Loads for Trailer Design
Loads for Trailer Design
(OP)
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.
Any help would be greatly appreicated.






RE: Loads for Trailer Design
RE: Loads for Trailer Design
rmw
RE: Loads for Trailer Design
For your total rig size, make sure you stay in the DOT limits of height, weight per wheel/axle, width, and length. Tax fees for escort vehicles, police permits, and bridge/rail/tunnel.town permits are nasty and take a while to get. So, permitting costs for over-sized trailers are a nightmare, and the permit delays of over-sized stuff adds to your client's overall costs.
Plan your skids so you build each by itself (what is his/your crane limits for weight and height-of-lift?) and then can put the loads on the trailer directly. A skid-mounted system will allow you more flexibility once you get to a non-standard jobsite and have to take everything off of the trailer to use it. I know you don't expect to run the gear that way, but it will happen.
RE: Loads for Trailer Design
The reason more detail is sought is that generators pose a particular problem in trailer mounting. They are very heavy concentrated loads and depending on the size, could require some VERY specialized equipment. (or not.)
We would need to know much more about your application before determining whether the previous post answers your equipment (standard-off the lot-equipment) or specialized.
rmw
RE: Loads for Trailer Design
The trailer we are being asked to reivew is 27 feet long, 6 feet wide, and constructed of 10 gage and 1/4" A1011 steel. We are not being asked to design the axles or wheels, just the frame of the trailer. It is not an option to use another trailer.
My basic question is are there any governing design codes, or standards that apply to the design of trailers. Can the AISC Steel Code and ASCE 7 be used to design the trailer? Does AASHTO have any design requirements?
The trailer we are designing is to support a 2,000 # enclosure, a 7,200 # generator, 4,100 # of fuel, and the trailer weight is 7,000 #. These are loads are distributed uniformly along the sides of the trailer.
Hope this helps to clarify.
RE: Loads for Trailer Design
However, you shall check the framing (using AISC manual) for different load arrangement, and check with trailer manufacturer for axial capacity. I would spread the loads using skids as mentioned above. Also, I would apply max impact factor unless you can positively control the driving speed.
RE: Loads for Trailer Design
36 Kips per axle???
Wouldn't that be 36 Kips per tandem?
The max that I am aware is allowed on any single axle is 20 Kips, but that is at a required minimum spacing.
And... how sudden was the stop that you had in mind DHK? Are you referring to a slam on the binders and screech the tires type stop, or a 'hits the freight train' type stop?
rmw
RE: Loads for Trailer Design
RE: Loads for Trailer Design
RE: Loads for Trailer Design
I guess I was referring to static loading, not live loads.
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Actually, I like this link better.
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rmw
RE: Loads for Trailer Design
No problem whatsoever.
I was responding to his 2nd post asking AASHTO. I think AASHTO truck load includes weight of both the truck and the freight, though it is considered live load in civil design. However, you were correct in pointing out that the design capacity of truck mechanism should be much lower than the AASHTO, which has some (unknown) safety factor built-in. We were on different tracks on this, but no conflict.
RE: Loads for Trailer Design
Your questions, line of questioning, and the info. given here as a basis for our comment, does not bode well for a good trailer design. The AISC Steel Code and a few good texts on strength of materials, steel design and welding should be a good starting point for steel design in general, but they don't really contemplate mobile equipment directly. I don't think you will find any load criteria for mobile equipment (a trailer) in ASCE 7, and maybe you shouldn't be taking on this project if you ask that question.
With a 20.3kip gross vehicle weight this will most likely be a tandem axle trailer. You have many choices for this type of running gear, you pick a package to fit your needs, and that will dictate a good part of your design. Their equip. should meet AASHTO bridge loading conditions and axle spacing, per their spec. sheet. Of course, this axle set can be overloaded, you design the trailer and locate the loads to prevent this. These kinds of loads are rarely uniform, in fact.
In your first post you say you are asked to design the trailer, and then in your next post you say you are going to review a very specific sized trailer. So, which is it, are you designing a trailer for these loads, or trying to put these loads on an existing trailer? A trailer constructed of 10 gage and 1/4" A1011 steel may not be the best choice for these types of concentrated loadings. I'm not sure I know what A1011 steel is, do you? You must design the frame or study the existing frame to take these fairly discrete loads at a limited number of load points. It's one thing to haul 4000# of sand or brick, a fairly uniform loading; and quite another to secure a high C.G. 7200# generator or a sloshing fuel tank to a few discrete cross members on the trailer frame. That's the real design problem. If you are starting from scratch, forget the 10ga., 1/4" and A1011 and use what you need for the job.
I've designed and built a lot of railcars, trailers and load handling equipment up to 700-800 ton capacity, much of this for the electrical and nuclear industries. I grant that these are exceptional loads and transport equipment. However, while your loads are pretty small, I have found that trying to fit special equip. to a trailer not designed for that equip. can be more work and more costly than building to the real needs, particularly when this sounds like it is intended as a dedicated trailer. Alternatively, at least, selecting a trailer better suited to accept your loads. Seems to me, your first task is to determine if an existing trailer is even fit for these loads and advise the client accordingly. To do a half baked job on a piece of dedicated equipment will make no one happy in the long run.
Good Luck
RE: Loads for Trailer Design
This paper suggests using the following static load cases:
1) Vertical load of 1g +/- 1.7g
2) Vertical load of 1g +/- 0.5g lateral load.
3) Longitudinal load of 0.65g
The paper also suggests using the following cases for fatigue analysis:
1) Vertical load of 1.15g +/- 0.625g
2) Lateral load of 0.5g +/- 0.25g
3) Longitudinal load of 0.25g
Any thouhgts?
RE: Loads for Trailer Design
RE: Loads for Trailer Design
you've got applied load factors, you've got (or can define) tie-down points on the trailer. remember to add in the sizeable tension load from the straps (chains?) holding the load down, then the trailer is "just" a beam.
how balanced to you assume the load to be ? ie, how much load on the hitch ? (i'm know it should be zero, but is it ever ?)
RE: Loads for Trailer Design
Vertical loads? An acrobatic aircraft is designed for 6g's which I think is too high for your trailer. Some sports cars are designed for 3g loads on their susbended parts and this might be a good starting point.
But what will you apply your g loadings to? Yield?
RE: Loads for Trailer Design
The biggest single issue is potholes.
On the minus side, you have no real suspension.
On the plus side you have big tires.
Cheers
Greg Locock
I rarely exceed 1.79 x 10^12 furlongs per fortnight
RE: Loads for Trailer Design
You might have a good handle on the actual loads to be carried, but the controlling design conditions are going to be the maximum bump you can tolerate, or the fatigue due to bumps in general. Ideally, those load conditions would be dictated in some industry or government standard. Are they? If you design a trailer for 1.8 times the dead weight, and someone runs it over a curb and it collapses, are you in the clear in your design?
RE: Loads for Trailer Design
I also recall a 1g lat/long load for securing cargo from somewhere...
RE: Loads for Trailer Design
RE: Loads for Trailer Design
Attached is the paper.
RE: Loads for Trailer Design
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.
RE: Loads for Trailer Design
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.