Code of Practice for Composite Construction in Structural Steel and Concrete

IS 11384: Scope & Key Specifications Summary

Scope (Clause 2.0 & 3.1):
Defines terms and symbols for composite steel-concrete beams design.

Key Symbols & Definitions

Material Standards (Clause 4.1)

• Steel: IS 800-1984
• Concrete & Reinforcement: IS 456-1978

Shear Connector Specifications (Clause 6.8 & Fig.1)

• Typical connectors: Stud, Bar, Channel, Tee
• Weld sizes depend on connector diameter (D):
  • Length of weld ( l = 2D - 12 ) mm
  • Size of weld ( = \frac{D}{2} + 2 ) mm
• Example: Tee connector size 100×100×10 mm

flowchart LR
    A[Steel Beam] --> B[Concrete Slab]
    B --> C[Shear Connectors]
    C --> D[Transfer Shear Forces]
    D --> E[Composite Action]

This summary covers the scope, symbols, material specs, and shear connector details essential for IS 11384 composite beam design. For detailed formulas and design methods, refer to specific clauses in the

IS 11384: Key Definitions, Symbols, and Specifications

1. Key Definitions (Clause 2.0 & 3.1)

• A = Area of top flange of steel beam in composite section
• As = Cross-sectional area of steel beam in composite section
• At = Cross-sectional area of transverse reinforcement (cm²/m)
• b = Breadth of flange in T-section
• be = Width of top flange of steel section
• dc = Vertical distance between centroids of concrete slab & steel beam
• ds = Thickness of concrete slab
• Es, Ec = Modulus of elasticity of steel and concrete respectively
• fck = Characteristic compressive strength of concrete (N/mm²)
• fy = Characteristic strength of steel (N/mm²)
• Fcc = Total concrete compressive force in composite beams
• Ls = Length of shear surface (mm)
• Mu = Ultimate bending moment
• n = Number of times transverse reinforcement crosses shear surface
• Ne = Number of mechanical shear connectors at cross-section
• Pc = Design ultimate strength of shear connector (kN)
• Q = Horizontal shear force (kN/m)
• tt = Average thickness of top flange of steel section
• Xu = Depth of neutral axis at ultimate limit state

2. Material Specifications (Clause 4.1)

• Structural steel per IS 800-1984
• Concrete & reinforcement per IS 456-1978

3. Shear Connectors (Fig. 1 & Clause 6.8)

• Typical connectors: Stud, Bar, Channel, Tee, Helical
• Example: Tee connector size 100 mm × 100 mm × 10 mm
• Weld sizes and directions specified for connectors (e.g., 10 mm fillet weld for stud connectors)

Summary Table of Symbols

IS 11384: Symbols and Notations (Clause 3.1)

Typical Shear Connectors (Fig. 1 Summary)

• Stud Connector: 10 mm fillet weld, direction of thrust shown.
• Bar Connector: 5 mm fillet weld full width.
• Channel Connector: 6 mm fillet weld.
• Tee Connector: 100×100×10 mm size, weld length ( I = 2D - 12 ) mm, weld size ( = \frac{D}{2} + 2 ) mm.

Important Notes:

• Units: Use consistent units (N/mm²

IS 11384: Materials and Workmanship Key Points

1. Applicable Standards (Clause 4.1)

• Structural Steel: Comply with IS 800-1984 (General Construction in Steel).
• Concrete & Reinforcement: Follow IS 456-1978 (Plain and Reinforced Concrete).

2. Important Symbols (Clause 3.1)

3. Shear Connectors (Typical Dimensions & Welds)

4. Workmanship Notes

• Design and execution must be by qualified engineers and experienced supervisors.
• Proper welding and connector installation per IS 800 and IS 11384 details are critical.

flowchart LR
    A[Materials] --> B[Structural Steel: IS 800-1984]
    A --> C[Concrete & Reinforcement: IS 456-1978]
    D[Workmanship] --> E[

Basis of Design - IS 11384 Key Points

1. Materials & Workmanship (Clause 4.1)

• Structural steel: Comply with IS 800-1984
• Concrete & Reinforcing steel: Comply with IS 456-1978

2. Symbols & Definitions (Clause 3.1)

3. Serviceability Limit States (Clause 5.2.2)

• Limit state of deflection
• Limit state of stresses in concrete and steel

4. Shear Connectors (Clause 6.8 & Fig.1)

• Typical connectors: Stud, Bar, Channel, Tee, Helical
• Weld sizes and lengths are specified, e.g.,
  • Fillet weld size = D/2 + 2 mm
  • Length of weld = 2D - 12 mm
  • Stud connector: 10 mm fillet weld

Quick Formula for Neutral Axis Depth (Xu)

[ X_u = \text{depth of neutral axis at ULS} = 0.87 \times f_y ]

Summary Diagram of Composite Section Parameters:

graph TD
  A[Top Flange Area (A)]
  As[Steel Beam Area (As)]
  At[Transverse Reinforcement Area (At)]
  b[Flange Breadth (b)]
  be[Top Flange Width (be)]
  dc

IS 11384: Design Assumptions for Limit State of Collapse in Flexure

Key Assumptions (Clause 8.1)

• Plane sections remain plane after bending (no warping).
• Max concrete strain at outer compression fiber = 0.0035.
• Tensile strength of concrete is ignored.
• Steel stress-strain curve as per Fig. 22B, IS 456-1978 (typical elastic-plastic behavior with yield plateau).

Important Symbols (Clause 3.1)

Typical Design Formula for Ultimate Moment Capacity (flexure)

[ M_u = 0.87 f_y A_s (d - \frac{x_u}{2}) ] where:

• (A_s) = area of tensile steel,
• (d) = effective depth,
• (x_u) = neutral axis depth.

Material Standards (Clause 4.1)

• Steel: IS 800-1984
• Concrete & Reinforcement: IS 456-1978

flowchart LR
    A[Concrete Slab] -->|Compression| B[Neutral Axis (Xu)]
    C[Steel Reinforcement] -->|Tension| B
    B --> D[Plane Sections Remain Plane]
    D --> E[Strain Distribution]
    E --> F[Max Concrete Strain = 0.0035]
    F --> G[Stress Block & Steel Stress-Strain Curve (IS 456 Fig.22B

IS 11384: Analysis of Sections for Ultimate Limit States (ULS)

Key Points from Clauses 7.2, 8.1, 8.2:

• Elastic properties of concrete and steel from IS 456 are used for analysis (Clause 7.2).

• Assumptions for ULS flexure (Clause 8.1):

  • Plane sections remain plane after bending.
  • Maximum concrete compressive strain, ε_cu = 0.0035.
  • Tensile strength of concrete is ignored.
  • Steel stress-strain curve as per IS 456 Fig. 22B.

• Plastic Neutral Axis (PNA) & Ultimate Moment:

  • Determined using Appendix A of IS 11384.
  • PNA location balances compressive and tensile forces.

Fundamental Formulas for Flexural ULS:

• Strain compatibility:

[ \frac{x}{d} = \frac{\epsilon_{cu}}{\epsilon_{cu} + \epsilon_{sy}} ]

Where:

• (x) = depth of neutral axis

• (d) = effective depth

• (\epsilon_{cu} = 0.0035) (max concrete strain)

• (\epsilon_{sy}) = steel yield strain

• Ultimate moment of resistance, (M_u):

[ M_u = C \times z = 0.36 f_{ck} b x \times z ]

Where:

• (C) = compressive force in concrete = (0.36 f_{ck} b x)
• (z) = lever arm ≈ (d - 0.42x)
• (f_{ck}) = characteristic compressive strength of concrete
• (b) = width of section

Table: Typical Values for Ultimate Moment Calculation

IS 11384: Limit State of Collapse in Flexure — Key Points

Assumptions (Clause 8.1)

• Plane sections remain plane after bending.
• Maximum concrete strain at extreme compression fiber: 0.0035.
• Tensile strength of concrete is ignored.
• Steel stress-strain curve as per IS 456:1978 Fig. 22B.

Design Approach (Clause 8.2)

• Use Appendix A to find:
  • Position of Plastic Neutral Axis (PNA).
  • Ultimate moment of resistance ( M_u ).

Ultimate Moment of Resistance ( M_u )

• Based on the equilibrium of forces: [ C_c = T_s ] Where:

  • ( C_c = 0.36 f_{ck} b x_u ) (compression force in concrete)
  • ( T_s = A_s f_{y} ) (tensile force in steel)

• ( x_u ) = depth of neutral axis (limited by code).

• Ultimate moment: [ M_u = C_c \times z ] Where:

  • ( z ) = lever arm (distance between compressive and tensile forces).

Connector Spacing for Composite Action (Clause 9.6)

• Max spacing ≤ 4 × slab thickness or 600 mm.
• Edge distance ≥ 25 mm.

Connector Strength Table (Sample for Headed Studs)

flowchart LR
    A[Applied Moment] --> B[Assume Strain Distribution]
    B --> C[Locate Plastic Neutral Axis]
    C --> D[Calculate Concrete Compression Force]
    C --> E[Calculate Steel Tension Force]
    D & E --> F[Check Equilibrium C = T]
    F --> G[Calculate Ultimate Moment M_u = C × z]

References:

• IS

IS 11384: Shear Connectors Design Summary

Key Definitions (Clause 2.2)

• Shear Connectors: Steel elements (stud, bar, spiral, tee, channel) welded to steel beam flange to transfer horizontal shear and prevent vertical separation.

Design Values (Clause 9.3)

• Design shear capacity = 67% of ultimate capacity (from tests).
• Table 1 (Fig. 1) provides design values for common connectors (stud, bar, channel, tee, helical).
• Other connectors require experimental shear tests per Clause 9.9.

Typical Shear Connectors (Fig. 1)

Shear Connector Testing (Clause 9.9)

• Test piece per Fig. 2, bond at steel-concrete interface prevented.
• Load applied uniformly to collapse in ≥10 minutes.
• Concrete strength at test ≈ beam concrete strength.
• Minimum 3 tests; design capacity = 67% of lowest ultimate load.

Limit State (Clause 10)

• Failure mode: vertical separation of concrete slab from steel beam.

Design Formula

[ P_{design} = 0.67 \times P_{ultimate} ]

Where:

• (P_{ultimate}) = ultimate shear capacity from tests or Table 1.

flowchart LR
    A[Steel Beam Flange] --> B[Shear Connector (Stud/Bar/Tee)]
    B --> C[Concrete Slab]
    C --> D[Horizontal Shear Transfer]
    B --> E[Prevents Vertical Separation]

**Use IS 11384 Table 1 and Fig. 1 for connector selection and design values. Experimental testing per Clause

IS 11384: Limit State of Collapse – Vertical Separation

Key Points from IS 11384 (Clause 10):

• Vertical separation refers to the failure mode where the concrete slab separates vertically from the steel beam.
• This is a critical limit state to ensure composite action between slab and beam.
• Design must ensure adequate shear connectors to prevent this separation.

Design Specifications:

• Use shear connectors (studs, channels) designed per Clause 9 (Shear Failure) to resist vertical separation.
• The ultimate shear capacity of connectors must be checked to prevent vertical slip.
• The design shear force (V_u) at the interface should be less than the total shear capacity of connectors.

Key Formula:

[ V_u \leq n \times P_u ]

Where:

• (V_u) = design shear force at interface
• (n) = number of shear connectors
• (P_u) = ultimate shear capacity per connector (from tests or IS 11384 Table)

Shear Connector Design (Summary):

Plastic Neutral Axis & Ultimate Moment (Appendix A):

• Used to locate plastic neutral axis for flexural collapse.
• Ensures composite section strength is fully utilized.

flowchart TD
    A[Concrete Slab] -->|Shear Connectors| B[Steel Beam]
    B -->|Vertical Separation Check| C{V_u ≤ n × P_u?}
    C -- Yes --> D[Safe Composite Action]
    C -- No --> E[Risk of Vertical Separation]

Summary:
To prevent vertical separation, design adequate shear connectors with verified ultimate capacity, ensure proper slab thickness and reinforcement, and check interface shear forces against connector capacity as per IS 11384 Clause 10 and 9.9.

IS 11384: Serviceability Limit State — Stresses & Deflections

Key Points from IS 11384:

• Serviceability Limit States (Clause 5.2.2):

  • (a) Limit state of deflection
  • (b) Limit state of stresses in concrete and steel

• Analysis (Clause 7.3 & 12.1):

  • Use elastic theory with:
    • Young's modulus from IS 456-1978
    • Modular ratio ( m = \frac{E_s}{E_c} )
  • Modular ratio values:
    • 15 for live load
    • 30 for dead load
  • Tensile stress in concrete is neglected

• Deflection Limits (Clause 12.1):

  • Maximum deflection ( \delta_{max} \leq \frac{L}{325} ) (L = span length)
  • Adopt steel structure deflection limits as reference

Typical Formulas:

• Modular Ratio: [ m = \frac{E_s}{E_c} ]

• Deflection Limit: [ \delta_{max} \leq \frac{L}{325} ]

• Stress in Steel and Concrete (Elastic Analysis): [ \sigma = \frac{M \cdot y}{I} ] where ( M ) = bending moment, ( y ) = distance from neutral axis, ( I ) = moment of inertia (transformed section)

Summary Table:

flowchart LR
    A[Loads] --> B[Calculate Modular Ratio]
    B --> C[Transform Section Properties]
    C --> D[Calculate Stresses (σ = M*y/I)]
    C --> E[Calculate Deflection]
    E --> F{Check Deflection ≤ L/325}
    D --> G{Check

IS 11384: Construction and Detailing Requirements - Key Specifications & Formulas

1. Materials and Workmanship (Clause 4.1)

• Structural steel: Comply with IS 800-1984.
• Concrete & Reinforcing steel: Comply with IS 456-1978.

2. Shear Connectors (Clause 10.1 & Fig.1)

• Minimum height of connector (stud, helix, channel, hoop): ≥ 50 mm.
• Projection into compression zone: ≥ 25 mm.
• Thickness of compression zone: At max bending moment section at collapse.
• Stud head diameter: ≥ 1.5 × stud diameter.
• Stud head thickness: ≥ 0.4 × stud diameter.

3. Symbols & Parameters (Clause 3.1)

4. Weld Length & Size for Connectors

• Length of weld, I: ( I = 2D - 12 , \text{mm} )
• Size of weld: ( \frac{D}{2} + 2 , \text{mm} )

Where ( D ) = relevant dimension of connector.

5. Typical Shear Connectors (Fig.1)

• Stud Connector: 10 mm fillet weld.
• Bar Connector: 5 mm fillet weld full width.
• Channel Connector: 6 mm fillet weld

IS 11384: Appendix A - Plastic Neutral Axis & Ultimate Moment of Resistance

Key Concepts:

• Plastic Neutral Axis (PNA): Axis dividing the transformed section into equal areas of compression and tension.
• Ultimate Moment of Resistance (Mu): Moment capacity at plastic state.

Determination of PNA & Mu:

Case (i) PNA within Concrete Slab

[ b d x \geq a A_s ] [ b X_u = a A_s ] Where:

• (b) = slab width
• (d) = slab thickness
• (a) = stress ratio (steel to concrete)
• (A_s) = steel area
• (X_u) = depth of PNA

Case (ii) PNA within Top Flange of Steel Beam

[ b d s < a A_s < (b d s + 2 a A_t) ] [ X_u = d_s + a A_s - b d g / 2 b a ]

• (d_s) = depth to steel flange
• (A_t) = steel flange area
• (b d g) = flange width × thickness

Steel tension force (F_e) balances concrete compression (F_{ce}) plus twice steel compression (F_{sc}).

Case (iii) PNA within Web of Steel Beam

[ a (A_s - 2 A_t) > b d s + b d g + 2 a A_t + 2 a (X_u - d_s - t_t) t_w ] [ X_u = d_s + t_t + \frac{a (A_s - 2 A_t) - b d s}{2 a t_w} ]

• (t_t) = flange thickness
• (t_w) = web thickness

Ultimate Moment of Resistance (Mu)

[ M_u = \sum (Force \times Lever Arm) ]

Calculate forces from concrete and steel areas based on PNA position, then take moments about the centroid of compression.

Summary Table: