Keywords: finite element, OSB, timber, I-beam, opening, interaction, nonlinearity.

Oriented Strand Board (OSB) is an orthotropic material made of thin flakes of wood mainly from commercially grown trees [1,2]. Experiments show that OSB in tension behaves almost linearly up to failure, whilst in compression it exhibits obvious plasticity [3]. One of the major applications of OSB is as a web material in timber I-beams. These have been extensively used in North America and Europe during the past two decades, particularly for use as roof and floor joists.

Openings in webs, usually square or circular shaped, are needed to allow services to pass through. By accommodating services within the depth of the floors the overall structural height can be reduced or greater headroom provided. However the presence of openings makes the stress distributions in the web more complicated and generally reduces the load carrying capability of a beam. When two or more openings are generated close to one another, interactions between them will occur.

In the current study, experimental work was undertaken on OSB webbed timber I-beams with openings spaced at different intervals along the length of the web. It was found that there were stress concentrations around an opening. In contrast to the Vierendeel mechanism that indicates the formation of four plastic hinges in the stress concentration zones [4], the OSB was found to fracture from tension zones around an opening, with cracks developing towards the beam flanges in a direction roughly at 450 to the beam axis. A beam would collapse when cracks reached the flanges, together with some plasticity being developed in the compression zones. Interactions between circular and square openings were also investigated experimentally. Tests showed that interactions not only further reduced the load carrying capability of a beam, but also changed its failure patterns. In addition to the diagonal cracks mentioned above, a horizontal crack linking the two openings appeared when interactions became severe.

Incorporating the finite element (FE) package ABAQUS, 3-dimensional non- linear finite element models were developed to predict beam behaviour. In these, both the web and the timber flanges of a beam were modelled with 8-node solid elements [5,6], with a finer mesh being generated around an opening to accommodate stress concentrations. Both timber and OSB in compression were treated as elasto-plastic orthotropic materials to incorporate their plasticity, whilst they were treated as linear orthotropic elastic materials in tension, where the Improved Tasi-Hill criterion [7] was used to define fracturing of OSB. In order to judge whether a particular point in a beam, especially around an opening, is in tension or in compression, and thus to apply the right material constitutive model, a user defined constitutive model subroutine was developed. The FE models developed can be used to trace the propagation of cracks in OSB and to define the failure of a beam. The behaviour of an OSB webbed timber I-beam with openings can thus be numerically simulated. Good correlation between experimental results and numerical simulations was obtained. Interactions between openings of different shapes were investigated. Critical distances of openings, where interactions became severe, were also assessed by FE analysis.

1

D.R. Griffiths and H.G. Wickens, "CEC Programme: Design stress for OSB- University of Surrey tests and reduction of results", Workshop on mechanical properties of panel products, March, 1995, Watford, UK

2

D. Zhou, "A study of oriented structural board made from hybrid poplar- Physical and mechanical properties of OSB", Holz als Roh-und Werkstoff 48: 293-296, 1990. doi:10.1007/BF02626522

3

E.C. Zhu, Z.W. Guan, P.D. Rodd and D.J. Pope, "A constitutive model for OSB and its application in finite element analysis", submitted to Holz als Roh-und Werkstoff, 2003. doi:10.1007/s00107-004-0513-y

4

K.F. Chung, T.C.H. Liu, A.C.H. Ko, "Investigation on Vierendeel mechanism in steel beams with circular web openings", Journal of Constructional Steel Research, 57, 467-490, 2001. doi:10.1016/S0143-974X(00)00035-3

5

Hibbitt, Karlsson & Sorensen, Inc., "ABAQUS/Standard User's Manual Version 6.2", 2001.

6

Hibbitt, Karlsson & Sorensen, Inc., "ABAQUS Theory Manual Version 6.2", 2001.

7

Z.W. Wang, "Composite Mechanics and Composite Structural Mechanics", China Mechanical Industry Press, 1990. (In Chinese)

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