Chapter 16, Question 5
1604.3 Serviceability. Structural systems and members thereof shall be designed to have adequate stiffness to limit deflections and lateral drift. See Section 12.12.1 of ASCE 7 for drift limits applicable to earthquake loading.

Q: Table 1604.3 uses L to refer to roof live load when it comes to the roof member deflection limits. However, the load combinations in Section 1605 use L_r to designate roof live load, and L to refer to live loads such as occupancy (floor) live loads. Does this mean that the deflection limits do not apply to roof members?

A: The deflection limits do apply to roof members as described in the table. In fact, the rafter span tables are based on these deflection limits for various conditions. The deflection limit predates the specific notation used for live loads in the IBC, which probably originated with ASCE 7. (L_r is not used in the 1994 UBC. See Section 1605, Tables 16-C and 16-D). The intent is that the deflection limit applies to the design roof live load (L_r), which could be a reduced live load. For example, if a ridge beam supporting an open rafter ceiling was designed for a reduced live load of 16 psf, it would be checked for l/180 based on deflection limit criteria. Refer to the discussion in the AITC Timber Construction Manual. The term "applied load" is used instead of "live load." Also refer to Design of Wood Structures by Breyer et al., where the authors refer to the deflection limits applying to L or L_r.

This question and answer are from the "2009 IBC Q&A — Structural Provisions." The question is a commonly asked question that arises in the application of the code provision during design and plan review. The IBC section excerpt is reprinted for easy reference, followed by the question and answer pertaining to the section.

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