CIB-W18/39-7-5 INTERNATIONAL COUNCIL FOR RESEARCH AND INNOVATION IN BUILDING AND CONSTRUCTION WORKING COMMISSION W18 - TIMBER STRUCTURES LOAD CARRYING CAPACITY OF JOINTS WITH DOWEL TYPE FASTENERS IN SOLID WOOD PANELS T Uibel H J Blaß Lehrstuhl für Ingenieurholzbau und Baukonstruktionen Universität Karlsruhe GERMANY MEETING THIRTY-NINE FLORENCE ITALY AUGUST 2006 Load Carrying Capacity of Joints with Dowel Type Fasteners in Solid Wood Panels T. Uibel, H.J. Blaß Lehrstuhl für Ingenieurholzbau und Baukonstruktionen Universität Karlsruhe, Germany 1 Introduction Solid wood panels with cross layers (Fig. 1) have been used more and more frequently in timber engineering in recent years. Their use in constructions requires their connection with each other and with other components of the construction. To that purpose dowel-type fasteners can be used. It is possible to position the fasteners perpendicular to the plane of the panels or in their narrow sides. In this paper only the first possibility will be discussed. The load carrying capacity for dowel-type fasteners is usually calculated according to Johansen’s yield theory [3], [4]. Embedding strength as well as the yield moment of the fasteners are important parameters in this calculation. Withdrawal strength is needed to calculate the load carrying capacity of axially loaded screws or nails. The aim of a current research project at the University of Karlsruhe is to develop a proposal for calculating the load carrying capacity of joints with dowel type fasteners in solid wood panels. Fig. 1: Solid wood panels with cross layers - two examples of products (pictures: Informationsdienst Holz) 1 2 Characteristics of the test material Solid wood panels consist of boards crosswise laminated with a minimum of three layers. Some panels are produced with gaps between the narrow sides of the boards. The maximum width of gaps is limited to 6 mm. In addition, grooves with a width of about 2,5 mm are sawn into the boards of some products. Fig. 2 shows cross-sections of different solid wood panel products. Table 1 gives some statistic information about the gaps. Fig. 2: Cross-sections of panels without and with gaps between boards and grooves Table 1: Width of gaps between boards of one layer for some different products Width [mm] of gaps in Build-up Outer layers Interlayers Centre layer product product 95% 95% 95% mean max. mean max. mean max. Manufacturer/ fractile fractile fractile 1 17-17-17-17-17 0,6 2,1 1,6 1,6 7,3 3,4 1,0 3,0 2,3 2 19-22-19 0,4 2,0 1,3 - - - 0,5 2,2 1,8 2 34-13-34-13-34 0,2 1,0 1,0 1,4 6,8 3,3 2,0 6,7 4,5 4 9,5-6,8-9,5-6,8-9,5 0 0 0 0,6 5,4 3,5 0 0 0 The characteristic density (at normal climate, 20°C/65% RH) of solid wood panels was determined by analysing altogether 2299 test specimens out of a range of products from different manufacturers (Table 2). This revealed a minimum 5th percentile density of 400 kg/m³ for product 2. Taking this result into account a characteristic density of 400 kg/m³ is proposed for solid wood panels made of European spruce (Picea abies). Table 2: Density of solid wood panels ρ Coefficient Manufacturer/ mean ρmin ρmax ρ0,05 n of variation product kg/m³ kg/m³ kg/m³ % kg/m³ 1 515 470 415 630 5,11 430 2 906 437 372 578 6,02 400 3 208 458 406 507 5,18 423 4 670 459 397 558 5,75 419 2 3 Embedding strength 3.1 Test set-up To determine the embedding strength of solid wood panels with cross layers 620 embedment tests according to EN 383 [5] were carried out, involving tests with a load under 0°, 45° and 90° to the grain of the outer layers. Furthermore the position of fasteners was varied, as shown in Fig. 3. They were placed in areas without gaps (pos. 1), placed in gaps (pos. 2 to 4) or over gaps (pos. 5). For tests with fasteners loaded parallel and perpendicular to the grain direction of the outer layers it was possible to apply the geometry of test specimens as specified in EN 383. To carry out embedment tests with fasteners loaded under 45° to the grain the size of test specimens had to be increased due to plug shear failure in outer layers. F F Richtung b 90° Richtung F Richtung a 45° F Richtung a F0° 1 2 3 4 5 Tested positions of dowel type fasteners Tested directions Fig. 3: Positions of fasteners and load direction in embedment tests, schematic sketch 3.2 Results for dowels For dowels it was possible to develop the following two models for embedment strength on the basis of a multiple regression analysis of 438 test results. In the first model the embedment strength as given in equation (1) is independent of the build-up of the panels. The correlation coefficient r is equal to 0,75. The embedment strength depends on the diameter d of the dowel, the density ρ of the solid wood panel and the angle α between load and grain direction of the outer layer. 0,035⋅⋅⋅ (1- 0,015 d) ρ1,16 f = h,pred 1,1⋅ sin22α +cos α in N/mm² (1) r = 0,75 Additionally the second model (eq. (2)) takes into account the build-up of the panels as defined in Fig. 4. t0,1 tttt90,1 0,i90,n-1 0,n t Fig. 4: Definition of thickness of layers for eq. (2) and (3), in case of a panel with 5 layers 3 ⎡⎤n n-1 t ⎢⎥∑t0,i ∑ 90, j 1,16 ⎢⎥i=1 j=1 fh,pred = 0,037⋅⋅⋅⋅ (1- 0,016 d) ρ + in N/mm² ⎢⎥t1,2sin⋅⋅()22α +cos α t1,2cos ⋅⋅() 22α +sin α (2) ⎢⎥ ⎣⎦ r = 0,74 The validity of equation (1) and (2) is limited to panels which fulfill the following conditions: • Maximum thickness of one layer: 40 mm • Ratio of layers with different grain directions as defined in Fig. 4: t 0,95 <∑ 0,i < 2,1 (3) ∑ t90, j Fig. 5 shows the results of embedment tests over the predicted values for model 1. The characteristic embedment strength on basis of equation (1) for a characteristic density of 400 kg/m³ can be calculated according to the following expression: 0,031⋅⋅⋅ (1- 0,015 d) ρ1,16 1- 0,015⋅ d f32==⋅k in N/mm² (4) h,k 1,1⋅⋅ sin22α +cos α 1,1 sin 22α +cos α 60 60 50 50 40 1-5_85-24 40 1-5_85-20 30 2-3_78-24 30 in N/mm² 2-3_60-16 N/mm² in h,test 3-3_17-16 h,test f f 20 20 4-5_42-24 4-5_42-20 10 4-5_25-16 10 4-3_12-12 4-3_12-8 0 0 0 102030405060 18 20 22 24 26 28 30 fh,pred in N/mm² fh,k in N/mm² Fig. 5: Comparison of test results and predicted resp. characteristic values for dowels 3.3 Results for screws and nails On the basis of a regression analysis of 179 tests with screws and nails the embedment strength can be derived as: f=0,13d⋅⋅-0,53ρ 1,05 h,pred in N/mm² (5) r = 0,83 A comparison of predicted values and test results is shown in Fig. 6. The correlation coefficient was determined as r = 0,83. At present the validity of equation (5) has to be limited to panels with layers thinner than 7 mm. In equation (5) the embedding strength is independent of the angle α. This result corresponds to the research results of Blaß and Bejtka [1], [2] for self-tapping screws. 4 60 60 50 50 40 40 30 30 in N/mm² in N/mm² h,test f h,test f 20 20 3-3_17-Sc12 10 4-3_12-Sc8 10 4-3_12-Na4.2 4-3_12-Na6 0 0 0 102030405060 10 15 20 25 30 35 f in N/mm² fh,pred in N/mm² h,k Fig. 6: Comparison of test results and predicted resp. characteristic values (screws/nails) By inserting a characteristic density of 400 kg/m³ in equation (5) the characteristic embedment strength can be proposed as: -0,5 1,05 -0,5 fh,k = 0,112⋅⋅ d ρ k =60d ⋅ in N/mm² (6) 4 Load carrying capacity 4.1 Calculation Model The load carrying capacity of joints in solid wood panels under lateral load can be determined according to Johansen’s yield theory. For connections with screws and nails in solid wood panels with thin layers (ti ≤ 7 mm) it is possible to use the embedment strength from equation (5) resp. (6) for the calculation. In other cases the embedment strength depends on the angle between load and grain direction so that additional investigations are necessary. Here obviously the embedment strength of a layer loaded in grain direction is larger than of one loaded perpendicular to the grain (see Fig. 7). In the following the load carrying capacity of dowels in a steel-to- solid-wood-panel connection with an inner steel plate is derived exemplarily. According to Johansen’s yield theory three failure modes are possible. The load carrying capacity is the minimum value of these three modes. Failure mode 1 is characterized by an embedment failure of all layers as shown in Fig.