Four-Point Bending Test of Glulam Beams

A glulam beam is tested under four point bending to determine its bending strength parallel to the grain in accordance with EN 408:2010+A1:2012. This test also allows calculation of the global modulus of elasticity (MOE) and evaluation of deflection for serviceability limit state design checks in EN 1995-1-1. The associated assignment details are provided below.

Background

The four-point bending test is used to determine:

Bending strength parallel to the grain
Global Modulus of Elasticity (MOE)
Load-deflection behaviour
Failure mechanisms and modes
Serviceability performance for design according to EN 1995-1-1 (Eurocode 5)

Learning outcomes

By the end of this content, learners should be able to:

Develop an understanding of a typical four-point beam bending setup
Identify key stages of failure in a glulam beam
Interpret provided load – deflection data
Calculate the bending strength and modulus of elasticity of a glulam beam
Assess the structural performance of the beam based on measured properties and observed failure behaviour.

Materials Provided

Test overview video
Load versus deflection data for four glulam beams: Glulam Beam Test Data
Spreadsheet template for calculations: Calculation Template
Photos of beams during and post-testing

Test overview video

Galway_beams_TimberEDU.mp4

TEST DESCRIPTION AND OUTCOMES

Test specimens

The beam specimens being tested are 98 x 125 x 2300 mm glulam beam sections manufactured using Irish-grown Sitka Spruce, either:

Unreinforced - see Figure 1 (2 beams: Beam B6 and Beam B23)
Reinforced with two 12 mm diameter Basalt Fibre Reinforced Polymer (BFRP) composite rods in the bottom layer - see Figure 2 (2 beams: Beam B2 and Beam B5)

Note: Test data is provided for four beams, which are a subset of a larger test campaign at University of Galway.

The timber strength class can be assumed as C16 (EN 338) as this is typical of Irish-grown Sitka Spruce. The beams were conditioned at a temperature of 20 ± 2°C and at a relative humidity of 65 ± 5% prior to testing.

Some further details can be found in: O’Ceallaigh, C. et al. (2025). Creep Behaviour and Creep Recovery of FRP Reinforced Timber Elements. In: Fink, G., Jockwer, R., Cabrero, J.M. (eds) Holistic Design of Taller Timber Buildings. Springer Tracts in Civil Engineering. Springer, Cham. https://doi.org/10.1007/978-3-032-02098-7_35, and O'Ceallaigh, C., Harte, A.M., Sikora, K., &McPolin, D. (2014). Enhancing low grade sitka spruce glulam beams with bonded-in BFRP rods. Paper presented at the COST Action FP1004 Experimental Research with Timber conference.

Figure 1 Unreinforced Glulam Beam Cross Section (Beam B6 and Beam B23)

Figure 2 Reinforced Glulam Beam Cross Section (Beam B2 and Beam B5)

Test apparatus

Test machine: Dartec Universal testing machine (Figure 3)
Measurements:
- Dartec load and cross head deflection
- Micro-Epsilon optoNCDT 1420 laser deflection on underside of beam at midspan (Figure 4)
Data acquisition: National Instruments multi-function DAQ device: NI USB-6210 (Figure 5)

Figure 3 Universal test machine

Figure 4a Deflection Measurements using a Micro-Epsilon optoNCDT laser

Figure 4b Deflection Measurements using a Micro-Epsilon optoNCDT laser

Figure 5a Control system and data acquisition device

Figure 5b Control system and data acquisition device

Loading Procedure

The test specimen (beam) is symmetrically loaded in bending at two points over a span of 18 times the depth as shown in Figure 6. Load is applied at a constant loading-head movement so adjusted that maximum total load Fmax is reached within (300 ± 120) s (or 5 ± 2 minutes). This is equivalent to a loading rate of approximately 10 mm/minute (or 0.167 mm/s; < 0.003 x h limit) for the glulam beam being tested here. The beams are expected to exceed their elastic limit once deflection exceeds 15 mm.

The mode of failure and the growth characteristics at any fracture section of each test piece shall typically be recorded where information is available. Failures that originate from other types of behaviour other than bending (flexure) shall be reported.

Load (F) and deflection (w) are measured and recorded as follows:

(i) At the universal testing machine (Dartec) crosshead (F and w)

(ii) At the beam midspan using a laser or LVDT (w)

Figure 6 Standard test arrangement for measuring bending strength in accordance with EN 408:2010+A1:2012

Experimental Data and Measurements

Quantities measured during testing or to be calculated are listed below.

Test Data is available here ->

Galway_beam_data_TimberEDU.xlsx

Photos of beams during testing and showing damage post-testing

Figure 7a Reinforced beam B2 photos

Figure 7b beam B23 photos

Figure 7c Reinforced beam B5 photos

Figure 7b Beam B6 photos

ASSIGNMENT

Review the test apparatus observed in testing and the test method in accordance with EN 408:2010+A1:2012.
Complete the calculations in Table 1 for each of the four beams. Review the significance of each quantity in terms of the test method and/or structural design.
Plot a graph load (F) versus deflection (w), with w1, w2, F1, F2, and Fmax annotated.
Describe and analyse the failure behaviour of the beam, referencing your load-deflection plot. Figure 7 photos can also be referred to.

OPTIONAL DESIGN EXERCISE

Assuming strength class C16 properties, in accordance with Eurocode 5 (IEN 1995-1-1):

Determine the design bending strength of the homogeneous glulam beam at the ultimate limit state
Assuming a floor application where the beam experiences a characteristic permanent action of 1.0 kN/m and a characteristic medium-term variable action of 2.5 kN/m, what is the longest distance, l, that this beam section (98 mm x 125 mm) can span without exceeding the serviceability limit state (SLS) deflection limits of Eurocode 5?

*Note: Bearing length can be ignored in span length calculations.