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State of Oregon
Building Codes Division Staff 
Technical Advisory

   
The following advice applies to current code and conditions described in the question. Applications for similar situations may have different determinations.
   
   
PROGRAM: One and Two Family Dwelling Program
SUBJECT: Concrete Foundation Walls
SOURCE: 1996 One and Two Family Dwelling Specialty Code (OTFDSC)
REFERENCE: Section 404.1
DATE OF ISSUE: May 29, 1997
PREPARED BY: C. Darin Wilson, P.E., Code Specialist, (503) 373-1354
   
QUESTION: Section 404.1.1 of the OTFDSC references Table 404.1.1a and Table 404.1.1b for requirements for concrete foundation walls. What are the minimum wall thickness' required for concrete foundation walls supporting wood frame construction?

DETERMINATION: For concrete foundation walls supporting wood frame construction, the most restrictive applicable requirements of Table 404.1.1a, Table 404.1.1b and Table 403.2 shall be used to determine minimum wall thickness.

ANALYSIS: The determination above results from review and analysis by staff as follows:

  1. Section 404.1.1 references Table 404.1.1a and Table 404.1.1b for requirements for concrete foundation walls. These tables are used to determine the minimum wall thickness' due to unbalanced fill conditions in various seismic zones.
  2. Section 403.1 references Table 403.1 and Figure 403.1 or Table 403.2 for concrete footings.
  3. Table 403.2 is a specific Oregon amendment for foundations supporting wood frame construction. It contains additional requirements for concrete foundation walls due to number of floors supported by the foundation.
  4. Table 403.2 is essentially identical to Oregon Structural Specialty Code (OSSC) Table 18-I-D.
  5. Section 103 requires that where different sections of the OTFDSC specify different methods of construction, the most restrictive shall govern. In this case, requiring greater width foundation walls of structural plain concrete provides greater structural integrity in higher seismic regions, such as Seismic Zone 3.
  6. Due to OTFDSC Section 103 and the structural benefits of wider unreinforced concrete foundation walls, the most restrictive applicable requirement from Tables 404.1.1a, 404.1.1b, and 403.2 shall be used to determine the minimum wall thickness' required for concrete foundation walls supporting wood frame construction.
  7. Example 1: A one-story wood frame structure with a plain concrete foundation is proposed in Seismic Zone 3. The height of unbalanced fill on the foundation wall is 2 feet. Table 404.1.1b requires a minimum 6-inch unreinforced wall. Table 403.2 also requires a minimum 6-inch wall. Therefore, a minimum 6-inch unreinforced wall is required.

    Example 2: A one-story wood frame structure with a plain concrete foundation is proposed in Seismic Zone 3. The height of unbalanced fill on the foundation wall is 5 feet. Table 404.1.1b requires a minimum 8-inch wall and reinforcing. Table 403.2 only requires a minimum 6-inch wall. Therefore, a minimum 8-inch reinforced wall is required.Example 3: A two-story wood frame structure with a plain concrete foundation is proposed in Seismic Zone 3. The height of unbalanced fill on the foundation wall is 2 feet. Table 404.1.1b requires a minimum 6-inch unreinforced wall. Table 403.2 requires a minimum 8-inch wall. Therefore, a minimum 8-inch unreinforced wall is required.