International Building Code

SUBJECT: Wood Supporting Masonry or Concrete
CHANGE TYPE: Modification

2304.12 Wood Supporting Masonry or Concrete.
Wood members shall not be used to permanently support the dead load of any masonry or concrete.

Exceptions:
1. Masonry or concrete nonstructural floor or roof surfacing not more than 4 inches (102 mm) thick is permitted to be supported by wood members.

2. Any structure is permitted to rest upon wood piles constructed in accordance with the requirements of Chapter 18.

3. Veneer of brick, concrete or stone applied as specified in Section 1405.5 having an installed weight of 40 pounds per square foot (psf) (1.9 kN/m2) or less is permitted to be supported by an approved treated wood foundation when the maximum height of veneer does not exceed 30 feet (9144 mm) above the foundation. Such veneer used as an interior wall finish is permitted to be supported on wood floor construction. The wood floor construction shall be designed to support the additional weight of the veneer plus any other loads and to limit the deflection and shrinkage to 1/600 of the span of the supporting members.

4. Glass unit masonry having an installed weight of 20 psf (0.96 kN/m2) or less is permitted to be installed in accordance with the provisions of Section 2110. The wood construction supporting the glass unit masonry shall be designed for dead and live loads to limit deflection and shrinkage to 1/600 of the span of the supporting members.

2304.12 Long-Term Loading. Wood members supporting concrete, masonry or similar materials shall be checked for the effects of long-term loading using the provisions of the NDS. The total deflection, including the effects of long-term loading, shall be limited in accordance with 1604.3.1 for these supported materials.

Exception: Horizontal wood members supporting masonry or concrete nonstructural floor or roof surfacing not more than 4 inches (102 mm) thick need not be checked for long-term loading.

CHANGE SIGNIFICANCE:

The provisions in the 2003 International Building Code were intended to address concerns with long-term creep in wood members supporting concrete and masonry, by imposing a restriction against wood members being used to permanently support dead load of masonry or concrete, followed by a list of exceptions to the restriction.

Under sustained loading, wood members exhibit additional time-dependent deformation (creep), which generally develops over long periods of time. The tabulated modulus of elasticity design values, E, in the National Design Specification (NDS) are intended to be used to estimate immediate deformation under load. Where dead loads or sustained live loads represent a relatively high percentage of total design load, creep is an appropriate design consideration that is addressed within the NDS. The total deflection under long-term loading can be estimated by increasing the initial deflection associated with the long-term load component by 1.5 for seasoned lumber or 2.0 for unseasoned or wet lumber or glued laminated timber. Section 3.5.2 and Appendix F of the NDS contain provisions to account for the time dependent deformations known as creep.

The reference standard for the design of the wood structures has been updated to the 2005 NDS with 2005 NDS Supplement.