Offshore timber slab by using screwed timber planks

Transcription

1 Offshore timber slab by using screwed timber planks Sandoz J. L. 1, Duro S. 2 SUMMARY At the occasion of the Swiss National Expo in 2001, which will be developed on three lakes near Neuchâtel, six hectares of offshore platforms have to be built. Because of the light sanded soil properties, light timber structures erected on piles have been preferred. Furthermore, all exhibition constructions, including the offshore platforms have to be removed in 2002, after the event. The offshore slab has been developed by using vertical timber planks screwed together. In order to increase the inertia of the slab, the planks are screwed displaced one from another. The crossing area is reduced to the minimal surface, regarding to screw efficiency. For a plank depth of 200 mm, the final inertia of the slab reaches 340 mm. INTRODUCTION In structural engineering, slabs are very often erected by using concrete, for span going from 4 to15 m. Concrete slab are efficient from the structural point of view because of the possibilities to reach an isotropic behaviour. For large span the real isotropy is not reached, but the load distribution reduces the beam associated stresses. When timber is used for vertical nailed plank slabs following a construction made like figure 1, the slab is not at all isotropic, but is working like a beam (on two or three supports). In the eighties and the nineties, many pilot projects have been erected by using massive floor, according to figure 1. [1] Figure 1 - Usual Timber deck by using vertical nailed plank. For the larger span, up to 10 m, the timber plank slab is up graded by using timber-concrete mixed systems, in order to have timber in tension and concrete in compression [2,3]. The connector for the efficiency of the mixed cross-section can be of several types. One developed at IBOIS, at the chair of timber construction is presented figure 2. Figure 2 - Example of a mixed timber-concrete deck connector, working in tension [2,3]. 1 Director CBT (Concept Bois Technologie), start-up new technology, CH-1025 Saint-Sulpice 2 Scientific assistant, Chair of timber construction, Swiss Federal Institute of Technology, CH-1015 Lausanne

2 If the slabs were erected by using only planks, the size of the plank would rapidly increase. For a 12 m span floor and for usual live load (500kg/m 2 ) the width of the plank according to figure1 is reaching 280 mm. The industry is not ready to furnish this kind of timber for a competitive price. THE O PORTUNE CONCEPT At the occasion of the Swiss National Exposition in 2002, which will be developed on three lakes near Neuchâtel, five hectares of offshore platforms have to be built. Because of the light sanded soil properties, light timber structures erected on piles have been prepared. Furthermore, every exhibition constructions including the offshore platforms have to be removed in 2003 after the event. In order to have an offshore platform with a span of 6.10m built for heavy track loads (4x90 kn), a new concept of timber slab has been designed; it is: the O Portune LVL-timber system, as figure 3. LVL (Kertopuu Q 27 mm) 205 mm 340 mm 70 mm Planks 60/205 mm 60 mm Figure 3 - Cross-section of the O Portune timber deck concept, using shifted planks screwed together and reinforced on the Y direction by a LVL Kertopuu Q of 27 mm. The planks are shifted one from another in order to maximize the final depth of the slab. By using standard planks with a width of 205 mm, the depth of the floor reaches 340 mm. This kind of design is the first step but it is not yet sufficient for a good isotropic effect. The isotropic effect is gained by using a LVL (Kertopuu Q, 27 mm) crossed on the longitudinal plank axis. The planks are screwed together and then the LVL is screwed on the plank edge. The size of the plank is 60/205 mm in order to receive the LVL connectors without cracks. The O Portune timber slab concept works on both X and Y directions. When a concentrated load of 90 kn is applied on a square surface 40x40 cm, the load is diffused by the system. Calibrated tests have been done following figure 4, in order to measure its diffusion effects and then to calibrate a design model m 1.80 m 2.10 m 4 Charges by sheets (40 cm x 40 cm) 3 m 12 m Figure 4 - Static system for a real size sample of O Portune slab, length 12m, width 3m, on three supports.

3 The sampling is a real size, and has a width of 3 meters. The length is 12 meters on three supports, giving a span of 6m. The deformation measurement sensors are located following figure 5 in order to have the diffusion form and the general answer of the slab under the four concentrated loads. Sensor 12.0 m 3.0 m Figure 5 - Distribution model of the deformation sensors under the O Portune bended sample, allowing to modelize the distribution on Y of the concentrated loads. The figure 6 gives the deformation profile for a test made in bending following the Swiss Code SIA160 Loads applied to structures [4] y = -0,16x 4 + 3,16x 3-22,29x ,12x - 30, Déformations F=150 kn 1300 mm F=150 kn 15 Série1 Polynomial (Série1) mm N capteur sur la largeur de la dalle Figure 6 - Transversal deformation made in bending for a slab on three supports, and for a span of 6m. The concentrated load (40cm) is spread on a band of 1.50m. It appears that the concentrated load is spread on a band from 40 cm to 1,50 m. By using polynomial fit regression, the answer theoretical function is given by equation (1). RELIABILITY OF THE PARALLEL SYSTEM The O Portune concept works as a parallel system where several planks work together. For such a system, the 5% percentile bending strength is much higher than a single member of the same quality. The parallel system effect has already been used for the Multi Glued Laminated Timber (5). For usual medium to low timber quality, the qualitative evolution of a parallel system, regarding to the nominal probability density function, is shown by figure 7.

4 Figure 7 - Qualitative probability density functions for a timber quality; a) piece for piece; b) in a parallel system. This effect can be used for the design model. Furthermore, the reliability of such systems can be observed when they have loaded to failure. On Figure 8, we can observe that the tested slab has reached a failure in one in the weakest plank. But, after this failure the hyper staticity of the system has allowed to increase the ultimate load of 10% more before reaching a failure by shear stresses. Actually, the screws number has been under-designed in order to calibrate the ultimate shear strength of this kind of connectors Force en kn Déplacement en mm Figure 8 - Load deformation classic answer of the O Portune slab, showing a first plank failure about 10% before reaching the ultimate load. CONCLUSION A new concept of timber slab for high performance has been developed for temporary structure or for structural decks based on parallel shifted planks screwed together, reinforced by a cross LVL connected on the plank edge. The LVL cross panel is bringing a better isotropy of the slab and contributes to diffuse the concentrated loads (track load of 4 x 90 kn). Based on the parallel system reliability this kind of slab reaches a high performance for the ultimate bending strengths and the failure theory can be the shear stress and the ultimate limits of the screw connectors. REFERENCES [1] J. Natterer, Th. Herzog, M. Volz, 1994: Construire en bois 2. Ed PPUR, CH Lausanne. ISBN [2] M. Hoeft, 1994 Zur Berechnung von Verbundträgern mit beliebig gefügten Querschnitt. Thèse N r EPF- Lausanne - CH

5 [3] J. Natterer, 1997: Concepts and details of mixed Timber Concrete structures. AIPC conference Sept. Innsbruck - Austria [4] SIA 160, 1989 : Actions sur les structures porteuses. CP 8039 Zürich, CH [5] J.L. Sandoz, 1998: Glued Laminated Timber : Reliability and High Performance. WCTE 1998, Aug Montreux- CH