Keywords: shear constant, timber beam, torsion test method, photogrammetric method, stereovision.

The shear modulus is a fundamental mechanical property of wood that is used in general timber design [1,2,3]. Compared with other engineering materials, timber has relatively low shear stiffness in comparison to its modulus of elasticity, and so shear deformation contributes a more significant portion of the flexural deflection. Specifically, the shear modulus is critical when designing for the lateral-torsional stability of joists, particularly those with a long span and no lateral supports [4]. The shear modulus is also significant in the design of the serviceability of wood-joist floors [5], and is an input into analytical [6] and finite element [7] models to predict the vibrational serviceability. However, obtaining the shear modulus is limited to either shear block [8] or bending tests [9].

In a torsion test, measuring the rotations of the samples effectively at various locations is the key to the determination of the shear modulus and its variations. The inclinometer is one of the best instruments for this purpose. However, for some size of samples, especially, the small clear samples, it is very difficult to put on more than one inclinometer. So it will be difficult to measure the variation or the distribution of the shear modulus in the samples. To overcome this difficulty, the authors propose a photogrammetric method to measure the surface rotation of the samples. With this new approach, the rotations of any point on the sample surface in the field of view can be measured from the photographs taken by two cameras simultaneously.

In this paper, an experimental study is described which was conducted to determine the shear modulus of clear wood timber using torsion tests. The main objective was to investigate the relationship between modulus of elasticity and shear modulus. Another objective was to compare the measurements of the sample rotation from the inclinometer and photogrammetry methods.

1

A.M. Khokhar, H. Zhang, D. Ridley-Ellis, J. Moore, "The shear strength, and failure modes, of timber joists obtained from the torsion test method", Proceedings of the 11th World Conference on Timber Engineering, Riva Del Garda, Italy, 2010.

2

A.M. Khokhar, H. Zhang, D. Ridley-Ellis, "Shear strength of timber joists obtained from torsion tests", WG meetings COST Action E53-Quality Control for Wood and Wood Products, Lisbon, Portugal, 2009.

3

A.M. Khokhar, H. Zhang, D. Ridley-Ellis, J. Moore, "Determining the shear modulus of Sitka spruce from torsion tests", Proceedings of the 10th World Conference on Timber Engineering, Miyazaki, Japan, 2008.

4

EN 1995-1-1:2004, "Eurocode: Design of Timber structures - Part 1-1: General - common rules and rules for building", European Committee for Standardization, Brussels, Belgium, 2004.

5

R.O. Foschi, "Structural analysis of wood floor system", ASCE J. of Structural Engineering, 108(7), 1557-1574, 1982.

6

Y.H. Chui, "Application of ribbed-Plate theory of predict vibrational serviceability of timber floor systems", The Proceedings of 7th World Conference on Timber Engineering WCTE, 4, 87-93, 2002.

7

L. Jiang, L.J. Hu, Y.H. Chui, "A finite element model for wood-based floor structures with lateral reinforcement", ASCE Journal of Structural Engineering, 130, ST 7, 1097-1107, 2004.

8

American Society for Testing and Materials, "Standards Methods of Testing Small Clear Specimens of Timber", ASTM D 143-94, Vol 04, Serial Designation, Conshohocken, PA, USA, 1952.

9

American Society for Testing and Materials, "Standard Methods of Static Test of Lumber in Structural Size", ASTM D 198-94, Vol 04.10 Wood, West Conshohocken, PA, USA, 1996.

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