Timber strength distributions

When fitted functions are used in reliability analysis, it is essential that the fit is good in lower tail area, the lowest values being most important. Lognormal distribution seems to fit well to the tail of strength data of industrial products like LVL and it may be used also for sawn timber when a COV is used based to the tail fit. When the number of experiments allows, determination of 5% fractile of strength should be based on the function fitting on the lower tail of the strength values, for instance 10%. All smooth functions fitted to tail data gave good estimate of 5% fractile. When 5% fractile is determined from a function fitted to all data, up to 5% error was made (in one case 9%). 3 parameter Weibull distribution gave in all calculated cases 5% fractile within an accuracy of 3 % when compared to nonparametric distribution.