MONOTONIC TESTS OF STRUCTURAL CARPENTRY JOINTS

Transcription

1 MONOTONIC TESTS OF STRUCTURAL CARPENTRY JOINTS Pedro Palma 1, João Ferreira 2, Helena Cruz 3 ABSTRACT: An experimental campaign on traditional diagonal front notched timber joints was carried out in order to assess their rotational behaviour. This campaign included ten different test conditions, comprising some of the most frequently observed configurations of the rafter and tie beam joint in Portuguese roof structures. The joints' mechanical behaviour was appraised with regard to their geometric parameters, presence of metal fastening devices and moisture content of timber. The experimental results show that the joint's response is different when opening or closing the skew angle and that specific load bearing mechanisms dictate the different performances exhibited by each typology. KEYWORDS: timber structures, connections, carpentry joints, tests, mechanical behaviour 1 INTRODUCTION BACKGROUND AND OUTLINE Modern standards and building codes focus primarily on modern industrialized mechanical or bonded timber joints, providing little or no guidance to design engineers regarding traditional timber carpentry joints; likewise, appraising engineers often face difficulties when making assumptions about carpentry joints in ancient timber structures, especially if calculation checks are to be carried out. Therefore it is necessary to establish reliable design and detailing rules and to provide rational criteria for the assessment of joints in existing structures, as well as recommendations for repair or strengthening interventions. The rafter and tie beam joint in traditional roof structures allows a wide range of possible layouts [1], with or without different metal parts. These joints are sometimes exposed to severe environmental conditions and, due to their location near the supports, withstand the highest internal forces in the roof trusses. For structural analysis purposes they are usually assumed as either pinned or rigid, when their actual behaviour lies somewhere in between [2-5]. The metal parts are often applied disregarding their influence in the joint's stiffness and failure mode, consequently influencing the overall structural behaviour. 1 Pedro Palma, Timber Structures Division, Laboratório Nacional de Engenharia Civil, Av. Brasil 101, Lisboa, Portugal. ppalma@lnec.pt 2 João Ferreira, Department of Civil Engineering and Architecture, ICIST/IST/UTL, Av. Rovisco Pais, Lisboa, Portugal. joao.ferreira@civil.ist.utl.pt 3 Helena Cruz, Timber Structures Division, Laboratório Nacional de Engenharia Civil, Av. Brasil 101, Lisboa, Portugal. helenacruz@lnec.pt The experimental campaign carried out at the Structures Department of the National Laboratory for Civil Engineering (LNEC) intended to characterize the rotational behaviour of some of the most frequently observed configurations of the rafter and tie beam joint in timber roof structures, concerning the influence of different factors related to the joint geometry (such as the notch depth and the presence of interior mortise and tenon), commonly employed metal fasteners (stirrups and clamps) and timber moisture content (due to the frictional nature of the joint's response when increasing the skew angle). A literature review was first carried out regarding the rafter and tie beam joints, namely their common typologies, behaviour, design errors and repair/strengthening methods. Afterwards, the layout of the experimental campaign was defined, accounting for the data collected in the previous step and the test specimens were then fabricated and subsequently conditioned to the required moisture content levels. The test set-up design [6] was focused on the specific requirements of these tests, namely the application of loads parallel and perpendicular to the rafter and the need to quickly assemble and disassemble the test specimens. Finally, the obtained test results are presented and discussed. 1.2 RAFTER AND TIE BEAM JOINTS Carpentry joints often connect timber elements without any other devices other than notches in the connected members. These joints rely on the timber-to-timber compression and friction forces to keep facing surfaces in close contact and seldom in metal fasteners Typologies The three main geometrical configurations of the rafter and tie beam joint are the widespread front notched joint (Figure 1 a)), the rear and the double notched joints. An

2 interior lengthwise mortise and tenon connection is also quite common (Figure 1 b)). Figure 3: Compression stresses in the notch Figure 1: Front notched joints without metal parts: a) without mortise and tenon; b) with mortise and tenon Metal fixings are often present in carpentry joints. In the experimental campaign the two commonly observed typologies presented in Figure 2 were analysed. These are intended to provide the basic behaviour model of each typology, although small variations are to be expected as a result of different geometric details. Figure 2: Front notched joints with metal parts: a) stirrups; b) clamps Stirrups (Figure 2 a)) were widely applied in ancient Portuguese timber joints and are frequently used as a reinforcement technique to simple carpentry joints. They act partially as a steel-to-timber double shear connection and prevent out of plane displacements and the undocking of the joined elements. The stirrups used in the tested joint specimens were made of two welded steel flat bars (3 50 mm 2 ), with holes for threaded steel bolts (Ø 8 mm). Clamps (Figure 2 b)) embracing the rafter and the tie in the notched area are also quite common and, although they can be materialized in many different ways, their function is essentially to restrain the relative displacements between the rafter and the tie by compressing a certain area of the outer surfaces of both members. In this campaign, two lateral bars (tension ties, Ø 8 mm) attached to thick metal plates (10 40 mm 2 ) that rested on the rafter s upper surface and the tie s lower surface were used. The ties were placed perpendicularly to the rafter and a small notch for the thick metal plate was made in the tie specimen Detailing and design The most widespread design procedure [7-11] consists in the definition of load paths between members, by decomposing the internal forces in components perpendicular to the notched surfaces (Figure 3) and by defining appropriate shear surfaces to accommodate those stresses (Figure 4). Detailing rules found in the related bibliography account for the notch depth and the heel length as related to the skew angle and the cross section's dimensions [8,12,13]. Figure 4: Shear stresses in the heel 2 EXPERIMENTAL CAMPAIGN 2.1 TEST PROGRAM AND MATERIALS The test program included ten different test conditions, comprising five configurations without metal parts and five with steel fixings (Figures 5, 6 and 7 show all the tested situations, where w stands for moisture content of timber and t v is the notch depth). Three replicas were made for each test situation, making a total of more than sixty tests. The connections were built with maritime pine wood (Pinus pinaster Ait.). Timber used had an average density of 550 kg/m 3 (mass and volume measured at 12 % moisture content) and care was taken to assure that all triplets of specimens had a similar average density. In all cases, joints were produced when the timber moisture content was around 16 to 18 % as this would likely correspond to the general practice. Afterwards, joints were conditioned to the target moisture content before testing. Figure 5: Tested configurations: joints without metal parts

3 Figure 6: Tested configurations: joints with steel stirrups Figure 7: Tested configurations: joints with steel clamps The base geometry of the test specimens, used in every tested configurations, is shown in Figure 8 a). The variations tested only in joints without metal parts are shown in Figures 8 b) and 9. Figure 10: Test setup (figure adapted from a scheme by Oliveira, F. [6]) The pneumatic actuator applies a force of 15 kn in the rafter specimen (representing the effect of self-weight and the dead loads in common timber roof trusses) which induces compressive stresses of 1.3 MPa. Transversal displacements in the rafter are imposed at a rate of 0.2 mm/s, at a distance of about 0.95 m from the intersection of the members axis. This is done monotonically (either opening or closing the skew angle) until failure of the joints (F u < 0.8 F max ) or the actuator s limit (around 100 mm) is reached. Figure 8: Joints with: a) t v = 4,5 cm; b) t v = 3,0 cm (dimensions in cm) Figure 9: Joint with mortise and tenon, with t v = 4,5 cm (dimensions in cm) 2.2 EQUIPMENT AND TEST PROCEDURE The equipment developed for the experimental campaign [6] consists of a metal rig, which holds the specimen, a pneumatic actuator, composed by an air compressor connected to two air bellows which apply the compression force in the rafter, and a hydraulic actuator, which applies the transversal displacements in the rafter (Figure 10). The air bellows are held against the cross section of the rafter specimen (Figure 11) by two hinged ties, parallel to the rafter, connected to the metal rig. Figure 11: Pneumatic actuator (figure adapted from a scheme by Oliveira, F. [6]) The test procedure is as follows: i) placing of the tie part in the metal base; ii) positioning of the rafter part and adjustment the the pneumatic actuator; iii) adjustment of the height and pressure of the air bellows to obtain the desired compression force in the rafter; v) attachment of the hydraulic actuator to the rafter specimen; vi) positioning of the LVDTs; vii) imposing of the displacements transversely to the rafter member.

4 2.3 RESULTS The test results are presented in the following sections, by comparing to each other the results obtained for different joint configurations Joints without metal parts Influence of moisture content When opening the skew angle, the moisture content increase has no significant influence in the joint's performance, but when closing the skew angle it introduces a saw-toothed profile in the load-slip curve (Figure 12), although it does not change the joint's strength or overall stiffness. the static ductility and in the 20 % moisture content joints (Figure 15) it smooths the saw-toothed profile of the load-slip curves and also increases the joint's strength. These results show that the presence of mortise and tenon, when adequately executed, can improve the joint's response. Figure 14: Joints without metal parts: influence of mortise and tenon, with w = 12 % Figure 12: Joints without metal parts: influence of moisture content, with t v = 45 mm Influence of notch depth Notch depth influence was studied only for the 12 % moisture content joint specimens (Figure 13). When opening the skew angle it has no effects in the joint's behaviour since for both geometries the contact area in the front notch decreases progressively throughout the test. On the other hand, the closing mode performance increased with notch depth (mainly the maximum displacement), because a smaller notch depth induced an earlier loss of equilibrium, completely undocking the rafter from the tie. The deeper notch geometry also led to an increased strength. Figure 15: Joints without metal parts: influence of mortise and tenon, with w = 20 % Joints with metal parts Influence of the stirrups Stirrups had a major influence in the joint's performance, regardless of the moisture content, both when opening and closing the skew angle: in most circumstances, the stirrups increased both the strength and the stiffness (except when closing the skew angle with w = 12 %) of the tested joints, as compared to joints without metal parts (Figures 16 and 18). Figure 13: Joints without metal parts: influence of notch depth, with w = 12 % Influence of mortise and tenon The presence of mortise and tenon has no impact when opening the skew angle, either for 12 or 20 % moisture content, but when closing the skew angle it has significant consequences in the joint's behaviour. In the 12 % moisture content joints (Figure 14), when closing the skew angle, the mortise and tenon greatly enhances Figure 16: Stirrups' influence, with w = 12 % A major drawback occurred in two of the tested joints where, due to the inadequate assembly (stirrups applied before compression of the rafter), there were gaps

5 between the rafter and the tie specimens in the notch area (Figure 17). Influence of clamps versus stirrups For the 12 % moisture content joints (Figure 20), when opening the skew angle, the joint's strength is similar in both configurations, although the clamps generate brittle failures (Figure 21 a)) while the stirrups do not. When closing the skew angle, the clamped joints exhibit much higher strengths and failure modes are ductile, but there is an early loss of contact between the front notch's surfaces (Figure 21 b)). Figure 17: Joint with stirrups: gaps in the notch area These poorly assembled joints showed much lower stiffness and strength values (dash-dotted lines in Figure 18) when compared to the other properly assembled joints, showing the great importance of a good match between the surfaces in the notch. Figure 20: Joints with metal parts: clamps and stirrups, with w = 12 % Figure 18: Stirrups' influence, with w = 20 % Influence of assembling stirrups before or after the loading of the rafter The assembling of the stirrups before or after the compression loading of the rafter denotes the difference amongst the stirrups applied in an early stage of the roof construction, or much later as reinforcement of the joint. This aspect was only examined for the 12 % moisture content joints (Figure 19). Figure 21: Joints with clamps: a) brittle failure (opening skew angle); b) loss of contact in the notch surfaces (closing skew angle) In the 20 % moisture content joints (Figure 22), when opening the skew angle, there is a reduction of strength in the clamped joints and an increase in the joints with stirrups, as compared to behaviour at 12 % moisture content, leading to similar values for both typologies. The clamped joints display, as in the 12 % moisture content joints, brittle failure modes (Figure 21 a)). Figure 19: Stirrups assembled before versus after the loading of the rafter, w = 12 % Test results show that assembling the stirrups after the loading of the rafter increases the joint's strength and, when closing the skew angle, leads also to an increase in the initial stiffness. This is due to the previous compression of the rafter providing a better fit of the notch surfaces, therefore mobilizing the wood-to-wood bearing and friction strength mechanisms in the notch, in addition to the steel-to-timber double shear strength of the stirrups. Figure 22: Joints with metal parts: clamps and stirrups, with w = 20 % When closing the skew angle, the swelling of wood promoted by the increased moisture content lead to a tighter fit between the metal parts and the timber members, which increased the strength of the joints with stirrups and decreased the strength of the clamped joints, although the latter displayed an ever increasing loaddisplacement curve, due to compression perpendicular to

6 grain under the thick metal plates and in the rear area of the notch. 3 CONCLUSIONS The main conclusions regarding the joint's behaviour obtained from the experimental campaign are listed below. In the joints without metal fixings: moisture content increase from 12 to 20 % did not reduce neither the strength or the stiffness of the tested joints, but introduced a saw-toothed profile in the load-displacement curves; notch depth decrease from 45 to 30 mm had no influence in the joint's performance when opening the skew angle, but reduced the strength and the ultimate deformation when closing the skew angle; mortise and tenon increased the joint's static ductility when closing the skew angle, and smoothed the saw-toothed profile in the joints with 20 % moisture content, but did not affect the performance when opening the skew angle. In the joints with metal fixings: stirrups greatly increased the joint's strength; a perfect match between the notched surfaces is crucial, since small gaps between elements induce great losses in the joint's strength and stiffness; applying the stirrups after compressing the rafter led to an increase in strength and stiffness, when compared to the joints assembled before the rafter's loading, in particular when closing the skew angle; clamps led to similar strengths to that of stirrups for 12 % moisture contents, but allowed brittle failure modes when opening the skew angle. When closing the skew angle of the clamped joints, there was an early contact loss between the surfaces in the front notch (in both 12 and 20 % moisture content joints); the increase in moisture content from 12 to 20 % was beneficial for joints with stirrups, due to a tighten fit of the notch surfaces, whereas it penalized the clamped joints that became more prone to brittle failure in tension perpendicular to grain, when opening the skew angle. [3] Senno M., Piazza M.: Behaviour and rehabilitation of queen post timber trusses: A case study. In: Structural Studies, Repairs and Maintenance of Heritage Architecture VIII, , [4] Branco J.: Influence of the joints stiffness in the monotonic and cyclic behaviour of traditional timber trusses. Assessment of the efficacy of different strengthening techniques. PhD Thesis, University of Minho, [5] Palma P., Cruz H.: Mechanical behaviour of traditional timber carpentry joints in service conditions results of monotonic tests. In: From Material to Structure - Mechanical Behaviour and Failures of the Timber Structures, [6] Palma P., Cruz H., Oliveira F., Morais P.: Sistema para ensaio de ligações estruturais em madeira. In: A Instrumentação Científica e a Metrologia Aplicadas à Engenharia Civil, pages , [7] Köhler N., Heimeshoff B.: Untersuchung über das Tragverhalten von zimmermannsmäßigen Holzverbindungen. Technischen Universität München, [8] J. Ehlbeck. Carpentry joints. In Timber Engineering STEP 1. Centrum Hout, Almere, Netherlands, [9] Blaβ H.J., Ehlbeck J., Kreuzinger H. and G. Steck: Erläuterungen zu DIN 1052: Bruderverlag, [10] Racher P.: Assemblages traditionnels : comportement des assemblages par queues d aronde. Complexe Universitaire des Cézeaux, [11] Hirsi H.: The carpentry joints in timber structures. Helsinki University of Technology, [12] Götz K., Hoor D., Möhler K. and Natterer J.: Construire en bois: Choisir, concevoir, réaliser. Presses Polytechniques et Universitaires Romandes, [13] SE-M Seguridad estructural - Estructuras de Madera. Ministério de Vivienda, ACKNOWLEDGEMENTS The authors wish to thank the Portuguese Science and Technology Foundation (Project POCI/ECM/56552/2004) for the financial support towards this research. Thanks also go to the LNEC Scientific Instrumentation Centre (Paulo Morais and Fernando Oliveira) for their most valuable contribution in the design and set up of the test equipment. REFERENCES [1] Palma P., Cruz H.: Comportamento de ligações tradicionais em estruturas de madeira. In: 4ªs Jornadas Portuguesas de Engenharia de Estruturas, [2] Parisi M., Piazza M.: Mechanics of plain and retrofitted traditional timber connections. J. Struct. Engrg. 126: , 2000.