ASCE 7-22 Flat Roof Snow Load Versus Minimum Snow Load

To view the figures and tables associated with this article, please refer to the flipbook above.

Snow load design changes to the American Society of Civil Engineers’/Structural Engineering Institute’s ASCE/SEI 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures include new strength-based ground snow load (GSL) maps based on the risk category of a building (Figs. 1-2). The new maps are also used in the 2024 International Building Code (IBC).

The introduction of strength-based GSL values has led to modifications in the load combinations. For allowable stress design (ASD), the snow load adjustment factor is 0.7. ASCE 7-22 Equation 7.3-1 converts ground snow load to flat roof snow load with a 0.7 exposure reduction factor. This should not be confused with the ASD modification. ASCE 7-22 Section 7.3.3, Minimum Snow Load for Low-Slope Roofs, p_m, further requires a minimum snow load independent of additional adjustments. Application of the adjustment factors is critical for the new snow load design provisions.

ASD Load Combinations

ASCE 7-22 Section 2.4.1, Basic Load Combinations, includes ASD load combinations. Equations 3a, 4a, and 6a are excerpted here and show the new adjustment factor of 0.7 for strength-based GSL:

ASCE 7-22 Equation 7.3-1 converts ground snow load to flat roof snow load (pf):

p_f = 0.7 C_e C_t p_g

where:
C_e = Exposure factor
C_t = Thermal factor

ASCE 7-22 commentary contains the following regarding the 0.7 adjustment factor in Equation 7.3-1:

The normal, combined exposure reduction in this standard is 0.70 as compared with a normal value of 0.80 for the ground-to-roof conversion factor in the 1990 National Building Code of Canada. The decrease from 0.80 to 0.70 does not represent decreased safety but arises because of increased choices of exposure and thermal classification of roofs (i.e., five surface roughness categories, three roof exposure categories, and four thermal categories in this standard vs. three exposure categories and no thermal distinctions in the Canadian code).

In summary, the 0.7 factor in ASCE 7-22 Equation 7.3-1 is based on exposure and thermal conditions, whereas the new 0.7 factor in the ASD load combinations is based on converting from a strength design basis to ASD basis.

Evaluation of both p_f and minimum roof snow load, p_m, is required along with sliding, drifting, unbalanced snow loads, etc. Much of the confusion regarding application of adjustment factors seems to relate to p_m. Per ASCE 7-22 Section 7.3.3, when p_g > p_m,max, then p_m = p_m,max. However, the last sentence of the section states the following: “This minimum roof snow load shall be a separate uniform load case.” And the ASCE 7-22 commentary Section C7.3.3 states, “In such areas, single storm events can result in loading for which the basic ground-to-roof conversion factor of 0.7, as well as C_e and C_t factors, are not applicable.” Some designers have incorrectly assumed that this is referring to the ASD load combination adjustment. The actual intent is to have a minimum strength-based snow load included in the roof design. As such, the ASD load combination D + 0.7S would be used for both p_f and p_m. The comparable strength design load combination 1.2D + 1.0S would also be used for p_f and p_m.
McGraw-Hill’s Structural Load Determination–2024 IBC and ASCE/SEI 7-22 by David A. Fanella includes background on this issue that is excerpted here:

“The purpose of the minimum snow loads is to account for important situations that may develop on roofs that are relatively flat. For example, in regions where p_g < p_m,max, a single storm event can result in loading where the ground-to-roof conversion factor of 0.7 and factors Ce and Ct are not applicable, resulting in a roof load equal to at most p_g.

“The minimum snow load is a uniform load case to be considered separately from any of the other applicable load cases. It need not be used in determining or in combination with drifting, sliding, unbalanced, or partial snow loads.”

To easily determine new ground snow loads based on location and risk category, the ASCE Hazard Tool (ascehazardtool.org) is a free resource referenced in both the IBC and IRC. Note that the hazard tool also provides p_g(asd) values; however, as noted earlier, determination of IRC GSL needs to be based on Risk Category II.

Table 1 provides a simple example of various snow load values and ASD load combination 3a for Richmond, VA. This location is chosen not only to see the difference in GSL based on Risk Category but also to help compare p_g and p_m. See Figure 3 for determination of Risk Category II values for this location using the ASCE Hazard Tool. Assume C_e = 1.0, C_t = 1.2 and roof dead load (D) = 20 psf.

In Table 1, p_g is greater than p_m,max for all risk categories. Therefore, p_m = p_m,max for all risk categories. Note that when S = p_m, the snow load, S, does not include the 0.7 GSL to roof snow load factor, whereas when S = p_f the snow load, S, includes the 0.7 GSL to roof snow load factor. For this specific location, the load case using pm controls for Risk Category I buildings but does not control for other risk categories.

Table 2 reveals the point where the GSL would equal the minimum snow load based on varying exposure and thermal coefficients (p_g = p_m/[0.7C_eC_t]). Whenever GSL is greater than the values shown in the table, the GSL will control over pm. Whenever GSL is less than the tabulated value, p_m controls. The highlighted row in Table 2 corresponds to the C_e and C_t assumptions in Table 1.

Structural engineers should be aware of significant changes in the 2024 IBC Chapter 16 and ASCE 7-22 for snow loads. Snow loads are now based on the risk category of the structure and use strength-based design values. With the introduction of strength-based GSL values, the load combinations have been modified. For allowable stress design, the adjustment factor for snow loads is 0.7. The conversion from ground snow load to flat roof snow load also uses a 0.7 exposure factor. These two factors are independent of each other, even though they have the same value. ASCE 7-22 further requires a minimum snow load exempt from certain adjustments. Proper application of the adjustment factors is critical for accurate designs.
The 2024 IBC also includes a GSL conversion to ASD. This conversion is necessary for IBC and IRC provisions that rely on ASD GSL. The ASCE Hazard Tool (ascehazardtool.org) is a free resource for determining snow loads, including ASD GSL, based on location and risk category. ■

About the Authors

John “Buddy” Showalter, PE, M. ASCE, M. NCSEA, (bshowalter@iccsafe.org) is Principal Staff Engineer of ICC’s Technical Product Development Group.

Donna E. Adams, PE (dadams@dunbarstructural.com) is a Senior Associate with Dunbar Structural.