《木结构 Timber Engineering》课程教学资源（参考文献）Contact-free measurements and numerical analyses of the strain distribution in the joint area of steel-to-timber dowel joints

tio697-0o ORIGINALARBEITEN.ORIGINALS Johan Sjodin·Erik Serrano·Bertil Enquist Contact-free measurements and numerical analyses of the strain distribution in the joint area of steel-to-timber dowel joints the joint area was studied ment s deutung hat. The casuremet sytem a Die numerischen Ergebnisse fuhrten zusammen mit den be- Ito be a le com eilangimVetndnestcrcichfestgecl,Darberhinasfand particularly useful for studying the man heraus,dass S nungen und Zugdehnungen paral tat is of special importance Several observations of considerable interest were made on stimmung mit fruheren Untersuchungen festgestellt.dass bei den the of numerical results in nation with results c groBeren V bindungen ein Sprodbruch auftra tion in the joint area was detected.for example.and shear srains to the grain 1 Introduction ence the failure mode of the ioint. onmade The design rules contained in the european timber code Ecs (Eurocode 52004)for multiple dowel-type joints loaded parallel 99)which ing th y (Joh an lyse der he vels.Restrictions related to spacing.end and edge di es and e m tet.Die Dehnungsverteilung im Bereich der Verbindung wurde untersuch ateengdirectedinrec stigations Messsvstem erwies sich als wertvolle u herkomm rience of craftsmen and the results of joint tests conducted in yed in nissen von Parameter- the laboratory.Theoretical meth 1998, ·B.E tresses perpendicular to the grain as well as shear stre el-type Brittle dowel-type joints(orbol in the literature:see eg the and()The ilure of a large glulam structure Thi tle(R ivctra Mecaics Lund Univriy Lund Sweden of the behavior of multiple dowel-type jonts

DOI 10.1007/s00107-006-0112-1 ORIGINALARBEITEN · ORIGINALS Holz als Roh- und Werkstoff (2006) 64: 497–506 Johan Sjodin · Erik Serrano · Bertil Enquist ¨ Contact-free measurements and numerical analyses of the strain distribution in the joint area of steel-to-timber dowel joints Published online: 12 April 2006 © Springer-Verlag 2006 Abstract Two different dowel-type joints of differing size were loaded in tension parallel to the grain. The strain distribution in the joint area was studied using a contact-free measurement sy￾stem. The results were compared with those of numerical analy￾sis. The measurement system was found to be a valuable comple￾ment to traditional measurement techniques and also to numeri￾cal analyses performed in parametric studies. The measurements obtained were shown to be particularly useful for studying the highly nonlinear behavior of timber that is of special importance in characterizing timber dowel joints. Several observations of considerable interest were made on the basis of numerical results in combination with results of the contact-free measurement system. A non-uniform strain distribu￾tion in the joint area was detected, for example, and shear strains and tensile strains, both parallel and perpendicular to the grain were found to be concentrated in the areas most likely to influ￾ence the failure mode of the joint. A general observation made was that the larger size joint failed in brittle manner, an observa￾tion supported by previous research. Beruhrungslose Messung und numerische Analyse der ¨ Dehnungsverteilung in Stahl-Holz-Stabdubelverbindungen ¨ Zusammenfassung Zwei verschiedene Stabdübelverbindungen unterschiedlicher Größe wurden auf Zug in Faserrichtung belas￾tet. Die Dehnungsverteilung im Bereich der Verbindung wurde mittels eines berührungslosen Messsystems untersucht und mit den Ergebnissen aus einer numerischen Analyse verglichen. Das Messsystem erwies sich als wertvolle Ergänzung zu herkömm￾lichen Messtechniken sowie zu den Ergebnissen von Parameter￾studien. Die Messungen erwiesen sich als besonders hilfreich zur J. Sjödin (✉) · B. Enquist School of Technology and Design, Växjö University, Lückligs plats 1, 351 95, Växjö, Sweden E-mail: johan.sjodin@vxu.se E. Serrano SP Swedish National Testing and Research Institute, Borås, Sweden E. Serrano Division of Structural Mechanics, Lund University, Lund, Sweden Untersuchung des stark nicht-linearen Verhaltens von Holz, das für die Charakterisierung von Holzdübelverbindungen große Be￾deutung hat. Die numerischen Ergebnisse führten zusammen mit den be￾rührungslosen Messungen zu einigen interessanten Erkenntnis￾sen. So wurde zum Beispiel eine ungleichmäßige Dehnungsver￾teilung im Verbindungsbereich festgestellt. Darüber hinaus fand man heraus, dass Schubdehnungen und Zugdehnungen paral￾lel und senkrecht zur Faser konzentriert in den für den Bruch maßgebenden Bereichen vorkamen. Generell wurde in Überein￾stimmung mit früheren Untersuchungen festgestellt, dass bei den größeren Verbindungen ein Sprödbruch auftrat. 1 Introduction The design rules contained in the European timber code EC5 (Eurocode 5 2004) for multiple dowel-type joints loaded parallel to the grain are based on the Johansen yielding theory (Johansen 1949), which assumes a rigid plastic behavior of the timber and the dowels. Restrictions related to spacing, end and edge di￾stances and timber thickness are also included in EC5 in order to help avoid brittle failure modes. In addition, the current ver￾sion of EC5 contains informative design rules aimed at reducing occurrence of the block-shear/plug-shear failure mode at which attention has been directed in recent investigations. According to Schmid et al. (2002), most timber design codes are based on the experience of craftsmen and the results of joint tests conducted in the laboratory. Theoretical methods that have been employed in this area include the approach of Jorissen (1998), which is based on the theory of beams on an elastic foundation, for analyzing stresses perpendicular to the grain as well as shear stresses in dowel-type joints. Brittle failures in dowel-type joints (or bolted joints) in contradiction to what design codes predict are reported in the literature; see e.g. the experimental results of Quenneville and Mohammad (2000). The failure of a large glulam structure was described as being brittle (Ranta-Maunus and Kevarinmäki 2003). This exemplifies the need for more adequate knowledge of the behavior of multiple dowel-type joints

498 2 Experimental 2.1 Specimens studies of joints to gather data other fjont' The two types of joints used in this study are shown in Fig.1. ploy in order to fill this gap.Some studies in which measur this type have been oints glulam beams manufactured by Moelven Tore boda Limr AB (L40 quality according to Swedish regulations method,comparing the results with those OClAecsaneash A ts of nd65%TeihaehmidyRH)ntl12%moistueegiim results were limited to the area close to the bolt.In order to (.6%)was reached,prior to further preparatior dicular to ntact-fre s the minimum for type fo 27 of dete dowels were 12mm in diameter with a tolerance of 0.2 mm ations and strains within largc arcas.and of maintaining 3 a high level of accuracy and spatial resolution in a large number from the loading machine.In systems of this sort,which in 2.2Test setup onare,strong ve been directed at making In the present paner one such mmercial contact-free mea oints onected to the loading system by a fork co ection. surement system is used for studying the joint area in two types Fig.2.For type I joints,the other end was first reinforced P sed forh o illustrate the advantage and the po ssibilities of such a mea mens were inserted about Im into the grip of the testing ystm.which ma hinc..the joints esults with those of ment methods and also to a displ ent-control rate of 0.5 mm/min for e 1 an with those of numerical analysis. 09mm/n for ypen Falrlah 15 mm 8网 45mm1■ mm ● 2 600 mm 1 x 70mm1 ↓8mm 12mm ahc1。 252m 2000mm

498 There is a gap between investigations involving experiments and those based on a theoretical approach. Despite the fact that theoretical methods can provide detailed results it is still rather unusual in experimental studies of joints to gather data other than that of a joint’s global load-displacement response. Modern contact-free measurement techniques should be possible to em￾ploy in order to fill this gap. Some studies in which measurement techniques of this type have been used for timber joints have been reported in the literature. For example, Masuda and Ta￾bata (2001) studied single-bolt joints by use of a digital image correlation method, comparing the results with those of theore￾tical analyses. A close correlation between results of the mea￾surements and of the theoretical analyses was obtained, but the results were limited to the area close to the bolt. In order to gain a better understanding of the overall performance of timber joints, a larger area around the joint needs to be studied. This is possible today thanks to advanced contact-free measurement sy￾stems often commercialized and frequently used as established techniques in areas other than timber engineering. Advantages of these new systems include the possibility of determining de￾formations and strains within large areas, and of maintaining a high level of accuracy and spatial resolution in a large number of separate instances during loading. Also, such systems can si￾multaneously record data such as load and displacement readings from the loading machine. In systems of this sort, which in addi￾tion are user-friendly, strong efforts have been directed at making it easy to evaluate the measurement data. In the present paper, one such commercial contact-free mea￾surement system is used for studying the joint area in two types of dowel joints loaded in tension parallel to the grain. The aim is to evaluate the results from a general point of view and also to illustrate the advantages and the possibilities of such a mea￾surement system, which does not appear to have been used pre￾viously for dowel-type joints. This will involve comparing the results with those of traditional measurement methods and also with those of numerical analysis. Fig. 1 Joint geometries used Abb. 1 Art und Abmessung der untersuchten Verbindung 2 Experimental 2.1 Specimens The two types of joints used in this study are shown in Fig. 1. The type 1 joints, made of Norway spruce, had a mean den￾sity of about 450 kg/m3 at 12% moisture content. For type 2 joints, spruce glulam beams manufactured by Moelven Töre￾boda Limträ AB (L40 quality according to Swedish regulations, approximately the same as the European GL32 quality) were em￾ployed. All specimens were stored in a standard climate of 20 ◦C and 65% relative humidity (RH) until 12% moisture equilibrium (±0.6%) was reached, prior to further preparation. The distan￾ces a3,t, a4,c and a1 were set according to the minimum values as given in EC5. For type 1, the dowel spacing perpendicular to the grain was twice the minimum distance prescribed by EC5, whereas the minimum distance was used for type 2. The side members were 20 mm thick for type 1 and 30 mm for type 2. The dowels were 12 mm in diameter with a tolerance of ±0.2 mm. The holes in the steel plates, which were cut out by laser, were set to 12.2 mm, with a tolerance of ±0.1 mm. The steel plates were used as templates for drilling the holes in the timber. 2.2 Test setup Eight and five replicates for types 1 and 2, respectively, were loaded in tension. The slotted-in steel plate for both types of joints was connected to the loading system by a fork connection, see Fig. 2. For type 1 joints, the other end was first reinforced with plywood and was then connected to a similar fork. For type 2 joints, a wedge device was used for the other end where the specimens were inserted about 1 m into the grip of the testing machine, see Fig. 2. During the loading procedure, the joints were subjected to a displacement-control rate of 0.5 mm/min for type 1 and 0.9 mm/min for type 2 joints. Failure was usually reached within

499 Load en CCD-Kae LOAD CCDECAME which is incuded in the ARAMIS-ystem For eac in the contact zone hetween the dowels and the timber the lo be determined and be converted to relative displacements and strains.The calibration volume selected results ed. the tes approximately 3.0mm for type was measured by two inductive gauges attached on oppo site sides of the joints.see Fig.2. 2.3 Contact-free measurement system 3 Numerical analyses ARAMISTM ABAQUS In 20wsus for ting a random or regular pattem.which is applied to the surfac sion in the joint area.shown in Fig.3.consists of apr oximately and deform ment red.Linear-elastic pehavior was surface.at different occasions during loading.see Fig.2.The assumed for the wood,which for the area close to the dowels air in th mag y loading.the 3D coordinates of these facets on the specim en's EL=15160.E7=505ER=505.G1R=950.GL7=950an lation and 3D GT=5 MPa. al dire s of the N ig. imila Th any tran =0.50.7=0.50 and var=0.70 luded the joint area and part of the stel plate;see the he load vas applied as a uniform deformation (of the was achi the on the area around the dowels.This was done in order to o stiffness.K.of these springs was calculated using the expres ain better ast of the gray sca ung the facets and =(E The in dots were applied by spraying black paint on the surface from d material were e modeled by contact elements involving fric a distance ion in the tangential direction of the dowels.The coefficient of also log on was set te nd fo restricted mainly to a qualitative discussion of them

499 Fig. 2 a Load-setup for type 1 joints, b load-setup for type 2 joints, c appli￾cation of inductive gauges (similar in the case of both types) and d type 2 joint, into which random patterns are applied, and the CCD-cameras used Abb. 2 a Versuchsanordnung für Prü- fungen des Typs 1, b siehe a für Typ 2 c Anordnung der Induktionswegauf￾nehmer (für beide Typen ähnlich) und d Verbindung vom Typ 2 mit aufge￾brachten stochastischem Muster und eingesetzten CCD-Kameras 5–15 minutes for type 2. For type 1, the time to failure was usually longer and in some cases, owing to large deformations in the contact zone between the dowels and the timber, the loa￾ding procedure was ended before a complete failure by tearing the test specimen apart occurred. During the tests, the load was recorded and the relative displacement between the plate and the timber was measured by two inductive gauges attached on oppo￾site sides of the joints, see Fig. 2. 2.3 Contact-free measurement system The measurement system ARAMISTM manufactured by the company GOM was employed. The system is based on evalua￾ting a random or regular pattern, which is applied to the surface and deforms along with the material. The placing of two CCD cameras (1280 by 1024 resolution) in front of the specimen at different angles enables stereoscopic pictures of the patterned surface, at different occasions during loading, see Fig. 2. The first digital-image processing step defines macro-image facets in the image pair in the original, unloaded state. For each stage of loading, the 3D coordinates of these facets on the specimen’s surface are calculated accurately using image correlation and photogrammetric principles. On the bases of these 3D coordina￾tes, the 3D displacements, rotations and in-plane strains as well as the shape of the specimen can be calculated with a high degree of spatial resolution. The test arrangements for all the joints were similar. The CCD cameras were calibrated to a measurement volume that in￾cluded the joint area and part of the steel plate; see the area inside the dashed lines in Fig. 2. A random pattern was achie￾ved in two steps. First a matt light-colored paint was sprayed on the area around the dowels. This was done in order to ob￾tain better contrast of the gray scale defining the facets and also to reduce the shininess of the timber. After this, small black dots were applied by spraying black paint on the surface from a distance. The cameras were triggered every five seconds, also log￾ging analog signal readings of displacement and force from the loading machine. The test images were then processed by the software, which is included in the ARAMIS-system. For each pair of images, 3D coordinates for a large number of facets could be determined and be converted to relative displacements and strains. The calibration volume selected results in a spatial re￾solution of approximately 3.0 mm for type 1 joints and 4.2 mm for type 2. According to the manufacturer, the strain accuracy is approximately 0.02%. 3 Numerical analyses The FE-software ABAQUS (ABAQUS Inc. 2003) was used for the 2D numerical analyses of the joints. The element subdivi￾sion in the joint area, shown in Fig. 3, consists of approximately 8200 elements in the case of type 1 joints and 19 000 elements in the case of type 2. In both cases, 6-node second-order tri￾angular elements were employed. Linear-elastic behavior was assumed for the wood, which for the area close to the dowels is an approximation. The moduli of elasticity and the shear moduli for defining the orthotropic wood material were set to EL = 15 160, ET = 505 ER = 505, GL R = 950, GLT = 950 and GRT = 95 MPa, being estimated from the European EN 338 standard. The indices L, R and T denote the longitudinal, radial and tangential directions of the timber, respectively (Fig. 3). No consideration was taken of the difference between the radial and the tangential direction. Thus, T in Fig. 3 represents any trans￾verse direction in the timber. The Poisson’s ratios used were set to vL R = 0.50, vLT = 0.50 and vRT = 0.70. The load was applied as a uniform deformation (u) of the dowels (Fig. 3). Elastic springs were used for type 2 to simu￾late the stiffness of the steel plate in the L-direction. The elastic stiffness, K, of these springs was calculated using the expres￾sion K = (Eplate × Aplate)/Ldowelspacing. The interaction between the dowels, which was assumed to act as rigid bodies, and the wood material were modeled by contact elements involving fric￾tion in the tangential direction of the dowels. The coefficient of friction was set to 0.4. Because of these various approximations in the modeling, the evaluation of the numerical results will be restricted mainly to a qualitative discussion of them

500 Fig.3 Abb.3 El ype G Fig.4 Ex ults.b i RK CRACK 0 123456789 Displacement(mm) Type 05000 2 Dis Type2 e The results for type I joints indicate ductile behavior.This 4 Results and discussion bu lure of 4.1 General eached for the type l ioints two initial cracks usually develo he nedte ped close toeach dowel,see Fig.4.Note that these cra cks are no of the disp

500 Fig. 3 Element subdivision in the joint area for both types of joints Abb. 3 Elementeinteilung im Verbin￾dungsbereich beider Typen Fig. 4 Experimental results for the specimens: a load-displacement re￾sults, b initial crack and c final fai￾lure modes Abb. 4 Versuchsergebnisse: a Kraft￾Weg-Diagramm, b erster Riss und c Bruchbild 4 Results and discussion 4.1 General The load-displacement curves presented in Fig. 4 are based on the mean value of the displacement readings of the two inductive gauges applied to each specimen (Fig. 2). The results for type 1 joints indicate ductile behavior. This was, however, not caused by plastic hinges developing in the dowels, but by the embedment failure of the wood which led to marked local deformation. Soon after the ultimate load had been reached for the type 1 joints, two initial cracks usually develo￾ped close to each dowel, see Fig. 4. Note that these cracks are not located at the dowel centers. This behavior is caused by friction between the dowels and the timber (Rodd 1973). If no friction is

501 the do ing the embedment capacity due tothe area between the initial crack choepCbep sidelReenint ion for the fact that the load usually decreased afer them od wasreached The load-displacement behavior for a type 1 ad of t st to ally by splitting (Fig.4).This cause d the load to drop.Note wing possible rotation and eccentricity in the joint t in some je of the dowels ts the lond-displacemen t results as registered h ctile behavior could be bse MIS-system.This involves calculating the change in length he load The s located on the timber close to the on the failure mode was the so-called block-shear failure,but usually pper picture in Fig. ength in the load-direction points in the ARAMIS-system Type1,Upper side Type1,Lower side 25 20 20 10 asured,Gauges 456 t (mm) lacement (mm 350 Type2.Upper side 350 Type2,Lower side 0 300 250 200 200 150 10 100

501 present, only a single crack develops located centric in relation to the dowels. Two developing cracks have the effect of reducing the embedment capacity due to the area between the initial cracks are taking most of the load, since after the cracks have been initia￾ted, the dowels can more easily push the wood aside (Rodd 1973). This seems to be a plausible explanation for the fact that the load usually decreased after the ultimate load was reached. The development of the two cracks, as described above, in type 1 joints, was usually followed by the final failure mode, usually by splitting (Fig. 4). This caused the load to drop. Note that in some joints the initial cracks in front of the dowels seemed to be propagating up to the final failure, whereas in some cases the final failure seemed to be initiated from the end grain. For type 2 joints, almost no ductile behavior could be obser￾ved. Instead, failure occurred at the ultimate load suddenly and the specimens could then no longer transmit load. The primary failure mode was the so-called block-shear failure, but usually in a combination with the so-called row-shear-out failure mode (Fig. 4). Note that prior to final failure the general tendency was Fig. 5 Load-displacement results measured by both the ARAMIS-system and the inductive gauges Abb. 5 Vergleich der mittels ARAMIS-System und Induktionswegaufnehmern bestimmten Kraft-Weg￾Diagramme that an initial crack developed; see Fig. 4, followed almost di￾rectly by final failure. 4.2 Comparing load-displacement results of the ARAMIS-system and of the inductive gauges The load-displacement behavior for a type 1 and a type 2 joint is presented in Fig. 5. The results for each of the two gauges are presented there instead of the mean value, in contrast to Fig. 4, allowing possible rotation and eccentricity in the joint area to be studied. In order to compare results of the traditional mea￾surement method with those of the contact-free method, Fig. 5 also presents the load-displacement results as registered by the ARAMIS-system. This involves calculating the change in length in the load direction between points located on the steel plate and points located on the timber close to the respective gauges, see the upper picture in Fig. 5. The results shown in Fig. 5 indicate no marked rotation or ec￾centricity in the joint area between the steel plate and the timber