Cheap way to build a floating dock

[dr1step]

what size aluminum channel would I need to be as strong as a wood 2x8

 

[berkeman]

Welcome to PF.

This thread may get closed at some point as being too dangerous, but let's get a little farther first. Can you upload a sketch of this dock design? Use the "Attach files" link below the Edit window to upload a PDF or JPEG sketch. It would help if it were a dimensioned sketch.

Is this dock on a lake, or in a sheltered cove/marina on the ocean?

 

[Baluncore]

Welcome to PF.

That will depend on what you mean by a "floating dock".
Is the 2x8 measurement in inches? 50 mm x 200 mm?
Also, the position of the member in the structure is important.

 

[dr1step]

Welcome to PF.

This thread may get closed at some point as being too dangerous, but let's get a little farther first. Can you upload a sketch of this dock design? Use the "Attach files" link below the Edit window to upload a PDF or JPEG sketch. It would help if it were a dimensioned sketch.

Is this dock on a lake, or in a sheltered cove/marina on the ocean?

Ok this dock is going to be on an inlet off the gulf of mexico. No real current light tide movement. Let me tell you about my dock building backround. I've been building floating docks on the upper hudson river for over 30 yrs. Ive seen a lot of tide and current stresses let alone wind damage. This is my personal winter cabin in florida. I've alway build docks like a big wood deck with floatation. I'm trying to get away from all that wasted weight and materials. I'm building a 16 ft. X 8 ft dock simple with billet floatation. I'm thinking as simple alluminum X frame and aluminum angle around the perimeter to make the box. Then 1 in box aluminum welded across the frame for purlings to mount the decking to. Misc hangers will need to be created to mount the billets. I just don't know the size of the X framing i need. 3 x 2 x .25 ? Less? More?
Looking for an educated guess

 

[berkeman]

I've been building floating docks on the upper hudson river for over 30 yrs. Ive seen a lot of tide and current stresses let alone wind damage.

It sounds like you have a good background for this.

this dock is going to be on an inlet off the gulf of mexico

How did the local docks fare during the recent 2 hurricanes in the Gulf? Did most of them ride out the hurricanes and their storm surges okay?

 

[Baluncore]

I have worked out that "billets" refers to the foam blocks used for buoyancy. What are the dimensions of those blocks, how thick are they vertically?

The buoyancy, below, must couple up through the superstructure to the deck that carries the live load. I have not understood your meaning of "x-frame", nor the way it works with, and connects to the rest of the structure. To be worth the investment, materials must be employed to spread the loads efficiently between the various layers of the structure.

You will need to produce a sketch of the structure that includes the distribution of the buoyancy.

 

[Tom.G]

In general, Salt air and Aluminium are not very compatible. Not recommended!

Although I have heard that there is one specific grade that works outdoors about four miles from the ocean. This was from a welding instructor whose day job was maintenance at an oil refinery.

Also, when wet, any other conductor in contact with the Aluminium will create a battery. You then have electrolysis to worry about.

 

[dr1step]

here's a simple question. what size aluminum square or rectangle stock would equal a 2x8 ? There must be a formula or table somewhere. Some engineering book

 

[berkeman]

here's a simple question. what size aluminum square or rectangle stock would equal a 2x8 ?

Before or after it corrodes in the salt air?

In general, Salt air and Aluminium are not very compatible. Not recommended!

 

[Baluncore]

here's a simple question. what size aluminum square or rectangle stock would equal a 2x8 ? There must be a formula or table somewhere. Some engineering book

It is not that easy. What is the tree species and Flex grade of the 2x8 ?

Timber is specified by flexibility, in a dry construction, where there is a structural framework. The regional code will specify different span or performance, for different parts of the structure, where load is taken in different ways. Timber will tend to move out of the way, to share the load.

Aluminium alloy must be specified for marine use, and then selected based on weldability, fatigue, and fracture propagation. Aluminium will bend or break, depending on its heat grade, when stress exceeds a limit.

Where different construction technologies are mixed, they do not meet well. How you join aluminium to the rest of the foam or timber structure will determine the aluminium wall thickness. If bolts pass through different parts of the structure, you must reinforce those parts of the section, where point loads are applied.

Your first structure will need to be heavier in aluminium than you may discover is optimum later on. That is because failure is expensive when you must rebuild rather than simply replace weak parts in the middle of a structure.

If you want lower cost, use timber construction above the waterline, with plastic polymer below the waterline. Select your materials wisely.

 

[Rive]

Aluminium alloy must be specified for marine use, and then selected based on weldability

Keep in mind that the welding material/technology also must be appropriate for a marine grade result

Where different construction technologies are mixed, they do not meet well. ... If bolts...

Using the right type of fasteners (as material and technology) also an art in marine (any corrosive) environment. Even if both a steel bolt and an aluminium sheet are marine grade, once used together they may be a disaster.

Wood and plastic are soo much simpler.