Friday, May 25, 2012

Where are the shear walls?

More than a few times I have been asked: where are the shear walls in this building? In California, the majority of wood framed shear walls in new construction consist of plywood sheathing.  It is not surprising that the less experienced framer who walks into a rehabilitation or remodel is surprised to see that there is no plywood sheathing in the wall studs.

Stucco and gyp-board shear walls are still allowed by the 2010 California Building Code (CBC), but after the 1994 Northridge earthquake, the strength capacity for these materials was significantly reduced and limited to single story applications. Stucco and gyp-board sheathing is now rarely specified by engineers since more available walls and more available length is required.

The diagonal wood brace was cut for new door opening
Diagonal braces in wood framed construction started phasing out in California almost 30 years ago.  It is not surprising that the younger generation of framers are at first puzzled to see a plank of wood carefully carved into the wall studs. Diagonal braces are still popular in other parts of the country that are not at risk of seismic activity.  

If you are working in a wood framed remodel or tenant improvement for an old wood framed building, be considerate of existing shear wall systems that did not use plywood sheathingAlthough stucco/gyp-board sheathing and diagonal braces may have little to no value in current codes, these materials were used for shear walls in hundreds of thousands of buildings in California. Earlier editions of the code allowed them and they remain to have "real" value for the existing building inventory. Although their seismic performance has proven to be poor in some cases, they have also effectively provided building safety in the majority of the existing buildings.