Comparative structural analysis of hinged jointed and fixed jointed geodesic domes with respect to AISC 360-10 specificatons

Since the ancient times, doctrines of structural design and structural member design have changed frequently. These changes have in common in the aspect of their aim. All changes have been made in order to improve the design and to fulfill the recent requirements of the design trends. Designing harmoniously with the structure and with the nature of the structural material is crucial to get the integrality in the design. Compatibility of the design with the nature also enables to get an optimum design in the aspect of building costs. In the history of the structural design and the structural member design, mankind has always pursued to get more effective and more natural behaved structures. The first form of this pursuit can be claimed to be the arch concept. With the accumulation of the knowledge obtained from the arch designs, a more complex form, "dome structures" showed up in the scene. As the dome designs had been elaborated, structurally more efficient types of the domes were formed. Speaking of structurally more efficient dome forms, the geodesic dome types appeared. Geodesic forms and geodesic dome structures are formed with the inspiration of molecular forms of the materials, the atomic forms, honeycombs, crystal structures, etc. The modern types of the geodesic forms and especially geodesic domes have been derived from the early studies and works of Sir Buckminster Fuller in 1950's. He has many patents and projections about geodesic forms and this visionary man's contribution to this knowledge have been enlightening the related studies, even today. In this study, the idea of geodesic forms is explained. The frequency concept which determines the spherical features and the stability behavior of the dome structure is defined. Examples of well known geodesic dome structures in the world and related to this, examples of their joint connection types are given. This thesis is prepared to explain the geodesic dome concept, define the natural behavior of the geodesic structure, explain the method of forming a geodesic structure and make a comparison between two analysis outputs of alternative geodesic structural analysis models with same polyhedron frequencies, load definitions, load application approaches, geometry and with different connection types. In this study, two similar "12V frequency" geodesic domes which have "50m" in diameter and "15.2m" in height formed and modeled with the "Geodesica" software. The output file from the software was imported firstly to AutoCAD software and then to SAP2000 analysis software. With this procedure, the geometry of the analysis models are obtained. In addition to this, manually modeling & normalizing the geodesic forms are explained using diverse frequency models. Forming the geometry is explained step by step. In the load definitions process, "snow, wind" loads are obtained from ASCE 7-10 (Minimum design loads for buildings and other structures). For determining the seismic loads, DBYBHY 2007 (Specification for structures to be built in disaster areas) is used. For temperature difference loads (self straining loads) an assumption is made. The load combinations are taken from the combination tables of ASCE 7-10 (Minimum design loads for buildings and other structures) and derived to bring in compliance with the analysis model. Applications of loads to points, areas and joints are monitored to clarify the procedure. All loads are applied to the joints of the analysis model except for the self straining forces (loads from temperature differences) In the analysis part; response spectrum analysis, modal analysis and buckling analysis are performed for both models. Regarding to the outputs of these analysis, the frame sections are optimized. As the sections are optimized, the results are collected to be used in the calculations. For the analysis model with hinged joints (M2-2 and M3-3 released), the calculations of stress checks are made with respect to AISC 360-10 (Specification for structural steel buildings) and allowable stress design (ASD) criteria. Frames are calculated in two stress conditions. Which are axial pressure and tensile. Modal frequencies and buckling behavior outputs are collected to be used in the results section. For the analysis model with fixed joints (with no moment releases), the calculations of stress checks are made with the stress check module of SAP2000 analysis software. Besides, "all load related" and "structural characteristics" of the model is made with respect to ASCE 7-10 (Minimum design loads for buildings and other structures) and AISC 360-10 (Specification for structural steel buildings), stress check modules are compiled and the stress analysis & checks worked properly. After determining the stress values of the structural members for both analysis models, a sample hinged connection design is given in order to monitor the design criteria of AISC 360-10 (Specification for structural steel buildings) for joints and welds. After all process is completed, a clear result emerged. The geodesic dome with the hinged connection members work properly and distribute loads, forces and stresses between the structural members with a high performance. The seismic performance and structural stability is solid. However, with many hinges in the connections, the buckling analysis showed that the structure is slightly vulnerable to the buckling loads. In order to increase the buckling factor of the structure, greater cross sections for the frames are selected. The increased sections are located on the lower/bottom elevations of the structure. Due to this increase in the cross sections, the weight of steel structure also increased and the capacity/stress ratios declined which makes the system members work with a lower capacity ratio due to the specified loads. The geodesic dome with fixed connection members work properly and distribute loads, forces and stresses between the structural members with a high performance as well as the analysis model with the hinged jointed frames. The seismic performance and structural stability is solid. In the buckling analysis, great numbers for factor values are obtained. This system is compiled with frame sections with lower cross sections. Analysis model with fixed joints gives an output of total material weight significantly lower when compared to the analysis model with hinged joints. However, producing and mounting the fixed connections in a geodesic dome is a really big challenge. The production and erection processes of this type of structure should be performed with a really high precision in order to get a structural behavior as simulated in the analysis model. Selecting the joint type for the frames will be made not only with respect to the weight and primary costs of the building but also to the mounting & erection issues and secondary costs.