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re: C & C vs MWFRS

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Conrad,

I understand what you are saying from your perspective, but maybe I was not clearly stating the facts.

 

This was an actual case where the Florida Board had several counts (charges) against an engineer. One of the counts was for not considering C and C loading on roof members and their connections. Since I have never seen any MWFRS loading that would be greater than a C and C loading, the latter would always govern. I understand that with larger trib areas like 500sf they begin to converge. I am simply stating what was an official ruling by the FL Board of Engineers as to what loading should be considered. This was not a disagreement with a building official, of which we have nearly no interaction, nor do we submit calculations to the bldg department though we are required to provide wind loading info on our drawings (and provide pressure tables if there are elements to be designed by others). I don’t think the average building official here knows any differences between the two anyway, you could likely use whatever you wanted as long as you seal the drawings. The responsibility falls upon you to do it the “right way”, which according to the Board is C and C.

 

To me the difference between the two is hardly worth arguing or discussing anyway unless we are talking a major structure or something design that will be used hundreds of times. With very large structures they usually use wind tunnel pressures anyway. I know I have always designed roof members and connections as C and C as well as any other FL engineer I have worked with or talked to, but I have read yours and others arguments on the subject so understand your viewpoints. There have been some great theoretical discussions. But as I have seen many failures after hurricanes and thunderstorms at connections of wood trusses particularly, a slight amount of overdesign of a roof structure and the connections is a small price to pay (if it were allowed) to go with a slightly larger uplift pressure. Roof members and their connections, particularly trusses and uplift straps, are a very small percentage of the cost of your average home or small commercial building. They will spend way more on the kitchen then the whole roof….

 

Happy New Year to all!

 

Andrew Kester, PE

Orlando, FL