5 Jan 2010 22:55 UTCTue 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^
6 Jan 2010 15:38 UTCWed 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.
6 Jan 2010 17:33 UTCWed 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
6 Jan 2010 18:41 UTCWed 6 Jan 2010 - 6:41 pm UTC
Great. How long is it.
6 Jan 2010 18:55 UTCWed 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
6 Jan 2010 20:00 UTCWed 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.
6 Jan 2010 20:48 UTCWed 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
7 Jan 2010 16:38 UTCThu 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.
7 Jan 2010 18:32 UTCThu 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
7 Jan 2010 18:57 UTCThu 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.