Analysis of Catenary Shaped Timber Structures

Sammanfattning: In the design of a building there are many parameters that effect the material efficiency. The choice of structural solution and material are two of the parameters that will be investigated further in this rapport. Suspension structures is one type of structural solution that has high material efficiency, here the bending stiff properties of timber could also be utilized in order to increase the stiffness of the structure. The use of suspension principle on roof structures is however not frequently used as it can give rise to unfavourable load cases which of course causes increased uncertainties. Some reference building has in spite of this been identified and increased understanding about how suspension structures in timber could function and what possible weaknesses this type of structure can have. With some ideas of how a suspension structure in timber could be designed, different load cases were studied, especially those that were expected to be important for design. Here the uneven snow load, wind uplift and dynamic instability from wind can be highlighted as potential problem areas. In order to get a better understanding of the snow drift on a suspension roof, a scale experiment was performed that showed that most of the snow drift is produced with wind oncoming perpendicular to the roofs parabolic shape and in proximity to the separation zones. The test also showed that a double-sided roof slope resulted in a less unfavourable snow load and that approximating the snow load with an butterfly roofs can give an approximation on the unsafe side. When the loads on the structure were determined, three different factors that affected the load carrying cross section was evaluated. Firstly the support stiffness was studied, here a lower support stiffness meant bigger displacements and bending moments, while its impact on the normal force were negligible. Secondly the member shape was investigated, the conclusion from this test was that a small catenary shape corresponded to an increase in normal force but at the same time resulted in less deflection, bending moment and increased eigenfrequency. Here the result was calculated with both computer aided FE analysis and analytical expressions. A comparison between the result of FE- and analytical calculations, showed a good agreement for calculations of normal force, while the displacement and bending moment showed larger differences between the different calculation methods. Thirdly the effect of slenderness for the cross-section was examined. In this test a slender cross-section means a high bending stiffness which resulted in that a bigger part of the load was resisted through bending. Finally a preliminary design on the catenary "timber-cable" was performed, where the information from the investigations could be applied. Biggest difference compared with the reference building was a reduction of catenary shape, which decreased the deformations and made it possible to utilize both the normal force- and bending moment capacity efficiently. The designed building take advantage of a green roof in order to counteract the large suction forces that can occur for this roof shape. In this way, the green roof result in a more slender load carrying cross-section in combination with having several other benefits. With a span of 70 m the final cross-section was designed with a glulam beam with the dimensions of 250 x 300 mm, with a center to center distance of 800 mm. In the load carrying structure two layers of plywood was also used which mostly contribute to the stability of the building. To be able to compare the the material efficiency of the timbercables, the height of the load carrying elements instead would have used a a truss design. With a truss design the hight of the load carrying element would require several meters which clearly shows the advantage of timber cable concept.