Performance of Nailed Gusset Joints in Ridged Timber Portals

Journal of Forest Engineering »13 Performance of Nailed Gusset Joints have generally been the weakest link in timber structures which has necessitated research Joints in Ridged Timber Portals into the various types of mechanical connectors. The performance of the joints, in general, is determined A. Kermani1 by the material properties of the timber and connec Napier Polytechnic of Edinburgh, UK tor, the joint configuration and the loading condi tions. and General research studies by Bryant et al [5], B. S. Lee Mack [8,9], Morris [10,11,12], Moss and Walford B. S. Lee and Associates, [13], and Smith et al [15,16] on nailed timber joints Osmotherley, UK have developed theoretical and experimental un derstanding of the strength and stiffness of simple nailed joints subjected to short term loading. The effect of loading condition, moisture content, nail ABSTRACT diameter and length, and timber species on simple nailed joints have been examined. Emphasis so far, The structural behaviour and performance of by most investigators, has been placed on the more nailed plywood gusset knee joints of timber portal general aspects of the nailed joint behaviour rather frames are investigated. An analytical study has than on the applied research studies, which for the shown, and experimental study has confirmed, that purpose of the present investigation refers to the the behaviour of the joints in terms of strength and moment carrying haunched knee joints in timber stiffness, distribution of stresses, and rotation of nail frames. groups within the gusset haunches, is directly related to the joint's component arrangements, plywood Limited studies carried out so far, have mainly grain orientation and the dowel action of the nails. been concerned with the development of simple design methods for nailed plywood gusset knee An empirical method is presented which permits joints. Two significantly different methods have calculation of the short-term ultimate strength of evolved from studies at Timber Research And De plywood gusset plates, for various component ar velopment Association, UK [6] and American Ply rangements of the knee joints, based on the strain wood Association[14] which have been successfully distribution along the critical section of the haunched used in design and construction of timber portal gussets. It is shown that a considerable gain in ulti frames for more than two decades. mate strength and performance can be achieved if a correct joint configuration is used. The method developed at TRADA assumes that the gusset strut acts as a prop to support the shear Keywords: nailed joints, plywood gussets, timber frames. forces on the rafter and leg members, producing triangular tensile stress distributions in the plywood INTRODUCTION gussets. APA's method is based on an idealised strain distribution curve, obtained from test results, For many years timber portal frames with nailed along the centre line of the haunch plates. In this haunched knee joints have been used extensively in method the gusset strut serves only to stiffen the farm and industrial buildings, as they provide eco compressive edge of gusset plates against buckling. nomical buildings that can be easily constructed by Batchelar and Cavanagh [1] and Batchelar[2] have semi-skilled labour. The use of timber as a cost also made a study of nail jointed timber portals using effective alternative to other materials such as steel laminated timber sections and reported similar and concrete, depends to a significant extent on the conclusions to those of the APA. structural efficiency of the joints, the cost of the components and the ease and speed of fabrication. In the last two decades only limited attempts have been made to investigate the real structural behaviour of the nailed plywood gusset knee joints 'The authors are respectively; lecturer, Department of Civilfo r timber portal frames. What investigations have Engineering, and Consulting Engineer, formerly Head of been carried out have been mainly concerned with Department of Civil and Structural Engineering, and Building, the development of design procedures. The exami- Teesside Polytechnic, UK. 14 • Journal of Forest Engineering. Figure 1: Details of a typical knee joint. 10 6 15 | 1 v L 2.35mm round nails A in 175 1 < - >* Section A-A Journal of Forest Engineering • 15 nation of the present design methods shows discrepancy between the methods and the assumed and real behaviour of these struc tures. The performance of the stiffener strut in the knee joints is not yet fully established and differs between the design methods. The effect of plywood gusset grain direction J- on the distribution of stresses and on the »•*' strength and stiffness of the haunched knee joints has been neglected, and so far no attempt has been made to determine the effects of member arrangements on the structural behaviour of haunched joints to justify the preference of one joint's member -v arrangement to another. Type(A) Type(B) In this paper results of the experimental work on a set of nailed plywood gusset knee joints are described and compared with those of finite element analysis. Also an empirical formulation for the prediction of the short-term ultimate strength of the plywood gussets of the knee joints is presented. DESCRIPTION OF TEST JOINTS The joints were designed as a part (knee joint) of a half scaled 12m span timber portal frame with hinged feet, in accordance with BS 5268 [3]. The height of the frame from floor to eave was 3.6m and to apex was 6.0m. Type(C) Type(D) The dimensions of a typical knee joint of the twelve specimens tested are shown in Fig Figure 2. Knee joint types. ure 1. The effective leg lengths shown were chosen in order to produce, when loaded, a correct combination of forces and moments as those Type(D):Joints with rafter leg resting over the produced in the actual portal knee joints. This was column leg and the touching ends being due to the fact that joints with equal leg lengths, as perpendicular to the column leg. tested by previous investigators (1,2,14), were con sidered to be an unlikely representation of the actual Materials frame behaviour. TIMBER: The timber members were cut from Classification of the joints British grown Douglas fir grade SS, 75 x 250mm sections. The timber sections were air-dried and The joints were classified on the basis of their allowed to come to equilibrium moisture content in component arrangements, see Figure 2, into the fol the laboratory before fabrication and were then tested lowing types : within 72 hours. The moisture content at the time of testing was around 14 ± 2%. Type(A): Joints with mitred timber legs along the centre line of the gusset plates. PLYWOOD: With the exception of joint Al for Type(B): Joints with timber legs being alternately which 9mm 5-ply Finnish conifer plywood was used, lapped over eachother. 6.5mm 5-ply sanded Finnish birch- faced plywood Type(C): Joints with rafter leg passing over the was used for the gusset plates of all joints. A total of column leg. 145 specimens, based on BS 5412 [4], were made and 16 • Journal of Forest Engineering. 80 was to determine areas of high stress concentra + Tensile Q Compressive tions, and the magnitude and direction of the 70 A In-plane bending principal stresses developed during elastic 60 loading on the plywood gusset plates irrespec tive of grain direction effects. The results of these 50 analyses were then used in experimental work for the correct positioning of the strain gauges 40 on the plywood gussets. Also the effects of vari 30 ous joint component arrangements on the distri bution of the stresses on the gusset plates were 20 examined. In Figure 4, the stress vectors on the plywood gussets of joints Type (A) with and 10 without the stiff ener strut in position are shown. 0 0 15 30 45 60 75 90 ii. The members and gusset plates of the joints PLYWOOD GRAIN DIRECTION (degrees) were considered to be homogeneous and orthotropic. The structural properties of the ma Figure 3. Effect of grain direction on strength of the terials and load-slip characteristics of nailed- plywood. joints were determined from the tests and used in the analysis [7]. In this analysis the non-linear behaviour of the joints was examined and the the tensile, compressive, and in-plane static bending effects of the plywood grain directions were strengths of the plywoods with respect to their grain considered. In Figure 5, the effects of the ply directions were determined. In Figure 3 the signifi wood grain direction on the stress distribution cant effects of grain orientation on the strengths of within the plywood gussets of joints Type (A) Finnish birch-faced (6.5mm, 5-ply) plywood are are compared. shown. EXPERIMENTAL VERIFICATION NAILS: 2.35mm diameter, 40mm long round wire nails were used for all joints. The gusset plate nailing pattern was arrived at by assuming a pattern Twelve moment resisting portal knee joints were and then determining its moment, thrust, and shear constructed and tested. Details of the joints are sum resistance. By trial and error it was possible to arrive marised in Table 1. The data acquired during the at a nail pattern in which the load on the average nail joint tests consisted of the following: did not exceed the safe lateral strength, for the size used, as specified in BS 5268 [3]. A typical nailing a. The hydraulic pressure in the tension ram, ap pattern for the tested joints is shown in Figure 1. plied by a hand pump, measured by means of a pre-calibrated proving ring. The Design Load for the joints tested was equivalent to 4.82kN FINITE ELEMENT MODELLING load produced in the tension ram.

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