Framing a small 3x5' platform

curls00 · 04-07-2009, 02:25 PM

It isn't "home" related, but this project is for my friends' fishing boat. We need to re-frame a casting platform and re-use the existing, carpeted top piece (very expensive marine-grade carpet, glued on).

The casting platform is 3'x5' in size, and the existing platform top is marine-carpeted 1/2" pressure-treated plywood. It's not quite stiff enough, so our plan is to supplement it with another 1/4" piece of plywood underneath to give rigidity ("subfloor"). The 1/4" piece will also allow us to "hide" all of the fasteners that will join the floor to the frame below.

============= <-- 1/2" carpeted pressure treated plywood
============= <-- 1/4" pressure treated plywood "subfloor"
************* <-- aluminum frame (approx 2"x3", probably 1/8" wall thickness. Used to be the posts and rails for a privacy fence in my backyard. The cross-section looks like a "U" but with the tops turned inwards at 90*, almost making a complete rectangle).

What I think might work and am looking for opinions on, is to join the "subfloor" (1/4" plywood) to the aluminum frame.

Then once that is done, join the floor to the subfloor with as few fasteners as possible, all countersunk bolts (less slip/trip hazard, and looks better) with wide washers underneath, and nylock nuts.

What type of fasteners should we look for? I'm thinking that because we're dealing with PT plywood (ACQ treatment, contains copper), and aluminum (although it is painted), we need some kind of coated or stainless fasteners? What thickness of bolts should we look for? This platform is only 18" from the floor of the boat (16' freshwater boat, not some large fishing vessel). I don't think the strength of the bolts should matter too much since the aluminum frame will be taking most, if not all, of the weight on the platform. The MAX weight that will ever be on it is 350 lbs, but normally there will only be one person, max 210 lbs, on this platform at one time.

47_47 · 04-08-2009, 07:57 AM

I wouldn't mix the aluminum with the pressure treated (even though it is painted). Can you get and use marine plywood instead of the PT?

I don't think that adding only 1/4" plywood will strengthen your platform. 1/4" + 1/2" does not equal 3/4" strength. Even adding a layer of 3/4" ply would bow under this span. Is there a way to add more bracing (joists) to decrease the span secured with galvanized joist hangers or 'L' brackets?

As for fasteners, use stainless steel machine screws with a fender washer and a nylon lock nut.

curls00 · 04-08-2009, 05:21 PM

After thinking about it some more, we decided we might look at not using the 1/4" and just re-using the 1/2" PT plywood. It wasn't too bouncy at all and that was with simple 2"x2" (1.5x1.5 finished) cedar. With the aluminum, I expect the frame itself won't bow/flex at all (not detectable, anyhow).

I'd have the long rear edge sitting on the bench (100% supported along that rear section), and then 3 aluminum frame rails about 2.5' in length running forward/back, and meeting at a 90* angle, the front 4.5' frame rail, which would have two legs on it. That means the platform is roughly divided into two sections, each with a frame around it. Each section would be about 2.5' x 2' in size between its surrounding frame. I can't see a 1/2" plywood section of that size flexing much according "The Sagulator" here http://www.woodbin.com/calcs/sagulator.htm#edge_strip

I know that's for shelving, but using a 30" square "shelf" to represent one half of the platform, and assuming the frame we build properly supports it, using pine as the shelf material, and a 2x2 "oak" front strip (to represent the front portion of the frame), the deflection with 400 lbs of load, centre-weighted, is only 0.01".

curls00 · 04-08-2009, 05:21 PM

It was also suggested to use 6 mil poly sheeting between the aluminum and the PT to get quite a bit of extra life out of the aluminum. Would that work to prevent the reaction for the mostpart?