Code of Practice for Composite Construction in Structural Steel and Concrete
1985 Edition

Overview of IS 11384: Scope & Terminology

Scope (Clauses 2.0 & 3.1): Defines terminology and symbols essential for composite steel-concrete beam design.

Principal Symbols and Their Meanings

Material Standards (Clause 4.1)

• Structural steel as per IS 800-1984
• Concrete and reinforcement conforming to IS 456-1978

Shear Connector Details (Clause 6.8 & Fig.1)

• Common types: Studs, bars, channels, tees
• Weld length and size depend on connector diameter D:
  • Weld length (l = 2D - 12) mm
  • Weld size (= \frac{D}{2} + 2) mm
• Example: Tee connector with dimensions 100×100×10 mm

flowchart LR
    SteelBeam[Steel Beam] --> ConcreteSlab[Concrete Slab]
    ConcreteSlab --> ShearConnectors[Shear Connectors]
    ShearConnectors --> ShearTransfer[Shear Force Transfer]
    ShearTransfer --> CompositeAction[Composite Structural Action]

This section provides foundational scope, symbols, material requirements, and connector details critical for IS 11384 composite beam design.

IS 11384: Definitions, Symbols, and Standards

Key Terms (Clauses 2.0 & 3.1)

• A: Top flange area of steel beam in composite section
• As: Steel beam cross-sectional area
• At: Area of transverse reinforcement (cm²/m)
• b: Flange breadth of T-section
• be: Width of steel section top flange
• dc: Vertical centroid distance between concrete slab and steel beam
• ds: Concrete slab thickness
• Es, Ec: Young’s moduli of steel and concrete respectively
• fck: Characteristic concrete compressive strength (N/mm²)
• fy: Characteristic steel yield strength (N/mm²)
• Fcc: Total concrete compressive force in composite beams
• Ls: Shear surface length
• Mu: Ultimate bending moment
• n: Number of transverse reinforcement crossings
• Ne: Count of mechanical shear connectors at a section
• Pc: Design ultimate strength of shear connector (kN)
• Q: Horizontal shear force (kN/m)
• tt: Average top flange thickness of steel section
• Xu: Depth of neutral axis at ultimate limit state

Material Specifications (Clause 4.1)

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

Shear Connectors (Clause 6.8 & Fig. 1)

• Types include stud, bar, channel, tee, helical
• Example connector: Tee, 100×100×10 mm
• Weld sizes defined, e.g., 10 mm fillet weld for studs

Symbol Summary Table

IS 11384: Symbol Definitions (Clause 3.1)

Common Shear Connector Types (Fig. 1 Summary)

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

Notes

• Ensure consistent unit usage (N/mm² preferred).

IS 11384: Material Specifications and Workmanship Guidelines

1. Applicable Standards (Clause 4.1)

• Structural steel must comply with IS 800-1984
• Concrete and reinforcing steel to conform with IS 456-1978

2. Key Symbols (Clause 3.1)

3. Shear Connector Details

4. Workmanship

• Execution by skilled engineers and supervisors
• Correct welding and connector installation critical per IS 800 and IS 11384

IS 11384: Basis for Design

1. Materials and Construction Quality (Clause 4.1)

• Structural steel per IS 800-1984
• Concrete and reinforcing steel per IS 456-1978

2. Symbols and Definitions (Clause 3.1)

3. Serviceability Limit States (Clause 5.2.2)

• Deflection limitations
• Stress criteria for concrete and steel

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

• Standard connectors: stud, bar, channel, tee, helical
• Weld dimensions:
  • Fillet weld size = D/2 + 2 mm
  • Weld length = 2D - 12 mm
  • Stud connectors use 10 mm fillet weld

Neutral Axis Depth Approximation:

[ X_u = 0.87 , f_y ]

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[Centroid Distance (dc)]
  ds[Slab Thickness (ds)]

IS 11384: Assumptions for Ultimate Flexural Limit State (Clause 8.1)

Principal Assumptions

• Plane sections remain plane post bending, i.e., no warping occurs.
• Maximum concrete strain at extreme compression fiber is 0.0035.
• Tensile strength of concrete is disregarded.
• Steel follows stress-strain behavior per IS 456-1978 Fig. 22B (elastic-plastic with yield plateau).

Symbols (Clause 3.1)

Typical Ultimate Moment Capacity Formula

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

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

Material Standards

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

flowchart LR
    ConcreteSlab[Concrete Slab] --> NeutralAxis[Neutral Axis (Xu)]
    SteelRebar[Steel Reinforcement] --> NeutralAxis
    NeutralAxis --> PlaneSections[Plane Sections Remain Plane]
    PlaneSections --> StrainDistribution[Strain Distribution]
    StrainDistribution --> MaxConcreteStrain[Max Concrete Strain = 0.0035]
    MaxConcreteStrain --> StressBlock[Stress Block & Steel Stress-Strain Curve]

IS 11384: Section Analysis at Ultimate Limit State (ULS)

Key Points (Clauses 7.2, 8.1, 8.2)

• Elastic material properties for steel and concrete follow IS 456 (Clause 7.2).

• ULS flexure assumptions (Clause 8.1):

  • Plane sections remain plane.
  • Max concrete strain at compression face = 0.0035.
  • Concrete tensile strength neglected.
  • Steel stress-strain per IS 456 Fig. 22B.

• Plastic Neutral Axis (PNA) and ultimate moment determined using Appendix A.

• PNA balances compressive and tensile forces.

Core Flexural ULS Equations:

• Strain compatibility: [ \frac{x}{d} = \frac{\varepsilon_{cu}}{\varepsilon_{cu} + \varepsilon_{sy}} ] where:

• (x): neutral axis depth

• (d): effective depth

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

• (\varepsilon_{sy}): steel yield strain

• Ultimate moment capacity: [ M_u = C \times z = 0.36 f_{ck} b x \times z ] where:

• (C) concrete compressive force = (0.36 f_{ck} b x)

• (z) lever arm ≈ (d - 0.42x)

Typical Parameter Values:

graph LR
    A[Strain Compatibility] --> B[Locate Plastic Neutral Axis]
    B --> C[Calculate Concrete Compression]
    B --> D[Calculate Steel Tension]
    C & D --> E[Check Force Equilibrium]
    E --> F[Determine Ultimate Moment Capacity]

IS 11384: Flexural Limit State of Collapse

Assumptions (Clause 8.1)

• Plane sections remain plane under bending.
• Max concrete strain at compression face is 0.0035.
• Concrete tensile stresses are ignored.
• Steel follows the stress-strain curve of IS 456 Fig. 22B.

Design Methodology (Clause 8.2)

• Use Appendix A for:
  • Determination of Plastic Neutral Axis (PNA) location
  • Calculation of ultimate moment capacity (M_u)

Ultimate Moment Computation

• Equilibrium of forces: [ C_c = T_s ] where

• (C_c = 0.36 f_{ck} b x_u) is concrete compression

• (T_s = A_s f_y) is steel tension

• (x_u) is neutral axis depth, limited per code

• Ultimate moment: [ M_u = C_c , z ] where (z) is lever arm between compressive and tensile force centers.

Shear Connector Spacing (Clause 9.6)

• Maximum spacing is the lesser of 4 times slab thickness or 600 mm.
• Minimum edge distance of 25 mm.

Sample Connector Strengths (Headed Studs)

flowchart LR
    AppliedMoment --> AssumeStrain[Assume Strain Distribution]
    AssumeStrain --> LocatePNA[Locate Plastic Neutral Axis]
    LocatePNA --> CalcCompForce[Calculate Concrete Compression]
    LocatePNA --> CalcTensForce[Calculate Steel Tension]
    CalcCompForce & CalcTensForce --> CheckEquilibrium[Check C = T]
    CheckEquilibrium --> CalcMu[Calculate Ultimate Moment]

IS 11384: Overview of Shear Connector Design

Definitions (Clause 2.2)

• Shear connectors are steel elements (studs, bars, spirals, tees, channels) welded to steel beam flanges to transfer horizontal shear forces and prevent vertical separation.

Design Capacities (Clause 9.3)

• Design shear strength is taken as 67% of ultimate capacity from testing.
• Table 1 (Fig. 1) lists design values for typical connectors (stud, bar, channel, tee, helical).
• Connectors not in the table require experimental shear testing (Clause 9.9).

Typical Connector Types and Welds (Fig. 1)

Testing Protocols (Clause 9.9)

• Use test specimens per Fig. 2 with prevented steel-concrete bond.
• Apply uniform load until failure occurs over at least 10 minutes.
• Concrete strength at test must be similar or less than beam concrete strength.
• Minimum of 3 tests required.
• Design capacity = 0.67 × lowest ultimate test load.

Limit State (Clause 10)

• Failure characterized by vertical separation at steel-concrete interface.

Design Calculation

[ P_{design} = 0.67 \times P_{ultimate} ] where (P_{ultimate}) is ultimate shear capacity from tests or tables.

flowchart LR
    SteelBeamFlange --> ShearConnector[Shear Connector (Stud/Bar/Tee)]
    ShearConnector --> ConcreteSlab
    ConcreteSlab --> ShearTransfer[Transfers Horizontal Shear]
    ShearConnector --> PreventSeparation[Prevents Vertical Separation]

IS 11384: Limit State for Vertical Separation Prevention

Overview (Clause 10)

• Vertical separation denotes the detachment of the concrete slab from the steel beam flange.
• This limit state ensures composite action integrity.
• Adequate shear connectors must be provided to mitigate this failure mode.

Design Requirements

• Employ shear connectors (studs, channels) per Clause 9 to resist vertical shear.
• Verify ultimate shear capacity of connectors surpasses interface shear forces.
• Design shear force at interface (V_u) must satisfy: [ V_u \leq n \times P_u ] where
• (n) = number of connectors
• (P_u) = ultimate shear capacity per connector

Detailing Guidelines

Appendix A Usage

• Locates plastic neutral axis for flexural collapse
• Confirms full utilization of composite section strength

flowchart TD
    ConcreteSlab --> ShearConnectors[Shear Connectors]
    ShearConnectors --> SteelBeam
    SteelBeam --> CheckSeparation{Is \(V_u \leq nP_u\)?}
    CheckSeparation -- Yes --> Safe[Safe Composite Action]
    CheckSeparation -- No --> Risk[Risk of Vertical Separation]

IS 11384: Serviceability Limits for Stress and Deflection

Key Provisions

• Serviceability limit states (Clause 5.2.2):

  • (a) Deflection limits
  • (b) Stress limits in concrete and steel

• Analysis based on elastic theory (Clauses 7.3 & 12.1):

  • Use Young’s modulus values from IS 456-1978
  • Modular ratio (m = \frac{E_s}{E_c}):
    • 15 for live loads
    • 30 for dead loads
  • Tensile stresses in concrete are ignored

• Deflection limits (Clause 12.1):

  • Maximum deflection (\delta_{max} \leq \frac{L}{325}) where (L) is span length
  • Deflection limits follow steel structure guidelines

Typical Equations

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

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

• Stress in steel or concrete under bending: [ \sigma = \frac{M y}{I} ] where (M) is bending moment, (y) distance from neutral axis, (I) moment of inertia of transformed section.

Summary Table

flowchart LR
    Loads --> CalculateModularRatio
    CalculateModularRatio --> TransformSectionProperties
    TransformSectionProperties --> CalculateStress
    TransformSectionProperties --> CalculateDeflection
    CalculateDeflection --> CheckDeflectionLimit
    CalculateStress --> CheckStressLimits

IS 11384: Key Requirements for Construction and Detailing

1. Materials and Workmanship (Clause 4.1)

• Use structural steel as per IS 800-1984
• Concrete and reinforcement per IS 456-1978

2. Shear Connector Detailing (Clause 10.1 & Fig. 1)

• Minimum overall height of connectors (stud, helix, channel, hoop) ≥ 50 mm
• Minimum embedment into compression zone ≥ 25 mm
• Compression zone thickness corresponds to max bending moment section at collapse
• Stud head diameter ≥ 1.5 × stud diameter
• Stud head thickness ≥ 0.4 × stud diameter

3. Symbols and Parameters (Clause 3.1)

4. Weld Specifications for Connectors

• Weld length (I = 2D - 12) mm
• Weld size (= \frac{D}{2} + 2) mm

Where (D) is the relevant connector dimension.

5. Typical Shear Connectors (Fig. 1)

• Stud connector: 10 mm fillet weld
• Bar connector: 5 mm full-width fillet weld
• Channel connector: 6 mm fillet weld

IS 11384: Appendix A — Plastic Neutral Axis (PNA) and Ultimate Moment Capacity

Concepts

• Plastic Neutral Axis is the axis dividing the transformed section into balanced compression and tension areas.
• Ultimate moment resistance (M_u) is the moment at plastic collapse.

Determination of PNA and Moment Capacity

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 plastic neutral axis

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 - \frac{b d g}{2 b a} ] where:

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

Steel tension force balances concrete compression plus steel compression forces.

Case (iii): PNA within Steel Beam Web

[ 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} ] where:

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

Ultimate Moment Calculation

[ M_u = \sum (Force \times Lever Arm) ] Calculate forces from concrete and steel areas based on PNA location, then compute moments about centroid of compression.

Summary Table