Live Load on Parking Structures: To Reduce or Not To Reduce
It is sometimes interesting, and always a learning experience, to explore how a certain code provision has evolved over several years.
The current codes do not allow live load reduction unless the member supports two or more floors. And then only a 20% maximum reduction is allowed. (2012 and 2015 IBC Section 1607.10.1.3 and Section 1607.10.2 Item 2).
While live load reduction requirements for parking garages has remained constant over the last few code cycles it wasn’t always that way. Both the 1997 UBC and the 1997 SBC had live load reduction based on tributary floor area. Both permitted what then used to be the uniform design live load of 50 psf to be reduced up to 40 percent (down to 30 psf) for parking structure decks. The 1996 BOCA had live load reduction based on influence area, as in ASCE 7. Live load reduction was not permitted for parking structure decks.
When the first (2000) edition of the IBC was published, it contained live load reduction based on influence area, as in ASCE 7, as well as alternate live load reduction based on tributary area, as in the 1997 and prior editions of the UBC. If one did live load reduction of ASCE 7, no reduction of the 50 psf design live load was permitted for parking structure decks. However, if one did alternate live load reduction, the 50 psf of uniform design live load was permitted to be reduced to a value that could be as low as 30 psf.
The 50 psf uniform design live load of ASCE 7-98 for parking structure decks was reduced to 40 psf in ASCE 7-02. This was reflected in 2003 IBC table 1607.1. However, under alternate live load reduction, the 2003 IBC still allowed up to 40 percent reduction for parking structure decks. So reduced design live load could be as low as 24 psf. This was corrected in the 2004 Supplement, which added that the reduction could not exceed 10 psf “for horizontal member in passenger vehicle garages.” The reduced design live load thus could not be below 30 psf.
The 2006 IBC added the following Item 3 under 1607.9.2 Alternate floor live load reduction:
“A reduction shall not be permitted in passenger vehicle parking garages except that the live loads for members supporting two or more floors are permitted to be reduced by a maximum of 20 percent.”
The above provision continues until now. In the 2015 IBC, the provision appears as Item 2 under Section 1607.10.2. Thus, since the 2006 IBC, no reduction in the 40 psf uniform design live load for parking structure decks has been permitted – either under basic live load reduction or under alternate live load red
See the table below that summarizes the live load and allowed live load reductions.