FSI STEEL STACK STRUCTURAL FEA ANALYSIS

Chimney miniature

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. 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. In this article a brief outline about the designing process and considerations is provided based on the experience gathered by the SDEA engineering team along different projects.

The main goal to achieve for the structure is to keep its design stresses under certain load combinations below a maximum allowable value. 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 first step is to determine the load combinations that the structure is going to be subjected to. Standards as “ASCE 7” or national regulations define the basic group of combinations to be considered during the assessment of the design, for example:

Load Combinations

The whole set of load combinations should be taken from the standards, from project’s characteristics or from customer’s requirements.

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. Next, a summary of three of the most common loads shared for the most part of the projects is presented:

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. A constant when calculating stresses for different loads is that the worst situation is the one to be considered. 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 static structural FEA model can be used to precisely calculate the stresses induced by this load on every section of the stack.

The second common load is the wind load, or better said, are the wind loads. Wind load can be divided in two different loads: static and dynamic. 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. 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 other wind load is the dynamic one, and it has to be considered due to the lightweight and flexible nature of steel stacks. It is defined as the load produced by the oscillations generated by the wind-structure interaction. “ASME STS-1” standard contains a set of rules to calculate the dynamic wind response. 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. More details about the dynamic wind can be seen in this article.

Static wind profile

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. In terms of the FEA model, a coupled modal analysis plus response spectrum model is required. The pseudo-spectral acceleration (Sa) used as input for the seismic response spectrum is calculated according to the following equation:

Spectral acceleration

Where g is the gravity acceleration. R and U are factors that depend on the structure itself, being R the seismic force reduction factor that depends on the geometric characteristics of the structure and U the use factor, which defines the category of the building depending on its function and importance. The ground environment is defined by the factors Z and S. The seismic zone factor Z depends on the magnitude of the seismic forces presented on the region where the stack structure is going to be standing, and the ground factor S is a value used to define the geological conditions of the terrain.

U.S. Seismic Zones Map

The last factor C is the seismic amplification factor. It is the non-constant value that provides Sa with the frequency dependant behaviour. An example of a real pseudo-spectral acceleration is showed in the next graph:

Seismic pseudo spectral acceleration

Both the dynamic and earthquake loads have a paramount importance when designing steel stack structures due to its slender and flexible nature. Frequency excitations close to the structure’s natural mode of vibration can lead it to failure because of the appearance of resonance phenomenon. To mitigate this effect, cost-effective solutions as tuned liquid dampers (TLD) are widely used. More information about TLDs can be found in further articles.

Once every relevant load is calculated and combined according to appropriate normative, the maximum allowed values have to be obtained to assess the design, with the idea of improving it in case of not meeting the requirements, or optimizing it reduce the project expenses! These value’s calculation methodology is described in standards such as “ASME STS-1”, and are directly related to the structure’s parameters such as diameter, plate thickness, material properties, etc. Various values have to be obtained depending on the project conditions, being the most common maximum allowable values for longitudinal compression and the longitudinal compression and bending combination.

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