How to determine structural loads Part 2: Snow and rain loads in accordance with 2006 IBC

August 2008 » Feature Article
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.
David A. Fanella, Ph.D., S.E., P.E.

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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.

In Part 1 of this article, information was provided on how to determine ground snow loads, flat roof snow loads, sloped roof snow loads, partial snow loads, and unbalanced snow loads in accordance with 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), which is referenced by Section 1608.1 of the 2006 IBC.

Part 2 of "How to determine structural loads" continues the discussion started in Part 1 (which was printed in the July issue of Structural Engineer) on the determination of snow loads and also presents the code 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, pg (Section 7.2).
2) Determine flat roof snow load, pf by Eq. 7-1 (Section 7.3).
3) Determine sloped roof snow load, ps 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).

Steps 1—5 were discussed in Part 1; steps 6—10 are discussed below. All figures and equations refer to those within ASCE/SEI 7-05, unless noted otherwise.

Step 6: Snow drifts—The following two types of drifts can occur on lower roofs or on adjoining roofs:

  • windward drifts, where wind deposits snow from higher portions of the same building or an adjacent building or terrain feature (for example, a hill) to a lower roof; and
  • leeward drifts, where wind deposits snow from the windward portion of a lower roof to the portion of a lower roof adjacent to a taller part of a building.

Windward and leeward drifts are illustrated in Figure 7-7.

Loads from drifting snow are superimposed on the balanced snow load, ps, (see Figure 7-8). The height of the balanced snow load, hb, is equal to ps (or, the flat roof snow load pf if applicable) divided by the snow density γ, which is determined by Eq. 7-3. As shown in Figure 7-8, hc is the clear height from the top of the balanced snow load to the top of the closest point on the adjacent upper roof. Drift loads need not be considered where hc/hb < 0.2.

For leeward drifts, the drift height, hd, is determined from Figure 7-9 where lu is the length of the upper roof in the direction of analysis. In lieu of using the graph, Figure 7-9 contains an equation for hd.

The drift height for windward drifts is also determined from Figure 7-9. In this case, lu is the length of the lower roof. According to Section 7.7.1, the windward drift height is equal to three-quarters of that which is determined by Figure 7-9. The larger of the leeward and windward drift heights is used in design.

Where h hc, the drift width w is equal to 4hd , and where hd > hc, w is equal to 4hd2/hc and the drift height is taken as hc. In both cases, the drift width need not exceed 8hc. In situations where w is greater than the length of the lower roof, a triangular distribution is used over the entire drift width, which means that the drift load at the far edge of the roof is not equal to zero.

Where a higher roof or terrain feature is separated from a lower roof by a horizontal distance s, drift loads on a lower roof can be reduced by the factor (20—s)/20. It is evident from this equation that drift loads on lower roofs need not be considered where s is greater than or equal to 20 feet.

Section 7.8 contains requirements for drift loads on sides of roof projections such as mechanical units and parapets. The drift height is to be taken as three-quarters the drift height from Figure 7-9 where lu is the length of the roof upwind of the projection. Drift loads are not required on sides of roof projections that are less than 15 feet long.

Flowchart 7 can be used to determine drifts on lower roofs.

Step 7: Sliding snow—The load caused by snow sliding off a sloped upper roof onto a lower roof is determined by the provisions of Section 7.9. The provisions are applicable to slippery upper roofs (see Section 7.4 for definitions of slippery roofs) with slopes greater than 1/4 on 12 and to all other roofs with slopes greater than 2 on 12.

The total sliding load per unit length of eave, which is uniformly distributed on the lower roof a distance of 15 feet from the upper roof eave, is equal to 0.4pfW, where W is the horizontal distance from eave to ridge of the upper roof. Sliding loads are added to the balanced snow load.

In cases where the width of the lower roof is less than 15 feet, the sliding load can be reduced proportionally. For example, if the width of a lower roof is 12 feet, the reduced sliding load is equal to (12/15) x 0.4pfW, which is uniformly distributed over the 12-foot width.

Step 8: Rain-on-snow surcharge load—A rain-on-snow surcharge load of 5 pounds per square foot (psf) must be added to the balanced snow load on roofs with slopes less than W/50 that are located where 0 < pg ≤ 20 psf (Section 7.10). This surcharge load need not be used in combinations with drift, sliding, unbalanced, or partial loads.

Step 9: Ponding instability—According to Section 7.11, progressive roof deflection and ponding instability from rain-on-snow or from snow meltwater must be investigated for roofs with a slope less than 1/4 inch per foot. Roof structures in such cases must possess adequate stiffness to prevent localized overloading that can lead to failure.

Step 10: Existing roofs—Where a new roof is constructed within 20 feet of an existing roof, Section 7.12 requires that the existing roof be investigated for potential additional snow loads due to drifting or sliding. When applicable, owners or agents for owners of an existing building must be advised of the potential for increased snow loads on their roof.

Rain loads
Provisions for design rain loads are contained in Section 1611 of the 2006 IBC. The nominal rain load, R, which is determined by 2006 IBC Eq. 16-36, represents the weight of accumulated rainwater assuming that the primary roof drainage system is blocked: R = 5.2(ds + dh), where ds is the depth of water on the undeflected roof to the inlet of the secondary drainage system when the primary drainage system is blocked and dh is the additional depth of water on the undeflected roof above the inlet of the secondary drainage system at its design flow. The constant in 2006 IBC Eq. 16-36 is equal to the unit load per inch depth of water = (62.4 pounds/cubic foot)/(12 inches/foot) = 5.2 pounds per square foot/inch. An undeflected roof refers to the case where deflections from loads (including dead loads) are not considered when determining the amount of rainwater on the roof.

The primary roof drainage system is designed for a specific rainfall intensity and the area of the roof that it drains. Chapter 11 of the International Plumbing Code contains requirements on the design of roof drainage.

Where primary roof drainage is blocked, water will rise above the primary roof drain until it reaches the elevation of the roof edge, scuppers, or other secondary drains. The depth of water above the primary drain at the design rainfall intensity is based on the flow rate of the secondary drainage system, which depends on the type of drainage system.

Once the rain load R is determined, it is combined with the other applicable loads using the appropriate load combinations in Section 1605 of the 2006 IBC.

Additional information on snow and rain loads, including worked-out example problems, can be found in the ICC publication Structural Load Determination Under 2006 IBC and ASCE/SEI 7-05, 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.


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