Splitting of timber beams loaded perpendicular to grain by connections – Combined effect of edge and end distance

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Abstract

The paper presents experimental results of tests on splitting of laminated veneer lumber beams subjected to loading perpendicular to grain by single bolts with small edge and end distances. Two different edge distances were used, and a total of eight different combinations of edge and end distances were tested. Material properties such as perpendicular-to-grain tensile strength and mode I fracture energy of the tested beams were also determined by testing. The experimental results are compared with a simple analytical model based on quasi-nonlinear fracture mechanics, which takes into account the effect of the edge distance as well as the end distance. A generalized model is proposed, which applies to small and large edge distances and to small and large end distances, and which contains as a special case the linear elastic fracture mechanics model, which is currently used as the basis for calculation of the splitting capacity in the Europeans and Canadian timber design codes.

Highlights

► We consider splitting of timber beams loaded by a bolt near the edge and near the end. ► We propose a generalized quasi-nonlinear fracture mechanics model. ► We validate the model by testing. ► We point out some problems with existing models and design codes.

Introduction

Timber has been used for centuries for structures. Like all building materials, timber has its advantages and disadvantages. The strength-to-weight ratio is high for timber subjected to normal stresses parallel to the grain direction, but timber is particularly weak when subjected to normal stresses perpendicular to the grain direction. Compression stresses perpendicular to the grain direction may cause significant deformations both instantaneously and over time, and cause serviceability problems. Tensile stresses perpendicular to the grain direction may cause brittle failure and are of concern for human safety. Such particularities of building materials must be considered and taken into account when designing structures. However, it is only in recent years that structural timber design codes have started explicitly taking into account the risk of splitting of beams loaded perpendicular to the grain by connections, and most design codes still rely solely on specified minimum edge distances. In the European [1] and Canadian [2] timber design codes, an explicit equation based on linear elastic fracture mechanics (LEFM) has recently been included. The equation is the result of an analysis of a simply supported beam loaded at mid-span [3], [4], but is in [1] also used for other support conditions, such as simply supported beams loaded at other locations than mid-span, and cantilever beams. The model [3], [4] takes into account the effect of the edge distance on the splitting failure load, but it does not take into account the effect of the end distance. The effect of the end distance may be of particular importance for moment-resisting connections [5] and for supports near the beam end.

There seems to be only one analytical model available at present which considers the influence of both edge distance and end distance on the splitting capacity of beams loaded perpendicular to grain by connections [6]. The model is a so-called quasi-nonlinear fracture mechanics model, which is based on the theory of a Timoshenko-beam on a Winkler-foundation. The purpose of the present paper is to experimentally validate an approximate solution, which is sufficiently simple to form the basis for practical design.

Section snippets

Theory and background

The European timber design code [1] requires that the maximum design shear force, Vmax,d, on either side of a connection must fulfil Eq. (1), where subscript d denotes design values and subscript k denotes characteristic values, and where b is the width of the beam, h is the depth of the beam, and he is the loaded edge distance (i.e. the distance from the loaded edge to the center of the fastener farthest from the loaded edge).Vmax,dRd,Rk=bC1he1-heh,C1=14N/mm1.5

According to Eq. (1), a

Materials and methods

Splitting tests were conducted on laminated veneer lumber (LVL) specimens made of New Zealand grown Radiata pine (Pinus radiata). Further, material property tests were conducted to determine among others the perpendicular-to-grain tensile strength and the mode I fracture energy of the LVL.

The LVL used had a density of 593 kg/m3 at a moisture content of 9%. The dynamic MOE in the grain direction was measured using a log-grader and found to be E = 15.9 GPa (the manufacturer specifies 13.2 GPa for the

Discussion

In Fig. 5 are shown the results of the splitting tests compared with the theoretical predictions based on Eq. (4). The following material properties have been used in Eq. (4): E = 15.9 GPa (measured), G = E/20 (assumed), ft = 1.5 MPa (measured), Gf = 0.95 Nmm (measured).

The agreement between experimentally obtained failure loads and theoretical predictions is surprisingly good considering the fact that all material properties except the shear modulus have been measured and no fitting of parameters has

Conclusion

Experiments were conducted on laminated veneer lumber beams made of New Zealand grown Radiata pine (Pinus radiata) in order to determine the splitting failure load for a case where a single bolt loads the beam near the edge and near the end. The purpose of the tests is to validate a simple quasi-nonlinear fracture mechanics model, which takes into account the effect of loaded edge distance as well as the end distance. Material property tests were also conducted to determine the

Acknowledgements

The funding of the work presented was due to the New Zealand Structural Timber Innovation Company, STIC, and the European Union’s Structural Funds – The Regional Fund, and is greatly appreciated by the authors. Likewise do the authors wish to express their gratitude to The University of Auckland undergraduate students Aaron Arnold and Kyle Rolland, who did much of the experimental work as a part of their final year project, and to The University of Auckland undergraduate student Rowan Ballagh

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