Seismic Evaluation, Repair and Strengthening of Masonry Buildings - Guidelines

IS 13935: Scope Summary & Key References

• Scope (Clause 1.4):

  • Provides guidelines for reinforcing horizontal and vertical seismic belts per IS 4326.
  • Applies to typical buildings within span and height limits of IS 4326 and IS 13935.
  • For special buildings exceeding these limits, specialist analysis is required.

• Referenced Standards:

  IS No.	Title
  1893 (Part 1): 2002	Criteria for earthquake design of structures
  4326 : 1993	Earthquake resistant design & construction of buildings
  13828 : 1993	Improving earthquake resistance of low strength masonry buildings

• Key Notes:

  • Reinforcement of seismic bands (horizontal & vertical) follows IS 4326.
  • Special analysis for buildings with larger spans/heights beyond IS 4326 scope.
  • Definitions and seismic design criteria as per IS 1893 (Part 1).

Typical Reinforcement Concept (per IS 4326 referenced):

Rounding Off (per IS 2:1960):

• Final calculated values should be rounded to the same number of significant digits as specified values.

flowchart TD
    A[IS 13935 Scope] --> B[Reinforcement per IS 4326]
    A --> C[Special analysis for large spans/heights]
    B --> D[Horizontal seismic bands]
    B --> E[Vertical seismic bars]
    A --> F[Reference IS Codes]
    F --> G[IS 1893 (Part 1)]
    F --> H[IS 4326]
    F --> I[IS 13828]

Summary: IS 13935 guides seismic reinforcement referencing IS 4326, with special provisions for atypical buildings, and aligns with IS 1893 for earthquake design criteria.

IS 13935: Occupancy and Importance Classification Summary

1. Occupancy Classification (Clause 2.0)

Any building with more than 100 occupants may be treated as Important.

2. Importance Factor (Clause 7.1)

• Important buildings have an importance factor (I) = 1.5.
• Ordinary buildings have I = 1.0 (default).

3. Seismic Zone Factors (IS 1893 Part 1)

4. Design Seismic Force Formula

[ F = I \times Z \times S_a \times W ] Where:

• (F) = design seismic force,
• (I) = importance factor,
• (Z) = zone factor,
• (S_a) = spectral acceleration coefficient,
• (W) = seismic weight of the structure.

5. Special Hazard Considerations (Clause 3.0)

• High water table within 5 m (liquefiable soil),
• Landslide-prone sites,
• Severe vertical and plan irregularities.

flowchart TD
    A[Building Occupancy] -->|>100 occupants| B[Important Building (I=1.5)]
    A -->|<100 occupants| C[Ordinary Building (I=1.0)]
    B & C --> D[Calculate Seismic Force: F = I × Z × Sa × W]
    D --> E[Design Structure for Seismic Loads]

Summary:
Use I=1.5 for important buildings to increase seismic design forces by 50%. Classify occupancy based on occupant count and building function as per Clause

IS 13935: Seismic Hazard and Site Conditions - Key Points

1. Seismic Hazard Classification (Clause 2.0)

• Occupancy Types:
  • Important Buildings: Hospitals, schools, emergency services, large halls, power stations, VIP residences, etc.
  • Ordinary Buildings: Buildings with <100 occupants.
• Special Hazards:
  • High water table within 5 m + sandy soil → Liquefiable site.
  • Landslide-prone site.
  • Severe vertical and plan irregularities (Yes/No).
• Falling Hazards:
  • Chimneys, parapets, cladding, others.

2. Site Conditions (Clause 1.5)

• Soil Types and Foundation:
  • Refer Table A-7.2.5 for soil classification.
  • Important for foundation design and seismic response.
• Vertical Irregularities:
  • Check for sudden changes in stiffness or mass in vertical direction.
  • Formula for vertical irregularity check:
    [ K_i < 0.8 (K_{i+1} + K_{i+2} + K_{i+3}) ] where (K_i) is stiffness of storey (i).

3. Rapid Visual Screening (Form 1)

• Document building parameters:
  • Number of stories, soil type, foundation, wall and roof types.
  • Presence of seismic bands (plinth, lintel, eaves, gable).
  • Reinforcement detailing at corners, junctions.

Summary Table: Seismic Hazard Indicators

flowchart TD
    A[Building] --> B[Check Occup

IS 13935 Key Points on Falling Hazards & Non-Structural Components

1. Occupancy Classification (Clause 2.0 Table)

• Important Buildings: Hospitals, schools, emergency facilities, large halls, power stations, VIP residences, etc.
• Ordinary Buildings: Occupants < 100, other general buildings.

2. Falling Hazards (Clause 2.0 Table)

Common falling hazards include:

• Chimneys
• Parapets
• Cladding
• Other non-structural elements

3. Special Hazards Affecting Falling Hazards

• High water table & liquefiable soils
• Landslide-prone sites
• Severe vertical and plan irregularities

4. Recommendations (Clause 7.2.4)

• In seismic zones IV & V, identify and report falling hazards explicitly in survey reports.
• Ensure non-structural components are secured against seismic forces.

Typical Design Considerations for Non-Structural Components

• Seismic Anchorage: Use flexible connections and anchors to prevent detachment.
• Mass & Stiffness: Minimize mass and avoid rigid attachments to reduce inertial forces.
• Inspection & Maintenance: Regularly check for damage or looseness.

Example: Seismic Force on Non-Structural Components

[ F = C_s \times W ]

Where:

• (F) = Seismic force on component
• (C_s) = Seismic coefficient (depends on zone & occupancy)
• (W) = Weight of the non-structural component

flowchart TD
    A[Building Occupancy] --> B[Identify Falling Hazards]
    B --> C{Hazard Type}
    C -->|Chimneys| D[Secure Anchors]
    C -->|Parapets| D
    C -->|Cladding| D
    C -->|Others| D
    D --> E[Design for Seismic Forces]
    E --> F[Inspection & Maintenance]

Summary: IS 13935 emphasizes identifying falling hazards (chimneys, parapets, cladding) especially in important buildings and seismic zones IV & V, with recommendations for anchorage and reporting in survey documents.

Damageability Assessment of Masonry Buildings — IS 13935 Key Points

1. Damageability Grades & Retrofitting (Table 1)

Note: For G4 & G5, aim for non-collapse performance only, especially in Seismic Zones IV & V.

2. Assessment Factors (Clause 7)

• Earthquake intensity (with importance factor 1.5 per IS 1893)
• Building typology & configuration
• Construction quality & maintenance

3. Crack Restoration Techniques (Fig. 1)

• 1A: Epoxy/grout injection in cracks
• 1B: Cement mortar + flat chips for wide cracks
• 1C: Cement mortar + wire mesh for cracks
• Steps: Remove plaster → Clean cracks → V-groove joints → Apply mortar/chips → Wire mesh front/back → Cement plaster

4. Probable Damageability (Clause 5.0 & 5.1)

5. Key Specification for Anchoring Bars (Clause 6.4.1)

• Drill hole > bar diameter
• Fill with epoxy or high-strength grout
• Insert bar, hold until grout sets

flowchart TD
    A[Start: Visual Screening] --> B{Damage Grade?}
    B -->|G1| C[No Retrofitting]
    B -->|G2| D[Stabilize Non-structural Elements]
    B -->|G3| E[Structural + Non-structural Retrofitting]
   

IS 13935: Structural Repair Techniques - Key Points

1. Structural Repair of Minor and Medium Cracks (Clause 6.2)

• Minor cracks: Usually repaired by epoxy injection or cementitious grout.
• Medium cracks: Require routing and sealing or stitching with steel bars.
• Ensure removal of loose material and proper surface preparation.

2. Structural Repairs/Restoration (Clause 4.2)

• Remove damaged concrete.
• Clean and treat reinforcement to remove corrosion.
• Use compatible repair materials (cementitious or polymer-modified mortars).
• Maintain original structural integrity and durability.

3. Fibre Reinforced Plastics (FRP) (Clause 5.8)

• FRP used for strengthening beams, columns, walls.
• Advantages:
  • 2 to 10 times stronger than steel plates.
  • Weight only 20% of steel.
  • High corrosion resistance.
• Application:
  • Bonded with epoxy resin to concrete surface.
  • Used as external reinforcement or confinement.

Typical Repair Steps with FRP:

flowchart TD
    A[Surface Preparation] --> B[Epoxy Primer Application]
    B --> C[FRP Sheet/Plate Placement]
    C --> D[Epoxy Saturation & Curing]
    D --> E[Final Inspection & Testing]

Summary Table: Crack Repair Methods

For detailed design and application, refer to IS 13935 clauses and relevant epoxy/FRP manufacturer guidelines.

IS 13935: Evaluation of Building Irregularities - Key Points

1. Types of Irregularities (Ref: Clause 7.1, IS 1893 Part 1)

• Plan Irregularities (Table 4):

  • Torsion irregularity
  • Re-entrant corners
  • Diaphragm discontinuity
  • Out-of-plane offsets
  • Non-parallel lateral force resisting systems

• Vertical Irregularities (Table 5):

  • Mass irregularity
  • Vertical geometric irregularity
  • In-plane discontinuity in vertical lateral force resisting elements

Buildings with these irregularities require detailed seismic evaluation.

2. Damageability Grades of Masonry Buildings (Table 10, Clause A-5)

• Note: "Few" = 5-15%, "Many" = 50%, "Most" = 75% affected.

3. Special Notes

• Buildings with vertical irregularities in Zones III, IV, and V require special seismic design.
• Buildings with plan irregularities may suffer damage one grade higher.
• Buildings in liquefiable or landslide-prone areas require special evaluation.

4. Quick Screening Parameters (Form 1)

• Building use, stories, soil type, foundation type,

IS 13935: Rapid Visual Screening (RVS) for Seismic Hazards – Key Points

1. Purpose & Scope (Annex A-1)

• RVS identifies seismic vulnerability without detailed structural calculations.
• Focuses on primary lateral load-resisting system and building attributes affecting seismic performance.
• On-site inspection taking a few hours per building.

2. Seismic Zones in India (Annex A-3)

3. Building Types & Vulnerability (Annex A-4, Table 8)

4. Damageability Grades (Annex A-5)

• Five grades G1 to G5 based on MSK/European Intensity scales (Table 9 & 10 in IS).
• Used to estimate expected damage level.

5. Key RVS Survey Form Elements

• Building identification (name, address, use)
• Structural details: wall type, thickness, mortar type, seismic bands
• Soil & foundation type (Clause 1.5, Table A-7.2.5)
• Sketch plan with dimensions
• Presence of vertical irregularities (FIG. 23)
• Notes on hazards: falling hazards, soft stories

6. Foundation Strengthening (Clause 14)

• Techniques: underpinning, drainage improvement, apron provision, RC strips on wall footings (FIG. 21)
• Connection of new walls to old (FIGs 18-20) via steel ties, concrete infill, keys.

Summary Formula/Criteria:

• Foundation Soil Factor (Ki) for vulnerability check:

  [ K

IS 13935: Strengthening Methods for Masonry Walls and Openings

Key Strengthening Techniques:

1. Strengthening Masonry Arches (Clause 9.5)

• By Ties: Use steel flat irons or rods (Fig. 6) to tie arch stones and prevent cracking.
• Insertion of Beam: Insert steel beam lintel above arch to avoid thrust.
• Ties and Bearing Plates: Use steel ties with bearing plates to reduce arch thrust.

2. Seismic Belts Around Openings (Clause 10)

• Purpose: Reinforce jambs/piers between openings in seismic zones (Cat D & E).
• Mesh Specifications:

• Vertical seismic belts must cover jambs and piers on both sides (Fig. 11).

3. Integral Box Action (Clause 11)

• Use horizontal bands and vertical steel bars at corners/T-junctions.
• Pre-stressing or seismic belts improve lateral strength.

4. Stitching and Through Bond Elements (Clause 9.4)

• Install 'through' stones or RC headers at ~1 m spacing with 500 mm stagger.
• Drill holes, insert hooked 8 mm bars, fill with 1:2:4 concrete, cure 10 days.

5. Grouting (Clause 9.1)

• Drill 2-4 holes/m², inject water then cement grout (1:1 mix) at 0.1-0.25 MPa pressure.
• Improves cohesion and fills hollowness.

Important Tables Summary:

IS 13935: Seismic Belts Around Openings – Key Specifications

Location of Seismic Belts (Clause 11.3.1)

• On both faces of walls just above lintels of doors/windows.
• Below floor or roof level.
• Roof belt can be omitted if RCC slab is used.
• Not required at plinth level unless height > 900 mm.
• At eave level of sloping roofs and near top of gable walls, below roof.

Belt Dimensions and Reinforcement (Clause 3.25)

• Gauge (g) of wires:
  • g10 = 3.25 mm
  • g11 = 2.95 mm
  • g12 = 2.64 mm
  • g13 = 2.34 mm
  • g14 = 2.03 mm
• N = Number of longitudinal wires at 25 mm spacing.
• H = Height of belt in micro-concrete (mm).
• Transverse wires spaced up to 150 mm.
• Mesh must be galvanized for corrosion protection.

Typical Arrangement (Fig. 10)

• Seismic belts are reinforced mesh bands embedded in micro-concrete.
• Ties and rafters connected to belts ensure structural integrity at openings and roof edges.

flowchart LR
    A[Wall] --> B[Seismic Belt Above Lintel]
    A --> C[Seismic Belt Below Floor/Roof]
    A --> D[Seismic Belt at Eave Level]
    B --> E[Longitudinal wires @ 25 mm spacing]
    B --> F[Transverse wires @ ≤150 mm spacing]
    E & F --> G[Galvanized Mesh in Micro-concrete]

Summary: Provide galvanized wire mesh seismic belts just above openings and below roofs, with specified wire gauges and spacing, ensuring enhanced earthquake resistance.

Achieving Integral Box Action (IS 13935 - Clause 11)

Integral box action improves lateral strength and stability of bearing wall buildings by ensuring walls act as a unified shear-resisting box.

Key Methods:

• Pre-stressing: Apply tension to horizontal bands to clamp walls.
• Horizontal Bands: Provide continuous reinforced concrete or steel bands at floor/roof levels.
• Vertical Reinforcement: Place vertical steel bars at corners and T-junctions of walls to strengthen shear walls.

Specifications:

• Horizontal Bands: Use RC ring beams or cast-in-situ concrete topping with:
  • 40 mm thick concrete topping
  • 6 mm diameter bars @ 150 mm c/c both ways (Clause 12.5)
• Vertical Reinforcement (Table 7):
  Vertical bars or mesh in vertical belts at corners depending on building category and storeys, e.g.:

Additional Recommendations:

• Use seismic belts (mesh reinforcement) around door/window openings (Clause 10).
• Connect perpendicular walls at corners and T-junctions using tie rods or seismic belts (Clause 8.1).

Simplified Concept Diagram:

graph LR
A[Walls] --> B[Horizontal Bands (RC Ring Beams)]
A --> C[Vertical Reinforcement at Corners]
B & C --> D[Integral Box Action]
D --> E[Lateral Strength & Stability]

Summary:
Integral box action is achieved by combining horizontal RC bands, vertical reinforcement at critical locations, and proper connection of walls to form a rigid box resisting lateral loads effectively.

IS 13935 - Modifications of Roofs and Floors (Clause 12.5)

Key Specifications:

• Prefabricated units (RC T or channel, wooden poles & joists):
  Integration is essential to ensure structural continuity.

• Timber elements:
  Connected by diagonal planks nailed and spiked to an all-round wooden frame at ends.

• Reinforced Concrete (RC) elements:

  • Either 40 mm cast-in-situ concrete topping with 6 mm dia bars @ 150 mm c/c both ways
  • Or bounded by a horizontal cast-in-situ RC ring beam embedding ends of RC units.

Table 7: Vertical Reinforcement in Vertical Belt at Corners (Clause 11.4)

Bars spacing typically 300-650 mm depending on category and storey.

Summary Diagram of RC Integration for Prefab Units:

flowchart LR
    A[Prefabricated RC Unit] --> B[Ends Embedded in RC Ring Beam]
    A --> C[40 mm Cast-in-situ Concrete Topping]
    C --> D[6 mm dia Bars @ 150 mm c/c both ways]
    E[Wooden Poles & Joists] --> F[Diagonal Planks Nailed]
    F --> G[Spiked to Wooden Frame at Ends]

Use these guidelines to ensure proper load transfer and structural integrity when modifying roofs or floors with prefabricated or timber elements.

IS 13935: Use of Advanced Materials in Repair and Strengthening

Key Materials & Properties (Clause 5.1 & 5.8)

• Cement & Steel: Basic repair materials; steel forms include bolts, rods, angles, expanded metal.
• Admixtures: Used to enhance mortar/concrete properties (non-shrinkage, bond).
• Fibre Reinforced Plastics (FRP):
  • High strength-to-weight ratio (2-10× steel strength).
  • Weight ~20% of steel.
  • High corrosion resistance.
  • Bonded using epoxy mortars.

Repair Techniques (Clause 6.3)

For cracks >5 mm or crushed concrete:

• Remove loose material.
• Fill with expansive cement mortar or quick setting cement.
• Provide additional shear/flexural reinforcement if needed.
• Cover reinforcement with mortar for protection.
• For severe damage, replace member or part.
• Use steel mesh on walls/floors, fixed and plastered for strengthening.

Typical Repair Material Properties

Conceptual Repair Flowchart

flowchart TD
    A[Damage Assessment] --> B{Crack Width > 5mm?}
    B -- Yes --> C[Remove loose material]
    C --> D[Fill with expansive/quick setting mortar]
    D --> E{Additional reinforcement needed?}
    E -- Yes --> F[Add shear/flexural steel reinforcement]
    F --> G[Cover with mortar]
    E -- No --> G
    B -- No --> H[Injection repair or minor patching]
    G --> I[Severe damage?]
    I -- Yes --> J[Replace member/portion]
    I -- No --> K[Apply steel mesh + plaster for walls/floors]

Summary: Use FRP for high-strength, corrosion-resistant strengthening; expansive mortars for crack filling; steel reinforcement for structural support; and steel mesh for surface strengthening.

IS 13935: Anchorage and Connection Improvements - Key Points

1. Mesh and Reinforcement Specifications (Clause 3.25 & 10)

• Mesh: Gauge 10 (3.25 mm dia) galvanized mesh with 25 mm wire spacing.
• Longitudinal wires (N): Number varies per design.
• Belt width (B): Micro concrete belt width split half on each adjoining wall.
• Transverse wires spacing: Up to 150 mm.
• For openings (doors/windows):
  • Category D & E: Gauge 10 mesh, 8 vertical wires @ 25 mm, belt width 200 mm or Gauge 13 mesh @ 25 mm, belt width 250 mm.
  • Category C: Gauge 13 mesh, 10 vertical wires @ 25 mm, belt width 250 mm.

2. Bar Diameter for T-Junctions

• Single bar: HSD or TOR type.
• Two bars at T-junction:
  • One 10 or 12 mm bar = Two 8 mm bars.
  • One 16 mm bar = Two 12 mm bars.

3. Mechanical Anchors (Clause 5.7)

• Use mechanical anchors with wedging action for shear and tension.
• Chemical anchors with polymer adhesives are alternatives.

4. Through Bond Elements in Stone Walls (Clause 9.4)

• Holes ~75 mm diameter drilled through walls.
• Insert 8 mm hooked mild steel bars.
• Fill with 1:2:4 concrete mix.
• Cure for 10 days minimum.

5. Retrofitting Actions for Roofs and Floors (Table 4 & 5)

6. **Integral Box Action

IS 13935 - References and Related Standards

Key Related Standards Referenced:

Important Notes:

• Reinforcement of horizontal and vertical seismic belts follows IS 4326.
• For buildings exceeding IS 4326 scope (large spans/heights), specialist analysis is recommended.
• Rounding off numerical values should comply with IS 2:1960 rules.

Typical Reinforcement Specification (from Table 7 for Vertical Bars at Corners):

Summary Diagram: Reference Flow for Seismic Design

graph LR
A[IS 13935] --> B[IS 1893 (Part 1): Earthquake Design Criteria]
A --> C[IS 4326: Earthquake Resistant Construction]
A --> D[IS 13828: Masonry Building Guidelines]
B --> E[General Provisions]
C --> F[Horizontal & Vertical Seismic Belts]
D --> G[Low Strength Masonry Improvements]

Use these standards collectively for seismic design and detailing per IS 13935. Always check for latest editions.