A soft story building
To provide a “Seismic Retrofit” for a building means to add lateral stability to its structure by strengthening its walls (adding “shear value”) as well as transferring some of the load onto its foundations in order to prevent structural failure in the event of seismic movement or better known as an Earthquake.
Some wood frame buildings (constructed prior the year 1978) are defined as Soft Story buildings. A soft story building is a multi-story building in which one or more floors have windows, wide doors, large unobstructed commercial spaces, or other openings in places where a shear wall would normally be required for stability as a matter of earthquake engineering design.
If the open area of Soft Story structure has immediate walls next to it, those walls can be retrofitted by replacing the finishes of those walls (stucco or drywall) with sturdier plywood and anchoring them to the foundation. Adding shear walls — walls built for the sole purpose of adding lateral stability — can also be very effective, provided they don’t interfere with the structure’s function.
Steel moment frame
In the event of a Soft Story building where the openings are too wide or there aren’t any walls immediately attached to the opening, a steal moment frame reinforcement is required in order to meet the lateral stability required by the building Code. The steal frame must be bolted onto a plate and anchored to the foundation of the building. Such frames may be more expensive and dangerous to install than other techniques, due to the fire hazard and noxious fumes generated from welding them in place. The frame can be delivered to the jobsite either as a two columns and a connecting beam, or fully pre-assembled into a single unit.A full frame has two columns at the height of the soft story floor (roughly 8’) and one beam in a width of the open bay (anywhere between 8’ to 14’). Moment frames can be bolted on the job site or welded in the fabrication process and delivered to the site in one piece.
In addition to strengthening the walls or installing a moment frame, continues foundations must be poured under the frame (or next to the shear wall) using a minimum 3000 PSI concrete with a large number of rebars to transfer some the load into the foundation.