structural grading of old chestnut elements by bending and
Transcription
structural grading of old chestnut elements by bending and
STRUCTURAL GRADING OF OLD CHESTNUT ELEMENTS BY BENDING AND COMPRESSION TESTS Beatrice Faggiano1, Maria Rosaria Grippa1, Anna Marzo1, Federico M. Mazzolani1 ABSTRACT: The paper deals with the mechanical characterization of timber elements based on bending and compression tests, according to UNI and ISO codes. The investigations were performed on both structural elements and defects-free specimens, made of old chestnut wood, aiming at evaluating the influence of typical defect patterns and wood anatomy on the elastic characteristics, strength properties as well as failure mechanisms of the material. In particular, the behaviour in compression of clear wood under uniaxial loading is studied with different orientations as respect to the longitudinal, radial and tangential directions. The experimental results are examined in relation with the standard values assumed by Italian codes for the quality classes assignment by visual grading. KEYWORDS: Chestnut wood, Bending tests, Compressive behaviour, Mechanical properties, Failure mechanisms. 1 INTRODUCTION123 In the field of analysis and restoration of ancient timber constructions, the assessment of the mechanical properties of the elements becomes difficult due to the strength high variability of the material among and within members. Timber strength grading is usually determined by means of visual inspection, through the identification of the critical areas, defects and decay. Accordingly, the UNI 11035-2 (2003) standard provides strength visual grading rules and characteristics values for Italian structural timber population, whereas the UNI 11119 (2004) code establishes procedures and criteria in order to assess the performances of timber members by means of in situ inspection, providing the admissible mechanical properties for three category classes. However, the estimation of the serviceability properties of new and/or ancient timber constructions by means of the visual grading method is not entirely reliable due to many factors influencing the mechanical properties and, further, the biased influence of the human factor. Moreover, the information is mostly qualitative. Therefore, a careful material characterization is needed by means of standardized laboratory tests, even only for few sacrificial specimens. In this context, aiming at the structural identification of old chestnut elements (Castanea sativa Mill.), a wide experimental campaign, including destructive tests in bending and compression, was carried out at the Laboratory of the Department of Structural Engineering 1 Beatrice Faggiano, Maria Rosaria Grippa, Anna Marzo, Federico M. Mazzolani, Department of Structural Engineering (DIST), University of Naples “Federico II”, Naples, Italy. Email: [email protected]; [email protected]; [email protected]; [email protected]. (DIST) of the University of Naples “Federico II”. The research activity was developed in the framework of the Italian project PRIN 2006 “Diagnosis techniques and totally removable low invasive strengthening methods for the structural rehabilitation and the seismic improvement of historical timber structures”, Prof. M. Piazza coordinator, Dr. B. Faggiano scientific responsible of the research unit UNINA (University of Naples “Federico II”) [1]. The methods specified in UNI EN 408 (2004) and UNI ISO 3787 and 3132 (1985) devoted codes were used for identifying the stiffness and strength properties of the material, together with typical failure mechanisms. In this paper, the local and global bending behaviour of full-scale beams is described. The mechanical evaluation in compression parallel to grain is analyzed by comparing the overall behaviour of structural elements with the one of defect-free specimens, pointing out the influence of defects in timber performance [1-3]. Furthermore, the orthotropic nature of the clear wood is investigated by means of compression tests perpendicular to grain in radial and tangential loading orientations. For each type of test, the experimental results are examined through test statistics, they being compared with the standard properties assumed by Italian codes. 2 MATERIAL AND TEST METHODS Structural elements in actual dimensions were provided from chestnut (Castanea sativa Mill.) timber trusses roof of an ancient masonry building of Naples, which was dismantled in a recent restoration intervention. From the trusses struts, 10 elements were selected for bending tests (specimens type SA-B). They had an irregular circular shape with mean diameter (D) ranging between 15 to 16.5 cm and length equal to about 19D. From the king posts, 14 samples were obtained for compression tests parallel to grain (specimens type SAC). They had a nearly square cross-section, characterized by large rounded edges, with a mean equivalent diameter (D) ranging between 14.5 to 16 cm and length of about 6D. The SA specimens had standard dimensions according to UNI EN 408. The conservation state of the elements was examined by means of visual inspection, checking wood features and defects, mainly consisting in knots, cracks, ring shakes, slope of grain, biological damage and holes, due to nails and insect attacks. Therefore, as a result of the visual structural grading, according to UNI 11119 standard, all specimens were assigned to the third category class, also providing a quick means for identifying critical areas. After the destructive tests, by cutting the undamaged parts of SA-C samples, 20 structural elements in small dimensions, 5×5×30 cm sized, were obtained for longitudinal compression tests (specimens type SS-C). Furthermore, several defect-free specimens of clear wood were also extracted, with no macroscopic defects and alterations (specimens type DF). They had 40 mm height and 20×20 mm2 cross-sectional area with the longitudinal axis being oriented along the grain, according to UNI ISO Italian codes. Therefore, in order to study the behaviour in compression of the base material in both parallel and perpendicular to grain directions, three groups of specimens were prepared taking into account the orientation of the annual rings with respect to the direction of the applied load. The first group consists of 33 specimens for longitudinal tests (type DF-CL), whereas the second and third groups of 22 specimens for both radial (type DF-CR) and tangential (type DF-CT) loading orientations. The geometric features of the specimens are illustrated in Figure 1, whereas in Table 1 the moisture content (MC) and density (ρ) values, together with the test types in bending (B) and compression (C) are summarized. 3.1 TESTING EQUIPMENT AND SET-UP Bending tests were performed under force control on 10 beams in actual sizes (type SA-B), using the Mohr Federhaff AG testing machine, a loading cell HBM of 740 kN and displacement transducers (LVDT). According to the four-points static scheme, the tested beams were loaded with two concentrated forces, applied in the third of the beam span by interposing a rigid steel frame between the actuator and the specimen (Fig. 2). Figure 2: Bending tests: test arrangement and set-up. 3.2 ELASTIC CYCLES Three cycles in elastic ranges, equal to 3-6-9 kN, were carried out using a quasi-static loading procedure and limiting the maximum force within the elastic conventional limit (0.4 Fmax). Displacements (w) and corresponding applied loads (F) have been fitted in F-w curves (Fig. 3), in order to evaluate both local (Em,l) and global (Em,g) elastic moduli, according to UNI EN 408. 14 F [kN] 12 a = 6D F/2 L1 = 5D LVDT1 a = 6D LVDT1 Av. LVDT 2, 3 LVDT 4 F/2 D LVDT2 F/2 10 LVDT4 LVDT3 F/2 L = 18D 8 6 4 SA-C SA-B 3 BENDING TESTS 2 w [mm] L ≈ 6D 0 D L ≥ 19 D DF b D 10 14 20 33 22 22 MC [%] 11-12 11-12 10-11 9-11 9-11 9-11 ρ [kg/m3] 526-638 530-642 483-641 421-720 432-638 396-668 6 8 10 12 14 16 18 3.3 FAILURE CYCLES Table 1: Specimens physical properties and test types. n. 4 2b SS-C Figure 1: Specimens geometric features. Specimens type SA-B SA-C SS-C DF-CL DF-CR DF-CT 2 Figure 3: Bending tests: typical elastic F-w curves. b L = 6b 0 B x Test types C // C ⊥ x x x x x After the elastic cycles, it was possible to evaluate the bending strength (fm) and the post-elastic response by increasing the load up to failure in several loadingreloading cycles, reaching the maximum force within 300±120 sec. (UNI EN 408). For each specimen, the destructive tests results are provided in terms of applied force (F) versus loading-actuator displacement (w) (Fig. 4a). F-w envelope curves of all tested beams are depicted in Figure 4b. It is generally observed that the initial growing branch presents a linear behaviour up to the maximum applied load; after the second or third cycle, an evident reduction of strength occurs, together with important actuator displacements. For almost all the specimens the failure modes were triggered around large knots located at the central zone and at the tensile edge of the beam cross-section, manifested by tearing of the more stressed fibres. In any case an evident buckling in compression was observed, followed by the propagation of slip phenomena (Fig. 4c). 50 F [kN] 40 I cycle II cycle III cycle IV cycle V cycle VI cycle VII cycle VIII cycle IX cycle 30 20 w [mm] 20 40 60 80 F [kN] 40 100 120 Failure cycles fm (I) fm (II) fm (III) 32.23 14.20 12.30 41.09 35.34 25.61 46.91 46.90 36.00 11.63 26.24 35.58 4.1 STRUCTURAL ELEMENTS 0 0 5-perc. average max CV [%] Elastic cycles Em,l Em,g 11023 10885 12535 12849 15291 15374 10.68 11.02 4 COMPRESSION TESTS 10 50 2 Table 2: Bending tests results [N/mm ]. a) SA-B; n=10 30 4.1.1 Testing equipment and set-up The compression tests parallel to grain on the structural elements in actual (type SA-C) and small (type SS-C) sizes were carried out under force control using the Mohr & Federhaff AG machines of 5000 kN and 400 kN capacity, respectively. In any case, the hydraulic press is constituted by a top fixed head and a moveable loadingbase plate. Loading cell HBM and displacement transducers were used during the tests (Fig. 5). 20 SA-C SS-C 10 w [mm] 0 0 20 40 60 80 100 120 b) c) Figure 4: Bending tests: a) Typical F-w failure cycles; b) Envelope curves; c) Failure modes. 3.4 RESULTS AND DISCUSSION Table 2 provides the mean statistical parameters of bending tests results. It can be stated the agreeable homogeneity of the bending behaviour in terms of both stiffness (Em,l; Em,g) and maximum strength (fm(I)), as the pleasant coefficients of variation (CV) confirm. However, the reduction of the load bearing capacity, after the peak load reached during the tests, is emphasized by the strong strength variation in both second (fm(II)) and third (fm(III) failure cycles. By comparing the experimental results with the standard values assumed by the Italian codes, the following observations can be made: ▪ The experimental average global elasticity modulus is about 22% and 60% higher than the mean values provided by UNI 11035-2 (11000 N/mm2) and UNI 11119-III (8000 N/mm2), respectively; ▪ Good agreement is found between the experimental 5-percentile strength and the standard characteristic value of UNI 11035-2 (28 N/mm2); ▪ The safety coefficient, obtained by the ratio between the experimental 5-percentile and admissible bending strength provided by UNI 11119-III (8 N/mm2), is equal to 4. Figure 5: Compression tests on structural elements: test arrangement and set-up. 4.1.2 Destructive tests The investigations on the behaviour in compression of the structural elements type SA-C were performed in several failure cycles. In each step the load was increased up to failure with a constant velocity gradient, so that the maximum force was attained within 300±120 sec. (UNI EN 408). The experimental data are fitted in stress-strain diagrams. The behaviour in compression in post-peak field was examined by means of envelope curves (Fig. 6a), which are plotted in Figure 6b for all tested samples. The global response reveals a initial linear portion, after that distinct non linearities are manifested, with softening branches characterized by a abrupt reduction of the stress carrying capacity. Collapse modes observed were mainly characterised by the attainment of the splitting almost parallel to grain, accompanied by fractures in the radial direction, cleavages along the annual growth rings or in multidirections, developed due to the presence of cracks and ring shakes (Fig. 6c). The stress-strain curves of the elements in small dimensions (type SS-C) are shown in Figure 7a. After a linear elastic region, the crushing strength is reached. Then, a distinct load drop followed by a almost linear decrease occurs. The complex collapse mechanisms exhibited were often caused by the propagation of splitting phenomena in specimens having macroscopic fissures prior to test. The so-called “wedge split” mode was developed in some elements, it being characterized by a not-well defined direction of the split. In any case, local plasticization shear bands appeared on specimen surface, due to buckling phenomena (Fig. 7b). 40 I cycle II cycle III cycle IV cycle V cycle VI cycle VII cycle VIII cycle IX cycle envelope σc,0 2 [N/mm ] 30 20 10 4.2 DEFECT-FREE SPECIMENS 4.2.1 Testing equipment and set-up The compression tests parallel and perpendicular to grain on defect-free specimens were conducted under force control with the universal machine Mohr & Federhaff AG of 400 kN capacity, equipped by an upper spherical bearing plate to improve the alignment of the element and promote uniform stress distribution on cross-section surfaces (Fig. 8). Deformations were measured by a displacement transducer placed in the arms of the test machine. The orientation of the annual growth rings with respect to the direction of the applied load was taken into account, so that three groups of samples were tested, respectively, in longitudinal (L), radial (R) and tangential (T) directions (Fig. 8). εc,0 [%] 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 a) 40 SA-C; n=14 σc,0 2 DF-CL [N/mm ] 30 L DF-CR R DF-CT T average curve 20 Figure 8: Compression tests on defect-free specimens: test arrangement and set-up. 10 εc,0 [%] 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 b) c) Figure 6: Compression tests on structural elements in actual sizes (SA-C): a) Typical σc,0-εc,0 failure cycles; b) Envelope curves; c) Failure modes. 60 SS-C; n=20 σc,0 2 50 [N/mm ] average curve 40 30 20 10 εc,0 [%] 0 0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 b) Figure 7: Compression tests on structural elements in small sizes (SS-C): a) σc,0-εc,0 curves; b) Failure modes. a) 4.2.2 Destructive tests Defect-free specimens were tested up to collapse in order to evaluate stiffness and strength properties of the clear material. The load was applied at a constant loadinghead movement so that in longitudinal compression the maximum force was reached between 90 and 120 sec. (UNI ISO 3787), while, in transverse compression, it was attained within 90±30 sec. (UNI ISO 3132). For longitudinal tests, in Figure 9a all stress-strain diagrams are represented, together with the average curve, which is assumed to characterize the mechanical behaviour in compression parallel to grain of the clear old chestnut wood. As it can be seen, the material exhibits moderate ductility beyond the linear response. Moreover, at deformations larger than 2.0%, a stress capacity reduction is generally manifested. The failure in compression along the grain direction was progressive, usually deemed to be an effect of shear, so-called shearing mode. In fact, for most of the specimens, beyond the maximum stress, structural changes began with the formation of one or two principal gross shear bands, which consisted in fracture cleavage approximately perpendicular to the longitudinal axis on the radial plane and 45° to 70° inclined as respect to the grain orientation on the tangential plane. Other failure modes were also manifested, such as crushing, splitting and combined modes (Fig. 9b). Typical stress-strain radial curves up to 30% total deformation are highlighted in Figure 10a. Considering that the wood microstructure in the radial direction can be regarded as a sandwich structure, consisting of dense latewood layers arranged in series between weak earlywood bands, three deformation levels can be easily identified: 1) elastic deformation, with a initial maximum stress value and next sudden load drop due to buckling of a portion of rays arranged in a growth ring of the weakest earlywood layer; 2) plastic level, with a plateau described by an irregular saw-tooth shape, corresponding to the fracture of individual cell walls; 3) densification and compaction of the material, with additional failures in the same or in several other earlywood layers, corresponding to a stress quick increment. As a result of these structural changes, the tested radial specimens assume a deformed shape crushed in the transversal direction (Fig. 10b). latewood. In fact, with increasing load a separation between these layers was often observed (Fig. 11b). 15 DF-CT; n=22 σc,90 T 2 [N/mm ] 12 average curve 9 6 3 εc,90 T [%] 100 0 σc,0 DF-CL; n=33 average curve 0 2 2 4 6 8 10 12 14 16 18 20 [N/mm ] 80 a) 60 40 b) 20 εc,0 [%] 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 a) Figure 11: Compression tests on tangential defect-free specimens (DF-CT): a) σc,90-εc,90 curves; b) Failure modes. 4.3 RESULTS AND DISCUSSION b) Figure 9: Compression tests on longitudinal defect-free specimens (DF-CL): a) σc,0-εc,0 curves; b) Failure modes. 14 DF-CR; n=22 σc,90 R 12 2 [N/mm ] 10 8 6 4 2 εc,90 R [%] 0 0 3 6 9 12 15 18 21 24 27 30 a) b) Figure 10: Compression tests on radial defect-free specimens (DF-CR): a) Typical σc,90-εc,90 curves; b) Failure modes. All experimental curves of tested tangential specimens and the average one are depicted in Figure 11a. As it appears, in tangential compression the elastic phase gradually tends to a plateau zone where the strain increases rapidly at a constant load. This behaviour can be explained by the early bond failure between alternating layers of earlywood and 4.3.1 Compression parallel to grain The mechanical characteristics of tested old chestnut timber under compression parallel to grain is analyzed by comparing the experimental results obtained by destructive tests performed on the structural elements, types SA-C and SS-C and defect free specimens, type DFCL. In Table 3 the basic statistics for both modulus of elasticity (Ec,0) and strength (fc,0) are given, whereas the stress-strain average curves are shown in Figure 12. It is evident that the three groups of samples exhibit different responses in terms of load carrying capacity, being the stiffness properties nearly similar each other. In particular, the presence of natural defects, wood degradations and geometric irregularities, typical of ancient timber members, seems to reduce of about three times the compression strength of the base clear material. Furthermore, while the stress-strain curve of clear wood shows a plastic branch, highlighting a ductile behaviour, the same curves of the structural elements reveal a brittle performance, due to a drastic reduction of crushing strength after the peak load. By examining the test results, it is worth noticing that: ▪ The strength values of the specimens in actual dimensions (SA-C) are affected by higher variability in terms of coefficient of variation (CV); ▪ The experimental 5-percentile strength value of specimens type SA-C and the characteristic value assumed by UNI 11035-2 (22 N/mm2), are very similar each other; ▪ By comparing the admissible crushing strength of UNI 11119-III (7 N/mm2) with the 5-percentile laboratory value, the obtained safety coefficients are equal to about 3, 5 and 6.5 for actual (SA-C), small (SS-C) and defect-free (DF-CL) specimens, respectively. Table 3: Experimental results of compression tests 2 parallel to grain [N/mm ]. 80 σc 70 parallel to grain 2 [N/mm ] perpendicular to grain (T) 60 Elasticity modulus Ec,0 SA-C 4828 5582 6739 9.84 5-perc. average max CV [%] SS-C 4619 5713 7358 11.98 DF-CL 4169 6555 9045 18.82 Strength fc,0 50 SS-C 37.67 44.05 56.46 11.98 40 SA-C 20.13 24.40 33.97 17.21 DF-CL 45.63 59.89 75.74 14.27 DF-CL; n=33 30 20 DF-CT; n=22 10 εc [%] 0 0 80 60 actual dimensions small dimensions defect-free 50 DF-CL; n=33 σc,0 70 2 [N/mm ] 0.5 1 1.5 2 2.5 3 3.5 4 Figure 13: Comparison between σ-ε average curves of defect-free specimens tested in compression parallel and perpendicular to grain. 40 30 5 CONCLUSIONS 20 SS-C; n=20 10 SA-C; n=14 εc,0 [%] 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Figure 12: Comparison between σc,0-εc,0 average curves of specimens tested in compression parallel to grain. 4.3.2 Compression perpendicular to grain The laboratory results of transverse compression tests, performed on both radial (DF-CR) and tangential (DFCT) defect-free specimens, are provided in Table 4 in terms of modulus of elasticity (Ec,90) and conventional proportional stress (fc,90). The following observations can be drawn: ▪ Similar responses are provided by both stiffness and strength properties in radial and tangential direction, whereas the tangential tests are affected by higher coefficients of variation (CV); ▪ The average modulus of elasticity provided by UNI 11035-2 (730 N/mm2) is 1.3-1.6 times the experimental values; ▪ The 5-percentile strength results are comparable with UNI 11035-2 value (3.8 N/mm2); ▪ The strength safety coefficient is equal to 1.2-2.0, being the admissible value equal to 2 N/mm2, according to UNI 11119-III. In Figure 13 the stress-strain average curves of longitudinal and tangential defect-free specimens are depicted. The tested specimens show very low compressive elastic and strength properties when loaded in perpendicular direction, the transverse behaviour being significantly influenced by the wood anatomy. In fact, the fc,0/fc,90 and Ec,0/Ec,90 are equal to about 11 and 14, respectively. Table 4: Experimental results of compression tests 2 perpendicular to grain [N/mm ]. Elasticity modulus Ec,90 5-perc. average max CV [%] DF-CR 343 545 823 21.21 DF-CT 211 463 850 34.78 Strength fc,90 DF-CR 3.99 5.33 6.69 14.60 DF-CT 3.23 5.62 11.25 41.27 In this paper experimental investigations on old chestnut wood are illustrated. Firstly, the mechanical identification in bending and compression parallel to grain of elements having structural sizes and natural defects is provided. Furthermore, compression tests on chestnut clear wood in different orientations, such as longitudinal, radial and tangential, are described showing the highly orthotropic material behaviour not only for elasticity or strength but also for the deformation patterns. The laboratory results emphasize that the wood mechanical characterization by means of tests on clear small specimens can be inadequate to have reliable information on structural timber, so much depending of the presence of faults. Furthermore, the obtained safety coefficients demonstrate that the structural capacity of the material seems significantly underestimated by means of the visual grading, according to the UNI 11119 current code. ACKNOWLEDGEMENTS Authors acknowledge the PRIN 2006 Italian project which founded the research activity [1]. REFERENCES [1] Faggiano B., Grippa M.R., Marzo A., Mazzolani F.M.: Experimental evaluation of the mechanical properties of wood by means of non-destructive compared techniques for the characterization of existing wooden structures. In Consolidation of timber structures, ed. M. Piazza, Hevelius Publisher, pp: 25-78, Italy, 2009 (in Italian). [2] Faggiano B., Grippa M.R., Marzo A., Mazzolani F.M.: Mechanical identification by NDT of old chestnut structural timber. Proc. of the First International Conference PROHITECH09. Rome, Italy, 22-24 June, 2009, F.M. Mazzolani Editor, Vol. 1, pp. 295-300. [3] Faggiano B., Grippa M.R., Marzo A., Mazzolani F.M.: Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements. Proc. of 3rd International Conference on Advances in Experimental Structural Engineering. San Francisco, United States, 15-16 October, 2009.