Unbalanced Roof Snow Load

Unbalanced snow load from drifting and sliding snow.

Unbalanced roof snow load.

Except for sawtooth type roofs the roof s windward portion is the snow source and the leeward portion accumulates a percentage of the drifted snow. The unbalanced loads in section 7 6 of asce 7 16 are in most cases drift loads. Since the roof slope of 30 2 7 on 12 is greater than 2 83 unbalanced snow loads must be considered see figure 1 and as a result minimum roof loading need not be considered. Different unbalanced loading provisions are present for hip and gable roofs curved roofs sawtooth roofs and dome roofs.

Therefore the roof snow load of 23 1 psf is the governing uniform or balanced load. Minimum roof snow load thermal factor unbalanced load drift load on adjacent roof parapet wall rtu drift sliding load on adjacent roof ponding faq s 7 thermal factor usually the roof snow load w o drifting is less than the ground snow load but with special circumstances p r p g 8 thermal factor in the seaw report biggest differences. If unbalanced snow loading isn t required or specified the truss designer may enter the 25 psf snow load as a top chord live load tcll set the load duration factor to 1 15 for snow and turn snow loading off completely. When snow accumulates at different depths in different locations on a roof it results in high and concentrated snow loads that can potentially overload the roof structure.

Different depths in different locations on a roof resulting in differential snow load. The asce 7 16 standard requires both balanced and unbalanced snow load case scenarios for a structure s design consideration. Or the 25 psf snow load could be entered as a roof snow load with the unbalanced snow loading option turned off. Unbalanced snow loading is the condition in which snow accumulates at.

Calculate sloped roof snow load. While this may be more intuitive for flat or even gable hip type roofs the determination of snow loads is increasingly more difficult for arch roofs due to complex geometry. However with guidance from the asce 7 16 on snow load calculations for curved roofs and rfem s efficient load application tools it s possible to consider both balanced and unbalanced snow loads. Unbalanced snow loads due to the variability of shapes and geometries of roofs and their interaction with differing wind directions unbalanced snow loads can differ quite a bit.