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A two-story apartment building with tuck-under parking: the apartments sit on a ground floor that is mostly open for cars, held up by a few posts along the front. That open, flexible story is the “soft story”: in a strong earthquake it can lean over and collapse. The retrofit adds a new concrete footing, two steel moment frames across the open front, and plywood shear walls on the back and side walls. The existing building is white. The parts are below, in the order the steps use them.
≈3–6 weeks of construction for a building like this, after engineering and permit
This guide shows the sequence, not a design. Every soft-story retrofit is designed by a licensed engineer for that building; frame sizes, footings, connections and wall lengths come from the approved plans.
An engineer surveys the building, measures the existing framing and foundation, and designs the retrofit: where steel frames go, how big the new footing is, and which walls get plywood. The city reviews the plans and calculations before the permit is issued.
≈2–6 months for engineering and plan check, depending on the city · Engineered plans + calculations · permit before work starts
Parts: 1 set × Engineered plans, calculations and permit
Los Angeles and a number of other Southern California cities require many older wood-frame buildings with a soft, weak or open-front ground floor (in Los Angeles, generally those permitted before 1978) to be retrofitted. Many cities also require a plan for keeping tenants safe and housed while the work is done.
Where the steel frames will stand, the parking slab is saw-cut and broken out, and a trench is dug for a new concrete footing (a grade beam) that will anchor the frames. Here it runs 33 ft along the front, 18 in wide and about 2½ ft deep.
≈2–3 days · Grade beam trench ≈33 ft × 18 in × 2½ ft deep
Footing size and depth come from the engineer. Buried lines are located before the saw cut, and where the trench runs next to an existing post footing the plans may call for temporary shoring. Parking is usually closed on the work side while the trench is open.
A rebar cage goes into the trench: long bars top and bottom, tied with closed loops. Anchor rods for each steel column are held in exact position by a steel template, so the column base plates will drop right over them.
≈1–2 days, plus inspection · 4 templates · 16 anchor rods · bar sizes per the plans
Parts: ≈20 bars × Rebar cage: 20 ft bars, cut and bent into bars and ties; 16 × Anchor rod, set in a steel template; 4 × Anchor-rod template
The building inspector, and a special inspector where the plans call for one, check the rebar and anchor rods before the pour.
Concrete fills the trench up to the top of the parking slab, worked in around the cage and the anchor rods. The rods stick up out of the new concrete, ready for the columns.
≈1 day, then curing · 33 × 1½ × 2½ ft ≈ 124 cu ft ≈ 4.6 yd³ (≈5 yd³ ordered)
Parts: ≈5 yd³ × Concrete, ready-mix
The steel goes up only after the concrete has reached the strength the engineer specifies.
Each frame is two steel columns and a beam joined with rigid, moment-resisting connections, welded or bolted per the plans. Each column arrives with its base plate welded on, drops over the anchor rods and is leveled on nuts; the gap under each plate is then packed with non-shrink grout. The frames resist sideways racking without blocking the parking.
≈2–4 days · 2 frames · 4 columns · 2 beams
Parts: 4 × Column base plate (shop-welded) and leveling nuts; 4 × Steel column (wide-flange); 2 × Steel moment-frame beam (wide-flange)
Field welding and high-strength bolting are checked by a special inspector, as the plans require. Frame sizes and connection types come from the engineer.
A frame only helps if the earthquake load can get into it. A wood nailer is bolted to the top of each beam, and the floor framing above is fastened to the nailer with steel straps or clips, so the second floor and the frames move together.
≈2–3 days · 2 nailers · 12 bolts · 12 straps in this example
Parts: 2 × Wood nailer bolted to the beam; 12 × Bolt, nailer to beam flange; 12 × Steel strap or clip, nailer to floor framing
The garage ceiling is opened at the frame line to reach the floor framing (new blocking between joists is common), then closed back with fire-rated drywall, because that ceiling separates the parking from the apartments. The plans may also call for collectors that drag load into the frames from farther along the floor.
The solid back and side walls are turned into shear walls: structural plywood nailed closely along every edge to the studs and plates, new anchors bolting the sill plate down to the existing footing, and a hold-down at each end so the wall resists lifting. Clips at the top tie each wall to the floor framing above, so the load can reach it.
≈2–3 days · ≈405 sq ft of plywood · 15 sheets (14 by layout + 1 spare) · 6 hold-downs · 15 anchors
Parts: 15 × Structural plywood, 4×8 sheet; 6 × Hold-down and its anchor; 15 × New sill anchor with plate washer
Shear-wall nailing, anchors and hold-downs are inspected before anything covers them. If the footing under a wall is too small, the engineer will call for a new one first.
Along the way, special inspectors check the welds, high-strength bolts, epoxy anchors drilled into existing concrete and concrete where the plans require it, and the building inspector checks each stage against the approved plans. The final inspection closes the permit; in Los Angeles, the city then issues a Certificate of Compliance for the soft-story order.
Several short visits, spread over the job · Special inspection reports + final inspection
Keep the inspection reports, the signed-off permit and any Certificate of Compliance with the building’s records; buyers, lenders and the city may ask for them.
That’s a soft-story retrofit: a new grade beam, two steel moment frames across the open parking, the floor tied to them, and plywood shear walls on the solid walls. The ground floor is now far less likely to collapse in a strong earthquake. Retrofits like this are designed to keep the building standing so people can get out; some damage can still happen.