Section 1805: Footings and foundations

January 2008 » Columns
For engineers new to the 2006 International Building Code or even for current users, finding the requirements for footings supporting light frame construction that compare to previous model codes may not be easy.
John R. Henry, P.E.

For engineers new to the 2006 International Building Code (IBC) or even for current users, finding the requirements for footings supporting light frame construction that compare to previous model codes may not be easy. The following is a series of questions pertaining to foundation walls and footings supporting light frame walls.

Answers to FAQ’s:

Q:
One of the legacy code tables gives the minimum depth of footings supporting light frame walls in terms of the number of floors supported plus one roof/ceiling. Notice that the footing depth is not given in IBC Table 1805.4. What is the minimum depth of footings supporting light frame walls?
A: Footnote "a" of Table 1805.4.2 refers to Section 1805.2 for footing depth, which specifies a minimum footing depth of 12 inches or the depth of the frost line, whichever is deeper. There are exceptions for Occupancy Category I buildings or buildings sited on solid bedrock.

Q: What is the minimum required width of footings?

A: Section 1805.4.1 states that the minimum width of footings is 12 inches. Section 1805.4.2 permits the use of Table 1805.4.2 for footings supporting light frame walls. Footnote "c" permits interior stud bearing walls to be supported by isolated footings provided the width and length are twice the minimum width given in the table. See also discussion of concrete footing requirements in Seismic Design Category C, D, E, and F below.

Q: IBC Table 1805.4.2 does not specify the minimum thickness of stem walls used in raised floor foundations as one of the legacy codes did. What is the minimum stem wall thickness for conventional raised floor foundations under the IBC?

A: Footnote "e" of Table 1805.4.2 refers to Section 1805.5 for foundation wall thickness, which in turn refers to Section 1805.5.1.1 and the foundation wall tables. The minimum thickness at the top of a foundation wall cannot be less than the thickness of the wall supported. For masonry foundation walls, the minimum nominal thickness of 8 inches is given in Table 1805.5(1) for unbalanced backfill height less than or equal to 4 feet. For concrete foundation walls, Table 1805.5(5) specifies a minimum thickness of 7.5 inches.

Q: Are there any special seismic requirements for these concrete foundation walls in Seismic Design Category D, E, and F?

A: Yes. Section 1805.5.5.1 contains specific seismic requirements for foundation walls. For buildings in Seismic Design Category C, D, E, and F, the foundation wall tables are not permitted unless they meet the specific requirements for plain concrete stipulated in Section 1908.1.15. Plain concrete basement and foundation walls are permitted for one- and two-family dwellings three stories or less in height provided the following requirements are met:

* For dwellings in Seismic Design Category D and E, the height of the wall is limited to 8 feet, the minimum wall thickness is 7.5 inches, and the maximum retained unbalanced fill height is 4 feet.

* The wall must be reinforced with two #5 bars around all door and window openings extending 24 inches beyond the corners in accordance with Section 22.6.6.5 of ACI 318.

Q: Are there any special seismic requirements for plain concrete footings in Seismic Design Category D, E, and F?

A: Yes. Section 1805.9 refers to Section 1908 for additional requirements for footings in Seismic Design Category C, D, E, and F. Section 1908.1.15 states that plain concrete continuous footings are permitted in Seismic Design Category C, D, E, and F provided the following requirements are met:

* Plain concrete continuous footings require two continuous longitudinal bars (not less than #4) with an area of not less than 0.002 times the cross-sectional area of the footing; footings greater than 8 inches thick require one longitudinal bar at the top and one at the bottom of the footing.

* Where plain concrete footings and stem walls are used, one longitudinal bar may be located at the top of the stem wall and one at the bottom of the footing.

* Where a slab on grade is cast monolithically with the footing, one longitudinal #5 bar may be located at either the top of the slab or at the bottom of the footing.

* For detached one- and two-family dwellings three stories or less in height constructed with stud bearing walls, plain concrete footings without longitudinal reinforcing are permitted.

Additionally, plain concrete isolated footings supporting columns or posts are permitted in Seismic Design Category C, D, E, and F provided the following requirements are met:

* The projection of the footing beyond the post or column does not exceed the thickness of the footing (P < T).

* For detached one- and two-family dwellings three stories or less in height, the projection of the footing beyond the post or column is permitted to exceed the thickness of the footing.

Disclaimer: The building official has the final authority in rendering interpretations of the code as prescribed in IBC Section 104.1. Opinions by ICC staff are based solely on the published model code and do not consider federal, state, or local amendments, ordinances, or policies.

John R. Henry, P.E., is the principal staff engineer at the International Code Council’s Los Angeles office. He can be reached at jhenry@iccsafe.org.

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