PSI - Issue 3

Alberto Lorenzon et al. / Procedia Structural Integrity 3 (2017) 370–379 A. Lorenzon et al. / Structural Integrity Procedia 00 (2017) 000–000

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availability of high resistance materials. However, the increase in the size of civil structures makes them more subject to fatigue and to wind action. Major international standards such as EN 1993-1-1:2005+A1:2014, Annex C have recently reinforced the request to consider the fatigue phenomenon for the design of large steel structures. The size of the structure it is implicitly part of the concept of Consequence Class and the large size almost automatically leads to fatigue calculations. In order to obtain the local stress in correspondence of a joint of a large structure, FEM model reduction techniques such as Guyan or Craig-Bampton methods (Bampton and Craig, JR. (1968)) need to be used - even if not many structural software implement superelements sub-modeling efficiently. Whereas an order of magnitude of approximately 5000 tons of steel for a large steel structure, the number of welds can be estimated roughly at a ratio 1-10, and then a number of welds equal to about 50000 can be expected. As shown in Colussi et al. (2017), a complete nominal stress based fatigue analysis all around a welding would require 20 checks, each of which could potentially be characterized by a different load spectrum, so this type of calculation easily becomes computationally demanding. The size of the structure also dramatically affects the potential number of imperfection as shown in Davidenkov et al. (1946). In order to guarantee the safety of a structure with respect to the fatigue phenomenon it is, in the end of the process of fabrication, necessary to perform inspection on the welds in workshop, where the inspection class is function of the fatigue utilization factor of each weld, as indicated by prEN 1090-2:2016, Annex L, Table L.1, and the utilization factor is based of fatigue calculations. The inspection may, eventually, lead to repair as explained in Jonsson et al. (2016), Thus, although the calculation of fatigue of large steel structures a has been a widely studied topic, there are still present several challenges to the industry, especially when the size of the structures becomes a relevant factor. Moreover, as the fluctuating nature of wind load generates the fluctuating stresses in structures, it becomes necessary to consider the phenomenon of wind-induced fatigue in the design of steel and aluminum structures. The introduction of fluctuating action of the wind introduction represents a very complex challenge because, as reported by many authors, (Holmes (2002)) the treatment of the damage under the dynamic loading of wind is still a not sufficiently developed subject. This consideration is true both from the scientific point of view, and from the regulatory point of view. In fact, a practical tool which enables to determine wind loads for the fatigue analyses of large steel structures, is not yet available to the engineering community. Among the (few) tools provided by Eurocode it should be noted the EN 1991-1-4 Annex B, Clause B.3 where an equation is provided an equation that calculates the number of cycles of an effect of wind during a period 50 years. Such equation does not, however, take into account any information about wind features and structural response (except for the effect S k due to a 50 years return period wind action). Some approaches have been proposed for a more accurate evaluation of wind-induced fatigue loading of steel structures and many of these methods makes use of wind tunnels to evaluate relevant features of the flow. Recently many Computational Fluid Dynamic (CFD) methods for the simulation of turbulence have gained appreciation for their ability to correctly represent flow characteristics with reduced cost in comparison to wind tunnels. Therefore, their use could potentially be extended to conduct fatigue analyses of steel structures exposed to dynamic wind action. Following a literature search, no cases of use of turbulence models for CFD to conduct fatigue analyses have been found by the authors. It is therefore of interest of this paper make a first step to link the topic of fatigue calculations of large steel structures to the subject of CFD. A brief review of the turbulence models for CFD that have been successfully applied to civil applications is here provided. These models are presented from the perspective of their use in place (or in combination) of the wind tunnel in order to perform fatigue analysis of the structure. To conduct this evaluation, we mention some studies in the scientific literature in which a test in the wind tunnel have been performed in order to calculate features of the flow that lead to the wind-induced fatigue spectrum. Consequently, we observe the key flow properties that a CFD model needs to grasp in order to use them for this purpose. The purpose of this paper is not therefore to carry out a comprehensive review of the turbulence models for CFD. The description of the turbulence models is reduced to the essential and the same applies to the methods for the assessment of the fatigue spectrum.