Snow Loads: Guide to the Snow Load Provisions of ASCE 7-16

Structural engineering aims to design facilities such that structural capacity exceeds expected loads by a desirable amount. This guide specifically addresses the expected snow loads that ASCE/SEI Standard 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures provides, noting that lightweight roof framing systems are particularly sensitive to snow overload. ASCE 7-16 Commentary also notes the increased importance of snow overload as the live-to-dead load ratio increases. Another significant factor influencing the potential for snow-related structural collapse is the building’s thermal condition. A final factor to consider is the type of snow load. This chapter presents the snow load type that caused partial or complete collapse of more than 50 buildings. Practicing structural engineers involved in the design, analysis, and review of building structures are the intended users of this guide.

ASCE 7-16 specifies roof snow loads based on or related to the ground snow load, p_g. This approach, which mirrors Canadian practice, is used because of the abundance of information on ground snow measurements relative to information on roof snow loads. As winter progresses, the average ground snow density tends to increase with additional snowfall. Compacted sites are higher-elevation mountain sites with nominally continuous snow cover throughout the winter. ASCE 7-16 presents ground snow load information as a map and as tables for selected states. In ASCE 7-16, the ground snow load map is blank for Colorado, Idaho, Montana, New Hampshire, New Mexico, Oregon, and Washington. For sites in these states, users are directed to individual state tables with elevation and ground snow load for the 50 most heavily populated towns in the state.

Section 7.3 in ASCE 7-16 provides for calculation of the flat roof snow load, p_f, and prescribes a minimum roof snow load. The formula for flat roof snow loads incorporates the effects of wind exposure, the thermal conditions of the roof system, and the importance of the facility. Case histories show that the snow load on a roof is typically less than that on the ground provided that drifting is absent. The roof’s exposure to wind and its thermal characteristics influence the conversion factors. A table shows the wind exposure rating’s influence on the conversion factor and presents the conversion factor mean and standard deviation for three exposure ratings. Using thermal factors aims to quantify the differences in ground-to-roof conversion factors due to heat loss through the roof layer.

Section 7.4 of ASCE 7-16 explains how to convert flat roof snow loads into sloped roof snow loads using the roof slope factor, C_s. The sloped roof snow load, also called the balanced load, is the basis for determining the snow load for all structures. Heat flow from poorly insulated roofs melts some of the roof snowpack. The resulting melt water flows downslope toward the cold eave, where some refreezes if the ambient temperature is below 32°F. Such eave ice dams can hinder the sliding of roof snow. Section 7.4.5 specifies the structural load due to such ice dams. Section 7.4.6 is new to ASCE 7-16 and provides sloped roof snow loads for air-supported structures with slippery exterior fabric surfaces.

ASCE 7-16 Section 7.5’s partial loading provisions account for patterned or checkerboard loading. The Commentary explains that partial loading is required for continuous structural members, such as roof purlins in metal building systems, where reduced snow loading on one span increases stress or deflection in an adjacent span. Although partial loading for a roof can be compared with the checkerboard loading pattern used for floor design, an actual checkerboard pattern is not prescribed for roof snow. The general patterns presented in Cases 1, 2, and 3 could also result from roof snow removal operations. The removal of the roof snow from one portion resulting in an increase in stress or deflection at another portion may occur in other structural systems besides purlins and frames in metal building systems.

The unbalanced loads in Section 7.6 of ASCE 7-16 are in most cases drift loads. Except for sawtooth-type roofs, the roof’s windward portion is the snow source and the leeward portion accumulates a percentage of the drifted snow. This situation is termed unbalanced because the leeward portion has more snow than the windward portion. This chapter discusses unbalanced snow loading on hip and gable roofs (Section 7.6.1), curved roofs (Section 7.6.2), sawtooth roofs (Section 7.6.3), and dome roofs (Section 7.6.4). For hip and gable roofs, the unbalanced load results from drifting that removes snow from the windward side of the ridge line. Some of this snow flux eventually settles in the aerodynamic shade region on the leeward side, downwind of the ridge. Unbalanced loads for domes are based on the corresponding unbalanced loads for curved roofs applied on a 90° downwind sector.

Modern code provisions that relate drift size to the upwind snow source area have decreased the amount of snow-related roof damage roof step drifting causes. Drifts accumulate differently when the higher-level roof is on the windward side than when the higher-level roof is on the leeward side. Windward drifts often start as quadrilateral shapes because a vortex forms when wind impinges on the vertical wall directly beyond the drift. ASCE 7-16 provisions relate a roof step drift’s size to how much snow is in the source area upwind of the step. The roof step relations are empirical and based on analysis of case histories. Choosing the larger independent value for the design drift loading is based on the empirical nature of the roof step drift provisions.

The drift on the upwind side of rooftop units (RTUs) is a windward drift as it is for parapet walls. The drift on the downwind side is a somewhat reduced leeward drift composed of snow originally on the roof upwind of the RTU plus a small contribution from snow originally on top of the RTU itself. For simplicity, ASCE 7-16 requires a windward drift on each side of the unit, based on the larger of the two fetch distances. These roof projection drifts follow the same provisions as windward roof step drifts. Significant differences exist between leeward drifts at roof steps and windward drifts at a parapet wall, roof projection, or roof step. Another difference is the available case-history information. The final significant difference between leeward and windward drifts involves trapping efficiency and growth with time.

The design snow load on a nominally flat roof and on a sloped roof differ theoretically. On sloped roofs, snow simply slides off or does not stick in the first place on very steep slopes. From a structural standpoint, snow sliding off a roof is beneficial if the sliding snow does not collect in an undesirable location. ASCE 7-16 has design load provisions for snow that slides onto an adjacent roof. Snow that slides off a roof and collects against a wall is another concern. In this instance, the snow pile exerts a lateral load on the wall. ASCE 7-16 provides lower bounds where sliding loads need not be considered. The sliding snow load is superimposed on the lower roof’s balanced load. Sliding loads can also occur on lower-level roofs that are separated from but close to a higher-level roof.

A rain-on-snow surcharge occasionally contributes to roof collapse. Section 7.10’s rain-on-snow provisions are intended to cover such loading situations. The chapter describes four possible rain-on-snow scenarios. In Scenarios 1 and 2, the rain refreezes in the ground snowpack and will likely be captured in ground snow load measurements. In Scenarios 3 and 4, ground snow load measurements would not capture the rain flow through the ground snowpack. In Section 7.10 of ASCE 7-16 the rain-on-snow provisions envision rain percolating through the roof snowpack creating a rain-on-snow surcharge composed of two portions. The first portion is the load caused by vertical percolation through the upper unsaturated layer. The second is the load caused by rain flowing downslope in the saturated layer immediately above the roof.

Sections 7.11 and 8.4 of ASCE 7-16 require a ponding analysis for susceptible low-sloped roofs and for roofs where water can be impounded. The designer must use the full roof snow load in the ponding instability analysis. Section 8.4 of ASCE 7-16 further requires the designer to assume that the primary drainage is blocked. The roof slopes below which bays become susceptible are a function of the roof framing. For a new facility adjacent to an existing facility with a lower roof, Section 7.12 requires the designer either to (a) analyze and possibly reinforce the existing roof for the expected drift or sliding loads or (b) inform the owner of the existing facility of the new loads. Finally, the chapter presents problems on susceptible bays’ purlin-parallel and purlin-perpendicular geometries.

Open-frame equipment structures (OFESs) primarily differ from more typical buildings in that an OFES may or may not have flooring on various levels, typically does not have exterior (perimeter) walls, and commonly has exposed cable trays and piping. ASCE 7-16 requires the flat roof snow loads to be calculated with C_t = 1.2 at the topmost level with flooring. If wind-walls are present at this top-floor level, drift loads are required as per Sections 7.7 and 7.8. Section 7.13.2 requires the flat roof snow load to be applied along all such open-floored edges. If the piping or trays are at the same nominal elevation and the gap between them is small, falling snow could bridge the gap, increasing the snow load on both. This behavior resembles that resulting in a snow cornice on the leeward side of a mountain ridge.

This chapter presents three snow load design examples. The first is a gable roof metal building with a roof step located where ground snow load is moderately heavy. It illustrates, among other things, how to determine various types of drift loads. The second is a pole barn with a hip roof located in an area with low ground snow load. It shows, among other things, how to determine unbalanced loads on hip roofs and how to check for minimum roof snow loads and rain-on-snow surcharges. The third is a ski chalet where ground snow load is moderately heavy. It explains, among other things, the simplified unbalanced load provisions for roof rafters common to such structures. The chapter summarizes the load cases from the heated storage building purlins.

This chapter presents answers to snow loading questions which ASCE 7-16 provisions do not specifically address. As such, it does not interpret, either officially or unofficially, the ASCE 7-16 provisions. Rather, although the answers are generally consistent with the ASCE 7-16 provisions, they are the author’s opinions. In the chapter, the author replies to client questions about an apparent overload of an existing structure and installation of new rooftop units adjacent to an existing unit. He also recommends options to address significant sagging in the roof purlins. The author also explains which leeward step drift to use if the lower roof has a slope of 2 on 12 and which appropriate unbalanced load condition to use for the gable roof.