Plan View:Partial Perimeter Foundation System

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• 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.
      

(See continuation on slides 14-15)