Design is Everything
Engineering is literally everything behind a good seismic retrofit. A single engineering oversight can cause catastrophic failure. If an inexperienced engineer or design/ build contractor designs your retrofit you may end up spending thousands and getting nothing for it.
A designer must consider the following: What was the building code when the house was built? How did houses like yours perform in previous earthquakes? Does it have a cripple wall? If so, which of the 4 mudsill connections will work best? Is the cripple wall made of redwood or Douglas Fir? How tall are they? Is the outside of the house stucco or wood? Are the floor boards straight, or are they placed at an angle? Does the entire house have a cripple wall, or just part of it? Is the foundation concrete, brick, or capped concrete? Are the walls balloon framed or platform framed? If there is no cripple wall, which hardware will fit and how earthquake resistant is it? What does the photographic record and reports on observed damage tell us about failure points and how to deal with them?
In other words, there are a lot of things you need to consider. Only someone who has taken the time to research these things will know how properly design a retrofit.
Someone Already Did the Engineering
These 85 pages of calculations are the mathematical basis of Standard Plan A. There is no reason to pay someone for calculations if a nationally recognized retrofit engineer (Jim Russell) did the calculations for you. I was on the committee that put Standard Plan A together and Jim taught me how to retrofit homes using these calculations. We apply these calculations to every house we retrofit.
Substantiating Data for Cripple Wall Bracing and Sill Bolting for the Seismic Retrofit of One and Two Story Dwellings for Standard Plan A. If you want to see the full 80 pages of calculations you can do so here.
By Jim Russel P.E.
The Engineering Basis of Standard Plan A
The following calculations determine the seismic load demand to cripple walls and foundation sill plates for conditions commonly found in existing wood-framed residential buildings located in the San Francisco Bay Area and are the engineering basis behind Standard Plan A. These demands are the basis for the cripple wall bracing and foundation sill anchorage requirements contained in the East Bay and Peninsula Chapter of ICC Seismic Retrofit Provisions. Certain assumptions are made in the calculation of these demand loads.
- Wood structural panels are used to brace the cripple walls, and the buildings are limited to a maximum of two stories. Therefore, the R factor used is 5.5. (2001 CBC Table 16-N)
- The Redundancy Factor rho (r) = 1.0, because the cripple wall bracing lengths along each exterior wall in each axis are equal, or are nearly equal. (2001 CBC Sec.1630.1.1)
- The Near Source Factor (Na) = 1.3, to account for buildings that are located between 4 and 10 kilometers of a Type A fault. This value is less than the maximum Na = 1.5 specified for locations 2 kilometer or less from a Type A fault, but is greater than the Na = 1.1 value permitted for buildings that are, 1) located on soil classified not greater than type SD, 2) are not defined by the code as being irregular, and 3) have rho = 1.0. (CBC Sec.1629.4.2 and Tables 16-L, 16-M, and 16-S)
- New resisting elements are located at the building perimeter only, therefore, one-half of the total seismic load in each axis is resisted by each of two parallel perimeter wall lines.
- No reduction from current code force levels is being taken, as is permitted by Section 301.3 of the Guidelines for Seismic Retrofit of Existing Buildings. (ICBO, 2001)
Certain assumptions are made with respect to the capacities of the new materials added to strengthen the buildings. They include:
- Allowable stresses are increased by a factor of 1.33 for short term seismic loads, or are based on tabular values already adjusted for seismic loading (2001 CBC Table 23-II-I-1).
- For determining bolt capacities, foundation sill plates are considered to be tight grain Redwood. Based on observations, and some limited testing, the dowel bearing strength of this species is considered to be equivalent to Douglas Fir having a specific gravity of 0.50. Bolt capacity is determined using one-half of the allowable double shear capacity for a sill plate twice the thickness of the actual 2x sill plate (2001 CBC Sec 2316.2 Item 24, amending 1991 NDS Sec. 8.3), and taking a 1.33 increase for duration of load. The resulting sill bolt capacities are ½” diameter = 820 pounds; 5/8” diameter = 1,170 pounds.
- Other wood members transmitting loads are assumed to be Douglas Fir and nails are assumed to be common wire diameter.
The buildings used to develop the seismic forces assume a rectangular building footprint where:
• For one-story buildings the footprint sizes are: 1) 30 feet by 40 feet (1,200 square feet)
2) 30 feet by 50 feet (1,500 square feet)
3) 36 feet by 56 feet (2,016 square feet)
• For two-story buildings the footprint sizes are: 1) 30 feet by 30 feet (1,800 square feet)
2) 30 feet by 40 feet (2,400 square feet)
3) 30 feet by 50 feet (3,000 square feet)
The following assumptions have also been made regarding the construction of the houses:
- The floor to ceiling wall height is 8 feet.
- The roof slope is 4:12, with gable ends occurring on the short (transverse) side, and two foot eave overhangs on all sides.
- Four Cases of exterior and interior wall finish and roofing are considered.
A) Lightweight roofing (5 psf) of wood shake, wood shingle, or composition shingle, exterior wood sheathing or board finish, and ½” gypsum wallboard interior finish.
B) Lightweight roofing, exterior wood sheathing or board finish, and gypsum lath and plaster interior finish. This is considered the definition of “Light Construction”
C) Lightweight roofing, cement plaster (stucco) exterior finish, and gypsum lath and plaster interior finish.
D) Heavy roofing (11 psf) of concrete or clay tile, cement plaster (stucco) exterior finish, and gypsum lath and plaster interior finish. This is considered the definition of “Heavy Construction. Certain types of clay tile using mortar setting for the tile will exceed this unit weight and therefore should be excluded from using these prescriptive methods.
- Interior partitions are framed with 2×4 studs at 16″ o.c. with either ½” gypsum wallboard (for 3A) or 3/8” gypsum lath and gypsum plaster (for 3B, 3C or 3D) on each side. The lath and plaster is a heavier wall finish (4.5 psf) than standard ½” thick gypsum wallboard (2.2 psf). Ceilings below attics and below a second floor are assumed to be either ½” gypsum wallboard (for 3A) or 3/8” gypsum lath and gypsum plaster (for 3B, 3C or 3D).
- The assumed layout of interior walls in a single story building is two in the long (longitudinal) direction and three cross walls in the short (transverse) direction. The assumed layout of interior partitions in a two-story building are two in the long direction and three in the short direction at the upper floor level, and one in the long direction and two in the short direction at the first floor level.
- Exterior walls are framed with 2×4 studs at 16″ o.c. with either wood board or panel siding (for 3A or 3B), or cement plaster (stucco) exterior wall finishes (for 3C or 3D). The interior finish of exterior walls is assumed to be either ½” gypsum wallboard (for 3A) or 3/8” gypsum lath and gypsum plaster (for 3B, 3C or 3D). Attic gable end walls are assumed to be unfinished on the interior face.
- The site is assumed to have no slope along an exterior wall line greater than 1:10 and cripple walls are limited to 4 feet in height at any point.
TABLE 3A – SUMMARY OF UNIT LOADS
Assume SD soil with Ca = 0.44; Na = 1.3; I = 1.00; and R = 5.5; Conversion to ASD force level: 1 / 1.4 Seismic V = 0.186 W (2001 CBC Equation 30-5)
The basic unit dead loads used to calculate the seismic loading demand for Case 3A are:
Roof/ceiling system: Light roofing and gypsum board ceiling finish. Light roofing is defined as wood shakes over spaced sheathing or wood shingles or composition shingles over solid sheathing. Vertical load adjustment for 4:12 roof slope = 1.054
Light roofing system: 5.0 psf
Rafters & ceiling framing: 2.5 psf
Gypsum wallboard: 2.2 psf
Miscellaneous: 0.8 psf
Light roof Total: 10.5 x 1.054 = 11.0 psf
Second floor/ceiling system: Gypsum wallboard is assumed to be the interior ceiling finish.
Carpet and fiber pad or finished wood flooring: 1.5 psf
7/8″ thick wood subflooring: 2.5 psf
2 x 10 joists at 16″ spacing: 2.5 psf
Gypsum wallboard: 2.2 psf
Second floor typical Total: 8.7 psf(9.0 psf used)