When an earthquake shakes a house from side to side, something has to stop the building from tipping over or sliding off its foundation. In most California homes, that something is a shear wall. Shear walls are the main reason a wood framed house can ride out a quake, and understanding them helps explain why your engineer cares so much about where windows, doors, and garages go.

What a shear wall actually does

A shear wall is usually a wood framed wall sheathed with plywood. When an earthquake pushes sideways on the top of the wall, the wall has two jobs.

First, it resists overturning. The sideways push wants to tip the wall over, lifting up one edge. Second, it resists sliding. That same force wants to shove the whole wall sideways off its base. If either of those happens, the wall is not doing its job.

Hold downs and anchor bolts

Engineers stop overturning with hold downs. These are steel connectors placed at the edges of a shear wall that anchor it down and keep it from lifting.

Sliding is handled by anchor bolts. These run through the mudsill, the piece of wood at the bottom of the wall that sits on the foundation, and tie the wall to the concrete. Spacing those bolts correctly is part of the design.

There is one more piece that is easy to overlook: nailing. A shear wall only works if the plywood is nailed to the framing the right way, with the right nails at the right spacing. Skip that and the wall is not really a shear wall at all.

Stacking it up in multi story homes

In a two or three story house, those uplift and compression forces have to travel all the way from the roof down to the foundation. Engineers use straps from floor to floor to create a continuous line of tension, and posts or solid blocking to carry the compression. Even a modest two story home has many combinations of these connections to work out.

When a shear wall will not fit

Sometimes the architecture does not leave room for a shear wall. A wall of glass or a wide garage opening can eat up the space an engineer needs. When that happens, there are other tools.

A steel moment frame can do the job of a shear wall in an open wall. Instead of relying on plywood, it carries the sideways force through the stiffness of the frame itself, down into the foundation. Pre manufactured panels, in steel or in dense engineered wood, can also pack a lot of strength into a narrow space.

The garage problem and soft story risk

Wide garage openings are a real issue in older Bay Area homes. They leave very little wall for resisting earthquake forces, which creates what engineers call a soft story. New local ordinances are starting to require owners of these homes to retrofit them so they do not collapse in the next big quake. If your home has a large opening at the ground floor, it is worth asking an engineer about.

Spacial provides structural engineering for homeowners, architects, and builders across California.