3/1/12 Bracing for the 'Big One'

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Bracing for the 'Big One'By billkevinburnettCreated 2012-02-01 12:09

Q: I'm an architect whohas also done my fairshare of swinging ahammer. Recently, yourecommended fasteningshear ply with screws.The California BuildingCode and other U.S.building codes havealways had schedules for shear wall nailing (although they do permit other types of fasteners).

My structural engineers have always said that the shear value of nails is far better than the shear value ofscrews. The screws are far more brittle. This has been borne out in my own construction, where screwshave snapped off while being driven.

Because the person writing is in a high-risk seismic area, you were right to recommend plywood and tofasten it to the bottom plate. But he also needs to have the best fasteners for the job: nails.

A: You reference a recent column where a reader asked for clarification about diagonal bracing ("Don'texpect miracle from diagonal bracing [2]," Dec. 28, 2011). He lives in a high-risk seismic zone. Weexplained how the bracing is constructed but suggested that for a better job he consider installingstructural plywood shear panels instead.

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Although we proposed screws as alternative fasteners, we agree that nailing the panels according to acode-specified nailing schedule is the tried-and-true method.

Shear panels refer to structural panels that make up exterior walls. They provide protection from thelateral forces of earthquakes. They are normally constructed by nailing 1/2-inch plywood to all framingmembers running from the mudsill (the piece of wood bolted to the foundation) to the top plate.

Plywood can be placed either horizontally or vertically and must be nailed on all edges and in the field toframing members. This creates a rigid diaphragm. If an edge falls in the middle of a stud bay, blockingmust be installed to provide a complete nailing surface.

Our reader wanted to provide protection from the inside. To accomplish this, 1/2-inch plywood isinstalled vertically and nailed to all studs, the top plate and bottom plate. Understand that this onlyprovides partial protection.

It will eliminate cracks in the plaster or visible seams in drywall, but because it doesn't tie the mudsill andcripple wall (which are below floor level) to the stud wall, lateral forces in an earthquake may cause thebuilding to move.

There are a couple of other things to consider while the studs are exposed. Whether the job is attackedfrom the inside or from the outside, this is a great time to insulate and inspect and upgrade the electricalwiring.

We consulted a cousin, Dan Smith, who is a retired master electrician. He recommended that if the wiringin the wall is ungrounded knob and tube or old sheathed cable, consider replacing it. It's quite possiblethat the insulation around the wires has deteriorated, creating a hazard.

If the wiring is grounded three-wire Romex, you can add outlets or lights provided that the circuit is notalready maxed out. Dan's rule of thumb for a 15-amp circuit serving living areas (not kitchens or otherhigh-load areas) is 10 outlets.

This assumes a normal electrical load and provides a little "fudge factor" if the homeowner occasionallyplugs in an appliance with a heavier draw.

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Of course, as always, get a permit and have the work inspected whether you do it or hire it out.

Insulation is pretty straightforward. If there is none, add it. Consider using spray-foam insulation if it's nottoo costly, or use faced insulation batts and turn the kraft paper vapor barrier toward the home's interior.In either case, insulate after the new electrical work is completed.

If replacing interior lath-and-plaster walls, we suggest using unfaced batt insulation and a vapor barrier ofplastic sheeting, known as Visqueen. Seal all cuts and seams in the plastic with the appropriate tape.Install the sheeting and then the drywall, tape, texture and paint.

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