Any ME's out there?

[skid mark]

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]

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]

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]

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]

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]

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]

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]

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!

 

[skid mark]

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]

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.

 

[ManofMystery]

Re: Any ME\'s out there?

The CAD drawing of your proposed trusses looks just like the ones in the workshop I am building. I purchased mine as a complete kit (less lumber) from Miracle Steel. They use angle iron for the chords, and solid rod for the web.

You might give them a call -- their stuff is inexpensive, well-engineered, and they can supply all the right hardware.

 

[skid mark]

Re: Any ME\'s out there?

Hey ManOfMystery-

Question for you. What size is the angle iron for the chords and what are the dimensions of your trusses? How far will they be spaced apart? Do your trusses have a "double" chord with two members, or only one? Can you measure or do you know the thickness of the angle iron?

I know that the 1" tubing I'm using is pretty small, but I plan to stick on the safe side of things by spacing my trusses much closer together than is common with Perka and MiracleTruss which will space trusses up to twenty feet apart. That's really something that they'll support that much roof area. A thirty foot wide building with trusses twenty feet apart has each truss supporting (30x20) 600 sq feet of roof. At a conservative 20lbs per square foot, that's 12,000 lbs on each truss! Amazing.

I'm playing with some modeling software which will calculate the loads on trusses. I could easily change the members of my truss to whatever size angle iron you are using, adjust the loads for your spacing and run the numbers. This would allow me what kind of compression loads per sq inch they are running and I'll stay below that level.

If necessary, I'll put up six or eight trusses instead of the three that is common with the pro's.

Thanks for the help!

Mark

 

[Egon]

Re: Any ME\'s out there?

Roof design isn't about maximum truss spacing. All that means is more purlin.

There is a sorta economical meeting of the two which the Engineer has to find.

Egon

 

[Villengineer]

Re: Any ME\'s out there?

I'm not trying to be a stick in the mud, but you need to be really carefull with FEA and modeling programs. While the equations and programs are not beyond anyone who can do math or figure out software, they are limited to proper application and the info put into them. All it takes is one constraint or loading to be wrong and all the data you get is wrong. We check all our computer simulations against experience and even other calculation methods. There simply isn't a substitute for experience and training. I'm really don't want to be a killjoy, I just don't want you to put a whole lot of work and $ into this project only to have it turn out poorly, or even worse fail and hurt someone.

 

[skid mark]

Re: Any ME\'s out there?

Hi Egon-

I'm don't understand.

No matter if I go with MiracleTruss, Perka or a home-brewed truss, I'll use the exact same girts and purlins - 2x6's 24'OC for the roof and 2x4's 16'OC for the walls.

I'd like to use wood for this purpose because the insulation and decking options are better, IMO.

Thanks.

Mark

 

[skid mark]

Re: Any ME\'s out there?

Hey Villengineer-

You are right RE the FEA models. I understand that any incorrect assumptions and/or errors could drastically throw off the results.

I'd like to have more personal experience in this area, but, well, I'm working on it. I built a 1400sqft storage barn two years ago using lightweight trusses (of a different design) that were designed by an ME (Structural) in town.

That said, I'm not exactly blazing new trails here. I'd like to stray from the fewer/bigger truss design that Perka, etc use to have lighter, easier to handle trusses. The pro's of course want to use the minimal amount of materials and labor to build the trusses and erect a building. I don't necessarily share that goal. To me, building the trusses and putting them up by myself is great fun. I want to do as much of the work as possible by myself. But if I can't convince myself that the trusses I'm using will work out, it'd be an option to build them using the same materials, etc as the big guys. I'm keeping my options open for now.

Thanks for the advice! Keep it coming!

Mark

 

[Villengineer]

Re: Any ME\'s out there?

I would suggest discussing this with the structural guy who designed your other trusses. Since you've already used his services, he might be willing to help guide a little. He will definitely know the allowable stress levels for your area. I doubt that he will be willing to help on the design itself for free, but he might be willing to help you with the parameters. Another option would be to discuss him designing the trusses using your material and desired constraints. I would at least attempt to "bounce" it off him. He might be intrigued and help you quite a bit.

 

[Egon]

Re: Any ME\'s out there?

Mark:

Uh- egg on my face! Misread your dimensions and became a little concerned.

My appologies for wasting your time.

Egon

 

[skid mark]

Re: Any ME\'s out there?

Villengineer-

I'm thinking about paying him a visit. He designed the trusses that I produced last time. His company makes lots of metal buildings using these same trusses. Left unsaid in my previous note was how I "consulted" with him then. I did so by taking a tape measure, a camera and a note pad to a couple of the buildings he built and closely examing them so that I could carbon copy his design.

Is this a bad thing? Since the building I made was for my own use and I'm not in competition with him, I have no problem with it. In fact, I assume that "reverse engineering" as such is commonly done even within competing companies and is just the way things go. You tell me, can you patent or somehow secure the rights to the design of a metal truss? Food for thought I guess...

But he's probably the area expert in these lightweight trusses. If he'd take the time to sit down with me to talk trusses, I'd be happy to pay for his time.

Thanks again.

BTW: Egon - no problem!

 

[Villengineer]

Re: Any ME\'s out there?

You are correct in that reverse engineering is a very common occurance. Many companies purchase competitors models and disassemble them to see how they accomplished various tasks. The problem is that unless your application is identical to the one you are using for reference you can't just copy and apply. That's why companies use engineers to do this and not just guys who can measure and record. Additionally, most companies don't want to simply offer the same, they usually want to improve on the design to gain an edge.
To answer your question on patents, I doubt that most companies would attempt to pursue patents on most trusses unless it was radically different than anything that's been done before. Very few trusses fall into that catagory and patents are $, time consuming, and usually delay the release of the product. I would imagine that only a few truss companies have the resources to attempt this process as often you devote the resources only to find out that it's not patentable.