Roof Live Load Reduction Formula

I ignore the slope factor altogether for snow load reduction which adds another measure of conservatism.

Roof live load reduction formula.

The formula can be written as. It is maximum for slope ratios of 12 12 or greater. Live load reduction 0 25 15 a i 0 25 15 1200 0 25 0 433 0 683 amplified loads per linear foot. P s c s p f where.

6 3 1 2 10 a reduction factor for imposed loads for area aa may be used and should be determined using aa 1 0 a 1000 0 75 na 1 bs en 1991 1 1. Live load reduction is a reduction in live load that contributes to the actions on a member. If the roof design load is 20psf and you are considering the compression in a web member that is located near the mid span than i would not use live load reduction for the compression in that member. The probability of full live load on large areas in certain cases is unlikely.

To do this i use conservative too heavy dead loads and full snow loads regardless of pitch. If the roof has such an occupancy then the area reduction for roof live loads may not be taken. L r l o r 1 r 2 where l r shall not be less than 12 psf and not more than 20 psf r 1 1 for a t less than or equal to 200 psf. The sloped roof snow load is calculated using equation 7 4 1.

Roof slope factor c s the roof slope factor is dependent on various roof properties including temperature shape and material. For uses other than storage where approved additional live load reductions shall be permitted where shown by the registered design professional that a rational approach has been used and that such reductions are warranted. R 2 max min 1 2 0 05f 1 0 6 where f is the rise in the slope ratio f 12. The live loads for members supporting two or more floors are permitted to be reduced by a maximum of 20 percent but the live load shall not be less than l as calculated in section 1607 10 1.

However in my practice i typically use the horizontal run of the roof for both types of load. Roof live load may be reduced by the following equation. 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 method given below complies with the bs en 1991 1 1 uk national annex but differs from that given the eurocode.

Dead load 1 2 70 psf 15 ft 1260 plf live load 1 6 0 683 50 psf 15 ft 819 6 plf w u 1 2 d 1 6 l 1260 819 6 2079 6 plf 2 08 k ft required moment lrfd mu wu l 2 8 2 08 x 40 2 8 416 k ft. C s roof slope factor p f flat roof snow load.