Response Study on Earth Station Antenna Support Structure by Simulating Actual Earthquake Spectra

Abstract

Complex real life structural analysis problems can now we handled easily using a galore of powerful computer hardware packed and zapped with the bewitching computer graphic facilities. Better insight into the behavior of the structural aspects can be now explored, up to the hilt. In addition to that, the engineering ingenuity helps in actually zeroing down on the keen issues related to design of complex structure with respect to the given design specifications.

With the advent of high speed digital computers and with the volcanic proliferation in the domain of computers, it is now possible to handle real life complex structures along with the soil mass. These 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 displacement and rotational compliances to be met with.

In this piece of work, an attempt is made to study the given antenna support structure in its totality particularly in the Eigen value domain including the detailed studies of the structural response due to the Bhuj earthquake spectra. As per the standard literature and interaction with IIT- Roorkee, the fact dawned upon us that for antenna support structure with stringent rotational and displacement 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 of 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 torsional modes, and their structural response due to the actual earthquake spectra of Bhuj.

In this piece of work, the case study has been picked up regarding the suitability of a building frame, vis-a-vis, the conventional robust conical massive concrete pedestal as an antenna support structure of high speed tracking antennae, under earthquake forces.

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 the center of gravity locations using mass less 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. 4. Investigate the structural dynamic response of the frame w.r.t the actual earthquake spectra for Bhuj experienced in 26th January 2001.

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 incorporates the theoretical aspects regarding analysis. Different methods for dynamic analysis and discussion about the response spectrum analysis Chapter-4 deals with the basic theoretical aspects of Finite Element Modeling.. Chapter-5 deals with the Eigen value analysis of an antenna supporting structure i in component approach as well as in holistic approach.. Chapter-6 deals with the study of response in both x-direction and z-direction in component approach for two different forcing functions. Chapter-7 deals with the study of response in both x-direction and z-direction in holistic approach for two different forcing functions. Chapter-8 summary and conclusions of the study are drawn. Chapter-9 further development or future scope of the study is drawn.

For this submission, the scope of the work is to generate the finite element model of the RCC staging framework as per the topology given in the SP-22 problem. This model of staging is further to be integrated with the FE Model of raft and soil mass and objective is to study the structural response of the holistic model and compare w.r.t the pointing error specification of the antenna and suggest the suitable foundation.

On this model the translational mass of the antenna structure is 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 including the response studies w.r.t IS: 1893 and Bhuj earthquake spectra.

The aim of study is to find out the suitability of the tall building frames for mounting the earth station antenna in the earthquake region. The structural responses are also studied as per the advice of IIIT-R in order to check the pointing error specification for the antenna structure. This study is also carried out to avoid semi resonance problem for high speed tracking antennas & radars mounted on the RCC frames / pedestals, during earthquake time.