Slide 5 of 13
Notes:
- NEW PRESCRIPTIVE RETROFIT METHOD
- Assumes that houses have existing perimeter foundations and cripple walls
- These standards require:
- That a completely new continuous foundation be installed along all the exterior walls, OR
- That a retrofit using a partial perimeter foundation be designed by a licensed architect or engineer and be approved by the local building official.
- Before using this method:
- The prescriptive methods and details described in this section are consistent with the Uniform Code for Building Conservation (UCBC) and City of Los Angeles standards
- Methods use partial perimeter foundation system to provide equivalent earthquake resistance below the first floor level.
- Permission to use this partial perimeter system without a specific design prepared by a licensed design professional must always be obtained from the local building official.
- PARTIAL PERIMETER VS. CONTINUOUS FOUNDATION
- Where a continuous foundation exists or is added
- Where a continuous foundation exists or is added, the primary retrofit elements used in the UCBC, are also parts of the partial perimeter system. They include:
- A foundation constructed with a concrete footing and either a poured concrete or grouted masonry stem wall.
- Foundation sill plates and sill bolts with plate washers.
- Sheathed cripple wall with connections between the wall and floor framing.
- Note that a new cripple wall is not always necessary.
- A new foundation stem wall can be extended to the underside of the existing floor framing depending on how high the existing floor is above the surrounding exterior grade.
- Generally, if the floor is more than 3 feet above grade at any point, a cripple wall will be necessary.
- Additional retrofit elements
- Several additional retrofit elements are needed for a partial perimeter system that do not occur in the UCBC prescriptive standards. They include:
- New hold-downs at the ends of new sheathed cripple walls.
- Straps to connect the existing floor framing beams and joists together and to connect them to the new partial foundations and cripple walls.
- These are load path elements needed to compensate for the use of a partial foundation system compared to a continuous foundation system
- Certain elements must be stronger or larger
- Certain elements of a partial perimeter retrofit must be stronger or larger than those prescribed in the UCBC. They include:
- Nails used to attach plywood to the cripple wall framing must be 10d common instead of 8d common.
- The minimum width of the new footing for a one-story house must be 15 inches rather than 12 inches.
- All foundation sill bolts must be 5/8-inch diameter instead of 1/2-inch, and their spacing is reduced from 6 or 4 feet on center to 2-’6” or 2’-0” on center.
- HOW PARTIAL PERIMETER EQUALS CONTINUOUS FOUNDATION
- As discussed earlier in this presentation:
- The standard retrofit method for a house with an existing continuous foundation and cripple walls:
- Adds wood structural panels in sections along all the perimeter cripple walls
- Prescribes a minimum number of sill bolts connecting the foundation sill plate to the continuous foundation.
- Foundation itself should not need to be strengthened
- Exception: houses with existing brick or other unreinforced masonry foundations
- The foundation’s primary task is to:
- Resist the sliding forces transferred from the foundation sill bolts into the foundation
- Resist sliding against the surrounding soil equal to or greater than the sum of all the sill bolt forces.
- How does the foundation resist these forces?
- The bottom surface of the foundation resists these forces by friction between it and the ground.
- In addition, the vertical face of the foundation that is below the ground surface also participates in that resistance by pushing against the adjacent soil.
- Sliding friction and lateral bearing against the soil are the final link in the load path.
- The amount of surface area a foundation must provide to resist a specific amount of earthquake load is based on the characteristics of the soil that determine its sliding and lateral bearing resistance.
- Chapter 18 of the Uniform Building Code provides numerical values for sliding and lateral bearing resistance of various soil types.
- Partial perimeter system
- Minimum depth, width and lengths of foundation used in the partial perimeter system are based on soils having the least resistance.
- Based on calculations, a continuous foundation is not required to provide the necessary resistance for average sized houses up to two stories in height.
- The minimum foundation consists of four separate, 15 inch wide “L” shaped footings, one at each of the building corners.
- For a one-story house the length of each leg of the “L” must be 8’-0”
- For a two-story house each leg must be 12’-0”.
- Along each leg of the one story house foundation four sill bolts are required
- Along each leg of the two-story house foundation five bolts must be provided to attach the new foundation sill plate.
- One-story post and pier house with existing floor framing within 3 feet of the ground
- When a one-story post and pier house has its existing floor framing within 3 feet of the ground surface, constructing a new cripple wall above the new partial foundation should not be necessary.
- The existing floor joists and girders can be directly connected to the new foundation sill plate.
- One-story post and pier house with existing floor framing greater than 3 feet of the ground
- For floor framing located grater than 3 feet above the ground surface, a new continuously sheathed cripple wall located above the entire length of each of the new “L” shaped foundations will be needed.
- The length of sheathing provided by the partial perimeter system is equivalent to the length provided by the multiple individual sheathed sections prescribed in the UCBC retrofit standards for a house with a continuous foundation and cripple wall.
- Moisture effects on wood materials
- Post and pier type foundations are commonly found in coastal areas
- The partial perimeter system was specifically developed to address existing post and pier houses.
- It recognizes that post and pier type foundations are commonly found in coastal geographical locations where moisture content of the air and the ground are very high for much of the year.
- High moisture levels can have a very undesirable effect on wood framing members and will also decrease the strength of nailed or bolted connections in the wood.
- Very important to examine all the existing wood members used in load path
- One of the most important considerations when retrofitting any wood framed building is to examine all the existing wood members to be used in the load path
- Determine if they need to be replaced because of fungus infections, commonly called “dry rot”, that destroy the wood fibers.
- Particular attention to presence of “dry rot” must be paid while inspecting all existing wood members used in the retrofit of these coastal houses.
- Unlike a house with a continuous perimeter foundation and cripple wall, where the underfloor space is fully enclosed and weather protected, the underfloor area of a partial perimeter foundation retrofit is open to the exterior along a substantial length of the perimeter.
- Moisture content of wood does not remain stable.
- Moisture content of wood in damp climates does not remain stable.
- Undergoes cycles where it is very moist and then dries.
- As moisture content changes, wood fibers alternately swell and shrink
- Changes the holding power of nails and the tightness of bolts in the wood.
- Addressed in the Building Code by the use of a “Wet Service Reduction Factor” to reduce the strength of nailed and bolted connections.
- All the retrofit connections in the prescriptive partial perimeter system involving nails or bolts in wood have been adjusted to lower their strength to 75 percent of normal to account for this effect.
- This is the principal reason that 10d nails are needed to attach sheathing to cripple wall framing, and why 5/8-inch sill bolts are typically used.
- Type of sheathing prescribed for use on cripple walls of a partial perimeter system was chosen to address the moisture exposure issue.
- Exterior grade plywood is specified because it has a very high durability for exposure to moisture.
- Plywood rather than Oriented Strand Board, known as OSB, was selected because OSB has different moisture expansion characteristics that make it less desirable for use where its moisture content is expected to vary.
- For a more detailed discussion of sheathing materials see the Shear Walls section titled How to Install Sheathing.
