Loads for Trailer Design 4

[DHKpeWI]

A client wants us to design a trailer to pull a generator, fuel tank, and enclosure. Are any references for trailer design? Specific questions: what percent increase of load should be used to account for impact loads? What about the impact of a sudden stop on the frame?

Any help would be greatly appreicated.

 

[tigermoth]

DHKpeWI - For these static loads, I assume you would let the stresses go up to the allowables stated in general structural design codes eg roughly 2/3 yield for bending (tension side). Do I understand you to say that the max static download on the chassis would be 2.7g? Seems high - we generally use 1.75 - 2.0g for trailers on air suspension. (Although manufacturers of hi-tensile truck chassis often specify a SF of 4.5 - 5.0 on yield for the vertical static case, this presumably really covers a multitude of other loadings including torsion etc). For welded high-tensile steel chassis, fatigue will dominate anyway. Would be interested in seeing that paper.

 

[DHKpeWI]

tigermoth:

Attached is the paper.

 

[dhengr]

DHKpeWI:

That looks like a very good paper, well worth your studying. I have just quickly perused it, at this point, but it seems to include the important points in the design and manufacture process. It generally conforms to the approach I have taken to these kinds of problems. In figs. 23 & 25 they certainly found the areas that I would look at first, as potentially being problem areas. Again, you must have your strength of materials, theory of elasticity, structural design and analysis, welding design, fatigue and the like, well-in-hand; and AISC and the like are good guides to general steel design, but you should look for industry standards, etc. for design criteria. If WI indicates a mid western state, I’d look a little harder for US trailer manufacturer industry standards.

It sounds like tigermoth might actually be in the over-the-road trailer design and manufacturing industry, and I would certainly defer to his knowledge and experience if that be the case. I would agree with him that 2.7g on the total load sounds high. My experience has been in the 1.7 to 2 range and I call those load factors (forget the g) and would as likely as not design to yield. But then, take great care in my detailing and welding as they relate to fatigue. Once tigermoth has seen the paper I would be interested in his interpretation of how they arrived at the load cases in this paper, particularly load cases M1 & M5, pgs. 31 & 32. After the advice I’ve given above, maybe I should dig out my own structural dynamics and vibrations books.

I’m still not sure if you are designing a trailer from scratch, or making your equipment fit on an existing trailer. Is this a dedicated trailer for your client’s long term needs? 10ga. and 1/4" sounds a bit light to me, it may not be over stressed on day one, but will it rust for ten years and still carry the load. If this is a dedicated trailer for company use, job to job, it may see fewer potholes and miles per year than an over-the-road trailer. This might alter your fatigue and load factor thinking a bit, and light weight (vs. payload) may not be as important as durability on the constructions site. I’m not at all sure that hitch load should ever be zero and you must look to the hitch design and hauling truck used for max. hitch load.

 

[tigermoth]

Very interesting paper. A vertical static load factor of 2.7 does seem high. Not clear from the paper what their "allowable" stresses are - looks like 300-350MPa for steel which has 645 MPa yield - ie allowable = 0.55 x yield, giving a total safety factor on yield of 4.9 for this load case? Always some ambiguity about what to do with high localised static stresses which show up in FEA - probably safe to let these go up to yield, depending on mesh refinement level.
The 2.7g may have originated from Dangerous Goods tankwagon codes, which tend to specify high load factors.
The Queensland (Australia) guidelines for trailer chassis design specify a total safety factor of 3.0 on yield. I wouldn't go below this. But remember that fatigue and deflection may be more important than the static vertical case.