This question seems to come up quite often: what importance factor should be used for a large office building? The answer is apparently less than clear. Here’s my take:

The intent of the 2006 IBC is to require Occupancy Category III (with its attendant 1.15 wind importance factor) for buildings “whose primary occupancy is public assembly with an occupant load greater than 300” (emphasis added). The principal staff engineer for ICC gave the following explanation in the November 2006 issue of Structural Engineer magazine:

Table 1604.5 (S7- 04/05)—The 2006 IBC criterion for Occupancy Category III buildings has been changed to read “covered structures whose primary occupancy is public assembly with an occupant load greater than 300” to achieve consistency with the legacy model codes and the NEHRP Provisions. The 2003 IBC table stated, “buildings where more than 300 people congregate in one area,” which was confusing and led to inconsistent interpretation and enforcement of the provision. It appeared to apply to any occupancy classification, even large commercial buildings where an occupant load greater than 300 in a single room is not unusual. That interpretation was a departure from past practice under the legacy codes; therefore, the language was changed in the 2006 IBC to better reflect the intent.

Even though the language of ASCE 7-05 is less clear, the intent of the code writers is evident: large office buildings should be in Occupancy Category II and have an importance factor of 1.0, even if certain rooms or areas can have more than 300 occupants. Note that the 2009 and 2012 IBC retain this same language. Only buildings whose primary occupancy is public assembly AND with an occupant load greater than 300 must be Occupancy Category III.

Disagree? Leave a comment and let me know why.