Guidelines for Improving the Cyclonic Resistance of Low-Rise Houses and Other Buildings/Structures
2004 Edition

Overview of Scope in IS 15498

• Scope Details: Specifies requirements relating to structural components and materials aimed at improving cyclone resistance.

• Rounding Procedures:

  • Adhere to IS 2:1960 standards for rounding figures.
  • Maintain the same number of significant digits as indicated within this standard.

• Referenced Codes:

  • Annex A includes related standards referenced herein.
  • Users should confirm and apply the most current editions of these referenced documents.

Compliance Highlights

For formulae or tabulated data, specify the particular structural element or material covered by IS 15498.

Key Aspects and Equations on Cyclonic Wind Field

1. Distribution of Wind Speeds (Clauses 3.1 & 3.2):

• Maximum Wind Radius (RMW): Distance from cyclone's eye center to point of highest tangential wind speed.
• Velocity Within RMW: Increases linearly from zero at cyclone center to peak at RMW.
• Velocity Beyond RMW:

[ V(r) = V_0 \left(\frac{r_0}{r}\right)^x ]

where:

• (V(r)) = wind speed at radius (r)
• (V_0) = peak wind velocity at radius (r_0)
• (r_0) = radius of maximum wind
• (x) = exponent in power law (typically 0.4 to 0.6)
• (r) = radial distance from cyclone center

2. Computation of Design Wind Speed in Cyclone Areas (Clause 1.15):

[ V_a = f \times k_1 \times k_2 \times k_3 \times V_b ]

where:

• (V_a) = design wind speed at height (z) (m/s)
• (f) = cyclonic risk amplification factor
  • Residential: 1.0
  • Industrial: 1.15
  • Critical post-cyclone structures: 1.30
• (k_1) = probability factor (risk coefficient)
• (k_2) = terrain, height, and building size factor
• (k_3) = topographic factor
• (V_b) = basic wind speed (from IS 875 Part 3)

3. Additional Notes:

• Values for (k_1, k_2, k_3, V_b) per IS 875 Part 3.
• Special structures like chimneys and towers have distinct requirements.
• Wind speed varies with elevation above ground.

flowchart LR
    A(Cyclone Center) -->|Radius r| B(Max Wind Radius \(r_0\))
    B -->|Max Wind Velocity \(V_0\)| C(Wind Speed \(V(r)\))

Design Wind Speed Calculation (IS 15498)

Formula for design cyclonic wind speed at height (z):

[ \boxed{ V_a = f \times k_1 \times k_2 \times k_3 \times V_b } ]

Details:

Additional Information:

• Inside cyclone eye radius, wind speed increases linearly from zero.
• Special structures warrant code-specific wind speed considerations.
• Cyclone winds have higher turbulence, justifying the factor (f).

Wind Speed Calculation Flow

flowchart LR
    Vb[Basic Wind Speed (Vb)]
    k1[Probability Factor (k1)]
    k2[Terrain/Height/Size Factor (k2)]
    k3[Topography Factor (k3)]
    f[Enhancement Factor (f)]
    Va[Design Wind Speed (Va)]

    Vb --> Va
    k1 --> Va
    k2 --> Va
    k3 --> Va
    f --> Va

Essential Formulas and Guidelines for Wind Pressures (IS 15498)

1. Design Wind Speed (Clause 1.15):

[ V_a = f \times k_1 \times k_2 \times k_3 \times V_b ]

Parameters:

• (f): Cyclone risk factor
  • Residential: 1.0
  • Industrial: 1.15
  • Critical structures post-cyclone: 1.30
• (k_1): Probability (risk) factor
• (k_2): Terrain, height & size factor
• (k_3): Topography factor
• (V_b): Basic wind speed (IS 875 Part 3)
• (V_a): Design wind speed at height (z)

2. Calculation of Wind Pressures and Forces:

• Apply wind pressure coefficients from IS 875 (Part 3) for both overall and local pressures.
• For clustered buildings:
  • Corner units: multiply pressures by 1.5.
  • Interior units: apply shielding factor 0.8.
• Roof uplift forces must be securely transferred to foundations using robust connectors (e.g., GI bolts, metal straps).

3. Structural Recommendations:

• Minimum parapet height of 600 mm for load-bearing masonry.
• Anchor roof slabs firmly to continuous lintel bands with proper ties.
• Employ ductile detailing per IS 13920 in cyclone-prone areas, even if wind load governs design.
• Provide bottom chord bracing in roof trusses to prevent buckling.
• Design rounded building corners to reduce aerodynamic drag.

Cyclonic Risk Enhancement Factor Summary

Wind Pressure Calculation Flow

flowchart TD
    Start[Start: Basic Wind Speed (Vb)] --> ApplyF[Apply Cyclonic Risk Factor (f)]
    ApplyF --> ApplyK1[Apply Probability Factor (k1)]
    ApplyK1 --> ApplyK2[Apply Terrain/Height/Size Factor (k2)]
    ApplyK2 --> ApplyK3[Apply Topography Factor (k3)]
    ApplyK3 --> CalculateVa[Calculate Design Wind Speed (Va)]

IS 15498: Planning Guidelines for Cyclone Resilient Buildings

Essential Planning Instructions (Clause 6)

• Site Selection:

  • Choose stable, natural ground; avoid partially reclaimed land.
  • Avoid flood-prone lowlands and ridge tops susceptible to high winds.

• Building Shape and Orientation:

  • Favor regular, compact, symmetrical layouts; circular or polygonal preferred.
  • Avoid re-entrant corners and irregular zigzag layouts.
  • Where wind direction is known, orient the smallest building face towards prevailing winds.

• Building Layout:

  • Cluster arrangements preferred over linear rows to mitigate tunnel wind effects.
  • Provide cross walls or pilasters in walls exceeding 3.5 meters in length.

• Roof Design:

  • Use hipped roofs instead of gabled for superior wind resistance.
  • Avoid roof slopes less than 1:3, except for large-span roofs.

• Openings and Walls:

  • Limit total openings to no more than 50% of wall width.
  • Position openings in load-bearing walls at least h/6 from inner corners, where h is storey height.

• Spacing Between Buildings:

  • Maintain at least twice the building width for spacing to enhance shielding.

• Flood Protection:

  • Employ raised earth mounds, stilts, or retaining structures in coastal flood zones.

Planning Features Summary

Formula for Minimum Distance of Openings from Inner Corner:

[ \text{Distance} \geq \frac{h}{6} ]

IS 15498: Guidelines for Non-Engineered Building Methods

Key Provisions (Clauses 3.5 & 7):

• Long Wall Support: Walls longer than 3.5 m require cross walls or integral pilasters to enhance lateral stability.

• Site Location: Avoid flood-prone low grounds and ridges with high wind exposure.

• Roof Pitch: Minimum slope of 1:3; hipped roofs recommended to reduce uplift forces.

• Wall Openings: Keep openings below 50% of wall width; position openings at least h/6 away from inner corners.

• Building Spacing: For spacing less than twice the building width, interior buildings benefit from shielding (pressure factor 0.8), while corner buildings face full loads (factor 1.5).

• Flood Mitigation: Use elevated earthen mounds or stilts with bracing; posts must be anchored at minimum 900 mm depth with at least 22,500 mm² bearing area.

• Anchoring and Connections: Secure roofs with diagonal organic ropes replaced annually or cyclone bolts; limit roof overhang to 450 mm or tie back overhangs.

Practices Summary

Anchorage Details:

• Four anchor poles per post, installed at two vertical levels, to secure structural stability.

IS 15498: Semi-Engineered Construction Practices and Key Calculations

Anchorage of Roof to Masonry (Clause 1.5 j):

• Anchorage reinforcement disperses at a 2 vertical to 1 horizontal ratio.
• Shear strength of masonry is excluded from uplift resistance calculations.
• Effective masonry weight above anchorage equals 1.5 times the uplift force.

Structural Elements and Connections:

• Use 3 mm by 25 mm mild steel plates for longitudinal reinforcements at roof crown and eaves.
• Anchor to foundations with galvanized or mild steel straps (30 mm × 24 gauge) and heavy gauge wire ties (16 gauge).
• Triangular roof frames spaced at a maximum of 2.0 m for sloped roofs.
• Connections must safely transfer uplift forces to foundations via metal straps, bolts, nuts, and steel flats.

Hollow Concrete Block Masonry (Clause 3.0 d):

• Reinforced pilasters spaced no more than 3.0 m apart.
• Pilasters anchored into foundations and integrated with lintel bands and bond beams.

Roof and Wall Connections (Clauses 3.0 e, f):

• Strong connections between wooden roof and wall members.
• Roof uplift forces transmitted safely to foundations.
• Roofs anchored to continuous lintel bands with cyclone bolts if walls are strong brick masonry.

Planning and Layout (Clause 3.5):

• Walls longer than 3.5 m require cross walls or pilasters.
• Roof pitch preferably greater than 1:3; hipped roofs preferred due to lower suction.
• Openings in load-bearing walls less than 50% width and positioned at least h/6 from corners.
• Building clusters: corner buildings subject to 1.5 load factor; interior buildings receive shielding factor 0.8.

Summary Table: Anchorage and Construction Specifications

IS 15498: Engineered Construction – Core Principles

Structural Reinforcement and Pilasters

• Reinforced hollow concrete block masonry with pilasters spaced at maximum 3.0 m.
• Reinforcements anchored into foundations and connected to lintel bands and bond beams.

Roof Structural Connections

• Roof uplift forces must be securely transmitted to foundations.
• Ensure robust connections between wooden roof and wall components.
• Anchor roofs to continuous lintel bands using cyclone bolts if brick walls are strong.

Load Factors for Building Clusters

• Corner buildings subjected to a pressure multiplier of 1.50.
• Interior buildings benefit from shielding factor of 0.80.

Anchorage and Load Dispersion

• Anchorage reinforcement disperses at a 2 vertical to 1 horizontal angle.
• Shear strength of masonry is neglected; effective masonry weight is 1.5 times uplift force.

Roof Truss Bracing

• Provide bracing for the bottom chord of trusses to avoid buckling and distribute horizontal loads.

Summary Table: Load Factors for Clustered Buildings

Connection Details Overview

• Use U-hook bolts with washers for sheet fixing.
• Employ GI/MS straps and metal brackets for roof-to-wall connections.
• Pilasters and bond beams must be continuous and well integrated.

flowchart TD
    A[Roof Uplift Forces] --> B[Strong Roof-Wall Connections]
    B --> C[Force Transmission to Foundation]
    C --> D[Anchorage Reinforcement]
    D --> E[Load Dispersion (2V:1H)]
    E --> F[Foundation and Bond Beam Integration]

Important Specifications for Roof and Wall Fixings (IS 15498)

1. Roof Anchorage into Masonry (Clause 1.5 j)

• Reinforcement dispersion angle: 2 vertical to 1 horizontal.
• Masonry shear strength is disregarded.
• Effective masonry weight above anchorage:

[ W_{eff} = 1.5 \times \text{uplift force} ]

• Anchorage reinforcements must resist uplift securely.

2. Triangular Frames for Sloped Roofs (Clauses 2.0 f, g)

• Maximum spacing between frames: 2.0 m.
• Frames support cross beams, connected by metal straps, bolts, nuts, and steel flats.
• Triangular frames anchored to bond beams at eaves; bond beams connected to wall posts via U bolts.

3. Restraining Bands for Tiled Roofs (Clause 1.2 a)

• Bands spaced between 1.2 m and 1.5 m.
• Band dimensions approximately 100 mm × 50 mm with embedded 10 mm diameter bar.
• Hip, valley, and ridge tiles set in continuous mortar bands.
• Nails through tile holes into mortar provide shear connection.

4. Foundation Anchorage (Fig. 5 & Clause 2.0 g)

• Bond beams connected to foundations using U bolts.
• Holding down bolts designed with a safety factor of 2.0.

5. Connection Components (Figs. 4, 6, 11, 12)

• Use 3 mm × 25 mm MS plates longitudinally.
• GI/MS straps 30 mm × 24 gauge for connections.
• Wire ties: 2 turns of 16 gauge or 4 turns of heavy wire.
• For thatched roofs, wire ties secure rafters, braces, and posts.

Summary Table of Typical Connection Elements

Bracing and Anchorage Essentials (IS 15498)

1. Bracing Details

• Triangular frames for sloped roofs spaced at a maximum of 2.0 m.
• Frames must support cross runners securely.
• Connections use metal straps, bolts, nuts, and steel flats to ensure structural integrity.

2. Anchorage Specifications

• Triangular frames firmly anchored to bond beams at eaves.
• Bond beams connected to main wall posts with U bolts.
• Anchorage reinforcement disperses loads at an angle of 2 vertical to 1 horizontal.
• Shear strength of masonry is ignored; effective masonry weight considered as 1.5 times uplift force.

3. Typical Dimensions and Materials

• Bond beams approximately 100 mm × 50 mm with a 10 mm diameter reinforcement bar.
• Metal straps of 30 mm × 24 gauge GI/MS.
• MS plates sized 3 mm × 25 mm for longitudinal ties.
• Holding down bolts designed with factor of safety of 2.0.

4. Connection Practices

• U bolts preferred for asbestos sheet roofs.
• Pilasters in hollow concrete block walls spaced at maximum 3.0 m.
• Roof uplift forces must be effectively transmitted to foundations via strong connections.

Summary Table: Anchorage and Bracing Specifications

Materials and Construction Best Practices (IS 15498)

1. Anchorage to Masonry (Clause 1.5 j)

• Reinforcement dispersion angle: 2 vertical to 1 horizontal.
• Shear strength of masonry disregarded in uplift calculations.
• Effective masonry weight above anchorage equals 1.5 times the uplift force.
• Use cyclone bolts to anchor roofs to continuous lintel bands in strong brick masonry walls.

2. Anchorage Details (Figs. 4–6)

• Employ 3 mm × 25 mm mild steel plates longitudinally at roof crown and eaves.
• Use 30 mm × 24 gauge GI/MS straps for connections.
• Secure rafters, braces, and posts with heavy gauge wire ties (16 gauge, 2 to 4 turns).

3. Semi-Engineered Construction (Clause 3.5)

• Walls longer than 3.5 m require cross walls or pilasters.
• Roof pitch preferably greater than 1:3 to reduce uplift.
• Hipped roofs favored over gabled roofs due to lower suction pressures.
• Openings in load-bearing walls limited to less than 50% width and placed at a minimum distance of h/6 from corners.
• Avoid building on flood-prone or low-lying sites; use raised mounds or stilts with bracing in surge zones.

4. Non-Engineered Construction (Clause 7)

• Limit roof overhang to 450 mm or secure with tie-backs.
• Anchor posts to a minimum depth of 900 mm using four anchor poles.
• Protect mud walls with stone or brick revetments and waterproof plaster coatings.

Summary Table: Key Anchorage and Construction Parameters

Multi-Hazard Approaches for Cyclone and Flood Resistance (IS 15498)

Essential Guidelines (Clauses 3.5 and related):

• Walls exceeding 3.5 m in length should include cross walls or pilasters to mitigate wind damage.
• Avoid constructing on low-lying flood-prone sites or ridge tops in hilly terrain.
• Roofs should have slopes greater than 1:3, except for large-span applications.
• Hipped roofs are preferred due to approximately 80% of gabled roof suction pressure.
• Limit total openings in frontal walls to less than 50% of wall width; openings should be positioned at least h/6 away from inner corners.
• Maintain inter-building spacing at or above twice the building width to reduce wind forces; closer spacing offers shielding for interior buildings.
• In flood-affected coastal areas, employ raised platforms, stilts (avoiding masonry below surge levels), and appropriate bracing for seismic resistance.

Critical Dimensions and Limits:

Aerodynamic and Construction Notes:

• Employ diagonal rope patterns to secure thatched roofs.
• Treat posts below ground with coal tar up to maximum anticipated flood level.
• Use stone or brick revetments and waterproof plaster on mud walls.

References for Load Estimation:

• Wind pressure coefficients from IS 875 (Part 3).
• Use 80% of gabled roof suction pressure figures when specific data for hipped roofs are unavailable.

flowchart TD
    A[Walls > 3.5m] --> B[Cross Walls or Pilasters]
    C[Roof Slope < 1:3] --> D[Avoid except for Large Spans]
    E[Openings < 50% and ≥ h/6 from Corners]
    F[Inter-Building Spacing ≥ 2× Width for Shielding]
    G[Flood Protection Measures: Raised Platforms/Stilts]

IS 15498 Cross-References to Other Indian Standards

The standard cites several Indian Standards crucial for cyclone-resilient design and testing, including:

Frequently Cited Codes:

• IS 875 (Part 3): Wind loads on structures
• IS 456: Code of practice for plain and reinforced concrete
• IS 1893: Criteria for earthquake-resistant design
• IS 800: Code of practice for general steel construction
• IS 13827: Guidelines on enhancing earthquake resistance of low-strength masonry
• IS 2: Rules for rounding numerical values

Rounding Off Specifications (IS 2:1960):

• Final numerical values must be rounded to the same number of significant digits as prescribed.

Rounding Rules Summary

For particular formulas or tables, specify the clause or subject (e.g., wind load computations or structural details).

Composition of the Committee for IS 15498 (Annex B)

The Cyclone Resistant Structures Sectional Committee (CED 57) formulated IS 15498. Key members included:

Summary:

• The committee comprised experts from government, research, academia, and industry.
• Chaired by Dr. Prem Krishna.
• Ensured multidisciplinary expertise for robust cyclone-resistant design guidance.

For wind load formulas or design tables, IS 15498 refers users to IS 875 (Part 3) for pressure coefficients and load calculations.

flowchart LR
    A[Committee] --> B[Government]
    A --> C[Research Institutes]
    A --> D[Academic Bodies]
    A --> E[Industry]
    A --> F[BIS Directorate]

Highlights from Annexes and Illustrations in IS 15498

1. Wind Pressure Modifiers (Clause 1.50)

• Corner roof pressure factor: 1.50 for outer rows in industrial sheds.
• Shielding factor for interior buildings: 0.80 for gabled roofs.

2. Roof and Wall Fixing Components

• Use 30 mm × 24 gauge GI/MS strips for roof attachment.
• Apply 8 mm diameter GI U-hook bolts; J-hooks are not recommended.
• Fixings per roofing sheet: 4 at corners, 3 at ridges, 2 in interior sections.

3. Structural Elements

• Reinforced concrete masonry pilasters and continuous bond beams (Fig. 9).
• Tie-down bolts connecting walls to foundations (Fig. 10).
• Roof-to-wall connections with galvanized straps or wooden cleats (Figs. 11 & 12).
• Mandatory bottom chord bracing for trusses; upper chord bracing recommended (Fig. 13).

4. Planning Guidelines (Table 6, Clause 3.5)

5. Flood and Multi-Hazard Provisions

• Elevated ground or stilts in flood-prone regions.
• Minimum parapet height of 600 mm for load-bearing masonry.
• Ductile detailing as per IS 13920 in seismic zones III and higher.

Summary of Illustrative Figures

flowchart LR
    RoofFixing[Roof Fixing Details] --> ConcreteStrips[Concrete Strips & Hook Bolts]
    RoofFixing --> FixingQuantities[Quantity of Fixings per Sheet]
    WallFixing[Wall Fixing Details] --> PilastersBondBeams[Pilasters & Bond Beams]
    WallFixing --> TieDownBolts[Tie-Down Bolts]