RELIABILITY-BASED ANALYSIS AND CALIBRATION OF EUROCODE 5 DESIGN CRITERIA FOR A SOLID TIMBER PORTAL FRAME

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Department of Engineering

ABSTRACT
In this study, laboratory tests were conducted on some commonly used timber species (Alstonia boonei, Triplochiton Scleroxylon, Terminalia Ivorensis, Terminalia superba and Lophira Alata) in Nigeria. The tests were in accordance with, EN 408, EN 13153-1 and ASTM D-143). The test results were analysed in accordance with Eurocode 5. Eurocode 0, JCSS and EN 384, with the aid of Easyfit statistical package. The statistics of the reference properties (density, modulus of elasticity and bending strength) of each specie were generated, and EN 338 strength class assigned to each species after adjustment to 18% Nigerian reference moisture content. The mean values of the densities are: 360.76kg/m3, 380.25kg/m3, 472.10kg/m3, 533.89kg/m3, 955.93kg/m3, for the Alstonia boonei, Triplochiton Scleroxylon, Terminalia Ivorensis, Terminalia superba and Lophira Alata respectively. The corresponding coefficients of variation are 4%, 16%, 18%, 19% and 4%. Likewise, the mean values of moduli of elasticity are 8192.0N/mm2, 6137.80N/mm2, 12161.0N/mm2, 11614.0N/mm2 and 22750.0N/mm2, for Alstonia boonei, Triplochiton Scleroxylon, Terminalia Ivorensis, Terminalia superba and Lophira Alata, with the corresponding coefficients of variation of 6%, 27%, 22%, 21% and 12% respectively. Also, the mean values of bending strengths are 43.09N/mm2, 54.07N/mm2, 70.49N/mm2, 83.63N/mm2 and 97.79N/mm2, for the Alstonia boonei, Triplochiton Scleroxylon, Terminalia Ivorensis, Terminalia superba and Lophira Alata, with the corresponding coefficients of variation of 12% 16%, 19%, 19%, and 16% respectively. Three theoretical distribution models (normal, lognormal and gumbel) were fitted to the reference material properties, using Kolmogorov Simonov test. Normal distribution was found to be the most fit for the timber density, and the best fit theoretical distribution 7 model for modulus of elasticity and bending strength is lognormal distribution. Based on the class limits of EN 338, strength class D18 was assigned to Alstonia boonei, Triplochiton Scleroxylon and Terminalia Ivorensis. Terminalia superba,, was assigned to strength class D24 and Lophira Alata was assigned to strength class D60. The derived material properties (tension and compression strengths parallel and perpendicular to grain, shear strength and shear modulus) were generated from the reference properties based on EN 384. The generated statistical data on the material properties and load statistics reported in various international references were used for the reliability-based analyses and calibration of the Eurocode 5 design criteria for a three-hinged solid timber portal frame. Thirty two failure modes were identified for the frame and limit state function developed for each failure mode. Component and system reliability analyses were implemented using first order reliability method and genetic algorithms. The rafter-column joint failure mode was found to be predominant. The implied safety for the predominant mode was found to correspond well with the frame implied system reliability. This is an indication that, the system reliability of timber structures is the reliability of its critical mode of failure. Uncertainty sensitive mathematical models for the prediction of material safety factors were developed. The models were recommended for use in a proposed Nigerian national annex to the Eurocode 5