The flat roof snow load, r f, on a roof with a slope equal to or less than 5 degrees (0.09 rad) (1 inch per foot = 4.76 degrees) shall be calculated in accordance with Section 7.3 of ASCE 7.
The value for the snow exposure factor, Ce, used in the calculation of rf shall be determined from Table 1608.3.1.
|TERRAIN CATEGORYa||EXPOSURE OF ROOFa,b|
|Fully exposedc||Partially exposed||Sheltered|
|A (see Section 1609.4)||N/A||1.1||1.3|
|B (see Section 1609.4)||0.9||1.0||1.2|
|C (see Section 1609.4)||0.9||1.0||1.1|
For SI: 1 mile = 1609 m.
- The terrain category and roof exposure condition chosen shall be representative of the anticipated conditions during the life of the structure. An exposure factor shall be determined for each roof of a structure.
- Definitions of roof exposure are as follows:
- Fully exposed shall mean roofs exposed on all sides with no shelter afforded by terrain, higher structures or trees. Roofs that contain several large pieces of mechanical equipment, parapets which extend above the height of the balanced snow load, hb, or other obstructions are not in this category.
- Partially exposed shall include all roofs except those designated as "fully exposed" or "sheltered."
- Sheltered roofs shall mean those roofs located tight in among conifers that qualify as "obstructions."
- Obstructions within a distance of 10 ho provide "shelter," where ho is the height of the obstruction above the roof level. If the only obstructions are a few deciduous trees that are leafless in winter, the "fully exposed" category shall be used except for terrain category "A." Note that these are heights above the roof. Heights used to establish the terrain category in Section 1609.4 are heights above the ground
The value for the thermal factor, Ct, used in the calculation of rf shall be determined from Table 1608.3.2.
For SI: 1h • ft2 •°F/Btu = 0.176 m2 •K/W.
|All structures except as indicated below||1.0|
Structures kept just above freezing and others with cold, ventilated roofs in which the thermal resistance (R-value)between the ventilated space and the heated space exceeds 25h • ft2 • °F/Btu
|Continuously heated greenhousesb with a roof having a thermal resistance (R-value) less than 2.0h • ft2 • °F/Btu||0.85|
- The thermal condition shall be representative of the anticipated conditions during winters for the life of the structure.
- A continuously heated greenhouse shall mean a greenhouse with a constantly maintained interior temperature of 50°F or more during winter months. Such greenhouse shall also have a maintenance attendant on duty at all times or a temperature alarm system to provide warning in the event of a heating system failure.
The value for the snow load importance factor, Is, used in the calculation of rf shall be determined in accordance with Table 1604.5.2 based on the Structural Occupancy Category determined in accordance with Table 1604.5. Greenhouses that are occupied for growing plants on production or research basis, without public access, shall be included in Structural Occupancy Category I.
Roofs with a slope less than 1/2 inch per foot (2.38 degrees) shall be designed for a rain-on-snow surcharge load determined in accordance with Section 7.10 of ASCE 7.
For roofs with a slope less than 1/4 inch per foot (1.19 degrees), the design calculations shall include verification of the prevention of ponding instability in accordance with Section 7.11 of ASCE 7.
The snow load, ps, on a roof with a slope greater than 5 degrees (0.09 rad) (1 inch per foot = 4.76 degrees) shall be calculated in accordance with Section 7.4 of ASCE 7.
The effect of not having the balanced snow load over the entire loaded roof area shall be analyzed in accordance with Section 7.5 of ASCE 7.
The extra load caused by snow sliding off a sloped roof onto a lower roof shall be determined in accordance with Section 7.9 of ASCE 7.
The design of enclosed buildings more than 250 feet (76 200 mm) in plan dimension shall provide for the forces and/or movements resulting from an assumed expansion corresponding to a change in temperature of 40°F (4.44°C). For exterior exposed frames, arches, or shells regardless of plan dimensions, the design shall provide for the forces and/or movements resulting from an assumed expansion and contraction corresponding to an increase or decrease in temperature of 40°F (4.44°C) for concrete or masonry construction and 60°F (15.55°C) for metal construction. For determination of the required anchorage for piping, the forces shall be determined on the basis of temperature variations for the specific service conditions. Friction forces in expansion bearings shall be considered.