814 815 Cracks in wooden structures - outdoor exposure and simulation - part 2

Published: 24 August 2012

The aim is to develop knowledge of cracking and degradation of the wooden structures of various influencing factors. The project will provide a basis for lifetime assessment of timber structures in the outdoor environment.

Area: Durability of wood
Budget: 3,755,000 SEK
Timetable: January 2011-December 2012
Coordinator: Karin Sandberg, SP Wood
Project Manager: Anna Pousette, SP Wood and Mats Ekevad, LTU

Background

An outdoor experiments with wooden beams and wooden poles started in 2007 with the aim to study how cracks in wooden structures outdoor evolve over time, and how moisture influences cracking, beams future strength and risk of rotting.


The outdoor experiments provide a basis for inspection supervision, maintenance guidelines and advise how damage is prevented. During inspections of outdoor structures of wood we needed to assess e.g. bearing beams of life remaining. Cracks in wooden structures affect the resistance by opening the protective paint film so that water can penetrate and separate the surface and color film apart. A crack can also cause water to penetrate into the wood with a risk of rot and deterioration resistance as a result.

Knowledge of cracks is important to many structures, such as wooden bridges, wooden balconies, decks and railings, where the issue is the same. The issue is how resistance is calculated and assessed, and how the cracks will develop under different conditions. For inspectors out in the field limits for different types of cracks, depending on the design, wood materials, fractures number, size and location of the structure facilitates asessments. Therefore, simple rules and methods needs to be developed.

The experiment may help to explain the wood properties such as shear strength with respect to cracking. It is extremely important that information about how the cracks affect the shear strength is investigated because the new Eurocodes for design which fully applies fr.om 2011 indicates a reduction factor, KCR, for shear. The KCR factor have nationalal variations  and according to Swedish rules it shall be 0.67 compared with 1.0 in some other countries such as Denmark and Finland. Shear capacity is reduced thus by 33%. The large reduction in Sweden means larger beam heights and make it more expensive to use glulam in the saddle beams and continuous beams with large spans. It may also have secondary negative impact on sales of beams and joists of wood and the ability to build large wooden structures.

Purpose, objectives

The aim is to develop knowledge of cracking and degradation of the wooden structures of various influencing factors. The project will provide a basis for lifetime assessment of timber structures in the outdoor environment. The vision is an engineering tool for calculating the resistance and durability of structures as a function of time and with respect to various conditions, including cracking and moisture migration.


One goal is to develop recommendations for inspections of supporting outdoor structures made of wood, with instructions on how to measure and assess cracks and estimated carrying capacity and remaining service life. The experiments will provide a basis for simulation models and lifetime calculations for structural wood in an outdoor environment above ground. Simulations will provide answers to how sprickutvecklingen happening and what significance cracks for stiffness and load capacity.

Previous projects in the field

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