A new ductile approach design of joints assembled with screw connectors

Transcription

1 A new ductile approach design of joints assembled with screw connectors Roberto TOMASI Researcher Dpt. Mech. & Struct. Engineering, Trento, Italy Maurizio PIAZZA Professor Dpt. Mech. & Struct. Engineering, Trento, Italy Albino ANGELI Engineer Rotho Blaas Company, Italy Mario MORES PG Student Dpt. Mech. & Struct. Engineering, Trento, Italy Summary Although timber, thanks to the excellent ratio between strength and density, has been traditionally considered an interesting building material for earthquake resistant structures, compared to other building materials (steel or reinforced concrete), it shows a brittle behaviour, with a poor attitude to dissipate energy. The only possibility to obtain energy dissipation in timber structures is limited, essentially, to plastic deformations occurring in mechanical joints assembled with dowels, nails or screws. Dowel-type connectors can show different ductile behaviours, depending on some geometrical and mechanical parameters. Several of them, such as the slenderness, the presence of a continuous thread, the screw inclination with respect to the shear plane, are not completely considered in European Building Standards. Taking into account some recent theories reported in scientific literature, an experimental survey on more than fifty push-out and push-pull specimens assembled by means of inclined screws has confirmed the good behaviour of this kind of joint in terms of ductility, strength and initial stiffness, while the cyclic tests have highlighted the energy dissipative capabilities of the joints. Experimental results have confirmed the behavior of inclined screws with continuous thread, as predicted by the F.E.M. model, hereafter illustrated. The goal of the research is the validation of the behavior of these kinds of screw connections used for assembling composite structures or beam-to-column joints, when high ductility demand is requested. Keywords: innovative timber structures, mechanical connections, structural analysis and testing, seismic design, ductility. 1 Introduction At present, the most commonly utilized theoretical model for determine the load carrying capacity of dowel-type connectors, is the European Yield Model (Johansen, 1949), which is based on limit state design. The parameters affecting the ultimate load of joints are the wood embedding strength and the fastener bending capacity. Recent studies (Blass and Bejtka, 22; Kevarimäki 22), showed that joints made with inclined screws offer important advantages compared to traditional ones. Bejtka and Blass have presented a modified form of Johansen model. According to this model, strength of the joints are strictly related to the angle between the screws axis and the force direction, reaching the maximum values with angles approximately equal to 6. In the Johansen modified equations there is an additional parameter μ for the friction between the joint elements, and a withdrawal parameter f a,mod,i,j for different failures modes (considering the withdrawal behaviour of the screws depending on lateral load).

2 2 Experimental investigation 2.1 Specimen geometry In order to investigate the behaviour of connections with inclined fasteners, more than fifty specimens have been prepared for both static and cyclic test. Some preliminary test have been done on specimens where the two screws were positioned symmetrically with different inclination of the screws: the coordinate system adopted in the paper, and some specimen configurations are illustrated in Figure 1 and Figure 2 (for instance the specimen P:8 is assembled utilizing 8 screws with an inclination of 9 between the axis connector and the shear plane). In some cases different screws inclinations have been used in the same specimen: for instance the specimen P: is assembled utilizing 4 screws with an inclination of 9 and 4 screws with an inclination of 6. Concerning the joint geometry, if the angle α defined in figure 1 is or 15, WT-T-19 screws are used; if α is 3 or 45, WT-T-22 screws are used. 2.2 Material Main and side members are made with glued laminated timber of Norway Spruce (Picea Abies Karst), strength class GL24h (according to European standard pren 1194:2), with 12% moisture content. Self-tapping screws with continuous thread are used (SFS Intec type WT-T19 and WT- T22); strength properties are derived through specific tests. Tab. 1 Material. Type of screws D1 D2 L L1 = L2 Glulam class ρ k [Kg/m 3 ] Moisture [%] WT-T-19 8,2 8, GLh WT-T-22 8,2 8, α < α > 45 D2 shear plane interface compression screw tension screw S2 grain L D1 S1 Fig. 1 Details of test set up and of the specimens (left); the type of screw utilized (centre);the coordinate system with the definition of the α angle (right above); the cyclic test protocol according to pren (right below).

3 2.3 Test set-up and protocol Both static and cyclic experiments have been performed under displacement control, by means of a servo-mechanical testing machine (see Figure 1). Information force and slip of the connections are derived, respectively, through a load cell and four L.V.D.T. transducers positioned symmetrically along the two shear planes. The cyclic tests have been performed following the loading procedure reported in the European standard EN (Figure 1). The static ductility of the joint (ability to undergo large amplitude slip without a notable reduction of the strength) is defined as Ds = Vu / Vy, where V u is the maximum deformation, and V y is the limit elastic deformation: both parameters have been worked out according to the method illustrated in EN Static test results During static tests, no specimen has shown brittle failures (i.e. splitting or plug-shear), even if large displacements have been applied (more than 8mm) Fig. 2 Details of test set up for the specimens P:8-, P:8-3, P: , and the monotonic test results.

4 2.5 Ciclic test results During cyclic tests, each specimen collapsed for the typical failure of connectors due to oligocyclic fatigue. νeq,3,2, α V y νeq,3,2, α V y νeq,3,2, α V y Fig. 3 Details of test set up of the specimens P:8- (left above), P:8-3 (right above), P: (left below), and definition of equivalent viscous damping ratio ν eq, according to EN (right above). Tab. 2 Experimental results for monotonic and cyclic tests, and ductility class according to European standard pren 1998: 23, where H stands for high capacity, and M stands for medium capacity. P:8- P:8-15 P:8-3 P:8-45 P: P: P: P: P: P: P: V y (mm),5,5,6,3,4,4,5,4,3,1,2 F y (kn) 53, 61,6 121,3 112,2 119,5 113,6 55,6 73,9 84,7 44,1 56,2 F max (kn) 142, 116,7 153,2 152,4 193, 23,9 116,5 117,8 127,8 113,7 18,7 F u (kn) 131,6 99,1 122,6 121,9 168,5 214,8 18,1 96,5 15, 11,6 11,6 V u (kn) 3, 3, 6,8 6,1 3, 3, 3, 3, 3, 3, 3, D s 58, 62,1 1,9 18,5 74, 69,3 56,5 85,1 89,6 276,7 171,7 F y / V y (kn/ mm) 6,4 8, 12,3 21,3 12,3 8,2 6,5 13,1 15,8 25,4 2,1 ν eq (%) EC8 class - H H M M H - H H M - H

5 In Table 2, an estimation of energy dissipation during the hysteresis cycles is reported, by means of the equivalent viscous damping ratio ν eq measured for every third repeated cycle in the loading procedure of Figure 1, where, according to the EN 12512, ν eq is defined as νeq = Ed /(2 π Ep ), where E d is the energy dissipated per half cycle, and E p is the available potential energy (Figure 3). It is worth noting that the values carried out from experimental results are far more promising, from the energy dissipation point of view, compared to the values reported in some literature. According to Chopra (1995), ν eq = 1% 15%, for dowel type connection, ν eq = 15% 2%, for nailed type connections. Finally, a comparison is made with the model developed by Bejkta and Blass; respect to the experimental results, this model shows a good predictive capacity, as clearly shown in Table 2 (specimens with single inclination of the screws for failure mode 3). The following assumptions are made: friction coefficient between timber members μ equal to,25; embedding strength f h,i costant for every inclination; withdrawal parameters f α,mod,i,3 for failure mode 3 equal to the withdrawal capacities of the screws, derived for the different angles α from EN 1995:24 standard. The third assumption is justified for the existence of a smoothed shank between the threaded parts, in contact with the embedded areas (see Figure 1). 3 Numerical analyses In order to better investigate the behavior of the connectors under large displacements, some F.E.M. model were developed, using the software ANSYS v9. Because the global answer of a joint can be seen as the sum of the behaviors of single screws, separated models are studied for screws at an angle of, 15, 3 and 45, either in tension or in compression. In this way, it is possible to implement a numerical bi-dimensional model. In particular: Timber zones are modeled through an orthotropic material, with an elasto-plastic behavior for each stress component; mechanical properties are derived from European standard EN 1194:2. The choice to utilize simple elasto-plastic law instead of more complicated failure criteria is justified by the great ductility shown by all specimens, without brittle failures. Screw is modeled in order to obtain a member with the same bending capacity of the thread root of the screw. The withdrawal strengths of the screws are modeled through link elements, which behaviors exhibit a maximum value and a subsequent capacity decrease, according to Bejkta and Blass. The maximum values are taken equal to the withdrawal strengths, according to EN 1995:24 standard model tension screws experimental compression screws model experimental Fig. 4 Model results for P:8- (left) and P:8-15 (right), under monotonic load. A sensitivity analysis has been made for all FEM models, where main mechanical parameters have been changed within a suitable range. The model with screw at is also compared with a more refined three-dimensional model. The numerical results can be considered good, as confirmed by Figure 4.

6 4 Discussion of results As reported in Table 2, joints with screws close to show a good ductility capacity, while joints with screws close to 45 have a great initial stiffness, but associated to a more brittle behavior. Joints with screws arranged at different angle along the shear plane show, at the same time, good stiffness and ductility. In particular it is demonstrated that the behavior of these joints can be obtained from the combination of the behaviors of single inclination screws: so, the specimen P: can be regarded as the simple sum of the specimens P:4- and P:4-3. As a result, an ideal elasto-plastic behavior for a connection can be hypothesized through an appropriate choice in the arrangement of the screws. Moreover, under large displacements, some specimens show a particular behavior: P:8-15 and P:8-3, in fact, present a particular shape of the Force - Slip pattern, with an initial force peak value, followed by a softnening phase and a subsequent increase of the capacity. The reason is due to the compression screws that, in presence of great deformations, change their state of stress and become in tension. As reported in Figure 3 and in Table 2, joints with screws close to show good dissipative capacities in cyclic tests, with low degradation of hysteretic loops for each displacement increment. Relevant degradation phenomena affect joints with screws close to 45 : the screws, in fact, are subjected to major axial displacements, causing a fast decrease of their withdrawal strength. The numerical analysis confirms the experimental data: the developed simplified numerical model is able to predict the behavior of each connector under great displacements. 5 Acknowledgments The research has been partly financed by the Administration of the Provincia Autonoma di Trento, through the Research Project named CODULE, and by the Italian Emergency Management Agency Dipartimento della Protezione Civile through the ReLUIS Consortium, under the three years Research Project ReLUIS - DPC. The Company Rotho-Blaas is grateful acknowledged for the support during experimentation, providing material and other equipment. 6 References [1] Johansen, K. W., Theory of timber connections, IABSE (1949) Publication no. 9: [2] Blass H. J., Bejtka, I., 22. Joints with inclined screws. Proceedings of CIB-W18 Timber Structures, Meeting 35, Paper , Kyoto, Japan, 12 pp. [3] Kevarinmäki, A., Joints with inclined screws. Proceedings of CIB-W18 Timber Structures, Meeting 35, Paper , Kyoto, Japan, 12 pp. [4] EN 1194, Timber structures, Glued laminated timber, Strength classes and determination of characteristics values, [5] pren FINAL DRAFT Timber Structures Test methods Cycling testing of joints made with mechanical fasteners. June 21. [6] Eurocode 8 pren Design of structures for earthquake resistance - Part 1: General rules, seismic actions and rules for buildings, Final Draft, December 23. [7] Chopra, A.K Dynamic of structures: theory and applications to earthqueake engineering, Prentice Hall, Englewood Cliffs, New Jersey, USA, 844 pp. [8] Eurocode 5 EN Design of timber structures Part 1-1: General Common rules and rules for buildings. For Publication, November 24. [9] Tomasi Roberto. Ductility and pseudoductily in timber elements: state of the art, research and development of innovative technologies (In Italian). PdD thesis, University of Trento, Faculty of Engineering, discussed in February 24