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Aluminium channel for deck support/drain - how much weight can it take


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I need to remove and refurbish the steel channel sections which support my deckboards and which currently provide a less-than-ideal drainage arrangement.

 

I have the option of some aluminium U-section channel which is the same dimension as the steel - measuring  3" (76mm) wide x 1.5" (38mm) deep 1/4" (6mm thick).   I don't have any further detail on what sort of grade it is so let's assume it is nothing special (not architectural grade or wotnot)

 

The deckboards sit on those four channels, each channel is 63" (1.6m) long, supported each end in steel sockets.  They are supposed to drain rainwater, so are orientated with the open side to the channel upwards. 

 

There are two load scenarios - the first is with the deckboards down, spreading weight across those four channels (plus some support at the ends)

 

The second is a simple standing weight on one of those channels when the deckboards are up, to access the engine. Potentially that could be my full weight on one foot in the centre (weakest unspported point) of the channel.  

 

I have found various deflection tables online but they are totally baffling so I was after any experience/common sense response around this as an idea....  too weak?  strong enough? 

 

 

 

If i had the channel in hand I would be able to do my own engineering test (i.e. stick it on a couple of blocks at the right measurements and stand on it a bit) - but i don't and i wanted a sense of whether it would be strong enough before going to get it....

 

any ideas?

 

 

 

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If it is the same size as the steel it will not be as strong as the steel, by quite a long way.  I suspect the Al channel   will be strong enough, but very "bouncy".  I would also be worried about the effects of  landing on the deck after a rapid descent from a dockside ( aka 'jump').

 

Aluminium is also more expensive than steel, but you may have a 'deal' lined up.  You may have a dissimilar metal corrosion problem where the Al meets the steel.

 

I could dig the formulas out and do the stress and deflection calcs, but really don't have the time for a couple of days at least, sorry.

 

N

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Thanks - this is what I was expecting...  it's a bit thicker gauge but not a lot.  It is free, but requires a fair drive to collect so probably not worth pursuing.

 

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25 minutes ago, TandC said:

Thanks - this is what I was expecting...  it's a bit thicker gauge but not a lot.  It is free, but requires a fair drive to collect so probably not worth pursuing.

 

 

Aluminium channel that size and 6mm thick should still be pretty strong and rigid, if not as much so as steel which is about 3x stiffer -- so it'll deflect 3x as far as the same thickness steel under the same load. Or 2x as far if it's 50% thicker, you can see where this is going... 😉

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4 hours ago, TandC said:

I need to remove and refurbish the steel channel sections which support my deckboards and which currently provide a less-than-ideal drainage arrangement.

 

I have the option of some aluminium U-section channel which is the same dimension as the steel - measuring  3" (76mm) wide x 1.5" (38mm) deep 1/4" (6mm thick).   I don't have any further detail on what sort of grade it is so let's assume it is nothing special (not architectural grade or wotnot)

 

The deckboards sit on those four channels, each channel is 63" (1.6m) long, supported each end in steel sockets.  They are supposed to drain rainwater, so are orientated with the open side to the channel upwards. 

 

There are two load scenarios - the first is with the deckboards down, spreading weight across those four channels (plus some support at the ends)

 

The second is a simple standing weight on one of those channels when the deckboards are up, to access the engine. Potentially that could be my full weight on one foot in the centre (weakest unspported point) of the channel.  

 

I have found various deflection tables online but they are totally baffling so I was after any experience/common sense response around this as an idea....  too weak?  strong enough? 

 

 

 

If i had the channel in hand I would be able to do my own engineering test (i.e. stick it on a couple of blocks at the right measurements and stand on it a bit) - but i don't and i wanted a sense of whether it would be strong enough before going to get it....

 

any ideas?

 

 

 

Your engin-ole sounds similar to mine (cruiser stern) although I have two hatches sitting on channels in good-ish condition. But I reckon I would never (or very, very rarely) actually stand / put body weight on the channel. So I would not see 'point deflection' as an issue. My routine for descending into the bowels is: foot on the deck, next on the weed hatch cover, then on the counter (timber step bridging fuel lines and some wires) then on a rear bearing cross-member (then baseplate if necessary). No weight on channel. As long as the weight of your hatches is evenly spread .....

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The silliest and I'd wager really common way of drainage via these supports that I saw the other day way angle iron set to make a V channel - all good - then removeable cross pieces above the engine. To fit the cross pieces the permanent sections were hacked out almost to the bottom (weakening them) and of course any water captured overflows at those points, never making it to the drain holes.

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As you don't provide your weight, taking a nominal 100kg (15+ Stone) in the middle of the beam, the maximum deflection of the aluminium channel would be almost 11mm. The beam wouldn't fail, but the deflection is more than I would be comfortable with, particularly as some of these drainage channels have very little overlap to the channels at their ends.

 

As the material is free, and if you have the clear space available below the deck, you could bond two aluminium channels back to back. This would reduce the maximum deflection to under 3mm (less than a single steel channel) and the beam would still be a third lighter than its steel cousin.

 

Yes, corrosion would occur where the aluminium contacts the steel, but 1) it will not happen overnight and 2) if the steel and aluminium are well painted then it would dramatically reduce the corrosion.

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19 minutes ago, MrBean said:

As you don't provide your weight, taking a nominal 100kg (15+ Stone) in the middle of the beam, the maximum deflection of the aluminium channel would be almost 11mm. The beam wouldn't fail, but the deflection is more than I would be comfortable with, particularly as some of these drainage channels have very little overlap to the channels at their ends.

 

As the material is free, and if you have the clear space available below the deck, you could bond two aluminium channels back to back. This would reduce the maximum deflection to under 3mm (less than a single steel channel) and the beam would still be a third lighter than its steel cousin.

 

Yes, corrosion would occur where the aluminium contacts the steel, but 1) it will not happen overnight and 2) if the steel and aluminium are well painted then it would dramatically reduce the corrosion.

You're quite correct, corrosion wouldn't occour overnight. I have aluminium deck plates resting on steel channel and I'm still waiting for the first sign of corrosion, mind you, they're only 26 years old. ( or will be in 2 days time)

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1 hour ago, Opener said:

Your engin-ole sounds similar to mine (cruiser stern) although I have two hatches sitting on channels in good-ish condition. But I reckon I would never (or very, very rarely) actually stand / put body weight on the channel. So I would not see 'point deflection' as an issue. My routine for descending into the bowels is: foot on the deck, next on the weed hatch cover, then on the counter (timber step bridging fuel lines and some wires) then on a rear bearing cross-member (then baseplate if necessary). No weight on channel. As long as the weight of your hatches is evenly spread .....

Thanks - sort of, but not really - I tend to remove all the deck boards (x3, which sit on top of 4 channels running lengthways: One under far left edge, middle left, middle right, far right edge).  With no deckboards, when doing maintenance etc - I do frequently step on the two in the middle sections and it isn't really doable to avoid that. 

 

27 minutes ago, MrBean said:

As you don't provide your weight, taking a nominal 100kg (15+ Stone) in the middle of the beam, the maximum deflection of the aluminium channel would be almost 11mm. The beam wouldn't fail, but the deflection is more than I would be comfortable with, particularly as some of these drainage channels have very little overlap to the channels at their ends.

 

As the material is free, and if you have the clear space available below the deck, you could bond two aluminium channels back to back. This would reduce the maximum deflection to under 3mm (less than a single steel channel) and the beam would still be a third lighter than its steel cousin.

 

Yes, corrosion would occur where the aluminium contacts the steel, but 1) it will not happen overnight and 2) if the steel and aluminium are well painted then it would dramatically reduce the corrosion.

 

I'm not 15 stone yet!   But, still feels a bit "bouncy"... could swap the two at the edges as they don't get walked on directly without deckboards and just clean up and retain the two central steel ones.  This saves a bit of work, saves a bit of unnecessary weight, and I can report back in 26 years on the corrosion!   

 

 

 

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2 hours ago, Opener said:

But I reckon I would never (or very, very rarely) actually stand / put body weight on the channel.

But if you stand on the deck boards directly above the middle of the channel it will experience virtually the same load anyway!

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34 minutes ago, David Mack said:

But if you stand on the deck boards directly above the middle of the channel it will experience virtually the same load anyway!

 But each deck board when down is supported on all four edges, so I'd expect it disperses even if standing with all weight in one foot directly above the centre of a channel beneath (as per if stepping onto that channel itself minus board) ... 

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Depends. If two separate deck boards meet over the channel in question then the weight will just go straight through. If the deck board is continuous across the channel to further supports either side, then a small amount will be carried by those supports, but most will still be taken by the centre channel. That's why I wrote virtually the same.

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There is also the potential for electrolytic corrosion; two dissimilar metals in a damp environment. However bedding the aluminium channel onto a layer of Sikaflex or similar adhesive where it joins the steel would prevent it.

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