steel strength vs size ?

[trailertrash]

I'm trying to build or rather have built a project out of 1" square tube steel that is pretty thick walled (1/8 " ?) now I'm ready to build the final model and was wondering where I can find info about using a bigger diameter size square tube to increase the welding surface without losing strength or increasing weight much if at all . I'm pretty sure the info is out there I just don't have a clue where. /forums/images/graemlins/confused.gif
Thank you

 

[hazmat]

You've come to the right place.

Do you have a picture or sketch of your creation? Lots of ways to optimize the strength to weight ratio.

TBN's resident Mechanical Engineer at your service /forums/images/graemlins/grin.gif

Got to go to bed now, describe your project in some better detail & I'm sure we'll be able to help you out.

 

[RonR]

Well, I'm not an eng (defer to hazmat), but I have welded a lot with 1" square tubing. If you are worried about weld joints at the corners, like a 90 degree joint, you can miter them at 45 degrees or even do a 90 degree joint and by welding around it , should have NEARLY the same strength as the other tubing.

If you are concerned about a butt joint welding it end to end, that might be more of a concern, but if there is the ability to add gussets to strengthen the joint, it should be fine. You could also insert some straps inside the tube and plug weld it to both sides of the joint, plus welding around the joint, to give it more strength.

The real question is what does it have to support. If it's a workbench or an implement stand, no problem. If it's a ground engaging implement, that might be risky.

It's worked well for me, your mileage may vary. Best wishes,

Ron

 

[Egon]

For larger stronger square tubing you may consider the type used for receiver hitches.

You could also go to a stell supplier and ask them.

Egon

 

[trailertrash]

I'll try to get a picture but in the meantime I'll try to explain this creation of mine. I run a lowboy trailer that we haul new trucks on, and with the new model trucks from IH there is no safe way to get in or out of them when they are in the air, they took the grab handles off and even the drip rails so we wind up being about 5 1/2 ft. in the air with no way up or down , gravity will get you down but thats painful ! There have been numerous attempts at ladders but none worked so I figured if you could'nt go down with a ladder go up and it seems like it will work , my prototype has been a sucess so far but I want to reduce the weight without losing the strength as it will need to support 350 lbs moving around on it to have any safety margin at all . The mount is simply a u shape made by welding 3 pieces of flat steel together , the t handle is a bolt that goes thru a nut welded over a hole in the top of the u bracket which is how it tightens on the lip, I hope this makes sense. The lip is part of the trailer and faces out , so what I do is slip the mount over the lip tighten the bolts and presto I've got a step, actually two of them the whole thing is around 30" tall which when added to the height of the trailer works very well and it is app.32" wide at the top . I can get the exact measurements tomorrow. When I started this project I used what I had laying around but now that I think it will work I'm ready to refine it to a finished product and let some of the other drivers try it and see what kind of input they have.

 

[rlw]

The formula for calculating bending stress is Mc/I. Where 'I' is the moment of inertia. Since 'I" is in the denominator of the equation, the larger it is, the lower the stress. Therefore, 'I' (moment of inertia) for a material's cross-section would be a good comparitor for sizing your material.
The formula for calculating 'I' is (b*h^3)/12. Since you have a square, its just b^4/12. This is for a solid square - since you are using tubing, you have to subtract out the inside.
So, you have: (b^4 - x^4)/12
where: b= outside dimension
x= inside dimension (b-2t), t=wall thickness

I've attached a sheet from a 'stock list' catalog with common square tubing sizes. It lists the wall thickness and weight per foot for several tubing sizes.

Now all you have to do is pick out some sizes, do the math, and compare 'I' values and weight/foot to pick your final material size.

Hope this helps and not confuses!

rlw

 

[RonR]

Wow, salute to rlw!

No offense meant by my smart **** comment. I'm sure you are correct, but I don't have clue what you described............. (grin).

Here's my response based on the description of the ladder project:

You'll never be able to stack enough fat guys on your step ladder at the same time (assuming it's like 24" wide and less than 10 ft tall and you have good weld joints) to cause 1" square tubing with .125 wall to fail.

I would more concerned with how it attached to the trailer so that doesn't fail or come undone and cause the ladder to crash down.

Disclaimer: I would still seek competent engineering approval for what you are doing, especially if you are having employees climb on this thing. Since most of the stuff I build will result only in MY injuries if it fails, I'm willing to risk more.

Best wishes,

Ron

 

[hazmat]

rlw answered for me! TBN has more than one resident engineer /forums/images/graemlins/grin.gif

your 1X1X1/8 has an I of .057 in^4. A 1.5 x 1.5 X 1/16 would be lighter with an I of .124. More than 2X stronger and the weight is slightly less. Also gives you 50% more weldable length.

As a point of reference, at my former job, we used 1.5X1.5 X1/16 steel tubing to fab roll cages for ambulances (vans with raised roofs). They have to statically (not moving) support 1.5X the van's weight = 11,000 #. It had about 8- 10 cross members, each about 60" long. Supported the weight with no problems.

Happy welding & send us a pic of the finished product (especially with the truck in the background).

 

[jmc]

Yeah but as your size increases, so does "c" in the numerator. So to compare two candidates in bending, compare bh^3 rather than bh^4.

As a practical matter, I would consider some 1 X 2, 2 1/2 , or 3 X 1/8 angle iron mounted tall side vertical. I'd guess its lower weight than an equivalent strength box and you can weld both sides. Also, some of the weight with a box is for strength in a direction you don't need it.

John

 

[yooperdave]

Structural steel tube (HSS) is hard to beat for many applications. It also tends to be one of the more expensive steels out there. A typical yield strength for hss tubing is 46 ksi while angle is lower grade material usually 36 ksi material.

Do a weight calculation prior to fabrication. The self weight of your project can get pretty heavy in a hurry.

I assume you will be using a wire feed welder due to the thinner wall thickness? You can still gets boatloads of capacity out of some 1/8" thick welds.

Good Luck

Yooper Dave