Explanatory Handbook on Indian Standard Code of Practice for Design Loads (Other than Earthquake) for Buildings and Structures

IS SP Part 64 – Scope Key Points & Tables Summary

Scope (Clause 6.2.3 & 1.0):

• Covers wind pressure calculations on various roof types: monoslope, free roofs, pitched roofs, and free-standing double slopes.
• Applies to roofs with open bays on all sides, considering height/width (h/w), length/width (L/w), and roof slope angles.
• Modifications needed for obstructions or stored materials under roofs.

Key Tables for Roof Wind Pressure (Clause references):

Important Specifications:

• Height (h): Defined as per Table 8, not simply lowest canopy point.
• Width (b): Main canopy width only, excluding fascia if inclined beyond edge.
• Frictional Force: For large roofs (Tables 9-14), distribute friction force:
  • 50% on windward 1/3rd surface
  • 50% on leeward 2/3rd surface
  • Act at mid-plane of these areas.

Wind Pressure Calculation (General formula):

[ p = 0.6 \times k_1 \times k_2 \times k_3 \times V^2 ]

Where:

• (p) = design wind pressure (kN/m²)
• (V) = basic wind speed (m/s)

Basic Wind Speed & Wind Map of India (IS SP 64 - Clause 3.1)

• Basic Wind Speed (Vb): Peak 3-second gust wind speed at 10 m height in open terrain (Category 2) for a 50-year return period.
• Range: 33 m/s to 55 m/s across India, divided into six zones.
• Data Source: Based on 47 DPA stations (1948-1983), analyzed using Gumbel distribution.
• Adjustments: Orography, Palghat Gap funneling, cyclonic storms (effective up to 60 km inland, 110 km for Bengal coast), dust storms, and Norwesters considered.
• Wind Speed at Height: Constant up to 10 m; velocity profile varies with terrain category above 10 m.
• Design Tip: Use higher speed if site lies on zone boundary without local data.

Velocity Profile over Terrain Categories (Fig. 8)

Basic Wind Speed Map Zones (m/s)

Formula for Design Wind Speed at height z (Vz):

[ V_z = V_b \times k_1 \times k_2 \times k_3 ]

• k1: Probability factor (risk coefficient)
• k2: Terrain, height, and structure size factor
• k3: Topography factor

flowchart LR
   

IS SP Part 64: Terrain, Height and Structure Size Factor (k₂)

Key Points from Clauses 3.4, 5.3.2 & 1.17:

• k₂ accounts for the influence of terrain roughness, structure height, and size on wind pressure.
• Valid for heights up to 50 m (or specified height).
• Terrain categories: 1, 2, 3, 4 (increasing roughness).
• Structure classes (size): A, B, C (large to small).
• For height ≤ 10 m, k₂ = 1.05 (Clause 5.3.2).
• Values can be linearly interpolated between heights (e.g., between 40 m and 50 m).
• Different k₂ values exist for 1-hour averaged wind speeds (Table 33).

Typical k₂ Factor Table (Excerpt from Table 2):

Note: Use linear interpolation between heights if exact height is not tabulated.

Formula for Linear Interpolation of k₂:

[ k_2 = k_{2h1} + \frac{(h - h_1)}{(h_2 - h_1)} \times (k_{2h2} - k_{2h1}) ]

• (h) = height of interest
• (h_1, h_2) = known heights bounding (h)
• (k_{2h1}, k_{2h2}) = corresponding k₂ values at (h_1, h_2)

Summary:

• Select terrain category (1–4) based on roughness.
• Select **structure

IS SP Part 64: Pressure Coefficients for Walls and Roofs

1. External Pressure Coefficients on Walls (Clause 6.2.2.1, Table 4)

• Use Cpe values from Table 4 based on wind direction and wall orientation.
• For end walls with openings ≤ 5% area, internal pressure coefficient Cpi = ±0.2 (Clause 6.2.3).

2. External Pressure Coefficients on Roofs (Clause 6.2.2.2, Table 5 & 15)

• Curved roofs converted to pitched roof above 3.5 m; coefficients per Table 5.
• Angles: θ = 0° or 90°, slope α = 22.6°.

For θ = 90°:

3. Internal Pressure Coefficient (Clause 6.2.3)

• For openings ≤ 5% of wall area:
  Cpi = ±0.2

Summary Formula for Net Pressure Coefficient:

[ C_{p(net)} = C_{pe} - C_{pi} ]

where:

• ( C_{pe} ) =

IS SP Part 64: Force Coefficients for Structural Members

Key Clauses & Tables:

• Clause 6.3 & Table 24: General force coefficients for structural members.
• Clause 2.7: Force and moment coefficients for members.
• Clauses 6.3.3.3 to 6.3.3.6 & Tables 26-32:
  • Force coefficients for unclad buildings, frameworks (single, multiple, lattice towers).
  • Effects of shielding and solidity ratios.
  • Global force coefficients for lattice towers (Tables 28-32).
  • Force coefficients for individual members and cables (Tables 26, 27).
• Clause 9.2.4 & Table 23:
  • Force coefficients for infinite length members.
  • For finite length members, modify coefficients by factor k based on slenderness ratio l/d or l/b.
  • If member end flow is blocked by plate/wall:
    • Double l/b for one blocked end.
    • Set l/b = 0 if both ends blocked (affects k).

Important Formula for Finite Members:

[ C_f = C_{f,\infty} \times k ]

Where:

• ( C_f ) = Force coefficient for finite member
• ( C_{f,\infty} ) = Force coefficient for infinite length (from Table 23)
• ( k ) = Modification factor depending on ( l/d ) or ( l/b ) and end conditions

Typical Values (Example from Table 23):

Summary:

• Use Table 23 for infinite length members.
• Modify using k factor based on slenderness and end flow conditions.
• Refer Tables 26-32 for frameworks and lattice towers.
• Consider shielding and solidity effects in multiple frames.

flowchart TD
    A[Start: Identify Member Type] --> B{Is member

Calculation of Design Wind Pressure as per IS SP Part 64

Key Formula:

Design wind velocity at height z: [ V_z = V_b \times k_1 \times k_2 \times k_3 ]

• (V_b) = Basic wind speed (m/s)
• (k_1) = Probability factor (life of structure)
• (k_2) = Terrain, height, and structure size factor
• (k_3) = Topography factor

Design wind pressure at height z: [ P_z = 0.6 \times V_z^2 \quad \text{(N/m}^2\text{)} ]

Example Calculation:

Given:

• (V_b = 47, m/s)
• (k_1 = 0.90) (for 25 years life)
• (k_2 = 0.98)
• (k_3 = 1.00) (flat ground)

Calculate: [ V_z = 47 \times 0.90 \times 0.98 \times 1.00 = 41.45, m/s ] [ P_z = 0.6 \times (41.45)^2 = 1031, N/m^2 ]

Design Wind Pressure Table (Excerpt):

Pressure Coefficients for Roof and Walls:

Gust Factor and Dynamic Effects (IS SP Part 64)

1. Gust Factor (G) Formula [Clause 9.3 & 8.3]

[ G = 1 + g_r \cdot r \cdot V \left[ B (1 + 0)^2 + \frac{SE}{B} \right] ]

Where:

• (g_r \cdot r) = gust response factor (from Fig. 8)
• (V) = mean wind velocity at height (h)
• (B) = background turbulence factor (from Fig. 9)
• (S) = size reduction factor (from Fig. 10)
• (E) = gust energy factor (from Fig. 11)

2. Key Parameters and Calculations

3. Example Calculation Steps

1. Calculate design wind velocity (V(h)) and pressure (p_z' = 0.6 V(h)^2).
2. Determine gust response factor (g_r r) from Fig. 8 based on height.
3. Compute background turbulence factor (B) from Fig. 9.
4. Calculate reduced frequency (f_r) and get size reduction factor (S) from Fig. 10

Wind Load Calculation - IS SP Part 64 Key Points

1. Basic Wind Pressure Calculation

• Design wind pressure ( p = 0.6 \times V^2 \times k_1 \times k_2 \times k_3 ) (N/m²)
  • (V) = Basic wind speed (m/s)
  • (k_1) = Risk coefficient
  • (k_2) = Terrain, height & structure size factor
  • (k_3) = Topography factor

2. Pressure Coefficients (Cp) for Buildings

• For rectangular buildings with ( h/w \leq 0.5 ) and ( 1 < l/w \leq 1.5 ), typical Cp values (wind at 0°) are:

• For wind at 90°, refer to the same table with different Cp values.

3. Design Wind Pressures for Cities (N/m²)

IS SP Part 64: Local Pressure Coefficients & Internal Pressures

1. Local Pressure Coefficients (Clause 4.2 & 6.2.2.1, 6.2.2.2)

• External Pressure Coefficients on Walls (Clause 6.2.2.1 & Table 4)
• External Pressure Coefficients on Roofs (Clause 6.2.2.2 & Table 5)

2. Key Table: External Pressure Coefficients for End Walls (Clause 1.5)

3. Internal Pressure Coefficient (Cpi)

• Depends on building openings.
• Typical values:
  • Closed buildings: ±0.2
  • Partially open buildings: ±0.55
  • Open buildings: 0.0

4. Pressure Calculation Formula

[ p = 0.6 \times V^2 \times C_p ]

Where:

• (p) = pressure (kN/m²)
• (V) = design wind speed (m/s)
• (C_p) = net pressure coefficient = external (C_{pe}) ± internal (C_{pi})

Summary:

• Use Tables 4 & 5 for walls and roofs external coefficients.
• Refer Clause 1.5 Table for end walls local coefficients.
• Combine external and

IS SP 64 (S&T): 2001 — Wind Loads on Special Structures

Key Formulas

• Design Wind Pressure:

[ P_z = 0.6 \times V_z^2 \times k_1 \times k_2 \times k_3 ]

Where:

• (V_z) = design wind speed at height z (m/s)

• (k_1) = risk coefficient

• (k_2) = terrain, height & structure size factor

• (k_3) = topography factor

• Wind Force on Element:

[ F = A_e \times P_{ax} \times C_f ]

Where:

• (A_e) = effective frontal area (m²)

• (P_{ax}) = design wind pressure (N/m²)

• (C_f) = force coefficient (depends on shape and aspect ratio)

• Moment at Base due to Wind:

[ M = \sum (F_i \times h_i) ]

Where:

• (F_i) = wind force at height (h_i)
• (h_i) = height of force application

Important Tables & Values

Refer Table 11 for detailed wind loads at floor levels.

Specifications Summary

• Terrain Category: Defines roughness; affects (k_2) factor.
• Wind Zone: Determines basic wind speed (V_b) (e.g., Zone 2 =

IS SP 64: Worked Examples & Illustrations Key Points

Reference Clauses & Tables for Roof Types:

• Clause 6.2.3 & 6.2.2: Wind pressure and force calculations.
• Table 7: Monoslope & free roofs.
• Table 8: Free standing double sloped roofs.
• Table 9: Pitched free roof with angle x = 30°.
• Table 10: Pitched free roof with angle xx = 30°, including effect of train or stored material.
• Table 22 & Clause 6.2.2.13: Additional coefficients and factors.

Typical Calculation Steps (Force Coefficient Method):

1. Determine Basic Wind Speed (Vb) from Clause 3.1.

2. Calculate Design Wind Speed (Vz): [ V_z = V_b \times k_1 \times k_2 \times k_3 ]

   • (k_1): Risk coefficient
   • (k_2): Terrain, height, structure size factor
   • (k_3): Topography factor

3. Calculate Design Wind Pressure (Pz): [ P_z = 0.6 \times V_z^2 ] (Pressure in N/m², velocity in m/s)

4. Apply Force Coefficients (Cf) from relevant tables (e.g., Table 7-10) based on roof type and angle.

5. Calculate Wind Force (F): [ F = P_z \times C_f \times A ]

   • (A): Effective surface area

Example Parameters (from Fig. 20, Example 5):

• Height (h = 6.0,m)
• Width (w' = 20.0,m)
• Length (w = 100.0,m)
• Number of units = 4
• Roof angle (\alpha = 12^\circ)

Useful Tables Summary:

Key Formulas, Tables, and Specifications from IS SP Part 64 Annexures & Tables

1. Roof Pressure Tables (Clause 6.2.3 & Clause 1.0)

• Table 7: Monoslope & Free Roofs
• Table 8: Free Standing Double Sloped Roofs (h/w: 0.25–1.0, L/w: 1–3)
• Tables 9 to 14: Pitched roofs and large canopies with open bays on all sides
  • Use Tables 9 & 10 for large canopies with b/d ≈ 5.0, considering wind angles 0°, 45°, 90°, 135°, 180°
  • Tables 11 & 12 for canopies inclined at 30°
• Friction Force Distribution: For large roofs (Tables 9–14), friction force is split:
  • 50% on windward 1/3 surface
  • 50% on leeward 2/3 surface
    Forces act at mid-plane of these areas.

2. Important Definitions (Clause 1.0 & Clause 2)

• Height (h): Not simply lowest canopy point; refer to Table 8 for precise definition per roof angle
• Width: Main canopy width excluding fascia if extended/inclined beyond canopy edge
• Roof Slope Angles: Tables vary with roof slope (e.g., 30°, 65°)

3. Wind Load Calculation Parameters

• Basic wind speed, terrain category, and structure class affect pressure coefficients (Clause 3 & 4)
• Gust factor method applies dynamic effects (Clause 6)

Example: Curved Circular Arch Roof (Clause 2.5)