Having laid out the preliminary architecture for my frame, I am now working my way from the roof cladding downward, applying stresses & calculating the necessary cross sections to carry them (and carrying the resultant members' weights/loads still further downward, iteratively)
The approach to determining and applying design loads per the ICC has me a bit mystified, however. I am an aerospace engineer by trade, more used to designing around given max/operating loads and theoretical material limits plus a standard safety margin than I am with the various coefficients and rules of thumb which appear to constitute analysis per the International Code. I wish to get some feedback on how well I'm tracking the intended usage of the code, or how far I'm adrift.
I'm in central Texas, so at least some of the load factors don't apply; the ones that seem to are:
-Dead load; weight of materials (I assume 5psf as a starting point for my roof cladding, which will be insulated steel sheet w/o decking; no hard figures on specific materials yet)
-Live load; appears to be plainly stated in the code per a table based on Tributary Area, however I'm a bit unclear on how Tributary Area is derived for selecting a value --I used the largest value, since my first roof face area is greater than the largest Tributary Area category (the magnitude of the chart categories suggest whole roof faces)
-Snow load; the Live Load exceeds the 5psi snow load per the climate map, so I understand it does not apply
-Wind load; this is the source of the most cornfusion, naturally. Even worse, considering how the maze of load areas, coefficients, and adjustments all appear to be arbitrarily defined, rather than based in any rhyme or reason (again, aerospace guy here, so I keep trying to rationalize the various criteria, and just confusing myself more, lol). The highest positive load is 10psf, regardless, since I appear to be in a 90mph wind zone & 14deg slope, so this value is what I am using for my initial calculations.
Where I am very lost, is how the various load zones are applied to the structure & calculations; am I only using them to drive the attachment fasteners for the outer layer, and using some constant pressure to drive the underlying structural elements, or are my rafters/purlins/etc being loaded at different rates depending where on the roof they lie? I know that realistically this is of course the case, but don't quite understand how the load zones approximate the varying pressure & turbulence of flow over the structure.
I'm even more in the dark as the to "effective wind area" criteria of the load chart, which as near as I can tell is equal to the span of the roof section squared, then divided by three. I can't make heads or tails of why this should make sense, nor why it should result in applied wind loading going down for a given structure (if anything, I would have thought larger facets would see greater negative pressure due to increased laminar flow over the continuous slope)
*Fear not, I know enough to realize these calculations are important and merit ultimate review by a PE. I also know enough to realize I should be capable, if not qualified, of determining these figures & avoid the need for the entire structure to be engineered by a third party