Any ME's out there?

[Villengineer]

Re: Any ME\'s out there?

After reading through this here are some thoughts I had:
1) Make sure you stress calcs are for welded connections in your truss. Many truss formulas are based on pinned connections. This will probably change your results. Especially since you now will have shear forces to deal with in your welds/tube walls.
2) I see some issues with welded joints that you should address. With such a thin wall your effective weld is very limited. Additionally, do you have sections of tubing long enough to make half span? If not, you're going to have some butt welds that you will have to make sure are strong enough.
3) Since your truss is at an angle remember that the total deflection is the resultant vector of both your DeltaX and DeltaY.
4) If you do a physical test on a short section you will have to remember that while it's true that the deflection varies linearly with load, for a simple uniformly loaded beam supported at both ends, it varies by the length to the 4th power at the point of max deflection (half the length). Just throwing a short section together and loading it may not really tell you anything.
Again, I'm no expert in trusses, just some things I see as a guy who designs welded structures. I'd rely on your SE friend as much as you can.

 

[skid mark]

Re: Any ME\'s out there?

Hey Rick!

I was 'noodling this today and was kind of wondering the same thing. So that's great to hear that you think that this test would be of some value.

One idea that I had that would be easy (relatively) and safe to test would be to use two flat trusses spaced just far enough apart (a little over five feet which is the spacing I'll use in the bldg) to allow me to park one of my old Willys Jeeps in between. The trusses could be just a couple inches above the concrete, perhaps on top of a 2x4. Jack up the Willys a few inches, place a support between the trusses (on top of the lower chords), beneath the tires and lower the Willys onto the supports. Repeat at the other end. So the weight of the (2500lb) Jeep would be supported by the truss.

This is different from the real world in that the load would be concentrated at the bottom of the truss instead of at the top and would be focused on two locations instead of spread throughout. As far as the last problem, according to my SE, the only way a roof in this area will see 20lb/ft of (snow) load is by some extremely lucky drifting so maybe that part is OK. I can place the forces in a new model at the same places and compare the actual versus modeled results.

The advantages of this test are that if something fails, the load will drop four inches instead of 5 feet and I won't have to load the truss by hand. If you're agreeing that this will help confirm the modeling I've done, this sounds like a good idea that isn't toooo much work!

</font><font color="blue" class="small">( Sorry, I was assuming that anybody on TBN had a tractor to use to move dirt. )</font>

Yeah, darn it. I get along just fine until I visit this web site and then I definitely get FEL envy! I have a decent older 45hp tractor, but no FEL. /forums/images/graemlins/frown.gif I've thought that another nerdy problem that will present me with more learning opportunities would be to build a FEL for my tractor! Maybe one of these days...

On the subject of trusses, testing, etc: I'm an instructor at the local college and teach Engineering and Math. Once per year we sponsor a competition for kids from the area schools to make little bridges out of 1/8" balsa wood sticks. Then they bring them to a competition where we load up their bridges between two fixed supports with a block down the middle that supports a bucket underneath. We slowly fill the bucket with sand (and sometimes steel bars) until the bridges fail. Then we weigh the bucket. Highest load/bridgeweight wins.

The bridges are limited in weight to 60grams. So 60g x (1lb/453g) = 0.13lbs. The winning bridge always tops out by supporting over 100lbs. That's ~770 times the bridges weight!

Heck, if my trusses do that well, that's 184,000lbs each. I've got it made... /forums/images/graemlins/wink.gif

Thanks again Rick for your help!

 

[skid mark]

Re: Any ME\'s out there?

Hi Villengineer!

Thanks for the great input!

RE #1: Welded connections: I have done that. Switched the model from a "true truss" with pinned connections to a frame with rigid/moment connections.

RE #2: half span tubing: I do have. The half spans are ~18ft each.

RE #3: "resultant vector of both your DeltaX and DeltaY". Good point. This gives a slightly larger deflection.

RE #4: I think that I follow. Are you saying that for a given truss, if you double the load at the point of max deflection, you'll double the deflection. But if you take the same load and double the length of the truss, the deflection will be multiplied by a factor of (2^4) = 16?

So try to stick with as long a truss as possible for the test or the deflections will be (had better be!) too small to measure...

Thanks a lot for the info!

 

[Villengineer]

Re: Any ME\'s out there?

phantom:
RE#2: Remember that your max length for the half span isn't 18'. It's the lenght of the hypotenuse. I don't remember your pitch so I can't tell you what it is, but I know you can figure it out.
RE#4:For a distributed load, yes. For a point load the deflection varies for the same load by the length cubed.

On a side note, when I was a soph in college we had to build trusses for our statics class. They were tested using an Instrom (sp?) complete with the chart to show the yield and exact failure points. I still have it somewhere.

 

[B7510]

Re: Any ME\'s out there?

I'm just one of those dumb old structural engineers but thought I would throw my few cents in just for grins.

First of all, have you considered the buckling length of the compression members? The top chord should be braced by what ever you are using for roof sheathing but you might need to do something wo reduce the unbraced length of any web members in compression. And don't forget, winds will create uplift loads on the roof, so you also may need bracing of hte bottom chord and other web members to keep them from buckling too. And make sure you put some X or V-bracing in each line of lateral braces. Without them the trusses will all fail at the same time, with the same member buckling in the same direction on every truss. Sort of like a line of drunks sitting on a bench, push on the first one and they all fall over.

Second, I would STRONGLY suggest that you load test one of these critters to confim capacity is above what you need. Use of water filled drums that are suspended just a few inches off the ground is probably a safe way to do it. You can gradually increase loads via a garden hose.

Third, you may not have codes, but you probably have insurance. Will they cover damage to the structure (and contents) if this thing dumps with a snow loaded roof or if it does a Dorothy and Todo thing during high winds.

You may want to just check with the local lumber yard and see what they normally design pre-engineered trusses for. That will give you a clue about what loads you are likely to be up against.

Good luck and remember the strucutral engineer's mantra;

WHEN IN DOUBT
MAKE IT STOUT
OUT OF THINGS
WE KNOW ABOUT

 

[yooperdave]

Re: Any ME\'s out there?

WHEN IN DOUBT
MAKE IT STOUT
OUT OF THINGS
WE KNOW ABOUT

sounds like a plan to me

/forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

Yooper Dave

 

[skid mark]

Re: Any ME\'s out there?

Good news.

The structural engineer that I've been working with has finished his analysis and given me a green light on the design of the trusses. We've also worked out a plan to test a couple prototype trusses. I was happy to see that his calculations paralleled mine on the design limits of the trusses.

If I can find the time, I'll do the tests over Spring Break in a couple weeks.

One thing that I thought I'd offer to anyone that's headed down a similar path. Check out this great book on The Design of Building Trusses. It was really fantastic. Explains the concepts involved, how to perform the calcuations, and even includes a fair amout of specs of various sizes of steel. It really helped to fill in the considerable gaps in my understanding of building trusses and (along with the SE) helped make up for my lack of personal experience in this area.

Thanks to all on this list for their help!

Mark

 

[TBAR]

Re: Any ME\'s out there?

I was impressed with this guys work.

LOOK HERE.


TBAR

 

[skid mark]

Re: Any ME\'s out there?

Howdy TBAR-

You and I must frequent the same lists!

The site you linked to is ... mine! That storage barn was my first crack at lightweight trusses. FWIW, I didn't design those, but "borrowed" the design from a local company.

Thanks for the reply anyway!

Best,

Mark

 

[TBAR]

Re: Any ME\'s out there?

Mark, ya sure get around.............. /forums/images/graemlins/grin.gif

Nice work incidentally. If you haven't noticed I have your web paged book marked.


TBAR