Tuesday, May 17, 2011

Lessons from a Tornado

The April 27 tornadoes left many opportunities to study structural performance of both commercial and residential buildings.  One area that I have observed was a residential development located just out of the path of an EF3 tornado that made its trail not far from my home.  With the exception of one building's collapse, damage to each home generally consisted of a portion of roof missing, as seen in the photo below. 

A close up inspection of the rafter heel condition at the top plate highlighted the failure of the roof system.  A birds-mouthed rafter with (3) toe-nails into the wall top plate is a common framing detail used in residential structures.  This detail was used throughout this development.  There appear to be two realistic modes of failure for this detail as highlighted in the pictures below.  The first mode is failure of the 2x rafter at the birds-mouth.  This failure mode occurred in a small portion of rafters witnessed. 

The second mode of failure is at the toe-nail connection.  Although this connection works reasonably well for lateral shear, the typical limit state for uplift loading is nail withdrawal from the wall top plate.  The nails in the picture below appear to have seen very little stress during failure.     

Athough this is a common detail, I avoid detailing a rafter/truss to top plate connection with toe-nails. A Simpson H2.5A anchor, available at local hardware stores, is inexpensive and provides a significantly improved connection capacity. This syle of clip results in a more consistent installation and brings a different connection failure mode that does not include nail withdrawal. For a small home with a simple framed roof, such as homes in this development, a small investment in H2.5A clips can have a major impact on structural performance.

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