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Article Retrofitting of Imperfect Halved Dovetail Joints for Increased Seismic Resistance

Miloš Drdácký and Shota Urushadze *

Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences, 190 00 Prague, Czech Republic; [email protected] * Correspondence: [email protected]; Tel.: +420-225-443-266

 Received: 27 December 2018; Accepted: 13 February 2019; Published: 18 February 2019 

Abstract: This paper presents possibilities for anti-seismic improvement of traditional timber carpentry joints. It is known that the structural response of historical roof frameworks is highly dependent on the behavior of their joints, particularly, their capacity for rotation and energy dissipation. Any strengthening, or retrofitting, approach must take into account conservation requirements, usually expressed as conditions involving minimal intervention. Several retrofitting methods were tested on replicas of historical halved joints within various national and international research projects. The joints were produced with traditional hand , and made using aged material taken from a demolished building. The paper presents two approaches, each utilizing different retrofitting technologies that avoid completely dismantling the joint and consequently conserve frame integrity. The energy dissipation capacity is increased by inserting mild steel nails around a wooden pin, and connecting the two parts of the halved joint. In the second case, two thin plates made of a material with a high friction coefficient are inserted into the joint and fastened to the wooden elements. This is done by removing the wooden connecting pin and slightly opening a slot for the plates between the halved parts. In addition, the paper presents an application for disc brake plates, as well as thin plates made of .

Keywords: carpentry halved joint; energy dissipation; seismic retrofitting

1. Introduction The behavior of carpentry joints in historical timber structures plays an important role in their overall structural response to applied loads, especially during seismic events. This has been the subject of several research projects, mostly in countries with high seismic activity, such as in the Mediterranean region. In these countries, roof framework joints employ a typical birdsmouth connection for joined timber elements, as shown in Figure1. For example, Parisi and Piazza [ 1] tested the rotational capability of such carpentry joints retrofitted with various metal connectors. They have also modeled friction joints in traditional timber structures [2]. Parisi and Cordié [3] further studied the elastic rotational stiffness and post-elastic behavior of double-step timber joints, and the reversed birdsmouth configuration.

Buildings 2019, 9, 48; doi:10.3390/buildings9020048 www.mdpi.com/journal/buildings

Article Retrofitting of imperfect halved dovetail carpentry joints for increased seismic resistance

Miloš Drdácký and Shota Urushadze *

Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences, 190 00, Czech Republic; [email protected]; [email protected] * Correspondence: [email protected]; Tel.: +420-225-443-266

Received: 27 December 2018; Accepted: 13 February 2019; Published: date

Abstract: This paper presents possibilities for anti-seismic improvement of traditional timber carpentry joints. It is known that the structural response of historical roof frameworks is highly dependent on the behavior of their joints, particularly, their capacity for rotation and energy dissipation. Any strengthening, or retrofitting, approach must take into account conservation requirements, usually expressed as conditions involving minimal intervention. Several retrofitting methods were tested on replicas of historical halved joints within various national and international research projects. The joints were produced with traditional hand tools, and made using aged material taken from a demolished building. The paper presents two approaches, each utilizing different retrofitting technologies that avoid completely dismantling the joint and consequently conserve frame integrity. The energy dissipation capacity is increased by inserting mild steel nails around a wooden pin, and connecting the two parts of the halved joint. In the second case, two thin plates made of a material with a high friction coefficient are inserted into the joint and fastened to the wooden elements. This is done by removing the wooden connecting pin and slightly opening a slot for the plates between the halved parts. In addition, the paper presents an application for disc brake plates, as well as thin plates made of oak.

Keywords: carpentry halved joint; energy dissipation; seismic retrofitting

1. Introduction The behavior of carpentry joints in historical timber structures plays an important role in their overall structural response to applied loads, especially during seismic events. This has been the subject of several research projects, mostly in countries with high seismic activity, such as in the Mediterranean region. In these countries, roof framework joints employ a typical birdsmouth connection for joined timber elements, as shown in Figure 1. For example, Parisi and Piazza [1] tested the rotational capability of such carpentry joints retrofitted with various metal connectors. They have also modeled friction joints in traditional Buildings 2019, 9, 48 2 of 12 timber structures [2]. Parisi and Cordié [3] further studied the elastic rotational stiffness and post-elastic behavior of double-step timber joints, and the reversed birdsmouth configuration.

FigureFigure 1. Typical 1. Typical birdsmouth birdsmouth joint joint and Mediterranean Mediterranean roof roofstructure structure schemes. schemes.

Buildings 2018, 8, x; doi: FOR PEER REVIEW www.mdpi.com/journal/buildings Branco et al. [4] performed a series of tests on joints subjected to static and repeated loading to assess the impact of various strengthening methods. Palma et al. [5] published an extensive review of references, as well as the results of their own experimental research in rotational behavior of and tie beam connections, including a study on the efficiency of some typical strengthening techniques. Similarly, Poletti et al. [6] studied traditional timber joints under cyclic loading, and possibilities of their repair or strengthening. All these works, for the most part, present techniques that are barely applicable to the structural restoration of historic timber structures. Low slope roofs are typical in the Mediterranean region, in contrast to Central and Northern Europe, where steeper roofs are common and the roof framework structures use different details, especially halved dovetail joints with a higher rotational stiffness. Real joints in historical structures have many imperfections, for reasons ranging from low-quality carpentry work to material defects and degradation. These imperfections substantially influence the stiffness of roof trusses [7], decrease the safety of historical structures, and worsen the response of the roof frames in the event of seismic loading. A study of ways to improve the energy dissipation of carpentry joints was, therefore, carried out within the EC 7th Framework Programme NIKER project, which aimed to establish new integrated knowledge-based approaches for the protection of cultural heritage from earthquake-induced risks. The performance of roof framework systems under seismic or generally dynamic loading has scarcely been studied even though roofs are sensitive to horizontal loads, especially those perpendicular to the of the trussed frame [8], despite the importance of such research having been underlined [9]. The stable roof framework as a three-dimensional box, and its potential role in a seismic event, has been analyzed by Giurani and Marini [10] but, again, only for low slope roofs. Steep roofs have not usually been considered for seismic loadings. However, they are typically constructed in a much stiffer way than low slope roofs, and do influence the building’s performance. Their role in this respect has not been appropriately studied, and this is also not an aim of this paper. With regard to historical structures, conservation requirements constrain retrofitting approaches to minimize interventions. Intervention should not significantly alter the appearance and behavior of a structure, and solutions that do not involve the complete disassembly of existing structures are preferred. This is important in the case of structural restoration works. During an experimental campaign, a number of retrofitting processes were adopted and tested on replicas of historical halved joints that were made from old taken from a demolished building and produced using traditional carpenters’ tools and techniques. Presented, here, are two approaches that each utilized a different retrofitting technique.

2. Experiments

2.1. Test Specimens The experiments were carried out on replicas of traditional halved dovetail joints made from authentic timber (approximately 300 years old) taken from a demolished ancient building. The wooden elements had not been attacked by wood-destroying fungi or by wood-boring insects. Only drying fissures and cracks were present. Buildings 2019, 9, 48 3 of 12

2.1.1. Material Characteristics After the experiments on the joints were complete, the material characteristics of the joint were determined by testing the standard coupons taken from the joint assemblies. The material properties evaluated were wood density and strength properties. Material information, mainly the coefficients of friction of the plates used for increasing energy dissipation, was taken from the literature [11]. The mechanical quantities of the brake plates were taken from the manufacturer [12]. The mechanical properties of the materials are summarized in Table1.

Table 1. Material characteristics.

Buildings 2018, 8,Mechanical x FOR PEER REVIEW Property Spruce Oak Brake3 Plateof 12 Density—ρ (kg/m3) 340–450 700–750 1950 Table 1. Material characteristics. Compression parallel to grain—R (MPa) 15.63 – – Compression perpendicularMechanical to grain— PropertyR (MPa) Spruce2.09 Oak –Brake Plate – Modulus of elasticity—Density—Eρ(MPa) (kg/m3) 340––450 700–750 12,000 1950 – StrengthCompression in compression— parallel fto(MPa) grain—R (MPa) 15–.63 -- –-- 180 CompressionCoefficient ofperpendicular friction—µ to grain—R (MPa) 2–.09 -- 0.41 *-- 0.4 * FrictionModulus coefficient of of elasticity the oak parallel—E (MPa to)the grain 0.48,-- perpendicular 12000 to the grain-- 0.34. Strength in compression—f (MPa) -- -- 180 Coefficient of friction—µ -- 0.41* 0.4 2.1.2. Geometrical Characteristics * Friction coefficient of the oak parallel to the grain 0.48, perpendicular to the grain 0.34. The geometry of the joint assemblies varied, as did the extent of the wood’s deterioration. An 2.1.2. Geometrical Characteristics angle of 45◦ between the joined members was adopted. This is close to the angles most widely used in Central EuropeanThe geometry roofing of the joint frames. assemblies The varied, geometry as did of the the extent specimens, of the wood’s the locationsdeterioration. of An the acting forces, andangle the of 45° potentiometer between the joined are depicted members in was Figure adopted.2. The This specimens is close to the were angles produced most widely with used intentional in Central European roofing frames. The geometry of the specimens, the locations of the acting forces, imperfections,and the as potentiometer far as the tightness are depicted of thein Figure coupled 2. The elements specimens was were concerned, produced to with model intentional some degree of joint degradation.imperfections For, as far this as research,the tightness only of the two coupled types elements of imperfection was concerned were, to considered—perfect model some degree joints with a tightof joint connection degradation. betweenFor this research joined, only short two struts types of in imperfection the overlap, were and considered imperfect—perfect joints joints exhibiting a slightwith loss a oftight contact connection in the between abutment joined ofshort the struts joined in the elements. overlap, and The imperfect imperfections joints exhibiting were produced a duringslight high-quality loss of contact carpentry in the abutment work and, of the therefore, joined element theirs. The range imperfection was limiteds were to produced local defects during of 3 mm high-quality carpentry work and, therefore, their range was limited to local defects of 3 mm maximum. The geometric imperfections exhibit rather high slippage during the change in direction of maximum. The geometric imperfections exhibit rather high slippage during the change in direction the loadedof the arm loaded rotation, arm rotati as shownon, as shown in Figure in Figure 8 and 8 Figureand Figure 9, compared 9, compared to t ao perfectlya perfectly mademade joint, e.g., as shown ine.g., Figure as shown 13. in The Figure increase 13. The in increase overall in deformation overall deformation increment increment of a roofof a roof truss truss due due to to joint joint slippage decreasesslippage with decreases an increase with of an slippage increase of value. slippage This value. dependence This dependence is not linear,is not linear, and theand greatestthe greatest difference in the overalldifference deformation in the overall has deformation been analyzed has been only analyzed around only the around change the from change perfect from to perfect imperfect to joint geometryimperfect [7]. joint geometry [7].

Figure 2. Geometrical scheme of the joint, the potentiometer location and location of the acting force, Figure 2. Geometrical scheme of the joint, the potentiometer location and location of the acting force, the the directiondirection of of the the joint’sjoint’s rotation rotation,, and and the corresponding the corresponding moment. moment.

2.2. Joint Retrofitting Several retrofitting techniques for increased energy dissipation were suggested and tested, as described above. However, for historic roof frameworks, only those which were acceptable from an aesthetic point of view were relevant for further development, especially in cases where the historic structures were exposed and accessible to visitors. Furthermore, the requirements of minimum Buildings 2019, 9, 48 4 of 12

2.2. Joint Retrofitting Several retrofitting techniques for increased energy dissipation were suggested and tested, as described above. However, for historic roof frameworks, only those which were acceptable from an aesthetic point of view were relevant for further development, especially in cases where the historic structures were exposed and accessible to visitors. Furthermore, the requirements of minimum Buildingsintervention, 2018, 8, x re-treatability, FOR PEER REVIEW and easy implementation were decisive for the selection of the4 tested of 12 techniques. Only the two most-suitable approaches for practical applications will be presented, intervention, re-treatability, and easy implementation were decisive for the selection of the tested as follows. techniques. Only the two most-suitable approaches for practical applications will be presented, as follows. 2.2.1. Metal Nails around a Wooden Pin 2.2.1. Metal Nails around a Wooden Pin In the first case, energy dissipation capacity was increased by inserting mild steel rods (nails) around In the woodenfirst case, pin energy that dissipation connects the capacity two parts was ofincreased the halved by inserting joint, as shownmild steel in Figurerods (nails3a,b.) aroundNo dismantling the wooden was pin necessary that connects for this the retrofitting two parts intervention.of the halved Thejoint 6, as mm shown diameter in Fig (D)ure nails 3 a,b. were No dismantlingapplied into was pre-bored necessary holes, for which this retrofitting allowed for intervention. minimum spacing.The 6 mm When diameter using (D) six nails nails, were the standard applied intorecommendations pre-bored holes, (10xD which along allowed grains, 3xD for acrossminimum grains) spacing. were observed.When using When six using nails, eight the nails, standard due recommendationsto the grain inclination, (10xD thealong distance grains, along3xD across the grain grains) was were only observed 6xD. . When using eight nails, due to the grain inclination, the distance along the grain was only 6xD.

(a) (b)

FigureFigure 3. 3.(a ()a) Retrofitting Retrofitting usingusing sixsix mild steel nails; nails; (b (b) )retrofitting retrofitting using using eight eight steel steel nails. nails.

2.2.2.2.2.2. Friction Friction Joints Joints InIn the second retrofitting retrofitting approach approach,, the the connecting connecting wooden wooden pin pin wa wass removed removed from from the the joint, the halvedhalved parts parts we werere slightly separated, separated, and and two two thin thin plates plates we werere then then inserted into into the the opened slot and fastenedfastened to to the the wooden wooden elements, elements, as asshown shown in Figures in Figures 4 and4 and 5 a5,b.a,b. The The pla platestes were were made made of material of material with awith high a friction high friction coefficient. coefficient. Car disc Car brake disc plates brake and plates thin andoak thinplates oak were plates used. were The used.joint wa Thes then joint fixed was andthen tightened fixed and with tightened a steel withbolt, awhich steel bolt,was whichpre-stressed was pre-stressed to a certain todegree. a certain The degree.screw bolt The enables screw boltthe joint’senables pre the-stress joint’s level pre-stress to change, level which tochange, influences which not only influences the stiffness not only of the the joint, stiffness but also of thethe joint,friction butal forcealso thebetween frictional the plates. force betweenSeveral bolt the pre plates.-stressing Several moment bolt pre-stressingvalues were chosen moment and values tested ( werefrom chosen115 up toand 240 tested Nm), (fromwhich 115 gen uperated to 240 a stress Nm), level which on generatedthe friction asurfaces stress level of around on the 0 friction.43 –0.90 surfaces MPa. of around 0.43–0.90 MPa.

Buildings 2018, 8, x FOR PEER REVIEW 5 of 12 Buildings 2018, 8, x FOR PEER REVIEW 5 of 12 Buildings 2019, 9, 48 5 of 12 Buildings 2018, 8, x FOR PEER REVIEW 5 of 12

Figure 4. Schematic sketch showing insertion of friction plates into a halved , without Figure 4. Schematic sketch showing insertion of friction plates into a halved dovetail joint, without Figurecomplete 4. Schematic dismantling sketch, which showing is the main insertion advantage of friction of this plates approach. intoa halved dovetail joint, without completeFigure 4. dismantlingSchematic sketch, which showingis the main insertion advantage of frictionof this approach. plates into a halved dovetail joint, without complete dismantling, which is the main advantage of this approach. complete dismantling, which is the main advantage of this approach.

(a) (b) (a) (b) Figure 5. (a) Joint retrofitting(a) with brake friction plate and replacement of the(b )oak pin by a pre-stressed Figure 5. ((aa)) J Jointoint retrofitting retrofitting with with brake brake friction friction plate plate and and replacement replacement of of the the oak pin by a pre-stressedpre-stressed steel screw bolt; (b) an oak plate for joint retrofitting with friction plates. steelFigure screw screw 5. (a bolt;) bolt; Joint (b ( b)retrofitting )an an oak oak plate plate with for for brakejoint joint retrofitting friction retrofitting plate with withand friction replacement friction plates. plates. of the oak pin by a pre-stressed steel screw bolt; (b) an oak plate for joint retrofitting with friction plates. 2.3.2.3. Test Test Arrangement Arrangement 2.3. Test Arrangement 2.3. Test OnArrangement the basis of positive experience gained from previous tests performed at the Institute of On the basis basis of positive positive experience gained from previous tests tests performed performed at at the the Institute Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences (ITAM) [7,13], a similar test set- TheoreticalOn the and basis Applied Applied of positive Mechanics Mechanics experience of of the the gainedCzech Czech Academy from Academy previous of of Sciences Sciences tests ( performedITAM (ITAM)) [7,13 [7 at,],13 athe], similar a Institute similar test testset of- up was constructed, as shown in Figure 6. This set-up enabled cyclic loading and pseudo-dynamic upTheoreticalset-up was was constructed, constructed,and Applied as asshown Mechanics shown in inFigure of Figure the 6Czech6. .Th Thisis Academyset set-up-up enable enabled of Sciencesd cyclic ( loading loadingITAM) [ 7 and and,13], pseudo-dynamic pseudoa similar-dynamic test set- behavior of the halved dovetail joints to be simulated separately from the roof frame. The joint samples behaviorup was constructed,of of the the halved halved as dovetail shown dovetail joint in jointsFigures to be to 6simulated. be Th simulatedis set -separatelyup enable separately dfrom cyclic the from loading roof the frame. roof and frame.The pseudo joint- The dynamicsample joints for testing were placed into a special testing rig that enabled pseudo-dynamic cyclic loading. It also forbehaviorsamples testing forof were the testing halved placed were dovetail into placed a specialjoint intos ato testing special be simulated rig testing that separately rigenable thatd enabled pseudo from the pseudo-dynamic-dynamic roof frame. cyclic The loading. cyclic joint sample loading. It alsos ensured static stability of the samples and their responses in only the direction of loading. The cyclic load ensureforIt also testingd ensured static were stability staticplaced of stability intothe sample a special of thes and samplestesting their rigresponses and that their enable in responses onlyd pseudo the direction in-dynamic only of the loading. cyclic direction loading.The ofcyclic loading. It load also was applied using a servohydraulic MTS Systems Corporation actuator (cylinder) (MTS headquarters, Thewasensure cyclicappliedd static load using stability was a servohydraulic applied of the sample usings aMTSand servohydraulic theirSystems responses Corporation MTS in only Systems actuatorthe direction Corporation (cylinder) of loading. ( actuatorMTS The headquarters cyclic (cylinder) load, 14000 Technology Drive, Eden Prairie, MN, USA, 55344) with a capacity of 25 kN, attached to a steel frame. (MTS14000was applied Te headquarters,chnology using Drive a 14000servohydraulic, Eden Technology Prairie, MTS MN Drive, , SystemsUSA Eden, 55344 Corporation Prairie,) with MN,a capacity actuator USA, of 55344) 25(cylinder) kN, with attached ( aMTS capacity to headquarters a steel of frame. 25 kN,, The rotational responses of the joints were measured indirectly by means of a MEGATRON Elektronik The14000attached rotational Technology to a steelresponse Drive frame.s, Edenof Thethe Prairie, rotationaljoints wMNere, responses USAmeasured, 55344 ofindirectly) with the jointsa capacity by were means of measured 25 of kN, a MEGATRON attached indirectly to a steel Elektronik by meansframe. GmbH & Co. KG SPR 18-S-100 (5kΩ) potentiometer (MEGATRON Elektronik GmbH & Co. KG ▪ GmbHTheof a rotational MEGATRON & Co. response KG ElektronikSPRs 18of- Sthe-100 GmbHjoint (5kΩ)s w &ere potentiometer Co. measured KG SPR indirectly 18-S-100(MEGATRON by (5 means kΩ ) El potentiometerektronik of a MEGATRON GmbH (MEGATRON & Elektronik Co. KG ▪ Hermann-Oberth-Strasse 7 ▪ 85640 Putzbrunn / Munich, Germany). HermannGmbHElektronik &- Oberth Co GmbH. KG-Strasse &SPR Co. 187 KG -▪S 85640-100Hermann-Oberth-Strasse (5kΩ)Putzbrunn potentiometer / Munich, G(MEGATRONermany 7 85640). Putzbrunn/Munich, Elektronik GmbH Germany). & Co. KG ▪ Hermann-Oberth-Strasse 7 ▪ 85640 Putzbrunn / Munich, Germany). Buildings 2019, 9, 48 6 of 12 Buildings 2018, 8, x FOR PEER REVIEW 6 of 12

Figure 6. The test set-up. Figure 6. The test set-up. The specimens were cyclically loaded, and the load deformation curves were registered. The load Thewas specimens applied to were the joint cyclically using the loaded, actuator and attached the load to the deformation oblique beam. curves The intensity were registered.of the actuator’s The load was appliedforce to was the controlled joint using by the its prescribedactuator attached displacement. to the The oblique amplitude beam. of the The controlled intensity displacement of the actuator’s force wasincreased controlled for each by itscycle, prescribed with a constant displacement. step equal to The 4 mm. amplitude The frequency of the of each controlled cycle was displacement 0.1 Hz. increasedDuring for each the tests, cycle, the with forces a needed constant to achieve step equal the desired to 4 mm.displacement The frequency of the cylinder of each and the cycle change was in 0.1 Hz. During thethe tests,lengththe of the forces potentiometer needed were to achieve recorded the, asdesired shown in displacement Figure 7. of the cylinder and the change Buildings 2018, 8, x FOR PEER REVIEW 7 of 12 in the length of the potentiometer were recorded, as shown in Figure7.

Figure 7. Time histories of the measured quantities during cyclic tests. Figure 7. Time histories of the measured quantities during cyclic tests.

3. Results Damping property is a fundamental factor influencing a structure’s seismic resistance. Effective dissipation of energy by a structural element, e.g., a joint, can significantly decrease a structure’s vibration level and lessen internal forces. The dissipative properties of the investigated halved dovetail joints can be described by the area of hysteresis loops, as shown in Figure 8a,b,9,10,11. Here, the hysteresis loops area representation of how the actuator’s moment of force about the axis of pin M is dependent on the rotation of joint Δα for one loading cycle. Both of the retrofitting methods tested (see Discussion) were effective from an energy consumption point of view. The results showing the changes in hysteresis loops after joint retrofitting are presented in Figures 8a,b,9,10,11. Figure 8b clearly shows an apparent increase in the energy needed to achieve the required rotation (displacement), in contrast to the unreinforced joint, represented in Figure 8a. In this case, the cyclic loading continued after the maximum testing load was reached, following the stability of the response loop. A negligible change in the hysteresis loop was observed. Similar positive effects were attained in tests with the inserted friction plates. A typical example of the behavior of a joint with two brake plates inserted in the gap between overlapping parts of the joined elements is shown in Figure 10. As shown in Figure 11, an almost identical dissipation energy was achieved with the two oak plates (Figure 11), even though the pre-stressing forces applied to the joining bolts were not identical. Buildings 2019, 9, 48 7 of 12

3. Results Damping property is a fundamental factor influencing a structure’s seismic resistance. Effective dissipation of energy by a structural element, e.g., a joint, can significantly decrease a structure’s vibration level and lessen internal forces. The dissipative properties of the investigated halved dovetail joints can be described by the area of hysteresis loops, as shown in Figures8a,b,9–11. Here, the hysteresis loops area representation of how the actuator’s moment of force about the axis of pin M is dependent on the rotation of joint ∆α for one loading cycle. Both of the retrofitting methods tested (see Discussion) were effective from an energy consumption point of view. The results showing the changes in hysteresis loops after joint retrofitting are presented in Figures8a,b,9–11. Figure8b clearly shows an apparent increase in the energy needed to achieve the required rotation (displacement), in contrast to the unreinforced joint, represented in Figure8a. In this case, the cyclic loading continued after the maximum testing load was reached, following the stability of the response loop. A negligible change in the hysteresis loop was observed. Similar positive effects were attained in tests with the inserted friction plates. A typical example of the behavior of a joint with two brake plates inserted in the gap between overlapping parts of the joined elements is shown in Figure 10. As shown in Figure 11, an almost identical dissipation energy wasBuildings achieved 2018, 8, withx FOR thePEER two REVIEW oak plates (Figure 11), even though the pre-stressing forces applied8 to of the 12 Buildingsjoining 2018 bolts, 8, werex FOR notPEER identical. REVIEW 8 of 12

(a) (b) (a) (b) Figure 8. (a) Hysteresis loops of a joint before retrofitting with steel nails; (b) Hysteresis loops of a joint after FigureFigureretrofitt 8. 8.ing (a()a withHysteresis) Hysteresis steel nails. loops loops of ofa joint a joint before before retrofitting retrofitting with with steel steel nails; nails; (b) Hysteresis (b) Hysteresis loops loops of a joint of a after joint retrofittafter retrofittinging with steel with nails. steel nails.

Figure 9. Figure 9.Hysteresis Hysteresis loops loops of of aa typicaltypical imperfect halved dovetail dovetail joint joint before before retrofitting retrofitting.. Figure 9. Hysteresis loops of a typical imperfect halved dovetail joint before retrofitting.

Figure 10. Hysteresis loops of a joint retrofitted with brake plates. Figure 10. Hysteresis loops of a joint retrofitted with brake plates. Buildings 2018, 8, x FOR PEER REVIEW 8 of 12

(a) (b)

Figure 8. (a) Hysteresis loops of a joint before retrofitting with steel nails; (b) Hysteresis loops of a joint after retrofitting with steel nails.

Buildings 2019, 9, 48 8 of 12 Figure 9. Hysteresis loops of a typical imperfect halved dovetail joint before retrofitting.

Buildings 2018, 8, x FOR PEER REVIEW 9 of 12 FigureFigure 10. Hysteresis 10. Hysteresis loops loops of of aa joint retrofitted retrofitted with with brake brake plates. plates. Buildings 2018, 8, x FOR PEER REVIEW 9 of 12

FigureFigure 11. Hysteresis11. Hysteresis loops loops of a a joint joint retrofitted retrofitted with withoak plates oak. plates. Figure 11. Hysteresis loops of a joint retrofitted with oak plates. In theIn friction the friction joints, joints, energy energy dissipation dissipation depended on on the the forces forces pressing pressing the two the surfaces two surfaces together. together. The resultingThe Inresulting the increase friction increase joints, in energyin energy energydissipation, dissipation dissipation depended, during during cycling on cycling, the, offorces the of pressingtested the tested joint the retrofitt two joint surfacesing retrofitting variations together. variationsis is shownTshownhe inresulting Figurein Figure increase 12 12.. ComparingC ompariin energyng thedissipation the results results of, during the ofeffect thecycling of effect two, of oak the of plates twotested oakin joint relation plates retrofitt to intheing relationbrake variations plates to is— the brake at an identical pre-stressing force—showed an obviously higher efficiency for the brake plates. plates—atshown an inidentical Figure 12. pre-stressingComparing the results force—showed of the effect anof two obviously oak plates higher in relation efficiency to the brake for plates the brake— plates. at an identical pre-stressing force—showed an obviously higher efficiency for the brake plates.

Figure 12. Energy dissipation curves for joints, both unreinforced and reinforced with various types of FigureFigurestructural 12. Energy 12. Energymodification dissipation dissipations. curves curves for joints joints,, both both unreinforced unreinforced and reinforced and reinforced with various with types various of types of structuralstructural modifications. modifications. 4. Discussion 4. DiscussionRotational stiffness and slipping of joints were studied in detail by Drdácký et al. [7] for both a typical baroqueRotational and gothic stiffness roof and framework slipping of onjoints which were halvedstudied dovetail in detail jointsby Drdácký were widelyet al. [7] used.for both The a typical results baroqueshowed andthat gothictight, well roof-made framework joints with on whicha reduced halved chance dovetail of slipping joints decrease were widelyd the used.overall The roof results frame showeddeformation that tight, under well horizontal-made joints wind with or earthquake a reduced loads, chance while of slipping also providing decrease reasonablyd the overall high roof rotational frame deformationstiffness and under capacity. horizontal Highly wind skilled or earthquakecarpenters wereloads, capable while also of producingproviding reasonablyflawless joints high which rotational were stiffnessfree of gaps and betweencapacity. the Highly individual skilled elements. carpenters were capable of producing flawless joints which were free ofPerfectly gaps between made thejoints individual may exhibit elements. a sufficiently high rotational capacity, as shown in Figure 13, and as alsoPerfectly shown made by Wald joints et al.may [13] exhibit, and can a sufficiently be modeled high in a rotationalrather simple capacity, way using as shown a component in Figure model 13, anding asapproach, also shown e.g. by, after Wald Vergne et al. [13] [14],, and which can berepresents modeled ain good a rather simple for the way description using a component joint behavior model undering approach, e.g., after Vergne [14], which represents a good tool for the description joint behavior under Buildings 2019, 9, 48 9 of 12

4. Discussion Rotational stiffness and slipping of joints were studied in detail by Drdácký et al. [7] for both a typical baroque and gothic roof framework on which halved dovetail joints were widely used. The results showed that tight, well-made joints with a reduced chance of slipping decreased the overall roof frame deformation under horizontal wind or earthquake loads, while also providing reasonably high rotational stiffness and capacity. Highly skilled carpenters were capable of producing flawless joints which were free of gaps between the individual elements. Perfectly made joints may exhibit a sufficiently high rotational capacity, as shown in Figure 13, and as also shown by Wald et al. [13], and can be modeled in a rather simple way using a component modelingBuildingsBuildings 2018 2018 approach,, ,8 8, ,x x FOR FOR PEER PEER e.g., REVIEW REVIEW after Vergne [14], which represents a good tool for the description1010 joint ofof 1212 behavior under repeated loading, as shown in Figure 14. In such a model, the joint was divided into repeated loading, as shown in Figure 14. In such a model, the joint was divided into components, which components,repeated loading which, as were shown represented in Figure 14. by In a force–deformationsuch a model, the joint diagram. was divided It is supposed into compone that thents , which were represented by a force–deformation diagram. It is supposed that the dowel resists the shear force resistswere represented the shear force by a and force clearly–deformation fixes the diagram. position It of is the supposed connection’s that t centerhe dowel of rotation.resists the The shear results force and clearly fixes the position of the connection’s center of rotation. The results presented in Figure 14 show presentedand clearly in fixes Figure the 14position show of that the more connection’s testing of center the materials’ of rotation. characteristics The results presented are required, in Figure especially 14 show that more testing of the materials’ characteristics are required, especially the behavior of wood under thethat behavior more testing of wood of the under materi concentratedals’ characteristics compression, are required to be, especially able to describethe behavior the contactof wood in under the concentrated compression, to be able to describe the contact in the rafter dovetail indent. The tested rafterconcentrated dovetail compression, indent. The tested to be composed able to describe joints theare morecontact complex in the rafter and, therefore, dovetail indent. an experimental The tested composed joints are more complex and, therefore, an experimental investigation was adopted without investigationcomposed joints was are adopted more withoutcomplex anyand, attempt therefore to, createan experimental computational investigation models. was adopted without anyany attempt attempt to to create create computational computational models. models.

FigureFigureFigure 13. 13.13. Example ExampleExample of ofof an anan experimentally experimentallyexperimentally attained attainedattained moment momentmoment rotation rotationrotation diagram diagramdiagram of ofof a aa perfectly perfectlyperfectly made mademade halved halvedhalved dovetaildovetaildovetail jointjoint [7]. [[7].7].

Moment,Moment, kNm kNm 22 11 00 Behaviour predicted -1-1 Behaviour predicted byby component component model model -2-2 -3-3 -4-4 BehaviourBehaviour observed observed by by experiment experiment -5-5 -6-6 -- 6 6 -- 4 4 -- 2 2 00 2 2 4 4 6 6 Rotation,Rotation, degrees degrees Figure 14. Comparison of the predicted component-based model for cyclic loading in the moment rotationFigureFigure 14. 14. diagram Compari Compari insonson Figure of of the the 13 predicted [predicted7]. component component--basedbased model model for for cyclic cyclic loading loading i nin the the moment moment rotation rotation diagramdiagram in in Figure Figure 13 13 [7]. [7].

InsertingInserting nailsnails aroundaround aa woodenwooden pin pin is is aa modificationmodification of of the the modernmodern design design of of seisseismicallymically resistant resistant frameframe corners corners, ,Ceccotti Ceccotti A. A. [15], [15], Kasal Kasal et et al. al. [16 [16]]. .The The positive positive effect effect of of this this design design has has been been published published (for (for anan example,example, StehnStehn andand BörjesBörjes [1[177]])). . OurOur eexperimentsxperiments havehave alsoalso demonstratdemonstrateded thethe effectivenesseffectiveness ofof thisthis design,design, asas shown shown in in FigFigureure 1515a,b.a,b. The The nails nails additionally additionally strengthen strengthen and and integrate integrate thethe joint joint, , andand may may replacereplace a a degraded degraded wooden wooden pin. pin. AsAs thethe timbertimber structurestructure movesmoves, ,the the connected connected timber timber strutsstruts rotate rotate mutuallymutually in in the the joint joint, , andand tend tend toto deformdeform the the inserted inserted steel steel rods rods plastically, plastically, w whichhich absorbs absorbs the the energy energy, ,as as shown shown in in Figure Figure 15 15b,c.b,c. Buildings 2019, 9, 48 10 of 12

Inserting nails around a wooden pin is a modification of the modern design of seismically resistant frame corners, Ceccotti A. [15], Kasal et al. [16]. The positive effect of this design has been published (for an example, Stehn and Börjes [17]). Our experiments have also demonstrated the effectiveness of this design, as shown in Figure 15a,b. The nails additionally strengthen and integrate the joint, and may replace a degraded wooden pin. As the timber structure moves, the connected timber struts rotate mutually in the joint, and tend toBuildings deform 2018 the, 8 inserted, x FOR PEER steel REVIEW rods plastically, which absorbs the energy, as shown in Figure 15b,c.11 of 12

(a) (b) (c)

FigureFigure 15. 15.X-ray X-ray presentationpresentation of of the the deformed deformed mild mild steel steel nails nails after after the the cycling cycling tests tests of a ofjoint a joint:: the (a the) X- (raya) X-rayrecording, recording, (b) perpendicular (b) perpendicular view viewof the of joint the plane, joint plane, and (c and) cross (c)-sectional cross-sectional view of view the joint of the plane joint. plane.

AA rather rather complex design, design, supported supported by by appropriate appropriate numerical numerical modeling modeling,, is required is required for an optim for anal optimalinteraction interaction between betweennails and nailswood. and The wood. effect can The be effect improved can be by improved inserting and by insertingfixing fiber and-reinforced fixing fiber-reinforcedplastic (FRP) sheets plastic on (FRP) the contact sheets on surfaces the contact of thesurfaces halved ofjoint the counterparts. halved joint This counterparts. requires partial This requiresdismantling partial of the dismantling joint, but prevents of the joint, the origination but prevents and thepropagation origination of fissures and propagation or cracks. of fissures or cracks.Similarly, the approach based on improvement of friction between the contact surfaces of the halved jointSimilarly, requires moderate the approach intervention, based on i.e. improvement, temporary removal of friction of the between wooden joining the contact pin, and surfaces insertion of theand halvedfixation joint of thin requires plates moderate in the slot intervention, between the i.e., two temporary joint components. removal ofThe the result woodens are joiningpresented pin, for and an insertionunreinforced and joint fixation, as shown of thin in plates Figure in 8 the; for slot brake between plates, theas shown two joint in Figure components. 10; and for The oak results plates are, as presentedshown in forFigure an unreinforced11. joint, as shown in Figure8; for brake plates, as shown in Figure 10; and for oak plates, as shown in Figure 11. 5. Conclusions 5. Conclusions This experimental investigation focused on determining the effectiveness of several joint modificationsThis experimental with respect investigation to their dissipative focused onproperties. determining The best the results, effectiveness and the of highest several level joint of modificationseffectiveness, were with obtained respect tofor their joints dissipative combined with properties. plates having The best a high results, friction and coefficient the highest and levela steel ofbolt. effectiveness, Tightening werethe nut obtained on the bolt for jointsthat is combined replacing withthe pin plates influences having the a highpre-stress friction of the coefficient joint, i.e. and, the afriction steel bolt. force Tightening between the the plates. nut on The the frictional bolt that force is replacing increases thein direct pin influences proportion the to pre-stressan increase of in the the joint,degree i.e., of thepre friction-stress. However, force between compressive the plates. deformation The frictional of the wood force increaseslimits the maximum in direct proportion possible value to anof increasefriction force. in the Fully degree fastening of pre-stress. the brake However,plates to the compressive wood using deformationscrews was the of most the wood effective limits method the . maximumPlates made possible of oak can value be ofused friction as an force.alternative Fully to fastening brake plates the, brake however plates, their to damping the wood efficiency using screws is not wasas high the. mostReinforcing effective the method. joints with Plates nails madegave g ofood oak results can, be as useddid the as use an alternativeof a combination to brake of nails plates, and however,inserted plates. their damping efficiency is not as high. Reinforcing the joints with nails gave good results, as did the use of a combination of nails and inserted plates. Author Contributions: M.D. and S.U. have done research and analyses, interpreted the results, as well as writes all sections of the paper. Original Draft Preparation, Writing-Review & Editing, M.D.

Funding: This study was supported by the EC FP7 Collaborative NIKER project (Grant agreement No. 244123) [18], [19], the grant project DG18P02OVV040 “Monuments in motion”, NAKI program II, provided by the Ministry of Culture of the Czech Republic and the ITAM institutional fund RVO 68378297.

Acknowledgments: The writers would like to thank master Stejskal for careful preparation of the test specimens, to Dr. Jaroslav Valach for x-ray photography and Marek Eisler for language and style correction.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Parisi, M.A.; Piazza, M. Seismic behavior and retrofitting of joints in traditional timber roof structures. Soil Dyn. Earthq. Eng. 2002, 22, 1183–1191. 2. Parisi, M.A.; Piazza, M. Dynamic modeling of friction joints in traditional timber structures. In Proceedings of the Eurodyn’99 Conference, Prague, Czech Republic, 7–10 June 1999. Buildings 2019, 9, 48 11 of 12

Author Contributions: M.D. and S.U. have done research and analyses, interpreted the results, as well as writes all sections of the paper. Original Draft Preparation, Writing-Review & Editing, M.D. Funding: This study was supported by the EC FP7 Collaborative NIKER project (Grant agreement No. 244123) [18,19], the grant project DG18P02OVV040 “Monuments in motion”, NAKI program II, provided by the Ministry of Culture of the Czech Republic and the ITAM institutional fund RVO 68378297. Acknowledgments: The writers would like to thank master carpenter Stejskal for careful preparation of the test specimens, to Jaroslav Valach for X-ray photography and Marek Eisler for language and style correction. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Parisi, M.A.; Piazza, M. Seismic behavior and retrofitting of joints in traditional timber roof structures. Soil Dyn. Earthq. Eng. 2002, 22, 1183–1191. [CrossRef] 2. Parisi, M.A.; Piazza, M. Dynamic modeling of friction joints in traditional timber structures. In Proceedings of the Eurodyn’99 Conference, Prague, Czech Republic, 7–10 June 1999. 3. Parisi, M.A.; Cordié, C. Mechanical behavior of double-step timber joints. Constr. Build. Mater. 2010, 24, 1364–1371. [CrossRef] 4. Branco, J.M.; Piazza, M.; Cruz, P.J.S. Experimental evaluation of different strengthening techniques of traditional timber connections. Eng. Struct. 2011, 33, 2259–2270. [CrossRef] 5. Palma, P.; Garcia, H.; Ferreirac, J.; Appletond, J.; Cruz, H. Behaviour and repair of carpentry connections—rotational behaviour of the rafter and tie beam connection in timber roof Structures. J. Cult. Heritage 2012, 13S, S64–S73. [CrossRef] 6. Poletti, E.; Vasconcelos, G.; Branco, J.M. Performance evaluation of traditional timber joints under cyclic loading and their influence on the seismic response of timber frame structures. Constr. Build. Mater. 2016, 127, 321–334. [CrossRef] 7. Drdácký, M.; Wald, F.; Sokol, Z. Sensitivity of Historic Timber Structures to Joint Response. In Proceedings of the 40th Anniversary Congress of IASS Madrid, Madrid, Spain, 20–24 September 1999; Astudillo, R., Madrid, A.J., Eds.; 1999; Volume 2, pp. G1–G10. 8. Parisi, M.A.; Chesi, C.; Tardini, Ch. Inferring Seismic Behavior from Morphology in Timber Roofs. Int. J. Archit. Heritage 2012, 6, 100–116. [CrossRef] 9. Parisi, M.A.; Piazza, M. Seismic strengthening and seismic improvement of timber structures. Constr. Build. Mater. 2015, 97, 55–66. [CrossRef] 10. Giuriani, E.; Marini, A. Wooden roof box structure for the anti-seismic strengthening of historic buildings. Int. J. Archit. Heritage 2008, 2, 226–246. [CrossRef] 11. Aira, J.R.; Arriaga, G.; Íñiguez-González, G.; Crespo, J. Static and kinetic friction coefficients of Scots (Pinus sylvestris L.), parallel and perpendicular to grain direction. Materiales de Construcción 2014, 64, 1–9. [CrossRef] 12. Available online: http://www.engineershandbook.com/Tables/frictioncoefficients.htm (accessed on 20 December 2018). 13. Wald, F.; Mareš, J.; Sokol, Z.; Drdácký, M. Component method for historical timber joints. In The Paramount Role of Joints into the Reliable Response of Structures; NATO Science Series; Baniotopoulos, C.C., Wald, F., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2000; pp. 417–424. 14. Vergne, A. Testing and modelling of load carrying behaviour of timber joints. In Proceedings of the International Conference Control of Semi-Rigid Behaviour of Civil Engineering Structural Connections, Liege, Belgium, 17–19 September 1998; p. 4. 15. Ceccotti, A. Structural Timber: Characteristics and Testing. Struct. Eng. Int. 1993, 3, 95–98. [CrossRef] 16. Kasal, B.; Pospíšil, S.; Jirovský, I.; Heiduschke, A.; Drdácký, M.; Haller, P. Seismic performance of laminated timber frames with fiber-reinforced joints. J. Earthq. Eng. Struct. Dyn. 2004, 33, 633–646. [CrossRef] 17. Stehn, L.; Börjes, K. The influence of ductility on the load capacity of a glulam truss structure. Eng. Struct. 2004, 26, 809–816. [CrossRef] Buildings 2019, 9, 48 12 of 12

18. D 6.4 (2010) Experimental Result on Structural Connections. Deliverable 6.4, EC FP7 Collaborative Project NIKER—New Integrated Knowledge Based Approaches to the Protection of Cultural Heritage from Earthquake Induced Risk (Grant Agreement No.: 244123). Available online: www.niker.eu/ (accessed on 20 December 2018). 19. Drdácký, M.; Urushadze, S.; Wünsche, M. Retrofitting of imperfect carpentry joints for increased seismic resistance. In SAHC 2012 “Structural Analysis of Historic Constructions”; Jasienko, J., Ed.; Dolnoslaskie Wydawnictwo Edukacyjne: Wroclaw, Poland, 2012; pp. 1485–1492.

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