2009 IBC Sections 1607.9 and 1607.11 Reduction inLive Loads

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April 2011 » Columns » CODE SIMPLE

S. K. Ghosh, Ph.D.

IBC Sections 1607.9 (floors) and 1607.11 (roofs) allow live loads set forth in IBC Table 1607.1 to bereduced. Live loads are permitted to be reduced because, for the most part, the likelihood that the entiretyof a given floor or roof area will be fully loaded with the design live loads is low. There are many rules setforth in the code for when, and by how much, live loads may be reduced. See the table below to clearlyunderstand these rules and why they are necessary.

Rule Reduction of Floor Live Loads Reduction of RoofLive Loads

Reason

Section 1607.9.1Based on InfluenceArea

Section 1607.9.2(ALTERNATE) Basedon Tributary Area

Section 1607.11.2Based onTributary Area

1 (KLL)(AT) needs to begreater than 400square feet.

Tributary area A needsto be greater than 150square feet.

For flat roofs, ATneeds to begreater than 200square feet.

A minimum area isnecessary before it canbe assumed that anentire area will not befully loaded with thedesign live loads.

2 Reduction cannotexceed 50 percent forelements that supportloads of a single floor.

Reduction cannotexceed 40 percent or23.1 (1 + D/L0) percentfor horizontalmembers.

20 psf of roof liveload may not bereduced to lessthan 12 psf.

This ensures that ahorizontal structuralmember, such as a beamor a slab, will bedesigned for a minimumlive load.

3 Reduction cannot bemore than 60 percentfor elements thatsupport loads of two ormore floors. forvertical members.

Reduction cannotexceed 60 percent or23.1 (1 + D/L0) percent

20 psf of roof liveload may not bereduced to lessthan 12 psf.

This ensures that avertical structuralmember, such as acolumn or wall, will bedesigned for a minimumlive load.

4 AT for one-way slabs,for use in reductioncalculation, cannotexceed the slab spantimes a width of 1.5times the slab span.

Tributary area A forone-way slabs, for usein reductioncalculation, cannotexceed the slab spantimes 0.5 times theslab span.

No rule This takes into accountthe lower redundancy of(possibility of loadredistribution in) one-wayslabs compared to two-way slabs.

5 Live loads greater than 100 psf cannot be reduced, except that liveloads for members supporting two or more floors may be reduced byas much as 20 percent (plus one more exception).

In storage-typeapplications with heavierlive loads, severaladjacent floor panelsmay be fully loaded.

6 Live loads in passenger vehicle garages cannot be reduced, exceptthat live loads for members supporting two or more floors may bereduced by as much as 20 percent.

Passenger vehiclegarage decks often arefully loaded.

7 Live loads of 100 psf,or on areas wherefixed seats arelocated, cannot bereduced in Group Aoccupancies.

Live loads cannot bereduced in Group Aoccupancies.

Live loads of 100psf or more onareas of roofsclassified asGroup Aoccupancies shallnot be reduced.

Because of largeconcentrations of peoplein Group A occupancies,it is likely that the entirearea under considerationwill be fully loaded.

There are changes in the live load reduction provisions between the 2006 IBC and the 2009 IBC, the mostsignificant 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 withsimilar provisions in ASCE 7-05 Section 4.8. “One-way slabs” is added to IBC Table 1607.9.1 to make itconsistent with Table 4-2 of ASCE 7-05. In the 2006 IBC, Section 1607.9.1.4 prohibited live load reductionon one-way slabs, except for certain heavy live load scenarios. The 2009 IBC permits the reduction of liveloads on one-way slabs using Equation 16-24 with a KLL value of 1. However, new 2009 IBC Section1607.9.1.1 imposes a restriction on the value of the tributary area, AT, of a one-way slab that can be usedin 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 Aoccupancies instead of assembly occupancies, as was done in 2006 IBC Section 1607.9.1.3, in order toclearly define the scope of the provision. Because there are public assembly uses with occupant loads lessthan 50 and categorized as Group B that do not warrant the prohibition, specifying Group A occupancyunambiguously applies the provision only where it is applicable. The scope of this provision now isrestricted even further by applying it to live loads of 100 pounds per square foot (psf) only, instead of the2006 IBC requirement of 100 psf or less. The only exception to this is an area where fixed seats arelocated. Even though the live load for fixed seats in an assembly area is 60 psf (Item 4 in Table 1607.1), itwas 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 alternatefloor live load reduction applicable to live loads exceeding 100 psf where the usage is not storage and aregistered design professional approves such a reduction through a rational approach. This revisionmakes 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 fabricconstruction supported by a lightweight rigid skeleton structure are now specifically included within thescope of live load reduction provision of this section. The language in Item 29 in Table 1607.1 implies thatreduction is permitted for these kinds of roofs, but no clear indication was given in the 2006 IBC regardinghow to carry out the reduction. It has always been the intent of the code to apply the provisions of Section1607.11.2.1 to the above mentioned roof category. However, since “awnings and canopies” are distinctlyseparate from “ordinary flat, pitched, and curved roofs” in Item 29 of Table 1607.1, this intent was notautomatically conveyed. This oversight now is fixed.

A change in the language also clearly specifies that greenhouses are just one example of the type ofstructures that use special scaffolding for maintenance and repair purposes. Thus, the requirement ofusing 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., andAliso Viejo, Calif. Presidents S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the developmentand interpretation of national structural code provisions. They can be contacted at skghosh@aol.com andsusandowty@gmail.com, respectively, or at www.skghoshassociates.com.