Subject: Re: More questions about rigid plywood Diaphragms
Date: Tue, 29 Jun 1999 07:23:56 -0700
Devils advocate. I have reservations about the wholesale classification of
light frame roofing systems as flexible.
May we recognize that roofing systems typical to light frame construction
are composed of ductile frame elements: tied, sheeted, braced, and pinned
about a level boundary perimeter (perfect world). The individual components do
not possess the properties of classical rigid elements, however, as a system,
the components certainly perform as a framed entity. The model is best
represented by a trussed roof system with multiple and evenly spaced
intermediate points of bearing with a simple perimeter.
On cut-roofing applications, the model becomes respectable frame as straps,
collar ties, ridge ties , purlin struts, kickers, braces, blocking (stepped
applications), straps, ceiling ties and chords (more?) are added. Inherent
redundancies make it all work. The top is sheeted, the bottom is sheeted,
intermediate supports are numerous and it has historically performed as a
laterally rigid diaphram.
Got a calc for that? When pigs fly. Have some extra recreational time for a
I do not believe that the code references the consideration of rigid
horizontal diaphrams because we have a tidy FBD for every element in a roofing
system, especially light frame construction. The historical performance of
these systems and the investigation of failures has provided the present basis
for the recognition of these systems to be considered as rigid diaphrams. I
would like to support the continued recognition of these systems as rigid