[DEW]

I was wondering what is the best material to use for a beam in the horizontal position, supported at both ends. Deflection wise is an i beam stronger than heavy wall square or rectangle tubing? I'm trying to find out pound for pound whats the best for the least amount of money, like everyone else
Thanks,
Doug

[InspirationTool]

You can get what you need to calculate it here:

http://www.efunda.com/math/areas/Com...tion_Index.cfm

You are looking for the best moment of interia for the money.

You'll want to use a spreadsheet.

-Jeff

[lakeside]

here the I-beam setup I used. Almost have my new playroom under house

[DareBee]

As far as structural steel goes, the engineers calculate strength by the pounds/foot of the structural member. Having said that it is the vertical web that creates the load bearing strength. Ie using heavy flat bar the "flat" way is baaaad.
I very strongly recomend that you check with tlocal building codes. Your snow load factor is likely different than mine is.

[lakeside]

Before I started my little digging project. I had an engineer over to look. Based on his math I would have had to use 3 19 1/4 LVL beams per selection to span 20' at a cost of $205/section. The cost of the steel at the time was $215/section.He suggested 12" I-beam 8' apart to replace carrier beams that where rotted. If I rebuild in future the middle beam comes out and I can span new building with 2 x 12 15'

[JerryFlyGuy]

There are several ways of looking at structural design. Usually us hobby type people look mainly at the cost of the project. This of-course translates into weight as we know steel is sold based usually on weight. If we sit down and look at the bare numbers typically for the same weight the beam will have more strength, however beam isn't w/out its little vice's either and the bottom line really depends on the application. One good rule of thumb is that if the beam can EVER be placed under some type of torsion or twisting load, use HSS [Hollow structural sections or tubing] If its purely a static application, beam can usually work. Beam are usually more work in making connections, usually it requires bolt up clips/plates/angles as has been shown in the pic's on this thread. Tubing can be just butted and welded. Both can be easily D & T'd, however the lighter walled tubing's might not lend themselves to this as well as the heavyer sizes.

I'd like to clarify a bit what DareBee said. If you are looking at two HSS sections of material, both are the same weight but one is rectangular [tall and skinny] and one is sqr, the tall/skinny piece will usually be stronger over the sqr pc of the same weight, but only when orientated so that the load is imparted on the skinny face, parrallel to the major length/side. ie; If the tube[when viewed from the end] is tall and skinny, and the load is imparted from above [or below] the member will take more load than if that same part was loaded from the side. The example of the flatbar is also a good illustration. A typical I-beam is another good example, you can see why the beam would be stronger when loaded on the flanges, across the depth of the beam, vs from the side where it would bow fairly easily..

Another option is to build a beam, from HSS members in a trussed style. This is actually how I'm building my system, its 20' long and has about 1/2 the weight of a standard beam of the same strength, it has taken alot more work to build however.

my 2 cents..

Jerry [hope I've helped and not confused the issue.. ]

[Geof]

I was wondering what is the best material to use for a beam in the horizontal position, supported at both ends. Deflection wise is an i beam stronger than heavy wall square or rectangle tubing? I'm trying to find out pound for pound whats the best for the least amount of money, like everyone else Thanks, Doug

On a pound for pound basis the I beam is going to deflect the least, rectangular tube with the longest dimension vertical is next and square tube will deflect much more. As Inspiration Tool says you are looking for the best value of I the moment of inertia. An I-beam is designed to give the best I by having a larger proportion of the cross-sectional area further away from the centerline of the beam. Rectangular tube with the longest dimension vertical is next and square is worst because the I for regular sections depends on the third power of the vertical dimension.

On a dollar for dollar basis if you get some dimensions and prices for I beams and rectangular tube do the calculations and compare I per dollar you might find rectangular tube is lower cost.

Whatever you discover it is essential that if you are using the beam to support a structure or for lifting something you need to get a professional engineering opinion.

[miljnor]

ignore everything else and the math is all about how much cross section in the direction of the load. Period!

I beams are designd for most of the load against the tall single part of the I. The more complex the shape the more complex the math. But it all boils down to cross section under the load.

[Geof]

ignore everything else and the math is all about how much cross section in the direction of the load. Period!

I beams are designd for most of the load against the tall single part of the I. The more complex the shape the more complex the math. But it all boils down to cross section under the load.

I do not like to disagree with such a vehement opinion but in my humble opinion and phrasing it as respectfully as possible it is only half right. The stiffness of a beam depends on the product of the cross section and the distance, in the direction of the load, of each element of the cross section from the neutral axis.

[DEW]

My thinking was that an I beam has 1 web and the rectangle tubing has 2-one on each side. So the tubing would be stronger. This application is for the rail/beam on a bridge saw. Approx. 14' long. supported on both ends. I don't want hardly any deflection in the z axis but more importantly none in the y axis. It has to cut in a straight line period. Again, it seems as if the rectangle tubing would be stronger, my thoughts.
Regards,
Doug

[DareBee]

Considering the beam of a saw will see potential twisting load a tube is your best choice - heavy (it would have saved a lot of people a lot of typing if you had divulged this application up front - me I don't like to type and keep my answers short and over-simplistic in most cases anyway :-) )

[Geof]

My thinking was that an I beam has 1 web and the rectangle tubing has 2-one on each side. So the tubing would be stronger. This application is for the rail/beam on a bridge saw. Approx. 14' long. supported on both ends. I don't want hardly any deflection in the z axis but more importantly none in the y axis. It has to cut in a straight line period. Again, it seems as if the rectangle tubing would be stronger, my thoughts.
Regards,
Doug

I will second DareBee; if you give more detail you can get a more focussed answer. For this application you are probably better to go with a trussed structure as JerryFlyGuy suggests. Part of the reason I make this suggestion is that the amount of deflection in the Y direction depends on the spacing of your guide rails and the distance from the guide rails to the point where the load is applied. Closely spaced rails are not suitable and the only way you can get them well separated is with a tall beam, i.e., large rectangular tube; which is heavy. A trussed structure is more work but much less weight.

[DEW]

My apologies for the inconvenience. I started out thinking about this topic as a general one covering rails, bridges, and base for the rails. I think all of the answers apply but they get more detailed with the saw bridge and deflections under load. Hence my last reply.
Thanks,
Doug

[JerryFlyGuy]

Geof, thanks for jumping in here.. I always like to read your well written explanations

Miljnor, according to your explanation, a sqr solid bar of the same cross-sectional area as a beam, would have the same strength? If only this was true, imagine all the pain of dealing w/ complex shapes that could be elliminated! The cross sectional area has some bearing, esp in tension and compression loads, however in a beam calculation where your trying to elliminate as much flexture as possible [ keep it stiff] the cross sectional area in realation to the depth of section and/or it centroid is where the strength is, I can built a beam from 1/8" steel that is far stronger than a 4" I-beam if given the time and willingness to go the mile for complexity.. it would use a fair bit less mat'l, weigh less and be stiffer.. all w/ less x-sectional area than the beam. Take a pc of paper.. and you can flex it fairly easily.. take that same pc of paper and fold it into a long tube like box.. and it will be far stiffer.. and would in theory 'bare more weight' even if it's so small its hard to measure..

If one wants to learn more about how all this works.. go to your local hardware store and for a few dollars buy some yardsticks. Then using them.. make several shapes.. a I beam, a tube [sqr and a rect.] and maybe a channel.. make sure you compare apples to apples in that each pc must have the same x-sectional area [ number of yard stick's as viewed by the end profile] and see which is stronger, which twists easier than others.. if you really want to get scientific.. use a 10lb weight and measure deflections.. etc.. it would be a great little learning excercise.. You'll be amazed by what you think would make something stronger.. and what actually does make things stronger..

DEW, the vertical web's would seem to make things stronger [I also thought this when I first started into mechanical design] however if we added mat'l to the side of a tube and measured its stiffness and then took the same amount of mat'l and added it to the top and bottom, you'll find a significant increase in stiffness over the tube w/ the mat'l on the sides, I've actually done this to a beam which had too much deflection, we added a fb top and bottom to the beam and the deflection was less than 1/4 of what it had been previously. The web's actually only carry the shear load between the top and bottom flanges. Typically a beam's web is 1/2 of less of the thickness of the flange
on a beam. Where as, a tube has a uniform wall thickness and "wastes" some of this thickness where it doesn't need it.

As DareBee has already stated.. in your case the tube is better, not because it is stronger than the beam [in simple term's], but because it will resist the twisting moment added to the structure while the saw is in operation, your still going to have to spec a tube which will take the calculated loads of the saw and deliver the needed stiffness.

Another thought to concider is tolerances. The structure is going to deflect, period. If a fly lands on a 6ft cubed block of steel, the block deflects, infact.. if you really want to get down to the nitty gritty.. the changes in atmosphereic pressure deflect the block.. its not measureable.. but its there.. You have to decided what is the allowable tolerance for your system and then design accordingly.. if 0.0001" deflection at full load isn't acceptable.. then make changes to get your structure to the point where it has reached your tolerance. You have to sit down and quantify what those tolerance's are, before you ever start. Calling for a unit that cuts a straight line doesn't give you that tolerance. Saying that you want a machine which can cut a straight line w/ less than 0.001" deviation from center, in 10" is a tolerance.. Then the big hurdle is building to arrive at your decided tolerance.

Jerry [Sorry DareBee.. I tend to get windy... ]

[miljnor]

Miljnor, according to your explanation, a square solid bar of the same cross-sectional area as a beam, would have the same strength? If only this was true, imagine all the pain of dealing w/ complex shapes that could be eliminated! The cross sectional area has some bearing, especially in tension and compression loads, however in a beam calculation where your trying to eliminate as much flexure as possible [keep it stiff] the cross sectional area in relation to the depth of section and/or it centroid is where the strength is, I can built a beam from 1/8" steel that is far stronger than a 4" I-beam if given the time and willingness to go the mile for complexity.. it would use a fair bit less mat'l, weigh less and be stiffer.. all w/ less x-sectional area than the beam. Take a pc of paper.. and you can flex it fairly easily.. take that same pc of paper and fold it into a long tube like box.. and it will be far stiffer.. and would in theory 'bare more weight' even if it's so small its hard to measure..

Stiffness is not strength and no matter how you make it the Material properties won't change (unless of course you laminate/alloy them with something else, but that is a different argument.) I agree with your statement you can make something stiffer with a complex shape, but that gives you a trade off of fragility. if you make a honeycomb structure and one part of it fails the structure tends to fail very rapidly afterwards.

Were in a solid square piece of material, you have a toughness equal to its material composition and a stiffness of its cross-section. The more complex the shape (with the same weight) you will get more stiffness at the trade off of more easily damaged structure.

I do not like to disagree with such a vehement opinion but in my humble opinion and phrasing it as respectfully as possible it is only half right. The stiffness of a beam depends on the product of the cross section and the distance, in the direction of the load, of each element of the cross section from the neutral axis.

That’s pretty well what I meant by direction of the load but I am not so eloquent with words.

I made the mistake of over simplifying to the point of abstraction!

It's all good! :cheers:

Ps: I wasn't all the vehemet anyway!

[Geof]

....if you really want to get down to the nitty gritty.. the changes in atmosphereic pressure deflect the block.. its not measureable....]

I know it seems like biting the hand that compliments you, to mix a metaphor, but I find it necessary to insert a minor correction. Atmospheric pressure works on all sides so it does not exert a net load on the beam.

I do have to agree though you do get windy . I may have to hone my typing skills to keep up.

[miljnor]

its not measureable.. but its there..

My wife tells me that about heaven and god all the time!

A professor that I new that was into practical application more than theory allways told me: "a differnce that makes no difference is no difference"

[JerryFlyGuy]

Atmospheric pressure works on all sides so it does not exert a net load on the beam..

True true... but!.. the atmoshpereic pressure is acting to compress the entire block, making it smaller in all dimensions.. [not to mention the effects of its own weight...Then you could take into account the effects of thermal expansion and differental heating.. oh.. we could make it complicated

I do have to agree though you do get windy . I may have to hone my typing skills to keep up.

Baaaa... I get it from my father.. I can honestly say.. I come by it honestly.. my mother was a school teacher.. a stickler for details.. so this also has an impact.

Miljor, your correct in both accounts, however your talking in philosophy.. I'm talking in physic's btw.. thanks for correcting me on stiffness and strength.. I often exchange the two [always in error] when trying to keep my fingers up to the speed at which my brain is spewing out junk...

Jerry [trying to be short and sweet.. at 6'3 and 220# ]

[Geof]

Stiffness is not strength
Ps: I wasn't all the vehemet anyway!

Okay I concede the Ps and you are correct about stiffness is not strength although a very good principle to follow in designing almost anything is: If it is stiff enough to perform its function correctly it is almost certainly strong enough. One time when this does not work is what you referred to; a stiff but fragile structure if you accept that fragile implies it is suceptible to a buckling mode failure.

[miljnor]

There is almost always a trade off between stiffness, weight and fragility.

from a practical point of veiw "big and strong you can't go wrong!" So the math becomes irrelevant. But if your designing an aircraft or rocket ect.... then "light and frail because the pilot can always bail"

I beams can usually be found cheaply if your not in a hurry you just have to start looking around at structural steel places for "take offs"

I didn't finish engineering school so I asked my partner (who has a masters in it and is older than me ) and he says that for load bearing the I beam is the master (of course he always prefaces it with the big and strong coment.) and if your using it for a crane it also has niffty place to put the wheels of your roller!

but what do I know I am after all a college drop out! I don't know nuttin!

but I do like a good tussle! (flame2)