ENGINEERING NO CRIPPLE WALL RETROFITS

By Jim Russel P.E. (P.E. stands for professional engineer)

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.

They include:

1. 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)

2. 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)

3. 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)

4. 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.

5. 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)

Seismic V = 0.186 W (2001 CBC Equation 30-5)

Certain assumptions are made with respect to the capacities of the new materials added to strengthen the buildings.  They include:

1. 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).

2. 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.

3. 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:

1. The floor to ceiling wall height is 8 feet.

2. The roof slope is 4:12, with gable ends occurring on the short (transverse) side, and two-foot eave overhangs on all sides.

3. 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.

4. 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 are 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).

5. 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.

6. 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.

7. 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)