexagrip laying on the angle the forces are all in compression. An adhesive between the two isn't going to add anything. 

[David Mack]

4 minutes ago, blackrose said:

 

As I suggested earlier, a section of the two materials will be formed just be laying the hexagrip on top of the angle without any adhesive. If a load is applied to 18mm hexagrip laying on the angle the forces are all in compression. An adhesive between the two isn't going to add anything. 

Oh yes it is. If you simply place one over the other, when they bend under load the bottom face of the hexagrip will be in tension and so stretched, the top face of the aluminium will be compressed, and will therefore be slightly shorter and so the two surfaces slide over one another. Bond the two together to prevent the sliding and the combined section is much stiffer.

Take a look at any riveted steel bridge girder. The top and bottom flanges are held to the vertical web plate by closely spaced rivets in each face of an angle iron. Those rivets do the same job carrying the shear force between the bottom of the top flange / top of the bottom flange and the top and bottom of the web plates. Do you think the bridge would carry the same load without them?

[blackrose]

3 hours ago, David Mack said:

Oh yes it is. If you simply place one over the other, when they bend under load the bottom face of the hexagrip will be in tension and so stretched, the top face of the aluminium will be compressed, and will therefore be slightly shorter and so the two surfaces slide over one another. 

 

Assuming they will bend under load. I don't think that's a given considering the rigidity of the L section aluminum angle and the ply. Even if they do bend I'm sure the bolts I will use to put the two components together will do a better job than your adhesive. Any decent structural adhesive will have to bed flexible to accommodate the differential thermal expansion and contraction of metal and wood, so if it does bend you're back to square one with as you say, the two surfaces sliding over each other. Much better to screw or bolt them together I'd have thought.

 

3 hours ago, David Mack said:

 

Take a look at any riveted steel bridge girder. The top and bottom flanges are held to the vertical web plate by closely spaced rivets in each face of an angle iron. Those rivets do the same job carrying the shear force between the bottom of the top flange / top of the bottom flange and the top and bottom of the web plates. Do you think the bridge would carry the same load without them?

 

Rivets yes, screws or bolts yes, but not glue.

Edited March 6, 2021 by blackrose

[Old Son]

Surely it depends what glue you use? I suggest you do it how you intended to do it before you asked your original question.

[blackrose]

2 hours ago, Old Son said:

Surely it depends what glue you use? I suggest you do it how you intended to do it before you asked your original question.

 

Thanks I will. I suggest you read my original question. It was about whether to use decking slats or one piece plywood and that question was resolved. It wasn't about whether to glue the wood on. I never asked that question because I know I'm not using glue. 

Edited March 6, 2021 by blackrose

[Murflynn]

18mm hexagrip placed loose on the framework will be just fine.

no need for structural calculations, glue or fairy dust.

if the ply has to be retained one bolt at each end (into the supporting angle iron) will be fine.

 

............................  of course some of the respondents would like to turn this into a PhD thesis. 

 

[Sea Dog]

I think we're in danger of over-thinking this: its a shelf that needs to be robust enough to sit on. There are no realistic weight limitations in a big fat steel boat - if you have to go into stress calculations and the efficacy of bonding, you're into airframe territory and common sense has departed. Just sayin'... 

 

(Pretty much like @Murflynn said whilst I was typing!)

Edited March 7, 2021 by Sea Dog
In brackets

[Murflynn]

trouble is there are a bunch of folk out there who are thoroughly bored with life and have nowt better to do than demonstrate that they went to skule.   

[LadyG]

I think it is too flimsy, just sayin'

It's a steel boat, why use aluminum?

[Murflynn]

2 hours ago, LadyG said:

I think it is too flimsy, just sayin'

It's a steel boat, why use aluminum?

 

Mike - if you need a second opinion, the good ladyship has provided it.  

[Alan de Enfield]

8 minutes ago, Murflynn said:

 

Mike - if you need a second opinion, the good ladyship has provided it.  

 

And it is worth exactly what you paid for it.

[blackrose]

On 07/03/2021 at 08:23, Sea Dog said:

I think we're in danger of over-thinking this: its a shelf that needs to be robust enough to sit on.

 

 

I think it will have to deal with a bit more than just sitting on it. When you step down quickly you're effectively jumping on it. That's a much more challenging dynamic load as opposed to sitting on it which is a static load.

On 07/03/2021 at 08:15, Murflynn said:

18mm hexagrip placed loose on the framework will be just fine.

no need for structural calculations, glue or fairy dust.

if the ply has to be retained one bolt at each end (into the supporting angle iron) will be fine.

 

 

Thanks that's all I need to know.

22 hours ago, LadyG said:

I think it is too flimsy, just sayin'

It's a steel boat, why use aluminum?

 

Too flimsy? Based on what exactly?

 

Aluminium doesn't rust, it doesn't need constant painting, it's easier to work with, it's lighter and easier to take apart when the deck needs painting. 

 

You don't have other materials on your steel boat? I assume your deckboards are all made of steel?

 

 

Edited March 8, 2021 by blackrose

[blackrose]

20 hours ago, Murflynn said:

 

Mike - if you need a second opinion, the good ladyship has provided it.  

 

I'm fine with people giving their opinions, but I'll only take seriously the opinions which have some understanding and knowledge behind them. I think the fact that some people have benches/steps made of unsupported 18mm phenonic ply (albeit not quite as wide as mine), reassures me that my fully supported constructions will be strong enough. 

Edited March 8, 2021 by blackrose

[blackrose]

Nearly finished the steps/benches. In the end I used the Iroko decking as I already had it and the buffalo board was going to cost more than £80. The width of the starboard bench meant I needed a full 8' x 4' sheet of buffalo board but I'd only have used half of it. Anyway, I think the decking looks better. I cut all the angles using an electric mitre saw. Just waiting to borrow a circular saw so I can cut narrower planks at the front.

 

At the moment the decking is just laying on the frame but I'm going to lay each plank on 4 washers so that water isn't trapped between the wood and the frame. I'm using stainless screws coming up from underneath the frame to hold the planks in place so you won't see any fixings from the top.

 

I jumped up and down on the starboard bench with a neighbour and although it flexed it seemed strong enough.

 

 

Edited March 21, 2021 by blackrose