Prefabricated wood I-Joist

[awa5114]

what is the "proper" way to determine the shear/flexural/axial capacity of prefabricated wood I-Joists per NDS? When dealing with these sorts of members is the only way to determing their capacity based on manufacturer data or could there be a way to calc them based on NDS? I see there is a chapter dedicated to them in NDS but cannot determine if it is at all useful...

 

[KootK]

I'm gonna tentatively say that the manufacturer's data is the only legit way to go. That said:

1) If you had test data, there are established procedures for converting that to design capacities. Of course, getting the test data from the manufacturer would be orders of magnitude more difficult that just getting their design tables.

2) One could certainly make a reasonable estimate using the flange axial capacities for flexure and the web material in shear. The hang up in an accurate assessment is mostly the glued joint I believe. Probably not a huge deal for uniform loads but, potentially a deal breaker for serious point loads and any loads that might be suspended from the bottom flange etc.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Enginerdad]

I would be very careful trying to calculate something like that from scratch. Like KootK said, there are some unknowns, such as glue strength and web shear buckling that aren't covered in the NDS. Some manufacturers for these products don't even have theoretical ways of calculating the capacity of their members; they rely entirely on empirical data from testing. Sounds like a whole lot of liability for you to do all that work and come up with any answer different from what the manufacturer says.

 

[XR250]

One thing that has always bothered about I-joists is how much vertical reaction they can take at the supports. It seems ridiculously high for a piece of tall, skinny OSB. I could never get those numbers to work. I queried Boise's tech. department and they apparently use a "K" of 0.5 for web buckling - not 1.0 like I was using in my calcs. They feel that the flange is adequate to provide fixity at the top and bottom. I have seen the webs buckle a number of times in heavily loaded i-joists and the skinny flange has actually rotated - which seems to put a hole in their theory or laboratory testing.