Any ME's out there?

phantom309 · Jan 30, 2004

Any ME\'s out there?

Howdy. Am in the process of building a new workshop. I'm currently finishing up the design of some lightweight trusses to use for roof rafters. It's my goal to keep them at about 200lbs or so so that I can lift them into place with a boom pole and not have to rent any equipment.

For the nerdy fun of it, I thought I'd try to calculate the load these trusses can carry versus my normal "overbuild everything and cross your fingers approach".

I blew the dust off of an old Statics & Dynamics text and started to perform a Finite Element Analysis on the nodes to calculate the forces and ultimately the stresses on the members of the trusses. What a pain! I ended up moving the calculations to a spreadsheet which helped some and also found a great tool on the web to help with this to verify my calculations.

So far so good.

The trusses are going to be made out of 1" 14ga tubing. 14ga is 0.083in thick. So the four sides of the tubing gives 4x0.083 = .33 in^2. So about a third of a square inch.

I've read that mild steel has a strength of about 55,000 lbs per square inch. So my 14ga tubing might, in theory support a load of 55,000/3 = 18,000lbs. But I'm guessing that that is the load at which it will fail. To provide a margin of safety, I'm wondering what would be a conservative tension load to apply to 1" 14ga tubing. My gut says I should be OK at half of the 18,000lbs. So 9,000lbs in tension. Does this sound OK?

What about a compresssion load? I'm more in the dark here.

I've hit the web looking for technical data on the tubing and everything points to the Hollow Structural Sections Connection Manual by AISC which I don't have.

I think that I can take care of the calculations, but just wanted some target numbers for maximum tension and compression loads and would also like to bounce some ideas RE welding the joints, bracing, etc to make sure that I'm headed in the right direction.

In the end, I'll put the trusses fairly close together to reduce any live loads, but I'd like to have some numbers to back up my plans.

Thanks in advance for any help!

Mark

Deerlope · Jan 30, 2004

Re: Any ME\'s out there?

Your math looks good but your profile tells me nothing. Don't know what area that you live in but you would be better served to ckeck the snow load if any and the wind sheer in that area.That is how most buildings are designed.

jmc · Jan 30, 2004

Re: Any ME\'s out there?

phantom309,

So far, your analysis has ignored the span and the heigth of the truss. Trusses don't fail in pure tension or compression. You need to account for bending and shear. You also need some help with an appropriate safety factor to anticipate weakening from welds and the possibility of your brother-in-law hoisting his engine from a truss while you're on vacation. You're right to get help but it ought to be professional.

John

dummy · Jan 30, 2004

Re: Any ME\'s out there?

Is there a way to eliminate the goof factor? /forums/images/graemlins/confused.gif On the serious side, Engineering this would be a structural issue moreso than mechanincal though, wouldn't it?

Villengineer · Jan 30, 2004

Re: Any ME\'s out there?

I'm an ME, but I can't help you a whole lot b/c you need to reference your local codes and permit office. They will require specific features no matter what the actual necessary structure needs to be. Additionally, most codes require trusses to be certified by a PE registered in your state or region. FYI, the AISC manual won't be of any help as 3x3 @ 3/16W is the smallest they give column data on.

Junkman · Jan 30, 2004

Re: Any ME\'s out there?

I thought that you were looking for a Medical Examiner /forums/images/graemlins/confused.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/grin.gif

jmc · Jan 30, 2004

Re: Any ME\'s out there?

Dummy,

Mechanical, Structural, and Civil Engineers are often lucky in the sense that they can run the numbers, find the appropriate size material, come up with a little extra money to buy the next bigger size, and get an extra 200% strength in the process. But my heart goes out to Aeronautical Engineers and phantom309 who have to worry about weight as well. They know how to sweat.

John

dummy · Jan 30, 2004

Re: Any ME\'s out there?

Meant to say that on the forum to look under structural engineering. (once more, not eliminating the goof factor) No matter which discipline, schmoozing the bean counters is the pits!

phantom309 · Jan 31, 2004

Re: Any ME\'s out there?

Hi Villengineer-

No permits/codes here. We're welcome to build any piece of junk that we can dream up! I just don't want it to fall down on my head.

I've since heard that I should try to stay below 17Ksi in tension. So with my 1/3 of a square inch tubing, I'm planning to try to stay under 5Ksi on the members in my trusses in tension.

I'm now just looking for a rule of thumb to guide me RE the compression values.

I'll offer a little more detail on the trusses if it will help.

The trusses use continuous 1" tubing for both the top and bottom chords, both of which are double. So two tubes up top and two below, with a single tube in between to form the web. Check out the screenshot from the CAD program I'm using here. Obviously, this truss is half of the full span of 36'. The roof is at a 2.66/12 slope (4' rise in 18').

For better insulation/decking, I'm using 2x4s for wall girts and 2x6's for roof purlins. Although you can't see the dimensions in the picture, the vertical members in the web are 24" OC (and the horizontal ones in the web of the walls are 16" OC) to support the girts/purlins so this gives you a pretty good idea how the trusses are shaping up.

Thanks for any help you can give!

Mark

Villengineer · Feb 2, 2004

Re: Any ME\'s out there?

To be honest with you I don't what your safety factors (determine allowable stress) should be. I know what we use for equipment, but I'm pretty sure that structures use different ones because they have to account for a lot more loading requirements than we do.(ie snow, wind, etc..) I'm just not comfortable trying to guide you through this process as this isn't my area of expertiece. However, my opinion is that your tubing isn't really suited to this application. The moment of inertia is really small and you're probably going to have problems with bending stress on the lengths you're talking about. Additionally, due to the span you're probably going to have some welds that aren't ideal from a stress standpoint. I know that having a bunch of material makes it tempting to try this, but I would suggest that either buying pre-engineered trusses or having a structural engineer design your trusses would be the way to go. Oh yea, if you continue to go on your own, just so you calculate your stress correctly the area of your tubing is less than 0.304in^2 due to the corner radiuses. Sorry, I can't, with good conscience, be more helpfull.