The Basics of a Seismic Retrofit
A seismic retrofit is the process where a home or building is upgraded structurally to better withstand the seismic forces it might experience from an earthquake. Reinforcement and additional connections are made at weak points so that the building is tied together and anchored to the foundation.
Common failure modes for residential homes are: the home sliding off the foundation and basement walls (also called cripple walls or pony walls) collapsing. To improve upon these weaknesses, anchors bolts are added and shear walls are installed. Anchor bolts prevent the home from sliding on the foundation while shear walls carry the load of the upper story and safety transmit it to the foundation. To create a shear wall we reinforce the existing studs between the foundation and the floor joists above with sheathing and add framing angle brackets.
The details such as location of hardware, how many, and what type are prescribed by engineering calculations to ensure they meet the code.
Photo credit: Simpson Strong-tie