Understanding tensile stregth, yield strength, and need advice for project:

Hi,
I am not an engineer but trying to understand bolt strgth, tensile strength og steel and some other varribles for something I am wanting
to construct. I am wanting to build a car dolly and/or dual axle trailer. I have welders and have welded on hobby projects but by no means consider myself an expert welder. My idea is to both weld and bolt a square togther to form the frame. I have some 3 inch square tubing laying around, I believe is is 1/8 thick but not sure. It came from some material stand at work that got cut up.
My idea was to get some scrap 1/8 inch plate and torch cut two L brackets and weld them togther to form a inch L bracket. These would go on all 4 corners of the square I would bolt each L with bolts through the L and through the 3 inch tubing. Before doing this I would insert a section of black pipe through all pieces for the bolt to ride in. After bolting and everything being square I would weld all joints with 6011 or 6013 rod with either my AC buzzbox or the mIller generator welder I just aquired on DC. (Not welded with it much yet) (I have trouble welding with 7018 for some reason.
My question becomes, what size bolts would be adequate? I get confused when I see specs on bolts such as shear strength, tensile strength, yield strength, etc? I suspect the bolt need only be as strong as the streth of the bolt steel around it. I am having trouble finding the specs for mild steel strength but think it might be around 38,000 PSI? Is this true?
Any help is appreciated!
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Metals have several strength figures:
Ultimate strength = psi at which a sample breaks under load Yield strength = psi at which a sample fails to return to its original shape/size when load is removed Proportional limit = Psi at which the sample deviates from Hook's law, i.e., deflection is no longer directly proportional to psi
However, knowing these numbers does not solve your trailer problem. In order to decide analytically how strong a bolt you need, you would have to do a complete stress analysis, which is not practical in most cases. You either need to overbuild it, or test it. or both.
What also plays into this is the strength of your welds. You are getting around this by welding AND bolting, but I have seen structures fail in totally unexpected ways, which could bite you in the ass--bending or buckling in a way that you didn't foresee. A trailer is a hazardous device, and you should be very careful.
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wrote:

Someone explained tensile strength, etc. in another post. Sheer strength is frequently calculated as a percentage of tensile strength and sometimes measured.
But what you are talking about would requite a complete stress analysis and calculations for each individual stress point, beam and brace.
In addition you don't seem to understand very much about stress and strength of materials - you talk about welding and bolting the same joint. Why? Do you think that the weld, which if properly done is actually stronger then the parent metal, isn't going to hold?
Given your obvious shortcomings in the field of engineering, the most logical thing to do is go and find an existing trailer that is being used for whatever purpose you intend to use yours and copy it exactly.
You probably will think that I am being sarcastic but I am giving you the best advice possible.
Cheers,
John D. Slocomb (jdslocombatgmail)
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Well, perhaps short of either using engineered plans or buying a trailer outright...
I'll also introduce the concept of the "factor of safety" to the discussion. Instead of using the 30ksi yield strength, common practice is to use a design strength 1/2 to 1/4 of that (or even less, depending on circumstances, and I won't even get close to overhead lifting and elevators) so that when the unexpected happens--say, a sudden shock of a pothole--there is some slight chance of your trailer, already loaded up to the design load, not failing immediately.
It's also very easy for the amateur to overbuild something to the point where it weighs two or three times what it needs to, yet still manages to be insufficient to the task due to poor design details. Especially when trying to economize by using found materials.
Like bolting square tubing through both walls, such that the force of the bolt when sufficiently tightenned collapses the tubing.
May I also suggest that while you're busy researching those smaller details, you also look into the larger: ballance, weight distribution, hitch weight, and their effect on towing stability, and hitch length on the ability to turn a corner without taking out your own taillights. Then, trailer registered gross weight and the requirements of your local DMV for brakes, fenders, and inspections. Above a certain weight, you may also fall under seperate DOT regulations for brakes. Not to mention DOT required lights and reflectors, and how those requirements change as trailers get wider.
Oh, and run it all by your insurance agent. Might be a good time to up your liability coverage...
--Glenn Lyford
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My only reason for bolting was that I can weld but I have had no training and worry about the stregth of my welds. As far as design my idea was to copy the rough dimensions a car dolly at u haul. Mine would be a simple square with removeable ramps.
Does the idea of an L bracket with bolt and welds not seem feasible?
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wrote:

Welds are extremely easy to test. Weld two tags together and then try to break them apart. If the weld breaks it is no good and if it doesn't it is a good un.
And yes that is how they tested welds for years and still do, for that matter. There are formalized methods of stressing the weld joint but essentially it is just about as I described it. Can you break your own welds? If not then they are good. If you can then better put in some practice. Or maybe even take a course...
Cheers,
John D. Slocomb (jdslocombatgmail)
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I can weld where I cant brake it apart with 6011. I have 7018 but worry since I have had then awhile about moisture and the weld craking over time.
Also, I am not sure the force I am using to try to bend the weld and the force a weld will have on a trailer are the same thing.... Maybe I am wrong.
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wrote:

Why insist on using 7018? After all, you are welding mild steel and the 60 series rods were used for that for years and years and certainly the welds in old steel welded structures don't seem particularly prone to breaking. Another point you might consider, nearly all the welded steel barges constructed in Asian yards are welded with 60 series rods, and certainly in the Singapore yards, at least, the barges meet formal classification society standards.
But if you have old 7018 and worry about moisture content then either bake some rods, (you can do this in a kitchen stove, and run some tags or just throw it away (nothing more valuable then a welding rod you aren't sure of) and buy a new box.
Certainly the breaking force used in testing welds may be different then forces imposed in various parts of a trailer frame but the point in testing welds is to ensure that your welds are done correctly. Of course you select a rod to have similar properties to what you are welding but a properly done weld will actually be stronger then the parent metal so your worries should be about the type and thickness and size of the metal you select, not the welds.
Frankly, you don't know what you are doing and don't have the knowledge to figure out what you don't know. I really, really, recommend that you go find a trailer like the one you envision building, and sweet talk the owner into letting you make a complete construction drawing of it - right down to part numbers for the brakes and axles. Then build an exact copy.
Cheers,
John D. Slocomb (jdslocombatgmail)
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Oh, and I said earlier that to prevent the tubing from collapsing, I would insert black iron pipe through the hole.
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Ok, missed that. Yes, that would help.
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