Don't expect miracle from diagonal bracing

Better seismic benefit comes from shear walls

By Inman News Feed
Add Comment Add Comment | Comments: 0 | Posted Dec. 28, 2011

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Better seismic benefit comes from shear walls

Bill and Kevin Burnett
Inman News®

Q: I read your article on diagonal bracing and was hoping you would clarify your instructions so I can explain them to my contractor.

I, too, have lath-and-plaster walls with many cracks. My house is located in Oakland, Calif., which is on an earthquake fault. I was not clear on the part where you said to place L-shaped metal bracing in the corners of the room and extend them diagonally from the top plate to the bottom plate of the framed wall. Is it possible to have a drawing?

A: Any contractor worth his or her tools should know immediately what we're talking about, but follow these instructions for a simple diagram that will make diagonal wall bracing crystal clear. Draw a 4-by-6-inch rectangle on a piece of paper. The top and bottom of the rectangle represent the top and bottom plates of a stud wall.

Then draw perpendicular lines from top to bottom every 1/2 inch. These are the wall studs. Next, draw a line from the top corner of the rectangle to the bottom corner. The diagonal intersects the studs. This is the diagonal bracing.

We also found a couple of websites that should make things clear for you. Go to Strongtie.com or Blueprints.

We're concerned that you might be expecting too much from diagonal bracing. If you're looking for protection from a big shaker, bracing will help, but it's not a cure-all.

Diagonal bracing can either be 1-by-4-inch wood notched and nailed or screwed into the studs or steel L-channel cut and nailed or screwed into the studs. Both provide some protection against horizontal movement, but neither is engineered to act as a shear wall load-carrying component.

Because you live on an earthquake fault, your house is more susceptible to earth movement than homes farther away. With the walls open you should take advantage of the opportunity to provide extra seismic protection.

Rather than just using diagonal bracing and installing drywall over the lath, consider removing the plaster and lath completely from the walls, then nailing 1/2-inch plywood or oriented strand board (OSB) to the studs and covering that with the appropriate thickness of Sheetrock. Plywood or OSB when nailed or screwed properly to the studs and the top and bottom plate creates a diaphragm that protects against vertical, diagonal and horizontal forces on the wall members. This will eliminate cracking from normal house movement.

To get the seismic benefit, the OSB must be nailed or screwed to the bottom plate. This means the baseboard must be removed, OSB and Sheetrock installed, and the baseboard reinstalled.

This is a big job, and we'd think twice about taking it on ourselves. Look for a good licensed and bonded seismic retrofitting contractor who's done a fair amount of work in your area.

Shear walls will cost a little more than diagonal bracing, but we believe it will be money well spent. While the walls are open, consider upgrading the electrical wiring. Knob-and-tube wiring should be replaced with Romex or armored cable and proper grounding should be installed. Also, insulation should be added to exterior walls.

                                     

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