Larch observation tower Tentech bv Postbus 619 2600 AP Delft the Netherlands Tel.: +31 15 2784034 Fax: +31 15 2784004 Contact: M. Peter de Vries Email: [email protected] Website: www.tentech.nl Tentech B.V. is a Design and Engineering company of lightweight structures with a large experience in membrane structures. Tentech was established in 1997 and has now a team of 7 people. Our work is distributed over different area’s of lightweight structures: • Tensile membrane structures • ETFE cushion roofs • Rental tent structures • Spatial (roundwood) structures Technical data project Physical location: Stiltegebied Holmers Halkenbroek, Drenthe Site owner: Staats Bosbeheer Architect, Engineer: ir. Peter de Vries Engineering office: Tentech BV, Delft Total weight of the structure: 16.5 tons Height of the structure: 14 m Largest diameter: 7 m Smallest diameter: 5 m Building costs: 137.000 Euro Building period: 09/2004 - 11/2004 Introduction: architectural design In the centre of the nature reserve HolmersHalkenbroek, situated in the province of Drenthe in the Netherlands, Tentech designed and engineered a public accessible wooden observation tower. The tower is constructed of slender Larch beams and has a spatial structure based on the geometry of a hyperboloid. In clear language the shape is a diabolic. A light steel stairway leads the visitor to the top level platform, approx. 14 m. above the ground. While the wide base of the tower makes sense from a structural point of view, the large area of the upper floor is clearly beneficial for the users of the structure. From the waist of the tower, the shape ‘reaches out’ to the surroundings. The orientation of the fence on the observation platform once again stresses this. The tower is situated in a silence area and can only be reached by foot or bicycle. Structural system of the tower There are two cooperating structural systems: a) the vertical columns and platform floors that support the stairs and b) the hyperboloid diagonal structure at the outside of the tower. The floor-column system has a pure vertical bearing constitution, while the outer diagonal structure bears it’s own weight, some of the floor loading of the top platform and all horizontal forces (lateral and torsional, caused by both wind and horizontal load by persons) on the structure. The used structural concept made it possible to build the tower with very slender beams. The diameter of the poles at the ground level is only 140 mm, with a length of approx. 4 meter, while in the storeys above the diameter is only 120 mm. Small diameter round wood The small diameter beams are produced of wood harvested in forest thinnings. Traditionally in structural applications where round wood from thinnings is used, the stems are debarked. This means the beams are slightly tapered. Milling the stems cylindrical has the disadvantage of loosing a large (up to 50%) part of the section. On the other hand however, milling has significant advantages with respect to the design of connections. Furthermore, using cylindrical sections the integration of the round wood stems with other, standard, building elements is much more straight forward. An other advantage of milling is the fact that all sapwood is removed, and stems consisting of only heartwood remain. This makes it possible to use the Larch beams unpreserved. For the tower all stems were milled cylindrical. Construction wood from thinnings is not available via the normal commercial routes. The material requires special selection techniques to ensure high enough strength. Using the results of the EC research project “Small diameter Roundwood in Constructions (1996-1999)”, is was possible to select wood with sufficient strength characteristics. Using ring width, maximum knot diameter and maximum initial curvature as selection criteria, the necessary 140 beams were selected from a batch of 180 poles. The EC5 strength class of the round wood was C24 and all beams used have initial curvatures less than or equal to 1/200. 92 Company Project Larch observation tower SCIA User Contest 2005 / Civil Engineering 2 Categorie
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