The International Code Council’s 2003 International Building Code (IBC) Table 1607.1, Item 16, sets forth a uniform live load of 40 pounds per square foot (psf ) for passenger vehicle parking garages, and “Footnote a” requires that garages restricted to vehicles accommodating not more than nine passengers be designed for a concentrated load of 3,000 pounds acting on an area of 4.5 inches by 4.5 inches. Both of these live loads were revised from those specified in the 2000 IBC, which were a live load of 50 psf and a concentrated load of 2,000 pounds acting on 20 square inches. The rationale for changing the live loads was based upon a publication which is available from the American Society of Civil Engineers (ASCE), Design Live Loads for Parking Garages: A Report to the Structural Engineering Institute of the American Society of Civil Engineers by Y.K.Wen and G.L.Yeo.This study justified a reduction in the live load from 50 psf to 40 psf and an increase in the concentrated load due to very heavy passenger vehicles such as sport-utility vehicles. The 2003 IBC-prescribed loads are the same as those in ASCE 7-02 and —05, Minimum Design Loads for Buildings and Other Structures.
There is one major difference between ASCE 7-02 and the 2003 IBC with respect to the application of passenger vehicle live loads. ASCE 7-02 does not allow a reduction in the live load (other than for members supporting two or more floors), whereas the 2003 IBC does. The 2003 IBC contains two methods for reducing live loads, the “General” method found in Section 1607.9.1 and the “Alternate” method found in Section 1607.9.2. The General method does not allow for the reduction of passenger vehicle parking garage live loads (other than for members supporting two or more floors); however, the Alternate method does.
The basic premise for live load reduction is that in the design of structural elements with large influence areas, it is highly unlikely that the floor will be fully loaded over the entire area. The proponents of no live load reduction for parking garage floors believe this basic premise does not always hold true for parking garage loads because vehicles typically are parked in regular patterns with garages often full.
Answers to FAQ’s: Q: I have always used the Alternate live load reduction provisions of Section 1607.9.2. When I combine the 2003 IBC-prescribed 40-psf live load with the 40-percent reduction, I’m only designing for 24 psf. This seems awfully low. Is this correct? A: Yes, you are correct. That is what the letter of the code allows. However, this was never the intent. Of the three legacy codes, the National Building Code, formerly published by Building Officials and Code Administrators International, Inc. (BOCA/NBC), strictly followed ASCE 7 and allowed live load reduction based on influence area. No live load reduction was allowed in parking garages or open parking structures, except that the design live load for members supporting more than one floor (columns and walls) could be reduced by as much as 20 percent. The Standard Building Code (SBC), formerly published by the Building Officials and Code Administrators International, Inc., allowed live load reduction of as much as 40 percent for horizontal members and 60 percent for vertical members, based on tributary area. There were no particular restrictions for parking structures.The Uniform Building Code (UBC), formerly published by the International Conference of Building Officials, contained the SBC provisions (live load reduction based on tributary area, no restrictions for parking structures), and also the BOCA/NBC provisions as an alternate (live load reduction based on influence area, no live load reduction for deck members in parking structures).
The live load reduction provisions of the 2000 IBC were almost the same as those of the 1997 UBC, except that tributary area-based live load reduction was now called “Alternate” live load reduction.
The above indicates that there is no experience concerning the performance of parking structure floors designed using a live load less than 30 psf. In the 2004 Supplement to the 2003 IBC, the inadvertent decrease to 24 psf was rescinded by specifying a minimum design live load of 30 psf.
Q: Is the 2006 IBC going to make any changes in passenger vehicle parking garage live loads? A: There is a new provision in 2006 IBC Section 1607.9.2 which reads as follows: “A reduction shall not be permitted in passenger vehicle parking garages except the live loads for members supporting two or more floors are permitted to be reduced by a maximum of 20 percent.” This eliminates the provision of the 2003 IBC, as modified by the 2004 Supplement, which allowed a designer to take advantage of the alternate live load reduction provisions to design the floors and beams of a passenger vehicle parking garage for a uniform live load of only 30 psf. It should be noted that this 2006 IBC change increases the design live load for parking structure decks from 30 to 40 psf in much of the country where the UBC and the SBC were, or still are, in use, or where the IBC already has been adopted.
S.K. Ghosh Associates, Inc., is a structural, seismic, and code consulting firm located in Palatine, Ill., and Laguna Niguel, Calif. President S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at firstname.lastname@example.org and email@example.com, respectively.