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Tongue and Groove Cedar Siding

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Does anyone have any information about the strength of vertical tongue and groove cedar siding acting as a diaphragm?  He is the situation:


I have been asked by a building contractor to determine if a barn which is already built needs additional lateral bracing.  This barn was originally designed by an engineer; however, there was no detail for diagonal bracing, or a plywood diaphragm.  I understand the original engineer thought they were installing a plywood diaphragm, however, after some design modifications, this detail did not end up on the final drawings.  The contractor wants to know if it is necessary to install diagonal bracing at this point, or if the siding provides adequate shear strength.  The wall studs are 2x8 at 16? oc, with a 14? wall height.  2x4 purlins are attached to the outside of the studs at 24? oc.  Cedar tongue and groove siding (7/8?) is attached to each purlin with 2-#9 galvanized screws per panel.  The panel width ranges from 4? to 8?.  The footprint of the barn is 36?x56? with various openings for windows and doors. 


Additional background information:  This barn is a 2 level bank barn, with the first floor being poured concrete walls with a stone finish, and the second floor a timber frame.  The second floor will probably be used as a recreation area.   This barn is for a ?hobby farm? and the owner prefers the way it looks with the inside unfinished.  He does not want to drywall or finish the inside.   Last winter, this barn was erected and the contractor was in the process of installing the roofing, when some temporary wall bracing was removed which resulted in the barn blowing over during a storm.  There was no siding installed when the barn blew over.  After this, the contractor began to question the lateral bracing of the finished structure.  The contractor rebuilt the barn, and then went back to the engineer for a bracing plan.  The original engineer provided a post-construction bracing plan, which amounted to diagonal ¼? x 3? steel straps tied to the inside of the wall.  The owner is not looking forward to installing these straps to the inside of the wall for aesthetic reasons. 


Coming up with an alternate lateral bracing system which does not require a major construction effort at this point (after construction) may be challenging.  I was wondering if anyone had any information about the strength of vertical cedar siding as a diaphragm.  I talked to a representative of Western Red Cedar Lumber Association, but the only information I got was this:  If Cedar Siding is used without a plywood diaphragm, and applied to outside purlins, it is recommended that the siding be attached to purlins spaced at 16? o.c.  Unfortunately, the as-built purlin spacing is 24?o.c.  Additional, this barn must be designed to the requirements of IBC 2003. 


I would appreciate any comments or advise on this subject.


With regards,


Molly H. Hughes

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