These prescriptive measures shall apply only to two-story buildings and only when deemed appropriate by the code enforcement official. These prescriptive measures rely on rotation of the second floor diaphragm to distribute the seismic load between the side and rear walls of the ground floor open area. In the absence of an existing floor diaphragm of wood structural panel or diagonal sheathing, a new wood structural panel diaphragm of minimum thickness of 3/4 inch (19 mm) and with 10d common nails at 6 inches (152 mm) on center shall be applied.
To qualify for these prescriptive measures, the following additional conditions need to be satisfied by the retrofitted structure:
- Diaphragm aspect ratio L/W is less than 0.67, where W is the diaphragm dimension parallel to the soft, weak or open-front wall line and L is the distance in the orthogonal direction between that wall line and the rear wall of the ground floor open area.
- Minimum length of side shear walls = 20 feet (6096 mm).
- Minimum length of rear shear wall = three-fourth of rear wall.
- No plan or vertical irregularities other than a soft, weak or open-front wall line.
- Roofing weight less than or equal to 5 pounds per square foot (240 N/m2).
- Aspect ratio of the full second floor diaphragm meets the requirements of the building code for new construction.
The anchor bolt size and spacing shall be a minimum of 3 / 4 inch (19 mm) in diameter at 32 inches (813 mm) on center. Where existing bolts are inadequate, new steel plates bolted to the side of the foundation and nailed to the sill may be used, such as an approved connector.
Shear wall top plates shall be connected to blocking or rim joist at upper floor with a minimum of 18-gage galvanized steel angle clips 4 1 / 2 inches (114 mm) long with 12-8d nails spaced no farther than 16 inches (406 mm) on center, or by equivalent shear transfer methods.
The shear wall sheathing shall be a minimum of 15 / 32 inch (11.9 mm) 5-Ply Structural I with 10d nails at 4 inches (102 mm) on center at edges and 12 inches (305 mm) on center at field; blocked all edges with 3 by 4 or larger. Where existing sill plates are less than 3-by thick, place flat 2-by on top of sill between studs, with flat 18-gage galvanized steel clips 4 1 / 2 inches (114 mm) long with 12-8d nails or 3 / 8 -inch diameter (9.5 mm) lags through blocking for shear transfer to sill plate. Stagger nailing from wall sheathing between existing sill and new blocking. Anchor new blocking to foundation as specified above.
Shear walls shall be provided with hold-down anchors at each end. Two hold-down anchors are required at intersecting corners. Hold-downs shall be approved connectors with a minimum 5 / 8 -inch diameter (15.9 mm) threaded rod or other approved anchor with a minimum allowable load of 4,000 pounds (17.8 kN). Anchor embedment in concrete shall not be less than 5 inches (127 mm). Tie-rod systems shall not be less than 5 / 8 inch (15.9 mm) in diameter unless using high strength cable. Threaded rod or high strength cable elongation shall not exceed 5 / 8 inch (15.9 mm) using design forces.