Transactions on the Built Environment vol 55, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509 Mechanics of traditional connections with metal devices in timber roof structures L. Candelpergher & M. Piazza Department of Mechanical and Structural Engineering, Trento UniversiQ Italy Abstract The preservation of ancient buildings (also if not monumental structures) has evolved, during recent years, from a mainly cultural issue into an economic opportunity and instrument of correct administration of the real estate. In this field the presence of wooden structures is significant. Structural timber, however, is affected by several problems related to the material itself. the structural elements and the connections. The arising of these difficulties often leads, in restoration design, to drastic solutions, such as the complete substitution of elements or whole sub-structures or, even worse? the use of elements made of other materials. Results, then, have been shown to be questionable not only from the aesthetic point of view but also from the strictly structural one. Justifications for similar interventions are very often related to the difficulty, complexity and burden of acquiring the understanding and knowledge about the existing structures. Within this framework, it appears necessary to struggle in deepening the methods to investigate mechanical and structural characteristics of existing structures, along with the techniques for effective restorations. This article reports the most recent results from the research held in Trento about connections typical of traditional timber roof structures (trusses) and especially about carpentry joints retrofitted with metal devices. The purpose of the specific research is to gain an understanding of the mechanisms and factors that influence the behavior of these connections and to set up synthetic models in order to characterize and verify the overall joint structural behavior. Traditional connections Most of the joints concerned, as the ones shohn in figure 1, provide connection between elements that are mainly subjected to axial loads. In practice, they are usually modeled as perfect hinges or, where a rotational stiffness is required for Transactions on the Built Environment vol 55, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509 4 16 Structaral Studies, Repairs and ,Maintenance ofHistorica1 Buildings equilibrium, a full moment transmission is assumed (Ehlbeck, Kromer, 1995). Actually, traditional joints, based on frictional behavior, are capable of internal constraints that could be hardly described with the classical hinged or fixed schemes. Such connections, indeed, show a limited but nevertheless non negligible rotational stiffness, so that they could be correctly classified as semi- rigid joints. Along with the monolateral nature of carpentry joints, relying on friction only, comes the need to employ metal reinforcements. This must be done in order to partially restore the continuity of the connection in case of exceptional loading events: unloading of compressed elements, whch warrant contact between surfaces and development of friction, may lead to disconnection of elements and, in extreme conditions, to structural collapse. The types of metal reinforcements commonly used in practice, however, do not transmit directly all forces across the joint, as modem devices do: they are thought to exclusively maintain the functionality of the connection under unusual conditions. N Figure l: Typical traditional timber roof truss, with details of carpentry connections based on the birdsmouth joint. The research presented here was essentially directed to parametric characterization of the mechanical behavior of reinforced connections. Results shown here refer to the most common type of connection in traditional timber trusses: the birdsmouth joint. The research path has gone through three subsequent phases: experimental tests on physical models in real size, numerical modeling and parametric analysis and eventually reinterpretation of results by means of simplified physical models. Experimental analysis The phase of direct experimental investigation was first started with tests on models of plain connections, built according to the two most frequently used geometries, i.e. nodes with skew angles (angle between the axis element lines) of 30" and 60". Afterwards, in two subsequent series, tests where carried out under monotonic and cyclic loading conditions on the same models reinforced by means of three different metal devices: bolt passing through rafter and chord, strip tightly binding the elements and couple of lateral stirrups (see figure 2). Transactions on the Built Environment vol 55, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509 Structrrral Studies, Repairs and Mazrttenance of Historical Buildings 4 17 Figure 2: Types of reinforcement under investigation: passing-through bolt (a), strip bmding the elements (b), couple of lateral stirrups (c). Tests on assembled connections were preceded by an accurate determination of the mechanical properties of the elements used for all h11 scale models. The wood used was obtained fiom the demolition of a building from XVII century, and was classified as coniferous timber of class from C18 to C22, according to the European Standard EN338. Loading conditions and measuring instrumentation are shown in figure 3. The assembled connections were placed on a steel test-stand that could be arranged, depending on the skew angle of the tested joint, so that the compressed rafter would always be initially vertical. By means of two independent hydraulic jacks an axial compression load (W, maintained constant during the whole test, and a transversal load (F),acting at the upper end of the rafter, were applied. The latter load generated a bending moment in the joint and could be monotonic or cyclic. Throughout all loading steps several measurements were taken, namely applied loads and corresponding displacements and deformations at significant locations. Direct results fiom such measurements were therefore expressed in terms of diagrams force F versus displacement s (channel "00" in figure 3). both monitored at the upper end of the rafter. The employment of strain gauges in the nodal region allowed to evaluate localized phenomena with particular regard to the strains perpendicular to fiber direction. Figures 4 shows some relevant F-s experimental diagrams: observing and comparing all the curves obtained in such way allowed a first comprehension: mainly qualitative, of the Influence on the joint behavior played by the reinforcements: first of all should be noticed the strongly non-linear rotational response of the plain connection: after an initially elastic phase, a sudden loss of stiffness follows at the time when the limit conditions of resistance based on friction only are reached along the contact surfaces. The roughly discontinuous response, more evident for the lower skew angle, is consequent upon phenomena of surface interloclung, plastic squashing, instantaneous losses of equilibrium localized in the contact areas. Adding a metal reinforcement allows to reach much higher strength than that in of the plain connection and to observe large excursions the post-elastic field, with values of rotational stiffness and ductility still very significant. Connections reinforced with bolt and binding strip appear to be, from this point of view, Transactions on the Built Environment vol 55, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509 4 18 Structural Studies, Repairs and Maintenance of Historical Buildings Figure 3: Loading conditions and measuring instrumentation (plain birdsmouth joint, skew angle a=60°). X / -Plain 0 Bolt % St~rru~s Strw Figure 4: Birdsmouth joint, skew angle a=30°, axial compression o,=O,SOMPa: Force - Displacement of rafter's upper end diagram for various types of reinforcement. retrofitting techniques of low impact but high effectiveness: they indeed do not alter the elastic response typical of the plain joints (stiffness remains almost identical), but play a substantial role when the bearing capacity due to simple friction comes to its limit, and this especially in the case of cyclic loading conditions. On the other hand the solution with lateral stirrups looks far less interesting, due to the radical modification, already at very low load levels, Transactions on the Built Environment vol 55, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509 induced by the reinforcement on the nodal response of the plain connection and, furthermore, due to the completely unsatisfactory performances shown under cyclic conditions (Parisi, Piazza, 2000). Numerical modeling The experimental measurements became the fundamental reference for the subsequent phase of the research: finite element numerical modeling of connections reinforced with bolt and binding strip (figure 5). Bi-dimensional models were set up, accurate enough so that they could simulate the experimental test conditions (finite element code Abaqus 5.8). Some aspects, typical of modeling continuous structural problems involving wood and complex geometries, are worth reporting here. ~igure5: Example of f~teelement model simulation of the experimental test conditions. Birdsmouth joint (a=60°) with passing-through bolt (left side) and detail of the finite element mesh (right side). The performed simulations took first of all into account the anisotropic nature of wood as a continuous material (Kollmann, Cote, 1984). The constitutive law was reproduced according to an orthortopic elasticity