Frame Design Concept and Question about Tube Sizing in Space Frame

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Many of you know I am working on designing a camper from scratch using a different approach than the manufacturers of these campers do. This camper will be designed for expeditions on rough back country roads (not rock crawling but just this side of it) I am thinking about the bearing frame of the camper which holds all the weight of the camper. This is the structure that sits in the bed of the truck and is what the weight rests on. So I thought a space frame might suit the bill nicely. I drew inspiration from aircraft design and roll cage design. I have quickly whipped up on in my CAD system and attached the image: As you can see the entire frame is designed using solid rods mostly for efficiency of modeling but the final frame will be of tube with each tube notched at the intersection and welded. My goal is to have the frame be able to support 3000 pounds which would be the weight of the camper plus 4 occupants. The rest of the camper will be modeled using a similar technique except in the non load bearing walls I would make the gaps between the tubes much larger and the spans much bigger. The frame is designed to leave a 1" gap around the camper frame to the truck bed to allow for shifting and torquing. I had also debated making the frame work as a drop in camper rather than slide in. This would mean the frame would fit over the wheel wells and would be lifted over them with the jacks and then dropped into place. In that case I would use triangles to span the frame that are a bit bigger and add some extra support over the wheel wells. Since I know there are people in here that have forgotten more about metal than I will know I was curious if you think I am barking up the wrong tree design wise.Assuming I am not barking up the wrong tree Id like your opinions on tube sizing. Assuming 1" outside diameter, what minumum wall thickness do you think I can get away with using mild steel or aluminum tubing. I have not made a decision as to which to use. I am much more experienced welding steel than aluminum and I dont have a TIG rig so I would have to mig the aluminum. At the same time I would like the lightest weight for the size.Thanks in advance.
Reply:1st you need an understanding of basic structures. The truss you have drawn up is all wrong. What you need to do is redraw your truss so the angles start from each end at the bottom and then go up. The way a truss works is that it takes the vertical forces and turns then into both horizontal and vertical forces, in tension or compression, depending on how the loads are applied.I've roughly redrawn the sides of your truss. The ends would be the same.The red lines will be what supports the "floor" if I'm understanding your rough idea. Each red line is a floor joist. Each joist will have to support part of that 3000 pound load. So each joist would carry roughly 1/5th of 3000 pounds, or 600 lbs, or 300 lbs supported on each end. Now each tube V of the truss needs to take that 300 lbs and split it up, so each tube takes 150 pounds.A quick graphic that shows how much area each floor joist will need to support. Each joist supports the floor area 1/2 way between it and the next, or an area equal to the space between two joists. The end sections only need to support 1/2 the area the main joists do since there is nothing on the outside they are supporting.This is all somewhat oversimplified, since my guess is that the trusses will directly support the walls/roof unlike your description. In general you can still use that idea as far as how much of the total the trusses will need to support, it will only be transferred differently.I wish I still had my writeup I did on basic trusses I did for another site. It would explain the concept better. 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
Reply:Thanks for the reply DSW. From what I gather from the forum you are indeed one of those people that know more about metal than I ever will. So please humor me with some answers if you have the time. (and I do greatly appreciate your time)1) Why does it matter if the diagonal struts start from the bottom left corner or top left corner. Wouldn't either way push the forces into the diagonal in compression? 2) Does it matter what angle the diagonal braces are relative to the main structure? Do they have to be 45 or can they be at another angle? The reason I ask is because I am trying to conceive of a system where the truss goes over the wheel well rather than straight. The idea being to make use of the entire truck bed and have the camper be "drop in" rather than "slide in". Consider the following image of a work in progress attempt to design a truss frame.You can see that I am trying to triangulate everything at 45 degrees but I need the gap on the bottom to clear the wheel well, then the camper would have one of these on each side and you would use the jacks to lift the camper over the wheel wells and then drop it right in place in the truck. The problem I am having is that the dimensions of the frame (82.5 long, 9" to clear the wells with flex room to spare, 25" in front of wells, 21" in back of them and 21" to clear the bed rails with 1" of room for twist and flex.) are such that they dont fit perfectly in the 45 degree paradigm. I suppose I could do this in two tiers and stick to 45s. What do you think is the best course of action? If I do two tiers is it best to put the shorter tier with more diagonals on the bottom or top? 3) Is it the case that with more diagonals I can go with thinner walled tube or lighter tube like aluminum? I am under the impression that the fewer diagonal braces I have, the thicker I need the walls of the tube to be. Yet weight is a major concern. If the whole camper frame weighs 1000 lbs, its going to be tough to get it in my weight target. 4) What do you think I need to be looking at for the minimum size tube (in aluminum or steel) to support the weight target I am looking at (3000# with the camper occupied with 3 adults and two small children and 2000# with the camper just sitting on the truck). I am much much better welding steel than aluminum so if I can get away with doing it in steel I probably will be if that means I can use less metal and thinner tubes to get relatively the same weight.Edit: if you find that writeup I would be very interested to read it.Edit 2: I have found a picture of a production slide-in camper frame and I am looking at the bottom of the camper that supports the weight. It seems to me that this design is weak, how can all those verticals (and precious few of them) be expected to hold the weight in anything like the shock of a rut in a dirt road. It seems to me like the camper is designed using house timber framing concepts but I dont understand how this is appropriate for a mobile vehicle. Am I wrong here ? If so how so? Last edited by yorzaxt; 11-26-2012 at 12:36 PM.Reason: Added more information
Reply:I'll try and cover as much as I can. It's been almost 20 years since I took all my structures classes when getting my Bach of Architecture and I haven't really used this all that much since then. Originally Posted by yorzaxt1) Why does it matter if the diagonal struts start from the bottom left corner or top left corner. Wouldn't either way push the forces into the diagonal in compression?  In some ways it doesn't matter, but it's not that efficient. Look at almost all truss bridges and you'll notice that they all taper down at the ends and don't end with a vertical column. I'm sure there are exceptions for special cases but in general that's not the way simple trusses are designed.2) Does it matter what angle the diagonal braces are relative to the main structure? Do they have to be 45 or can they be at another angle?  No the angle can be something other than 45 deg. The flatter the angle the less efficient it is. The steeper the angle, the more vertical load and the less horizontal load transfer. 45 deg makes the math simple and I'll try and cover that in a different post later.3) Is it the case that with more diagonals I can go with thinner walled tube or lighter tube like aluminum? I am under the impression that the fewer diagonal braces I have, the thicker I need the walls of the tube to be. Yet weight is a major concern. If the whole camper frame weighs 1000 lbs, its going to be tough to get it in my weight target. Sort of. Tube has two components that work hand in hand for strength. Diameter and wall thickness. These combined with the total length will determine if the tube will support the load in compression or not. I used to do a demo for Scouts where I'd stand on an empty soda can. Even though the walls were super thin, the short length combined with the diameter meant that the can could easily support my weight IF applied only in compression straight down. Tap the side of the can and distort it even a tiny bit, and it collapsed. That distortion could come from a lateral load such as if I was leaning to one side, or it could come from the tube bending if the length was too long for the wall thickness/diameter. Same idea works with say a 2x4. You can support your weight easily on a 6" 2x4, but try to stand on a 16' long one turned vertical, and the weight will bend and buckle the 2x because of it's length. Material is the same, only the longer length means that it will fail. Make sense?4) What do you think I need to be looking at for the minimum size tube (in aluminum or steel) to support the weight target I am looking at (3000# with the camper occupied with 3 adults and two small children and 2000# with the camper just sitting on the truck). I won't even guess. It's been way too long since I did those calcs and I don't have any of those tables any longer. Those I do have, I'd need a refresher course to be able to use again.I am much much better welding steel than aluminum so if I can get away with doing it in steel I probably will be if that means I can use less metal and thinner tubes to get relatively the same weight. Even though alum is stronger than steel by weight, it's not stronger than steel by volume. You need thicker alum or a larger size member to equal the same strength as steel in most cases. Alum also has fatigue issues if it flexes a lot. In some cases you can get a lighter member using steel than alum to take a given load, assuming the member is designed well.Edit: if you find that writeup I would be very interested to read it.I hunted to see if there was an archived page I could copy past from and I couldn't locate one. It will take me a while to rebuild that from memory, but I'll see what I can do at some point."Edit 2:  It seems to me like the camper is designed using house timber framing concepts but I dont understand how this is appropriate for a mobile vehicle. Am I wrong here ? If so how so? My guess is that they are using the skin to form a shear wall, sort of like plywood or cardboard. Corrugated cardboard is probably the best example of a shear wall except maybe a stress skin panel. You can support all sorts of weight with only paper and no frame. We used to get cardboard pallets that held 2 tons and you could triple stack the pallets. Take a standard moving box and put a sheet of plywood on top to evenly spread the load. Then start loading the pizz out of it. You'd be amazed at how much weight it would take to crush a simple cardboard box. The key to this, like the soda can is to make sure the sides don't buckle. As soon as the sides go out of vertical, the whole thing comes down. You do that by "bracing" the walls so the thin skin is supported by the frame. If the skin can't twist or bend, then it supports weight. The frame just has to support the skin in theory, but in reality it also carry's load.
Reply:Originally Posted by DSWEdit 2: It seems to me like the camper is designed using house timber framing concepts but I dont understand how this is appropriate for a mobile vehicle. Am I wrong here ? If so how so? My guess is that they are using the skin to form a shear wall, sort of like plywood or cardboard. Corrugated cardboard is probably the best example of a shear wall except maybe a stress skin panel. You can support all sorts of weight with only paper and no frame. We used to get cardboard pallets that held 2 tons and you could triple stack the pallets. Take a standard moving box and put a sheet of plywood on top to evenly spread the load. Then start loading the pizz out of it. You'd be amazed at how much weight it would take to crush a simple cardboard box. The key to this, like the soda can is to make sure the sides don't buckle. As soon as the sides go out of vertical, the whole thing comes down. You do that by "bracing" the walls so the thin skin is supported by the frame. If the skin can't twist or bend, then it supports weight. The frame just has to support the skin in theory, but in reality it also carry's load.
Reply:Welded moment connections ( except for 1 days coverage)  and complex composite frames went beyond what I learned in 5 semesters of basic structures.I still think fiberglass over foam would be the easiest solution to many of your issues of strength and weight. If you can paper mache, you can do basic fiberglass. ( Yes I know it's not welding... GASP!) 2" rigid foam covered with a shell of epoxy and glass cloth is very strong. The foam is easy to cut, easier to cut if you build a hot wire tool, but basic wood working saws or a knife will work. Mix up the 2 part epoxy and paint it on, and smooth on the glass cloth. Make sure all the glass is coated and wait until it dries. More layers, the more strength... West Epoxy systems has some good info on all of this. They are one of the easiest and best systems I've found for doing epoxy. If you need to add a frame or tie in floor supports, simply mix up some more epoxy and glue the frame to the shell, then glass over the frame to lock it in..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:.....uummm, you're stating 3000# total weight in the short bed of an F 250 which hasa nominal load rating of 1500#?Blackbird
Reply:Dave max payload on my F250 diesel 4x4 is just under 3K. The gas 4x2's have a heavier payload by a slight bit, say 3100lbs. 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
Reply:Originally Posted by DSWWelded moment connections ( except for 1 days coverage)  and complex composite frames went beyond what I learned in 5 semesters of basic structures.I still think fiberglass over foam would be the easiest solution to many of your issues of strength and weight. If you can paper mache, you can do basic fiberglass. ( Yes I know it's not welding... GASP!) 2" rigid foam covered with a shell of epoxy and glass cloth is very strong. The foam is easy to cut, easier to cut if you build a hot wire tool, but basic wood working saws or a knife will work. Mix up the 2 part epoxy and paint it on, and smooth on the glass cloth. Make sure all the glass is coated and wait until it dries. More layers, the more strength... West Epoxy systems has some good info on all of this. They are one of the easiest and best systems I've found for doing epoxy. If you need to add a frame or tie in floor supports, simply mix up some more epoxy and glue the frame to the shell, then glass over the frame to lock it in.
Reply:Originally Posted by yorzaxtHowever the problem isnt the weight and so on as much as it is the strength. The way I use trailers and campers on rough roads and in the back country your average vacubonded fiberglass jobs come apart rather quickly. They cant handle the vibration or shock. I have been through several campers and trailers and learned this lesson the hard way. .
Reply:Originally Posted by Broccoli1What were the failure points in the old trailers/Campers? ( besides cracked fiberglass)
Reply:I'm not sure if there is a design that would last with the abuse you are putting on the camper.Might be one of those things where buying  a used TC might be the route, just buy a new one when the old one falls apart.You mentioned Horses- how has the horse trailer held up?Ed Conleyhttp://www.screamingbroccoli.com/MM252MM211 (Sold)Passport Plus & Spool gunLincoln SP135 Plus- (Gone to a good home)Klutch 120v Plasma cutterSO 2020 benderBeer in the fridge
Reply:Horse trailer does fine. The problem is that most campers on the market (short of custom) are fiberglass foam construction and even in that manner they are constructed on principles that apply more to houses than to something moving. Even looking at camper construction they frame like a stick frame building, not like an aircraft which is under a lot more dynamic load than a stick frame building. Furthermore, the RV industry has completely stalled in innovation except, again, for the very high end custom construction. I personally feel I can do better than them and do it well. I also dont feel that fiberglass construction is anything other than a gimick to get you to buy another trailer soon. The world of mechanics is full of great ideas that have never been tried and old ideas that ork much better but were abandoned. After all, if the trailers lasted 50 years, the trailer companies wouldnt have been able to sell as many trailers.
Reply:Originally Posted by yorzaxt The world of mechanics is full of great ideas that have never been tried and old ideas that ork much better but were abandoned. After all, if the trailers lasted 50 years, the trailer companies wouldnt have been able to sell as many trailers.