aluminum framing choices for floating docs and gangway

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North
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Location: Nova Scotia

aluminum framing choices for floating docs and gangway

Post by North »

Hi Folks - my plans to build a boat cabin will have to wait another year.. My floating docks, built mainly from rough sawn 2"x8" 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' power boat (end will stick out a bit but tied 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 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 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 so. when i go with aluminum for framing i got some prices on rectangle box tubing and channel and am considering something like 5" x 2" channel x 1/4" thick or 5" x 2" rect 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 $200 per 20 ft length.. so if I needed about 4 lengths per section (running 2 12' outside long edges, 2 12' longitudinals and the the 6 ' ends and some other cross bracing) that would run about $800 per section - so $2400 for the 3 floating sections and another $1000 maybe for a beefed up gangway - total $3400 is 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 docs a few years ago.
thanks in advance for any advice and help!
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IMG_20160803_095959.jpg
IMG_20160803_095950.jpg

TAB
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Re: aluminum framing choices for floating docs and gangway

Post by TAB »

Do you need square tubing or will angle work ? It has to be cheaper than tubing

North
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Re: aluminum framing choices for floating docs and gangway

Post by North »

Good question. I Imagine a fairly heavy, and large dimension angle may work, but I think it would need a fair amount of more cross members, etc to make it rigid or strong enough to hold the pull of the boats in the wind versus the pulling on the gangway and crib.

I would love the input of the guys who have experience with these types of builds in aluminum. The floating docks will have 4 barrels under each, near each corner, so there will only be about 5 feet of span in the middle where weight will need to be supported without a barrel underneath, but there will be a lot of pull in all directions on the docks and gangway, so they need to be strong.
If the old docks had only been 2x 4 framing I might be more comfortable going lighter on the aluminum... but as it was 2 x 8 rough lumber framing it was pretty strong and i hate to go much weaker now.
should mention - 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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Bill Edmundson
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Re: aluminum framing choices for floating docs and gangway

Post by Bill Edmundson »

Use C-Sections "Channels". Point the legs out. Then all the intermediate stiffeners can be cut square.

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

North
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Re: aluminum framing choices for floating docs and gangway

Post by North »

Hi Bill - thanks for the input! That does seem like a good idea if I go with Channel. I could then weld pieces of plate on the edges of the legs, to form a complete box shape, where I need more strength like where I may bolt on galvanized hardware to join the docs together, or thicker pieces of aluminum. When I built the 32' steel trailer for my sailboat, I used 6" channel on the trails, turned in, and then used 5" rail for cross members, which sat nicely inside the larger channel and worked out well. For the docks, though, I think your idea has merit..

My main concern is how big of channel, or rectangular box tube to use.. and, I think it would be reasonable to use thicker material for the outside rails and then save some weight and cash and use thinner material for the inside longs, as they will also have cross members tying them to the sides as well.
For example 5" x 2" x 1/4" (or even 9/32" which is common here) for the outside rails and maybe 5" x 2" x 1/8" (or 3/16") for the inside longs. The main purpose of the inside longs (besides adding strength and stiffness to the outside rails and supporting deck boards every 2 feet) is to give the barrels somewhere to sit in and up against.. they sandwich in between the outside rails and the inside long members. I may even get away with something like 6" x 2" x 3/16" angle as it could be placed with the long side vertical and this would still hold the barrels in place.. They are lighter and cheaper than using channel or Box and if they cross braces every 3 feet, I wonder if they would provide nearly as much strength and rigidity as channel or box tubing..

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Bill Edmundson
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Re: aluminum framing choices for floating docs and gangway

Post by Bill Edmundson »

I haven't done a lot of design in aluminum. Generally, divide the length (in feet) by 2 and that is the depth in inches for the channels. That will keep it from being too bouncy.

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

Kevin Morin
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Re: aluminum framing choices for floating docs and gangway

Post by Kevin Morin »

North,
one direction of the loads (up and down) are being sustained by the individual section modulus of the individual pieces of extrusion. The other direction, horizontal or wind and wave load is being supported by a truss- that is the matrix of welded elements the full width of the dock, walk ways or ramp elements.

So that means the 5' to 6' wide 'truss' of the decks and ramps are wildly stiffer than the vertical loading without vertical trussing members?

This implies a 2x2" x 1/4" deck matrix would be fully strong enough to resist the horizontal loads of wind and waves of a dock..... but might not be sufficient to carry an individual's wt if they walked on that framing surface!

2" x 6" x 1/4", 6061 T6 angles welded into a rectangular frame will be adequate to carry good loads on top. And, if a set of tension rods, or inverted truss rods were put below the upper surface, welded to the vertical 6" legs- say a 12" or 16" truss for most spans; then the vertical load would be very adequate to carry the dock's surface loads even with long spans.

Last; by diagonal bracing each 'cell' or rectangular frame (do this with small section flat bar at the surface of the deck frame) ; then the horizontal loading due to wind, weather and boat surge in those conditions is 'triangulated' and absorbed with very narrow horizontal scantlings.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

Kevin Morin
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Re: aluminum framing choices for floating docs and gangway

Post by Kevin Morin »

North, in most of my dock walk ways, I made the tide adjusting ramps a truss of the handrail and deck. That is I designed and fabbed a trussed 'U' where the walking deck was the bottom of the U and the two handrails the actual trussed frames of the two vertical sides.

Here, in SC AK our tides are average 20-40' vertical depending on location, so these ramps move up and down every 6 hours quite a bit. Further, they are the main horizontal resistance to tide movement side to side and to any weather that comes into the locations where they're installed.

So we hinge the top and bottom and then add 'guy wires' from the top to the bottom - spread at the upper level and narrow on the dock or floating section as resistance to horizontal movement of the "float".

If you have a dock with a ramp? I'd suggest considering handrails as part of the overall truss design, as mentioned above, I've put the trusses below the top deck of a ramp as well- when added to 36" 40" high hand rails, these 30- and 40' long spans of walking ramps have been very reliably stiff and lasted a long time.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

North
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Re: aluminum framing choices for floating docs and gangway

Post by North »

Hi Kevin - thank you very much for the detailed response. The only part I don't understand, or have a hard time picturing is this part:

"And, if a set of tension rods, or inverted truss rods were put below the upper surface, welded to the vertical 6" legs- say a 12" or 16" truss for most spans; then the vertical load would be very adequate to carry the dock's surface loads even with long spans."
Could you explain that a bit more please?

Re: the ramp - it will be similar size to what is in the picture- 12' long by 6 ' wide in the middle, but changing to 10' on the ends so that they two sets of "hinges" to the crib and the doc are almost 10' apart rather than only 6' apart.

Yes, I planned on having a railing on at least one side, and have a general understanding that this will form a bridge to the deck and give it strength - if i needed to do railings on both side I can, but having one side open is nice sometimes in case i am bring something wide down to the dock.

If I understand you correctly, i can use something perhaps a bit higher sides than 2" x 2" x 1/4" like say 2" x 3" (or 4") would certainly hold the side to side loads and should be fine for walking as well, as its only a 6 feet span between where your weight would be over the barrels at either end.

Or, I can go with something like 6" x 2" x 1/4" angle for a frame and then have a lot of supporting members underneath (and diagonally above) to stiffen it up?

If you had time I would be willing to pay you to do a rough sketch or two of what you would design for this project!
say one using something like box tubing or channel or perhaps one using angle.... one thing - I would like to lean towards a design that does use a fairly high material, like the 6" angle, or perhaps even some 2" x 2" box tubing for the frame with maybe 4" X 2" angle welded below it so the total depth of the frame is 6" or more- reason being that the barrels need some side wall to wedge in between and stay in place.. this was 2" x 8" boards on the old docks.. This depth (ideally 8")could also be achieved I guess with some fairly lightweight bridging or rod on 30 or 45 degrees down to a lower "rail" of tubing or angle?

I don't want to cheap out on this project.. the wood docks I built last 10 years, but I want these ones to last 20 - 30+ years..
I learned a lesson when i built my 32' sailboat trailer - it worked out very well.. but i likely have 3000lbs of steel in it... overbuilt due to my lack of engineering knowledge.. this time I would prefer to pay someone like you to design it well from the start and use the correct material and design.. rather than me just building it very heavy materials. I do still want to err a bit on the side of overbuilt however...

Kevin Morin
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Re: aluminum framing choices for floating docs and gangway

Post by Kevin Morin »

North,
hopefully this image will post and do the 1000 words thing about explanation?

Image

I looked for elements of docks in my photos but didn't find to many. This one does however have one that explains the inverted truss remarks. This gangway/is about 40' long and has a truss below it, the truss is made up of angles and they support the truss- combined with handrails.

We always use the handrail as the 'rail way' to move items up and down from the dock/float. A small cart with hollow wheels will roll on the top of the railings' top pipe like a track and then be pulled or lowered when something is moved too big to fit in the walkway.

This gangway is solid decked with welded aluminum on 3" (I think) channels - but it was a series of four or so pieces the owner salvaged and we put together into one piece- scabbed together might be a better description?

hope this helps.

I don't very often do designs online, reason being I have not view, control or influence over the resulting builds - but I'd be first inline to catch it if for reasons way beyond my control- something went amiss?

Sorry my remarks weren't any more clear. If I remember out barrels under this float, we have to strap them in using 1x1/8" bolted to the frame (wood) in order to keep them implace. We also filled them about 1/4" full to make the top of the float lower to the water if I recall? While some framing can keep the barrels in place, we always strapped them in before the deck went on.

I designed and built the two boats on the left and a student/trainee/past employee did the skiff on the right of the float.

Not sure of why a 6' wide gangway is needed? At the lower center is one of the cables fixed to a steel stake on the beach and attached to the float so it can't drift side to side.


Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

North
Posts: 313
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Location: Nova Scotia

Re: aluminum framing choices for floating docs and gangway

Post by North »

Hi Kevin - the pics definitely explain it. Thanks.

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'. For the first few years I did also run 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 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 dine that in years either, as I felt the gangway was strong enough to hold the docks on it's own. These 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 like the cabin on the middle boat, but that is on hold for now..

I understand why you wouldn't want to do designs where you can't have control of the build factors.

I am not looking at having you or any engineer sign off on anything... but I do value your experience and opinion, and I if you can help me in deciding what material choices make reasonable sense, I would appreciate it.

I will try and draw up a sketch or two with material choices I am thinking of using, and hopefully you won't mind having look and perhaps telling me where I might want to make changes, etc. I understand that you undertake no liability for any suggestions!
Thanks again for your time!

North
Posts: 313
Joined: Mon Jul 15, 2013 6:29 pm
Location: Nova Scotia

Re: aluminum framing choices for floating docs and gangway

Post by North »

Hi Kevin - please see my crude drawings (done in paint program) .

Black lines would be my heavier material for the main frame and other colors represent lighter material. That could be rectangular tube, channel or angle...


Blue rectangles are the barrels on the floating dock sections.

Yellow circles are pipe sections welded in vertically (to be used for inserting removeable pipe bollards).

For the gangway, I did not add the railings or any spans/ tension bars underneath as it is only a 1 dimensional drawing.

I am hoping you will give input into what shapes and thicknesses would provide necessary strength, for the floating docks and gangway.
I understand you are not liable for any suggestions and cant control the quality of my welding or building, but I would appreciate if you would tell me what materials and thicknesses you would use for such a project please!
Attachments
Gangway design filled in.png
Gangway design filled in.png (7.96 KiB) Viewed 3388 times
Floating dock design filled in.png
Floating dock design filled in.png (11.54 KiB) Viewed 3388 times

North
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Re: aluminum framing choices for floating docs and gangway

Post by North »

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 crossbraces 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 alot of money using used round tubing for the inner frames and crossbracing 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.

Kevin Morin
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Re: aluminum framing choices for floating docs and gangway

Post by Kevin Morin »

North,
glad that you found some lower cost extrusions that may serve in your projects. It is kind of important for you to note that if someone were to have spent time replying to your request for 'help' in defining scantlings? Their entire contribution would evaporate as you've moved to a 150% change in direction (almost 180deg!) .... due to a fortunate find in materials available in your area?

This kind of circumstance makes any suggestions, contributions, or voices of experience into 'faint echo's of futility' for anyone who may feel like contributing to your threads?

As a result, while I do wish you "best of fortune", like my build sequence withdrawal some years past- I don't feel its productive to contribute to those who "don't have ears to hear".

Hope your dock re-build works well and holds your boat in the tidal flow?

Merry Christmas and my best wishes for a prosperous and safe boating New Year.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

North
Posts: 313
Joined: Mon Jul 15, 2013 6:29 pm
Location: Nova Scotia

Re: aluminum framing choices for floating docs and gangway

Post by North »

Hi Kevin- well, you are certainly entitled to your opinion of my strategies on asking fr and accepting or not accepting all help, and of course, are under no obligation to post or help.

That said, although I have not always taken all advice given in building my boat, I certainly have taken alot of it, for example:
- built a box keel based on your input and concern around the issues with a single plate keel
- rearranged stringers after members on another forum point point out errors/ concerns to me.
- used etching/paint for the bottom based on input from forums

For these docks, I really didn't get a firm recommendation from anyone on what dimensions would be best or strong enough.. but I did look at the examples you gave, where or how they added strength, and things to consider and have let that influence my choices going forward... the used material find was not ideal, but it will only be used for the internal framing and cross bracing. I am still buying new material or all outside framing, and will go with what I mentioned in my last post, unless someone with experience or knowledge would like to show me why I should go bigger, smaller, etc and I really would appreciate that help, from whomever will give it... If no one offers more advice, I have no choice but to go on alone, with my best guesses of what will do and perhaps end up over building / spending more than I would like...but I do feel I will end up with good, strong docks regardless... its just a matter of wishing I had validation or advice from those with more experience.
The old strong wooded docks were fine in the 5-6 ft tidal swing, and I am sure the new ones will handle it as well.... hopefully someone will look at my proposed choices and offer advice, before I need to buy materials in the next couple of weeks.

I wish you a Merry Christmas and Happy New Year as well! you may not realize it.. bit I have listened to a fair bit of your advice thus far (but not all.. as I don't quite need the quality and strength which you build for commercial users) and I have certainly learned alot in reading your many post on building and welding of aluminum boats.

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