Ideal Gas in Structural Analysis
In theory, an ideal gas consists of freely moving mass particles without extension in a volume space. In this space, each particle moves at a speed in one direction. The collision of one particle with another particle or the volume limitations leads to a deflection and a change in the speed of the particles.
The state of an enclosed gas can be described by the presumptions of the thermodynamic equilibrium. This results in the following general gas equation:
p ∙ V = n ∙ R ∙ T
where the state variables
p is the pressure
V is the volume
n is the is the amount of substance
R is the universal gas constant
T is the temperature
Ideal Gas Properties
When keeping certain state variables constant in the general gas equation, special properties of the ideal gas arise. It is useful to know these properties for using ideal gases in the structural analysis, and it helps to simulate certain load conditions accordingly.
Isothermal Change of State (Boyle-Mariotte)
If variables T and n are constant and acting pressure p is increased, volume V of the considered gas unit is reduced.
Isobaric Process (Gay-Lussac)
If variables p and n are constant and acting temperature T is increased, volume V of the considered gas unit increases.
Isochoric Process (Amotons)
If variables V and n are constant and acting temperature T is increased, pressure p of the considered gas unit increases.
Application in Structural Analysis
In structural analysis, encased gases are usually used to transfer the external forces. In this case, it is required that the force acting locally on a certain location on the volume casing is transported by the enclosed gas to all other sides of the volume casing.
This property is used, for example, in insulated glass panes or inflated membrane cushions. In both cases, the volume casing is described by the load-bearing elements and filled with a gas. The volume limitations consist of rigid shell elements in the case of insulated glass panes, and flexible membrane elements in the case of membrane cushions. However, to use one example, wind or snow load acts on one side of the volume limitation in both cases, and is transferred by the enclosed gas to the adjacent volume limitations.
Since the temperature does not change suddenly in the load situations considered in structural analysis, the ideal gas with isothermal properties is usually simulated in the volume casing.
Implementation in RFEM
Solids can be defined in RFEM. These solids are described by the surrounding surfaces. In this type of volume cell of the surrounding shell and solid components, the solid definition can be inscribed with the gas type. This resulting gas volume requires describing the enclosed gas and determining the atmospheric state variables. The atmospheric state variables have no impact on the enclosed volume and only describe the initial situation for the simulation.
In the assigned load cases, the corresponding solid load can be applied for each gas solid. To simulate open or closed solids, it is possible to specify the resulting pressures/volumes or the pressure/volume changes.
Dipl.-Ing. (BA) Andreas Niemeier, M.Eng.
Mr. Niemeier is responsible for the development of RFEM, RSTAB, and the add-on modules for tensile membrane structures. Also, he is responsible for quality assurance and customer support.
Do you have further questions or need advice? Contact us via phone, email, chat, or forum, or find suggested solutions and useful tips on our FAQ page, available 24/7.
With the activated option "Topology on form-finding shape" in the Project Navigator - Display, the model display is optimized based on the form-finding geometry. For example, the loads are displayed in relation to the deformed system.
- Although I have modeled two identical structural systems, I obtain a different shape. Why?
- How is it possible to make factorized combinations of a dead load in the context of form‑finding?
- I want to create a mapped mesh for a circular hole plate. Is it possible to generate such a mesh in RFEM?
- It seems that the members stay not deformed after my RF‑FORM‑FINDING calculation. What did I wrong?
- How do I model a tent roof with two cone tips?
- How do I model a suspended membrane roof structure with line supports?
- I would like to calculate and design "flying structures". What do I need for this?
- I have already divided my membrane roof into individual surfaces. Can I quickly create a cutting pattern from these surfaces?
- How is an inflatable object simulated in RFEM?
- Why does the FE mesh of my conical membrane look angular after the form-finding?
Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements