Method of test for nail-jointed timber trusses, Part I: Destructive test

IS 4924 Part 1 (1968) — Scope & Key Specifications

• Scope: Covers testing and evaluation of structural timber, focusing on deflection, load, and safety factors.

• Key Clauses:

  • Clause 3.81: Safety factor example: 420 > 2.5 (minimum factor of safety)
  • Clause 4.1: Deflection recording during load tests (see Appendix B)

• Appendix B (Clause 4.1) - Deflection Test Pro Forma:

• Safety Factor Calculation:
  [ \text{Factor of Safety} = \frac{\text{Ultimate Load}}{\text{Working Load}} > 2.5 ]

• Moisture Content & Defects: Must be recorded as part of destructive testing.

Summary Diagram of Deflection Test Setup

graph LR
A[Load Applied at Heel/Node Points] --> B[Timber Specimen]
B --> C[Deflection Measurement at Bottom Chord Center]
B --> D[Horizontal Slip at Lengthening Joints]
C --> E[Record Initial, Final, Residual Deflection]
D --> E
E --> F[Analyze Safety Factor & Structural Integrity]

This standard ensures timber structural safety by systematic load-deflection testing and documentation.

IS 4924 Part 1: Procedure of Test (Experimental Setup) for Nail-Jointed Timber Trusses

Key Points from the Standard:

• Scope: Destructive testing of fabricated nail-jointed timber trusses.
• Objective: To determine ultimate load capacity and failure mode.
• Rounding off results: Follow IS 2-1960 for rounding decimals in reported values.

Experimental Setup Essentials:

• Test Specimen: Full-scale fabricated truss as per design.
• Loading: Apply gradually increasing load until failure.
• Instrumentation: Use dial gauges/strain gauges to measure deflections and strains.
• Support Conditions: Simulate actual boundary conditions (usually simple supports).
• Load Application: Point loads at panel points or uniformly distributed loads depending on design.

Typical Test Procedure:

1. Initial Inspection: Check specimen dimensions and joint details.
2. Setup: Place the truss on supports; install measuring devices.
3. Loading: Increase load in increments; record deflection and load.
4. Failure Observation: Note mode of failure (e.g., nail withdrawal, timber fracture).
5. Data Recording: Capture load-deflection curve until collapse.

Important Formula (for Nail Withdrawal Capacity):

[ P = \pi d L f_s ]

• (P) = withdrawal load (N)
• (d) = nail diameter (mm)
• (L) = embedded length (mm)
• (f_s) = allowable withdrawal stress (N/mm²)

Reference Table (Typical Nail Sizes & Allowable Loads):

flowchart LR
  A[Test Specimen Preparation] --> B[Setup on Supports]
  B --> C[Install Measuring Devices]
  C --> D[Apply Incremental Loads]
  D --> E[Record Deflections & Loads]
  E --> F[Observe Failure Mode]
  F --> G[Analyze & Report Results]

For detailed values and precise procedures, always refer to the full IS 4924 Part 1 document and IS 2-1960 for rounding rules.

Guiding Attachment for Lateral Stability (IS 4924 Part 1)

• Purpose: Provide lateral stability to the truss under test without restricting vertical deflection.
• Method: Place a similar truss unit at a suitable distance, connected by hinged timber purlins simulating actual rigidity from purlins in use.
• Key dimension: Effective length of top chord between adjacent nodes ≈ 85 cm (Clause 86.6 f).
• Timber Spacing & Thickness:
  • Web members thickness ≥ 2.0 cm
  • Chord members thickness ≥ 2.5 cm
  • Spacing between members ≤ 3 × thickness
• Spacing of timber distance pieces in compression members: every 30 × d (d = thickness), minimum one at center.
• Nailing:
  • Minimum 2 nails at node points, 4 nails at lengthening joints
  • Nails arranged to pass load through centroid; prebore recommended to avoid splitting.
  • Nailing from both faces; protruding nails cut flush or clenched.

Summary Table: Timber Member Requirements

Lateral Stability Attachment Diagram

graph LR
    A[Truss Under Test] -- Hinged Timber Purlins --> B[Adjacent Similar Truss]
    B -- Simulates Rigidity --> A
    style A fill:#f9f,stroke:#333,stroke-width:2px
    style B fill:#bbf,stroke:#333,stroke-width:2px

This setup ensures lateral restraint without vertical deflection restraint, simulating actual in-service conditions.

For nail-jointed timber trusses, refer to Appendix A for factor of safety calculations and ensure timber is seasoned to service moisture content to minimize defects.

Deflection Measuring Equipment & Allowable Deflection (IS 4924 Part 1)

Key Points from Clauses:

• Clause 2.4:

  • Dial gauges fixed at the centre of bottom chords for vertical deflection.
  • Additional gauges at lengthening joints for measuring longitudinal slip.

• Clause 4.1:

  • Deflection recorded as per the pro forma in Appendix B.

• Clause 5.3 (Allowable Deflection Formula):

[ \delta = \frac{N \times F \times U \times L}{E \times A} ]

Where:

• (\delta) = allowable deflection (cm)

• (N) = number of planks in a member

• (F) = force induced in a plank due to design load (kg)

• (U) = force induced in member due to unit load (kg)

• (L) = length of member (cm)

• (E) = modulus of elasticity of timber (kg/cm²)

• (A) = total sectional area of the member (cm²)

• Clause 5.4:

  • Actual deflection must be compared with the allowable deflection (\delta).

Summary Table for Deflection Measurement Setup

flowchart LR
    Load[Applied Load] --> Truss[Timber Truss]
    Truss --> DialGaugeCenter[Dial Gauge at Centre Bottom Chord]
    Truss --> DialGaugeJoint[Dial Gauge at Lengthening Joints]
    DialGaugeCenter --> Record[Record Vertical Deflection]
    DialGaugeJoint --> RecordSlip[Record Longitudinal Slip]
    Record --> Compare[Compare with Allowable Deflection]

Note: Use timber (E) values from IS 883 or relevant timber properties for accurate calculation.

IS 4924 Part 1: Recording of Deflection and Failure

1. Recording of Deflection (Clause 4.1 & 2.4)

• Deflection Measurement Points: Dial gauges fixed at:
  • Centre of bottom chord (vertical deflection)
  • Lengthening joints of bottom chords (longitudinal slip)
• Data Recording: Use the pro forma in Appendix B to log deflections under various loads.

2. Recording of Failure (Clause 4.2)

• Observe ultimate failure modes visually at critical points:
  • Buckling, fracture, joint failure, or excessive deformation.
• Note the sequence and location of failure for interpretation.

3. Interpretation of Data (Clause 5.4)

• Compare actual deflection with allowable deflection (from Clause 5.3).
• Allowable deflection typically depends on span and load type (commonly span/250 to span/350).

Typical Allowable Deflection Formula (as per general practice):

[ \delta_{allow} = \frac{L}{250} \quad \text{to} \quad \frac{L}{350} ]

• Where L = span length of the truss.

Summary Table for Deflection Recording

flowchart LR
    Load --> Truss
    Truss -->|Deflection| DialGauge_Centre[Dial Gauge at Bottom Chord Centre]
    Truss -->|Slip| DialGauge_Joints[Dial Gauge at Lengthening Joints]
    Truss -->|Failure Observation| VisualInspection[Visual Inspection]
    DialGauge_Centre --> DataRecording[Record Deflection in Appendix B]
    DialGauge_Joints --> DataRecording
    VisualInspection --> FailureRecord[Record Failure Mode & Location]

Note: Always ensure dial gauges are calibrated and fixed firmly to avoid measurement errors.

IS 4924 Part 1: Record of Deflection – Key Points & Formula

1. Deflection Recording (Clause 4.1)

• Deflection under loads must be recorded using the pro forma in Appendix B.
• Use dial gauges fixed at:
  • Centre of bottom chords (vertical deflection).
  • Lengthening joints of bottom chords (longitudinal slip).

2. Allowable Deflection Formula (Clause 5.3)

[ \delta = \frac{N \times F \times U \times L}{E \times A} ]

Where:

• (\delta) = Allowable deflection (cm)
• (N) = Number of planks in the member
• (F) = Force in a plank due to design load (kg)
• (U) = Force in member due to unit load (kg)
• (L) = Length of member (cm)
• (E) = Modulus of elasticity of timber (kg/cm²)
• (A) = Total sectional area of member (cm²)

3. Comparison (Clause 5.4)

• Compare actual deflection with allowable deflection to assess serviceability.

Summary Table for Deflection Measurement

flowchart LR
    A[Load Application] --> B[Measure Deflection with Dial Gauges]
    B --> C[Record Deflection in Pro Forma (Appendix B)]
    C --> D[Calculate Allowable Deflection using Formula]
    D --> E[Compare Actual vs Allowable Deflection]
    E --> F{Is Deflection Acceptable

IS 4924 Part 1: Record of Failure - Key Points

1. Record of Failure (Clause 4.2)

• Ultimate failure of the timber truss is observed visually at various critical points.
• Failure modes include:
  • Crushing of timber
  • Rupture of tension members
  • Buckling of compression members
  • Joint failure

2. Record of Deflection (Clause 4.1 & Appendix B)

• Deflection under load is recorded systematically using a pro forma sheet.
• Deflection data helps interpret structural behavior before failure.

3. Apparent Factor of Safety (Clause A-2.1)

• Defined as:
  [ \text{Apparent Factor of Safety} = \frac{\text{Ultimate Load}}{\text{Working Load}} ]

4. Point of Failure (Clause 207.5 & Fig.4)

• Failure point is identified by visual signs such as cracks, excessive deflection, or joint slip.

Summary Table: Failure Observation Parameters

flowchart TD
    A[Load Application] --> B[Measure Deflection]
    B --> C{Visual Inspection}
    C -->|No failure| B
    C -->|Failure observed| D[Record Ultimate Load & Mode]
    D --> E[Calculate Apparent Factor of Safety]

Use these guidelines to systematically record and interpret failure in timber trusses per IS 4924 Part 1.

IS 4924 Part 1: Interpretation of Data - Key Points

1. Recording Failure (Clause 4.2):

   • Observe ultimate failure visually at various points on the truss.
   • Note mode and location of failure for analysis.

2. Data Reporting (Clause 5 & 0.6):

   • Round off final observed/calculated values per IS 2-1960 (Rules for rounding off numerical values).

3. Observation Table (Clause 4.1, Appendix B):
   Use the following tabular format for test data recording:

4. Additional Notes:
   • Record average moisture content and any defects.
   • Refer to Appendix A for example calculations of actual and apparent factors of safety.

Summary Table for Data Interpretation

flowchart TD
    A[Test Setup] --> B[Apply Loads at Heel & Node Points]
    B --> C[Measure Deflections & Slips]
    C --> D[Record Time Intervals]
    D --> E[Observe Failure Mode]
    E --> F[Tabulate Data]
    F --> G[Interpret Results per IS 4924 & IS 2-1960]

**Use this structured approach to ensure consistent,

IS 4924 Part 1 – Apparent Factor of Safety (AFS)

Key Concepts:

• Apparent Factor of Safety (AFS): Ratio of ultimate load to working load, as perceived from tests or calculations.
• Actual Factor of Safety: True ratio considering all influencing factors.

Formula:

[ \text{AFS} = \frac{\text{Ultimate Load}}{\text{Working Load}} ]

Where:

• Ultimate Load: Load at failure or maximum load capacity.
• Working Load: Load under normal service conditions.

Notes:

• Appendix A of IS 4924 Part 1 provides a worked example differentiating actual and apparent factors of safety.
• The apparent factor may differ from the actual due to test conditions, assumptions, or simplifications.

Practical Use:

• Use AFS for preliminary design checks.
• Confirm with actual factor of safety for final design.

flowchart LR
    A[Working Load] -->|Ratio| B[Apparent Factor of Safety]
    C[Ultimate Load] -->|Ratio| B
    B --> D{Design Decision}
    D -->|Sufficient| E[Proceed]
    D -->|Insufficient| F[Reassess Design]

For detailed calculations, refer to Appendix A of IS 4924 Part 1.

IS 4924 Part 1: Allowable Deflection in Nail-Jointed Timber Trusses

Key Formula (Clause 5.3):

[ \delta = \frac{N \times F \times U \times L}{E \times A} ]

Where:

• (\delta) = Allowable deflection (cm)
• (N) = Number of planks in a member
• (F) = Force in a plank due to design load (kg)
• (U) = Force in truss members due to unit load at deflection point (kg)
• (L) = Length of member (cm)
• (E) = Modulus of elasticity of timber (kg/cm²)
• (A) = Total sectional area of the member (cm²)

Additional Specifications:

• Clause 5.4: Actual deflection must be compared with allowable deflection.
• Clause 4.1: Deflection data should be recorded as per Appendix B pro forma.
• Clause 2.4: Use dial gauges at bottom chord center and lengthening joints for vertical and longitudinal deflection measurement.

Summary Table:

graph LR
A[Design Load] --> F[Force in Plank (F)]
B[Unit Load] --> U[Force in Member (U)]
N[Number of Planks (N)] --> Calc[Calculate Allowable Deflection]
L[Member Length (L)] --> Calc
E[Modulus of Elasticity (E)] --> Calc
A[Sectional Area (A)] --> Calc
Calc --> Delta[Allowable Deflection (δ)]

This formula ensures structural

IS 4924 Part 1 - Design & Construction Key Points

1. Material & Member Specifications (Clause 2.0)

• Minimum thickness:
  • Web members: ≥ 2.0 cm
  • Chord members: ≥ 2.5 cm
• Spacing between members: ≤ 3 × individual member thickness
• Knots & defects: Avoid serious defects at joints.
• Seasoning: Use seasoned timber with optimum moisture content for nail-jointed trusses.
• Distance pieces: Insert timber spacers in compression members at 30d spacing (d = thickness), minimum 1 at center; one spacer at center for tension members.
• Nail joints:
  • Minimum 2 nails/node, 4 nails/lengthening joint.
  • Nails arranged so force passes through nail centroid; account for eccentricity if not.
  • Prebore holes recommended to avoid splitting.
  • Nails driven from both faces; protruding nails clenched or cut flush.

2. Design Data Example (Appendix A)

• Timber species: Fir (Abies pindrow)
• Span: 3 m
• Total design load: 420 kg
• Modulus of Elasticity, E = 94,000 kg/cm²
• Allowable compressive stress parallel to grain, f_c = 60 kg/cm²

3. Deflection Checks (Clause 5.4)

• Compare actual deflection with allowable deflection from design.

Nail Joint Force Consideration Formula

[ F_{max} = \text{Force in member} + \text{Eccentricity correction} ]

• Arrange nails so load passes through centroid.
• If eccentricity ( e ) exists, adjust forces accordingly.

Summary Table for Thickness & Spacing

flowchart LR
    A[Timber Member] --> B[Thickness ≥ 2.0 cm (Web) or 2.5 cm (Chord)]
    B --> C[Spacing ≤ 3 × Thickness]
    C --> D[Insert Distance Pieces at 

IS 4924 Part 1: Worked Example of Safety Factor Calculations

Key Concepts from Appendix A (Clause A-2):

1. Actual Factor of Safety (FOS_actual):
   [ \text{FOS}_{actual} = \frac{\text{Ultimate Load}}{\text{Working Load}} ]

2. Apparent Factor of Safety (FOS_apparent):
   Defined at failure, considering the measured loads and stresses during testing:
   [ \text{FOS}_{apparent} = \frac{\text{Failure Load}}{\text{Working Load}} ]

3. Effective Length of Top Chord (l):
   Given as 86.6 cm, approximated to 85 cm between two adjacent node points (Clause 86.6 f)).

Typical Calculation Steps:

• Determine working load based on design requirements.
• Measure or calculate ultimate/failure load from tests or analysis.
• Compute FOS_actual and FOS_apparent using the formulas above.
• Compare factors of safety to ensure structural adequacy.

Summary Table Example:

flowchart LR
    A[Working Load \(P_w\)] --> B[Calculate FOS_actual]
    C[Ultimate Load \(P_u\)] --> B
    B --> D[FOS_actual = \(P_u / P_w\)]
    E[Failure Load \(P_f\)] --> F[Calculate FOS_apparent]
    F --> G[FOS_apparent = \(P_f / P_w\)]

Note: Use Appendix A in IS 4924

IS 4924 Part 1: Pro Forma for Deflection Recording

Key Specifications & Formulas

• Deflection Recording (Clause 4.1):
  Deflections due to loads must be recorded using the pro forma in Appendix B (not provided here, typically includes date, load details, deflection readings at specified points).

• Deflection Measurement (Clause 2.4):

  • Use dial gauges fixed at the center of bottom chords for vertical deflection.
  • Additional gauges at lengthening joints to measure longitudinal slip.

• Allowable Deflection Formula (Clause 5.3):

[ \delta = \frac{N \times F \times U \times L}{E \times A} ]

Where:

• (\delta) = allowable deflection (cm)

• (N) = number of planks in member

• (F) = force per plank due to design load (kg)

• (U) = force in member due to unit load (kg)

• (L) = length of member (cm)

• (E) = modulus of elasticity of timber (kg/cm²)

• (A) = total sectional area of member (cm²)

• Comparison (Clause 5.4):
  Actual deflection measured must be compared with (\delta). If actual > allowable, redesign or strengthening is needed.

Summary Table

flowchart TD
    A[Apply Design Load] --> B[Measure Deflection with Dial Gauges]
    B --> C[Record Deflection in Pro Forma (Appendix B)]
    C --> D[Calculate Allowable Deflection