A Beginner's Guide to ASCE 7-10

Chapter 5 - L: Live Loads

© 2012, T. Bartlett Quimby

BGASCE7-10 Section 5.5

Live Load Reduction

Last Revised: 11/04/2014

The probability of full live load on large areas, in certain cases, is unlikely.  In fact it become less likely as the size of the area supported increases. As a result the Standard allows for the reduction of live loads in consideration of the lower probability of experiencing the full live load over large areas at the same time.

The exceptions to this allowance are for live loads exceeding 100 psf, passenger vehicle garages, and places of assembly.  Occupancies exceeding 100 psf and places of assembly are generally likely to see the full load everywhere at the same time.  Garages are also likely to be filled often.

ASCE 7-10 4.7 addresses live load reductions.  The reduction equation is ASCE 7-10 equation 4.7-1.  The equation uses the concept of "influence area" as a major factor in computing the reduction that a member can use.

The definition of influence area, as found in the ASCE 7-10 C4.7.1 commentary, is "that floor area over which the influence surface for structural effects is significantly different from zero." This area is typically the total area supported by all the structural elements supported by the element under consideration. 

For example, while an interior girder's tributary are is half the area supported by the joists that the girder supports, the girders influence area is the total area supported by the joists that it supports.  In this case, the influence area is twice the tributary area of the girder. 

ASCE 7-10 Figure C4-1 illustrates the relationships between tributary and influence areas for various common structural elements in a framing plan.

Similar analysis can be used to relate a member's tributary area to it's influence area.  This has been done for common situations and the ratio of influence area to tributary area is given in ASCE 7-10 Table 4-2.

The term K_LLA_T in the load reduction formula is the influence area for an element.  For elements not considered in ASCE 7-10 Table 4-2 you can determine the area of influence and substitute that number directly for this term.  Cases where this might be necessary include structural elements with non-rectangular tributary areas, such as hip beams and locations of irregular framing.  Click here for a Powerpoint presentation that illustrates a few influence area determinations.

The Standard also limits the amount of live load reduction that can be taken depending on the number of floors supported by a member.

Heavy live loads and live loads in public assembly areas may not be reduced.