Scantlings for aluminum floating docks and gangway build

Hi Folks - My current floating docks, built mainly from rough sawn 2"x8" hemlock boards for framing, with large welded metals corners, axles and wheels (for hauling in and out each year) need replacing. There were several sections which were so rotten that I decided to completely tear then down and build aluminum framed docks for next Spring.

They will be supported with the current 45 gallon barrels which were under the old dock, as I don't have the budget to buy the fancy floating systems, and the barrels are paid for..
Current floating sections are 6 ' wide and have lengths of 8 ' for the two end sections and 12' for the middle section (so total 28' length) , which attaches to the gangway to form tee docks.

Planning on building 3 new floating sections 12' x 6' wide for a total of 36' length, as I will be relaunching our old 30' sailboat (sits on the outside of the docks) and also need room on one side of the inside edge to tie up the 25' aluminum power boat I built a few years ago (end will stick out a bit but will tie up fine along the side, and the tee attachment of the gangway will likely be off center about 4 - 6 feet to allow one end to be longer and give more room to the power boat).

Current gangway was very heavily built - it was 12' long x 6' wide in the middle but had wings extended on the top and bottom so that the attachment points to the Crib and the floating docs were 10 ' wide. It rises and falls with the tide, about 6' max tide. The ends were 4" x8" hemlock x 10' and had welded braces which attached to the main section and had numerous long through bolts and bracing attaching it all together, along with 1" x 5" decking screwed down to keep it rigid.

My main question or concern is what material / dimensions to use (for the floating sections) for the external rectangle framing and 2 longitudinals which run about 1/3 of the way inboard and keep the barrels in place, sandwiching them between these and the outside framing.
As mentioned, this framing was 2" x 8" boards on the old docks. When I go with aluminum for framing - I got some prices on rectangle box tubing and channel and was considering something like 5" x 2" channel x 1/4" thick or 5" x 2" rectangular tube by 3/16" or 1/4" thick.
Prices really climb of course, as dimensions increase.. for example the 5" x 2" channel x 1/4" thick is about $260 per 20 ft length.. so if I needed about 4 lengths per section (running 2 12' outside long edges, 2 12' longitudinals inside and the the 6 ' ends and some other cross bracing) that would run about $1040 per section - so $3120 for the 3 floating sections and another $1200 maybe for a beefed up gangway - total $4300 is much more than I had planned..

I would like some input please if i can get away with smaller dimensions of framing and/or thickness or any other input would be appreciated!
Corners will be gusseted of course and will have about 2' pieces welded down from corners to hold the aluminum tube axels, and will have bracing at 45 degrees to each side.
shapes come here in 6061 or 6063.
pics attached of old docks a few years ago.

The reason for the wide gangway (6' in the middle and 10' wide at the ends) is that unlike many I see around, mine does not roll out on the dock or connect loosely to a floating dock which has it's own support system to keep it roughly in place (such as mooring connections, pilings to ride up and down on, etc). I wanted the extra width so that it had more strength to hold the docks in place while the wind is blowing on the boat, and have less stress on the connection points. Perhaps a 4 wide gangway would have enough strength to hold everything in place fine? I just know the difference that I would feel in trying to hold something from twisting with my hands say 4ft apart on it versus 1 or 2' apart on it and I felt it couldn't hurt to go as wide as my crib which it attached to, which is 10'.

tidal swing is 4-6 ft - my pics are at high tide. I am about 1/4 mile up a tidal river and it has several bends in it - We do get some wind, but its a bit of a hurricane hole - no big waves.
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This is brackish water then? I'm asking because it sounds like you are planning on connecting aluminum dock material to steel drum material which will also have an aluminum boat and a shore connection. Galvanic corrosion could be an issue.

As for answering your specific question, I have little experience with this, but I like the wider gangway you described.

 

Have you seen the Rolling Barge website? They also use 45 gallon barrels for buoyancy.
https://rollingbarge.com/

 

Thanks for the quick replies! It is brackish water, but the barrels are plastic. Most of the aluminum will be above water, but the corners will have aluminum which goes into the water where the axles attach. I do have shore power for the sailboat, but normally don't have it connected all the time.
I have seen the rolling barge website but prefer to use wheels and axles to roll on rather than rolling on the barrels (barrels placed the opposite direction) .
I would like a closer view of their dimensions of their frames and the thickness.. as I am not sure how big and thick I should go..

 

If you built a dock similar to the ones shown on the Rolling Barge website, it should still be possible to put wheels and axles on the underside, so that you can wheel it out of the water for the winter?

The 'I' shaped dock is a good idea re spreading the loads at the ends. But it sounds like the wind loads on the end dock and the boat(s) moored there are all transferred to the base of the 'I' at the shore connection - which does put a lot of load on it.

Would it be feasible to drive a couple of piles for securing the dock at the end?
Or failing this, have a couple of anchors with chains to secure it, to reduce the loading on the shore connection?

 

I could drive pilings but it would be expensive. Or as I did for the first few years I did also run galvanized steel cables from each side of the crib out near the end points of the docks, but I haven't done that in years and it's been fine. I aslo fabbed up a large steel screw / helical mooring and attached a loose chain from that to one end of the docks, and had a cable and chain from a large rock to the other end of the docks, but haven't done that in years either, as I felt the gangway was strong enough to hold the docks on it's own. These cables and chains had to be kept loose enough that they only became close to tight at high tide, and were really just an insurance in case we got a huge wind and everything else became too stressed.

I guess my main observation is that the wooden and metal reinforced gangway worked fine holding the the wind stresses on the boat and docks for 10 years, so I would like to now select materials and have it designed/ cross braced / bridged enough that will give similar strength using aluminum framing.

I just have no idea what the appropriate scantling are for the framing of the docks and then gangway. I know the gangway needs to be pretty strong... and perhaps the middle dock (which attached to the bottom of the gangway) should likely have more reinforcing than the two end docs especially near the attachment points. I doubt if the end docs see too much strain as the boat will be tied to several bollards along the dock corners)

 

Decided to check a scrap metal dealer about an hour away today, as they said they had lots of aluminum on hand, at 1/3 of the price of new.. unfortunately, they only had one 20' length of 2.5" x 1/4" square tube, and nothing else like angle or channel.. so as I paid for that piece and was leaving, I decided to go back and look through the round tube/pipe leftovers.. I hadn't wanted to use pipe/tube, as it makes some aspects of fabrication more time consuming...or doesnt allow deck boards to site flat on it's edge..but after considering that they were 10' lengths and they had about 16 of them - it's approx 3 or 3.5" in diameter with 3/16" wall... I walked away with 400lbs of aluminum for a buck a lb. it is normally $1.5 / lb used, but it was a bit dirty so they only charge a buck a lb.
I plan on using all of this pipe for my inner framing / longitudinals and cross bracing and now I plan on buying 3" x 2" x 3/16" rect tube for the outer edge rails of the floating docks and 4" x 2" x 1/4" rect tubing for the outer edge rails of the gangway, I will add 1" x 1" x 1/8" angle on top of all outside rails so that the deck boards will slide under this edge, and not require screwing down. This will stiffen the frame vertically as I add another inch to it's height. I will add have cross braces every 3' on the docks and every 2' on the gangway.
I would appreciate input but I feel that this combination of buying new, adequately sized rectangular tubing for all outside frame rails while saving a lot of money using used round tubing for the inner frames and cross bracing is a good compromise. If I went lighter on the outside materials I could save more money, but I think I 'll sleep better at night with these choices.