The JCSS Probabilistic Model Code for Timber – Examples and Discussion

Authors

Jochen Köhler and Michael H. Faber

Abstract

During the last decades structural reliability methods have been further developed, refined and adapted and are now at a stage where they are being applied in practical engineering problems as a decision support tool in connection with design and assessment of structures. Furthermore, basic knowledge concerning the actions on structures and the material characteristics has improved due to increased focus, better measuring techniques and international research co-operation. This knowledge has now reached a level where it enables experts to take into account uncertainties in material properties and actions when assessing the load carrying capacity, serviceability and service life of structures. This is not least due to the fundamental works on structural reliability methods performed within the Joint Committee on Structural Safety (JCSS) including, among others, the basic reports on actions on structures, basic reports on material resistances and the guideline for reliability based assessment of structures. These documents provide general guidelines for the use of structural reliability methods in practical applications and at the same time these documents constitute the basis for ensuring that such analysis are performed on a theoretically consistent and comparative basis. The recent reinforcements in providing such a basis for design are condensed in the almost complete JCSS Probabilistic Model Code (PMC) [1]. In line with the ongoing joint efforts to complete the JCSS Probabilistic Model Code, a chapter about the probabilistic modelling of timber material properties has been added recently.
The proposed probabilistic model for timber material properties is structured into several levels of sophistication. The basic level reflects the recent practice for reliability based code calibration. The bending strength and stiffness and the density of timber are referred to as reference material properties and are introduced as simple random variables. Furthermore, several possible refinements are proposed. New information might be introduced, and it is shown how different types of new information can be integrated by using a Bayesian updating scheme. Refinements in regard to the modelling of damage as a consequence of time load duration are proposed. For the bending strength, a hierarchical spatial variability model is proposed and a method is presented for linking the properties of a cross section (which is considered as the reference starting point for the modelling of spatial variability) with the properties of a test specimen.
In this paper, several examples demonstrate the applicability of the probabilistic model code for different problems in timber research, engineering and code writing. General reference is made to the JCSS PMC and the interested reader is invited to obtain this document from JCSS

Published by/in

Proceedings WCTE 9th World Conference of Timber Engineering, Portland, Oregon, USA, August 6-10, 2006.