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DHKpeWI (Structural) (OP)
29 Dec 09 17:33
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.

 
cntw1953 (Civil/Environmental)
29 Dec 09 18:55
If he wants to move these items from one location to the other, why not look into the readily available low bed, extra wide trailer?
rmw (Mechanical)
29 Dec 09 21:17
Some of us work with generators that you could almost drive a truck and trailer through.  Just the rotor would be a sizeable load for a very specialized heavy haul trailer.  How about giving some particulars about your application so we can have a starting point.

rmw
racookpe1978 (Nuclear)
30 Dec 09 0:07
I'd recommend buying a "dumb trailer" and then plan on mounting your equipment and skids on the trailer: don't try to re-design the wheel(s) when you can buy a perfect good flatbed, lowboy, or specialized rig from the lot.  Complete with real wheels, DOT cert's, brakes, standard brake and trailer light hookups, standard sizes of brakes, tires, axles, and fifth wheel heights too, license plate holders, and lights.


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.   
rmw (Mechanical)
30 Dec 09 9:25
Take a look at the link in one of the replies in this thread.  thread404-258498: King pin disconnection from trailer  

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
Helpful Member!  DHKpeWI (Structural) (OP)
30 Dec 09 11:00
Hopefully this additional information will help clarify my question.

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.
cntw1953 (Civil/Environmental)
30 Dec 09 11:23
The AASHTO standard truck load (HS-20) per axile is 36 Kips, you are ok on that aspect.

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.  
rmw (Mechanical)
30 Dec 09 15:37
CNTW,

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
cntw1953 (Civil/Environmental)
30 Dec 09 15:57
Yes, if my memory is correct, 36 kips per axile, 18 kips per wheel, 6' (8'?) standard truck width (wheel to wheel). These information should be on AASHTO website. But, I have no clue on how it relates to design of trucks, which shall have its own safety factor before cited for bridge/roadway design use.
cntw1953 (Civil/Environmental)
30 Dec 09 17:00
Ok, I have to admit getting a bit older, it is 32kips/single axile, 16kips/wheel. See attached doc.
rmw (Mechanical)
30 Dec 09 18:15
Sorry,

I guess I was referring to static loading, not live loads.

http://ops.fhwa.dot.gov/freight/sw/brdgcalc/calc_page.htm

Actually, I like this link better.

http://ops.fhwa.dot.gov/freight/publications/brdg_frm_wghts/index.htm


rmw
cntw1953 (Civil/Environmental)
30 Dec 09 19:24
rmw:

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.

 
Helpful Member!  dhengr (Structural)
31 Dec 09 14:07
DHKpeWI:

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
DHKpeWI (Structural) (OP)
4 Jan 10 12:03
I found the following suggestions for load cases to use for trailer design.  These pages were taken from a paper on the use of high strength alloys in 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?

 
DHKpeWI (Structural) (OP)
4 Jan 10 12:19
Ignore attachment to last post
rb1957 (Aerospace)
4 Jan 10 13:11
sounds like you've answered your own question ...

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 ?)
Helpful Member!  Bobber1 (Structural)
4 Jan 10 17:17
Your lateral loads will come from tire friction which generally won't be more than about 0.75g. under braking or lateral but not both simultaneously.  A sports car can generate up to 1+g lateral but I don't think you've got a sports trailer there.

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?
GregLocock (Automotive)
4 Jan 10 18:46
Those loads are very low compared with a car suspension, which in the old days was 3g vertical 2g long, 1g lateral. That is ok for a circuit car, substantially more would be better for a production car.

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

JStephen (Mechanical)
5 Jan 10 0:36
Once upon a time, I got involved to a lesser extent on a similar problem, with a similar lack of information.  If you design, say, a building, you have building codes of various kinds that specify the design loads and specify the allowable stresses to be used to carry those loads.  The question is, is there a comparable specification for trailers?  Or does each manufacturer just rely on their own experience to make their own designs?

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?
Bribyk (Mechanical)
5 Jan 10 1:11
Surely there's an SAE Standard for trailer design.

I also recall a 1g lat/long load for securing cargo from somewhere...
Helpful Member!  tigermoth (Mechanical)
5 Jan 10 4:04
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 (Structural) (OP)
5 Jan 10 10:35
dhengr (Structural)
5 Jan 10 16:28
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 (Mechanical)
7 Jan 10 15:03
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.

 

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