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
