Seismic Evaluation and Strengthening of Existing Reinforced Concrete Buildings - Guidelines

IS 15988: Scope & Key Specifications Summary

Scope (Clause 1.2):

• Assesses existing buildings' life-safety performance, focusing on identifying unfavorable structural characteristics that may cause damage.
• References key IS codes for design and detailing:
  • IS 456:2000 – Plain & reinforced concrete practice
  • IS 1893 (Part 1):2002 – Earthquake resistant design
  • IS 13920:1993 – Ductile detailing for seismic forces

Important Symbols & Notations

Knowledge Factor (K) for Material Strength (Clause 5.5)

Effective Stiffness Values (Clause 7.2.3)

IS 15988 Key References: Formulas, Tables & Specifications

1. Effective Stiffness Values (Table 2, Clause 7.2.3)

2. Knowledge Factor (K) for Material Evaluation (Table 1, Clause 5.5)

3. Key Symbols & Notations (Clause 3.36 and others)

• Ac = Total cross-sectional area of columns
• Ag = Gross area of RC section
• As = Steel area in jacket
• Aw = Area of shear wall
• E = Modulus of elasticity (Concrete or Steel)
• fck = Characteristic compressive strength of concrete
• fy

IS 15988 - Definitions & Key Specifications

• Vertical Irregularity (Clause 3.36):
  Discontinuity in strength, stiffness, geometry, or mass in one storey relative to adjacent stories.

• Key Symbols:

• Knowledge Factor, K (Table 5.5):
  Reflects confidence in structural data/documentation, ranges from 1.00 (complete data + testing) to 0.50 (little knowledge).

• Referenced Codes:
  • IS 456:2000 (Concrete design)
  • IS 1893 (Part 1):2002 (Earthquake design)
  • IS 13920:1993 (Ductile detailing)

Summary Diagram: Vertical Irregularity Concept

graph TD
  A[Storey i] -->|Strength/Stiffness discontinuity| B[Storey i+1]
  B -->|Geometry/Mass discontinuity| C[

IS 15988: Preliminary Evaluation Summary

Purpose (Clauses 5.6.1, 6.1)

• Quick, conservative assessment of building's physical condition, robustness, structural integrity, and strength.
• Identify potential seismic vulnerability using:
  • Visual inspection
  • Simple calculations based on conservative parameters
  • Observed damage patterns from past earthquakes

Procedure (Clause 5.6.3)

• Establish actual structural layout.
• Assess key characteristics affecting seismic risk.
• Use approximate methods to screen buildings for detailed evaluation.

Outcome (Clause 5.6.2)

• If strength, stability, and integrity are acceptable, no further action.
• If deficiencies found, proceed to detailed evaluation unless:
  • Building is single/two-storey,
  • Floor area < 300 m²,
  • Not essential for post-earthquake emergency,
  • And seismic retrofitting is done.

Key Points:

Typical Simple Calculation Example:

• Approximate lateral strength check:

[ V_d = \sum (A_c \times f_{ck} \times \alpha) ]

Where:

• ( V_d ) = Design lateral strength
• ( A_c ) = Cross-sectional area of structural elements
• ( f_{ck} ) = Characteristic compressive strength of concrete
• ( \alpha ) = Reduction factor for conservatism

flowchart TD
    A[Start Preliminary Evaluation] --> B[Visual Inspection]
    B --> C[Identify Structural Layout]
    C --> D[Simple Strength & Stability Calculations]
    D --> E{Are Strength & Stability Acceptable?}
    E -->|Yes| F[No Further Action]
    E -->|No| G{Is Building Exempted?}
    G -->|Yes| F
    G -->|No| H[Detailed Evaluation Required]

For detailed formulas and parameters, refer to IS 15988 Clause 5.6 and associated

IS 15988 – Evaluation Process Key Points

1. Evaluation Procedure (Clause 7.2)

• Follow a systematic approach:
  • Preliminary evaluation (Clause 5.6.3)
  • Detailed analysis (Clause 8.4.3)
  • Performance criteria adherence (Clause 8.4.1)

2. Preliminary Evaluation (Clause 5.6.3)

• Initial assessment of structural condition.
• Identify deficiencies and potential failure modes.
• Collect data: material properties, geometry, loading.

3. Analysis Options (Clause 8.4.3)

• Choose analysis methods consistent with evaluation.
• Options include:
  • Linear elastic analysis.
  • Nonlinear analysis.
  • Load testing results incorporation.

4. Design Criteria (Clause 8.4.1)

• Strengthening design must meet the same performance criteria as evaluation.
• Ensure safety, serviceability, and durability.

Typical Evaluation Flowchart (Simplified)

flowchart TD
    A[Preliminary Evaluation] --> B[Detailed Structural Analysis]
    B --> C[Performance Assessment]
    C --> D{Meets Criteria?}
    D -- Yes --> E[Maintain/Monitor]
    D -- No --> F[Design Strengthening Measures]

Summary Table: Evaluation Steps

Note: Always refer to IS 15988 clauses for detailed methodology and safety factors.

IS 15988: Acceptability Criteria Summary

1. Definition (Clause 3.1)

• Acceptability criteria are limiting values of:
  • Drift
  • Strength demand
  • Inelastic deformation
    Used to judge if a structural component is acceptable.

2. Building Acceptability (Clause 6.3 & 7.3)

• A building is acceptable if:
  • It passes configuration checks (geometry, irregularities).
  • It meets global checks on axial and shear stresses.
  • It satisfies component-level criteria (strength and deformation).
  • Either of the two conditions in 7.3 (e.g., performance levels) are met.
  • Supplemental criteria per building type (Clause 7.4) are fulfilled.

3. Demand-Capacity Ratio (Clause 7.2.4)

• For each component:

[ \text{DCR} = \frac{\text{Demand (member action)}}{\text{Capacity (probable strength)}} ]

• Acceptable if: ( \text{DCR} \leq 1 )

Key Table: Example Acceptability Limits

Visualization of Acceptability Check Flow

flowchart TD
    A[Start: Structural Analysis] --> B{Check Configuration}
    B -- Pass --> C{Global Stress Checks}
    B -- Fail --> F[Not Acceptable]
    C -- Pass --> D{Component Demand-Capacity Ratio}
    C -- Fail --> F
    D -- DCR ≤ 1 --> E[Acceptable Building]
    D -- DCR > 1 --> F

Summary:
IS 15988 emphasizes checking drift, strength, and deformation limits, ensuring all global and local stress checks pass, and verifying DCR ≤ 1 for components to declare a building acceptable.

Detailed Evaluation of Primary Lateral-Force Resisting System (IS 15988)

Key Steps (Clause 6.5 & 7.2.3)

1. Modified Demand Lateral Force: [ V_{modified} = V_{base} \times \text{Occupancy Risk Factor} \times \text{Usable Life Factor} ]

2. Shear Stress Check:

   • Check shear stress in columns and walls against allowable shear stress.
   • Use member forces from linear static/dynamic analysis per IS 1893 (Part 1).

3. Axial Stress Check:

   • For moment frame columns, axial stress must satisfy: [ \sigma_{axial} = \frac{P}{A} \leq f_{allowable} ]

4. Component Strength with Knowledge Factor (Clause 7.2.1): [ \text{Effective Strength} = \text{Nominal Strength} \times \text{Knowledge Factor} ]

Important Table: Effective Stiffness Values (Table 2, Clause 7.2.3)

Analysis Procedure:

• Use linear static or dynamic analysis per IS 1893 (Part 1).
• Include all concrete and masonry elements in the model.
• Use stiffness values from Table 2 for member modeling.

flowchart TD
    A[Start:

IS 15988: Seismic Strengthening Options and Strategies (Clause 8.2 & 8.5)

Key Points:

• Strengthening aims to increase redundancy of lateral load resisting elements to prevent collapse.
• Strategies must correct or reduce seismic deficiencies identified during evaluation (Clause 7).
• Applicable mainly to Reinforced Concrete (RC) framed structures.

Common Strengthening Options (Clause 8.5):

• Addition of Shear Walls or Bracing: To enhance lateral stiffness and strength.
• Jacketing of Columns and Beams: Using concrete or steel jackets to improve capacity.
• Base Isolation: To reduce seismic forces transmitted to the structure.
• Strengthening Beam-Column Joints: Using FRP wraps or steel plates.
• Adding New Frames or Bracing Systems: To improve load paths and redundancy.

Design Methodology (General):

1. Evaluate existing deficiencies (strength, stiffness, ductility).
2. Select strengthening option(s) based on deficiency and structural configuration.
3. Calculate required capacity increase using seismic demand and capacity ratios.
4. Detail strengthening elements for ductility and compatibility.

Typical Formula for Capacity Increase:

[ \text{Required Strength} = \text{Seismic Demand} \times \text{Factor of Safety} ]

Where seismic demand is based on:

• Design Base Shear (V)
• Moment and Shear capacities of elements

Summary Table: Strengthening Options vs. Effect

graph LR
A[Seismic Evaluation] --> B[Identify Deficiencies]
B --> C[Select Strengthening Strategy]
C --> D{Options}
D --> E[Shear Walls / Bracing]
D --> F[Jacketing]
D --> G[Base Isolation]
D --> H[FRP Wrapping]
E & F & G & H --> I[Design

IS 15988 - Clause 8.4 & 8.5: Strengthening Methods and Design

1. Design Criteria (8.4.1)

• Strengthening design must meet performance criteria as per Clause 5 (Evaluation Process).
• Ensure structural safety, serviceability, and ductility comparable to or better than original design.
• Consider seismic demands, load combinations, and material properties.

2. Methods of Analysis

• Use non-linear static (pushover) or linear dynamic analysis based on building complexity.
• Verify strengthened member capacity against increased loads and seismic forces.
• Check for:
  • Flexural strength
  • Shear strength
  • Anchorage and bond capacity

3. Strengthening Options (8.5)

• Common techniques include:
  • Jacketing (concrete or steel)
  • Fiber Reinforced Polymer (FRP) wrapping
  • Adding new RC elements (beams, columns, shear walls)
  • Base isolation or energy dissipation devices

4. Key Formula for Strengthened Member Capacity

For flexural strength of a strengthened RC member:

[ M_u = \phi \times (0.87 f_y A_s d + 0.36 f_{ck} b x (d - \frac{x}{2})) ]

Where:

• ( \phi ) = strength reduction factor
• ( f_y ) = yield strength of steel
• ( A_s ) = area of tension steel
• ( d ) = effective depth
• ( f_{ck} ) = characteristic compressive strength of concrete
• ( b ) = width of section
• ( x ) = neutral axis depth

5. Typical Table: Strengthening Techniques & Applications

IS 15988: Supplemental Damping and Isolation - Key Points

1. Supplemental Damping and Isolation (Clause 8.3.1)

• Base Isolation:

  • Effective for stiff, low-rise, heavy mass buildings.
  • Reduces seismic demand on structural elements by decoupling structure from ground motion.

• Supplemental Damping (Energy Dissipation):

  • Effective for flexible structures with some inelastic capacity.
  • Reduces displacement demands by dissipating seismic energy.

2. Effective Stiffness Values (Table 7.2.3)

3. Design Notes

• Use response reduction factor R = 5 for moment-resisting RC frames (IS 1893 Part 1).
• Adjust R if supplemental damping/isolation deficiencies exist.
• Consider vertical irregularities (Clause 3.36) affecting stiffness and strength.

Practical Formula for Base Shear Reduction via Isolation:

[ V_b = \frac{W \times S_a}{R / q} ]

Where:

• (V_b) = base shear
• (W) = seismic weight
• (S_a) = spectral acceleration
• (R) = response reduction factor
• (q) = isolation factor (reduces effective seismic forces)

Conceptual Diagram of Isolation & Damping:

graph LR
A[Ground Motion] --> B[Base Isolation System]
B --> C[Superstructure]
C --> D[Supplemental Dampers]
D --> E

Addition of New Structural Elements (IS 15988: Clause 8.5.2)

Purpose:
To increase lateral force capacity by adding new elements like shear walls and steel bracing.

Key Specifications for Steel Bracing (Clause 8.5.2.2)

• Slenderness ratio:
  [ \text{Slenderness} \leq \frac{2500}{\sqrt{f_y}} ]

• Width-thickness ratio limits:

  Section Type	Max Width/Thickness Ratio
  Angle sections	(\leq \frac{136}{\sqrt{f_y}})
  Circular sections	(\leq \frac{8960}{\sqrt{f_y}}) (D/t ratio)
  Rectangular tubes	(\leq \frac{288}{\sqrt{f_y}})

• Chevron (inverted-V) braces:

  • Beam must resist unbalanced vertical load:
    [ P = P_y \text{ (tension)} \quad \text{and} \quad 0.3 P_{ac} \text{ (compression)} ]
  • Beam flange lateral force design:
    [ F = 0.02 \times f_b \times t_f ]

• Brace connections: Must prevent out-of-plane failure and brittle fracture. See Fig. 4 in IS 15988 for typical gusset and hinge details.

Effective Stiffness Values (Table 2, Clause 7.2.3)

IS 15988: Member Level Strengthening Key Points

1. Purpose (Clause 8.2.1)

• Strengthen only deficient members (columns/beams) in buildings with adequate global strength/stiffness.
• Focus on strength, stiffness, and ductility improvement.
• Retain existing lateral-force resisting system.

2. Strengthening Measures (Clause 8.2.1b & c)

• Jacketing (concrete or steel) of columns/beams to:
  • Increase strength and stiffness.
  • Improve ductility via confinement (especially for RC columns).
• Ductility enhancement without large strength increase is beneficial when only a few members are deficient.

3. Design Criteria (Clause 8.4.1)

• Follow same performance criteria as evaluation (Clause 5).
• Ensure strengthened member meets target strength, stiffness, and ductility.

Typical Strengthening Techniques & Effects

Simplified Formula for Jacketed Column Strength (Approximate)

[ P_{u,new} = P_{u,old} + A_{jacket} \times f_{jacket} ]

• (P_{u,new}): Ultimate axial load capacity after jacketing
• (P_{u,old}): Original ultimate axial load capacity
• (A_{jacket}): Cross-sectional area of jacket
• (f_{jacket}): Design strength of jacket material

flowchart TD
    A[Existing Building] --> B{Global Strength & Stiffness Adequate?}
    B -- Yes --> C[Identify Deficient Members]
    C --> D[Member Level Strengthening]
    D --> E[Jacketing, Wrapping, etc.]
    E --> F[Improved Strength, Stiffness, Ductility]
    B -- No --> G[Consider System Level Strengthening]

Summary:

Key Specifications & Formulas for Condition Assessment (IS 15988)

1. Visual Inspection Criteria (Clause 7.1.1)

• Concrete deterioration: No visible damage in vertical/lateral force elements.
• Cracks in boundary columns: Diagonal cracks ≤ 3 mm width.
• Masonry units: No visible deterioration.
• Mortar joints: Mortar should not be easily scraped; no erosion.
• Infill wall cracks: No diagonal cracks > 3 mm or out-of-plane offsets > 3 mm.

2. Material Strength Evaluation (Clause 7.1.2)

• Use non-destructive tests (NDT) on-site.
• Supplement with laboratory testing for quantitative strength.
• Evaluate concrete strength, steel yield strength (fy), and mortar integrity.

3. Important Symbols & Parameters (Clause 3.36)

4. Shear Capacity Formula (from IS 456 and related)

[ V = V_{concrete} + V_{steel} + V_{FRP} ] Where:

• (V_{concrete}) = Shear contribution by concrete
• (V_{steel}) = Shear contribution by steel stirrups
• (V_{FRP}) = Shear contribution by FRP sheets (if any)

5. Assessment of Useful Life

[ U = \frac{T_{rem}}{T_{des}} ]

• (T_{rem}): Remaining useful life
• (T_{des}): Design useful life
• (U): Useable life factor (indicator of structural viability)

Summary Diagram: Condition Assessment Workflow

flowchart TD
    A[Visual Inspection] --> B{Check Cracks & Deterioration}
    B -->|Pass

IS 15988: Irregularities and Load Path Checks - Key Points

1. Irregularities Definition (Clause 3.36 & 3.20)

• Vertical Irregularity: Discontinuity in strength, stiffness, geometry, or mass between adjacent storeys.
• Plan Irregularity: Horizontal irregularity causing mismatch between center-of-mass and center-of-rigidity → torsional demands.

2. Effective Stiffness Values (Table 7.2.3)

3. Load Path Checks

• Ensure continuity of strength and stiffness vertically and horizontally.
• Check shear transfer reinforcement:
  [ A_{vf} = \text{area of a single shear transfer bar} ]
• Use axial force due to overturning (F) to verify column and wall capacities.
• Calculate storey shear (V_i) and compare with allowable shear ( V_u ).

4. Key Notations

• ( A_c, A_g, A_s, A_{vf}, A_w ) — Areas of columns, gross section, steel jacket, shear reinforcement, walls.
• ( f_{ck}, f_y ) — Concrete characteristic strength, steel yield strength.
• ( E_c ) —

IS 15988: Annexes & Committee Composition - Key Points

1. Annex A: Committee Composition

• The Earthquake Engineering Sectional Committee (CED 39) comprises representatives from:
  • Government bodies (e.g., Atomic Energy Regulatory Board, Central Public Works Department)
  • Research institutions (e.g., IITs, Central Building Research Institute)
  • Industry experts (e.g., Gammon India, Engineers India Limited)
  • Professional societies (e.g., Indian Concrete Institute, Indian Society of Earthquake Technology)
• Chairperson: Dr. A. S. Arya (Building Materials & Technology Promotion Council)
• Members include senior engineers, researchers, and consultants ensuring broad expertise in earthquake engineering.

2. Pre-fabricated Steel Bracing (Clause 8.5.2.3)

• Bracing types: X-, V-, inverted V-
• Arranged inside a heavy rectangular steel frame, placed in frame bay and firmly connected.
• Spiral hoops (4 ¢) used for detailing corners (Fig. 5).
• Enables ease of construction and enhanced seismic performance.

3. Table: Effective Stiffness Values (Excerpt from Clause 7.2.3)

Summary Diagram: Pre-fabricated Steel Bracing Concept

graph TD
    A[Rectangular Steel Frame] --> B[X-, V-, Inverted V