Code of Practice for Use of Cold-Formed Light Gauge Steel Structural Members In General Building Construction

IS 801: Scope - Key Formulas & Specifications

Scope Summary:

• Design computations (load, stress, deflection) follow conventional structural design methods (Clause 5.1).
• Section properties (area, moment of inertia, section modulus, radius of gyration) are per conventional methods, using full or net sections unless reduced/effective widths are specified (Clause 5.2).
• Maximum allowable stresses as per Clauses 6.1 to 6.8.
• Special tensile strength to yield ratio correction if ratio < 1.35 (Clause 1.35).

Important Formulas for Stud Design (from Clause 8):

Notes:

• Slenderness ratio (L/r) of stud between attachments must not exceed specified limits.
• Attachments spacing limited by slenderness and lateral support requirements.
• Use net section properties if applicable, else full section.
• Wall and attachments provide lateral bracing modeled by Kw.

flowchart LR
    A[Stud Member] --> B[Cross Section Properties]
    B --> C[Area (A), I₁, I₂, r₁, r₂]
    A --> D[Length (L), Attachment Spacing (a)]
    E[Wall Material +

Here are the key definitions and formulas from IS 801 related to bending, axial stress, and buckling:

Key Definitions (Clause 3.0 & 6.1)

• Fb1: Max bending stress in compression when bending only exists, excluding lateral buckling.
• f: Axial stress = ( \frac{P}{A} ) (Axial load / cross-sectional area)
• fo: Max bending stress = ( \frac{M}{S} ) (Moment / section modulus)
• Ix, Iy: Moments of inertia about x and y axes.
• K: Effective length factor in bending plane.
• Lb: Unbraced length in bending plane.
• Mc, Mr: Elastic critical moments causing compression or tension on shear centre side.
• r_b: Radius of gyration about bending axis.
• S_ve: Compression section modulus = ( \frac{I_y}{\text{distance to extreme compression fiber}} )
• OTF: Average elastic torsional-flexural buckling stress.

Important Formulas (Clause 6.1 & 6.7.1)

• Axial stress:
  [ f = \frac{P}{A} ]

• Max bending stress:
  [ f_o = \frac{M}{S} ]

• Buckling stress:
  [ F' = 23 \left(\frac{K L_b}{r_b}\right)^2 \frac{E}{12} ] (May be increased by 1/3 per 6.1.2)

• Combined axial and bending stress (doubly symmetric shapes):
  [ F_b + (1 - A) F_{ox} \leq \text{allowable stress} ]

Notes

• Section modulus ( S ) for stiffened elements is based on effective widths.
• Elastic critical moments ( M_c, M_r ) account for shear center effects.
• Effective length factor ( K ) depends on end conditions.
• Use conventional structural design methods unless otherwise specified (Clause 5.1).

flowchart TD
    A[Axial Load P] -->|Stress| B[f = P/A]

IS 801 - Materials and Mechanical Properties Summary

1. Mechanical Properties Testing (Clause 9.1.3, 9.3)

• Tests on flat elements of formed sections per Clauses 9.3.2 & 9.3.3.
• Tensile specimens for virgin steel: minimum 4 per lot, taken longitudinally near coil ends.
• Compression tests (Appendix A):
  • Length ≥ 3× largest section dimension, but ≤ 20× least radius of gyration.
  • For ultimate compressive strength, length ≥ 15× least radius of gyration.
  • Load must be concentric with centroidal axis.

2. Allowable Design Stress (Clause 6.1, Table 2)

• Basic design stress F ≤ Fy (or average yield after cold work as per 6.1.1).

3. Notes on Virgin Steel Properties (Clause 9.3.3)

• Use virgin steel properties to compute increased yield and ultimate strength after cold forming.
• Follow IS 1079-1973 for hot rolled carbon steel sheet and strip.

Quick Formula for Basic Design Stress (Cold-Worked Steel):

[ F = \text{average yield point of full section} \quad (as ; per ; 6.1.1) ]

Summary Diagram: Mechanical Property Testing Flow

flowchart TD
    A[Virgin Steel] --> B[Tensile Test (4 specimens/lot)]
    B --> C[Determine Yield & Ultimate Strength]
    C --> D[Cold Forming]
    D --> E[Calculate Increased Yield Strength]
    E --> F[Allowable Design Stress F]
    F --> G[Design of Structural Members]
    
    subgraph Compression Testing
        H[Flat-end Specimen]
        I[Length ≥ 3× Largest Dimension, ≤ 20× Radius of Gyration]
        J[Load Applied Concentric to Centroid]
        H --> I --> J
    end

**References:

IS 801: Loads and Load Combinations - Key Points

1. Load Combinations (Clause 6.1.2.2):

• Allowable stresses for members under combined wind/earthquake and other loads can be increased by 33% over dead + live load stresses.
• This increase applies if the section is not smaller than that needed for dead + live load combination.
• Applies to primary/secondary roof members and connections under dead load, live load, and ponding.

2. Design Computations (Clause 5.1):

• Use conventional structural design methods for safe load, stress, deflection, unless otherwise specified.

3. Compression Members (Clause 6.6):

• Axially loaded compression members must be designed per IS 801 provisions (refer to Clause 6.6 for detailed formulas).

4. Effective Flange Width for Short Spans (Clause 5.2.5 & Table 1):

• For spans < 30 × flange projection (w), flange effective width is limited as per:

• L = span or distance between inflection points (cm)
• w = flange projection beyond web or half distance between webs (cm)

Summary Diagram of Load Combination Allowable Stress Increase

graph LR
    DL[Dead Load] -->|Base| StressCalc[Stress Calculation]
    LL[Live Load] -->|Base| StressCalc
    WL[Wind Load] -->|+33% Allowable Stress| StressCalc
    EQ[Earthquake Load] -->|+33% Allowable Stress| StressCalc
    Ponding -->|+33% Allowable Stress| StressCalc

IS 801 Design Procedure Summary

1. Design Basis (Clause 5.1)

• Use conventional structural design methods for load, stress, and deflection calculations.
• Refer to IS 800-1962 and IS 875-1964 for load considerations.

2. Allowable Design Stress (Clause 6.1 & Table 2)

• Design stress ( F ) must not exceed specified limits on net tension section or extreme fibers in flexure.
• ( F ) depends on the minimum yield strength ( F_y ) of steel.
• When cold working is used, ( F ) is based on the average yield point of the full section.

3. Notes:

• Use these stresses for tension, compression, and flexural members.
• Maximum allowable stresses are detailed in Clauses 6.1 to 6.8.

Key Formula for Basic Design Stress:

[ F = \text{Allowable design stress from Table 2 based on } F_y ]

flowchart TD
    A[Start: Structural Design] --> B[Refer IS 800 & IS 875 for Loads]
    B --> C[Calculate Loads, Stresses, Deflections]
    C --> D[Determine Steel Grade & Yield Strength \(F_y\)]
    D --> E[Select Allowable Stress \(F\) from Table 2]
    E --> F[Check Stresses ≤ \(F\)]
    F --> G[Design Safe and Compliant Structure]

This concise framework ensures safe, code-compliant design per IS 801.

IS 801 - Allowable Design Stresses (Clause 6)

Key Points:

• Basic Design Stress (F) applies to:
  • Net section tension members
  • Extreme fibers of flexural members (tension & compression)
• Stress must not exceed F, except where specifically allowed.

Formula for Basic Design Stress:

[ F = \text{Specified minimum yield point } (F_y) ]

If cold work of forming is utilized (Clause 6.1.1), use: [ F = \text{Average yield point of the full section} ]

Table 2: Basic Allowable Design Stress (F)

Additional Notes:

• Allowable stresses for webs of beams are specified in Clause 6.4.
• Cold work increases allowable stress only if conditions in 6.1.1.1 are met.
• All allowable stresses must be based on unformed steel properties unless cold work is verified.

This ensures safe design stresses for structural steel per IS 801 and IS 1079-1973 standards.

IS 801: Key Formulas & Tables for Bolted and Welded Connections

1. Bolted Connections (Clause 7.5)

• Tension Stress on Net Section (7.5.2):
  [ \sigma_t \leq 0.6 F ]
  where ( F ) = design strength of the material.

• Shear Stress on Bolts (7.5.4):
  Shear stress on bolt gross area under load shall not exceed:

2. Spacing of Connections in Compression Elements (Clause 7.4)

Spacing ( s ) shall not exceed the minimum of:

• Shear transmission requirement per design strength (7.2),
• ( 1680 \times \frac{t}{\sqrt{f}} ), where:
  • ( t ) = thickness of cover plate/sheet (mm)
  • ( f ) = design stress in cover plate/sheet (N/mm²)
• ( 3 \times w ), where ( w ) = flat width of narrowest unstiffened compression element, but not less than:
  • ( \frac{1590 t}{\sqrt{f}} ) if ( F_c > 0.54F )
  • ( \frac{1910 t}{\sqrt{f}} ) if ( F_c \leq 0.54F )

Note: For intermittent fillet welds parallel to stress, spacing = clear distance + 13 mm; otherwise, center-to-center.

Summary Diagram (Connection Spacing Limits)

graph TD
    A[Spacing s] --> B[Shear transmission limit]
    A --> C[1680 * t / √f]
    A --> D[3 * w (≥ min spacing)]
    D --> E[≥ 1590 * t / √f if Fc > 0.54F]
    D --> F[≥ 1910 * t / √f if Fc ≤

IS 801: Design of Beams and Bracing - Key Points

1. Bracing Forces (Clause 1.0 b)

• For a concentrated load ( P ) within 0.3a from brace:
  [ P_1 = 1.0 \times K' \times P ]
• For ( P ) located between 0.3a and 1.0a:
  [ P_1 = 0.67 \times P \times K ]
• Where:
  • ( a ) = bracing interval length
  • ( x ) = distance from load to brace
  • ( m ) = distance from shear center to web mid-plane
  • ( d ) = channel depth
  • ( I_x ) = moment of inertia (for Z-sections)

2. Spacing of Braces (Clause 8.2.1)

• Attach braces to top and bottom flanges at ends and intervals ≤ ¼ span length.
• For concentrated loads over ≤ 1/12 span, add an extra brace near load center.

3. Effective Flange Width (Clause 5.2.5, Table 1)

• ( L ) = span length (cm)
• ( u_t ) = flange projection width (cm)

4. Box Beams (Clause 8.3)

• Laterally unsupported length to web spacing ratio ≤
  [ \frac{175 \times 700}{F_y} ]

flowchart LR
    A[Concentr

IS 801: Testing and Quality Control Key Points

1. Mechanical Properties Testing (Clause 9.3 & 9.3.3)

• Tensile Tests: Minimum 4 specimens per lot, taken longitudinally at quarter-width near coil end.
• Compression Tests (Appendix A):
  • Specimen length: 3× largest dimension (max 20× least radius of gyration).
  • For ultimate compressive strength, length ≥ 15× least radius of gyration.
  • Load must be applied concentrically.

2. Full Section Tests (Clause 9.3.1)

• Yield Point Determination:
  • Tension: per 9.1.6.
  • Compression: smaller of max compressive strength/cross-sectional area or yield stress by:
    • Sharp yielding steel: autographic or total strain under load method.
    • Gradual yielding steel: strain under load or 0.2% offset method.
• Flange Yield Point: For bending stresses, test flange + web specimen with Q = 1.
• Acceptance Tests:
  • 2 tests per lot (30-50 tonnes), 1 test if <30 tonnes.
  • Lot = single production run from one heat/blow.

3. Evaluation of Tests (Clause 9.2)

• Mean of ≥3 specimens; deviation ≤ ±10%.
• If deviation >10%, test 3 more; use average of lowest 3.
• Load capacity must sustain:
  [ \text{Total Load} = 2 \times \text{Live Load} + 1.5 \times \text{Dead Load} ]
• For wind/earthquake, reduce factors by dividing by 1.5.
• No harmful local distortions at:
  [ \text{Load} = \text{Dead Load} + 1.5 \times \text{Live Load} ]

Summary Table: Compression Specimen Length

IS 801: Compression Testing - Key Formulas & Specifications

1. Allowable Average Compression Stress (Clause 7.67)

[ F_{a} = 0.522 F_y - 7.67 \sigma_{TO} ]

• For (\sigma_{TO} < 0.5 F_y): [ F_{a} = 0.522 \sigma_{TO} ]
• (\sigma_{TO}) = Elastic torsional-flexural buckling stress, calculated by: [ \sigma_{TO} = 28(\sigma_{0x} + \sigma_0) - \sqrt{(\sigma_{0x} + \sigma_0)^3} - 48 \sigma_{0x}^1 ]
• Parameters include:
  • (K) = effective length factor
  • (L) = unbraced length
  • (E = 2,07,4000 \text{ kgf/cm}^2)
  • (G = 795,000 \text{ kgf/cm}^2)
  • Section properties: radii of gyration, torsion constant (J), warping constant (C_w), thickness (t).

2. Compression on Unstiffened Elements (Clause 6.2)

For flat elements with width-thickness ratio (w/t):