Eigen Value Analysis Of An Antenna Supporting System Including Soil-Structure Interaction Behavioral Aspects – A Holistic Approach

Abstract

With the advent of high speed digital computer and with the volcanic proliferation in the domain of hardware and software it is now possible to handle real life complex structures along with the soil mass. This type of studies are very important for the complex Radar antennas, High speed tracking type earth-station antennas, Microwave transmission towers and large sized telescopes for deep space missions; where the structures have strict and stringent displacements and rotation compliances to be met with.

In this piece of work, an attempt is made to study the given structure in its totality particularly in the dynamic analysis domain. As per the standard literature and interaction with IIT, Roorkee, the fact dawned upon us that for such structures with stringent rotation and displacements compliances a more reliable theoretical as well as experimental approach has to be undertaken to generate the confidence level in the design before construction is taken up.

As per the instructions, IIT-R, the theoretical approach suggested was to study the elements of the structure in the component approach and then take all components together for the system approach. This system approach of all the components of the structure when taken together along with the soil mass gives the whole scenario. This whole scenario of predicting the natural frequency of the entire system by incorporating the flexibility of a large volume of soil mass is termed as holistic approach in determining the natural frequencies of the entire structural system. First few modes are the high-energy modes, which are having maximum amplitude and our study is particularly from the point of view of bending, compression and torsion modes.

In this piece of work, the case study is picked up regarding a high speed earth station antenna support frame, which can be either a RCC pedestal or can be RCC framework, supported on a suitable raft. Various design options were to be exercised for coming out with the right choice of the support frame topology in order to optimize the design. RCC pedestal option was taken up by the project whereas it was decided to study the RCC framework in this piece of work.

The approach synthesis for the final submission of the dissertation was decided as follows: 1.For the holistic analysis the scope of the present work is not to model the antenna system but consider the translational and rotational masses of the antenna structure on the RCC framework at center of gravity locations using massless beam concepts. 2.Consider the RCC staging as the standard SP-22 (S & T)-1982 problem in order to ratify the frequency results obtained in the theoretical study as given in the IS code using standard Stodola approach. Boundary conditions were exercised at the frame ends assuming that the base is of infinite stiffness as per the full fixity condition assumed by the code. 3.Consider the Finite Element Modeling of raft and soil mass and consider the effect of soil mass when it gets interacted with superstructure especially in the dynamic analysis domain. Parametric study was exercised in order to get suitable model of raft and soil mass for the holistic analysis.

Accordingly in the treatise, chapter-1 introduces the problem, and the objective of the study. Chapter-2 gives the details of literature review. Chapter-3 deals with the theoretical aspects regarding an eigen value analysis. Chapter-4 incorporates the study of behavior of an antenna supporting structure. Chapter-5 deals with the basic theoretical aspects of Finite Element Modeling. Chapter-6 incorporates the influence of soil on the behavior of supporting structure. Chapter-7 gives the details of a holistic approach. Chapter-8 conclusion of study is drawn Chapter-9 Future scope of study is given.

For this submission, the scope of the work was to generate the finite element model of the RCC staging framework as per the topology given in the SP-22 problem, to generate the FE Model of raft and soil mass.

The results were obtained after several iterations by exercising different boundary conditions and modeling techniques. As the in plane rotational stiffness of the beam element is relatively lesser than the rotational stiffness of plate as a continuum, it was necessary for us to physically incorporate the additional rotational constraint to improve the FEM modal and make it mathematically stable.

On this model the translational and rotational inertia of the superstructure was modeled for considering the effect of antenna coming on the slab. The detailed holistic approach has been taken up as a part of the final dissertation.

The aim or objective of study is to find out by what extent the natural frequency falls when the soil mass is incorporated in the model. This becomes important for the structure mentioned above which have specific azimuth & elevation servo motors. This study is carried out to avoid resonance problem for high speed tracking antennas & radars mounted as the RCC frames / pedestals.