Question: Steel tubing advice- 2x2,3x2,4x2

Tue 7 Sep 2010 - 5:35 pm UTC

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ANSWERED on Wed 6 Jan 2010 - 8:00 pm UTC by redhoss

Question: Steel tubing advice- 2x2,3x2,4x2

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Asked by mbutler315 on Tue 5 Jan 2010 - 10:55 pm UTC:

Hi I need to build a steel frame on a heavy duty trailer and would some
advice on material selection. This frame/cage will measure 66"Lx 60"W x 24"
high. On this frame will rest a piece of equipment that is 2500lbs.  If you
think "table" this frame would resemble an 8 legged table with 4 legs along
each 66" side. (See diagrams below.) Final weight of the trailer is a
consideration. 2x2  3/16 wall tubing has been suggested along with 2x4 3/16
wall rectangular tubing. I have calculated the deflection of 2x2 w/ 2500lbs
on a single 60" piece at .5". How do I take into consideration how the
weight is dispersed over the design below? Any help would be appreciated.
Thanks-Mark

____________________           ________________________
                               [       [        [     [
l     l       l     l          [       [        [     [
l     l       l     l          [       [        [     [
l     l       l     l                                  
l     l       l     l              ^side view^
____________________                                    

   ^ Top View^

________________
[              [
[              [
[              [

  ^Rear View^

Uclue Researcher Comment by Researcher admin on Tue 5 Jan 2010 - 11:27 pm UTC:

Hi mbutler315,

I edited your ASCII diagram because it was messed up by the automatic
line-wrapping. Please check that I haven't accidentally altered anything.

Uclue Researcher Request for clarification by Researcher redhoss on Wed 6 Jan 2010 - 3:38 pm UTC:

Hi mbutler315, your calculation of .5" deflection is correct. You ask how
the weight is dispersed. How long and wide is the piece of equipment. Is
the 2,500# weight evenly distributed along the length and width of the
equipment or concentrated in one area. I know that .5" of deflection
doesn't sound like much, but if you use L/360 you get 0.17" allowable.
Also, this is with no safety factor. Consider what happens when the trailer
hits a bump in the road and the mass of the equipment is displaced.
As you probably already know, Ix for the 2x4 tube is 3.66 compared to 0.64
for the 2x2. Quite a difference. My son builds trailers and we often
discuss designs similar to this. Answer my questions and we will come up
with a design.

Question clarification by mbutler315 on Wed 6 Jan 2010 - 5:33 pm UTC:

Redhoss- The peice of equipment can best be described as round, with a 54"
diameter, with its mass evenly distributed. Thank you- Mark

Uclue Researcher Request for clarification by Researcher redhoss on Wed 6 Jan 2010 - 6:41 pm UTC:

Great. How long is it.

Question clarification by mbutler315 on Wed 6 Jan 2010 - 6:55 pm UTC:

Hi Redhoss- The frame I need to construct is 66" L X 60" W. The equipment
that will go on top is perfectly round. Its diameter is 54" and will sit
on the center of the 66x60. So there would be 3" on each side and 6" in
front and in back. Thank you- Mark

Uclue Researcher Answer by Researcher redhoss on Wed 6 Jan 2010 - 8:00 pm UTC:

Okay Mark, each of the cross members will carry 1,250#. To account for the
inertia loading I would suggest a safety factor (commonly known as the
idiot factor) of 3. This would give us a load of 3 x 1250 = 3,750#. Holding
the maximum deflection to 0.17 requires using the 2x4x3/16 tubing with a
deflection of 0.15. This is what I would tell my son. Please ask for a
clarification if there is anything you don't understand. Good luck with
your fab work, Redhoss.

Request for clarification by mbutler315 on Wed 6 Jan 2010 - 8:48 pm UTC:

Thank you Redhoss. Would it be possible to increase the # of crossmembers
to 3 and add 2 lengthwise cross members to be able to use 2x2? The top
would then resemble a grid 4 "boxes" long by 3 "boxes" wide. So instead of
2 cross members it would have 5.  I guess my original question should have
asked for a design that would use 2x2.If I have to pay additional I will do
so gladly. Thanks Again- Mark

Comment by User myoarin on Wed 6 Jan 2010 - 10:14 pm UTC:

What's giving support against inertia:  braking, acceleration, sharp turns?

Uclue Researcher Answer clarification by Researcher redhoss on Thu 7 Jan 2010 - 4:38 pm UTC:

Sorry about the misunderstanding. Is the 2x2x3/16 a weight issue or do you
have the 2x2 already laying in your shop. If you haven't purchased the 2x2
and it is just a weight consideration, 4x2x1/8 would be a much better
choice.

Uclue Researcher Comment by Researcher redhoss on Thu 7 Jan 2010 - 4:44 pm UTC:

Hey myoarin, always good to hear from you. The inertia of the 2,500#
equipment does cause additional loads on the trailer frame. As you say this
happens during braking, acceleration and sharp turns. It also happens when
you hit a bump in the road. That is the one I would be most concerned with.
Depending on speed and severity of the bump, it can be a significant load.

Comment by User myoarin on Thu 7 Jan 2010 - 5:06 pm UTC:

Hi Redhoss,  

Thanks.  You're the expert (sought webwide).  I was thinking about diagonal
bracing, assuming that bumps would be just putting additional strain on the
vertical strength of the uprights, not so much stressing the joints.

Don't bother to tell me that my second guessing is off target  - has been
before.

Myo.

Request for clarification by mbutler315 on Thu 7 Jan 2010 - 6:32 pm UTC:

Redhoss- After a sleepness night of pondering I decided to use the 4x2. I
assume that if I were to add 2 additional cross members(4 total) under the
load that the deflection would be half of that if only 2 are used?
With regard to myoarins comment above, is there an advantage of diagonals
vs gussets?
Thanks again- Mark

Uclue Researcher Answer clarification by Researcher redhoss on Thu 7 Jan 2010 - 6:57 pm UTC:

The formula for your load application (concentrated load at mid-span) is:

D(deflection) = P(load) xl^3/48EI (you probably already know this since you
did a calculation on your own)

So, any factor you adjust the load by gives a proportional change in
deflection. Half the load means half the deflection. It certainly doesn't
hurt a thing to add diagonals or gussets. Both will make the frame more
rigid and not add too much weight. The more rigid the frame the less the
welds are stressed and my son doesn't like to repair cracks in welds(I will
bet that you don't either). I am glad that you decided to use the 4x2.
Sorry if I caused you to lose sleep.

Uclue Researcher Comment by Researcher redhoss on Thu 7 Jan 2010 - 7:01 pm UTC:

myoarin, thanks for your comments. I am not an expert at anything. I am
just a poor old retired engineer that trys to help people keep from killing
themselves or anyone else. People should only be killed in anger.

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