load bearing capacity of I beam and channel

[weisyboy]

gday

i am trying to work out if 2 lengths of Channel or I beam would be strong enough for my sawmill beam.

so here is what i have.

8m span supported 500mm from each end.

the actual load it will support is minimal, 200kg or so.

but im more worried about the resonance produced by the saw blade cutting, starting a bounce as it dose with the aluminum rails on the lucas.

i thought about using 2 rails with about 150mm between them.

the big question is would 300mm (12") x 150mm(6") x 10mm (3/8") channel be strong enough to not move/bounce.

would i beam be stronger than channel?

[DanG]

G'day Weisyboy.  If you're planning to use a single beam, the I-beam would be better.  I assume you're building a swinger, so you will have to be concerned about lateral forces as well as vertical.

We wrestle with the track bounce problem on the Mobile Dimension Saw too, and I've cogitated at length about ways to prevent it.  The actual weight bearing capacity is less of a concern than the stiffness of the beam.  MD recommends building what they call a "track rider shoe", which is just a temporary prop to place under the beam for extra support in the middle of the span.  It works well, but is inconvenient.  My thought is that if the saw could be suspended below the beam, rather than riding on top of it, you could add a truss to the topside to stiffen it up without adding a lot of weight.

[weisyboy]

i want to use 2 beams located close together, this fixed the probelm i was having with belt/shaft location and eliminated an idler shaft.

weight is not a problem, i would like to keep it below 2.5 ton but that should be easy.

[JohnG28]

In most cases the I beam will be stronger, but that could depend on the material that they are made out of, if one is different than the other.  The actual forces that the beam(s) will carry will is dependent on the material mostly.  If your talking steel I beams, with the dimensions you said, I can't see there being much problem with a 200 kg load, even when accounting for the added stress from the mill.  Even smaller I beams would work fine, and their shape lends them to being much stronger than a channel.

[Gary_C]

the big question is would 300mm (12") x 150mm(6") x 10mm (3/8") channel be strong enough to not move/bounce.

would i beam be stronger than channel?

There are no easy answers to your questions.

You would need to provide specific sizes for the I beam vs. Channel question but in general a channel is one half of an I beam that is split down the vertical rib so yes, the I beam is stronger. But it also depends on the weight per foot, the flange width, and the thickness of the flanges. That question is a relatively easy one to answer given all the sizes.

But the other question about the stiffness or bounce could be answered easily by saying NO, there is no beam that will not bounce. But it is a far more difficult to answer completely. There are so many things to consider to answer that question that even engineers can go wrong on that answer. It can involve engine speed and balance as well as motor mounts. Also type of rolling saw mounts, span lengths, harmonics, number of teeth in the saw blade and many other factors. In the end, the only way to get somewhat of an answer is by trial and error or building a scale model and testing your design.

One of the most famous "Oops" in engineering design was the Tacoma Narrows Bridge that was completed just before WWII. Here is a clip of what happened.



All the damage was caused just by the force of the wind blowing up Puget Sound. It turned out the bridge design was unstable in just the wind. It was plenty strong but the wind caused oscillations or standing waves in the structure and the oscillations grew ever larger till the disaster happened.

So as DanG said, It may be best to just put supports under the span to limit the bounce as much as possible.

[DanG]

Good post, Gary.  I certainly would never question your expertise in this area, but I will submit that the beam only needs to be stiff enough to resist the forces that will be applied to it.  What we are looking at is a relatively small circular blade, buried almost to the hub in a log, with a minimal number of teeth on the blade.  When the one tooth that is in the wood at a given instant is in its upward swing, it tends to pull the beam down.  When it exits the log, the pressure is released, only to be applied again by the following tooth, hence the bounce begins.  My thinking is that a truss, much like one would use on a roof, would provide enough stiffness to counteract these forces.  In order to do that, the saw would have to ride below the beam, rather than on top.  I'd like to hear your take on this theory.

[Gary_C]

Sorry but it makes no difference as far as stiffness or strength if the load is above or below the beam. The direction the load is applied and the point(s) where the load is located on the beam are important but not height. It could be 100 feet below or right on top, it's all the same.

Now having said that, the type of connection above or below could make a difference in the stability of the assembly. If the load was hung on a bunge cord 100 feet below the main beam, you could have severe bounce or stability problems. You could also have problems if the engine was mounted on top of the beam on rubber isolation mounts. So the type of connection to the beam could cause stability or bounce problems.

Of course stiffness and strength are two different terms with two meanings. Stiffness as described by the Modulus of Elasticity determines how far a material will deflect when you apply a load. And everything will deflect when you apply a load. Strength is a measure of the point where either the material will bend (yield strength) or break (ultimate tensile strength). In this case, I doubt you would have strength problems but you would sure want a stiff material so the assembly would not begin to bounce.

[DanG]

The point in putting the saw head below the beam is to allow a place to put the truss.  As my mill is set up, the engine, etc, rides along the top of the beam, leaving no place to put any bracing.  Mine is a space-frame type beam, 8"x10"x30'.  If the mechanics were slung below the beam, I could add a truss to stiffen things up.  I'm just asking if such a truss would add significant stiffness.

[Gary_C]

Oh sure, adding a truss would make the beam stiffer and reduce the bounce. And there are some other things you could do to reduce bounce. Things like stringing a cable in an arch thru or under the existing truss and tighten it up using an eye bolt on one end.

I can't seem to imagine your set up. Is your camera working? 

[weisyboy]

re: the I beam vs Channel, would 2 12" x 6" channels be stronger or weaker than 2 12" x 6" I beams.

i know this isnt something that is easy to work out, i have asked engineers but all have laughed. not wanting to help with my meager problem.

i would have 2 beams running parralell, with a peice of inverted angle iron attached to the top of each, the engine/mechanics would sit on a frame with V grove wheels that ran along these pieces of inverted angle, a frame would extend down between the 2 rails and have the saw blade sitting under that.

here is how it will be setup,


only with 2 narrower beams instead of 1 wide beam, doing this saves me a lot of headaches.

its the constant pulling force applied by the blade that i am worried abotu starting a bounce. the lucas has 2 - 8" x 3" x 1/8"thick aluminum box rails and they are not enough to stop the bounce, but im uping it a lot from there.

as you can see i cant have a support under the center it just wouldnt work and would get in the way.

trial and error is what i have been fearing with $1000 worth of steel and a lot of time attaching the angle and getting it straight.

[DanG]

The I-beams would definitely be stronger, but the channel should be strong enough, IMHO.  Of course, I'm not an engineer, so take that opinion for the two cents that it is worth.

The saving grace on the multi-blade saws is that edger that is always in play.  As you work across the log and develop a "shelf", the edger shaft acts as a damper and stops the bouncing, so it is only an issue on the first couple of boards.  You won't have that luxury with a swinger.

[JohnG28]

Gary, sounds like your an engineer.  Im studying civil engineering right now myself.  To the OP, if the most stiffness is what you are looking for then the double I, actually called W beams, is your best bet.

[Gary_C]

The I-beams would definitely be stronger, but the channel should be strong enough, IMHO.  Of course, I'm not an engineer, so take that opinion for the two cents that it is worth.

Who gave you two cents? 

Weisyboy I doubt the engineers did not want to help. For one thing they may not have been experienced in structural design. And for another your design is too complex and there are too many potential problems for any easy answer. But I will try to give you some ideas and point out some potential problems where I can. But I am not a structural design engineer either so my advice may be worth the same as DanG's.   

As to your question of I beam vs. channel, there are a few types of both and they are usually described by the depth of the member and the pounds per foot. So you really can't get top flange widths of 6 inches in either W Beams (wide flange beams) and especially not in C channels (structural). So that question really does not have a direct answer. And I hope you know that a W 12 x 26.0 lbs wide flange beam with a 6.49 inch wide flange and 8 meters long will weight 675 lbs.

So I think you are way over in the strength required, the weight is going to be unmanagable, and it probably will not be stiff enough to stop the bounce. And welding that angle to the top flange will warp that beam like a pretzel even with careful skip welding.

You need to start thinking stiffness as in light weight trusses rather than heavy steel beams. And your end supports are another thing you need to consider for stiffness. If your beams or trusses are just hanging on the end supports, they will add to the bounce factor. You need fixed end supports to stop the bounce.

Sorry for the negative report, but better now than later. 

[weisyboy]

weight is not an issue,

i can get an 1 Beam that is 301.8mm x 165.1mm with 6.1mm web and 10.5mm flange, a 8m length of this beam is 320kg, (700lbs.) so 600kg for both rails thats nothing i would still be happy with double that.

the other option was a steel web like some of the other mills have, but i have seen these in operation and they don't seam to work to well.

i have thought that if i use I beam and its not stiff enough, i could weld struts and angle stays in between the flanges.

the angle would be bolted to the I beam to avoid any warping.

[weisyboy]

end suports are fixed to the rail not just sitting on them, but they are fixed with VEE rollers as the beam has to move side to side with each cut.

[Stan snider]

Weisboy, I think you would want to consider boxed tubing. It is far stiffer pound for pound than beam or channel. Stan

[weisyboy]

i dont think i can get 300mm x 150mm RHS can I?

i thaught about using 2 150mm x 150mm and welding them along the middle, but i doubt that it will stay steright.

[Don_Papenburg]

If you wish to weld two members together  you need to tack the ends ,middle and then between the mid and ends.  start welding about one eight of the total distance from the end ,welding toward the end .  About two inches per bead. flip the weldment and weld the other side in the same place . then go to the other end and repeat the weld.  Move into the 1/4 spot and weld back to the end as before and repeat the sequence on each end till you reach te mid point  then go back to the first weld and start a new sequence of welds . Repeat till done  The thing is weld the same  amount on each side and each end  on every sequence .

I think that the double I beam would be your strongest setup .  If you would be fine with more weight why not go to a 14" beam ? the deeper the beam the less it will flex . Look at a couple of flatcars on the rail siding some time.  They have deep frames  and they don't bounce in the midsection with the abuse that the rail gives them

another way to eliminate warp is to make steel box spacers and then bolt the beams together say every four or five feet. through the upright portion.