TIMBER BEAMS STRENGTHENED BY ATTACHING PRESTRESSED … Timb… · Prestressed FRP, glulam beams,...

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Proceedings of the International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005) Chen and Teng (eds) © 2005 International Institute for FRP in Construction 465 TIMBER BEAMS STRENGTHENED BY ATTACHING PRESTRESSED CARBON FRP LAMINATES WITH A GRADIENTED ANCHORING DEVICE Maurice Brunner and Marco Schnueriger University of Applied Science Bern, Switzerland. Email: [email protected] ABSTRACT The bending strength of a timber beam could be greatly increased if the brittle tensile face would be adequately strengthened with prestressed carbon laminates of high strength. However, current technology is unable to overcome the premature failure of the prestressed beam due to delamination of the laminate. In a COST Project, the authors used a special device to bond the prestressed laminate to the timber in stages starting from the centre of the beam. There was no delamination. The strengthened beams were tested in bending: the results corresponded quite well to the calculated values. KEYWORDS Prestressed FRP, glulam beams, graduated anchoring device, bending tests INTRODUCTION A method sometimes used to improve the load-bearing capacity of glulam beams involves the inclusion of high- strength artificial fibres (FRP) on the tensile face (Tingly 1995, Romani and Blass 2001). In current engineering practice, the fibres are attached in a slack state. They could also be prestressed in order to make better use of their high strength and at the same time reduce the cross-section of FRP needed. Glulam beams loaded in the direction of the wood grain exhibit high strength. Theoretically, it should be possible to apply very high prestressing forces on relatively small cross sections. In practice, only a small prestressing force can be applied because of delamination dangers. Theoretical models show that the prestressing force is anchored over a very short distance at both ends of the beam when conventional gluing techniques are used (Triantafillou et al 1991 and 1992, Holzenkaempfer 1997, Luggin 2000). The resulting concentrated force transmission from the laminate end into the beam body induces high stresses in the direction perpendicular to the grain, where the cracking energy of timber is low. As fig. 1 shows, the danger of delamination is also well known when concrete beams are similarly strengthened. Acting within the framework of the COST action E13 "Wood adhesion and glued products", the authors initiated a research project to analyse a promising approach to the delamination problem: There was reason to believe that the gradiented anchoring technique developed for concrete structures could also be successfully applied to timber as well. The developer of the device, EMPA (Swiss Federal Materials Research Laboratory), was incorporated as research partner. Fig. 1: Delamination: a prestressed laminate is torn away from the concrete beam Fig. 2: Distribution of shear stress τ in the load- transmission zone Crack τ max

Transcript of TIMBER BEAMS STRENGTHENED BY ATTACHING PRESTRESSED … Timb… · Prestressed FRP, glulam beams,...

Page 1: TIMBER BEAMS STRENGTHENED BY ATTACHING PRESTRESSED … Timb… · Prestressed FRP, glulam beams, graduated anchoring device, bending tests INTRODUCTION A method sometimes used to

Proceedings of the International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005) Chen and Teng (eds)

© 2005 International Institute for FRP in Construction

465

TIMBER BEAMS STRENGTHENED BY ATTACHING PRESTRESSED CARBON FRP LAMINATES WITH A GRADIENTED ANCHORING DEVICE

Maurice Brunner and Marco Schnueriger University of Applied Science Bern, Switzerland. Email: [email protected]

ABSTRACT The bending strength of a timber beam could be greatly increased if the brittle tensile face would be adequately strengthened with prestressed carbon laminates of high strength. However, current technology is unable to overcome the premature failure of the prestressed beam due to delamination of the laminate. In a COST Project, the authors used a special device to bond the prestressed laminate to the timber in stages starting from the centre of the beam. There was no delamination. The strengthened beams were tested in bending: the results corresponded quite well to the calculated values. KEYWORDS Prestressed FRP, glulam beams, graduated anchoring device, bending tests INTRODUCTION A method sometimes used to improve the load-bearing capacity of glulam beams involves the inclusion of high-strength artificial fibres (FRP) on the tensile face (Tingly 1995, Romani and Blass 2001). In current engineering practice, the fibres are attached in a slack state. They could also be prestressed in order to make better use of their high strength and at the same time reduce the cross-section of FRP needed. Glulam beams loaded in the direction of the wood grain exhibit high strength. Theoretically, it should be possible to apply very high prestressing forces on relatively small cross sections. In practice, only a small prestressing force can be applied because of delamination dangers. Theoretical models show that the prestressing force is anchored over a very short distance at both ends of the beam when conventional gluing techniques are used (Triantafillou et al 1991 and 1992, Holzenkaempfer 1997, Luggin 2000). The resulting concentrated force transmission from the laminate end into the beam body induces high stresses in the direction perpendicular to the grain, where the cracking energy of timber is low. As fig. 1 shows, the danger of delamination is also well known when concrete beams are similarly strengthened.

Acting within the framework of the COST action E13 "Wood adhesion and glued products", the authors initiated a research project to analyse a promising approach to the delamination problem: There was reason to believe that the gradiented anchoring technique developed for concrete structures could also be successfully applied to timber as well. The developer of the device, EMPA (Swiss Federal Materials Research Laboratory), was incorporated as research partner.

Fig. 1: Delamination: a prestressed laminate is torn away from the concrete beam

Fig. 2: Distribution of shear stress τ in the load-transmission zone

Crack

τ max

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GRADIENTED ANCHORING TECHNIQUE A lot of international research work has been done on the strengthening of existing concrete structures. In Switzerland for example, the EMPA has developed a special prestressing device, the gradiented anchoring system, to apply a prestressing force in stages (Stöcklin + Meier 2001). The difficult task of anchoring a large prestressing force at the brittle laminate ends is solved elegantly by turning the laminate over two wheels with a rough surface (Figs. 3 and 4). Because of friction over the wheel section, the gripping force at the laminate ends is greatly reduced and thus readily manageable with clamps . The next task is to bond the prestressed laminate to the beam. As mentioned above in Introduction, when conventional gluing methods are used, the build-up of peak stresses may quickly lead to delamination failure. The electronically controlled device solves the delamination problem as follows: in the first bonding stage, the midsection of both the prestressed laminate and the beam are attached by activating the epoxy-based adhesive with heat. The prestressing force is then slightly reduced and the next section of the laminate is bonded to the beam. In effect, the prestressing force is anchored over a certain beam length. Fig. 5 shows test results where the shear stresses between the beam and the prestressed laminate are spread over a distance of about 500mm at the beam ends: in the middle part of the beam, the strains – and thus the stresses - in the laminate remain constant.

Fig. 3 EMPA Prestressing device with a gradiented anchoring system

Fig. 4 Detail of the device during the prestressing process of a timber beam

Fig. 5 Strain distribution in a prestressed FRP laminate along the beam length

THEORETICAL ANALYSIS OF PRESTRESSED TIMBER BEAMS

Calculation Model A calculation model is needed to estimate the load-bearing capacity of prestressed timber beams and for the analysis of test results. The discussion below is concerned with the ultimate limit state and it is only valid when the danger of premature failure of prestressed beams due to delamination is precluded. When a timber beam is strengthened on the brittle tension side, the ductile compressive face can be expected to "plastify". Inspired by the many refined calculation models for reinforced concrete, many timber researchers have proposed different calculation models for a plastic design approach for strengthened timber beams (Kuilen 1991, Tingly 1995, Brunner 2000, Lindyberg and Dagher 2000, Romani and Blass 2001). Most of the models make a clear distinction between the elastic-plastic stresses on the compressive face of the timber on the one hand, and the purely linear-elastic stresses on the tensile face. Some European authors such as Brunner and Blass use the assumptions depicted in fig. 6:

− linear strain distribution over the entire height of the beam − linear stress distribution on the tensile face, maximum value corresponds to the bending strength − initially linear, then constant stress distribution over the compressive face, maximum value corresponds

to the axial compressive strength − The stress σ in the FRP corresponds to the strain level ε attained:

σ = E .ε (1) The calculation is iterative. An assumption is made for the position of the neutral axis. The failure strain of the tensile face of the timber beam can be estimated from the bending strength and the Modulus of Elasticity: the

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characteristic strains in the compressive face of the timber and the strain in the artificial fibre can be calculated accordingly. From the stress-strain diagrams, the stress distributions in the composite materials can be calculated. The internal forces are calculated from the stress distribution in the cross-section and they must fulfil the equilibrium condition that, since there is no external axial force, the sum of the internal forces must be zero: Fc,T + Ft,T + Ft,F = 0 (2)

Fig. 6: Strain and stress distributions in a strengthened timber beam

The calculation is repeated for different assumptions of the position of the neutral axis until the above condition is fulfilled. The bending resistance is then calculated by multiplying the internal forces on the tensile face in both timber and laminate with the corresponding distances from the compressive force in the timber: MR = Ft,T . e1 + Ft,F . e2 (3) The calculation models mentioned above need only a slight modification in order to permit the calculation of timber beams strengthened with prestressed FRP laminates. Brunner 2002 for example refers to the calculation of prestressed concrete and modifies the calculation model depicted in fig. 6 by adding the initial prestressing force in the FRP to the additional force corresponding to the strain level. The distribution of the stresses in the timber remains essentially the same as depicted in fig. 6, though of course the neutral axis will be shifted downwards to accommodate the larger compressive zone needed to counter-balance the greater force in the FRP laminate. Calculation example of prestressed timber beam The calculation model described above was used to predict the load-bearing capacity of a strengthened timber beam which was later tested in bending (fig. 7). The timber has the following material properties:

• GL 32: Em=14 kN/mm2 • 5% fractile values according to Eurocode EN 1194: fc,k = 29 N/mm2, fm,k = 32 N/mm2 • Medium values expected in loading tests are about 33% higher than the 5% fractile values:

o fc= 39 N/mm2, fm= 43 N/mm2 • Characteristic strains:

o Tensile failure at εt = fm / E = 43/14’000 = 3.07 %o. o Yielding of compressive face at εc = fc / E = 39/14’000 = 2.79 %o

The FRP laminate used has the following properties:

• S&P-carbon laminate type 150/2000: • Cross-section 1.4x50mm • E=165 kN/mm2 • Initial prestressing force: 60 kN.

It is hereby assumed that failure of the tensile face of the timber beam at a strain of 3.07%o resp. a stress of 43 N/mm2 will induce collapse.

Neutral axis

f c,T

Legend: fc,T: Axial compressive strength of timberFc,T: Internal compressive force timber fm,T : Bending strength of timber Ft,T : Internal tensile force in timber ft,L : Tensile stress in FRP-laminate Ft,L : Tensile force in FRP-laminate

:

Stress distribution Internal forces: Strains

Fc,T

Ft,F

e1 e2

ft,T

ft,L

Ft,T

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The calculation is iterative. Assuming a height z1=91mm of the tensile face of the timber leads for example to the following results:

• z2=(39/43) x 91 = 83 mm ⇒ z3=200 – (91 + 83) = 26 mm • Maximum strain on the compressive face: εO = 2.79% x (26 + 83) / 83 = 3.66 %o • Strain in the FRP (additional to prestressing) = (91.7/91) x 3.07 = 3.09 %o

o ⇒ Additional stress in FRP = ε x E = 3.09%o x 165’000 = 510 N/mm2

Fig. 7: Calculation example of prestressed glulam beam

Internal forces:

• Concrete compressive face: o D1 = (26 x 140) x 39 / 1000 = 142.0 kN o D2 = 0.5 x (83 x 140) x 39 / 1000 = 226.5 kN o D(total) = D1 + D2 = 368.6 kN

• Timber tensile face: o Z2 = 0.5 x (91 x 140) x 43 / 1000 = 273.9 kN

• FRP: o Prestressing force Z0 = 60 kN o Additional force Z1 = 510 x (1.4 x 50) / 1000 = 35.7 kN

• Total tensile force: Z0 + Z1 + Z2 = 369.6 kN Since the compressive forces and the tensile forces are (nearly) equal, the iterative process can be ended. The distances between the forces can be found from a consideration of the geometry. The resultant compressive force D(total) for example has the following distance from the top of the beam: e1 = (142.0 x 13 + 226.5 x 54) / 368.6 = 38.1mm Similarly, it can be shown that the resulting total tensile force acts at a distance of about 178 mm from the top of the beam. The distance between the resulting compressive and tensile forces is therefore 140 mm and the expected failure moment of the prestressed glulam beam can be calculated as: MU = 368.6 x 0.140 = 52 kNm It is worth remarking here that the calculated maximum strain on the compressive face is only about 3.66 %o. Although the failure strain of structural timber under compressive loading is not listed in any norms and standards known to the authors, literature studies indicate that it may be close to the better researched values for small clear specimens, which many authors suggest lies at about 12 %o. Hence the compressive face of the timber specimen could readily accommodate larger forces. In other words, the prestressing force in the FRP laminate could be greatly increased before there would be any real danger of timber compressive failure. Advantage of strengthening with prestressed rather than with slack FRP-laminates

20

0 m

m

Z 1

5 0 m m

1 40 m m

S T R A IN S % o

S T R E S S E S N /m m 2

2 .7 9

3 .0 7 4 3

3 9

Z 2

Z 3

5 10

IN T E R N A L F O R C E S

D 1

D 2

Z 2

Z 1 + Z 0

εO

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In the timber industry, much research has been done on the strengthening of timber beams by attaching FRP in a slack form. The technique has been applied in structures, and some engineers expect an increased demand, particularly with regard to the rehabilitation of old structures. The gluing technologies used are still not very advanced and the amount of reinforcement possible is quite modest and usually much less that 1% of the cross-section of the timber beam. Lindyberg 2000 estimates that under future, idealized conditions, with 3.3% reinforcement, a maximum strengthening ratio of about 2.0 could be attained for glulam: this means that the load-bearing capacity of naked glulam beams could be doubled. Brunner 2002 published a similar study for timber beams strengthened with prestressed FRP. The results are summarized in table 1. The amount of FRP could be significantly decreased to about 1.2% in order to attain a maximum strengthening ratio of about 2.3 - 2.8 for glulam, whereby the higher the ratio the poorer the timber grade. Thus this technique may encourage future engineers to turn poor grade timber into higher grades. Table 1: Strengthening ratio for timber grades as listed in EN 1194, strengthened with prestressed carbon fibres

The calculation results indicate that the loading capacity of timber beams could be greatly enhanced with prestressing techniques whilst greatly reducing the amount of reinforcement necessary as compared to the current practice of using slack reinforcement. It is worthwhile to undertake research in order to solve the delamination problem, which has been a major stumbling block.

BENDING TESTS WITH PRESTRESSED GLULAM BEAMS

Test set up There was a need to verify if it would be possible to attach prestressed artificial fibres safely to a glulam beam of practical size without delamination. Bending tests were performed to demonstrate the increased effectiveness of strengthening glulam beams with prestressed fibres. Table 2 lists the test specimens used. The following materials were used; their material properties are listed in the calculation example above:

• Glulam: GL 32 • Carbon FRP, Type S&P 150/2000

In the first series of tests, six glulam beams were strengthened with a carbon FRP laminate prestressed with the gradiented anchoring system to 60kN. The adhesive prescribed for the system is a specially designed epoxy. The experiment was a complete success; there were absolutely no signs of delamination or of a significant loss of prestressing force after about three months. The six prestressed beams were tested in bending in accordance with the European standard EN 408 (Fig. 8). In a second test series, six glulam beams of the same grade and size were strengthened with slack (i.e. unstressed) carbon laminates of the same type and size. They were also tested in bending. For control purposes, six naked glulam beams of the same size and grade were also tested in bending.

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Test series Prestressing

force Adhesive used

naked beam - - Carbon laminate (FRP)slack

0 kN Epoxy

Carbon laminate prestressed

60 kN Gradient-system

Epoxy

Dimensions Beam GL 32: height 200mm width 140mm length 4000mm

Carbon FRP-laminate type S&P 150/2000: 50 x 1.14 mm length 4000mm

Table 2 Test specimens Fig. 8 Four-point bending test according to European standard EN 408

Test results and discussions As mentioned above, three types of test specimens were tested in bending:

• Six naked glulam beams • Six glulam beams each strengthened with a slack (unstressed) carbon FRP laminate • Six glulam beams each strengthened with a prestressed carbon laminate

All the specimens exhibited quite linear load-deflection behaviour. As indicated in fig. 9, even the prestressed beams only exhibited slight ductile behaviour, thus confirming the calculation results above that the amount of reinforcement used was too little to induce significant plastification of the timber compressive face. In all the test beams, failure was induced by the brittle breaking of the tensile face of the timber. In the case of the strengthened beams, the laminate broke immediately afterwards (fig. 10). The tough compressive face displayed signs of “plastification” in the form of buckled fibre, but it remained intact.

Fig. 9: Typical load-deflection behaviour of the

prestressed glulam beams Fig. 10: Typical failure of the glulam tensile face

followed by breakage of the prestressed FRP

Table 3: Positive results of strengthening glulam beams with slack and prestressed FRP strips.

Bending stiffness EI [kN.m2] Ultimate bending moment M [kNm] Test Series

Average test values Calculated values Average test values Calculated values

Naked glulam 1’200 1’300 41 40

With FRP slack 1’430 1’420 50 45

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With FRP prestressed 1’460 1’420 55 52

The results of the bending tests are summarised in Table 3. They demonstrate that the strengthening with a prestressed laminate is more effective than the use of a slack laminate: the bending resistance of the naked glulam beam was increased by 34% when strengthened with a prestressed laminate as against 22% when the laminate is not prestressed. There is quite good agreement between the test results and the calculated values.

CONCLUSIONS

The project aim was to analyze a possible solution to the delamination problems which may occur when glulam beams are to be strengthened with prestressed artificial fibres of high strength. The solution proposal was concerned with the gradiented prestressing device developed in Switzerland by the EMPA to strengthen existing concrete beams. Although the prescribed epoxy-based adhesive is seldom used in the timber industry, the tests demonstrate that the system could also be successfully used to strengthen glulam beams with prestressed artificial fibres of high strength. No delamination was observed. The test results confirm theoretical work that the use of prestressed carbon laminates will lead to a greater improvement of the load-bearing capacity of timber beams than when the laminates are bonded in a slack state. However, the degree of strengthening was rather low because the prestressing force was too small. In a follow-up project, studies are being carried out to attach several layers of prestressed carbon laminates glued on top of each other.

REFERENCES

M. Brunner (2000): “On the plastic design of timber beams with a complex cross-section”, WCTE-2000, Canada.

M. Brunner: “Theoretical strength limits of timber beams fortified with prestressed artificial fibres”, WCTE-2002, Malaysia.

Gustafson J. (2000): “Tests and Test Results on Mechanical Properties of Adhesive Bond Lines”, Chapter 2 of Final Report, COST E13, Version 4.

Holzenkaempfer P. (1997): „Ingenieurmodelle des Verbunds geklebter Bewehrung für Betonbauteile“, Deutscher Ausschuss für Stahlbeton, Heft 473, Beuth Verlag.

Kuilen, J. van de (1991): „Theoretical and experimental research on glass fibre reinforced laminated timber beams“, International Timber Engineering Conference, London.

Lindyberg RF & Dagher HJ (2000): "Probabilistic nonlinear model for reinforced glulam beams", WCTE2000, Whistler, British Columbia, Canada.

Luggin W. F. (2000): „Die Applikation vorgespannter CFK-Lamellen auf Brettschichtholzträger“, Dissertation, Universität für Bodenkultur, Vienna, Austria.

Romani M, Blass HJ (2001): “Design model for FRP reinforced glulam beams”, International Council for Research on Innovation in Building and Construction, Working Commission W18 - Timber Structures, Meeting 34, Venice, Italy.

Scherrer J. (2000): "FRP Fibre Reinforcement Polymer", S & P Clever Reinforcement Company, Brunnen, Switzerland.

Stöcklin & Meier U. (2001): “Strengthening of concrete structures with prestressed and gradually anchored CFRP strips”, IABSE International Conference, Malta.

Tingly D. (1995): “FIRP Reinforcement Technology Information Packet”, Science and Technology Institute, Corvallis OR, USA.

Triantafillou T.C., Deskovic N. (1991): "Innovative prestressing with FRP-sheets: Mechanics of short-term behaviour", Journal of Engineering Mechanics, Vol. 117, Nr. 7, pp1652-1672.

Triantafillou T.C., Deskovic N. (1992): "Prestressed FRP-Sheets as external reinforcement of wood members", Journal of Structural Engineering, Vol. 118, Nr. 5, pp1270-1284.

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