Some of this post will make better sense after reading my intro. I am building a 1971 Chevy 1 ton flatbed dually. Currently the frame rails are 1/4” C. I am going to box the rails from the cab back since they will be visible. My engineering question for y’all is: will a 3/16” rectangle tube be as strong or stronger than a 1/4” C. It is not C channel it is flat stock that has been bent into a C shape. Every time I have asked this question the comparison between C channel and tube is drawn. This is automotive frame rail not C channel. I had been reading some forums here and noticed a couple engineers on this site. I know this may be a shot in the dark but I want to make an informed decision when it comes to fabricating the truck frame. Making the boxed frame out of 1/4” when it may be strong enough using 3/16”.
You need to start by showing a picture of the materials and how you plan to use it, I don't know about the others here, but I'm a machinist type that likes drawings with dimensions and pictures. I can't read something and make a lot of sense of it.
I'm not a engineer but I can tell you is the material thickness is less important than the beam height, and since I gather that the height will be the same the strength will be pretty much close enough.
I’m no engineer either, but I do a lot of fabrication and enjoy building off road rigs. 2x4 or 2x6 0.120 wall is common for frames on something like a jeep, buggy, or small truck. I’d think 2x6 0.188 wall would be plenty adequate, but then again I don’t know what your intended uses are for your truck. 0.250 wall tubing starts to get real heavy weight wise.
Strength isn't the only factor in a vehicle frame, maybe not the most important even. Stiffness vs flexibility vs brittleness vs toughness are all important factors. Basically there's a lot of engineering that goes into a truck frame besides just the bending strength in one direction.
There should be plenty of information on strength of materials out there on the web these days. I always refer to my old textbook by Gere & Timoshenko - Mechanics of Materials => Appendix E - Properties of selected steel shapes. That gives me most everything I need to start an analysis. A quick study guide would be to find your associated beam loading diagram in the appropriate appendix, find the point of maximum stress to work around that point, get the structural piece's data and plug into Sigma=M*y/I for the stress in the beam. That should get you your bending point stress if I remember correctly. (been over 30 years) Don't forget to multipy what you want by a safety factor in order to get a strong enough beam design. You're not building a bridge but you do need to account for a rough ride on a bad road and heavier than designed load in case someone does something stupid. I'm sure there are plenty of online calculators and visual analysis tools out there these days to speed this process up.
I also forgot to mention if this is going to be a regular truck (not a drag truck) you need to account for flexibility, might crack if it's too stiff.
try over at www.eng-tips.com Lot's-O-Engineers there.
Reason C Channel was used is easier to install X Members while Self Cleans, Dumps trash that accumulates with minor accumulations that can be seen to be washed out, Boxed hold Dirt, any hole, end point of Mouth of channel will allow dirt In, in no way will it ever leave, and No way will know how fast is corrupting until done. Ford is dealing with that on the F150 line, as they rot from the inside and shear. Next up Box will be more rigid, how is connection point to Forward Frame to be accomplished as will eventually develop cracking there. Nice Look, some days a bad choice.
Not much different than trailers. If the height is the same the tube will be a lot stronger and also more ridged so it won't flex as much. I'd say even a size smaller tube would be stronger than channel. Ie/ 5" x 2" x .188" tube would be stronger than 6" x 2" x .250" channel. Not sure if the channel is 2" flange or not but most of the strength comes from the height. Think about it, in a trailer or frame application tube gives twice as many vertical members. The thinner wall thickness of the tube still adds up to 3/8" compared to the 1/4" of the channel. The bigger issue, that may not be much of an issue anyways, is weight. The tube will weigh more.
Welder Dave, the strength comes from the flanges, not the web. The web is merely there to keep the flanges separated and tied together. The problem is much more complex than a simple beam. The stamped shape of the C channel doesn't follow any of the structural shapes, so no book data is available. The wide flanges on the C will want to buckle, so need considerable bracing. The typical failure mode would have the tension flange bending to the center and the compression flange buckling. As the beam deforms it rapidly becomes weaker in section modulus, so failure will be catastrophic. If it was mine, I think I'd just box the existing rails with 3/16.
The strength for weight capacity in a truck or trailer frame application comes from the height of frame members. The flanges keep the frame ridged and prevent it from buckling sideways. Tubing makes it more ridged and adds a 2nd vertical member basically the same as boxing in a channel frame to make it stronger. A few years ago there were some poorly installed girders on a new bridge that buckled. The crew got in a hurry, because of bad weather coming and didn't put in the required bracing before installing the next girder. The flanges on the beam were quite narrow but the web was very tall to give the required weight capacity. I agree it's probably easiest to just box in the channel. Would be a good idea to drill some weep holes. I have a tubing trailer but all the members have small holes drilled in the bottom near the ends to let any moisture or condensation out.
Lots of good advice from all of above. But I'm puzzled by this statement .... So, from this statement, I'm getting that your frame boxing exercise is more for looks, than any strength addition? If additional strength is required, a simple sheet of plate, slightly bigger in height than the current frame rail height, welded on the inside face of the frame channel is all that's needed to strengthen the frame. A slightly larger height in the reinforcement plate, than the frame depth measures, gives you a projecting sharp edge top and bottom. This sharp edge and additional depth is what will provide the extra strength. You get more strength from sharp corners, than rounded ones. Remember that you need the chassis to flex under load and torque. Frame flexing is engineered into every chassis. Making the frame too rigid only results in frame cracking. Strengthening from the centre of the cab floor to the front spring perch of the rear axle is all that's required for increased frame strength. Strengthening any other section outside this area will be extra weight for no major strength or stiffness increase. A chassis bending downwards from heavy weight is by far the most common chassis problem. I'd like a dollar for every old truck I've seen with a plated chassis in the centre. Double sheeting steel materials and boxing them, only makes for improved rust pocket areas. If you box in a C frame, you need BIG drain holes along the frame to let the dirt and moisture get out, when you do a washdown. Landrovers used a box-section frame with inadequate drain holes. A lot of Landrovers broke in half with chassis corrosion after a few years. Landrover went to a galvanised chassis for military models to try and fix the problem.
Welder Dave, you need to do some research on topics like moment of inertia and strength of sections. The strength of a beam is typically around 80% to 90% from the flange material, the rest from the web. Yes, if you consider the three parts of a beam separately, the flanges bend much easier than the web. But when you assemble them into a beam, you are now trying to bend the flanges about a neutral axis that is in the center of the web. Bending a stack of paper can be used as an example. When the paper is free to slide over each other, the stack of paper bends easily. When you remove the center pieces of paper and replace it with a separator, and glue them together so they can't slide, the paper is now much more rigid.
Why are you doing this if its for looks you wont be able to see it unless your not going to have a bed on it of any kind. If you start it just behind the cab your going to make the frame weaker than what it was . You need to go as far forwards as you can right to were the frame turns down after going over the front end.
I don't know about your application-but I do know this. I've lengthened/shortened 73 truck frames {class 8 in my 49 years}. It's not {C} channel it's called formed frame rail. The other is boxing a frame reduces flex and tends to crack. Rail is based on grades grade 8-80,000 lb and so on. On lighter GVW trucks the best way is to do it like IH did, just plate the outside of rail and huck rivet, there's a reason IH did it that way-to reduce viatical cracks from flex. Have the pieces cut on a plasma table it will look a heck of allot better and get the strength you want. One of many I've done. *
Some of the Chevy one tons U Haul had were done this way but i think he is going for looks?
If your going from cab back it will crack right there. If your going for look get it in a guage
While I may not be a structural engineer, I’ve carried more than a few 20’ sticks of channel and box beams over my shoulders and you learn real quick which is the strongest side or you’re bouncing in the dirt. Lol
Yes pictures are worth a thousand words, I was really just cracking the ice with the first post to see if you guys would be interested in weighing in on my project. So here is the C30. It is a 1971 one ton dually that was converted to a camper at the Chevy dealership. It was first sold this way and was not a back yard job. The frame was stretched 18” via a sleeve that was inserted into the frame rail. As mentioned this was done at the dealership as part of an option package back in 71. The frame rails from the cab back are 12’ long. My intention is to take the splice out and bring it back down to an 8’ flat bed dump. This is going to be custom show truck (not a trailer queen). It is going to be a working truck so I want it to be able to haul 2000 pounds if needed. The reason for my question was that I wanted to make sure that reducing the thickness of steel was not going to compromise the strength. The frame is pretty ugly and does not present well, also the plan is to lower this truck pretty low so the geometry of the frame must change to drop over the rear axle. This is why a stiffer frame is needed. Simply putting a plate on one side is not possible for the geometry needed. The photo of the extreme lowered truck is for reference of where I am headed. It is not an exact concept but it is in the general area. The suspension is changing to a parallel four link and it will be on airbags. I plan on having plenty of weep holes and possibly having it galvanized then painted prior to assembly. Many more aspects to discuss but I have to get to work. Thanks for all of your input, this will be fun to design with y’all
I don't think that's what Dave was saying, he said the strength comes from the height, which is undeniable. Maybe stiffness even more so than strength? And also the weakness of just increasing strength with height.
Either way, this isn't for a structural engineer so much, they do buildings and the like. A mechanical engineer is what your after, specializing in truck frames.
With your paper example trying to bend the paper on edge standing up would have the greatest resistance to bending. The same reason why bridge girders are tall and can have narrow flanges. Flanges help prevent buckling. An I beam provides more rigidity than channel and an H beam provides the most rigidity in a single web beam.
Whatever you do you need to do it to the front spring perch otherwise it's just going to crack at the cab
If you're shortening the frame back to stock and only need to support 2000lbs. or so, I don't think you need to worry too much since the stock frame would carry a lot more than 2000lbs. before it showed any signs of weakness. If you still want to strengthen it, and can't box it, then double the frame with a plate along the side. If you wanted to get fancy you could have a formed C shape frame rail made to make a double frame just like a lot of vocational trucks have. To make frames stronger on dump trucks and such they either double the frame or make the frame rails taller. I seem to recall Ford had a taller section single frame than other trucks. The advantage to that over a double frame is less weight so you can carry more. Also you don't have to worry about rust jacking.
Ahhh! A hot rodder! Now, we've got the whole picture! Isn't there any hot-rodders in your area you can talk to, as regards new boxed frame requirements? First thing I would do, is find out your States legal restrictions or requirements as regards frame replacement. I know many U.S. States are pretty laid back when it comes to legalities of major chassis redesign, but I'd still talk to your local hot-rodders as to the local requirements for road registration.
This thread went off the rails.
Frame rails for sure.
Since this is going to be a show truck and you seem pretty set on tubing, I'm not sure how relevant this is but here is how I did something ‘sort of’ similar on my ‘79 F-150. It’s a trail rig and about as far from a show truck as you can get, but I did reframe it from the cab back and it has worked amazingly well. What may be relevant is how I attached the two sections of frame and the fact that after the joint you could build any sort of frame you want. Basically I used c channel sized to fit inside the existing frame. Slid it in far enough and used an angle pattern of grade 8 bolt to attach the two frame sections. You don’t want any bolt holes or welds making a vertical path up the frame, always work on angles. Here is a pic of the joint between the frames: I built the truck in 2005 and it has literally thousands of off-road miles on it now. Heaviest thing it hauls is some camping gear and a cooler, but it does get twisted, beat on and shock loaded something terrible. Here’s a pic of the rig:
Well the load I want to be able to haul is 2000, the truck weighs in at 5000, so there's that portion.
Haha, I am the hot rodder that people ask these questions to. But most hot rodders are shade tree mechanics and they are making guesses based upon experience. I am looking for more data driven results. Just to give a bit more of my pedigree, I have lengthened, shortened, notched, modified, and stiffened my fair share of vehicle frames. Being an X-ray certified welder I know how to engineer the joint so it will not crack anywhere. Since this is a one ton truck it will be working harder and carrying more than previous vehicles. The spirit of my OP was to find out if anyone here had run the numbers on strength of 3/16 box versus 1/4 "c" shaped frame. I know solid works has a tool that will do stress analysis if you give it all of the parameters but I do not have solid works. The boxing of the frame Is for looks but also for strength, I don't really want to make it a 8" frame if I don't have to. Having a 3"x6"x1/4" sq tube frame rail that is just as strong or stronger than the 3"x8"x1/4" c shaped frame would get me 2" closer to the ground. More than likely I will stick with the 8'' width, but while I am in the design phase I might as well look at all of the possibilities.
Maybe you need to have a chat with these guys? They manufacture complete new hot rod chassis for GM pickups, and they use new box section that is mandrel bent over the axles. They may be prepared to hand out some advice on what strength and thickness is needed. Scott’s Hotrods – 1967-1972 Chevy / GMC C10 Chassis Visit the post for more. scottshotrods.com
I think 6" x 2" x 3/16" tube would be more than adequate. The entire length isn't hanging behind the back spring mount.
I'd choose the 8" tall c frame in 3/16 or 1/4 over anything else, but I'm some random guy on the Internet. The frame will crack it won't matter who welds it as you will have this super stiff section of frame and it ends too short leaving regular frame to do all the flexing.
I had a 1967 c10 i shortened it and made a short bed out of it to start with then i lengthened it back to a long bed then i put 3/4 ton axles under it and used it to pull a gooseneck. I didnt plate it or anything. I never had a problem with it i drove it over 20 years and pulled several 16000# loads with it . If that was my pickup i would shorten it weld it back together and be done with it.
Yeah I am not too concerned about cracking. If the connection is designed correctly that spreads load out over a distance it will be fine. If welds failed anytime there was a thick to thin or stiff to flex connection I would be out of a job and they would start glueing all vehicles together like the F150. Just because the rails will be boxed and stiffer, doesn’t mean that the entire frame will not be flexible. The flex occurs while the vehicle maneuvers side to side, it bends and twists to keep the wheels planted on the ground. It is necessary for that flex to happen as a critical part of the suspension. Designing the back frame I intend to have the whole system allow for that dynamic. I put a whole new independent front suspension on my wife’s 65 mustang years ago with zero issues. All welded in place, the entire front clip was removed and the new welded in.
Not really.
Engineer here. I don't really get into truck frames much other than a fubar heavy truck frame we advised on. There is a LOT of interaction and trade off in a truck frame. Motive force through the drivetrain imparts torsion into the frame. Some might think reducing that is ideal but a good road frame (not race) usually needs to accept an amount of torsional deflection to increase cyclic fatigue. Typically if you have a C channel system, then to make part of the frame box, you will build load concentration right where that connection is, and that must be mitigated. However, there are thousands of backyard bandits that slap metal together and run it. That is not engineering. The way I try to explain this is if I bend a thin stick until it breaks, it will break roughly at the center, not surprise. But if I take another stick and put a steel tube collar in the area where it would be expected to break, where does it break now? Right at the edge of the tube collar. I mostly work with airplanes and buildings, but structural work is VERY relative in any field. If you build an infinitely torsionally rigid frame, you are very likely to end up with cracks as road vibrations are not able to be absorbed as elastic deformation in the frame. OEM frames typically combine box and open C with special care to transition the loads through gussets and such, or loading back through an alternate structure such as a cab, bed, etc. As for materials, ductile materials such as A36 are very common as they can handle more fatigue cycles. Some frames are heat treated, but you will never be able to achieve that in a garage, such as the issue we had with a semi truck frame. We actually spec'd a preheat of the steel to ensure rapid cooling and hardening did not occur. That is not much an issue with A36, but certainly is with the filler rod. That's why it's common to see cracks right in a weld. Steel's strength is in tension, and it can handle a LOT. So simply pulling on it in a linear plane, it can take a lot, but very rarely is steel loaded in pure tension. There is nearly always a bending component such as a trailer hitch. If you are just building a show truck to bed carry 2k lbs, you probably can't get in too much trouble. You will commonly see a radius in one side of a box section where is transisions to a C. That is to aid in the load transition. That might help you. But remember that a C section has nearly no torsional stiffness on it's own, while a box section most certainly does. If you are going to run a hot engine with high torque, think about the torsion events. I will further just touch on the word "strength" as it is commonly misapplied. strength is just a measure of how much force a piece of steel can take and still return to its original shape. IE elastic deformation. Low carbon annealed steel would have low relative strength, but higher on cyclic fatigue. As you impart heat treatments, you increase the strength and stiffness sure enough, but also increase embrittlement. Cycle count can reduce but sometimes due to the increase in stiffness, the actual measured deflection reduces so this is where careful engineering is applied. The right balance of stiffness, strength, and cyclic fatigue. There is a wild card in welding heat treated steel because the moment it is heated, it anneals, but under rapid cooling, hardening can/will occur and that requires very precise procedures if you are trying to achieve a certain result. In the case of welding higher carbon steels when cold, it can cool way too fast, creating a very hard but very brittle joint.
Are you a "Structural Engineer" ???
Yes. I don't mean to imply an ME can't approach this. The fields are indeed similar, but an SE is who would have oversight on something like this. Both have vast education in materials. We have also found that each material can bring huge education gaps. We had to take a crash course in wood engineering, which is largely applied differently, such as nail/screw/glue connections, allowables, etc.
Thanks fastline for weighing in on this. I think there are some basic rules working with steel and beams that apply in many different applications. There was a "semi famous" episode of American Hot Rod where the young kid in the shop wanted to hot rod a 34 Ford or something like that. The shop was offering a bunch of free help with the kids project. He wanted to put a V8 in it but didn't want to box the frame which is the standard practice. The kid had a big blowup because Boyd Coddington said no. The kid wanted the original look but Boyd said it won't be safe on the road without boxing the frame. I think the kid quit over it and just refused to accept the stock frame wouldn't support a modern powertrain. A lot of the drama might have been for TV but it was pretty obvious the stock frame wasn't adequate.
Yeah, it is always a mixed bag I try to stay out of anymore because there are a LOT of variables in something like this. The idea of "over building" something implies that someone knows the loads or original structure to make that statement. Within the building spectrum people say "it's been standing for years", in which I say, "because it hasn't seen a design level event yet". Remember it's always good......until it isn't. I've seen bandaid after bandaid from welding bandits that reinforce a structure, just to see the failure move to another concentration point. We simply can't operate in the "try it and see" space, though I do enjoy pushing new ideas and designs. But when talking about a roadster project with a V8, yeah, that is a LOT more torque (torsion) than that frame was ever meant for and starting over sounds good to me! I really like round tube for the uniform torsional force capacity but in a roadster, you also need to support the vertical weight of that engine bouncing down the road. I'm sure everyone here has seen those log truck races and that is a perfect display of the torsion going through a frame. I get so annoyed with the abused use of 'HP' because it has little bearing on a frame. It is all about torque and HP is just torque with a time component added (work done/time). Another fun example I just remembered was breaking the front spindle off the front of a 50HP tractor with a 2 bottom plow. The thing with frame torsion is gear reduction/torque multiplication. the frame sees many times more torque than the engine produces.
A lot of tractors the engine and transmission is a structural part of the chassis and there isn't a conventional frame. The oil pan is a heavy cast section with the front axle mounts incorporated into it. Sometimes you get failures because lack of grease and of course from abusing things beyond their design limits. I seem to recall reading about some people thinking frameless tractors were weaker but you don't hear of any tractors breaking apart very often. I've seen where front ends were pulled off if a tractor is stuck and when I was at a dealer saw about a 75 HP Kubota front wheel assist getting the transmission switched over to a new housing because somehow the front end loader carrying a bale came suddenly crashing down while driving on the road. That looked like a big job switching everything over. Amazingly I think the engine survived intact and that was why they repaired the tractor instead of writing it off. I think the owner was getting at least partial warranty for it because he claims he never touched the loader control. It just came crashing down. I can see an insurance company opting to repair it instead of giving the owner a brand new tractor.
Never said your welds would be the thing that cracks......
You are right! When I say "frame", that is really a chassis in which the engine/trans are one big torque tube. Just was an example of the torsion seen through that system. took a lot of force but much of that was due to the rolling force of that plow. They are known for it. One thing I never want to really do is have to 'split' a tractor.
Apparently it's not too bad of a job to split a tractor if you're set up right to do it. A modern large tractor with a cab and loader I think would be a royal PIA but maybe they designed all the linkages and electrical that has to come apart to be simple plug and play??
All I know is I have studied my big Case tractor and though I would do it if I had to, I would dread that like tax day. My Case is very poorly designed as they basically took an open cab rig and added the cab, making no provisions to work on anything. It is a cluster fk! Maybe Deere is better. I don't even want to talk about that machine. Couple a poor design with all the shade tree work on it, it's amazing it works! They hot wired a bunch of wires because, ya know, if a breaker blows, just bypass it! Burned up most of the harness and I spent days doing sister wiring so things would work. Luckily most of that tractor is mechanical. Electrically, she is fubar.
One of the major problems with doing things such as chassis strengthening is trying to find out what type of steel is used in the original chassis - and trying to find out what the type of steel is, that you're going to use. A lot of people simply pick up some steel scrap or offcuts, and utilise that for reinforcing - but unless the steel has markings on for grade or quality, you're flying blind. I find a lot of original steel grade markings on offcuts or scrap very quickly disappear with weathering and handling. So on that basis, using new steel with markings of known grade or quality is pretty important.
yield vs tensile strength test - Google Search Since we are chatting on steel engineering, I figured I might share a few things that might interest people and could be relative to the discussion. In the video (sorry man, that one is in jibborish), they are doing a tensile coupon test on what appears to be rebar, which is very common for the structural building world. 6min in for actual testing. I have never agreed with some of the terminology because it is confusing. They like to say "yield strength" and "tensile strength", yet this test does both, which both are measures of the stress/strain, and both are measures in tension. Yield would be the point at which tension is applied until the steel is permanently stretched or reaches the "plastic zone". The steel will continue to take tension load which elongating until the UTS or ultimate tensile strength is reached, which is the breaking point. The data collected is the yield, UTS, stress/strain curve, and elongation or displacement. Elongation can be highly important, especially in applications such as rebar. We may wish to have high elongation before UTS is reached, which could prevent a full collapse of a structure. If folks are not aware, in many applications, rebar lays dormant inside concrete until concrete cracks. Then the tension capacity is used. This is starkly different from pre/post tensioned rebar which is very common in bridges and high rise structures, which actually presets tension in the steel which makes the structure mucho betterlay! Relating some of this back to the frame work, steel can commonly be bought in the USA in "grades", like grade 40, 60, 80. That is 40ksi yield. Ksi is thousands of lbs per si in tension. This can help in selecting a grade for frame work. Most of these grades are considered low carbon and are largely ductile, meaning they will have high elongation but resist fatigue cracks better. What you have to watch for in auto frames is hydro or cold formed steel. Cold working steel is a fantastic way to impart additional strength by "work hardening" or stretching it. This not only forms the material for final shape, but it will get harder and stiffer. Unfortunately this can be highly problematic for repairs and some frames are considered non-repairable due to the inability to repair back to the same spec. But we make do when needed. The stretching or cold forming of materials is a function of the alloy ratio and elements in the alloy. Any high Nickel alloy is going to want to work harden. Just get it hot, let it cool, and your fkd! Copper is another and one can simply play with a piece of wire to learn this, but Copper can be annealed easily with a torch. Anyway, there is a little TMI that might interest someone. Who knows. I live it. I do know most people don't realize most materials we use are actually alloys or a mixture of elements to extract certain properties we want. If we want something that will get hard, strong, low elongation, but will accept the big bang at UTS with little overload capacity, we might grab 4140 and turn up the heat treat to even a 60 HRC and a super low temper. Not many applications where that is practical, but..... Also, as it relates back to frames, you will find some material shapes come only in different alloys and some of this is that only certain alloys can be done in certain shapes, but the specs are similar. Like steel plate vs steel tube, etc.
Thanks Fastline for that informative reply. I will be using A36 for the entire frame and the connection point will be spread over 18”. The inside plate of the box will gradually angle toward the outside vertical member of the C. I was not planning to have too many cross members in the rear portion allowing it to flex torsionally as the truck is driving. Any cross members would be bolted in not welded to allow for this movement. At the weld school where I teach we do a lot of bend testing and I am very familiar with the yield strength of A36 and how to effectively weld a joint so it will not crack but can flex with the rest of the joint. I will not be using higher carbon or heat treated steel, so no annealing issues. One concept I was entertaining was to cut large holes (4”) in the 8” vertical rails of the box frame and weld in the same thickness tube in the rail connecting the outside and inside verticals. Making hollow portions in the frame but they would all have tube inserts. My design had holes every 14”- 18” and as the frame rails narrow toward the back of the truck, the holes will gradually be smaller. It is kind of a cool custom look but form follows function so if you think this would not be advised I will listen to your advice. I had my buddy insert these values into his solid works program and there did not seem to be concentrated stress loads from this concept. You had mentioned rapid cooling from welding can and will occur, I know there is a certain amount of work hardening or embrittlement that can occur with the wrong weldment but I do not think there is enough carbon in A36 to really harden under heating and cooling. True?
Hard to mentally picture some of what you are explaining, if you could make a drawing or picture mock up? I think I understand the round tube cross members. If you are installing those into square tube, there would be some stress build up or actually reduced torsional support by making swiss cheese out of the square tube. It would take a little studying to determine the interactions but I think that can all be done safely. I would like to see any round tube cross members welded rather than bolted. Bolts in that arrangement are a guaranteed hot point in stress. A weld will distribute loads much better. You would also want to run any tube through the entire square tube, not just one side. Unless I am not quite understanding your plan? Now, with pull truck plans, I might shift tactics because you are actually removing tension (trailer pulling) capacity by making large holes in the frame rails. That is where you would assess if there is enough steel left to stay safely within margins. Most people probably don't know how to relate my above post with this build. Let''s assume you will use A36, 36ksi yield. You want to know if those bed rails will handle towing. Assume they are .125" thick, we will just use simple numbers here, 8" tall, 2" wide That is 20"x.125" = 2.5si But for now I am going to ignore those 4" cutout areas (other ways but we always start with worst case), that reduces to 12" of effective wall length * .125" = 1.5si. * the 36ksi we are using is 54k lbs in pure tension per frame rail at yield. Then we back that up because you NEVER push things to the failure point, we call them margins. I will apply a margin of 3 for now. 54k/3 = 18k lbs * 2 frame rails and we are at a reasonably safe 36k drawn, BUT there are other factors such as shock loads that could cause the margins to be increased. But this just shows how I might start to arrive at safe allowables. Also remember to assess racking, which would be forward/aft movement of the frame rails. This can usually be satisfied with some type of diagonal bracing. Some of this is likely driven through the engine and cab, or if there is a heavy flatbed being installed, that would certainly arrest it. As for A36, you are correct about the carbon and hardening. It is a general use steel with small concerns of cracking because it is so ductile or compliant. The welds become hot points because welding rod looks great on paper but is not nearly as compliant as A36. I am sure your build will be just fine though! I am just sharing data, not saying all is lost! Sometimes we gotta run the less than ideal hot glue because it is the only thing that we run in a certain weld position. Since you are a welder (many here are?), that is a huge step towards winning. Good understanding of clean materials, chamfers, fillets, rods, wire, heat, etc. All good stuff!
Yes you are right, I was typing my last response on my phone and knew the photo aspect would be helpful. Although I am not a draftsman and my computer does not have any design software, it does have some rudimentary shapes available. So here goes... To explain the simple sketch, The round tube in the frame rails are not at all connected to the cross members. The cross members were going to be bolted in from driver side rail to passenger side rail. The tubes in the frame are welded in place. The tube is in the middle of the frame rail so the reduction of side wall is offset by the addition of the tube. as you can see in the drawing, as the frame is reducing in size the closer to the back of the truck, so a 6" frame rail only gats a 2' tube through it. This design aspect is not imperative, it was actually considered that it may reduce the rigidity of the rectangle tube and allow the frame to flex a bit while being something that adds a unique feature to the frame. If the stress of towing would compromise the frame rail then I will abandon the idea. The Actual frame cross members I was going to bolt in place similar to how the factory does with hot rivets. Allowing a bit more torsional flex than a welded joint. As stated in my OP the frame rail will be 3/16 tube versus 1/4 C shaped stamped rail. I believe the gain in strength with a tube frame will offset any losses with the "swiss cheese" tubular rail. But this is the whole reason why I asked for any engineers to weigh in. I need to build this truck based upon good data and not back yard mechanics as you put it. That is not my style, I am not a weld it and see kind of fabricator I am a structural D1.1 code welder and I fully understand if the engineering is substandard or not even considered then bad things happen. I appreciate any and all intel you can provide.
Just a side note, in the 17 years I have been an instructor and the 30 years I have been a welder, only a hand full of engineers have taken my class. I always appreciate when pre or even established engineers take a welding class so they understand the welding process and their designs take into account application of the weldment.
Have had engineered blueprints you have to take back to them to get modified because they didn't allow access to weld some of the pieces in.
Yes I realize, but I also am not going to just weld a box to a C, I am going to gradually diffuse the stress point over 18". I understand your concern and I have experience in this situation but yes it is good to discuss these issues.
I think a lot of the required flex in a frame comes from careful placement and type of cross members. I think the frame rails stay pretty ridged for the most part especially on a large truck with a double frame. Some of the big logging trucks have tube and even I beam frames.