Need help with aluminum flatbed (pics/sketchup inside)

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Hi everyone, I am building an aluminum flatbed for my 2006 F250. It will be holding about 3500# on a regular basis. I am designing the flatbed myself, but I will be hiring someone else to do the welding. The reason I need help designing it is because I want to make the flatbed as low as possible, with wheel-well cutouts, instead of the normal flatbed build. The "normal" flatbed build usually consists of about 4" channel running along the frame vertically. Then on top of this usually 3" square tube spaced a 16" OD or something along those lines. The end result is 7" of height for the flatbed, where as I need a flatbed which only 3-3.5" high total. Here is an example of the type of flatbed I am talking about which I do not want In this version, you can see the channel runs and slats placed on top. I want these to be basically merged together. I want them to run on the same level as each other, to decrease height. This is where I need structural help. I am wondering what the best way structurally is to build something like this? I am going to be 3"x2 square tubing, 1/4" wall, for the "beams" running down the length of the frame. But since I can't lay anything on top of them, and need to weld to the sides, how can I make it strong enough? Are welds going to be just as strong in this case? Can I weld some 3"x2" to each of the sides? For a visual, here is a google sketchup I did. It shows 3"x2" frame, with 1/4" wall used. The holes down the middle of the frame are for a sliding ladder I am going to put in (2" high ladder that folds). Any other ideas would be appreciated. I am open to using steel if I need to, or using more solid aluminum. I just need it to be 3" tall maximum. Like I said, about 3500# on it daily, so I need to make sure it will hold up. Thanks very much.
Reply:3500lbs plus the weight of the deck on an f250?
Reply:The F-250 is rated at 3/4 ton or 1750 pounds. Ive overloaded my 3/4 Ton ever since Ive had it. In my State Nevada, they are more concerned about your tires being able to handle the weight. Ajd777, your gonna neeed ten plys.Your plan looks good from here but I want a bed to be flat so I can load from the side. I would also want a built in ramp that could be used as an xtension for loading xtra hay and stuf. Also, a hidden compartment to store keys, wallet, gun or whatever would be nice.
Reply:lhdmech - Yes 3500# plus the weight of the deck. My truck is actually an F350 for all intent and purposes. It had the F350 suspension and 19.5" wheels/tires (capable of handling tons of weight).Insaneride - I would love a flatbed too, but I am going to be using this for a camper (custom made) and so I need it to be as low to the ground as possible. To have a flat deck would require 4" more height and that increases the COG a lot.If anyone has any suggestions let me know. Will 3"x2" with 1/4" wall be enough ????? And how should I attack the side beams to the center ones. Just weld directly to, or should I notch them somehow?
Reply:Usually when I've seen decks like this done, they use channel and pierce  the main rails so the side pieces are continuous. One issue is that the side rails end up lower than the mains. Usually they deck the side rails so they are even with the top of the main rails in that case. Our Eager Beaver trailer is that way.Your "hollow" section for the ladder will be an issue with that design. I have a feeling it will even be an issue with your current design as you won't have any resistance to twisting on the back 1/2. If you load those rear areas, the deck will bow and twist. I wanted to do a similar thing with my custom steel utility body. The body company didn't recommend it for the reason given above. To do a design like you wanted, I'd have had to go up to 6" main rails with the 3" cross members over the mains to get enough room for the ramp, and still maintain the strength in the back of the body..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Originally Posted by DSWUsually when I've seen decks like this done, they use channel and pierce  the main rails so the side pieces are continuous. One issue is that the side rails end up lower than the mains. Usually they deck the side rails so they are even with the top of the main rails in that case. Our Eager Beaver trailer is that way.Your "hollow" section for the ladder will be an issue with that design. I have a feeling it will even be an issue with your current design as you won't have any resistance to twisting on the back 1/2. If you load those rear areas, the deck will bow and twist. I wanted to do a similar thing with my custom steel utility body. The body company didn't recommend it for the reason given above. To do a design like you wanted, I'd have had to go up to 6" main rails with the 3" cross members over the mains to get enough room for the ramp, and still maintain the strength in the back of the body.
Reply:Picts of my truck. Whole body is semi custom from Wilcox, based on their heavy crane body. Had them narrow the boxes and widen the bed to 60". Install the operable roof with raised shelves and doors over tailgate. Reinforce the side walls for tie downs. Originally I wanted to go with the door for a ramp under the floor behind the tailgate until they suggested otherwise. I wanted to keep the overall bed height down since climbing in and out of the back is hard on my back if it's acting up. The only thing I regret is that I didn't go a bit higher with the bed. The side exterior "shelves" are 1" below the tops of the doors. To put a long ladder on that "shelf" I have to pack it up with a 2x4 to gain enough room to open the door on the passenger side. They wouldn't raise the side boxes any taller than I already had them raised with out a large increase in price.Originally wanted to put the body on a F350 DRW chassis, but couldn't get any of the dealers to work with me. No one wanted to listen to me about what I'd use the truck for and the fact I knew exactly what it would end up weighing when everything was done. Body company would do it, but it would have cost $4K extra since they don't stock 350 class trucks. Total weight 4wd, diesel, F550 ( stocked chassis), 11' body, empty comes in at just under 11K. Loaded with tools I'm still under 12.5K.I'd have to agree with the others I think an F250 would be seriously over loaded. I run my F250 4x4 diesel heavy, ( Truck is 8.5K empty, 10K on average) but even loaded with extra ballast for plowing, I doubt I have that much weight in the bed even with the 100 gal fuel tank full. I've had that much weight in there on occasion, but even with the extra heavy springs I added when I had to replace the stock springs, I wouldn't want that abuse on the frame rails like that regularly. Extra heavy duty 350 dump truck springs ( 6 leaf rears, 4 leaf fronts IIRC) did level out the headlights with all the weight in the back compared to the stock springs.As far as flex. Without any center support, all you have is two pieces of flat stock. Try and bend a piece of flat and you'll see it bends very easily. You need that center web for stiffness. You can drill small holes in it, even get away with well designed larger holes, but cutting that much out removes all the strength of the tube or channel. Attached Images.No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan