The pseudo-spectral acceleration (Sa) used as input for the seismic response spectrum is calculated according to the following equation: In terms of the FEA model, a coupled modal analysis plus response spectrum model is required. Last but not least, we have the earthquake load, defined as the horizontal and vertical loads related to the response of the structure to seismic motions, and are calculated using the response spectrum method. More details about the dynamic wind can be seen in this article. A more complex CFD fluid structure interaction model should be used in this case to assess the structure-vortex interaction in order to then evaluate the stresses produced. “ASME STS-1” standard contains a set of rules to calculate the dynamic wind response. It is defined as the load produced by the oscillations generated by the wind-structure interaction. The other wind load is the dynamic one, and it has to be considered due to the lightweight and flexible nature of steel stacks. This pressure is obtained using standard’s equations, and usually depends on factors as the local wind speed, the geometry of the structure and the height of the point where the pressure is being applied. The first one represents the action of the constant part of the wind pressure and is calculated using a static FEA model where a height dependant pressure load is applied to upwind and leeward surfaces. Wind load can be divided in two different loads: static and dynamic. The second common load is the wind load, or better said, are the wind loads. A static structural FEA model can be used to precisely calculate the stresses induced by this load on every section of the stack. Thus, in this case unlike the following ones, effects that reduce the total mass such as corrosion have not to be taken into consideration to not underestimate the stresses induced by the dead load expected in a real situation. A constant when calculating stresses for different loads is that the worst situation is the one to be considered. The first one is the dead load, and can be defined as the weight of all material incorporated into the structure, including its fixed permanent equipment such as ladders, platforms, etc. Next, a summary of three of the most common loads shared for the most part of the projects is presented: Once the load combinations are defined and the most relevant loads are identified, it’s time to calculate each of these values separately by using a finite element analysis (FEA) method over a detailed 3D model. The whole set of load combinations should be taken from the standards, from project’s characteristics or from customer’s requirements. Standards as “ASCE 7” or national regulations define the basic group of combinations to be considered during the assessment of the design, for example: The first step is to determine the load combinations that the structure is going to be subjected to. That is a common design strategy through many sectors the difference here is the forces acting over the structure are somehow easy to misunderstand. The main goal to achieve for the structure is to keep its design stresses under certain load combinations below a maximum allowable value. In this article a brief outline about the designing process and considerations is provided based on the experience in FEA use gathered by the SDEA engineering team along different projects. This is particularly important for those located in areas subjected to heavy environmental loads, such as high activity seismic zone 4, due to the slender nature of this kind of structures thus low resonant primary modes prone to be easily excited by seismic or wind actions. When designing steel stack structures, several rules from standards as “ASME STS-1” or “ASCE 7” have to be taken into consideration to assure security during its useful life.
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