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Chapter 16, Structural Design, of the 2006 International Building Code (IBC) prescribes minimum structural loading requirements that are to be used in the design of all buildings and structures. The intent is to subject buildings and structures to loads that are likely to be encountered during their life span, thereby minimizing hazard to life and improving performance during and after a design event.

Parts 1 and 2 of this article focus on environmental loads due to snow and rain. Section 1608.1 of the IBC requires that design snow loads on buildings and structures be determined by the provisions of Chapter 7 of the 2005 edition of the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-05). Section 1611 of ASCE/SEI 7-05 contains requirements for design rain loads.

Snow loads
The following general procedure can be used to determine design snow loads in accordance with Chapter 7 of ASCE 7-05:
1. Determine ground snow load, p_g (Section 7.2).
2. Determine flat roof snow load, p_f by Eq. 7-1 (Section 7.3).
3. Determine sloped roof snow load, p_s by Eq. 7-2 (Section 7.4).
4. Consider partial loading (Section 7.5).
5. Consider unbalanced snow loads (Section 7.6).
6. Consider snow drifts on lower roofs (Section 7.7) and roof projections (Section 7.8).
7. Consider sliding snow (Section 7.9).
8. Consider rain-on-snow loads (Section 7.10).
9. Consider ponding instability (Section 7.11).
10. Consider existing roofs (Section 7.12).

Step 1: Ground snow loads, p_g — Ground snow loads, p_g, are obtained from ASCE/SEI Figure 7-1 or IBC Figure 1608.2 for the conterminous United States and from ASCE/SEI Table 7-1 or IBC Table 1608.2 for locations in Alaska. The snow loads on the maps have a 2-percent annual probability of being exceeded (which is a 50-year mean recurrence interval).

In areas of the United States where the ground snow loads are too variable to map, a site-specific case study is required (designated by "CS" on the maps; see Commentary Section C7.2 for more information). Numbers in parentheses on the maps in mountainous areas represent the upper elevation limits in feet for the ground snow load values that are given below the elevation.

Step 2: Flat roof snow loads, p_f — Flat roofs are defined as those where the roof slope is less than or equal to 5 degrees. The flat roof snow load, p_f, is determined by Eq. 7-1: p_f = 0.7 C_e C_t I p_g.

The exposure factor, C_e, is determined from Table 7-2 and depends on the terrain category and the degree of roof exposure to wind. Terrain categories B, C, and D are defined in Section 6.5.6 and are the same as those used for wind design. Definitions of partially exposed, fully exposed, and sheltered roofs are given in footnote "a" of Table 7-2. Values of Ce are smaller for unsheltered roofs located in more open terrains than for sheltered roofs in rougher terrains, since wind tends to blow more snow off the roof in the former case.

The 0.7 factor is a basic exposure factor that is used to convert a ground snow load to a roof snow load where C_e, C_t, and I are equal to 1.0. This factor in combination with C_e produce ground-to-roof reduction factors ranging from 0.49 to 0.84.

Thermal factors, C_t, are provided in Table 7-3 and vary from 0.85 for continuously heated greenhouses to 1.2 for unheated structures and structures intentionally kept below freezing. Snow is more likely to melt on warmer roofs resulting in smaller snow loads than those typically found on a cold roof.

Table 7-4 contains the importance factor, I, for snow loads, which is determined using the occupancy categories given in Table 1-1. In general, snow loads are to be larger for more important structures and smaller for those that are less important. An importance factor of 0.8 corresponds to a 25-year ground snow load and a factor of 1.2 corresponds to a 100-year ground snow load.

The minimum values of p_f given in Section 7.3 take into account the expected loads that can occur on a roof after a single large storm. In areas where p_g is relatively low (less than or equal to 20 pounds per square foot, psf), it is possible for the roof load pf to be equal to the ground snow load p_g. In such cases, it is required that p_f = I p_g. In areas where p_g exceeds 20 psf, it is unlikely that the roof snow load will equal the ground snow load, so p_f = 20I. It is important to note that the minimum roof snow load is a separate load case and it is not to be combined with drifting, sliding, or other types of snow loading and is applicable to only low-slope roofs, which are defined in Section 7.3.4.

Flowchart 1 provides the steps that are needed to determine p_f.

Step 3: Sloped roof snow loads, p_s—This snow load is also referred to as the balanced snow load, and it is assumed to act on the horizontal projection of a roof surface. The sloped roof snow load, p_s, is determined by Eq. 7-2: p_s = C_s p_f.

The factor C_s depends on the slope and temperature of the roof, the presence or absence of obstructions, and the degree of slipperiness of the roof surface. Roof materials that are considered to be slippery and those that are not are given in Section 7.4. Figure 7-2 contains graphs of C_s for various conditions, and corresponding equations for C_s are given in Commentary Section C7.4.

For portions of curved roofs that have a slope exceeding 70 degrees, C_s = 0. Balanced snow loads for curved roofs are determined from the loading diagrams in Figure 7-3 with C_s determined from the appropriate curve in Figure 7-2. Multiple folded plate, sawtooth, and barrel vault roofs are to be designed using C_s = 1, that is p_s = p_f. These types of roofs collect additional snow in their valleys by drifting and sliding, so no reduction in snow load based on roof slope is applied.

Step 4: Partial snow loads—The partial loading provisions of Section 7.5 must be satisfied for continuous roof framing systems and all other roof systems where removal of snow load on one span (by wind or thermal effects, for example) causes an increase in stress or deflection in an adjacent span. Only the three load cases given in Figure 7-4 need to be investigated; comprehensive alternate span (or, checkerboard) loading analyses are not required.

Partial load requirements need not be applied to gable roofs where the condition described in Section 7.5.1 is satisfied. Also, the minimum roof load requirements of Section 7.3.4 are not applicable in the partial load provisions.

Step 5: Unbalanced snow loads—Unbalanced snow load occurs on sloped roofs from wind and sunlight. Wind tends to reduce the snow load on the windward portion and increase the snow load on the leeward portion. This is unlike partial snow loading where snow is removed on one portion of the roof and is not added to another portion.

Steps 6—10, as well as the rain load steps, will be printed in August 2008. Additional information on snow loads, including worked-out example problems, can be found in Structural Load Determination Under 2006 IBC and ASCE/SEI 7-05, published by ICC in 2008.

David A. Fanella, Ph.D., S.E., P.E., is Associate Principal and Director of New Structures in the Chicago office of Klein and Hoffman, Inc. He can be reached at dfanella@kleinandhoffman.com.

Flowcharts reference for Part 1:
1: Flat roof snow load, p_f