What shear walls and hold downs do in a wood frame house

Hold downs for shear walls are heavy steel brackets that bolt wood framing to the concrete foundation. They stop the ends of the wall from lifting off the ground when high winds or earthquakes push the house sideways. Together with wood panels, these metal connectors help keep the building square and upright. Their design and installation belong to an engineer and a licensed pro.

Sideways forces from wind and earthquakes

Gravity pulls a house straight down. Standard wood framing carries this vertical weight easily. High winds and earthquakes introduce a different problem by pushing a house sideways. This lateral force tries to rack the walls out of square.

Understanding how a house is framed helps explain the risk. The exterior walls parallel to the wind must resist the massive pressure applied when a storm hits the broad side of the house. The roof structure also catches wind and can create uplift.

During an earthquake, the ground moves the heavy concrete foundation rapidly. The top of the house tries to stay still while the bottom moves, which creates a sideways shear force. Walls without lateral bracing can lean or fail. The framing needs specific stiff elements to stay rigid against these combined lateral and uplift loads.

What a shear wall is and how sheathing makes it stiff

A basic framed wall is just a row of vertical wood studs nailed between a top and bottom plate. A simple rectangle of studs can be pushed out of shape. Nailing a rigid sheet of plywood or oriented strand board across those studs turns that flexible rectangle into a stiff panel.

This assembly is a shear wall. APA, The Engineered Wood Association, says wood structural panel sheathed shear walls and diaphragms are the primary lateral load resisting elements in most wood frame buildings. The wood panel acts as a continuous web. It locks the vertical studs and horizontal plates together so they cannot shift independently.

The sheathing attachment matters a great deal, and the APA page says its calculator also determines the shear force for sheathing attachment. The engineer's plan sets the nail type and spacing, and a shear wall passes its load down through the nails, into the bottom plate and into the foundation.

Braced wall panels and the braced wall plan

Not every exterior wall in a house operates as a shear wall. Building codes require specific sections of wall to act as bracing. These designated sections are called braced wall panels. Where these panels go depends on the size of the house, the local wind zone and the seismic risk, and your building department or an engineer sets that.

A braced wall plan shows where the braced wall panels go and which connectors they need. Ask your local building department what plan and inspections apply to your house, because requirements differ by place.

Framers often use temporary wall supports during construction. These are long diagonal boards nailed across the bare studs to keep the walls plumb while building. A crew keeps these temporary braces until the permanent sheathing is in place.

Hold downs, straps and anchor bolts

A stiff shear wall transfers the sideways force to the ground. That transfer creates a new mechanical problem at the base of the wall. Pushing horizontally against the top of a stiff panel makes one bottom corner pivot downward and the opposite bottom corner lift upward. Heavy metal connectors help stop the wall from sliding or overturning.

Anchor bolts secure the bottom horizontal board, known as the sill plate, directly to the concrete foundation. California's Earthquake Brace and Bolt program says bolting a house to its foundation helps prevent sliding, and bracing cripple walls with plywood may keep it from toppling. An engineer or the building department sets bolt size and spacing for each house.

Hold downs handle severe lifting forces as massive steel brackets installed at the extreme ends of a shear wall. APA's shear wall calculator computes hold down force for uplift resistance at the ends of wall segments. Hold down details vary with the design, so they come from an engineer's plan and never from guesswork.

Steel tension straps tie the upper floors to the lower floors. In a two story house, the upper walls must pass their loads down to the walls below, and an engineer specifies the straps or other ties that do it.

Finding these parts in a garage or crawl space

Most shear walls are hidden by drywall on the inside and siding on the outside. You can still spot the hardware if you know where to look. An unfinished garage often leaves the bare wood framing exposed. Stand in the garage and look at the bottom of the walls near the large garage door.

A wide garage door creates a massive hole in the front wall. The narrow slivers of wood framing on either side must do a lot of work to keep the garage standing under lateral loads. You may see a steel hold down bolted to the concrete floor and attached to a post beside the door, though not every house has one.

You can also find structural connectors in an unfinished basement. Look at the top of the concrete foundation wall. You will see steel anchor bolts poking through the wood sill plate. Look closely if you have a raised foundation space under the first floor.

Many raised foundations have short wood walls, called cripple walls, framed between the concrete footing and the floor joists. They can be a weak point in older homes. Earthquake Brace and Bolt retrofits add structural plywood to these short walls and new anchor bolts to the sill plate. Earthquake Brace and Bolt retrofits are only for wood framed California homes built before 1980 on a raised foundation, and must follow chapter A3 of the California Existing Building Code, so elsewhere ask your building department.

Why new openings in braced walls need an engineer

A braced wall relies on a continuous sheet of wood nailed tightly to the framing. Cutting a large hole for a new window or a wide double door destroys that structural panel. The wall loses its stiffness and can no longer resist lateral forces. This is why cutting large openings in exterior walls should involve a permit and a structural review, so check with your building department.

An engineer must calculate how to replace the lost shear strength. The fix depends on the wall and might include added bracing or hold downs, which is the engineer's call.

You cannot safely guess how much bracing a wall needs. Cutting into a shear wall without an approved plan can leave the house weaker against wind and earthquakes. Have a licensed pro draw a new braced wall plan before altering an exterior wall. Taking out interior walls requires the same care, and removing a load bearing wall always involves an engineer.