Fir and chestnut timber beams reinforced with GFRP pultruded elements

Composites: Part B 38 (2007) 172–181 www.elsevier.com/locate/compositesb

Fir and chestnut timber beams reinforced with GFRP pultruded elements Marco Corradi *, Antonio Borri Civil and Environmental Engineering Department, School of Engineering, University of Perugia, Via Duranti 93, 06125 Perugia, Italy Received 27 February 2006; received in revised form 26 June 2006; accepted 16 July 2006 Available online 4 October 2006

Abstract High-performance fibers are being widely researched for repair and rehabilitation in civil engineering structures. The potential benefits, liabilities, and architectural considerations regarding the use of high-performance fibers for reinforcing wood beams are discussed. An experimental program based on a four- and three-point bending test configuration is proposed to characterize the stiffness and strength response of wood beams reinforced with pultruded GFRP (glass fibers reinforced polymers) elements. Improving wood mechanical characteristics through the use of fiber reinforced polymers often involves the use of adhesives, generally epoxy resins. For this reason mechanical, calorimetric and thermo-gravimetric analyses were performed on the resin utilized and bonding effectiveness was studied. Mechanical tests carried out on full-scale wood beams showed that the reinforcement with GFRP beams may produce strong increases in flexural stiffness and capacity. In addition, an analytical investigation based on a simple linear analysis was conducted to predict ultimate load. At the end of this paper results of the experimental program are presented and used for comparison with the analytical procedure.  2006 Elsevier Ltd. All rights reserved. Keywords: D. Mechanical testing; C. Numerical analysis; B. Elasticity

1. Introduction Wood is a construction material denoting high tensile strength. For this reason it was widely used in the past in many structural situations in which elements were subjected to flexion or tension loads. Wood is also a very efficient material. Its notable resistance under both compressive and tension loads must be considered as being nearly unique when compared with its limited weight density. In civil structures existing wood elements such as beams have usually been subjected either to replacement or reinforcement using classic techniques involving the use of common building materials, such as concrete or steel. It should also be noted that wood is characterized by long-term durability, as long as it undergoes correct maintenance. *

Corresponding author. Tel.: +39 075 585 3906; fax: +39 075 585 3897. E-mail address: [email protected] (M. Corradi).

1359-8368/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2006.07.003

With regard to new wood elements reinforcing wood elements through the use of other materials (steel, reinforced concrete, additional wood) is certainly not a new technique. As far as reinforcing with steel or aluminium we refer to works carried out by Sliker [1], Borgin et al. [2], and Hoyle [3] proposed inserting thin plates of steel or aluminium both vertically and horizontally between the lamellae of wood. Other experimental work and studies were carried out by Bulleit et al. [4] on glulam elements reinforced by the insertion of steel bars. Significant experimental work on reinforcing wood beams was carried out by Peterson [5] using epoxy adhesives to glue stressed steel plates to the area under tension in order to pre-stress the wood. Subsequent experiments, carried out to replace metal plates in wood beams, involved the use of high-resistant steel wire embedded in an epoxy matrix (Krueger [6], Krueger and Eddy [7], Krueger and Sandberg [8], Kobetz and Krueger [9]).

M. Corradi, A. Borri / Composites: Part B 38 (2007) 172–181

All of the above reinforcement techniques refer to interventions on new wood and in general the reinforcement was to be carried out previous to on-site mounting of the wood elements. Through the years proposals for various methods of reinforcing pre-existing wood beams employing traditional materials, both with and without metal alloys, have been made by Tampone [10]. It is clear that all the proposals involving the use of metal plates and bars can be adapted to the world of composites, substituting the above-mentioned with sheets and pultruded FRP elements, with some advantages (lighter weight, less invasiveness) as well as some disadvantages (costs, problems of joining and connections). Ad example, the use of aluminium profiles does not permit the achievement of these objectives due to lower in-site machinability, higher costs and weight density compared to composite materials. The use of FRP materials in reinforcing wood structures is not as of yet a widespread technique and, to some degree, adequate experimentation is needed prior to its use on a wide scale. This is particularly true for ‘antique’ wood elements, that is of existing structures, since there is already ample, well-consolidated experience in the field of new wood structures. The use of composite materials in wood reinforcement was first proposed in the 1980s by Meier [11], Triantafillou [12], Triantafillou and Plevris [13], Kropf and Meierhofer [14], Gentile et al. [15], Borri et al. [16], who applied composites based on glass (GFRP) or carbon fiber sheets (CFRP) epoxy-bonded externally on the tension zones and studied their effect on the mechanical characteristics of the reinforced wood elements. Corradi et al. [17] used composite materials (GFRP sheets) for in-plane reinforcement of existing wood beam floors. These studies have demonstrated that unidirectional composite sheets are ideal in tension reinforcement of wood beams, but are not able to provide a notable increase in flexural stiffness characteristics. When a reinforcement work is projected there are many critical issues including ensuring a durable bond between the FRP and wood given the shrinkage and swelling of wood due to moisture changes. Glass fibers are economically competitive and are characterized by high mechanical characteristics. However glass transition temperatures for epoxy resins are generally not elevated and equal to 30– 80 C. Recently Tascioglu et al. [18,19] found that E-glass fiber/phenolic resin matrix pultruded composite materials designed for wood reinforcement are susceptible to fungal penetration by common wood decay fungi highlighting the risk of strength decrease and moisture increase. In the interest of increasing stiffness characteristics of existing wood beams, in this paper we examine the effects of the use of pultruded elements placed in the compression zone on the failure mechanisms, stiffness characteristics and ductility of the reinforced elements as they are subjected to bending loads. An experimental program based on a four- and three-point bending test configuration is proposed to characterize the stiffness and strength response

173

of wood beams reinforced with pultruded GFRP elements. These elements do not function as a substitute for the wood beams but rather effect an increase in their capacity through the creation of a mixed wood–composite structure. Mechanical, calorimetric and thermo-gravimetric analyses were performed on the resin utilized and bonding effectiveness was studied. In addition, a numerical investigation was conducted to predict ultimate load and to establish a methodology for the optimum design of the hybrid FRPwood system. Finally the reinforcing scheme was applied to an historical building (Palazzo Collicola) located in Spoleto, Italy struck by the Umbrian earthquakes of 1997–98 in order to validate its practicality. 2. Experimental method The experimentation involved an initial mechanical characterization of both the wood material and the composite. In particular, a series of experiments were carried out on 13 wood beams for the purpose of analysing improvements in characteristics of stiffness and flexural strength resulting from reinforcement by means of pultruded composite elements. The wood beams, seven of which were of white fir (identified as A1, A2, A3, A4, A5, A6 and A7) and six of chestnut wood (C1, C2, C3, C4, C5 and C6), had square, unbevelled cross-sections, 180 · 180 mm and were each 3050 mm in length. Moreover, according to the UNI ISO 3130 [20], the percentage of humidity measured was 11.39%. The density of the fir beams was 452 kg/m3, while for those of chestnut it was 800 kg/m3. Reinforcement of the beams was carried out using nine glass fiber-based pultruded elements (GFRP) 2500 mm in length: five gray were supplied by Creative Pultrusion (identified as H1, H2, H3, H4 and H5) (type H), while four white were supplied by Atp Pultrusion (identified as I1, I2, I3 and I4) (type I). The I-type elements were obtained by means of a coupling of two white C-shaped elements, glued together by means of an epoxy system (Fig. 1). The density of the pultruded elements was 1810 kg/m3 for those produced by Creative Pultrusion (type H) and 1770 kg/m3 for those of Apt Pultrusion (type I). The pultruded elements were reduced due to execution of notches of the GFRP flange. In fact the common floor joists between

Fig. 1. Cross-section of pultruded elements: (a) type H, (b) type I (dimensions in mm).

174

M. Corradi, A. Borri / Composites: Part B 38 (2007) 172–181

wood beams and rafters, for the most part of existing ancient floors, necessitate the notching of the GFRP flange. 2.1. Mechanical characterization of the wood beams In order to find the Young’s modulus of wood EW and flexural stiffness kW a series of elasticity tests were executed on all 13 wood beams: four-point bending tests were carried out by effecting six load–unload cycles using loads in the range between 10 kN and 30 kN, obtaining for the elastic modulus the results of 11426 MPa for fir beams and 10251 MPa for chestnut beams. The span of each test was 3000 mm, while the load span was equal to 1000 mm. The beams were loaded with a 245 kN MTS actuator and a spreader beam. The spreader beam, centered about the mid-span, created a 1000 mm zone with a constant moment and zero shear. The test was then carried out with a constant load rate, so that initially the first load from 0 kN to 30 kN was reached in 45 s. The load–unload cycle between 10 kN and 30 kN was then repeated another five times, always with the same rate and therefore in cycles of 60 s. A second series of tests (failure tests) were carried out on four wood beams. The load was applied up to failure with a constant rate of 0.2 kN/s and the beam response to the applied load was measured by means of two inductive transducers (LVDT). The two LVDTs were positioned to effect the reading near the neutral axis, midway on the span on which the test was conducted (span equal to 3 m); the two LVDTs were placed on opposite sides of the wood beam to record eventual rotations of the beam during the test. The four-point bending configuration was used for these tests as well. Data acquisition was carried out with a 2 points/s sampling. It was then possible to evaluate the apparent stiffness (in particular the stiffness value was calculated for 1/3 of the maximum load and for the maximum load Pmax). P 1=3  P i d 1=3 P max  P i ku ¼ d max

k 1=3 ¼

ð1Þ ð2Þ

where Pi is the preload assumed equal to 3 kN, P1/3 is 1/3 Pmax and dmax and d1/3 are the midspan deflections at maximum load and at 1/3 Pmax. 2.2. Mechanical characterization of the pultruded elements The mechanical characteristics of the pultruded elements were measured by recoding the load–deflection

behavior and by compressing prismatic samples in an Instron testing machine. The references for the mechanical tests on the pultruded elements were EN 13706-2 (parts D and G) [21] and ASTM D 695 M [22] standards. In particular the following were carried out: 1. Determination of compression strength: 12 compression tests on 12 prismatic samples with constant rectangular cross-sections, a height of 40 mm, a length of 28 mm and a thickness equal to that of the flange of the pultruded element (12 mm in the case of the I type and 7 mm in the case of the H type) from which the sample was extracted; 2. Determination of the Young’s modulus, EP, and shear modulus, GP: three-point bending tests on three spans (2.5 m, 2.0 m and 1.5 m) were executed. In accordance with the above-mentioned standards, these tests were carried out using six load–unload cycles under controlled mid-span vertical displacements in 55 s, with displacements, respectively, between 1 mm and 12 mm, 1 mm and 10 mm, 1 mm and 7.5 mm. 3. Determination of the decrement in flexural stiffness kP following execution of notches in pultruded elements: four-point bending tests on a span of 2.5 m under six load and unload cycles were carried out on pultruded elements before and after execution of 80 · 30 mm notches at 50 cm intervals on the pultruded element flange. Table 1 shows the results for compression strength and the Young and shear moduli. Both I and H type elements are characterized by similar Young modulus values (respectively, 25407 MPa and 27031 MPa). Compression strength is higher for the H type and equal to 373.45 MPa. Before pultruded elements were applied to the wood beams, notches were made intended, in a simulation of a real case scenario, for the insertion of the wood rafters of a floor. The notches were executed with particular care to avoid any significant damage to the pultruded element (Figs. 2 and 3). The four-point bending tests highlighted a loss of flexural stiffness in the notched pultruded elements (20.2% for type H pultruded elements and 11.4% for type I). A synthesis of the results is presented in Table 2. It can be noted that type I elements result as much stiffer compared to H type elements. Reinforcement of the wood beam, by applying a notched pultruded element to the upper part of the beam (compressed area), was carried out using an epoxy glueing system saturant (produced by Mac Spa, Treviso, Italy) and 4.8-type steel screws with a diameter of 10 mm and a length of 80 mm.

Table 1 Mechanical characteristics of pultruded elements Composite element No.

Compression strength (MPa)

Young modulus EP (MPa)

Shear modulus GP (MPa)

Density (kg/m3)

H type I type

373.45 206.22

27031 25407

1640 1340

1810 1770

M. Corradi, A. Borri / Composites: Part B 38 (2007) 172–181

175

2.3. Characterization of the epoxy system

Fig. 2. Pultruded elements after execution of notches.

An epoxy system with room temperature polymerization was used to bond the pultruded elements to the wood parts. A series of five tensile strength tests and five compression tests were carried out in accordance, respectively, with ASTM D638 [23] and D695 standards to determine the mechanical characteristics of the epoxy resin. The tests on the resin samples highlighted a compression strength of 56.54 MPa, a tensile strength of 23.43 MPa and an elastic modulus of 2112 MPa. Furthermore, the epoxy system was examined using calorimetric, thermo-gravimetric and microscopic analyses. For the former, dynamic analyses were carried out between 30 C and +200 C at a heating speed of 10 C/min to determine both the polymerization heat, DHD, as well as the glass transition temperature, Tg. The calorimeter utilized was a Setaraam DSC92, capable of working in isothermic and dynamic conditions between 100 and +600 C. Thermo-graphic analyses were carried out using an Seiko Exstar 6000 TG-DTA, which permits a heating speed from 0.5 to 200 C/min and can function at a maximum temperature of 1200 C. The samplings were heated from 25 to 6000 C at a heating speed of 10 C/min and the weight-loss curve and its derivative were obtained. The results of the calorimetric tests showed polymerization heats DHD and glass transition temperature Tg equal to 350 J/g and 30 C, respectively. Since the Tg is, respectively, close to room temperature and 60 C, it can be hypothesized that the system is stable until 50 C. This hypothesis is confirmed by the results of the thermo-graphic analyses which show that thermic degradation of the material does not initiate until 50 C. At higher temperatures T P Tg the epoxy resin should become rubbery, with negative effects on the joint performances. Microscopic analysis (Fig. 4) showed that the penetration of the epoxy resin in the chestnut wood was negligible. This explains the necessity of using a mechanical anchoring

Fig. 3. Wood beams reinforced with GFRP pultruded elements. Notches permit the application of reinforcement without interfering with wood rafters.

Table 2 Flexural stiffness decrement after execution of notches Composite element No.

Flexural stiffness kP (N mm1)

H type I type

381 1467

a b

Before notch execution. After notch execution.

a

Flexural stiffnessb kP (N mm1)

Decrement (%)

304 1300

20.2 11.4 Fig. 4. Microscopic analysis: penetration of the epoxy resin into wood.

176

M. Corradi, A. Borri / Composites: Part B 38 (2007) 172–181

system made of screws to connect wood and composite elements.

32

2.4. Wood beams strengthened with pultruded elements

24 Load [kN]

A series of tests were carried out, subsequent to reinforcement with a pultruded GFRP element applied in the compressed zone, to determine the increase in stiffness and flexural strength deriving from application of the reinforcement. Analogously to the bending tests on the un-reinforced wood beams, the elasticity tests were executed using a four-point bending configuration, carrying out six load– unload cycles between 10 kN and 30 kN. Each pultruded element, 2500 m in length, was centered and applied to a wood beam 3050 mm long (Fig. 5). The procedure followed in executing the joining was articulated in various phases: (1) careful planing and cleaning of the surfaces to be joined (wood and GFRP elements); (2) positioning and centering the pultruded element in GFRP above the wood beam; (3) application of the epoxy resin for the bonding; (4) effecting the joining by applying screws with washers. A direct comparison of readings taken on the same wood beam prior to and subsequent to reinforcement highlights significant increases in flexural stiffness kR following application of the pultruded element (Table 3). It can be observed that the maximum effect, in terms of a percentage increase in flexural stiffness, was obtained for the beams reinforced with I type pultruded elements (+188%) with negligible difference between fir and chestnut beams. Reinforcement with H type elements caused an average stiffness increments of 83% for fir wood beams and of 58% for chestnut

C3 + I1

28

C3

20 16 12 8 4 0 0

2

4

6 8 10 12 Deflexion (midspan) [mm]

14

16

Fig. 6. Elastic tests: behavior of wood beam C3 before and after reinforcement.

ones. This is essentially due to the fact that I-type pultruded element has a greater cross-section area than the H-type pultruded elements used. Fig. 6 shows the load– deflection behavior of wood beam C3 prior to and subsequent to reinforcement with I1 pultruded element. The tests up to failure on the reinforced beams were carried out according to the same configuration as those on the un-reinforced wood beams. From the graph in Fig. 7 it can be seen how the failure test on all wood beams highlighted a notable increase, when compared to the un-reinforced beams, both in terms of the ultimate load Pmax as well as of stiffness k1/3 and ku and ductility. For all beams a flexural tensile rupture occurred in the load span near a knot, grain deviation or other defect. Fig. 8 shows a typical

1000

1000

Pultruded element Metal screws/250 L=80

2500 Wood beam 180x180x3050

280

180

(H-type reinforcement)

317 (I-type reinforcement)

No.2 LVDT 3000

Fig. 5. Test configuration of reinforced wood beams (not to scale, dimensions in mm).

Table 3 Results of elastic tests Flexural stiffness kW Un-reinforced wood beams (N mm1)

Flexural stiffness kP Pultruded elements with notches (N mm1)

Flexural stiffness kR Reinforced wood beams (N mm1)

kR/kW

A2 A3 A4 A6 A7 C2 C3 C5 C6

H1 H3 H2 I3 I4 I2 I1 H4 H5

A2 + H1 A3 + H3 A4 + H2 A6 + I3 A7 + I4 C2 + I2 C3 + I1 C5 + H4 C6 + H5

2.16 1.61 1.71 2.88 2.85 2.92 2.87 1.58 1.59

1127 2439 2253 2356 2442 2153 2252 2345 2478

305 291 303 1305 1288 1310 1293 311 295

18

2435 3935 3852 6790 6956 6294 6454 3701 3940

M. Corradi, A. Borri / Composites: Part B 38 (2007) 172–181

240 C3+I1 A7+I4 A6+I3 C2+I2

220 200 Load [kN]

180 160

A4+H2

140 120

C5+H4

100

A3+H3 C6+H5

80

A1

60

C4

40

– – 4.74 3.94 2.10 1.23 – – 1.81 1.31 1.30 3.72 3.78

Increment ku Reinforced/un-reinforced

260

A5

C1

20 0 25 30 35 40 45 50 55 60 Deflexion (midspan) [mm]

65 70

75

k1/3 (N mm1)

Increment k1/3 Reinforced/un-reinforced

ku (N mm1)

Fig. 7. Tests carried out on un-reinforced wood beams and reinforced.

Pmax (kN)

65.1 67.2 225.2 183.5 137.2 101.8 85.8 78.0 173.2 80.8 148.3 193.0 205.8

No.

C1 C4 C3 + I1 C2 + I2 C5 + H4 C6 + H5 A1 A5 A3 + H3 A2 + H1 A4 + H2 A6 + I3 A7 + I4

Table 4 Results of flexion tests

tensile rupture evidenced for fir beam A3 reinforced with H3 pultruded element. Failure caused a suddenly drop of beam capacity and the subsequent rupture of pultruded element between the flange and the web. Measurements in these tests indicated the greatest increase in strength and stiffness for the chestnut beams reinforced with I-type pultruded elements (Pmax increment 208%, k1/3 increment 270%, ku increment 334%) (Table 4). The ultimate capacity of chestnut beams reached 204 kN. Fir beams reinforced with I-type elements, except for A2 beam, also demonstrated a very interesting reinforcement action (Pmax increment 143%, k1/3 increment 242%, ku increment 275%). Moreover, the strengthened beams with I-type pultruded elements demonstrated a behavior almost completely linearly elastic for all four beams reinforced as a consequence of the impossibility for the wood to plasticize in the compressed area due to the presence of the pultruded element, which has a linearly elastic behavior. The firwood beam, A2, is characterized by low mechanical properties, highlighted both by geometric examination (a great number of knots and splits as well as the presence of grain deviation) and by the characterization test. For

Increment Pmax Reinforced/un-reinforced

Fig. 8. Flexural tensile rupture of A3 + H3 beam.

1491 1470 7011 5837 3102 1816 1971 1547 3187 2307 2284 6535 6647

15 20

– – 4.24 3.16 2.04 1.78 – – 1.87 1.21 1.99 3.39 3.45

10

1753 1829 7591 5665 3659 3190 1975 1988 3699 2393 3944 6721 6827

5

– – 3.40 2.77 2.07 1.54 – – 2.11 0.99 1.81 2.36 2.51

0

177

178

M. Corradi, A. Borri / Composites: Part B 38 (2007) 172–181

3. Analysis

100 90 80

A2+H1

Load P [kN]

70 60 50 40 30 A2 20 10 0 0

5

10

15 20 25 30 Deflexion (midspan) [mm]

35

40

45

On the basis of the experimental results for the compression strength of the pultruded elements in GFRP and the modality of collapse observed, it is possible to estimate the failure load as well as the flexural stiffness for each of the reinforced wood beams, hypothesizing a conservation of plane sections. The behavior of the pultruded element in GFRP is comparable to linear-elastic, while the behavior of wood is notorious for being practically linear-elastic under tension, while presenting linear-plastic behavior under compression (Fig. 10). In particular, the Bazan–Buchanan law for the wood [24,25] can be expressed by:

Fig. 9. Wood beam A2 before and after reinforcement.

rWc ¼ EW  eWc if eWc < eWc0 rWc ¼ rWc0  m  ðeWc  eWc0 Þ if eWc > eWc0 this reason it is not possible to effect a comparison with the failure loads of the other wood beams. The measured failure load of 80.8 kN is much lower than those of the other reinforced beams. However, a direct comparison between the behavior of the beam A2 before and after reinforcement is shown in Fig. 9, where it is possible to note the considerable increase in stiffness following application of the pultruded element. The timber beams, A4 and C6, were reinforced by applying an H-type pultruded element. The two elements, in wood and composite, were joined using only screws without applying the epoxy resin on the contact surface. The result of the flexure test highlights that the absence of the epoxy resin did not compromise the effectiveness of the reinforcement. The beam A4 + H2 failed at a load of 148.3 kN compared to the average capacity load of 81.9 kN for the un-reinforced beam A1 and A5. The effectiveness of the reinforcement is mainly determined by the presence of the metal screws which operated up to the failure of the reinforced beams. However, the differing flexural stiffnesses of the wood beam and the pultruded element, respectively, determined phenomena of withdrawal of the screws from the wood. The withdrawal of screws was particularly evident for fir beams reinforced H-type elements. This means that failure initiated at the screws in the GFRP-timber beams. In order to avoid these phenomena metal screws should be inserted into the wood at a slight angle (15–20%) with respect to the perpendicular and toward the supports. In fact, all five beams A2 + H1, A3 + H3, A4 + H2, C5 + H4, C6 + H5 (expect for C5 + A4 beam) clearly exhibit non-linear response as a consequence of partial withdrawal of the screws though local tearing and fracture of the wood. As a consequence the adopted screw spacing equal to 25 cm is to long for fir beams characterized by lower mechanical properties. Another possible solution is to increase screw size. It is also probable that a limited compression yielding of the wood occurred for timber beams reinforced with Htype elements.

ð3Þ

rWt ¼ EW  eWt Where m represents the slope of the plastic branch of the Bazan–Buchanan law: rWc0  rWcu m¼ ð4Þ eWcu  eWc0 and rWc and rWt are, respectively, the wood compression and tensile stress, EW is the wood Young modulus, eWc and eWt are the wood strains in compression and in tension. eWc0 is the strain value at yield stress rWc0. In the case of I-type reinforcement the wood material may be assumed to be completely in tension or only partially lightly compressed in a limited zone. As a consequence it is possible to assume that: ð5Þ

eWc max < eWc0 σW

EW εWcu

εWc0

1 εW

1 EW

σWcu σWc0

Fig. 10. Bazan–Buchanan law for wood.

M. Corradi, A. Borri / Composites: Part B 38 (2007) 172–181

i.e., that the wood does not yield under compression due to the presence of the pultruded element in the compressed zone. As far as the FRP material is concerned, the generic law will be given by: rP ¼ EP  eP

ð6Þ

a

y¼

h1 2

ð7Þ

AW þ EEWP AP

where h1 is the height of the pultruded element and h2 is the height of the wood beam. Based on this simple linear-elastic model, we may use experimental results of capacity P in order to estimate both the compression and tensile stresses for wood and the compression stress for the GFRP pultruded element: M  ðh1 þ h2  yÞ Jx M rWc ¼  ðy  h1 Þ Jx M EP rP ¼  y  Jx EW rWt ¼

ð8Þ

εPmax

d

h1

y h = h1+h2

c

εWc h2

εP εWcmax

FI FII

σWc0

σWt FIII σWtu STRESS

εWtu STRAIN

b

σPmax FIV

εWt

describing a linear-elastic behavior, having assumed the same elastic modulus EP for the wings and web of the pultruded element. Under these hypotheses, the position of the neutral axis is easily found: AW ðh1 þ h22 Þ þ AP EEWP

179

Fig. 11. Cross-section strain and stress analysis.

The numerical non-linear model takes into account the presence of upper reinforcement on the section under the hypothesis of preservation of plain sections and perfect adhesion between wood and pultruded element. Assuming the failure is not reached in the composite materials, it is possible to reduce the study of the problem to the following failure case: attainment of limit strain eWtu in compression region without exceeding limit strain in the compression region for wood or pultruded element. From the condition of equilibrium it follows that (Fig. 11): F I þ F II þ F IV ¼ F III

ð9Þ

where the forces in the compression region are given by:

Table 5 shows that the experimental and numerical values for chestnut and fir beams reinforced with I-type elements are similar and the error is always in the range 10–20%. The data obtained makes it clear that the linear-elastic model utilized is capable of providing acceptable results for the reinforced beams C3 + I1 and C2 + I2 for which the maximum calculated compression stresses of the wood reached (5.96 and 4.85 MPa, respectively) turn out to be sufficiently low so as to exclude the wood yielding. An acceptable error was also calculated for fir beams reinforced with I-type elements. With regard to H-type reinforcement, a non-linear behavior was observed during the experimental work. This non-linearity may be due to compression yielding of the wood and/or imperfect composite action between the wood and FRP as discussed above, which the approximated linear-elastic model cannot account for.

Table 5 Experimental vs. numerical: capacity values No.

Reinforcement

Experimental Pmax (kN)

Numerical Pmax (kN)

C (chestnut wood) A (fir wood) C (chestnut wood) A (fir wood)

I-type

204.3

159.4

I-type H-type

199.4 119.5

190.6 96.1

H-type

160.7

111.5

FI ¼

h   i9 8 1 <2rWc0  m eWtu yh  eWco = hy :

2

;

 b  ½y  h1  aðh  yÞ;

ð10Þ F II F IV

rWc0 ¼  b  aðh  yÞ; 2  3 2 1 eP max EP  eWtu EP yh hy 5ðh1  2dÞc ¼ eP max EP ad þ 4 2   y  h1 þ eWtu EP ad hy

and the force FIII in the tension zone is given by: eWtu  EW ðh  yÞ  b; F III ¼ 2 where: rWc0 a¼ EW eWtu eWtu eP max ¼ y ðh  yÞ

ð11Þ

ð12Þ

ð13Þ

ð14Þ ð15Þ

From Eq. (9) and from the stress–strain laws of materials it is possible to find the position of the neutral axis and the value of the ultimate bending moment of the section. Once the neutral axis position is found, it is possible to proceed to the calculation of the ultimate bending moment of the section, which can be expressed as follows: M u ¼ F III  s

ð16Þ

180

M. Corradi, A. Borri / Composites: Part B 38 (2007) 172–181

where s is the lever arm of internal forces FI, FII, FIII and FIV. For application of Eq. (16), it is necessary to know the limit and yield strain for wood in compression and the limit strain in tension. Considering that a major problem encountered when developing flexural strength models for wood is that tensile stress at failure is greater in bending than in axial tension, the tensile stress at failure rWtu (parallel to grain) was calculated from tests carried out on unreinforced beams (30.7 MPa for fir wood and 24.8 MPa for chestnut wood). The remaining data were assumed [26,27]: rWc0 ¼ 0:7  rWtu eWcu ¼ 2  eWc0

ð17Þ

rWcu ¼ 0:8  rWc0

4. An example of application The design criteria described above were applied to the timber floor of the XVII century palace (Palazzo Collicola) located in Spoleto, Italy. The reinforcement work of the floors was entirely focused on strengthening and stiffening of existing ancient beams, which poses construction challenges, including the field application of adhesive. To achieve the aim the supplementation of the beams shown in Fig. 12, where the common floor joists necessitate the notching of the GFRP flange, an attempt was made to avoid any work which would be particularly traumatic for the important historical structure. As a consequence an important issue of a reinforcement work was lightness and minimum increase in dead loads. The reinforcement with pultruded H-type elements is characterized by this property and by an high machinability of pultruded elements to be carry out in-site. Due to particular specifications of the local authority it is was not possible to operate at beam intrados (zone under tension) for the presence of decorations. The reinforcement of the compressed zone has the positive characteristic to be un-visible and

the use of pultruded elements induced very interesting increase in flexural stiffness and strength. Reinforcing operations, under the direction of Dr. Andrea Giannantoni, described in detail in [28] with full description of the case of study, turned out to be extremely fast and effective. 5. Conclusions The application of new, advanced materials for the reinforcement and seismic upgrading of civil engineering constructions can represent a valid alternative to traditional techniques. With regard to existing constructions, the use of composite materials presents positive characteristics in terms of reversibility, effectiveness, non-invasiveness, chemical stability and compatibility in a large number of reinforcement typologies. The flexural reinforcement of wood elements using pultruded elements in composite material is capable of determining significant increases in terms of strength, stiffness, and ductility when compared to the un-reinforced wood beams tested. In particular, the increase in flexural stiffness obtained via this method is capable of overcoming a lack typically present in existing wood beam floors. Moreover, this intervention is easily carried out on site, without having to dismantle the overlying part of the structure, and within a short time, depending exclusively on the curing times of the epoxy resins used for glueing. The series of tests performed on composite-reinforced and un-reinforced wood included calorimetric, thermogravimetric and microscopic analyses of the epoxy resin used to bond wood and composite elements. The results demonstrated that the bonding is effective at least up to 50 C, but the penetration of the epoxy resin into the wood is negligible. The joining of the elements was also effected through the use of screws in order to avoid problems connected to loss of adherence between the epoxy resin and wood surface. Moreover, even the use of these screws alone seemed capable of guaranteeing an effective joining of the wood beam and the composite element, but more tests will be necessary to validate this. The adopted screw spacing of 25 cm turned out to be adequate only for chestnut beams. The presence of soft wood like fir beams needs shorter screw spacings or higher screw sizes. However, it is advisable to place the screws at an inclination of 10–20 with respect to the perpendicular, and not directly perpendicularly, in order to avoid phenomena of screw withdrawal. Finally a linear (based on the hypothesis that compression stresses for wood are sufficiently low so as to exclude the wood yielding) and non-linear analyses furnished acceptable results in terms of estimated capacity of reinforced beams. Acknowledgements

Fig. 12. The in-site application of reinforcement: Palazzo Collicola.

Special thanks to engineers Dr. Andrea Giannantoni and Dr. Antonio Triboli for the cooperation and assistance given. ECT s.r.l. located in Jesi (Italy), ATP Pultrusion,

M. Corradi, A. Borri / Composites: Part B 38 (2007) 172–181

Creative Pultrusion, Ediltecnica s.r.l. are also thanked for their technical assistance during the strengthening operations. References [1] Sliker A. Reinforced wood laminated beams. Forest Prod J 1962;12(1):91–6. [2] Borgin KB, Loedolff GF, Saunders GR. Laminated wood beams reinforced with steel strips. J Struct Engng ASCE 1968;94(7): 1681–705. [3] Hoyle RJ. Steel-reinforced wood beam design. Forest Prod J 1975;25(4):17–23. [4] Bulleit WM, Sandberg LB, Woods GJ. Steel-reinforced glued laminated timber. J Struct Engng, ASCE 1989;115(2):433–44. [5] Peterson J. Wood beams prestressed with bonded tension elements. J Struct Engng, ASCE 1965;91(1):103–19. [6] Krueger GP. Ultimate strength design of reinforced timber: state of the art. Wood Sci 1973;6(2):175–86. [7] Krueger GP, Eddy FM. Ultimate strength design of reinforced timber: moment-rotation characteristics. Wood Sci 1974;6(4):330–44. [8] Krueger GP, Sandberg LB. Ultimate strength design of reinforced timber: evaluation and design parameters. Wood Sci 1974;6(4): 316–30. [9] Kobetz RW, Krueger GP. Ultimate strength design of reinforced timber: biaxial stress failure criteria. Wood Sci 1976;8(4):252–62. [10] Tampone G. The slow progress of the conservation of wooden structures and wooden architecture. In: Proceedings of the international conference in conservation of historic wooden structures, February 22–27, 2005. [11] Meier U. Bridge repair with high performance composite materials. Mater Tech 1987;4:125–8. [12] Triantafillou TC. Shear reinforcement of wood using FRP materials. J Mater Civil Engng, ASCE 1997;9(2):65–9. [13] Triantafillou TC, Plevris N. Post-strengthening of r/c beams with epoxy-bonded fiber-composite materials. Proc ASCE specialty conf on Advanced Composites for Civil Engng Struct. ASCE; 1991, p. 245–6. [14] Kropf FW, Meierhofer U. Strengthening, retrofitting and upgrading of timber structures with high-strength fibres. SEI 3/2000, 2000.

181

[15] Gentile C, Svecova D, Saltzberg W, Rizkalla SH. Flexural strenghtening of timber beams using GFRP. In: 3rd International conference in advanced composites materials in bridge and structures, proceedings, Ottawa-Canada, August 15–18, 2000. [16] Borri A, Corradi M, Grazini A. A method for flexural reinforcement of old wood beams with CFRP materials. J Compos, Part B 2005;36/ 2:143–53. [17] Corradi M, Speranzini E, Borri A, Vignoli A. In-plane shear reinforcement of wood beam floors with FRP. J Compos, Part B 2006;37/4–5:310–9. [18] Tascioglu C, Goodell B, Lopez-Anido R. Bond durability characterization of preservative treated wood and E-glass/phenolic composite interfaces. Sci Technol 2003;63:979–91. [19] Tascioglu C, Goodell B, Lopez-Anido R, Peterson M, Halteman W, Jellison J. Monitoring fungal degradation of E-glass/phenolic fiber reinforced polymer (FRP) composites used in wood reinforcement. Int Biodeter Biodegrad 2003;51:157–65. [20] UNI ISO 3130, Wood. Humidity determination. [21] EN 13706-2: Reinforced plastic composites – specifications for pultruded profiles – Part. 2: methods of test and general requirements, CEN (European Committee for Standardization), October 2002, 2002. [22] ASTM D 695M-91. Standard test method for properties of rigid plastics. [23] ASTM D 638-95 Standard test method for tensile properties of plastics. [24] Buchanan AH. Combined bending and axial loading in lumber. J Struct Engng, ASCE 1986;112(12):2592–609. [25] Buchanan AH. Bending strength of lumber. J Struct Engng, ASCE 1990;116(5):1213–29. [26] Boding J, Jayne BA. Mechanics of wood and wood composites. New York: Van Nostrand Reinhold; 1982. [27] Forest Products Laboratory. Wood handbook – Wood as an engineering material. Gen. Tech. Rep. FPL-GTR-113. Madison, WI: US Department of Agriculture, Forest Service, 1999. [28] Borri A, Giannantoni A. Esempi di utilizzo dei materiali compositi per il miglioramento degli edifici in muratura, Examples of possible uses of composite materials for seismic upgrading of masonry structures. In: XI Conference in ANIDIS, January 25th–29th, CD rom, Genova, Italy, 2004.