THREE DIMENSIONAL ANALYSIS OF LIGHTING TOWER FOR A SPORTS STADIUM

Abstract

Since the Ninth Asian Games held in New Delhi during the year 1982, the sports activity is continuously developing in our country. There is the growing need for the construction of more sports stadium to provide the facilities for various sporting events. The provisions of flood lighting in s. sports stadium is also considered necessary to make the stadium useful during night time. Generally, the structures used for flood lighting are either a concrete tower or a lattice steel tower. These towers are comparatively lighter structures and hence the seismic loads are not significant in the design of these towers. The wind loads invariably govern the design of these towers. In this dissertation, the analysis of an actual lighting tower for a sports stadium has been carried out. The stiffness matrix method has been used for the analysis. The tower consists of a main inclined boom with one end being pinned at ground level and supported by a prop' at the middle. The cantilever end of the boom is having a head frame for fixing the flood lights. The analysis of this tower is performed first by considering the structure as a space truss, but as the structure is made of welded connections, finally the tower is analysed as a space frame with six degrees of freedom, at each joint. A comparative study of the results of space truss and space frame analysis is presented in order to bring out the secondary effects. The analysis of the tower is done for different loads i.e, dead load, Live load, transverse and longitudinal wind (iii) loads. The wind loads have been considered as per the IS:875 (Part 3) - 1887. The temperature effects may also be significant in this type of structure. But special provisions are made at the top of supporting structure so that the boom can freely move in case of temperature variation. So, the temperature effects will not be of a high order. To study the dynamic response of the tower under wind, gust factor method has been used, as specified in the 1 S code. In this method, the equivalent static forces are obtained using the dynamic propertie.s of the structure and wind such as fundamental frequency of the structure, damping coefficient, hourly mean wind speed etc. In case of lattice steel towers, this method is appropriate as these towers are susceptible to the along-wind oscillations, where as concrete towers are generally prone to cross wind oscillations. The study shows that the primary forces are the axial forces in this structure. The forces are maximum above the prop support and reduce towards the two ends of the tower. The forces are maximum due to dead load. The forces due to wind load are approximately 60-70 percent of the forces due to dead load. The dynamic effect of wind increases the forces about 5-8 percent. The secondary stresses obtained in space frame analysis are mainly due to the bending moments. The stresses due to shear forces and torsion are not very high. Thus, it is necessary to consider the secondary stresses in the design of the members. Therefore, the structure having welded connections must be analysed as a space frame to investigate the secondary effects.