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re: C & C vs MWFRS

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I myself was unaware that it was debatable not to use both MWFRS and C&C
for elements that serve dual purposes. I use the MWFRS loads on diaphragm
and shear wall sheathing, and OT moment. I use C&C loads on everything else
the wind touches since it is the more critical loading. I thought ASCE 7
was explicit...maybe not. I would have been the guy saying that engineer
should have used the C&C loads.

Hard to image the difference would have caused a court case. Did something
fall down, or was it a mud fight where the warring parties through
everything out and the kitchen sink?

Rand



                                                                           
             "Andrew Kester,                                               
             P.E."                                                         
             <akester(--nospam--at)cfl.rr.c                                          To 
             om>                       <seaint(--nospam--at)seaint.org>                 
                                                                        cc 
             12/31/2008 01:22                                              
             PM                                                    Subject 
                                       re: C & C vs MWFRS                  
                                                                           
             Please respond to                                             
             <seaint(--nospam--at)seaint.or                                             
                    g>                                                     
                                                                           
                                                                           




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
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