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ASCE 7 Snow Load Calculation

Using the SkyCiv Load Generator in ASCE 7 Snow Load Calculations

To calculate the snow load pressures for a structure using SkyCiv Load Generator, the process is to define first the code reference. From there, the workflow is to define the parameters in Project Tab, Site Tab, and Building Tab, respectively. However, only users with a Professional Account or who purchased the standalone Load Generator module will be able to use this calculation. You can purchase the standalone module thru this link.

Site Data

Using ASCE 7, you just need to put the address of the location in USA, regardless if it is ASCE 7-10 or ASCE 7-16.

Figure 1. Site tab of SkyCiv Load Generator

The ground snow load values are derived from Section 7.2 of ASCE 7 utilizing the ground elevation of the structure taken from Google Maps API.

Building Data

The structure data and the wind and snow parameters are separated into different accordions. In order to calculate snow loads, the snow load checkbox should be checked.

Figure 2. Snow load parameters on SkyCiv Load Generator

Terrain Category for Snow Loads

This parameter is used in calculating Exposure Factor, Ce. Options for this factor are the following:

  • B, C, or D (from Section 26.7 of ASCE 7)
  • Above the treeline in windswept mountainous areas
  • In Alaska, in areas where trees do not exist within a 2-mile (3-km) radius of the site

Exposure Condition of Roof

Like the Terrain Category for Snow Loads, this parameter is also used in calculating Exposure Factor, Ce. The option for this selection and their corresponding definition from ASCE 7 are the following:

  • Sheltered: “Roofs located tight in among conifers that qualify as obstructions. Obstructions within a distance of 10 x height of the obstruction above the roof level provide shelter.”
  • Fully Exposed: “Roofs exposed on all sides with no shelter afforded by terrain, higher structures, or trees. Roofs that contain several large pieces of mechanical equipment, parapets that extend above the height of the balanced snow load (hb), or other obstructions are not in this category. If the only obstructions are a few deciduous trees that are leafless in winter, the “fully exposed” category shall be used.”
  • Partially Exposed: “All roofs except as indicated above.”

Thermal Condition

This parameter is used in getting the Thermal Factor, Ct. The options for this are the following:

  • Structures kept just above freezing and others with cold, ventilated roofs in which the thermal resistance (R-value) between the ventilated space and the heated space exceeds 25 °F × h × ft2/Btu (4.4 K × m2/W)
  • Unheated and open air structures
  • Structures intentionally kept below freezing
  • Continuously heated greenhouses with a roof having a thermal resistance (R-value) less than 2.0 °F × h × ft2/Btu (0.4 K × m2/W)
  • All other structures

Sloped Roof Condition

This parameter is used in getting the Roof Slope Factor, Cs. This parameter has two options: “Slippery” and “Other cases.”

Consider Unbalanced/Drifted Snow Load Case?

This parameter is a “Yes” or “No” to calculate for Unbalanced Snow Load Case. Upon selecting “No,” the balanced snow load case shall only be calculated. On the other hand, upon selecting “Yes,” the unbalanced snow load case table will show. There are three options for ASCE 7:

Snow Load Drift on Symmetric Gable Roof

This case is used to calculate the snow load drift on symmetric gable roof.

Figure 3. Parameters for calculating snow load drift on symmetric gable/hip roof.

Snow Load Drift on Asymmetric Gable Roof

This case is used to calculate the snow load drift on asymmetric gable roof as shown below.

Figure 4. Parameters for calculating snow load drift on asymmetric gable/hip roof.

Snow Load Drift on Lower Roof

This case is used to calculate the snow load drift on lower roof level and can also be used in obtaining drift on parapet walls for windward and leeward case.

Figure 5. Parameters for calculating snow load drift on lower roof.

Results

The snow load results will show the parameters in calculating the balanced snow load as shown below:

Figure 6. Snow load parameters used in calculation and the calculated balanced snow load.

Unbalanced Snow Load

If Unbalanced/Drifted Snow Load is selected to be considered, the results are tabulated on a table depending on the cases defined:

Figure 7. Sample results for unbalanced snow load on symmetric and asymmetric gable roof.
Figure 8. Sample results for snow load drift lower roof.

For additional resources, you can use these links for reference:

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