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Influence of timber density on the axial strength of joints made with glued-in steel rods: An experimental approach
ARTICLE IN PRESS International Journal of Adhesion & Adhesives 30 (2010) 380–385
Contents lists available at ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Influence of timber density on the axial strength of joints made with glued-in steel rods: An experimental approach M.D. Otero Chans n, J. Este´vez Cimadevila, E. Martı´n Gutie´rrez, J.A. Va´zquez Rodrı´guez ˜a, Department of Construction Technology, Architecture School, Spain University of A Corun
a r t i c l e in f o
a b s t r a c t
Article history: Accepted 20 November 2009 Available online 7 March 2010
Joints made with glued-in steel rods have many possibilities in the design of timber structures. They can be used for new buildings or for the rehabilitation of old structural elements damaged by the attack of biotic agents or humidity. Since the 1970s many studies have been carried out to characterize the strength of these joints when made with glued laminated timber (glulam). These studies hypothesize that the axial strength of joints made in glulam depends on some geometric parameters (anchorage length, steel rod diameter, adhesive thickness, etc.) as well as on timber density. For several years our research group has been studying the behavior of these joints when made in sawn timber, determining the influence of different geometric and material parameters on the axial strength of the glued-in steel rods. This work summarizes the experimental results of joints made in pieces of sawn timber of two species having different densities and mechanical properties. The experimental study was carried out for different geometric configurations: threaded steel rods of 10 and 12 mm diameter, epoxy adhesive of 1 mm thickness, and five anchorage lengths. The aim was to test the same specimen conditions for each timber species studied, tali and chestnut. The experimental results show that the axial strength of the joints does not increase linearly with timber density. This result contradicts many of the traditional design proposals suggested for joints made in glulam. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Epoxy B. Wood C. Destructive testing E. Joint design
1. Introduction Joints made with glued-in rods have great potential in the field of timber engineering. Initially, these joints were used in glulam to prevent cracks in areas submitted to tensile stresses perpendicular to the grain. Presently, they are also being used to design joints in planar or spatial structures consisting of rods under axial load; to design rigid joints (bending resistant) in frame structures; for the rehabilitation of damaged structural elements in buildings, this being their principal application, for the moment, in Spain [1]. Apart from being aesthetically pleasing, they are also effective in the case of fires since they are not exposed. One of the main reasons why these joints are generally not used is the inexistence of standards regulating their use. The standards applied to calculate values for timber structures normally do not consider this type of joint. The European Standard, Eurocode 5, only includes criteria for design in an Informative Appendix [2]. Hence, destructive tests must be performed to characterize the joint before it can be used, or the
n
Corresponding author. Tel.: + 34 981167000; fax: + 34 981167051. E-mail address: [email protected] (M.D. Otero Chans).
0143-7496/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2010.03.004
joints must be over-designed to avoid having to perform these tests. This lack of regulation limits the use of glued joints when designing structural joints for new constructions. Since the first experiences with the use of glued-in joints in Europe in the 1970s [3,4], several analytical and experimental studies have been performed to establish design criteria for their widespread use [5–7]. Most of these studies were carried out on glulam from softwood. This is a major inconvenience given it makes the extrapolation of the results to hardwood sawn timber difficult, this timber commonly used for the rehabilitation of existing structures. Given the lack of information available on glued-in joints, our research group has performed an extensive experimental study on these types of joints [8–10]. The main objective of this work was to be able to determine the failure load values with respect to the different material and geometric parameters involved in the joint: timber type and density, anchorage length, etc. Most of the existing design formulas proceed from experiments using glulam from softwood. These formulas consider timber density as a decisive parameter when calculating the axial strength of joints [2,3,7,11]. This criterion could be associated with the system commonly used to characterize the strength of structural timber, where timber density and the presence of
ARTICLE IN PRESS M.D. Otero Chans et al. / International Journal of Adhesion & Adhesives 30 (2010) 380–385
defects in the timber are fundamental aspects. The present study describes the work performed by our group to determine the relation between these parameters, density and defects, and the axial strength of joints made with glued-in rods in hardwood sawn timber.
2. Materials and test setup This paper studies the role of timber density in the axial strength of glued-in joints. Specimens were tested for two types of timber: chestnut (Castanea sativa) and tali (Erythrophelum ivorense). Chestnut was chosen because it is a medium-density hardwood traditionally used in our region (Galicia); hence, the results obtained could be useful for the design of joints in new constructions as well as in the rehabilitation of existing structures. tali, a hardwood of tropical origin, was selected because of its high density and great commercial availability in our country. The timber specimens were prepared in a sawmill, using timber with humidity values below 14%, achieved with an outdoor drying process. Each piece of timber was measured and weighed to determine the specific weight. During the measuring and weighing process, the humidity of the pieces was recorded using an electronic xylohygrometer, verifying that specimen humidity at hygroscopic equilibrium was in the range 1272%. Figs. 1 and 2 show the distribution of the densities obtained for each species, the values of mean density, characteristic density and a polynomial approximation of the distribution law. A detailed study of the presence of defects in each piece was also performed, keeping a photographic record of these so as to evaluate later the possible impact these defects may have on joint failure.
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Some chestnut specimens presented knots or splits. The size and position of these were recorded and specimens with significant flaws were rejected. The tali timber, with highly interlocked grain, turned out to be more regular than the chestnut, in terms of presence of defects, some specimens showing only small splits from the drying process. Galvanized threaded steel rods of 12.9 quality (UNE-EN-ISO 898-1; 1080 N/mm2 yield strength and 1200 N/mm2 ultimate tensile strength) were used. Threaded rods were chosen over corrugated rods (deformed bars) or plastic rods, given the transmission of the load to the adhesive is basically mechanical in threaded rods, going from one thread to the next. This load is then transferred to the timber by adherence. This mechanical transmission of the load between the threaded rod and the adhesive does not require treatment of the rod surface. In contrast, studies have shown that in cases of corrugated and plastic rods the surface must be treated previously by sandblasting or priming to guarantee good adhesion of the glue to the rods [12,13]. The adhesive used is a type of two-component epoxy. In previous works [14], our group performed an experimental analysis of glued-in joints using different types of structural adhesives: two-component epoxy, epoxy acrylate, polyurethane, polyester, and neoprene. Of all the adhesives used, the two-component epoxy-type provided the strongest joints. The different formulations of two-component epoxy adhesives used by the manufacturers Hilti, Sika, and Loctite were compared. The results showed that when a thin glueline (about 1 mm) is used no significant differences were produced between the two-component epoxy adhesives tested. For our tests, the adhesive HILTI HIT-RE 500 was chosen basically because of its viscosity, given it enables easy insertion of the rods. The gluing process was ˜a carried out at the Structures Laboratory of the University of A Corun ˜a) under (Laboratorio de Estructuras de la Universidad de A Corun conditions of controlled temperature and humidity (2072 1C and 6575% relative humidity). Using these materials, specimens were designed with different anchorage lengths and rod diameters. The geometric characteristics of the specimens tested are summarized in Table 1. The tests were designed to determine the influence the different geometric parameters involved in the joint have on the physical and mechanical characteristics of the timbers used.
3. Test results
Fig. 1. Density distribution of the chestnut specimens tested.
Fig. 2. Density distribution of the tali specimens tested.
The experimental work was carried out at the Materials Laboratory of the CITEEC (Center of Technological Innovations in ˜ a. Construction and Civil Engineering) of the University of A Corun The specimens were tested until failure in an Instrom universal press with 1000 kN tensile capacity. The load was applied by displacement control, at constant speed until failure occurred in 572 min, time established for short-duration assays. Load speed ranged from 0.6 to 1.2 mm/min, depending on the type of specimen. The tests were performed using a double tensile, pull–pull setup. The testing apparatus used a clamping system to fasten the threaded steel rods. This system allows possible imperfections in the alignment of the rods to produce bending stresses at the joint. To avoid this problem and guarantee a true tensile test an auxiliary device was designed to center the load. This device consists of a pair of cylinders where the clamps of the press are attached (Fig. 3). The cylinders have a greater inner diameter than the threaded rods. The threaded rods are introduced into the cylinders and they are anchored at the ends by means of a washer and nut system. Hence, the pressure of the clamps is exerted on the outer part of the cylinders and not on the rods. The tests
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Table 1 Geometric characteristics of the specimens tested. Dimensions in millimeters. Type Type Type Type Type Type Type Type Type Type Type
1 2 3 4 5 6 7 8 9 10
Number of chesnut specimens
Number of tali specimens
Dimensions of timber piece
Rod diameter
Hole diameter
Anchorage length
12 12 12 12 12 12 12 12 12 12
6 6 6 6 6 8 8 8 8 8
60 60 180 60 60 270 60 60 360 60 60 450 60 60 500 72 72 180 72 72 270 72 72 360 72 72 450 72 72 500
10 10 10 10 10 12 12 12 12 12
12 12 12 12 12 14 14 14 14 14
60 90 120 150 180 60 90 120 150 180
Fig. 3. Test device and specimen setup with the device used to center the pieces within the clamps: (a) Scheme of the size of the pieces: gripping cylinder, washer limit, and nut. (b) View of the pieces before setting up the specimens. (c) Assembled apparatus during the testing phase.
demonstrated the effectiveness of the system, since the rods act as if they were pinned at their ends, achieving pure tensile load behavior at the joint. Figs. 4 and 5 show the relation between failure load and timber density for each specimen tested. For an easier interpretation of the results, the two types of timber and the ten types of
specimens tested are presented independently. Thus, each graph shows the results for identical specimens, that is, with the same geometric characteristics and the same type timber, the only difference being the specific weight of each specimen. The graphs clearly show that the failure load values for each type of specimen were quite homogenous despite the significant differences in timber density for each one. Apart from failure load, failure mode was also recorded for each specimen, the most common being: failure due to shear stress at the timber surface in contact with the adhesive (Figs. 6a and b) and failure due to splits in the timber (Fig. 6c). Initially, failure due to tensile stresses at the steel rod (Fig. 6d) was also observed. In view of this failure, new specimens were prepared using 12.9 quality steel instead of the 8.8 initialed used. The most common failure was by shear stress at the timber surface and it was evident by the presence of loose fibers on the failed surface in some cases, and by pieces of timber stuck to the pulled-out rod in others. This type of failure was produced in 55% of the chestnut specimens and in 100% of the tali specimens. Given different types of failure modes were presented in the chestnut specimens, the possible relation between the type of failure produced and the failure load obtained was studied. The load values obtained for identical specimens were compared in those where different failure modes were produced. The results were quite homogenous, that is, specimens with identical geometric characteristics and those where different failure modes were produced reached similar maximum load values. For tali specimens, the uniformity in the type of failure produced could be related to the greater homogeneity of the timber, as observed by the absence of defects.
4. Discussion When analyzing the influence of the type of timber on joint strength two fundamental aspects were considered: the existence of defects in the timber and timber density. These two parameters
ARTICLE IN PRESS M.D. Otero Chans et al. / International Journal of Adhesion & Adhesives 30 (2010) 380–385
Fig. 4. Failure load values in relation to density for each chestnut specimen tested. The symbols indicate the different failure modes.
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Fig. 5. Failure load values in relation to density for each tali specimen tested. The symbols indicate the different failure modes.
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Fig. 6. (a–d). Specimens with different failure modes and some timber defects. (a): specimen with dry splits showing a failure mode due to shear in timber. (b): specimen with a knot showing a failure mode due to shear in timber. (c): splitting failure in timber. (d): specimen with a knot showing a tensile failure of the steel rod.
are considered in the strength grading of the timber and in the design proposals of glued-in joints, as explained previously. Initially, failure mode was analyzed for each specimen, comparing those with and without defects. From this analysis important conclusions were made, the first being that failure load was not lower in specimens presenting appreciable defects: basically knots or splits. This could be verified by comparing failure load values for each type of specimen. The second conclusion was drawn from observing how and where the failure occurred in specimens that already had defects. In all cases, failure mode and its location had no relation with the defects in the piece, indicating that the failure was not caused by the existing defects. Fig. 6 presents examples of failure mode and the location of defects in the timber. The influence of timber density on joint strength was studied from two perspectives. Firstly, the relation between the specific weight of each specimen and failure load was studied, in each case comparing specimens having the same geometric characteristics and type of timber. This relation was studied for ten types of specimens for each type of timber, as observed in Figs. 4 and 5. No relation was found between specimens obtaining maximum failure load and those with elevated specific weight. The graphs in Figs. 4 and 5 show how in some cases the higher density specimens reached lower failure loads, that is, the results reveal that for the same type of timber and remaining characteristics, high timber density does not give stronger joints. An example of this is observed in the
results of chestnut specimens with 10 mm rod diameter and 90 mm anchorage length. The relation between the specimen with the highest density and that with the lowest gave a value of 1.60. However, the relation between failure load for these specimens was 1.13. This lack of relation was observed for both chestnut and tali species. Secondly, the influence of timber density on the strength of the joint for both timber species used was studied. The mean and characteristic values of timber density in the specimens studied are shown in Figs. 1 and 2. The relation between the density values of tali and chestnut was 1.55 (870 kg/m3/560 kg/m3) for mean values and 1.70 (800 kg/m3/470 kg/m3) for characteristic values. If a linear relation were to exist between timber density and joint strength, the ratios between failure load for both species should be around these values. However, the actual values obtained experimentally were totally different. Fig. 7 presents (with white bars) for each type of specimen the relation between mean failure load for tali specimens and the mean failure load for chestnut specimens, being between 0.98 and 1.70. Then, the black bars represent the relation between the mean density of tali and the mean density of chestnut for each type of specimen. This relation ranged between 1.40 and 1.80, but with a clearly different distribution than that for failure loads. The graph confirms that there is no linear relation between mean timber density and failure load. An example of this can be observed for specimens with 10 mm threaded steel rods and 60 mm anchorage length, where the mean
ARTICLE IN PRESS M.D. Otero Chans et al. / International Journal of Adhesion & Adhesives 30 (2010) 380–385
Fig. 7. Ratios between density and between the values of failure loads for the two timber species used (tali values/chestnut values).
load value for both timber species was practically the same (ratio of 0.98), despite the density of tali specimens being 1.80 times higher than that of chestnut. Most of the available formulas consider that there is a linear relation between timber density and ultimate axial load [3,11]. The Informative Appendix of Eurocode 5 [2] considers that there could even be an exponential influence of density, which would make the influence of density on the load value even greater. However, as indicated before, the experimental results presented in Fig. 7 demonstrate that this approach is erroneous. The variations between the failure load ratios shown in the figure indicate that joint strength does not change linearly with the physical properties (density) of the timber. Finally, the results shown in Fig. 7 confirm that joint strength does not hold a linear relation with the mechanical properties of the timber species either. As mentioned before the ratio relating the mean load of both species (white bars) ranges between 0.98 and 1.70. If the properties of the timber had a linear influence on the load of the joint, the ratio of the strengths obtained for tali and chestnut would have to remain constant for all the specimens tested, a situation that was not produced, as seen in the graph. The data presented suggest that the influence of other geometric and mechanical parameters should be considered, apart from the factors already mentioned, to predict the axial strength of these types of joints.
5. Conclusions Our research group has carried out a thorough experimental analysis to evaluate the failure load of joints made with glued-in rods. The tests were performed by gluing threaded steel rods using epoxy adhesives in pieces of high-density hardwood of two different timber species. The failure load values obtained demonstrate the potential of these types of joints when designing structures of sawn or
385
laminated timber, or when a prosthetic section is needed to replace structural elements of buildings damaged by timber rot. The failure mode of the specimens tested seems to reveal a lack of relation between defects present in the timber and joint strength. In the case of knots and dry splits there is no appreciable relation between the type of failure and the location of the defects. These defects do not seem to weaken the strength of the joints either, where the failure was conditioned, generally, by the shear strength of the timber. When the failure loads of specimens with identical geometric characteristics prepared in the same timber species were compared, no linear relation was observed between specimen density and axial strength. The experimental results obtained for the specimens made of tali and chestnut timber clearly show that there is no linear relation between the axial strength of the joint and the density of the timbers used. Similarly, joint strength did not hold a linear relation with the mechanical properties of the timber species either. From the results, it is evident that a more in depth analysis, both theoretical as well as experimental, of these types of joints is necessary to correctly identify the parameters influencing the axial strength of these joints. This information can be used to develop new formulas for efficient joint design. Our group is presently working on this objective.
Acknowledgements This research was sponsored by the Ministry of Innovation and Industry (Xunta de Galicia) through the research project entitled ‘‘Sistemas de mejora de la tecnologia de la union con adhesives de barras de acero insertadas en madera’’ (Systems to improve the technology of joints made with steel rods glued into timber). References [1] /www.promaxsa.comS. [2] CEN/TC 250/SC5. Eurocode 5. Desing of timber structures. Part 1.1. General rules. General rules and rules for building. Final draft prEN 1995-1-1, 2001. [3] Riberholt H. Glued bolts in glulam. Department of Structural Engineering. University of Denmark.Serie R. No. 210; 1986. ¨ [4] Mohler K, Hemmer K. Bauen Mit Holz 1981;83(5):296–8. [5] Contribucion from Legnodoc srl: Evaluation of draft desing rules. Task 5: Physical testing and prototyping. Sub-task 5.3: Development of structural Eurocode-type principles. Formal Report 3 TRADA-LINCONS- CRAF: 199971216, April 2003. [6] J.G. Broughton & A.R. Hutchinson. Preliminary Test series. Experimental verification of design calculations—for the development of design procedures for repairs. LICONS (Low Intrusion Conservation Systems for Timber Structures), CRAF-1999-71216, Task 4.1, March 2004. [7] Feligioni L, Lasvisci P, Duchanois G, Ciechi M, Spinelli P. Holz als Roh-und Werktoff 2003;61:281–7. [8] Este´vez Cimadevila J, Va´zquez Rodrı´guez JA, Otero Chans MD. Int J Adhes Adhes 2007;27(2):136. [9] Otero Chans D, Este´vez Cimadevila J, Martı´n Gutie´rrez E. Int J Adhes Adhes 2008;28(8):457. [10] Otero Chans D, Este´vez Cimadevila J, Martı´n Gutie´rrez E. Mater Des 2009;30(4):1325. [11] Gerold M. Bautechnik 1992;69(4):167–78. [12] Barroso DJ, Negra~ o JH, Cruz. PJ. Avaliac-ao do comportamento de vigas de madeira lamelada pre-esforc-ada. CIMAD04. 1o Congreso Ibe´rico A Madeira na % Construc- ao. Guimaraes, Portugal: Universidade do Minho; 25–26 March 2004 p. 103–12. [13] Harvey K., Ansell M.P. Improved timber connections using bonded-in GFRP rods. In: Proceedings of the World conference on timber engineering, Canada, 2000. ˜a [14] Otero Chans D. Steel rods glued-in hardwood timber. Ph.D. thesis. A Corun University, May 2007.
Contents lists available at ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Influence of timber density on the axial strength of joints made with glued-in steel rods: An experimental approach M.D. Otero Chans n, J. Este´vez Cimadevila, E. Martı´n Gutie´rrez, J.A. Va´zquez Rodrı´guez ˜a, Department of Construction Technology, Architecture School, Spain University of A Corun
a r t i c l e in f o
a b s t r a c t
Article history: Accepted 20 November 2009 Available online 7 March 2010
Joints made with glued-in steel rods have many possibilities in the design of timber structures. They can be used for new buildings or for the rehabilitation of old structural elements damaged by the attack of biotic agents or humidity. Since the 1970s many studies have been carried out to characterize the strength of these joints when made with glued laminated timber (glulam). These studies hypothesize that the axial strength of joints made in glulam depends on some geometric parameters (anchorage length, steel rod diameter, adhesive thickness, etc.) as well as on timber density. For several years our research group has been studying the behavior of these joints when made in sawn timber, determining the influence of different geometric and material parameters on the axial strength of the glued-in steel rods. This work summarizes the experimental results of joints made in pieces of sawn timber of two species having different densities and mechanical properties. The experimental study was carried out for different geometric configurations: threaded steel rods of 10 and 12 mm diameter, epoxy adhesive of 1 mm thickness, and five anchorage lengths. The aim was to test the same specimen conditions for each timber species studied, tali and chestnut. The experimental results show that the axial strength of the joints does not increase linearly with timber density. This result contradicts many of the traditional design proposals suggested for joints made in glulam. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Epoxy B. Wood C. Destructive testing E. Joint design
1. Introduction Joints made with glued-in rods have great potential in the field of timber engineering. Initially, these joints were used in glulam to prevent cracks in areas submitted to tensile stresses perpendicular to the grain. Presently, they are also being used to design joints in planar or spatial structures consisting of rods under axial load; to design rigid joints (bending resistant) in frame structures; for the rehabilitation of damaged structural elements in buildings, this being their principal application, for the moment, in Spain [1]. Apart from being aesthetically pleasing, they are also effective in the case of fires since they are not exposed. One of the main reasons why these joints are generally not used is the inexistence of standards regulating their use. The standards applied to calculate values for timber structures normally do not consider this type of joint. The European Standard, Eurocode 5, only includes criteria for design in an Informative Appendix [2]. Hence, destructive tests must be performed to characterize the joint before it can be used, or the
n
Corresponding author. Tel.: + 34 981167000; fax: + 34 981167051. E-mail address: [email protected] (M.D. Otero Chans).
0143-7496/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2010.03.004
joints must be over-designed to avoid having to perform these tests. This lack of regulation limits the use of glued joints when designing structural joints for new constructions. Since the first experiences with the use of glued-in joints in Europe in the 1970s [3,4], several analytical and experimental studies have been performed to establish design criteria for their widespread use [5–7]. Most of these studies were carried out on glulam from softwood. This is a major inconvenience given it makes the extrapolation of the results to hardwood sawn timber difficult, this timber commonly used for the rehabilitation of existing structures. Given the lack of information available on glued-in joints, our research group has performed an extensive experimental study on these types of joints [8–10]. The main objective of this work was to be able to determine the failure load values with respect to the different material and geometric parameters involved in the joint: timber type and density, anchorage length, etc. Most of the existing design formulas proceed from experiments using glulam from softwood. These formulas consider timber density as a decisive parameter when calculating the axial strength of joints [2,3,7,11]. This criterion could be associated with the system commonly used to characterize the strength of structural timber, where timber density and the presence of
ARTICLE IN PRESS M.D. Otero Chans et al. / International Journal of Adhesion & Adhesives 30 (2010) 380–385
defects in the timber are fundamental aspects. The present study describes the work performed by our group to determine the relation between these parameters, density and defects, and the axial strength of joints made with glued-in rods in hardwood sawn timber.
2. Materials and test setup This paper studies the role of timber density in the axial strength of glued-in joints. Specimens were tested for two types of timber: chestnut (Castanea sativa) and tali (Erythrophelum ivorense). Chestnut was chosen because it is a medium-density hardwood traditionally used in our region (Galicia); hence, the results obtained could be useful for the design of joints in new constructions as well as in the rehabilitation of existing structures. tali, a hardwood of tropical origin, was selected because of its high density and great commercial availability in our country. The timber specimens were prepared in a sawmill, using timber with humidity values below 14%, achieved with an outdoor drying process. Each piece of timber was measured and weighed to determine the specific weight. During the measuring and weighing process, the humidity of the pieces was recorded using an electronic xylohygrometer, verifying that specimen humidity at hygroscopic equilibrium was in the range 1272%. Figs. 1 and 2 show the distribution of the densities obtained for each species, the values of mean density, characteristic density and a polynomial approximation of the distribution law. A detailed study of the presence of defects in each piece was also performed, keeping a photographic record of these so as to evaluate later the possible impact these defects may have on joint failure.
381
Some chestnut specimens presented knots or splits. The size and position of these were recorded and specimens with significant flaws were rejected. The tali timber, with highly interlocked grain, turned out to be more regular than the chestnut, in terms of presence of defects, some specimens showing only small splits from the drying process. Galvanized threaded steel rods of 12.9 quality (UNE-EN-ISO 898-1; 1080 N/mm2 yield strength and 1200 N/mm2 ultimate tensile strength) were used. Threaded rods were chosen over corrugated rods (deformed bars) or plastic rods, given the transmission of the load to the adhesive is basically mechanical in threaded rods, going from one thread to the next. This load is then transferred to the timber by adherence. This mechanical transmission of the load between the threaded rod and the adhesive does not require treatment of the rod surface. In contrast, studies have shown that in cases of corrugated and plastic rods the surface must be treated previously by sandblasting or priming to guarantee good adhesion of the glue to the rods [12,13]. The adhesive used is a type of two-component epoxy. In previous works [14], our group performed an experimental analysis of glued-in joints using different types of structural adhesives: two-component epoxy, epoxy acrylate, polyurethane, polyester, and neoprene. Of all the adhesives used, the two-component epoxy-type provided the strongest joints. The different formulations of two-component epoxy adhesives used by the manufacturers Hilti, Sika, and Loctite were compared. The results showed that when a thin glueline (about 1 mm) is used no significant differences were produced between the two-component epoxy adhesives tested. For our tests, the adhesive HILTI HIT-RE 500 was chosen basically because of its viscosity, given it enables easy insertion of the rods. The gluing process was ˜a carried out at the Structures Laboratory of the University of A Corun ˜a) under (Laboratorio de Estructuras de la Universidad de A Corun conditions of controlled temperature and humidity (2072 1C and 6575% relative humidity). Using these materials, specimens were designed with different anchorage lengths and rod diameters. The geometric characteristics of the specimens tested are summarized in Table 1. The tests were designed to determine the influence the different geometric parameters involved in the joint have on the physical and mechanical characteristics of the timbers used.
3. Test results
Fig. 1. Density distribution of the chestnut specimens tested.
Fig. 2. Density distribution of the tali specimens tested.
The experimental work was carried out at the Materials Laboratory of the CITEEC (Center of Technological Innovations in ˜ a. Construction and Civil Engineering) of the University of A Corun The specimens were tested until failure in an Instrom universal press with 1000 kN tensile capacity. The load was applied by displacement control, at constant speed until failure occurred in 572 min, time established for short-duration assays. Load speed ranged from 0.6 to 1.2 mm/min, depending on the type of specimen. The tests were performed using a double tensile, pull–pull setup. The testing apparatus used a clamping system to fasten the threaded steel rods. This system allows possible imperfections in the alignment of the rods to produce bending stresses at the joint. To avoid this problem and guarantee a true tensile test an auxiliary device was designed to center the load. This device consists of a pair of cylinders where the clamps of the press are attached (Fig. 3). The cylinders have a greater inner diameter than the threaded rods. The threaded rods are introduced into the cylinders and they are anchored at the ends by means of a washer and nut system. Hence, the pressure of the clamps is exerted on the outer part of the cylinders and not on the rods. The tests
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Table 1 Geometric characteristics of the specimens tested. Dimensions in millimeters. Type Type Type Type Type Type Type Type Type Type Type
1 2 3 4 5 6 7 8 9 10
Number of chesnut specimens
Number of tali specimens
Dimensions of timber piece
Rod diameter
Hole diameter
Anchorage length
12 12 12 12 12 12 12 12 12 12
6 6 6 6 6 8 8 8 8 8
60 60 180 60 60 270 60 60 360 60 60 450 60 60 500 72 72 180 72 72 270 72 72 360 72 72 450 72 72 500
10 10 10 10 10 12 12 12 12 12
12 12 12 12 12 14 14 14 14 14
60 90 120 150 180 60 90 120 150 180
Fig. 3. Test device and specimen setup with the device used to center the pieces within the clamps: (a) Scheme of the size of the pieces: gripping cylinder, washer limit, and nut. (b) View of the pieces before setting up the specimens. (c) Assembled apparatus during the testing phase.
demonstrated the effectiveness of the system, since the rods act as if they were pinned at their ends, achieving pure tensile load behavior at the joint. Figs. 4 and 5 show the relation between failure load and timber density for each specimen tested. For an easier interpretation of the results, the two types of timber and the ten types of
specimens tested are presented independently. Thus, each graph shows the results for identical specimens, that is, with the same geometric characteristics and the same type timber, the only difference being the specific weight of each specimen. The graphs clearly show that the failure load values for each type of specimen were quite homogenous despite the significant differences in timber density for each one. Apart from failure load, failure mode was also recorded for each specimen, the most common being: failure due to shear stress at the timber surface in contact with the adhesive (Figs. 6a and b) and failure due to splits in the timber (Fig. 6c). Initially, failure due to tensile stresses at the steel rod (Fig. 6d) was also observed. In view of this failure, new specimens were prepared using 12.9 quality steel instead of the 8.8 initialed used. The most common failure was by shear stress at the timber surface and it was evident by the presence of loose fibers on the failed surface in some cases, and by pieces of timber stuck to the pulled-out rod in others. This type of failure was produced in 55% of the chestnut specimens and in 100% of the tali specimens. Given different types of failure modes were presented in the chestnut specimens, the possible relation between the type of failure produced and the failure load obtained was studied. The load values obtained for identical specimens were compared in those where different failure modes were produced. The results were quite homogenous, that is, specimens with identical geometric characteristics and those where different failure modes were produced reached similar maximum load values. For tali specimens, the uniformity in the type of failure produced could be related to the greater homogeneity of the timber, as observed by the absence of defects.
4. Discussion When analyzing the influence of the type of timber on joint strength two fundamental aspects were considered: the existence of defects in the timber and timber density. These two parameters
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Fig. 4. Failure load values in relation to density for each chestnut specimen tested. The symbols indicate the different failure modes.
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Fig. 5. Failure load values in relation to density for each tali specimen tested. The symbols indicate the different failure modes.
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Fig. 6. (a–d). Specimens with different failure modes and some timber defects. (a): specimen with dry splits showing a failure mode due to shear in timber. (b): specimen with a knot showing a failure mode due to shear in timber. (c): splitting failure in timber. (d): specimen with a knot showing a tensile failure of the steel rod.
are considered in the strength grading of the timber and in the design proposals of glued-in joints, as explained previously. Initially, failure mode was analyzed for each specimen, comparing those with and without defects. From this analysis important conclusions were made, the first being that failure load was not lower in specimens presenting appreciable defects: basically knots or splits. This could be verified by comparing failure load values for each type of specimen. The second conclusion was drawn from observing how and where the failure occurred in specimens that already had defects. In all cases, failure mode and its location had no relation with the defects in the piece, indicating that the failure was not caused by the existing defects. Fig. 6 presents examples of failure mode and the location of defects in the timber. The influence of timber density on joint strength was studied from two perspectives. Firstly, the relation between the specific weight of each specimen and failure load was studied, in each case comparing specimens having the same geometric characteristics and type of timber. This relation was studied for ten types of specimens for each type of timber, as observed in Figs. 4 and 5. No relation was found between specimens obtaining maximum failure load and those with elevated specific weight. The graphs in Figs. 4 and 5 show how in some cases the higher density specimens reached lower failure loads, that is, the results reveal that for the same type of timber and remaining characteristics, high timber density does not give stronger joints. An example of this is observed in the
results of chestnut specimens with 10 mm rod diameter and 90 mm anchorage length. The relation between the specimen with the highest density and that with the lowest gave a value of 1.60. However, the relation between failure load for these specimens was 1.13. This lack of relation was observed for both chestnut and tali species. Secondly, the influence of timber density on the strength of the joint for both timber species used was studied. The mean and characteristic values of timber density in the specimens studied are shown in Figs. 1 and 2. The relation between the density values of tali and chestnut was 1.55 (870 kg/m3/560 kg/m3) for mean values and 1.70 (800 kg/m3/470 kg/m3) for characteristic values. If a linear relation were to exist between timber density and joint strength, the ratios between failure load for both species should be around these values. However, the actual values obtained experimentally were totally different. Fig. 7 presents (with white bars) for each type of specimen the relation between mean failure load for tali specimens and the mean failure load for chestnut specimens, being between 0.98 and 1.70. Then, the black bars represent the relation between the mean density of tali and the mean density of chestnut for each type of specimen. This relation ranged between 1.40 and 1.80, but with a clearly different distribution than that for failure loads. The graph confirms that there is no linear relation between mean timber density and failure load. An example of this can be observed for specimens with 10 mm threaded steel rods and 60 mm anchorage length, where the mean
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Fig. 7. Ratios between density and between the values of failure loads for the two timber species used (tali values/chestnut values).
load value for both timber species was practically the same (ratio of 0.98), despite the density of tali specimens being 1.80 times higher than that of chestnut. Most of the available formulas consider that there is a linear relation between timber density and ultimate axial load [3,11]. The Informative Appendix of Eurocode 5 [2] considers that there could even be an exponential influence of density, which would make the influence of density on the load value even greater. However, as indicated before, the experimental results presented in Fig. 7 demonstrate that this approach is erroneous. The variations between the failure load ratios shown in the figure indicate that joint strength does not change linearly with the physical properties (density) of the timber. Finally, the results shown in Fig. 7 confirm that joint strength does not hold a linear relation with the mechanical properties of the timber species either. As mentioned before the ratio relating the mean load of both species (white bars) ranges between 0.98 and 1.70. If the properties of the timber had a linear influence on the load of the joint, the ratio of the strengths obtained for tali and chestnut would have to remain constant for all the specimens tested, a situation that was not produced, as seen in the graph. The data presented suggest that the influence of other geometric and mechanical parameters should be considered, apart from the factors already mentioned, to predict the axial strength of these types of joints.
5. Conclusions Our research group has carried out a thorough experimental analysis to evaluate the failure load of joints made with glued-in rods. The tests were performed by gluing threaded steel rods using epoxy adhesives in pieces of high-density hardwood of two different timber species. The failure load values obtained demonstrate the potential of these types of joints when designing structures of sawn or
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laminated timber, or when a prosthetic section is needed to replace structural elements of buildings damaged by timber rot. The failure mode of the specimens tested seems to reveal a lack of relation between defects present in the timber and joint strength. In the case of knots and dry splits there is no appreciable relation between the type of failure and the location of the defects. These defects do not seem to weaken the strength of the joints either, where the failure was conditioned, generally, by the shear strength of the timber. When the failure loads of specimens with identical geometric characteristics prepared in the same timber species were compared, no linear relation was observed between specimen density and axial strength. The experimental results obtained for the specimens made of tali and chestnut timber clearly show that there is no linear relation between the axial strength of the joint and the density of the timbers used. Similarly, joint strength did not hold a linear relation with the mechanical properties of the timber species either. From the results, it is evident that a more in depth analysis, both theoretical as well as experimental, of these types of joints is necessary to correctly identify the parameters influencing the axial strength of these joints. This information can be used to develop new formulas for efficient joint design. Our group is presently working on this objective.
Acknowledgements This research was sponsored by the Ministry of Innovation and Industry (Xunta de Galicia) through the research project entitled ‘‘Sistemas de mejora de la tecnologia de la union con adhesives de barras de acero insertadas en madera’’ (Systems to improve the technology of joints made with steel rods glued into timber). References [1] /www.promaxsa.comS. [2] CEN/TC 250/SC5. Eurocode 5. Desing of timber structures. Part 1.1. General rules. General rules and rules for building. Final draft prEN 1995-1-1, 2001. [3] Riberholt H. Glued bolts in glulam. Department of Structural Engineering. University of Denmark.Serie R. No. 210; 1986. ¨ [4] Mohler K, Hemmer K. Bauen Mit Holz 1981;83(5):296–8. [5] Contribucion from Legnodoc srl: Evaluation of draft desing rules. Task 5: Physical testing and prototyping. Sub-task 5.3: Development of structural Eurocode-type principles. Formal Report 3 TRADA-LINCONS- CRAF: 199971216, April 2003. [6] J.G. Broughton & A.R. Hutchinson. Preliminary Test series. Experimental verification of design calculations—for the development of design procedures for repairs. LICONS (Low Intrusion Conservation Systems for Timber Structures), CRAF-1999-71216, Task 4.1, March 2004. [7] Feligioni L, Lasvisci P, Duchanois G, Ciechi M, Spinelli P. Holz als Roh-und Werktoff 2003;61:281–7. [8] Este´vez Cimadevila J, Va´zquez Rodrı´guez JA, Otero Chans MD. Int J Adhes Adhes 2007;27(2):136. [9] Otero Chans D, Este´vez Cimadevila J, Martı´n Gutie´rrez E. Int J Adhes Adhes 2008;28(8):457. [10] Otero Chans D, Este´vez Cimadevila J, Martı´n Gutie´rrez E. Mater Des 2009;30(4):1325. [11] Gerold M. Bautechnik 1992;69(4):167–78. [12] Barroso DJ, Negra~ o JH, Cruz. PJ. Avaliac-ao do comportamento de vigas de madeira lamelada pre-esforc-ada. CIMAD04. 1o Congreso Ibe´rico A Madeira na % Construc- ao. Guimaraes, Portugal: Universidade do Minho; 25–26 March 2004 p. 103–12. [13] Harvey K., Ansell M.P. Improved timber connections using bonded-in GFRP rods. In: Proceedings of the World conference on timber engineering, Canada, 2000. ˜a [14] Otero Chans D. Steel rods glued-in hardwood timber. Ph.D. thesis. A Corun University, May 2007.