IBC Sections 1607.9 (floors) and 1607.11 (roofs) allow live loads set forth in IBC Table 1607.1 to be reduced. Live loads are permitted to be reduced because, for the most part, the likelihood that the entirety of a given floor or roof area will be fully loaded with the design live loads is low. There are many rules set forth in the code for when, and by how much, live loads may be reduced. See the table below to clearly understand these rules and why they are necessary.
|Rule||Reduction of Floor Live Loads||Reduction of Roof Live Loads||Reason|
|Section 1607.9.1 Based on Influence Area||Section 1607.9.2 (ALTERNATE) Based on Tributary Area||Section 1607.11.2 Based on Tributary Area|
|1||(KLL)(AT) needs to be greater than 400 square feet.||Tributary area A needs to be greater than 150 square feet.||For flat roofs, AT needs to be greater than 200 square feet.||A minimum area is necessary before it can be assumed that an entire area will not be fully loaded with the design live loads.|
|2||Reduction cannot exceed 50 percent for elements that support loads of a single floor.||Reduction cannot exceed 40 percent or 23.1 (1 + D/L0) percent for horizontal members.||20 psf of roof live load may not be reduced to less than 12 psf.||This ensures that a horizontal structural member, such as a beam or a slab, will be designed for a minimum live load.|
|3||Reduction cannot be more than 60 percent for elements that support loads of two or more floors. for vertical members.||Reduction cannot exceed 60 percent or 23.1 (1 + D/L0) percent||20 psf of roof live load may not be reduced to less than 12 psf.||This ensures that a vertical structural member, such as a column or wall, will be designed for a minimum live load.|
|4||AT for one-way slabs, for use in reduction calculation, cannot exceed the slab span times a width of 1.5 times the slab span.||Tributary area A for one-way slabs, for use in reduction calculation, cannot exceed the slab span times 0.5 times the slab span.||No rule||This takes into account the lower redundancy of (possibility of load redistribution in) one-way slabs compared to two-way slabs.|
|5||Live loads greater than 100 psf cannot be reduced, except that live loads for members supporting two or more floors may be reduced by as much as 20 percent (plus one more exception).||In storage-type applications with heavier live loads, several adjacent floor panels may be fully loaded.|
|6||Live loads in passenger vehicle garages cannot be reduced, except that live loads for members supporting two or more floors may be reduced by as much as 20 percent.||Passenger vehicle garage decks often are fully loaded.|
|7||Live loads of 100 psf, or on areas where fixed seats are located, cannot be reduced in Group A occupancies.||Live loads cannot be reduced in Group A occupancies.||Live loads of 100 psf or more on areas of roofs classified as Group A occupancies shall not be reduced.||Because of large concentrations of people in Group A occupancies, it is likely that the entire area under consideration will be fully loaded.|
There are changes in the live load reduction provisions between the 2006 IBC and the 2009 IBC, the most significant of which can be summarized as follows:
Table 1607.9.1 Live load element factor, KLL — the live load reduction provisions are revised to align with similar provisions in ASCE 7-05 Section 4.8. “One-way slabs” is added to IBC Table 1607.9.1 to make it consistent with Table 4-2 of ASCE 7-05. In the 2006 IBC, Section 1607.9.1.4 prohibited live load reduction on one-way slabs, except for certain heavy live load scenarios. The 2009 IBC permits the reduction of live loads on one-way slabs using Equation 16-24 with a KLL value of 1. However, new 2009 IBC Section 1607.9.1.1 imposes a restriction on the value of the tributary area, AT, of a one-way slab that can be used in Equation 16-24. The restriction is the same as that found in ASCE 7-05 Section 4.8.5.
Section 1607.9.1.4 Group A Occupancies — 2009 IBC Section 1607.9.1.4 now refers to Group A occupancies instead of assembly occupancies, as was done in 2006 IBC Section 1607.9.1.3, in order to clearly define the scope of the provision. Because there are public assembly uses with occupant loads less than 50 and categorized as Group B that do not warrant the prohibition, specifying Group A occupancy unambiguously applies the provision only where it is applicable. The scope of this provision now is restricted even further by applying it to live loads of 100 pounds per square foot (psf) only, instead of the 2006 IBC requirement of 100 psf or less. The only exception to this is an area where fixed seats are located. Even though the live load for fixed seats in an assembly area is 60 psf (Item 4 in Table 1607.1), it was judged that the areas with fixed seats also warrant this prohibition.
Section 1607.9.2 Alternate floor live load reductions — a new exception is added to make the alternate floor live load reduction applicable to live loads exceeding 100 psf where the usage is not storage and a registered design professional approves such a reduction through a rational approach. This revision makes Section 1607.9.2 consistent with Section 1607.9.1.2 (2006 IBC Section 1607.9.1.1).
Section 1607.11.2.1 Flat, pitched, and curved roofs — awnings and canopies other than those of fabric construction supported by a lightweight rigid skeleton structure are now specifically included within the scope of live load reduction provision of this section. The language in Item 29 in Table 1607.1 implies that reduction is permitted for these kinds of roofs, but no clear indication was given in the 2006 IBC regarding how to carry out the reduction. It has always been the intent of the code to apply the provisions of Section 1607.11.2.1 to the above mentioned roof category. However, since “awnings and canopies” are distinctly separate from “ordinary flat, pitched, and curved roofs” in Item 29 of Table 1607.1, this intent was not automatically conveyed. This oversight now is fixed.
A change in the language also clearly specifies that greenhouses are just one example of the type of structures that use special scaffolding for maintenance and repair purposes. Thus, the requirement of using a minimum live load of 12 psf is not specific to greenhouses, but to all such structures.
S.K. Ghosh Associates Inc. is a structural seismic and code consulting firm located in Palatine, Ill., and Aliso Viejo, Calif. Presidents 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, or at www.skghoshassociates.com.