INFLUENCE OF JOINT COMPLIANCE ON THE BEHAVIOUR OF SPACE STRUCTURES

[yanburak]

INFLUENCE OF JOINT COMPLIANCE ON THE BEHAVIOUR OF SPACE STRUCTURES

Chapter 1 introduces space frames and double layer grids in particular, with the advantages of using double-layer grids. Jointing systems available around the world are briefed and the node connector used in the present study is introduced with a briefwrite-up on its advantages and disadvantages. This chapter also explores the available literature and, the scope and objectives of the thesis are mentioned.

Chapter 2 introduces 3D finite element models of the hollow spherical and octahedral nodes. The stiffness matrixes of these nodes have been derived by conducting analyses on the computer for six sizes each of the Octa and spherical nodes. Using the stiffness matrix of the node, a new method of incorporating this into the regular analysis of a space truss has been developed. The new method proposed yields realistic values for the forces in the members and takes into account the elastic deflections in the node under the action of member forces. Implementation of the proposed method has been carried out by a custom program using state-of-the-art object oriented programming techniques. A sample problem has been analyzed using this program to demonstrate the effect of including joint flexibility. The effect of flexibility of nodes on the effective length of compression members in double-layer grids has been evaluated. The effect of compliance on the dynamic characteristics of a space frame has also been evaluated for the sample space frame with flexible joints. The analysis program has been modified to
evaluate the natural frequencies of the system using rigid or flexible nodes.

The study of the Octa node and spherical node under the action of uniaxial compression and tension dominates the contents of Chapter 3. The two types of nodes have been analyzed using commercially available finite element software considering material nonlinearity. The stress patterns from the analyses have been examined thoroughly. Two consistent methods for fixing the load at yield in both uniaxial compression and tension have been proposed using the load-displacement curve. Yield loads for all the nodes have been evaluated using both the methods and the results agree well between the two methods. Three material yield values have been selected for each of the node size for evaluating the yield values viz. 240,320 and 415 MPa. The members of a double layer grid are connected to the nodes by bolts and holes are drilled in the nodes for this purpose. The bolthole patterns differ between two popular types of double-layer grids. Both these bolthole patterns have been modeled separately in the above exercise and the results for these two have been shown to be approximately the same. The effect of varying diameters of the boltholes on the response of the nodes has been examined. Relationships between the yield load, diameter, thickness and material yield have been developed using the method of least squares. The differences in the behaviour of the nodes under uniaxial compression and tension have been discussed. Ramberg Osgood type of relationships have been worked out for all the load-displacement curves obtained from the analyses. The simulation of non-linear behaviour of nodes with cracks with
plastic crack closing forces have been carried out with useful insights into the behavior of the two types of nodes in uniaxial compression and tension.

Chapter 4 is devoted largely for studying the two types of nodes under the influence of biaxial load combinations. The combinations studied are dual compression, dual tension and compression-tension. In all cases equal loads are applied along two orthogonal, directions in the horizontal plane. Stress patterns have been examined for each type of load combination and yield values for each case have been obtained using one of the methods proposed in chapter 3. These have been compared with the corresponding
uniaxial values in both compression and tension. Some useful inferences have been possible by studying the behaviour of the nodes under the various biaxial load combinations. In each case, relationships between the biaxial yield load, uniaxial yield load, diameter of node, thickness of node and material yield of node have been obtained using the method of least squares. The nodes have been analyzed under some selected Multi-axial loading and combinations of load which cause yield based on the second method proposed in Chapter 3 have been obtained and tabulated. However, a proper and thorough study of the nodes under multi-axial loading proved to be beyond the scope of this thesis.

Chapter 5 contains the contributions made towards developing new methods and algorithms for obtaining the several results of chapters 2,3 and 4, using object oriented programming (OOP) techniques. The contributions have been in Object Pascal, the underlying language of Delphi, a popular RAD tool developed by Borland/Inprise of USA. Several new modules have been developed to reliably handle the large amounts of data generated by the hundreds of analyses detailed in chapters 2,3 and 4. The ease with
which new methods were possible to be incorporated into existing software using OOP has been demonstrated, with source code exampIes. Comparisons with other types of tools available and the advantages of using OOP have also been demonstrated using the experience during the preparation of this thesis. A strong case for OOP as an indispensable tool for the researcher has been made.

Chapter 6: Several important conclusions and suggestions for future work have been made.

483 pages

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