Performance of shell foundations under static loads

To make safe and economical designs in engineering constructions is considerable significant. In soil mechanics and geotechnical engineering; especially soils with low bearing capacity and soils with settlement potential, the choice of the foundation system plays an important role both in terms of safety and economy. Foundations are the nonvisible section of engineering structures. However, they are the most important part of structrues. Theirs function is to transfer the load of the structure to the soil. A properly designed foundation transfers the load throughout the soil without overstressing the soil. However, geotechnical and structural engineers who design foundations must evaluate the two conditions together. These conditions are as follows: •Foundations must possess sufficient bearing capacity to safety against shear failure in soil. •Amount of settlement attached to structurel load must be within allowable limits as so not produce cracks and damage to a building. Shell foundations perform better in terms of bearing capacity and settlements compared to flat foundations when examined geotechnical behaviour in terms of the soil–structure interaction beneath the foundations. Shells are outstanding performers about to transfer the load of the structure to the soil. Their main advantage is closely related to their lightweight nature since, by definition, they are thin–wall structures requiring reduced quantities of concrete material in their construction. Consequently, the American Concrete Institute has defined a thin shell according to ACI 318–19 as: "A three–dimensional spatial structure made up of one or more curved slabs or folded plates whose thicknesses are small compared to their other dimensions. Thin shells are characterized by their three–dimensional load–carrying behavior, which is determined by the geometry of their forms, by the manner in which they are supported, and by the nature of the applied load." Reinforced concrete shell footings have been used since 1950s and have been subject to much research, especially due to the heavy superstructure loads perform better in transporting them to the soils with low bearing capacity. Numerical and experimental studies have shown the advantages of shell foundations in terms of geotechnical. Experiments have shown that shell foundations sitting on soft soils are more economical than flat foundations. Shells which arched, circular, triangular, conical, hyperbolic paraboloid, cylindrical, spherical dome, pyramid, square and strip type are used commonly shell foundations of structure. Nowadays, reinforced concrete shell systems are being used in Russia and also the United States, France and Kenya, mainly in Mexico and India. There are numerous successful shell foundation applications in and around the city of Calcutta, particularly in the Mexican city with soft loose deeper alluvial soils and under the Ganges River. Reinforced concrete shell systems are benefit economically on soft soils compared to other alternatives (raft, pile system, soil improvement etc.). Therefore, reinforced concrete shell systems are expected to spread in different countries. The standard for the design and construction of conical and hyperbolic paraboloid shell foundations in India was published in 1980. The performance of shell foundations in terms of bearing capacity and settlements is usually done with comparing it with the performance of flat foundations. The bearing capacity of the shell foundation which has the same dimensions in plan as the flat foundation is greater. The settlement of shell foundations is less than flat foundations under the same load. The reasons for main differences can be listed below: •Rigidity of the foundation •Contact pressure distribution •Soil stress distribution •Failure surfaces of soil under footings •Shell–soil contact surfaces •Geometry of the foundation. In addition, the shells exhibit a higher resistance to horizontal loads than the flat foundations due to rigidity of the foundation and large shell–soil contact surfaces, so that the shells will perform better in earthquake zones. The load transfer in the flat foundations is done with bending moment only. On the other hand, in the case of shell foundation systems, the axial normal forces and shear forces that occur in the average surfaces in addition to bending do the load transfer. In this way, it is possible to obtain ideal solution for large spans. By using the shell foundations, it is possible to form foundation systems, which are more suitable not only in terms of strength but also in terms of rigidity. This thesis introduces shell foundations in general and different types shell foundations in particular as an alternative to the conventional plain foundation. The different types of shells that may be employed in foundations are introduced with their geometry and applications under different situations. Shell foundations can be used as combined or isolated footing. This study explains a study on the geotechnical behavior of shell foundations using a nonlinear finite element analysis with a finite element code, PLAXIS. For the analysis of the finite elements under the static loads of the shell foundations, five different models, one of them flat foundation, were created and loose, medium–dense and dense sands were used. Numerical data obtained from analyzes are presented graphically and comparative evaluations are made. However, the shell efficiency factors and the settlement factors, which express the bearing capacity and settlement differences between the shell and the flat foundation, were calculated. The shell foundation is found to have a better load carrying capacity compared with the conventional flat foundation, which has the same dimensions in plan. It was also the settlements for shell foundations were less as compared to those under flat foundations. From the observations it is evident that the shell footings exhibit a higher load carrying capacity and good settlement characteristics as compared to those of its flat counterparts and thus shows to show that shell foundation could be considered as an economical alternative to conventional flat foundation. The aim of this thesis study; is an introduction to the foundations applications of reinforced concrete shells which will be an alternative to flat foundations used in their foundation designs. Thanks to the reinforced concrete shell foundation, settlements in the soil will be reduced in engineering structures where the superstructure loads are high. Furthermore, the higher the bearing capacity will be designed. In addition to these geotechnical advantages, economical savings will be achieved.