# Numerical Analysis of the Bottom Thickness of Closed Rectangular Tanks Used as Pontoons
## 1 Introduction  
Tanks as civil engineering structures have been used in numerous branches of industry and the economy for many years. Applications of tanks as civil engineering structures include swimming pools, fire tanks, clean water tanks, waste tanks and tanks for fuel rectification products. As there are many applications of these types of structures, the analysis of their static work is important [1]. The most frequently designed and produced tanks have a constant wall thickness, regardless of whether their cross-sections are rectangular or circular. If they are closed tanks, the cover is usually a plate based freely on the walls, whereas the bottom is a bottom plate that is usually thicker than the tank walls. The contact between the bottom plate and tank walls is assumed to be rigid in static analysis. Tanks with walls of variable thickness (i.e., of trapezoidal cross-section) are very rarely designed despite their optimal adjustment to stress distribution (from the point of view of calculation schemes). For tanks hydrostatically loaded, the load acting on walls increases with the depth (of foundations), and triangular load distribution means that the highest values of bending moments in the vertical cross-section occur at the point where the wall connects to the bottom, while the upper free edge of the tank has a zero value. Thus, structural and economic considerations should determine the selection of walls with a thickness that increases accordingly with the immersion of the tank [2, 3]. Changes ininternal forces occurring along the longer wall of the tank also apply to bridge structures, where the interaction between incoherent backfill (similar to a fluid) and spandrel walls is studied [4; 5]. Floats used for constructing floating platforms can be made of plastic, steel or aluminium sheets, as well as reinforced concrete. These structures often consist of two elements: a box and a cover plate. As a result, they can be treated computationally as rectangular open tanks with the upper plate resting freely on walls. The solution carries either the risk of inaccuracy in assembly or possible leaks and faster wear in use [6]. A better proposal, although it is more difficult in terms of calculation and performance, is to design and construct a monolithic reinforced concrete tank concreted entirely in one stage [7]. The structure is then computationally treated as a closed rectangular tank. Reinforced concrete pontoons intended for the construction of floating platforms should have adequate buoyancy and be stable; i.e., they should not show excessive tilt at partial load [6; 8]. Stationary floating structures are becoming increasingly popular around the world, stimulated by various reasons [9]. It is estimated that there are 40,000 to 50,000 such structures [10]. They are considered to be architecturally attractive with pleasing locations. On the other hand, their construction is associated with changes in water levels [11] or with high population density, and they are seen as a way of ensuring enough living space for people. Severe and rapid meteorological phenomena such as typhoons, hurricanes and floods affect many areas of the world. Climatologists predict that water levels will have risen by a meter by the end of the century. Amphibious construction is one of the building solutions oriented towards the urbanisation of floodplains. However, it is necessary to recognise the statics of their construction so that 'houses on water' can become a viable alternative to traditional construction [12]. The purpose of this article is to analyse the statics of closed rectangular tanks, the distribution of internal forces and the impact of bottom thickness on the values of occurring bending moments, the buoyancy and the stability of the pontoon. The subject matter of plate structures is well known in engineering literature. As for the design of tanks, scientific publications most often refer to open tanks, in which calculations are made using the finite element method.