Optimizing the design of timber components under decay and climate variations

International audienceThe durability of timber structures can be affected by the isolated or combined actions of loading, moisture content, temperature, biological activity, etc. This work focuses on the optimal design of new timber structures subjected to deterioration. Since the deterioration processes and the structural behavior of timber structures are complex, nowadays the deterioration models are not able to account for all influencing factors. Consequently, this study is based on an empirical model that was derived based in-lab experimental studies for the decay growth of brown rot in pine sapwood under variant climate conditions. Such a model is divided into two processes: (i) activation process and (ii) mass loss process. On the other hand, there are significant uncertainties involved in the problem. The uncertainties inherent to materials properties, models and climate are considered throughout a time-dependent reliability based-design optimization (TD-RBDO) approach. The TD-RBDO aims to ensure a target reliability level during the operational life. This approach is applied to design optimization of a new timber component subjected to different French climates. The performance of the optimized solution is compared with a traditional cross-section designed according to the Eurocode 5 in terms of safety. The overall results indicate that an optimized solution ensures a target reliability level during the whole structural lifetime

Theoretical analysis of the spatial variability in tillage forces for fatigue analysis of tillage machines

This paper presents a new theoretical model to describe the spatial variability in tillage forces for the purpose of fatigue analysis of tillage machines. The proposed model took into account both the variability in tillage system parameters (soil engineering properties, tool design parameters and operational conditions) and the cyclic effects of mechanical behavior of the soil during failure ahead of tillage tools on the spatial variability in tillage forces. The stress-based fatigue life approach was used to determine the life time of tillage machines, based on the fact that the applied stress on tillage machines is primarily within the elastic range of the material. Stress cycles with their mean values and amplitudes were determined by the rainflow algorithm. The damage friction caused by each cycle of stress was computed according to the Soderberg criterion and the total damage was calculated by the Miner's law. The proposed model was applied to determine the spatial variability in tillage forces on the shank of a chisel plough. The equivalent stress history resulted from these forces were calculated by means of a finite element model and the Von misses criterion. The histograms of mean stress and stress amplitude obtained by the rainflow algorithm showed significant dispersions. Although the equivalent stress is smaller than the yield stress of the material, the failure by fatigue will occur after a certain travel distance. The expected distance to failure was found to be df=0.825×106km. It is concluded that the spatial variability in tillage forces has significant effect on the life time of tillage machines and should be considered in the design analysis of tillage machines to predict the life time. Further investigations are required to correlate the results achieved by the proposed model with field tests and to validate the proposed assumptions to model the spatial variability in tillage force

Optimizing the design of timber components under decay and climate variations

International audienceThe durability of timber structures can be affected by the isolated or combined actions of loading, moisture content, temperature, biological activity, etc. This work focuses on the optimal design of new timber structures subjected to deterioration. Since the deterioration processes and the structural behavior of timber structures are complex, nowadays the deterioration models are not able to account for all influencing factors. Consequently, this study is based on an empirical model that was derived based in-lab experimental studies for the decay growth of brown rot in pine sapwood under variant climate conditions. Such a model is divided into two processes: (i) activation process and (ii) mass loss process. On the other hand, there are significant uncertainties involved in the problem. The uncertainties inherent to materials properties, models and climate are considered throughout a time-dependent reliability based-design optimization (TD-RBDO) approach. The TD-RBDO aims to ensure a target reliability level during the operational life. This approach is applied to design optimization of a new timber component subjected to different French climates. The performance of the optimized solution is compared with a traditional cross-section designed according to the Eurocode 5 in terms of safety. The overall results indicate that an optimized solution ensures a target reliability level during the whole structural lifetime

OPTIMAL AND RELIABLE DESIGN OF TIMBER BEAMS FOR A MAXIMUM BREAKING LOAD CONSIDERING THERMAL AND HYDROLOGICAL EFFECTS

International audienceThe use of of bio materials in sustainable construction aims to reduce the environmental impact of buildings. However, the wood material suffers from several drawbacks, such as the uncertainties of the timber mechanical properties, the knots of the material and the appearance of cracks. However, the timber elements exhibit micro-cracks, which can propagate due to fatigue, overload or creep loading. Thus, crack initiation is one of the most important factors involved in the collapse of timber component in building structures. To predict the crack initiation, many numerical methods have already been developed to characterize the mechanical fields in the crack tip vicinity [1]. In this work, energy method based on invariant integrals is used to estimate the fracture parameters such as energy release rate and stress intensity factors [2]. The analytical formulation of the T-integral to viscoelastic materials [3] is extended to A-integral in order to take into account the effect of thermal loading and the effect of moisture variation [4]. In fact, the study of the crack growth initiation and crack propagation in wood timber may consider the effect of temperature and the moisture content on the mechanical field distribution in the crack tip vicinity. Structural optimization is widely used for effective cost reduction of civil engineering structure. Several works have used the Deterministic Design Optimisation (DDO) approach to design timber trusses [5]. The Deterministic Design Optimization procedure is based on minimizing an objective function as the structural volume or cost subjected to geometric, stress and deflection constraints. These design conditions are considered in accordance with Eurocode 5 in order to satisfy the requirements of both the ultimate and the serviceability limit states. However, in the context of fracture mechanic limit state, the DDO-based on the partial safety factors is not conservative since these safety factors are not calibrated on the basis of the fracture mechanic limit state

Updating the Reliability of cracked timber structures by using experimental results and numerical fracture model

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