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Ken_Shea
05-04-2009, 12:31 AM
Which of these do you feel would be the strongest. my suspect is the box but am looking for others opinions.

It is channel, with the box, welded at external overlap joints, the I beam style welded center top an bottom and bolted together.

What I am after is, which would yield the least amount of amount of vertical deflection under load. Twist, I don't believe will be an issue.

Each channel will be 1.590 x 4.320 x 12'

Thanks
Ken

http://i187.photobucket.com/albums/x175/Ken_Shea/Channel.jpg

doctor demo
05-04-2009, 01:11 AM
I'm not going to guess ,but what are You building ?
I have a old structural steel handbook at work that has various examples of shapes and loading, but I'm sure someone will beat Me to an answer.

Steve

Ken_Shea
05-04-2009, 01:17 AM
Steve,
I knew some one would ask and was hoping to leave that out of the equation :D

A trailer, this would be for the perimeter box using internal horizontal single channel bracing every 16".

Thanks
Ken

barts
05-04-2009, 01:23 AM
Which of these do you feel would be the strongest. my suspect is the box but am looking for others opinions.

It is channel, with the box, welded at external overlap joints, the I beam style welded center top an bottom and bolted together.

What I am after is, which would yield the least amount of amount of vertical deflection under load. Twist, I don't believe will be an issue.

Each channel will be 1.590 x 4.320 x 12'

Thanks
Ken

http://i187.photobucket.com/albums/x175/Ken_Shea/Channel.jpg


What matters here is the moment of inertia of each section. The two shapes have almost the same section; the box is very slightly higher as each section is offset very slightly from the center - but it's not going to make an appreciable difference.

The ease of fabrication, bolting, etc, will trump any slight strength advantages, though.

Edit - for a trailer, I'd use either a single piece of channel or rectangular tube; the rain/salt/etc will raise hob with built-up sections.

- Bart

dp
05-04-2009, 01:31 AM
The box is vulnerable to flexing along the weld and the unwelded surface inside is going to flex it's adhacent side.

One side of the I beam can be drilled and the two welded along the side in addition to the joint weld. It can also be gusseted diagonally or vertically as needed at stress points.

Adding fasteners to the I beam can happen anywhere while the box beam doesn't allow access to nuts/bolds buried deep inside the box unless you drill access holes.

Ken_Shea
05-04-2009, 01:34 AM
Bart,
Ease of fabrication is a good point I had not given much consideration.

Both would have the same amount of welding, the box would require no extra hardware or drilling and would be quicker to put the internal bracing on.

Ken

Ken_Shea
05-04-2009, 01:44 AM
Dennis,
Additionally, what I had though of was to drill every 12" or so on the box top and bottom of the overlap so as to join weld the internal areas.

Won't have any real need to add fasteners that I can quickly think off. The axles are the torsion type that do not require shocks or leaf springs.

Ken

Evan
05-04-2009, 03:36 AM
The box section is much stronger. As an H beam with simple supports at each end if a load is placed in the center the stress will be compression on the upper flanges and tension on the lower. The upper flange will be susceptible to the same type of failure that occurs in slender member buckling under compression loads. The edges of the flanges are unsupported and will fail by buckling along the edges when the resonant frequency of the portions under compression load approaches zero.

With the box section there are no unsupported edges to buckle. That is why you see a box section vehicle chassis on vehicles such as my Land Rover. It is designed to be as light as possible and the strongest section for a given amount of material is a closed section such as box or tube.

[added] You can achieve nearly the same strength at much lower weight by welding together two L shapes instead of two C channels.

sbmathias
05-04-2009, 12:20 PM
The type of the load needs to be known. In reality, it probably is a combination of both bending and torsional loads. As others have said, there is not a great deal of difference between the two for bending loads, until you get to Evan's well-taken point of buckling on the compression flange. However, for torsional load, the boxed section is superior by orders of magnitude.

Pure torsion produces pure shear on the surface of a cylindrical member, and nearly so on a rectangular closed section. The material itself resists this shear stress. However, in the I-beam configuration, there are no "sides" between the flanges to resist this shear action, so an I-beam twists with relative ease compared to a closed section.

Another consideration which may be important in some cases, although probably not here, is that of lateral deflection. When a beam takes a vertical load, it is likely to deflect to the side. Think of supporting a yardstick at both ends, but with it standing on its edge. Push down in the middle. It readily deflects sideways. This is the reason for much of the bracing you see on steel bridges - to minimize the unbraced length. The two shapes being discussed here exhibit much different lateral strengths as well, with the boxed section being much stronger laterally.


[added] You can achieve nearly the same strength at much lower weight by welding together two L shapes instead of two C channels.
I have to disagree with Evan on this. I think most here will agree that the bending resistance of either of these shapes (box or I) depends highly on the upper and lower extremities. Since the boxed C channels have twice the material at these extremities as boxed L shapes, the C boxed channels have a much greater bending resistance.

barts
05-04-2009, 12:36 PM
In most trailer designs, the sections are not appreciably torsionally loaded;
the yield strength of the members is often less important than the amount of
deflection under load, not unlike most of the wood loaded in bending in a house. In general, box sections make for stronger trailers, but are more work, since keeping water out of bolt holes, etc, makes for a lot more fabrication effort. Also, corrosion prevention is a much more difficult problem if parts of the trailer are welded such that water can get in but paint cannot....

- Bart

Evan
05-04-2009, 01:25 PM
Corrosion in a box section can indeed be a problem. That is why the chassis on my Land Rover was hot dip galvanized when made.

The corrosion issue can be mitigated by coating the inside of the box section after fabrication with high solids zinc bearing paint. This can be poured in and poured back out, a very simple process.

As for the box made with two L flanges I would expect it to have about 80% of the strength of one made from two channels, given the same web thickness. Standard channel iron has a diminishing flange thickness toward the edges and will present a gap to be filled when welded as a box. Unequal leg angle iron is available with constant thickness legs.

This is probably a moot point as I am guessing that Ken already has the material on hand. Rather than be concerned about which is the strongest I would work from the principle of which is strong enough and the lightest.

There is another option and that is to weld the C channels flange to flange making a box section that is wider than the overlapping flanges. That will be far stronger for the same weight than either of the proposed sections.

barts
05-04-2009, 04:23 PM
Corrosion in a box section can indeed be a problem. That is why the chassis on my Land Rover was hot dip galvanized when made.

The corrosion issue can be mitigated by coating the inside of the box section after fabrication with high solids zinc bearing paint. This can be poured in and poured back out, a very simple process.



To a point.. but once welded into a frame, getting paint everywhere inside is difficult w/o lifting equipment. If you coat it before final fabrication, you end up w/ the paint burned off inside wherever the welds are.... awkward.

HD galvanized is indeed the best; it's just hard to find a place that will do an entire trailer frame.

- Bart

Robin R
05-04-2009, 06:04 PM
If it were me, I think I would weld it flange to flange and make it a sealed box, that way once the oxygen inside is used up, no more rust. Most attachment points could be welded on lugs, but if something needed to be through bolted, it would require a piece of tube with the required diameter, to be welded into the through holes.

hardtail
05-04-2009, 07:14 PM
I would say it depends on the forces subjected and application it's intended for, you don't see many cranes or heavy equipment lowboys that aren't constructed of beam material..........works well for vertical suspended loads. I doubt drilling of either is necessary and maybe undesireable.

wierdscience
05-04-2009, 09:06 PM
The I-beam section would be better in torsion which a trailer will see a lot of.As mentioned corrosion is a major factor,even with paint or galvanize the overlapped seams will rust unless they are seal welded.

McNeillMachine
05-05-2009, 12:03 AM
I might be a bit of a neophyte with steel channel, but all the ones I've seen are slightly tapered on the inside faces of the "C". Wouldn't that be an issue when welding a box channel out of them?

Ken_Shea
05-05-2009, 01:49 AM
Lots of good information, and thanks to everyone for the input.

The flange to flange welded box design and coated inside seems a good way to go with what I have.

Ken

Paul Alciatore
05-05-2009, 02:48 AM
Great question. Get a few feet of smaller sized channel, perhaps 1/2" and weld it up both ways. You should try Evan's suggestion also (I like it). Load a one or two foot long section of each until failure: same length for each of course. Then you will know.

And post the answer.

Evan
05-05-2009, 03:16 AM
Make sure you tack weld starting at both ends on both sides, then the middle both sides, then half way between on both sides and ends, then half way between those etc. Otherwise you will end up with a banana.

I am sure you know that but I mention it for the benefit of others that may be reading here. I learned a lot of this type of thing the hard way before we had resources like this.

Paul, the load test won't scale correctly unless the length to width ratio and the web thickness is the same. For members longer than about 10 to 15 times the maximum distance to the fiber radially the failure mode will be a type of slender member buckling. That is when the structure suddenly becomes unstable and will twist around the centroid and fail. That is also why the flange to flange section is much stronger because of the much higher moment of inertia gained by moving the webs further away from the central axis.

oldtiffie
05-05-2009, 03:34 AM
I realise that you seem to have settled on using "L" and "C" sections.

Have you considered using "C" section roof purlin and wall girt sections as used in many steel commercial buildings.

They a pre-coated (galvanised or painted), are readily available here in OZ at least as I guess would be the case in the US.

There are a range of section sizes and material thicknesses available. They are readily cut with common shop tools and are very easily fabricated (MIG mostly). With the down-turned flange-ends they are very stiff. Cleats are easily welded to (the inside of the "C") for connection to your cross-members (can also be "C") and all surfaces are visible and readily accessible for post-fabrication (re)surfacing. They will drain easily on a trailer and are easily cleaned with fresh water. There are no hidden spots for corrosion to form or be concealed.

I daresay that similar or "special" sections can be formed locally in the US to suit your need.

Bolting can be done using hi-strength bolts which when tightened/torqued correctly give a very efficient hi-strength rigid "portal-frame" type joint/connection.

My next choice would be rectangular hollow section steel.

Check this catalogue/manual for "C" (and "Z") section girts and purlins.

http://www.edconsteel.com.au/pdfs/information/purlin_and_girt.pdf

Preparation for and use of galvanising:
http://www.edconsteel.com.au/information2.htm#galv

Structural steel:
http://www.edconsteel.com.au/catalogue.htm

Stainless steel:
http://www.edconsteel.com.au/stainless_stel.htm

Other catalogues etc:
http://www.edconsteel.com.au/information2.htm

Google:
http://www.google.com.au/search?hl=en&q=one+steel+catalogue&meta=&aq=8&oq=one+st

Ken_Shea
05-05-2009, 08:56 AM
As was alluded too, the choice of materials was availability and not necessarily because it is what is best, have had the the axles also for a while, this will make good use (and get rid) of stuff !, getting sick of tripping over it.

The channel is already painted, but I want to coat the inside to cover the welded areas.

Evan, that banana effect can be put to good use when applied properly. Glad to get the reminder though, in the haste to get something done those factors are often overlooked. My approach is usually to just dive in and do it, with out really engineering the task, at least to some degree, that's why I was asking for others opinions here, should help cut down on those "wish I had done it this way" feelings.

Thanks for all the metal links OT.


Ken