CIVL 2020 Lab No. 3: Compressive Test of Wood Specimens

CIVL 2020 Lab No. 3: Compressive Test of Wood Specimens

Objectives

  • Upon successful completion of this lab module, students will be able to:

    • Set up and perform a laboratory test to determine the mechanical properties of a timber specimen loaded in compression both parallel and perpendicular to the grain.

    • Record appropriate data from the test and apply safety factors to this data to determine the design compressive stress.

Introduction

  • When timber is stressed in a manner that causes shortening of the fibers in the direction of the grain, it is identified as:

    • Compression Parallel to Grain: This occurs when structural studs or posts experience compressive stresses.

    • Timber exhibits surprising strength in compression parallel to the grain and is efficient as a compression member.

    • Efficiency Definition: Refers to the strength provided by the member relative to its weight, cost, and ease of installation.

  • This module focuses on timber subjected to compressive stresses parallel to the grain. Data manipulation allows for the determination of:

    • Compressive Strength

    • Modulus of Elasticity

  • It is essential to recognize that wood is organic and displays variability in its properties compared to concrete and steel.

  • Wood Structure: Wood resembles a bundle of straws glued together (with lignin) which forms its grain. This structure leads to its classification as an:

    • Anisotropic Material: This means it behaves differently in different directions.

    • Example of Anisotropy:

      • Crushability:

      • Easier to crush straws when pushed sideways (perpendicular to length).

      • Harder to crush when pushed along their length (parallel to length).

  • Compressive Strength Characteristics:

    • Stronger when loaded parallel to grain compared to perpendicular.

    • Case Study: A piece of clear Douglas Fir, dimensions 150mm x 38mm x 38mm (6" x 1½" x 1½") tested under:

    • Force of 112 kN (25,200 lbs) required to crush it when loaded parallel to grain.

    • For loads applied perpendicular to grain, failure occurred at approximately 20% of the parallel load.

  • Modes of Failure During Compressive Testing:

    • Crushing (a): Resulting in horizontal planes of crushed fibers.

    • Wedge Splitting (b): Characterized by a Y-shaped failure at the specimen's surface.

    • Shearing (c): Occurring at an angle more than 45 degrees to the load applied.

    • Splitting (d): Related to specimens with internal defects.

    • Combined Crushing and Splitting (e): Typically arises in cross-grained pieces.

    • Brooming (f): Common with elevated moisture content or improperly cut specimen ends.

  • Important Note: For characterizing defect-free wood, results from failures in modes (d), (e), and (f) are excluded from final data tabulations.

Review

  • Recollection from lectures regarding:

    • Modulus of Elasticity (E): Also known as Young's Modulus.

    • Definition: A measure of a material's stiffness.

    • Determined by plotting a stress-strain graph and finding the slope of the elastic portion.

  • Stress-Strain Curve for Timber: Helps in identifying:

    • Characteristics of brittle or ductile material.

    • Proportional limit, yield strength, and ultimate strength.

  • Refer to the textbook (Goodno, section 1.4) for a refresher on this topic.

The Laboratory Test

  • Test Setup for Parallel-to-Grain Testing:

    • Utilize a specimen sized 38 mm x 38 mm x 150 mm (may vary slightly).

    • Mount Select Structural Douglas Fir in a testing machine for parallel load application.

    • Compressometer: Dial gauge used to measure shortening of the specimen.

    • Necessary to record changes in length at predetermined load intervals to equate stress and strain.

  • During the test, take note of:

    • Rate of movement of the load pointer compared to the compressometer pointer after the load exceeds 40 kN.

    • The failure mode displayed by the specimen.

    • Instructor will guide on the setup and procedures for perpendicular-to-grain testing.

The Results

  • Understanding test results may be challenging due to the organic and non-homogeneous nature of timber, versus the more predictable properties of homogeneous materials like steel.

  • Instructions for Analyzing Parallel-to-Grain Results:

    1. Calculate stress corresponding to each load reading using the original cross-sectional area of the specimen.

    2. Calculate strain corresponding to each extensometer reading using the original specimen length.

    3. Plot the entire stress-strain curve by hand (no computer) on graph paper.

    4. Determine the modulus of elasticity, proportional limit, and ultimate stress, labeling each point on the graph.

    5. Develop a sketch and description of the failure mechanism as specified in the introduction.

  • Design Code Value and Safety Factor Calculation:

    • An allowable stress value for select Douglas Fir is 11.0 MPa.

    • Assume failure occurred just beyond the proportional limit of the specimen; calculate the corresponding safety factor against this permissible stress and analyze if it is appropriate, explaining your reasoning.

  • Connection Detail:

    • Figure presented shows a typical beam-to-column connection detail.

    • Assumptions:

      • Steel saddle remains intact.

      • Bolts carry no load.

      • Allowable stresses for compression are:

      • Parallel to grain: 9.7 MPa

      • Perpendicular to grain: 3.17 MPa

    • Calculate the magnitude of the load safely transferable between the beam and the column under these conditions.