Prof. Dr. Wieland Becker Dipl.-Ing. Jan Weber Verbindungsknoten aus Ultrahochfesten Betonen für räumliche Tragwerke aus Holz (Rundholz) Fachhochschule Trier Lehr- und Forschungsgebiet Holzbau 54293 Trier, Schneidershof D 105 w.becker@fh-trier.de
HP composite joints for spatial round wood truss structures Wieland Becker Chair of Timber Structures Jan Weber Research scientist Trier University of Applied Sciences Trier, Germany Summary Anisotropic material characteristics of wood still challenges the engineers work for engineered timber constructions, especially for 3 D-truss joints and spatial structures. It is well known, that there are some practical glued solutions for unidirectional wood-wood or wood-steel connections with nearly 100% efficiency, but spatial structures are usually fitted by expensive steel knods, where the shape of force trajectories is often unrespected. Although naturally dried round wood has outstanding mechanical values and a excellent energy balance, it is not respected in the current building markets and timber engineer constructions. One of the reasons is, that the jointing technology is usually done with manual methods which are not accepted for large constructions. The proposal presents a CAE/CAM process which creates High Performed (HP) composite joints with force- and formoptimized design and simple fastening methods via laminated bar connections. Tension-, compression-, shear- and bending forces can be resisted. The timber profiles are always used by longitudinal stress via laminated bar connections. Highly stressed sections are brought in the HP-Joints, which can be designed via FE-Methods under using homogeneous material values. The CAM-manufacturing process allows efficient individual solutions and designs where the joints can also be produced as castings in high units.
1. State of knowledge-current situation and use of round wood truss structures Although the high mechanical values of round wood is documented in EC 5 and the production process has low power requirements, round wood is usually not used and respected in current timber-engineer constructions. In rare cases where round-diameters are used, profiles are made of glue-laminated timber lamellas, which are crafted in cylindric form to girders or columns. Industrial use of logs or round wood-timber has the disadvantage, that the cutting to size of girder joints or girder connections can not be manufactured by joinery machines. An automatic cutting process in different angles is currently not be given. Manual cutting processes are not precise and reduce the application of round wood-constructions upto simple constructions in rural environment. Different statical and mechanical requirements as compression, tension or bending forces can not be taken over without large deformation behavour of the complete bearing system. Fig. 1 Conventional round wood truss joint Fig. 1 shows a conventional solution for round wood trusses which are manually fitted. Since the Olympic Games of Munich in 1972 it is well known in engineering of steel constructions, that framework jointing elements can be manufactured as steel casting elements. First applications where used for steel grids and membrane structures for tensional use. Between 1990 and 2010 a lot of wide spanned roofs and bridges as steel contructions with casting elements where realized. Fig.2 shows a detail and Fig.3 the casting-joints of Stuttgart Airport roof construction, which was built between 1981-1991 [1]. Fig. 2 Casting joint of Stuttgart Airport Fig. 3 Stuttgart Airport roof construction
2. Aims and object-composite joints with force- and formoptimized design According to the described solutions in steel a new type of high performed composite joints (HPTJ) for round wood-truss structures will be created. The design of timber composite joints allows joint-solutions in high performed concrete or high performed materials based on epoxyresin. The mechanical values for standard products can be assumed for compression strength of 135-150 N/mm2, for flexural strength of 35-45 N/mm2. Different load-bearing capacities as compression, tension or shearforce can be resisted by contact pressure or glued bars. If required, a pressure ring between the contact surfaces with ductile behaviour will resist also bending moments in the joints. In Fig.4 a 3D-joint for spatial structures as a standard detail for mainly compression strength is shown, Fig. 5 shows the function of usual load cases. Fig. 4 3-D joint for spatial structures Fig. 5 function of usual load cases Fig.6 shows a joint solution for outdoor use and also bending loads. The ductile behaviour of the joint is given with a pressure ring, witch can be choosed depending of the expected bending load. Fig. 7 shows the mechanical model of this proposal. As shown in Fig.6 the fastener can be fixed at the round wood diameter with a standard qualified screw-product. Fig. 6 Joint solution for bending load Fig. 7 Mechanical model with ductile behavour
3. Work packages The research focuses three main aspects: MECHANICAL TESTS HP-Concrete materials Concrete-steel connections Timber-steel connections Pressure-ring ductile behavour DESIGN OF SPATIAL STRUCTURE JOINT-MODELLING 3D-modelling HP-concrete joint Definition of Material parameters based on results of mechanical tests FE-Modelling Formoptimizing of the 3D-model MANUFACTURING CAM-processing of the 3D-model Mould + casting Connectiong of timber an concrete joints MECHANICAL TESTS OF SPATIAL STRUCTURES References [1] Schober, H.; Rohrknoten aus Stahlguss, Der Prüfingenieur, No.17, 2000, pp. 16-36. Lippert, P.; Rahmenecken aus Holz mit eingeklebten Gewindestangen, Diss. Universität Stuttgart, 2002. Mitteilungen des Instituts für Konstruktion und Entwurf 2002-4. Widmann, R., Steiger, R.; Eingeklebte profilierte Stahlstäbe, Tagungsband Holzbautag Biel, 2011. Strahm, T.; Verbindungen mit großer Leistung, Tagungsband Werkstoffkombinationen aus Holz-SAH, 41. Fortbildungskurs 2009, pp. 79-85.