General Live Load Reduction

For members supporting one floor, L shall not be less than 0.50L₀ (this is a 50% reduction). For members supporting two or more floors, L shall not be less than 0.40L₀ (this is a 60% reduction). To be consistent with the way the Live Load reduction is reported by the program for other Building Codes, the Live Load Reduction value will be computed and given as a percent reduction, rather than as a multiplier.

The Live Load Reduction value is further limited to the user specified maximum, if any.

There is a special provision in the code (IBC 2018 Section 1607.11.1.2 Heavy live loads) for live loads greater than 100 psf. In the program, this type of load is referred to as “Storage” live load and, for such loads a reduction of up to 20% (if allowed by Eq. 16-23) is allowed on members carrying storage load from two or more levels. The user must explicitly designate loads as Storage. Any live load designated by the user as Storage will be treated as explained whether the load is greater than 100 psf or not, and any load not specified by the user as Storage will not be treated as Storage even if it exceeds 100 psf.

Table 1607.11.1 gives values for K_LL. Theoretically it is the ratio of the Influence Area to the Tributary Area: $K_{LL}=\frac{A_i}{A_T}$. Typically the value for columns is 4, and the value for beams is 2. With these values, the equation gives the same reduction values as ASCE 7-95 and ASCE 7-93, which is what has been implemented for BOCA Live Load Reduction. When there are large cantilevered areas contributing to the area supported by the member, the values of 4 for columns and 2 for beams are considered unconservative. The table gives a value of 3 for edge columns with cantilever slabs, 2 for corner columns with cantilever slabs, and 1 for edge beams with cantilever slabs. The Code does not give any indication as to what fits within the definition of cantilever slabs. For example, it does not seem that a column supporting beams that carry a nominal slab edge would need to be included under this classification. The table also specifies a value of 1 for cantilever beams.

The program determines both the tributary area and the influence area for each member. K_LL is then calculated:

To conform to the table values, which show integer numbers, the program will round the calculated K_LL to an integer value as well. This is done as follows:

• If K_LL is greater than 3.85, it rounds up to 4, otherwise it rounds down to 3.
• If K_LL is greater than 2.85, it rounds up to 3, otherwise it rounds down to 2.
• If K_LLL is greater than 1.85, it rounds up to 2, otherwise it rounds down to 1.

The 0.15 tolerance for rounding up is to avoid unduly penalizing members that carry some nominal slab edge or very short cantilevers.

For loads on the cantilever portion of a cantilever beam, K_LL = 1.

K_LL is calculated independently for both Reducible Live Loads and Storage Live Loads based on their respective tributary and influence areas.

Alternate Live Load Reduction

An alternate live load reduction methodology is given in IBC 2018 Section 1607.11.2. It is based on the methodology specified by the UBC. Beginning with IBC 2015 a subtle but important change was made, making it identical to the UBC live load reduction method: previous to IBC 2015 the requirement stated that the reduction for horizontal members (i.e., beams) is limited to 40% and the reduction for vertical members (i.e., columns) is limited to 60%. Note that this meant that a column carrying load from only one level could be reduced up to 60%, and a transfer girder carrying load from more than one level could only be reduced up to 40%. Beginning with IBC 2015 the requirements now say that for members receiving load from one level only the reduction is limited to 40% and for members receiving load from two or more levels the reduction is limited to 60%. In prior versions of the program the requirements of the pre-IBC 2015 were implemented but beginning with V17.02 the requirements given in IBC 2015 and later are now used.

For loads specified as Reducible, any member supporting 150 square feet or more of Reducible live load is reduced per Equation 16-24: R = 0.08(A – 150) where A is the tributary area and R is the reduction percent, limited to a maximum of 40% for beams and 60% for columns as explained above. The reduction is further limited to that obtained from Equation 16-25: R = 23.1(1+D/L) where D/L is the ratio of the Dead Load to Live Load, and it is further limited to the user specified maximum, if any.

There is a special provision in the Code (Section 1607.11.2.1) for live loads greater than 100 psf. In the program, this type of load is referred to as ’Storage" live load and, for such loads a reduction of up to 20% (if allowed by Eq. 16-24 and Eq. 16-25) is allowed on members supporting load from two or more floors. The user must explicitly designate loads as Storage. Any live load designated by the user as Storage will be treated as explained whether the load is greater than 100 psf or not, and any load not specified by the user as Storage will not be treated as Storage even if it exceeds 100 psf.

Roof Live Load Reduction

The Live Load Reduction value will further be limited to the user specified maximum, if any.