Eindhoven University of Technology MASTER Lateral stiffness of hexagrid structures de Meijer, J.H.M. Award date: 2012 Link to publication Disclaimer This document contains a student thesis (bachelor's or master's), as authored by a student at Eindhoven University of Technology. Student theses are made available in the TU/e repository upon obtaining the required degree. The grade received is not published on the document as presented in the repository. The required complexity or quality of research of student theses may vary by program, and the required minimum study period may vary in duration. General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain ‘Lateral Stiffness of Hexagrid Structures’ - Master’s thesis – - Main report – - A 2012.03 – - O 2012.03 – J.H.M. de Meijer 0590897 July, 2012 Graduation committee: Prof. ir. H.H. Snijder (supervisor) ir. A.P.H.W. Habraken dr.ir. H. Hofmeyer Eindhoven University of Technology Department of the Built Environment Structural Design Preface This research forms the main part of my graduation thesis on the lateral stiffness of hexagrids. It explores the opportunities of a structural stability system that has been researched insufficiently. The hexagrid is a variant of the “diagrid”; one of the subjects in structural design that appeals me. The word “diagrid” is derived from “diagonal grid”, an efficient triangular structure that can guarantee the stability of a building. “Diagrids” are used for high-rise buildings where the horizontal wind pressure is normative. After some literature research it became clear that there is already done a lot of research on diagrids. For me this was a reason to look for another subject. In the library of our faculty I came across the book ‘Honeycomb Tube Architecture’ (HTA, Honeycomb Tube Architecture (Volume I), 2006). This book contains a collection of ideas, ambitions, designs and researches on the subject of hexagonal structures. Some assumptions made in the discussed researches lack a solid scientific basis. All together this book made me enthusiastic about the potential of hexagonal structures. Hexagonal structures are often visible in nature; this is probably not without a reason. I hereby would like to thank my graduation committee for all their support and enthusiasm. Each one of these members of this committee has helped me during but also outside of these planned meetings. I felt free to visit them anytime with questions or points of doubt. Besides my graduation committee other associate professors and teachers have helped me: dr.ir. J.C.D. Hoenderkamp for providing me helpful research papers, ir. H.J.M. Janssen for pointing me in the right direction concerning mechanical calculations, and ir. R.C. Spoorenberg for helping me with the use of ANSYS. Furthermore, I would like to thank all students and friends that helped me during the process with one of their specialties; structural and mechanical knowledge, graphical skills, listening, motivating and giving opinions. The person whom I discussed with most is Michael; my permanent neighbour at the university, graduating the same day as I do. Other names that I would like to mention are: Dennie, Douwe, Floor, Johan, Koen, Lianne, Linh Sa, Mark, Maud, Paul, and Rob. Finally, I would like thank my parents who have supported me since the early beginning of my study. Joost de Meijer July 2012 i Summary Next to the very common structural systems for lateral loads such as concrete cores, shear walls and rigid orthogonal frames, there are more possible options to ensure structural stability and stiffness. Some of these systems such as ‘diagrids’, ‘tube constructions’, and ‘mega trussed frames’ have been researched by numerous scientists. The lateral stiffness properties of a hexagrid have not yet received enough attention. A hexagrid is a hexagonal frame built up out of beam-like elements. It is as a variant on the diagrid, which has a triangular configuration of diagonal columns and horizontal beams. A combination of the hexagonal and triangular grid, a trihexagonal grid, is also examined; see Figure 1. This thesis focuses on ‘horizontal’ hexagonal grids comprised of only diagonal and horizontal elements. The objective of this thesis is to investigate the structural properties of hexagonal structures and compare their efficiency with other common structural systems. Hexagonal grid Triangular grid Trihexagonal grid Figure 1 Horizontal grids; the division into unit cells is visualized with dashed lines. For simplicity, the grids are split up into smaller pieces. These pieces are called ‘unit cells’. The unit cells include all geometric parameters of the grid-structure; see Figure 2 and Figure 3. Figure 2 Unit cell subjected to axial stress Figure 3 Unit cell subjected to shear stress In a standard stiffness test, to determine the elastic stiffness of a material, a specimen is loaded in an axial direction to determine the axial stiffness of the material in the direction of the loaded axis. In this thesis the unit cell is the specimen to be tested. The stress on the unit cell is translated to a concentrated load acting on the element in the unit cell at the border of the unit cell. The strain of the unit cell is caused by axial, bending, and shear deformations of the elements inside the unit cell. The deformations of the elements can, in most cases, be calculated using standard formulae. When slender elements are used, bending deformations are normative; axial and shear deformations of the elements are ignored. The effective stiffness of the unit cell is divided by the stiffness of the used material to obtain a ‘stiffness modification factor’ of the grid. The factors are now only dependent on the geometry of the unit cell and on Poisson’s ratio of the material used for the elements. ii Modification factors are derived for the axial stiffness as well as for the shear stiffness. The lateral displacement of a grid-structure can now be calculated using standard formulae for a cantilever column, see Figure 4. The effective stiffness of the grid contains the modification factor, the stiffness of the used material and the geometric properties of a corresponding solid with outer dimensions equal to the grid-structure. Figure 4 Grid structure (left) and corresponding solid translated to a cantilever column with effective stiffnesses. A parameter analysis of the unit cells is performed by plotting the formulae with an alternate angle of the diagonals. Overall, an increasing angle gives a higher axial stiffness modification factor and a lower shear stiffness modification factor. The stiffness modification factors of the unit cells are valid for an infinitely large grid. When the method of this thesis is applied to a finite structure, some discrepancies occur between the simplified hand calculations and the response of the structure in a finite element analysis. In preparation of the verification of the hand calculations, the influence of the boundary conditions and the method of applying loads are examined first. The loads and boundary conditions should be equally divided over the total structure as much as possible to avoid local extreme deformations. Secondly the width of the structure to be taken into account is researched by comparing the concept with the generally accepted Huygens-Steiner theorem. The obtained effective width of the structure is used to determine the cross-sectional area and the second moment of area of the corresponding solid. To evaluate and verify the concept of this thesis, the finite element program ‘ANSYS’ is used and its results are compared with the hand calculations. A systematic selection of structures with different dimension is evaluated. The displacements at the top of the structures obtained using ANSYS differ less than 10% with the hand calculations. The discrepancies are visualized by plotting the ratio of both values. The discrepancy is dependent on the slenderness of the structures; the ratio converges from 90% for stocky structures to almost 100% for very slender structures. In the final part of this thesis, a preliminary design is worked out. Horizontal as well as vertical loads are applied to the structure. At first, a regular hexagonal horizontal is considered; later the angle of the diagonals is optimized to obtain a more efficient structure. The efficiency of a grid structure is in this thesis the reciprocal of the relative density under condition that the horizontal displacement of the structure suffices the requirements. The proposed method of the thesis can be used as a quick method to determine preliminary section sizes and accompanying horizontal displacements. A hexagonal grid is less stiff than a triangular grid or trihexagonal grid. Acceptable results can be obtained when using relatively stocky elements. iii Contents Preface .............................................................................................................................. i Summary .......................................................................................................................... ii Contents .........................................................................................................................