Unbalanced Roof Snow Load

The asce 7 16 standard requires both balanced and unbalanced snow load case scenarios for a structure s design consideration.

Unbalanced roof snow load. A building constructed 40 years ago may not have been designed for snow loads as they are understood today. Snow load calculations in the skyciv load generator module are supported by reference codes such as asce 7 10 7 16 en 1991 1 3 nbcc 2015 and as nzs 1170 3 and is available on the standalone load generator only and professional. Different unbalanced loading provisions are present for hip and gable roofs curved roofs sawtooth roofs and dome roofs. Ps pf cs where ps is 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.

Summary of unbalanced snow load results. Only relatively recently have drifting and sliding snow loads been addressed in building codes. However with guidance from the asce 7 16 on snow load calculations for curved roofs and rfem s. If the ground snow load isn t specified then a ground snow load needs to be assumed and the truss designer and the building designer may not be on the same page as it relates to this design assumption.

Unbalanced roof snow loads were not introduced in ubc until 1988. These unbalanced snow loads are analyzed separately from the balanced snow load case and therefore are not additive. How are these loads determined and what factors affect unbalanced snow loads. 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.

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. This situation is termed unbalanced because the leeward portion has more snow than the windward portion. Unbalanced snow loads occur when wind redistributes this balanced snow from one area of the roof to another from the windward side of the peak to the lee ward side see fig 1 and 2. Unbalanced snow load on monoslope roof ajh1 structural 28 jul 17 17 17 given that the current unbalanced provisions that you are referring to are based on an upwind fetch distance i e the windward roof on the other side of a ridge there is no basis for that particular provision to be used for a monoslope roof.

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. Calculate sloped roof snow load. The determination of the drift load that is required for unbalanced snow load cases requires the use of the ground snow load pg not the roof snow load. Summary of snow load parameters used and the balanced snow load to be applied to the structure.