arched or cambered trailer frames 3

[Tmoose]

We were having a discussion today about the purpose of cambered main rails on trailers that haul around heavy equipment.
One fellow felt that the beam was stiffer as a result of the camber.
I thought it would have little or no effect on stiffnes unless a tension member went from end to end. So the arch is to end up with a flat floor or good ground clearance.

 

[rmw]

First off, the frame rail goes from one end of the trailer to the other, so it does go end to end, and the bottom flange is in tension under load.

Now, my qualifier. I am not a structural engineer, and maybe this should be posted in a fourm where they live, but the arched trailer is done on purpose to enable the beams to carry the load properly.

It is magical to see an arched trailer being loaded with with a point load, steel coils come to mind as one of best of examples, and the frame rails flatten out, only to return to their original arch upon having the load removed.

Now the structural guys will have to explain all the statics and stress and strain that goes with all that.

As I remember it, the bottom flange of the frame rail goes into a heck of a tension, and as your friend says, gets stiffer.

Same for heavy haul trailers, etc. It has to do with the load carrying characteristics of the trailer, and nothing to do with ground clearance.

In theory, a properly designed, properly loaded trailer frame will be perfectly horizontal, and the ground clearance will be equal all around the trailer.

If you are going to make the case that an "un-arched" trailer would 'sag' in the middle, and hence reduce ground clearance, I guess it could be made, but I don't think that is the case at all, based on experience with trailer manufacturers.

rmw

 

[ivymike]

If you are going to make the case that an "un-arched" trailer would 'sag' in the middle, and hence reduce ground clearance, I guess it could be made, but I don't think that is the case at all, based on experience with trailer manufacturers.

Oh really? 'cause that's exactly the case I was going to make. Ground clearance doesn't sound like as much of a concern to me as a flat deck, though. A flat deck "looks right", whereas a sagging deck looks overloaded. If you made the trailer flatten out as it approached design load, then a sagging trailer would be overloaded, and more easily spotted. Then again, if I remember my vehicle stability course, a distributed load is worse for trailer stability (something to do with comparing J to m*a*b), and a trailer will sag less under a distributed load.

Back in school we went over beam bending, and I don't recall the stiffness varying significantly vs deflection, as you suggest above ("...gets stiffer").

 

[MintJulep]

Quite probably because railroad flat cars have always been build with positive camber.

Structurally I don't think there is any real need to design for positive camber when empty.

 

[zeusfaber]

You really see the benefit of prestress when you're dealing with a very low deck low loader.

This time last year, I watched a trailer (which had started out with 3 or 4 inches of ground clearance) sag right onto the vehicle park as a new type of load was craned onto it. There followed an afternoon of (somebody else's) clever work with a gas torch to reshape the beam and adapt it to the new load.

Even if you don't ground out, heavy trailers which sag are far more likely to attract unwelcome attention from people with awkward questions to ask about axle weights.

A.

 

[MikeHalloran]

I'm guessing that the camber is provided so that a person with no engineering skills needs no measurements to decide when the trailer has pretty much reached its design load; the deck gets flat.

The beam doesn't get stiffer from the camber. The ground clearance argument has some validity, especially for lowboys, but there's no structural reason that the trailer couldn't start out flat when unloaded. But if that were so, truckers and loaders and load enforcement people would waste endless hours arguing about how much negative camber was acceptable for a given trailer with a given load.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[NormPeterson]

Support provided to a large, rigid flat-bottomed load is better and deck unit loading is more uniform if the deck is flat under load. I suspect that full-length contact between load and deck has something to do with friction damping of the spring (trailer beam) - mass (load) system as well.

Norm

 

[SomeYahoo]

I have a hard time believing that the sole reason is asthethics. I have seen the work that it takes to custom fabricate one of these arched beams, and you could make a trailer a heck of a lot cheaper with straight beams.

 

[robinhood4x4]

Maybe the bottom half of the beams are actually preloaded to compression so that when a load is applied the bottom goes from compression to tension? Doing this would lower the stresses on the top and bottom surfaces of the beams when placed under load. This could be done if the beams are 2 piece with the top section arched and stress relieved and then welded to a flat bottom section.

 

[TheTick]

Since this post firstr came up, I have been trying to remember the exact reasons for this. As I remember, it is to minimize the strain (and resulting stress). With an arched bed loaded to near-level, the top half is under compression and the bottom half is under tension. If the bed had gone from unloaded and level to level and sagging under the same load, the tension strain on the bottom of the bed would be greater.

 

[rmw]

I wish some structural eng'r would come over here from the structural forum and answer this. I think TheTick has come the closest, but I want to add two more things to the conversation.

One is, not only do you have the static loading of the trailer beams, there is the dynamic loading. Back to my 50,000 lb single steel coil loaded amidships on an arched trailer that brings it from its fully arched position to a level position, but then think about what this pair of beams is undergoing as this rig bounces down the road at 70 mph on the kind of roads that I have to use.

Now all of a sudden the stress and strin of this beam under load takes on some real intersting characteristics.

Secondly, ever notice that the ultra long concrete bridge beams that can be seen from time to time being trucked down the road are arched??

rmw

 

[GregLocock]

To be honest I think the Tick's explanation is awful (sorry mate).

Using simple beam theory the stress and strain in simple bending is proportional to the distance of the fibre from the neutral axis of the section. The large scale curvature of the beam does not affect this. If you don't like beam theory then replace the beam by a simple warren truss, and you'll see the same effect, in a manner that can be calculated by a mechie (grin).

Having said that, I have no better alternative except: If you had a long stiff heavy load, and placed it on a flat beam, it would only touch at the ends.

If the beam is curved then it initially contacts in the middle, and then as the beam bends the contact load will be distributed to some extent along the length of the beam or load.

I don't particularly like this explanation, it seems to rely too much on perfect matching of the load and the stiffness of the beam.







Cheers

Greg Locock

 

[flamby]

thread507-58876

You may find this useful.

Regards.

 

[dbuzz]

Pre-cambering of beams (and trusses) is for deflection control. You would need a large amount of camber for a beam to behave as an arch!

 

[flamby]

And most importantly the end conditions - pinned or fixed.

 

[jhardy1]

I think GregLocock is petty close to the money:

A bit of camber in a beam does not make it stronger or stiffer, unless there is sufficient "arching" to generate arch behaviour AND you have some means of preventing the ends from spreading, such as a tension tie. Neither condition applies to a cambered trailer rail, so it will deflect just as much as if it was dead flat.

A trailer body which hogs slightly in the middle looks OK. A trailer body with 50 mm of sag at midspan looks overloaded. Try and convince the police and other road users that your trailer is not overloaded when they can see it sagging!

If you put a long "rigid" load onto a precambered trailer, it will force the trailer to deflect down until the whole load is more or less uniformly spread over the length of the trailer. If the trailer is initially flat or sagged, the load will probably be totally concentrated at the ends, which will be stiffer than the midspan.

Sometimes, this is exactly what is intended. Check out a typical container trailer. Containers are designed to carry all their loads by the corner blocks, not by distributing the load along their length. The container sits on the trailer at its 4 corners only, with clear daylight between the bottom of the container and the top of the trailer. The trailer body retains its camber even when fully loaded in this case, because all of the loads go straight down though the corner blocks into the chassis and suspension, and none goes to the trailer rails.

 

[Ron]

I, too, think Greg has pretty much nailed it. It is primarily an aesthetic issue...with deflection being the key. If you pre-camber a member to an expected deflection, you can reduce the member size (thus weight), if deflection is the primary controlling factor.

IvyMike makes a good point about the "notice" factor....if it looks overloaded, it will draw suspicion, thus the authorities are more likely to take a look....keep in mind that overloaded trailers are a source of income to the government...they charge for overweight vehicles (handsomely in some cases) because of the "impact" that additional load has on the life of a pavement structure.

 

[DTGT2002]

I'll agree with Ron, and restate the obvious.

Trailer beams are cambered to save money.

There are many factors to be considered in designing a structural member. Quite often, the smallest member capable of supporting the load is not adequate due to reasons of serviceability i.e. too much deflection, obvious movement, etc.

I am not a designer of semi trailers, but if I could use a smaller member with added camber, versus a larger and heavier member, I'd go with the lighter, and spend the bonus money I got for allowing a higher load rating on the trailer due to lower self weight on a nice little vacation.

SO, having camber in a truck trailer potentially allows for a smaller member to be used without seeing the negative visual effects of deflection, and thus allows a light trailer weight and larger allowable shipping loads for a given gross vehicle weight.

The extra cost for cambering the beam is made up by the difference in beam size, and more so by the larger allowable payload of the trailer.

How about that?

Daniel Toon

 

[doejohn]

More agreement with recent posts (and with original post by Tmoose): it's a deflection thing.

Regarding some details raised in earlier posts:

Trailer beam resists load like a "beam", not an arch; besides the camber being too small for arch action, there's no mechanism to resist horizontal arch thrust.

Camber of beam doesn't increase stiffness, so relative deflection from unloaded condition isn't different between cambered and uncambered beams. But benefits from deflecting from the unloaded cambered shape include the "notice factor", and perhaps low trailer clearance during suspension compression over bumps--both mentioned in previous posts.

Cambered concrete beams mentioned earlier similar to trailer beams (load-induced deflection superimposed on pre-deflected shape), but different too: Camber in concrete beams achieved by prestressing eccentric tendons in beam to introduce net compression on unloaded beam, causing more favorable (to concrete) stress distribution under load. This is neither helpful nor feasible for steel beams.

 

[JAE]

Another structural input - just to confirm a few points that have been posed above:

1. Agree that this isn't an arch issue.
2. Agree that cambering doesn't do anything to reduce stress or deflection....just changes where the deflection starts.
3. Agree SOMEWHAT with the "notice the sag" point, but I think the following is another consideration:

The types of loads placed on the flatbeds are sometimes quite long, extending the full length of the trailer. If the load is fairly long, and the trailer frame is not cambered, then the load will cause a downward (smile-shaped) curve whereby the ENDS of the loaded element will become point loads, and the loaded element would be required to span from end to end, placing stress and bending in the load.

With a cambered beam, as the load is applied, the beam becomes MORE straight, not LESS straight, and there would be a tendency for the load to be more uniformly supported. Shims can always be used so this may not be a totally valid point, but it seems right.

Also, with a uniformly loaded frame, as the trailer goes over bumps and swales, I would think that a more uniformly loaded frame would not be as susceptible to cyclic excitation where the deflections would be magnified from free swings in deflections....again - I don't have any special knowledge in this - but it also seems to make sense.