Square tube vs I beam for trailer floor

[davejo]

I'm replacing a rusted out trailer floor, originally constructed with square and rectangular tubing. Crossmembers are 2x2x.180" 28 inch centers. 6 feet long

I have a good pile of tiny galvanized I beams 1.75x1.875x.25" taken from a fencing demolition job.

What issues do I need to consider in deciding if this is a good or bad idea to save about $300 by reusing these i beams?

 

[bigguns69]

If you are looking for same strength or better, you will want to compare the bending moment of inertia value, known as "Ix" between the two cross sections and make sure that the I-beam you have, has a value equal to or greater then the square tube. For the square tube you have listed the Ix and Iy values are the same at .668 in^4, since the tube is symmetrical. The smallest S type I-beam i can find listed is a 3x5.7 lb/ft with a Ix=1.68. Your will be smaller, probably about half of the 3" value which is larger then the tube. To be right you would have to compute this.

I would go with it and add a few more cross members.

 

[cmandp]

I would factor in the hassle of removing the galvanizing well away from where you are going to weld too. I assume you know the fumes from welding zinc coated steel are poisonous, but always worth mentioning.

 

[tlspeed1]

Besides the galvanizing, I would go with the I Beam. I Beam and Channel is the only way to build trailers. Tubes trap moisture and they rust as you have found out.

 

[koditten]

Use the I-beams. It works good. Weld outside if you can to avoid the fumes. Breathing carbon steel welding fumes isn't good for you either. Use common sense and mind the wind direction.

 

[Farmall450]

If you are looking for same strength or better, you will want to compare the bending moment of inertia value, known as "Ix" between the two cross sections and make sure that the I-beam you have, has a value equal to or greater then the square tube. For the square tube you have listed the Ix and Iy values are the same at .668 in^4, since the tube is symmetrical. The smallest S type I-beam i can find listed is a 3x5.7 lb/ft with a Ix=1.68. Your will be smaller, probably about half of the 3" value which is larger then the tube. To be right you would have to compute this.

I would go with it and add a few more cross members.

Gotta love the engineering toolbox, although they had Ix as 2.52

Agreed though, worth calculating his if he knows what he's got.

 

[davejo]

If I'm unable to find a book value Ix for my beams is there a way to calculate or just compare it with redneck engineering?

Place x pounds of weight in the middle of a beam of y length and measure how many z inches the beam deflects? I could compare equal lengths of my rusted tube to the tiny I beams in a shade tree, head to head shootout.

 

[BD1]

Welding galvanized with stick is the best. 6010 5P would be my joice. Remove what galvanizing you can and the 6010 will burn right through it as long as it not thick thick. You'll get lots of white smoke too so be ready. Do a test sample and see what you think.

 

[bigguns69]

davejo, apologize here for nerding out on you. Here is a link that you can watch to learn how to manually calculate moments of inertias for shapes. This is standard Mechanical engineering statics problem. I could figure it out for you but would need to know every dimension of the cross section of your beam and recognize that for beams and channels, the flanges generally have taper, so average width for the material thickness would be needed.

Alternate method would be to do what you suggested by comparing deflection by putting a known weight on the center of , square tube and then your i-beam, over a defined length, to get a warm fuzzy of which is stronger.

Welding galvanized aint no joke. Did a side job for someone years ago using galvanized. I could have done a better job of keeping the fumes away from me. I got sick from that event. Kind of like the flu.....

 

[theoldwizard1]

You need to "run the numbers" based on the video above. My gut says the I-beam will have a lot less "stiffness" than the 2x2 because it is 1.875" tall. The last few minutes of the video are worth watching (how to add strength) just by themselves.

Also remember the only purpose of the center section of an I-beam is to hold the top and bottom flange fixed in relationships to one another. This is why metal trusses used for flat roofs only have small vars connecting the top to the bottom on triangles.

I did this game on some aluminum and the results were was going to need a extremely tall I-beam that was basically not available.

 

[Farmall450]

You need to "run the numbers" based on the video above. My gut says the I-beam will have a lot less "stiffness" than the 2x2 because it is 1.875" tall. The last few minutes of the video are worth watching (how to add strength) just by themselves.

Also remember the only purpose of the center section of an I-beam is to hold the top and bottom flange fixed in relationships to one another. This is why metal trusses used for flat roofs only have small vars connecting the top to the bottom on triangles.

I did this game on some aluminum and the results were was going to need a extremely tall I-beam that was basically not available.

Well, the web is largely for shear forces -- not just a spacer like you envision a truss. I suppose I see where you're coming from, but think of a 2x4 flipped on end...it still significantly contributes to resisting bending, even if not as much as the fixed ones above/below.

 

[davejo]

Flange is 3/8 heel tapering to 3/16 toe so I averaged to 1/4 thickness.

Web is 1/8 thick.

Using the method in the interesting video I get Ix of .609 for the little I beams

 

[bigguns69]

If you are dead set on using the i-beam you have, I would add a few more beams to your trailer deck and move on.

 

[Farmall450]

Flange is 3/8 heel tapering to 3/16 toe so I averaged to 1/4 thickness.

Web is 1/8 thick.

Using the method in the interesting video I get Ix of .609 for the little I beams

Yeah, so you're going to want 9% more of them -- but you already knew that. Agreed with above poster.

 

[Bretny]

Box steel is a very poor choice for trailers. You can never paint the inside and you can never seal the ends enough. C channel or other such steel is a good choice.

 

[Chilliwack Murray]

Box steel is a very poor choice for trailers. You can never paint the inside and you can never seal the ends enough. C channel or other such steel is a good choice.

I’ll second that. Almost every failed trailer frame I’ve ever seen was tube. Channel frame trailers last almost forever unless they are left in the weeds or damaged in some way.

If you do use tube, be sure to drill some drain holes in a few non structural points so it can drain and even be flushed out if it’s been in the salt.

 

[ddawg16]

Lots of valid points above.

When you say 6', I"m assuming that will be how wide it is?

If so, I think it will work well with lots of cross bracing. Wood or metal floor?

The I-beams will not be as easy to attach a wood floor to. You will have to hit the I-beam on either side. If metal....then just weld.

Lots of cross bracing.....

What are you going to carry? If a tractor or some vehicle....most of the loading will be towards the sides.