Unbalanced Roof Snow Load

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.

Unbalanced roof snow load.

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. Unbalanced snow loading is the condition in which snow accumulates at. The unbalanced loads in section 7 6 of asce 7 16 are in most cases drift loads. Different unbalanced loading provisions are present for hip and gable roofs curved roofs sawtooth roofs and dome roofs.

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. 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. Therefore the roof snow load of 23 1 psf is the governing uniform or balanced load. 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.

Calculate sloped 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. 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. Unbalanced snow load from drifting and sliding snow.

Unbalanced snow load poses a greater risk to the roof structural system than a uniform snow load. 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. The asce 7 16 standard requires both balanced and unbalanced snow load case scenarios for a structure s design consideration. Ps pfcs where psis in pounds per square foot of horizontally projected roof area 3 check for unbalanced snow load assuming wind can blow from any direction hip and gable roofs curved roofs multiple folded plate roofs sawtooth roofs barrel vault roofs and dome roofs see section 7 6 of asce 7.