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Re: RE Plywood Roof Diaphragms

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If the roof was pitched, this was probably due to the plywood roof sheathing acting as a beam by folded plate action.  There is an interesting and short article in the May 1977 Forest Products Journal (citation below) illustrating that the plywood decking on a pitched truss roof system was sufficient to support the roof gravity design load (albeit with substantial deflection) even though the two walls providing support to all of the trusses were removed, leaving only the end trusses over the gable end walls supported.  The tested system was only a 12 x 12 ft roof system with trusses at 1 ft o.c., and this should not be counted upon for typical applications without adequate engineering and construction details.  However, it shows the potential for significant redundancy in some roof systems due to folded plate beams formed by the roof sheathing.

Nicol-Smith, C. A.  1977.  Two-way action of pitched roofs.  Forest Prod. J. 27(5):55.

Original message from Digest for August 7, 2002
>---------------------------------------------------------------
>Message:0027                           27 
>--------------------------------------------------------------- 
>From: MSSROLLO(--nospam--at)aol.com 
>To: seaint(--nospam--at)seaint.org 
>Subject: Re: RE Plywood Roof Diaphragms/ Swedge Anchors 
>
>Part of the roofs I have looked at over the years were houses 
>caught up in the December 2000 tornado that hit Tuscaloosa, AL. 
>The houses immediately next to those directly hit by the 
>tornado still appeared to be intact although they did have 
>mostly projectile damage and some isolated areas where roof 
>sections got lifted off.  Garages were most noticeable because 
>the garage doors buckled and typically over garages have the 
>least amount of rafter bracing.   
>
>I saw one truss roof I wished could have gotten a picture of.  
>Just happen to drive past it and someone I knew asked me to 
>stop and explain why it had not fallen.  The steep trusses over 
>the garage did not have any wall support on one side for about 
>20'.  The rest of the garage (about 12') still had the wall.  
>The roof was drooped down about 2' but had not fallen.  It 
>appeared the horizontal bridging on the bottom chord (one 2x4 
>at third points) was acting as a compression strut and the 
>plywood deck was the tension tie of a "deep cantilever beam". 
>The top plates of the wall were still attached to the trusses 
>also although the studs were gone for the 20' section.   
>
>I agree those that buildings in my area rarely experience their 
>full design wind or gravity loads.   I am alway curious what 
>would happen in this area if full design loads ever occured.  
>bet lot of things would have problems. 
>
>Ron Martin
>                 

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