Tornado Protection: What did we learned from 2011 tornado’s

The recent “Journal of Light Construction” took a look at the damage inflicted on new wood frame homes by the April 2011 tornado’s in North Carolina and Alabama. 

According to analysis by the National Weather Service nearly 95 percent of tornado’s are rated EF0, EF1, or EF2; these weaker tornado’s produce winds that a well-built home should be expected to withstand.

In response to the recent storms, the APA has developed construction recommendations designed to strengthen the overall structural shell so that it can withstand the forces of tornado’s and hurricanes. 

Rectangular Cut Nails

The picture shows the failure of rectangular cut nails to hold the bottom plate to the foundation.    The recommendation is to use 6 inch “J” bolts on 48 inch centers.  The “J” bolts use a square washer and bolt the bottom plate to the foundation. 

Non-Structural Sheathing

The picture shows walls that were blown out in areas where there was not structural grade sheathing attached to the frame.  The new recommendation includes the use of structural sheathing around the entire home.    

Sheathing Detached

The picture illustrates how walls failed if the sheathing was not adequately attached to the bottom and top plate.  The APA recommendation was to more thoroughly attach the sheathing to the bottom and top plate.

Metal Ties

The last photo shows that the framing anchors stayed attached to the top plate but the wind force pulled the roof and top plate from the frame.  The recommendation was to continue to use metal ties or hurricane straps but instead of applying them to inside of the top plate they should be nailed to the outside of the top plate in alignment with the load path through the structural sheathing.

These recommendations provide a prescriptive approach with details that rely on standard framing and sheathing materials, with a minimum of additional hardware.  Although Pinnacle Custom Homes already does most of the recommendations we have implemented remaining recommendations which will optimize the structural performance of homes without great expense to our customers.

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