MiTek Engineering software gives you two options to input snow load – Ground Snow and Roof Snow (flat roof snow). What is the difference between ground snow and flat roof snow?
Ground snow load, pg, is obtained directly from the American Society of Civil Engineers Standard (ASCE 7) Figure 7-1 for the contiguous United States and Table 7-1 for Alaska. There are areas on the map listed as “CS” (Case Study) – they are regions that experience abnormal weather patterns and therefore ground snow cannot be directly found on the map provided in ASCE 7. Ground snow within the CS areas shall be approved by the authority having jurisdiction.
Flat roof snow, pf, is ground snow multiplied by factors that account for roof snow loss – some snow is blown off the roof and some is melted off by heat escaped from the space below the roof. The flat roof snow load (roof with slope equal to or less than 5 degrees) is calculated using formula 7.3-1 of ASCE 7:
𝑝f = 0.7𝐶e𝐶t𝐼s𝑝g
Where:
𝐶e – Exposure factor
𝐶t – Thermal factor
𝐼s – Importance factor
𝑝g – Ground Snow load
Exposure factor, 𝐂𝐞, shall be determined from Table 7-2 in ASCE 7 and is based on the Terrain exposure category (Surface Roughness category in ASCE 7-16) and Roof exposure category.
Terrain exposure category (Surface Roughness category in ASCE 7-16) is dealing with the obstructions to wind within a 2600 feet distance of the structure or 1500 feet for structures with a mean roof height of 30 feet or less. Exposure B refers to urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger. Exposure C is for open terrain with scattered obstructions having heights generally less than 30 feet. This category includes flat open country and grasslands. Exposure D is for flat unobstructed areas and water surfaces outside hurricane prone regions, it involves a structure a close distance (typically within 600 feet) from an “open waterway” one mile or more across. This category is readily distinguishable, where the locally enforced code very likely has considered this in their requirements.
The Roof exposure category deals with obstructions right next to the structure. Sheltered roofs are those that are tight in among conifers and fully exposed roofs are those that have no shelter from terrain, higher structures, or trees. Partially exposed roofs are all of those that do not meet one of the first two definitions.
For example, per ASCE 7 Table 7-2, Ce=1.2 for a building in terrain category B with a sheltered roof, but for fully exposed roof in terrain category B, Ce=0.9. If a building has no cover from the trees or other buildings, the snow is much more likely to be blown off by the wind.
Thermal factor, 𝐂𝐭, shall be determined from table 7-3 in ASCE 7. The Thermal condition deals with how much heat escapes from the space below the trusses up to the roof to melt the snow off. The IRC states that the minimum insulation to be used in the attic space in normal heated residential structures is R-30, R-38, or R-49 depending on location. ASCE says that if an R value of 25 or greater is used between the heated and ventilated space, a Ct factor of 1.1 is to be used. Therefore, a Ct factor of 1.1 should be used on almost all residential structures. On structures that are unheated such as most barns, the Ct factor should be 1.2. A Ct factor of 1.0 may apply to certain commercial structures but this would be the exception rather than the norm.
Importance factor, 𝐈𝐬, shall be determined from ASCE 7 Table 1.5-2 based on the Risk/Occupancy Category from Table 1.5-1. Per ASCE 7, the Risk/Occupancy Category is a categorization of buildings and other structures for determination of flood, wind, snow, ice, and earthquake loads based on the risk associated with unacceptable performance.
For example, per ASCE 7 Table 1.5-2, Is=0.8 for an agricultural building with a low risk to human life in the event of failure, Is=1.2 for an essential facility, like a hospital or fire station. The higher importance factor creates proportionally higher design loads to improve the building’s structural reliability during and after a major environmental event.
The snow load that is applied to the truss is not the ground snow load. In most cases it is the flat roof snow. The flat roof snow load is calculated using formula 7.3-1 of ASCE 7 and is the unfactored, balanced design snow load that will be applied to the truss. For sloped roofs (a roof with a slope greater than 5 degrees), the flat roof snow load can be modified to account for slope and the roughness characteristics of the roof.
Let’s run an example to see how important it is to input correct information in the Snow Loading section of MiTek Engineering software.
Consider trusses for a fully exposed, unheated agricultural building with a low risk to human life in the event of failure in an area with ground snow of 30 psf (pounds per square foot) and terrain category C. If you input occupancy/risk category II, flat roof snow will be 22.68 psf. But if you change category to I (low risk to human life in the event of failure), flat roof snow will be 18.14 psf.
MiTek is not specifying how to load trusses. All these parameters must be verified by the authority having jurisdiction and/or by the building owner/designer. Overestimation of snow load can unnecessarily increase the cost of construction; underestimation of snow load can result in costly repair if trusses are built.
For additional information, or if you have questions, please contact the MiTek Engineering department.