aisi through fastened requirements

huangyhg

aisi through fastened requirements
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.