Code of practice for composite construction

IS 3935: Scope - Key Points, Formulas & Tables

Scope (Clause 1):
Covers design and construction requirements for composite steel-concrete members, including prefabricated and in-situ concrete components.

Key Symbols (Clause 3.1)

Basic Design Considerations (Clause 5.1)

• Composite action between steel and concrete must be ensured.
• Shear connectors (studs, channels) transfer horizontal shear.
• Permissible stresses and deflections must comply with limits.
• Use equivalent transformed section for analysis.

Important Formula for Shear Connector Capacity

[ Q = q \times A_c ]

• (Q): safe shear resistance per connector
• (q): permissible shear stress in concrete
• (A_c): effective concrete area around connector

Summary Table: Connector Dimensions

flowchart LR
    Steel_Beam -->|Composite Action| Concrete_Slab
    Steel_Beam -->|Shear Connectors (Q)| Concrete_Slab
    Shear_Connectors -->|Transfer Shear (St)| Composite_Interface

IS 3935 - Definitions & Symbols (Clauses 2.0 & 3.1)

Key Symbols and Their Meanings:

Notes:

• These definitions form the basis for design calculations of composite steel-concrete members.
• Refer IS 3935 clauses 5 & 6 for design formulas involving these parameters.
• For material properties, IS 456 and IS 800 should be consulted as per clause 5.1.

flowchart LR
    A[Composite Section] --> B[Steel Flange (b)]
    A --> C[Stud Connector (d, H)]
    A --> D[Channel Shear Connector (h, t, L)]
    A --> E[Concrete Slab (fcu, q)]
    A --> F[Shear Forces (St, V)]
   

IS 3935: Symbols and Notations (Clause 3.1)

Key Formula for Anchor Connector Shear Resistance (Clause 6.5.7)

[ Q = K \times A_t \times Ø_{st} ]

• (A_t) = Cross-sectional area of anchor bar
• (K) = Coefficient based on anchor type:

Notes:

• (d) = diameter of bar.
• Anchor angle (vertical or ~45°) does not affect (Q).
• Use IS 456 and IS 800 for complementary design provisions.

flowchart LR
    A[Anchor Bar] --> B[Cross-sectional Area \(A_t\)]
    B --> C

IS 3935: Key Materials Specifications & Formulas

1. Structural Steel (Clause 4.2.1)

• Steel must comply with:
  • IS 226-1962 (Standard quality)
  • IS 961-1962 (High tensile)
  • IS 2062-1962 (Fusion welding quality)
  • IS 1977-1962 (Ordinary quality, Designation St 440)

2. Reinforcement Steel (Clause 5.1 references)

• Use as per:
  • IS 432 (Mild & medium tensile steel bars)
  • IS 1786 (Cold twisted bars)
  • IS 1566 (Hard drawn wire fabric)
  • IS 6003 (High tensile bars for prestressing)

3. Concrete

• Concrete cube strength (fcu) = Crushing strength of 150 mm cube at 28 days (standard measure)

4. Symbols & Parameters (Clause 3.1)

5. Important Formulas

• Safe shear resistance of connectors:

  [ Q = q \times A_c ]

  where (A_c) = effective shear area of connector.

• Horizontal shear at interface:

  [ S_t = \frac{V}{b} ]

• Moment of inertia (I) for transformed composite section is calculated by converting concrete slab area into equivalent steel area using modular ratio.

Summary

• Use IS 226/961/2062 for steel quality.
• Reinforcement per IS 432/1786.
• Concrete strength

IS 3935: Design Requirements Overview

1. Basic Requirements (Clause 5.1)

• Ensure composite action between steel and concrete.
• Design for serviceability (deflection, crack control) and strength.
• Consider differential shrinkage and creep effects.
• Use permissible stresses as per material properties.

2. Design of Shear Connectors (Clause 6.5)

• Shear connectors ensure composite action by transferring longitudinal shear.

• Design shear connectors for shear force (V) using:

  [ V_{connector} = 0.29 \times f_{u} \times A_{w} ]

  where:

  • ( f_u ) = ultimate tensile strength of shear connector steel
  • ( A_w ) = cross-sectional area of the connector weld or stud

• Spacing and number of connectors depend on:

  • Shear force demand
  • Concrete slab thickness
  • Steel beam dimensions

3. Flange Width of Composite Beams (Clause 5.10)

• Effective flange width ( b_{eff} ) is limited by:

  [ b_{eff} = \min \left( \frac{L}{4}, \frac{b_s}{2} + 15d, b_s \right) ]

  where:

  • ( L ) = span length
  • ( b_s ) = width of steel beam flange
  • ( d ) = slab thickness

Summary Table: Shear Connector Design

flowchart LR
    A[Steel Beam] -->|Shear Connectors| B[Concrete Slab]
    B -->|Composite Action| C[Composite Beam]
    C -->|Load

IS 3935: Prefabricated Steel and In-Situ Concrete Composite Members

Key Concepts:

• Composite members combine prefabricated steel/prestressed/reinforced concrete units with in-situ concrete to act monolithically.
• Shear connectors ensure composite action by transferring shear at the interface.

Important Formulas & Specifications

1. Ultimate Horizontal Shear Stress at Interface

Calculated as per Clause 6.5.2 (not fully detailed here), but design must ensure:

• If shear stress > No-slip permissible shear (Table 1), assume slip occurs.
• Design with frictional shear resistance = 10 kg/cm².
• Remaining shear resisted by steel connectors stressed up to 1340 kg/cm².
• Interface shear must not exceed Maximum permissible shear from Table 1.

2. Table 1: Permissible Shear Stress at Ultimate Load (kg/cm²)

Design Notes:

• Use monolithic action principle to increase efficiency.
• Shear connectors must be designed to transfer interface shear without slip.
• Prefabricated units can serve as formwork, reducing construction time.

flowchart LR
    A[Prefabricated Steel/Concrete Unit] -->|Shear Connectors| B[In-Situ Concrete]
    B -->|Composite Action| C[Monolithic Structural Member]
    C -->|Load Transfer| D[Increased Structural Efficiency]

This summary aids in designing composite members per IS 3935, focusing on shear transfer and permissible stresses at the steel-concrete interface.

IS 3935: Prefabricated Prestressed/Reinforced Concrete & In-Situ Concrete Composite Members

Key Specifications & Formulas

• Composite Member Definition (Clause 2.2):
  Structural members combining prefabricated units (steel, prestressed/reinforced concrete) and cast-in-situ concrete acting monolithically.

• Ultimate Horizontal Shear Stress at Interface (Clause 7.2.1):
  Use formula from Clause 6.5.2 (not provided here) to calculate shear stress (\tau_u).

  • If (\tau_u) > permissible no-slip shear (Table 1), assume slip occurs.
  • Design for frictional shear resistance = 10 kg/cm² plus steel shear connectors stressed up to 1340 kg/cm².
  • Interface shear should not exceed maximum permissible shear from Table 1.

Table 1: Permissible Shear Stress at Interface (kg/cm²)

Design Notes

• Shear connectors must be designed to resist the balance shear after frictional resistance.
• Limit shear stress at interface to avoid slip or failure.
• Follow IS 456 for concrete design and IS 1343 for prestressed concrete detailing.

flowchart TD
    A[Prefabricated Prestressed/Reinforced Concrete] --> B[Cast-in-Situ Concrete]
    B --> C[Interface Shear Stress Calculation]
    C --> D{Shear Stress > No Slip Limit?}
    D -- Yes --> E[Design for Slip: Friction + Shear Connectors]
    D -- No --> F[Design for No Slip Condition]
    E --> G[Check Max Permissible Shear]
    F --> G

This ensures monolithic action and safe load transfer in composite members.

IS 3935: Shear Connectors (Clauses 6.4 & 6.5)

Types of Shear Connectors (Clause 6.4.2)

• Flexible Connectors:

  • Studs (Fig. 5) and Channels (Fig. 6) welded to prefabricated units.
  • Resistance mainly by bending of connectors.

• Bond/Anchorage Connectors: (Fig. 7)

  1. Mild steel bars welded as vertical/inclined loops or stirrups.
  2. Inclined bars welded at one end, bent at the other.
  3. Bar stirrups welded at each loop.
  • Resistance by bond and anchorage action.

• Other Mechanical Devices: To resist horizontal shear and prevent vertical separation.

Design Requirements (Clause 6.5) - Key Points

• Connectors must transfer horizontal shear effectively between prefabricated and in-situ concrete.
• Spacing and size depend on shear force and connector type.
• Weld quality and embedment length critical for performance.
• Typical shear capacity of studs can be estimated by:
  [ V_u = 0.5 \times A_s \times f_y ]
  Where:
  • (V_u) = ultimate shear capacity per connector
  • (A_s) = cross-sectional area of connector
  • (f_y) = yield strength of connector steel

Summary Table Example for Stud Connectors

flowchart LR
    A[Prefabricated Unit] -->|Welded Studs/Channels| B[Shear Connectors]
    B -->|Transfer Shear| C[In-situ Concrete]
    B -->|Prevent Vertical Separation| C

Note: Refer to IS 3935 figures for

Bond Strength at the Interface (IS 3935 - Clauses 7.2, 7.2.1, 6.5.7, 2.5)

1. Ultimate Horizontal Shear Stress Calculation

• Use formula from Clause 6.5.2 (not provided here, typically:
  [ \tau = \frac{V}{A} ] where (V) = shear force, (A) = shear area at interface)
• If calculated shear stress > permissible shear (Table 1, no slip), assume slip occurs.
• Design with frictional shear resistance = 10 kg/cm² + steel shear connectors stressed to max 1340 kg/cm².
• Inter-face shear ≤ maximum permissible shear (Table 1).

2. Table 1: Permissible Shear Stress at Interface (kg/cm²)

3. Shear Resistance of Anchor Connectors (Clause 6.5.7)

[ V_{safe} = K \times A_t \times f_{sy} ]

• (A_t) = cross-sectional area of anchor bar
• (K) = coefficient depending on anchor type:

4. Additional Design Checks (Clause 2.5)

• Longitudinal shear stress between connectors ≤ 2.5 × permissible shear stress of concrete.
• Projected area along slope 1:5 between connectors ≥

IS 3935: Design of Slabs (Clauses 5.9 & 6.3)

Key Points for Slab Design:

• Slab Types: One-way and two-way slabs depending on support conditions.
• Thickness (h): Minimum thickness generally taken as l/20 to l/25 (l = span length).
• Load Considerations: Dead load + live load + impact/load factors.

Design Formulas:

• Bending Moment for One-Way Slab (simply supported):
  [ M = \frac{w l^2}{8} ]
  where ( w ) = uniform load per unit length, ( l ) = span length.

• Bending Moment for Two-Way Slab (continuous):
  Use coefficients from IS 456 or IS 3935 tables based on aspect ratio ( \frac{l_x}{l_y} ).

Reinforcement:

• Minimum steel as per IS 456:
  [ \rho_{min} = 0.15% \text{ to } 0.25% ]

• Main steel placed in tension zone, distribution steel at top.

Clause 6.3: Slab and Haunch

• Haunch provided at supports to resist negative moments.
• Haunch thickness and length depend on moment magnitude.

Typical Table (Moment Coefficients for Two-Way Slabs):

Moment = Coefficient × w × l²

graph TD
A[Load on Slab] --> B{Slab Type}
B --> C[One-Way Slab]
B --> D[Two-Way Slab]
C --> E[Bending Moment: M = w*l²/8]
D --> F[Use Moment Coefficients from Table]
F --> G[Design Reinforcement]
E --> G
G --> H[Provide Haunch at

IS 3935 - Clause 5.6: Permissible Stresses

1. Permissible Stresses in Concrete (Clause 5.6.1)

• Compression:
  [ f_{c,perm} = \frac{f_{ck}}{m} ]
  where ( f_{ck} ) = characteristic compressive strength of concrete,
  ( m ) = factor depending on the type of stress (usually 3 to 5).

• Tension:
  Concrete is assumed to have negligible tensile strength; tensile stresses are generally not permitted.

2. Permissible Stresses in Steel Reinforcement (Clause 5.6.2)

• For mild steel or HYSD bars:
  [ f_{st,perm} = \frac{f_y}{\text{Factor of Safety (FoS)}} ] where ( f_y ) = yield strength of steel (e.g., 250 MPa for mild steel, 415 MPa for HYSD).

• Typical FoS = 1.5 to 1.67.

Summary Table:

flowchart TD
    A[Material] --> B[Concrete]
    A --> C[Steel Reinforcement]
    B --> D[Compression: f_ck/m]
    B --> E[Tension: ~0]
    C --> F[Permissible Stress: f_y / FoS]

Note: Always refer to the exact clause for specific factors and conditions.

IS 3935 Key Points on Modulus of Elasticity and Modular Ratio

1. Modulus of Elasticity (E)

• Taken as per relevant IS codes:
  • For steel: Refer IS 800 or IS 2062.
  • For concrete: Refer IS 456 (latest version).
• Typical values:
  • Steel, ( E_s \approx 2.0 \times 10^5 ) MPa
  • Concrete, ( E_c ) varies with grade, e.g., for M20 concrete, ( E_c \approx 25,000 ) MPa

2. Modular Ratio (n)

• Defined as the ratio of modulus of elasticity of steel to concrete:

  [ n = \frac{E_s}{E_c} ]

• For precast and cast-in-situ concrete, modular ratio is based on their respective moduli ( E_1 ) and ( E_2 ):

  [ n = \frac{E_{\text{precast}}}{E_{\text{cast-in-situ}}} ]

3. Equivalent Section Concept

• For composite sections (e.g., steel + concrete slab), convert concrete area ( A_c ) to steel equivalent area ( A_s ) by:

  [ A_s = \frac{A_c}{n} ]

Summary Table

flowchart LR
    A[Steel Modulus, \(E_s\)] --> B[Modular Ratio, \(n = \frac{E_s}{E_c}\)]
    C[Concrete Modulus, \(E_c\)] --> B
    B --> D[Convert Concrete Area to Steel Equivalent: \(A_s = \frac{A_c}{n}\)]

Note: Always verify values with the

IS 3935 Deflection Considerations: Key Points

1. Deflection Calculation Basis

• Live Load Deflection (Clause 5.8.1): Use moment of inertia (I) of transformed composite section with full modulus of elasticity (E) of concrete.
• Dead Load Deflection (Clause 5.8.2): Use moment of inertia of transformed composite section with full E.
• Dead Load Deflection for Shored Beams (Clause 5.8.2.1): Use half the modulus of elasticity (0.5E) for concrete.

2. Limiting Deflections (Clause 5.8.3)

3. Deflection Formula (for beams)

[ \delta = \frac{5 w L^4}{384 E I} \quad \text{(Uniform load, simply supported beam)} ]

Where:

• ( \delta ) = deflection
• ( w ) = load per unit length
• ( L ) = span length
• ( E ) = modulus of elasticity (adjusted as above)
• ( I ) = moment of inertia of transformed section

flowchart TD
    A[Calculate Moment of Inertia (I)] --> B[Select Modulus of Elasticity (E)]
    B --> C{Load Type}
    C -->|Dead Load| D[Use full E or 0.5E if shored]
    C -->|Live Load| E[Use full E]
    D & E --> F[Calculate Deflection (\delta)]
    F --> G[Check Limiting Deflection]
    G -->|Within Limits| H[Design OK]
    G -->|Exceeds Limits| I[Modify Section or Load]

Summary: Use transformed section properties and appropriate E values for deflection calculations. Ensure deflections do not exceed limits in Clause

IS 3935 - Welding and Fabrication of Shear Connectors: Key Points

1. Material & Welding (Clause 6.4.1.1)

• Shear connectors must be weldable steel.
• Weld capacity ≥ shear resistance of connectors.
• Welding follows relevant IS standards (e.g., IS 816 for welding).
• For studs: fusion welding needed; maintain ≥ 15 mm gap between adjacent stud heads.

2. Welded Stud Connector Design (Clause 6.5.8.1)

• Stud head diameter = d + 12 mm; head height = 12 mm.
• Steel properties:
  • Ultimate strength = 4600 kg/cm²
  • Yield point = 3500 kg/cm²
  • Elongation = 20%
• Safe shear resistance formula (for H/d < 4.2):
  [ P = 0.5 \times f_u \times A_s ] where:
  • (P) = safe shear resistance
  • (f_u) = ultimate tensile strength of stud steel
  • (A_s) = cross-sectional area of stud shank

3. Spacing & Layout

• Maintain adequate spacing to avoid welding defects.
• For studs, ensure minimum 15 mm gap between heads.
• Weld studs directly to structural members without bending ends.

4. Connector Types (Fig. 7)

• Bond Type Connectors (7A)
• Composite Dowel and Anchor Connectors (7B)
• Spiral Connectors (7C)

Summary Table: Stud Connector Dimensions and Properties

flowchart LR
    A[Shear Connector Steel] --> B[Welding to Structural Member]
    B --> C[Weld Capacity ≥ Shear Resistance]
    C --> D[Stud Connectors]

IS 3935 - Precautions and Detailing: Key Points

Though IS 3935 does not have a dedicated clause titled "Precautions and Detailing," relevant specifications can be drawn from composite and prestressed concrete detailing clauses:

Key Detailing Specifications (Clause 7.4.1)

• Castellation Depth: Minimum 25 mm
• Castellation Length: ≈ 4 × depth
• Centre-to-centre Spacing: ≤ lever arm or 60 cm, whichever is less

General Design & Detailing Guidelines (Clause 5.1)

• Follow IS 456-1964 for concrete detailing (reinforcement cover, bar spacing, anchorage)
• Use IS 1343-1960 for prestressed concrete detailing
• Use IS 800-1962 for steel structure detailing

Shear Connectors (Clause 6.4 & 6.5)

• Proper welding and spacing to ensure composite action
• Design per IS 3935 shear connector specifications

Typical Castellation Detailing Formula:

Summary Diagram: Castellation Detailing

graph LR
A[Castellation Depth ≥ 25 mm] --> B[Length ≈ 4 × Depth]
B --> C[Spacing ≤ min(Lever Arm, 600 mm)]

Note: Always ensure compliance with IS 456 and IS 1343 for reinforcement detailing, cover, and anchorage to prevent corrosion and ensure structural integrity.