Any ME's out there?

[skid mark]

Re: Any ME\'s out there?

Villengineer-

Thanks for your correspondence on this. I appreciate your opinions.

FWIW, the last building I built was identical to the ones I looked at and the only change that I made was to use more trusses and place them a little closer together.

I've enjoyed the learning process I've gone through so far on this project, including this thread. I've bought and am reading some excellent references including this great book and have enjoyed trying to learn new things.

I want to take the time to have fun learning about and building this new building. I'll take the time to do it right.

That said, there is a little voice in me that says that I'm taking this a little too seriously (I tend towards obsessive/****). Perhaps my past experiences have not been shared by others thanks to building codes (some folks have them, I don't), but living where I do, I've had lots of experiences putting up storage sheds, cattle turnouts, hay barns, etc over the years.

In every case, those were planned on the back of an envelope, materials were bought, poles were planted in the ground, etc and the thing was put together. If it wiggled a little, a brace was added. Sizes of materials were arrived at by guestimate and past experience. When in doubt, upgrade to the next size of materials and all will be fine.

I heard one time that "Anybody can build a bridge that will stand up. But it takes an engineer to build one that will just barely stand up". I think that applies to your typical barn/shed/etc. When in doubt, overbuild. You'll spend more money on materials (something the pro's work to avoid), but the cost savings of DIY will pay for the materials and if you enjoy the work, great.

Without exception, all of the thrown together buildings that I've put up over the years have held up fine, including an all time record snow three years ago.

But I acknowledge that using metal trusses is a different world. Except for the barn two years ago, everything I built was a pole barn. I've not had a problem with any of them, but I have seen a couple members sag a little under a big load in the older buildings. I'm guessing that pole barn construction would tend to give more warning and fail more "gracefully" than a metal truss that might show no signs of fatigue and then fail catastrophically.

So I'll keep at the books. It's part of the fun of doing something new.

Mark

 

[AV8R62]

Re: Any ME\'s out there?

A little common sense goes along way
It is your building and only you know the risk you are comfortable with .
I have seen many home built trusses, some look like you could park a tank on them and others look like they could fall under their own weight but all of them worked as far as I know.
if you build one that is not strong enough you will probably know it before you build the second one and then you can ajust your design .
remember you are not building a Church, school or hospital
it is a barn.
enjoy your project

 

[slowzuki]

Re: Any ME\'s out there?

I'm a mechanical engineer as well and I say you should contact a structural guy or pick up some civil engineering text books.

You should be looking at buckling of compression members as well. Also, please read more on ulimate tensile strength vs yield strength of materials. Safety factors are applied to yield strength in this this of design.

It really isn't ethical for me to offer any numbers for your design.
Please talk to a structural engineer.
Ken

 

[Villengineer]

Re: Any ME\'s out there?

</font><font color="blue" class="small">( remember you are not building a Church, school or hospital
it is a barn )</font>
Why would it matter what kind of building it is? If it falls on you, chances are you're still dead. Buildings bigger than a small shed aren't the best project for the trial and error school of design.

 

[AV8R62]

Re: Any ME\'s out there?

chances are he will not buid something that will fall (common sense)

And if it does chances are no body will be in it.

 

[Villengineer]

Re: Any ME\'s out there?

Those are some awefully big assumptions to make. Especially when you make them for someone else's life. This exact reason is why most areas have building codes, including buildings such as barns.

 

[AV8R62]

Re: Any ME\'s out there?

Back to my original point it is not someone else's barn it is his barn.
building codes change almost daily if a house or barn is more than a few years old it is not built to the latest spec's
but we still use them

 

[yooperdave]

Re: Any ME\'s out there?

There are importance factors applied depending on occupancy.

Esential facilities such as hospitals have a higher importance factor than a agricultural building which has a lower factor.

If you value your life, or the lives of other humans which may occupy your building, you may wish to provide more structural capacity.

My personal pole barn has much more vertical and lateral load capacity than any of the neighbors - I feel that I am worth the extra expense /forums/images/graemlins/cool.gif.

Yooper Dave

 

[skid mark]

Re: Any ME\'s out there?

Here's an update. I phoned a local ME (structural) who runs a small one-man shop. We're going to get together this weekend.

He said that any analysis he does would take him a while because he uses graphical methods to evaluate the forces and reactions in the truss. I know just enough about the graphical methods to know that I wouldn't want to evaluate the forces in a clothes line that way. Ugh!

I asked if he'd ever seen any computer based tools made for this purpose and he said that he had not. When I asked if he'd like to, he said sure! So I'm pretty excited. I bet that his jaw will hit the floor when he watches how easy it is to set up a truss/frame, put various loads on it and test the loads individually and in combination. And the real beauty of using SW to do the analysis is that as soon as you see a trouble spot in the design, you quickly make a change, and run the numbers again. Very fun stuff.

If he's impressed, I'll volunteer to help get him set up with the 30 day eval of the product I'm using and walk him through the process of setting everything up, evaluating, etc. With a little luck, I'll be able to roll back his bill by my consulting with him!

I'll let you know how it goes.

Mark

 

[AV8R62]

Re: Any ME\'s out there?

phantom309

Are using Solid Works ?
I have been using it for about two years now and it is a very powerful tool and more fun than Autocad and Cadkey

 

[skid mark]

Re: Any ME\'s out there?

Howdy AV8R62-

The CAD program I used to come up with the initial truss layout is available at: http://www.cadstd.com/. The "lite" version of their product is free to all. It's limited, but suited my needs fine.

To analyze the trusses, I'm using The VisualAnalysis4 program from: http://www.iesweb.com/. This product is available for download but is limited to a 30 day trial period. It's very nice!

I learned autocad years ago in an engineering design graphics class and we actually have a license to AutoCad 2003 here (didn't discover until I'd already started using the above CAD program). Wow, Autocad has packed on a lot of features since the early days when I used it!

Mark

 

[AV8R62]

Re: Any ME\'s out there?

Yes it sure has changed in the last 12 years but is simple compared to Solid Works

 

[keeney]

Re: Any ME\'s out there?

Compression yield is generally the same as tension for most metals. However, the issue you need to be concerned with is that compression members in certain shapes and geometries can fail by buckling.

In a buckling failure, once the member flexes to the side past a certain point (perhaps not even reaching the yield point yet), the geometry of the buckle works like a lever to quickly push the material in the area of the buckle well past its yield point.

To get an understanding of this, play with a plastic soda straw. Although it can hold a heck of a lot of force in tension, it is slender enough to easily buckle under compression. Its length to diameter ratio is way too slender - well over 25:1.

Another type of buckling is demonstrated by an aluminum pop can. In theory, the wall cross section area multiplied by the yield strength of the aluminum alloy used should be able to support a couple of hundred pounds. Also, the can is plenty wide enough compared to its length to not buckle as a solid coulmn. However, in practice, it is almost impossible to stand on a pop can without buckling it. In this case, its not the slenderness ratio of the whole can, but the slenderness ratio of the very thin wall of the tubing vs its length.

Unfortunately, determining the buckling factor for a complex shape like a hollow round tube starts involving how it is attached at the ends, its length vs its diameter, etc. Anticipating all possible side loads like wind, vibration, uneven loading, etc. can be tricky. Torsion, residual stress in the welds, uneven thermal expansion and many other things can also be an issue for designs very close to the buckling threshold.

In most designs, members are significantly over-designed in terms of buckling so as to avoid having to do more than a simple estimate. This does not necessarily always mean making the member heavier. Somethimes a simple trade of tubing diameter vs wall thickness can make a shape much less prone to buckling using the same amount of meterial.

There are some rules of thumb concerning slenderness ratios where buckling can be ruled out as a failure mechanism.

I would recommend that you get your own "experience" by building the first article of your design and testing it before committing to building and using the remainder. Be sure that the testing mimicks how the truss will actually be installed in terms of any roof decking fastened to it (or not fastened if that is the case). The decking can be significant in preventing the truss from twisting sideways. Many truss designs include some amount of truss-to-truss bracing to prevent twisting as well.

For a flat truss design you can test it yourself pretty easily on the ground: Build a section (at least two trusses worth) of your roof a foot off the ground on some cinder blocks. Pile dirt on it in a level pile. Keep track of the weight of the dirt by how deep it is. Most dirt, rock, concrete, sand, etc weighs between 100 and 150 lbs per cubic foot. When loaded with the design load, measure how far the truss has deflected in the middle (assuming it has not failed at this point).

DON'T CRAWL UNDERNEATH TO MEASURE, DUH!

Compare the actual "experience" at the design load with the predicted deflection from the computer model. If they match within a factor of 2x, then you have confirmed that you are at least in the right ballpark.

- Rick

 

[skid mark]

Re: Any ME\'s out there?

Hi Rick-

Thanks for the note.

The Structural Engineer I met with Friday is off on his own now doing calculations to parallel mine. My questions for him were a little out of the norm for him, but he seemed pretty knowlegeable. He thought that the truss design looked sound but he (like me) wanted to dig up the specifics on the one inch tubing I'm using.

All of the calculations so far look very encouraging. With a 20lb/ft live load, 5lb/ft dead load and 15lb/ft wind load, the maximum deflections we calculated were DX=0.24in and DY=0.39in. Max tensions were very reasonable and the max compressions were in members only 16" long.

From what I've read, as the L/r (slenderness) ratio gets smaller and smaller, the allowable axial compression approaches that of the yield strength (Fy) with a limit of about (0.6)(Fy) for completely braced members (L/r = 0).

Using values of 0.04 for the moment of inertia (I) and 0.28 for area (A), the radius of gyration (r) for my members is: r = sqrt(I/A) = .378

Calculating L/r gives a slenderness ratio of 16/.378 = (approx) 42. The book I have on truss design shows the normal range for truss compression elements is between 40 and 200, so my 42 is actually at the conservative end of the slenderness ratio chart, thanks to the short length of the compression members.

With a slenderness ratio of about 42, the book shows an allowable axial compression stress of 19.0K per square inch.

The highest compression stress that I have in my trusses are about 25K (7K force divided by 0.28 area), but these members are double chords so the stress is shared by two members which gives us about 12.5K stress each which on paper is safely below the max stress of 19.0K per tube.

I always like to build things with a large factor of safety. I don't have enough experience in this area to know if these numbers support that or not. Given that the numbers are within the limits shown in my references and the analysis was run with a record-busting 20lbs/ft of snow combined with a 90mph wind, I think that I should be fine.

One remaining question is that I'm not sure of is I'd like to confirm that the steel in my tubing has the same (or higher) specs as the ATSM A36 steel that is the baseline for the calculations in my book. I'm digging into that one now...

Oh, one other question. I'd like to confirm what should serve as a reasonable maximum deflection of any node in the truss is when running an analysis. The ME (structural) I spoke with Friday said that you should stay below L/256, where L is the span of the truss. In my case, that's 432/256 = 1.7inches. That seems a little high to me and my numbers are well below that, but I thought I'd mention it to see if that rings a bell with anybody.

Still plugging away....

Mark

 

[yooperdave]

Re: Any ME\'s out there?

Are your tubes hot rolled or cold rolled?

Each one has its own specification.

I usually use hot rolled with ASTM A500, GRADE B, Fy = 46 ksi.

Also make sure you get your units right. You speak of stresses but use forces for your results.

Good Luck

Yooper Dave

 

[skid mark]

Re: Any ME\'s out there?

I'm tracking down the materials specs now.

You are right that I was a little lazy with the units. At the end of my last note, I was talking about stresses, but left off the /in^2.

 

[keeney]

Re: Any ME\'s out there?

mark,

For a floor that people would walk on, L/256 would be marginal and feel too bouncy. For a roof, I wouldn't think it matters as much, and is probably plenty stiff.

Even if you don't do a full-scale test, I would still suggest that you verify your calculations with at least a test of one small section of your design just to make sure you are on the right track.

- Rick

 

[Egon]

Re: Any ME\'s out there?

Are not allowable deflection ratio's set by the intended use of the structure with definite's specified.

Egon

 

[skid mark]

Re: Any ME\'s out there?

Thanks for the notes Rick & Egon-

Thanks for the confirmation RE L/256. I thought that I remembered the SE saying something about L/256 for a roof with no decking attached to the underside and a larger denominator (3xx?), smaller deflection if decked. I thought to myself but didn't ask that if decked with sheetrock, less deflection would be necessary to keep the seams from cracking?

So your comments both support what I thought that I'd heard.

Rick-

I would like to do a "real world" test and have been thinking about your advice. Funny you mentioned a limited test because that is really my only option. With my roof trusses having a span of 36 feet and a rise of 4 feet, if that was put on cinder blocks, the peak would still be plenty high to crush me if a rafter collapsed and fell to the side. But if I took a half of a rafter, that'd would only have a height of about 19". If this collapses, I'll only have crushed legs instead of be killed. /forums/images/graemlins/smile.gif

So if I go this route and test a half-rafter, would the goal be to load up two of them (side by side, with decking, etc) and simply compare the measured deflection numbers against a computer model of the same scenario?

With a 5000lb load on a 36foot span, the model says that the truss will deflect less than a half an inch. A similar load on an 18 foot span (made of TWO trusses, side by side) will, I assume not deflect the truss enough to be able to accurately measure?!?

Again, I wouldn't mind taking the time to do a test like this, I'm just trying to find the best way to do so and avoid having to dig up several tons of dirt by hand! Ugh!

Added in edit: I guess what I'd like to do is to build the whole thing out of match sticks and then apply a ten pound load to that to test. hahaha...

Thanks again!

Mark

 

[keeney]

Re: Any ME\'s out there?

<font color="green"> Again, I wouldn't mind taking the time to do a test like this, I'm just trying to find the best way to do so and avoid having to dig up several tons of dirt by hand! Ugh!
</font>

Sorry, I was assuming that anybody on TBN had a tractor to use to move dirt.

You could also do a much more limited test of a smaller section of the truss made out of the same materials and the same fabrication techniques. Maybe a 10-foot piece. At least a couple of repeats of the webbing pattern.

Theoretically, the deflection should be linear with load up until the yield point, so you can confirm your model with any load assuming you can measure it accurately at small deflections.

You could also build the building and test it after it was built to confirm things. If it isn't as stiff as you predicted, weld-on some more steel or add some poles in the middle or something.

- Rick