Connections Special elements in the structure are the nodes that connect the round wood beams. These nodes have large eccentricities (the heart lines of beams that are jointed in a node cross, rather then that they cut in the node). To prevent instability in these joints, a moment resisting connection couples the nodes in the outer structure to the floor-columns structure that surround the stairs. By having the round wood beams cross in the joints, and making a smart choice for the heart line of the connections, the axial and lateral coupling could be carried out on a standard way. This approach made it possible to use identical parts for all round wood connections. The detailing of the coupling blocks in the nodes is such that it is possible to disassemble each round wood beam in the structure, and have it replaced easily. This aspect of the nodes is important, as the wood in the tower is not treated with preservatives. During the drying of round wood, cracks develop along the length of the stem. The cracks are caused by differences in radial and tangential shrinkage of wood. This phenomenon is characteristically for round wooden poles in which the pith is still in the section. When making bolted or dowelled joints in such beams, the cracks lead to problems. Therefore another jointing principle was developed. In the EC research project mentioned earlier, extensive research was done to the ‘block shear joint’. This joint is less sensitive to the cracks in the round wood. Based on this jointing technique the (patented) construction node was developed that was used in the tower. The principle of the jointing technique is also applicable for other structural elements, like trusses. ESA-Prima Win In ESA-Prima Win a complete model of the tower was built. The eccentricities in the joints of the round wood beams are of key importance on the structural behaviour of the tower. Therefore the eccentricities are all in the calculation model. Self weight and loading of the stairs are essential loading components of the structure. To design the stairs and have overall clarity in the calculation note, the stairs are part of the model. Special modelling was used to correctly predict the forces on the (concrete) foundation. The eccentricities in the joints and other connection detailing cause bending moments in the round wood beams. The ‘block shear joint’ connections are designed essentially for axial loads, and have only limited bending moment capacity. The extreme slenderness of the beams, large initial curvature of the round wood, high design tension and compression forces and the bending moments that result from the eccentricities in the connections, made it necessary to perform design calculations in which the influence of the initial curvature is accounted for. In the ESAPrima Win model the round wood elements were given initial curvatures, corresponding to the measurements we made during the selection of the stems. The 2nd order frame calculation was used to do the analysis in ESA-Prima Win. Calculations of the non-linear combinations with initial curvature were carried out using the ‘Timoshenko’ algorithm. Light weight structures like this tower are sensitive to dynamic response of horizontal loading by wind and/or persons. Notably in this case, were also the eccentricity in the nodes has negative influence. To analyse this aspect of the structure, the horizontal deformations of the structure were studied carefully, and hand calculations were made to verify correct dynamic behaviour. We clearly lacked the ESA-Prima Win eigenfrequency module here, but unfortunately the engineering budget was insufficient to purchase the module for this project. ESA-Prima Win 3.50 modules used Base 3D frame Dynamic document 2nd order frame with initial curvature 93
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