Prepared by Marcia C. Belcher, PE

Problematic Snow Loads

Problematic Snow Loads

Snow Loads

Snow loads can be structurally problematic due to drifting and build-up.

Load provisions of ASCE 7-05 provide guidance for determining the magnitude of those loads based on geographic location.

In colder regions, the peak snow load is not the result of a single event.

-Result of accumulation from many storms over the course of a winter season.

Minnesota MetrodomeDecember 2010 Collapse

The double layer, fibreglass reinforced fabric roof is designed to work on positive

air pressure created by 20 90hp fans which keep the 250-ton structure inflated.

Snow Loads on Structures

Snow load is affected by:◦ Roof slope◦ Roof sheathing materials◦ Thermal characteristics of the structure◦ Exposure of structure to wind

All these have an impact on the amount of snow that may be present on a roof over the course of a winter season.

ASCE 7-05 accounts for each of these factors.

Basis for the snow load computation in ASCE 7-05 is the ground snow load.

ASCE 7-05 Figure 7-1 shows a map of the United States with contours for ground snow load.

(See IBC Chapter 7 handout)

Ground Snow Loads

ASCE Snow Loads Map

•Contour lines indicate 50 year mean snow recurrence interval:

•In other words, a 2% probability of being exceeded in any given year

•In the mountainous western US there are areas listed as being "CS".

-Case studies are required in these areas.

Flat Roof Snow Load:

Slope < 5°◦ ps=0.7CeCtIpg

Ce= exposure factor

Ct= thermal factor

I= importance factor

Pg=ground snow load

◦ Let’s take a look at the coefficients.

Calculating Roof Snow Load: Flat Roof

Step 1: Determine Occupancy Category & Importance Factor

ASCE Table 1-1 and 7-4

ASCE 7: Table 1-1

Step 2: Determine Thermal Coefficient Ct

ASCE Table 7-3

Drift on adjacent roof

FROM ASCE Document:

Exposure A. Large city centers with at least 50 percent of the buildings having a height in excess of 70 feet (21 356 mm). Use of this exposure category shall be limited to those areas for which terrain representative of Exposure A prevails in the upwind direction for a distance of at least 0.5 mile (0.8 km) or 10 times the height of the building or other structure, whichever is greater. Possible channeling effects or increased velocity pressures due to the building or structure being located in the wake of adjacent buildings shall be taken into account.

Exposure B. Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger. Exposure B shall be assumed unless the site meets the definition of another type exposure.

Exposure C. Open terrain with scattered obstructions, including surface undulations or other irregularities, having heights generally less than 30 feet (9144 mm) extending more than 1,500 feet (457.2 m) from the building site in any quadrant. This exposure shall also apply to any building located within Exposure B type terrain where the building is directly adjacent to open areas of Exposure C type terrain in any quadrant for a distance of more than 600 feet (182.9 m). This category includes flat open country, grasslands and shorelines in hurricane-prone regions.

Exposure D. Flat, unobstructed areas exposed to wind flowing over open water (excluding shorelines in hurricane-prone regions) for a distance of at least 1 mile (1.61 km). Shorelines in Exposure D include inland waterways, the Great Lakes and coastal areas of California, Oregon, Washington and Alaska. This exposure shall apply only to those buildings and other structures exposed to the wind coming from over the water. Exposure D extends inland from the shoreline a distance of 1,500 feet (460 m) or 10 times the height of the building or structure, whichever is greater.

Step 3: Determine Terrain Category

Step 4: Determine Exposure Coefficient Ce

ASCE Table 7-2

Step 5: Determine Ground Snow Load From Maps

ASCE Table 7-1

ps=0.7CeCtIpg

Step 6: Combine Coefficients and Ground Snow Load

Apply one more coefficient “Cs” fromTable 3.4:

This table is based on surface of roof, roof obstructions and thermal factors

Thus the roof snow load becomes:

ps=0.7CeCtIpgCs

For Slope Roofs…..

Cs = Roof Slope Factor

Determine Sloped Roof Coefficient: Cs

Slippery Roof with an Obstruction