Code of Practice For Design Loads (Other Than Earthquake) For Buildings And Structures, Part 4: Snow Loads
1987 Edition

Overview and Applicability (Clause 4.2.1)

• Addresses snow load calculation on various roof types including:
  • Flat and single-sloped roofs
  • Simple or multiple pitched roofs
  • Multi-span roof structures

Shape Factors for Roof Slopes (Table 4.2.1)

Additional Coefficients for Multi-level Roofs (Clause 4.2.4)

• Calculated as: H_w = 1 + ((m1 + m2) × (4 - 2h))/l
• Where m1 and m2 depend on roof profile (values between 0.3 - 0.5)
• Restrictions on dimensions and adjustments for snow transfer due to wind and temperature.

General Principles (Clauses 4.1 & 4.2)

• Snow generally deposits evenly on roofs in calm conditions.
• Wind action causes redistribution, leading to drift loads, especially on complex or multi-level roofs.
• Shape coefficients (μ) incorporate effects of roof geometry and drifting.
• Snow distribution along eaves is presumed uniform.
• Due to limited empirical data, nominal shape coefficients are provided for design use.

Shape Coefficients for Basic Roof Types

• Refer to slope-based tables for coefficients M1 (positive slope) and M2 (negative slope).

Multi-level Roof Considerations

• Total coefficient μ = μ_s (sliding snow) + μ_w (wind-induced load).
• Parameters include height differences, roof lengths, and slope angles with defined restrictions.

Formula for Roof Snow Load (Clause 3.1)

S = p × S₀

• S: Design snow load on roof (Pa)
• p: Shape coefficient accounting for roof geometry
• S₀: Ground snow load at location (Pa)

Ground Snow Load Determination

• Based on maximum snow depth and density.
• For Indian regions such as Jammu & Kashmir, Himachal Pradesh, and Uttarakhand, consult DRDO's Snow and Avalanches Study Establishment and the Indian Meteorological Department.

Shape Coefficient Application

• Reflects effects of sliding, drifting, and accumulation on different roof types.

Notes

• Design should consider the maximum of snow load or imposed loads per IS 875 Part 2.
• Snow load is modeled as uniformly distributed pressure over the plan area.

Shape Coefficients for Flat and Monopitch Roofs (Clause 4.2.1)

• Coefficients vary with roof slope angle as detailed in tabulated form.

Multi-level Roofs (Clause 4.2.4)

• Total shape coefficient combines sliding and wind-induced components.
• Wind pressure coefficient limited between 0.8 and 4.0.
• For slopes above 15°, an additional load equal to 50% of the maximum adjacent upper roof load is linearly distributed.

Wind Pressure Coefficients (Clause 4.3)

• Vary with roof exposure and shape.
• Detailed values available in Appendix A of the standard.

Key Formulas for Roof Slopes (Clause 4.2.1)

• Asymmetrical roofs treated as halves of symmetrical roofs.
• Considerations on wind load coefficients and height restrictions included.

Considerations for Curved Roofs (Clause 4.2.3)

• Wind pressure coefficients derived based on roof slope and curvature.
• Two main cases: positive and negative slopes.
• Use shape coefficients akin to simple pitched roofs for external pressure calculations.
• Approximate curved roof slope by an equivalent angle for coefficient application.

Key Concepts (Clauses 4.2 & 4.3)

• Shape coefficients depend on roof form, slope, and wind exposure.
• Possible reduction of coefficients by 25% if roofs are fully exposed and free from snow-trapping projections.
• For multi-level roofs, total snow load is the sum of sliding and wind drift components.
• Restrictions apply on roof dimensions and wind pressure coefficients.

Typical Coefficient Ranges

Important Points (Clause 4.2.4)

• Total wind pressure on roof is sum of sliding (H_s) and wind-induced (H_w) loads.
• Height difference factor h_t influences wind load; restrictions on horizontal length l (5 m < l < 15 m) and wind pressure coefficient u_w (0.8 < u_w < 4.0).
• For roof slopes above 15°, an additional load equal to 50% of max load on adjacent upper slope is applied.
• Shape coefficients outlined in tabulated form for various slope ranges.

Effects of Projections and Obstructions (Clause 4.2.6)

• Local roof features influence wind pressure and snow accumulation.
• Use shape coefficients from Clause 4.2 for slopes and configurations.
• For multi-level roofs, wind pressure is combined from sliding and wind components.
• Additional loads due to obstructions include a 50% increase on adjacent upper slope loads, distributed linearly.
• Refer to Appendix A for detailed wind pressure coefficient calculations.

Guidelines (Clause 5.1)

• Ice thickness considered between 3 mm and 10 mm depending on site.
• Ice density assumed as 0.9 g/cm³ (900 kg/m³).
• Effective wire diameter increases by twice the ice thickness: d_ice = d + 2t
• Ice load per unit length calculated using: w = π × d_ice × t × ρ × g
• Wind force calculations on wires use the enlarged diameter d_ice.
• Designers should verify local climatic conditions to select appropriate ice thickness.

Key Equations and Limits (Clause 4.2.4)

• Total snow height H_a = H_s + H_w
• Wind-induced height H_w = 2h, constrained by 5 m < l < 15 m
• Shape coefficient μ_w limited to 0.8 ≤ μ_w ≤ 4.0
• For slopes exceeding 15°, an additional load of 50% of maximum adjacent upper roof load is linearly distributed.
• Interpolation methods suggested when horizontal length to snow drift ratio is below 13.
• Tabulated coefficients for simple roof slopes provided for reference.