AISI and Through Fastened Roofs!!!!!!!

[SmithJ]

Hi All,

Section C.3.1.3 of the AISI '96 code provides numerous conditions for a roof panel to be considered through fastened. One of the requirements (#8) states that "For continuous span systems, the longest member span shall not be more than 20% greater than the shortest span."

I have found that this requirement is easily failed. Take for an example a 112ft wide building. I have endwall spacings of 16', 15', 15', 20', 15', 15' and 16'. These spacings are pretty much a fixed requirement of openings in the wall.

Does anyone understand the basis of this requirement? How stringent is this in terms of the ability of the screw down sheets to proveide bracing for the flanges of the girts? My judgement tells me that I can use through fastened design factors on my secondary design for this wall and not worry about discrete braces for the girts. Am I mistaken in this assumption?

And, if my assumption is acceptable, how far can this requiremnt be stretched? If I have a system that passes all the other requirements but only fails this one, at what point do I have to introduce discrete braces ....30%, 40% etc? Say on a 120ft wide building, Endwall spacing was 10', 20', 20', 20', 20', 20' and 10'. Would this require discrete braces?

Thanks in adavance for any and all input.

JS.

 

[j19]

To the best of my knowledge all of the criteria listed in this section of the AISI is based on test results and the material (members)that was tested. The rule of thumb that I have heard from researchers is that you can extrapolate 10% beyond the given limitations. For example, even though the criteria calls for a maximum member depth of < 11.5", you can still use the criteria for a 12" deep cold-formed member. Of course I don't think you will find that written down anywhere. If you want more information in order to make a judgment call you may want to talk with Roger LaBoube at the Univ. Missouri-Rolla. I think he was one of the researchers that came up with this criteria and he has always been very helpful in answering questions.

 

[ChuckerD]

Was my post so controversial that it got removed?

Anyway, the AISI spec defines a reduction factor R that is applied based on the purlin or girt conditions defined in section C3.1.3. The reduction factor is applied to the full allowable strength on the section being analyzed. This eliminates the need to do a full analysis of the unbraced lengths of the member to determine its capacity.