I beam vs sq. or rectangle tubing

[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]

Originally Posted by 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

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]

Originally Posted by 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.

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]

Originally Posted by 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

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.