Determination of the modulus of elasticity in bending of structural timber - comparison of two methods

The current method for determination of modulus of elasticity (MOE) in Europe, EN 408, does not specific enough instructions to obtain comparable results between institutes. There are different interpretations especially regarding the placements of deflection yokes and transducers and also regarding the evaluation of test results. This paper proposes a modified test method, similar to the one in the USA and Australia. The advantages would not only be consistent and reliable results but also a possibility to compare test results between Europe and North America. In order to obtain correction factors to transfer results from the current EN 408 method, called local MOE (EL), to the proposed method, called global MOE (EG), 800 beams were tested. The tests were performed at the Norwegian Institute of Technology (NTI), the Swedish National Testing and Research Institute (SP), the Swedish Institute for Wood Technology Research (Trätek) and the Technical Research Centre of Finland (VTT). An attempt to take characteristics, such as density, knot size, timber depth and sawing pattern into account was made. The results show that high or low density has no effect on the EL/EG ratio but the knot size seems to have. From where timber originates, pith or bark, does not have a clear influence. The differences are however large. Based on all material there seems not to be any effect of timber depth, but looking at each institute separately, there is. Finally, an equation for transferring test results from the current method into the proposed method was determined. This should be handled with care due to different assessment methods for EL at different institutes. INTRODUCTION The method for determining the modulus of elasticity in bending (MOE) in the European standard EN 408 does not give consistent and reliable results (Boström et al. 1996; Boström 1997; Boström 1999; Solli 1996). The determination of the MOE is based on measurement of a deflection over a relatively short span between the loading points. The deflections are small, often less than 1 mm. Hence the method is sensitive to measurement errors. Such errors can be caused by twisting of the timber during the test (Solli 1996). In addition to this, Boström et al. (1996) have shown that different MOE-values are obtained depending on where on the beam the deflection yokes are placed. It seems that measuring deflection on the top surface gives a higher MOE than at mid depth. Whether the deflection is measured on both sides of the beam or only on one also seems to have an influence (Solli 1996). It is evident that the method in EN 408 does not particularise enough where and how the measurement of deflection should be done. The method is open for different interpretations and as a result of this the MOE-values are likely to differ depending on where the tests are carried out. This is unacceptable and has lead to discussions regarding changes of the test procedures and the experimental set-up in EN 408. The objective of this project was to propose a modified test method for MOE and determine the influence it will have on the test results. It was also an aim to determine an equation or ratio to transfer the properties determined using the current EN 408 into results complying with the proposed version. The proposed test method should reduce the disadvantages of the current EN 408 to a minimum and possibly also correspond to other methods used for MOE determination. In Australia and North America the mid-span deflection in relation to the supports is measured. This is often referred to as a ”global” measurement and could possibly be an alternative to the ”local” measurement in EN 408. 1 MSc, SP Swedish National Testing and Research Institute, Box 857, S 501 15 Borås, Sweden 2 PhD, SP Swedish National Testing and Research Institute, Box 857, S 501 15 Borås, Sweden METHOD Experimental set-up: Testing according to the current EN 408 The EN 408 standard specifies a four-point bending test, with loads applied at the third points. The distance between supports is 18 times the beam depth, see Figure 1. The deformation is measured over a length equal to five times the beam depth centred within the test span, i.e. the deformation of point B is measured relative to points A and C. The standard does neither specify at what depth the points A, B and C should be located, nor if the deformation should be measured on both sides of the beam or not. However, at SP and NTI the deflection was measured on both sides of the specimen, while at VTT it was done on only one side. At all institutes but Trätek the points A, B and C were located at the neutral axis. At Trätek the measurement was done on the tension side as they have the experience that the repeatability is better when doing so. In all tests, the worst defect was placed centred between the loading points and randomly placed in the compression or tension zone. The resulting modulus of elasticity according to EN 408 will henceforth be called the local modulus of elasticity.