Resources

Seismic Retrofit Basics

Portland Bureau of Development Services teamed up with Steve Gemmell to learn about what it takes to attach a home to its foundation.

Glossary of Seismic Terms

SR WALL SYSTEM

The SR wall system is a combination of concrete and wood framing used to sandwich an existing foundation in place against the soil on the outside of the house. It uses a closed cell expanding foam technology that keeps the foundation pressed against the soil and attached to the wood framing of the new SR wall. The SR wall itself is a combination of wood sheer wall framing on top of new concrete footings that is attached to the house framing as well as the old foundation. 

SHEER WALL

Sheer wall typically consists of a wood frame plywood sheeted wall that is fastened down to a footing used to keep a structure from sheering off its foundation. 

FOUNDATION

The foundation is the support structure of a building that sits directly on the soil of the property. High density. 

SILL PLATE AND JOIST SYSTEM

The SR wall is actually attached to the sill plate joist system above it; it is the attachment to the house. The joist system refers to the floor joists that hold up the floor membrane of the first and second floors. 

The important thing in seismic retrofitting about the joist system is that the first floor joist system is the first real structure of the house. That’s really important because there’s another piece of wood that sits below that above the foundation called the sill plate. The sill plate was originally introduced to keep the floor joist system from moving while building a house. So the sill plate had no structural value at the time. What’s fundamental to that is that most seismic residential retrofits are a two step process…

You have to make a connection from the foundation up to the sill plate as your first connection to the wood framing and then from the sill plate up into the floor joist system because the sill plate isn’t connected to the house well enough if hit by a earthquake. If we just connect the foundation to the the sill plate only and not the sill plate to the joist system the house can still slide off the sill plate. 

SEISMIC RETROFIT

The connection of the wooden structure of the building down to its masonry foundation. 

SOFT STORY AND PORTAL FRAME RETROFITS

The concept of having all the posts holding up a building with no lateral support creates a “soft story.” For example, with a garage you have a big hole. The garage seismic retrofit becomes more costly because it’s actually a soft story.