Simplified Frame Analysis - Simplified Design of Reinforced Concrete Buildings - page 28

2.2.3 Live Load Reduction for Columns, Beams, and Slabs

Most general building codes permit a reduction in live load for design of columns, beams and slabs to account
for the probability that the total floor area “influencing” the load on a member may not be fully loaded
simultaneously. Traditionally, the reduced amount of live load for which a member must be designed has been
based on a tributary floor area supported by that member. According to ASCE/SEI 7-05, the magnitude of live
load reduction is based on an influence area rather than a tributary area. The influence area is a function of the
tributary area for the structural member. The influence area for different structural members is calculated by
multiplying the tributary area for the member A T , by the coefficients K LL given in Table 2-4, see ASCE 4.8.

The reduced live load L per square foot of floor area supported by columns, beams, and two-way slabs having
an influence area (K LL A T ) of more than 400 sq ft is:

⎛ ⎞

15
K LL A T

ASCE (Eq. 4-1)

L = L 0 0.25 ⎜ + ⎟

⎝ ⎠

where L o is the unreduced design live load per square foot. The reduced live load cannot be taken less
than 50% for members supporting one floor, or less than 40% of the unit live load L o otherwise. For other

limitations on live load reduction, see ASCE 4.8.

Using the above expression for reduced live load, values of the reduction multiplier as a function of influence
area are given in Table 2-5.

Table 2-4 Live Load Element Factor K LL

Element K LL

Interior columns 4

Exterior column without cantilever slabs 4

Edge column with cantilever slabs 3

Corner columns with cantilever slabs 2

Edge beams without cantilever slabs 2

Interior beams 2

All other members not identified above including: 1

Edge beams with cantilever slabs

Cantilever beams

Two-way slabs

The above limitations on permissible reduction of live loads are based on ASCE 4.8. The governing general
building code should be consulted for any difference in amount of reduction and type of members that may be
designed for a reduced live load.