CICE 2010 - The 5th International Conference on FRP Composites in Civil EngineeringSeptember 27-29, 2010 Beijing, China

Flexural behavior of FRP reinforced glubam beams

Quan Zhou (quanzhou516@163.com ) Institute of Modern Bamboo, Timber and Composite Structures (IBTCS), Hunan University, Changsha, 410082, Hunan, China.

Yan Xiao(yanxiao@usc.edu) Department of Civil Engineering, University of Southern California, Los Angeles, CA, 90089, USA. Institute of Modern Bamboo, Timber and Composite Structures (IBTCS), Hunan University, Changsha, 410082, Hunan, China. ABSTRACT: A new type of rectangular glued laminated bamboo (glubam) beam had adopted in the world first truck-safe modern bamboo bridge in Leiyang, Hunan, by the authors. The glubam beams can further be enhanced by FRP. This paper analyzes the effect of some parameters, such as FRP thickness, span-depth ratio, strengthen measures, types of node on bending properties. The basic mechanical model is established to pre-dict the failure. Through the experimental work, the flexural stiffness and ultimate load of FRP reinforced beams are compared with those of unreinforced beams. The result show that the analytical model can forecast the flexural behavior of FRP reinforced glubam beams well.

1 INSTRUCTIONS

From the beginning of last century, due to the en-ergy-saving, environmental protection, excellent structural performance, etc., wood has been widely used in building structures. However, because of the shortage of domestic timber resources, wood struc-ture has been developed slowly in China. Despite the high tariffs for imports of timber, a large number of Europe, the United States, Japan's wooden houses have been brought in as a result. But, most of timber is used only for the expensive villas.

The research on wood and timber structures also falls behind, while the national wood-structure stan-dard often referring to foreign standards. For the limitations of traditional wooden structures, some experts and scholars began to research the bamboo as a substitute of wood, which would alleviate the shortage of fine structural timber. China is in abounding with bamboo, but most of it remains in the original bamboo stage. In recent years, research-ers continue to explore new types of manufactured bamboo production all over the world. S.Rittironk & M. Elnieiri investigate Laminated Bamboo Lumber as an alternative structural material. It takes a differ-ent approach from conventional raw bamboo struc-ture, which is an alternative manufactured bamboo. They proposed that LBLs structural properties and superior quality compared to wood lumber in terms of higher strength, higher density, lower shrinkage, and dimension stability, have been proven through many studies. J. Correal & L. Lopez introduced an-other type of structural bamboo material, glued laminated Guadua (GLG), which has comparable

mechanical properties to structural Colombian wood. In some case, the mechanical properties of the GLG are better than those of the best structural wood in Colombia.

The Institute of Modern Bamboo, Timber and Composite Structure (IBTCS) conducted a compre-hensive research program, with the goal to develop modern bamboo structures for building and bridge. A modern bamboo pedestrian bridge had been com-pleted, using glued bamboo (glubam) as the main material. The bridge had much lighter superstructure and was easier to construct compared with conven-tional steel or concrete in terms of same load condi-tion. After the successful completion of the first modern bamboo pedestrian bridge in 2006 [Zhou et al. 2007], the authors were given the opportunity to design and construct a truck loaded 10 m long bridge in the Village of Daozi, Leiyang, Hunan Province. The bridge was a single lane bridge to cross the Xunjiang river and connect the rural road-way network in the local region, as a part of the ag-riculture infrastructure development by the local government. Different from the former bridge, the girders of this bridge were glubam beams with FRP reinforcement.

In this paper, flexural behaviors of glubam beams were studied through tests, compared with those of FRP reinforced beams, which could be taken as a reference of production, design and application of this kind of structure.

2 EXPERIMENT DESIGN

The glubam was made from Phyllostachys edulis, which was sourced from Yiyang city of Hunan prov-ince. The resin used in glubam was domestic glue named ESA-T, particular for FRP bonding. Some small clear specimens of glubam had been tested to obtain the main physical and mechanical properties of glubam, in which the values was the mean tested value, while the other materials referring to litera-tures (see Table 1). In addition, Elastic modulus of CFRP was 220Gpa and tensile strength was 2.6Gpa as obtained by test. T able 1. Basic material properties of laminated bamboo

Materials In-plane compres-

sive strength (MPa)

In-plane tensile

strength(MPa)

Bend-ing

strength

(MPa)

Elas-tic

(GPa)

Den-sity (kg/m3)

Plybamboo 54 80 75 9.4 880 Typical Bamboo

Clum

55 124 - 17 650

880 Douglas Fir 48 - 83 12 497 West White

Pine 35 - 64 10 398

Table 2. Details of large tested beams

* the cross-section rate of FRP to bamboo, ** the length between bearing plate and nearest load point.

There were two groups of glubam beam, and first group had 11 non-reinforced specimens and 2 FRP reinforced specimens. All of those were produced by researched in laboratory under 25 centigrade, while heaters were employed to stabilify the temperature. FRP reinforced beams were tested about one month after they cohered. The specimens design was de-veloped referring to the code ASTM D3737-03, ASTM D7199-07, ASTM D7341-08 and GB/T 50329-2002.

There were some assumptions in experiments: 1) the cross-section of girder remain plane after bend-ing, 2) the adhesive layer between FRP and bamboo was linear-elastic body, without regard to its thick-

ness, and bond connects bamboo sheets well, 3) FRP was considered a linear-elastic material, 4) the theo-retical value mentioned in this paper was base on the hypothesis of ideal joint in bamboo beams

This paper analyzes the effects of several parame-ters, such as FRP reinforcement, span to depth ratio, strengthen measures, types of node and so on, on bending properties. Table 2 shows the details of the former group, all of which were large scale beams ranged from B1 to B13, while another group stating the small scale beams ranged from S1 to S15. And then, qualitative analysis of large specimens is car-ried out.

3 RESULT AND ANALYSIS

3.1 Experimental results The length of pure bending zone is one meter in large tested beams. All of non-reinforced specimens broke in the tensile zone when its tensile strain reached maximum. At the beginning of test, bamboo was in elastic stage. With the load increasing, it showed some plastic behaviors and the flexural stiffness decreased. When bamboo beams cracked, noises came from the bottom and deflection was ob-vious. The beams collapsed quickly at the moment of failure. On the other hand, FRP reinforcement in-creased the ultimate strength of the glubam beams. Moreover, cracking noises came later than former and the failure didnt happen only in the compres-sive zone. Some crack came up in the top of beams, and the deflection of mid-span was less than non-reinforced beams.

The ultimate design condition was checked based on the following simple procedure:

S Mu yf (1) a / M P umax (2)

where Pmax = the ultimate design strength; Mu = mo-ment of flexure subject to Pmax; a = length between bending plate and nearest load point; fy = bending strength of plybamboo.

3.2 Analysis of experimental results 3.2.1 Results of 3.5m span beam tests Ultimate strength of 3.5m span beam calculated from formula (1) and (2) was 360KN, and that of B1 was 340kN which was close to the theoretical value. Finger-zone prematurely approached to failure in tension, resulting in the compression zone of bam-boo quickly reached the limits of strain. Figure 1. Experiment equipments

Cross-

section di-mension

/mmmm

Span / mm

Reinforce-ment rate (*) ) / %

a** / mm

B1 100600 3500 0.037 1250 B2 100600 3500 - 1250 B3 100600 3500 - 1250 B4 100600 4500 - 1750 B5 100600 4500 - 1750 B6 100600 4500 - 1750 B7 100600 4500 - 1750 B8 100600 4500 - 1750 B9 100600 4500 0.037 1750

B10 100600 4000 - 1500 B11 100600 4000 - 1500 B12 100600 4000 - 1500 B13 100600 4000 - 1500

T able3. Test results of 3.5m-span beams.

Joint- length (mm)

Bolt Reinforcement FRP

Initial stiffness (kN/mm)

Ulti-mate load

B1 20 7.556 390 B2 30 - - 7.096 260 B3 20 - 6.189 237.86 Joint effect factor J was took into calculation, and revision formula was showed below:

J P P maxJ (3) where J = 0.75 when there is two finger-area overlap in bending zone, else J = 1 when other conditions.

The theoretical result of B2 multiplied 0.75 is 270, close to test result. Moreover, bearing capacity of B3 was close to B2, but 20mm finger-joint lead to de-flection increased quickly after cracking. However, it presented good ductility when the specimen stiff-ness decreased gradually.

In addition, initial stiffness of B1 was 7.556KN/mm, higher than other two beams, due to the strengthening of FRP. As the cracks increasing, FRP reached the strain limit. When load reached 340KN, a sudden failure occurred. The effect of FRP reinforcement was not obvious for capacity, but it could significantly improve the specimen stiffness. B2, with 7.096KN/mm initial stiffness, implied that 30mm length finger-joint performance was better than 20mm, also the cracks developed slower than B1, obviously. However, with no bolts to strengthen nodes, resistant ability to shear between sheets ex-tremely decreased, and a sharp increase turned up in the deflection. As a result, after the specimen had been loaded exceed 200kN, stiffness was declining as the finger-joint cracks expanding. Finally when the load reached 260KN, the cracks at the bottom suddenly expanded near the vicinity of the neutral axis. After that the specimen broke and the experi-ment ended. The initial stiffness of B3 was 6.189, lowest in this group.

3.2.2 Results of 4.5m span beam tests Calculated load carrying capacity of 4.5 m clear span beam was 257.1kN. Because finger-zone ex-isted in B4 and B6, their capacity reduced to 192.9kN according to equation (1)(3). Initial stiff-ness, damage loads and mid-span deflection of each specimen's are listed in table 2. Specimens B5 and B9 were the counterpart testing cases with the main difference being the existence of FRP reinforcement in B9. As a result, initial stiffness of B9 exceeded 21.5% over B5 and 15.7% of ultimate load. Obvi-ously, FRP played a significant role in improving the stiffness and slowing the destruction.

B5 held the lowest initial stiffness and load carry-ing ability due to short finger-joint. Moreover, De-spite of FRP reinforcement enhanced the joint strength in B9, stress-concentration subject to large

force in compressive zone led to the crush of bam-boo beam and specimen damage quickly and this behavior belongs to brittle failure. In the other hand, the experiment data of 30mm finger-joint beam B7 and B8 surpassed theoretical values by 26.98% - 27.18% of initial stiffness and 16.1% - 28.53% of ul-timate load. Table 4. Test results of 4.5m-span beams

Joint- length(mm) Bolt

Reinforce- ment

FRP Initial

stiffness(kN/mm)

Ultimate load

kNB4 30 - 8.791 222.5 B5 20 - 6.220 168.5 B6 30 - 4.722 242.5 B7 30 - 9.113 298.54 B8 30 - 9.128 330.5 B9 20 7.556 195

Through comparison of specimens, it could be seen that ductility of glubam beams of 30mm finger-joint was better than 20 mm finger-joint. In addition, the ductility of B5 was not poor but its bearing ability was worse than others and it destructed too early in the experiment, which attributed to short length of joint. B4, B6, B7, B8, with 30mm length joint, put out fine ductility, even better than the FRP rein-forced B9, and possessed higher ultimate load. Al-though joint-finger cracked early in B6, the crack exploded for a long time until it was crushed. To sum up, all of above implied that longer finger-joint performed better ductility in the bending condition.

3.2.3 Results of 4m span beam tests It is can be seen from the experiments that each of tested initial stiffness was similar to calculated re-sults. In addition, as there was no joint-fault over-lapped in this group of beams, we only took the fin-ger length into consider. Then, the initial stiffness and carrying ability of 30 mm beams exceeded 20mm beams by 3.72%-9.96% and 60.4%-42%, re-spectively. Moreover, those of FRP reinforced glubam beams surpassed non-reinforced by about 10% and 42%. Obviously, members which did not reinforced with bolt and FRP had poor characteris-tics, extremely inappropriate to be adopted in prac-tice and design, since their abilities were poorer than others. T able 5. Test results of 4 m-span beams

Joint- length(mm)

Bolt Rein-force-ment

FRP rein-forcement

Initial stiffness(kN/mm)

Ultimate load

kNB10 20 - 10.462 212 B11 20 - 10.462 239.5 B12 30 - 11.504 340 B13 30 - - 10.851 170.5

3.2.4 Small cross-section beam A four point load method, was used to test speci-mens. The clear distance between the edges of the bearing plate and the nearest loading point was about one-third of the length of beam, equal to those between load points. Moreover at least a lateral sup-port located at space between the reaction and the load point.

Deflectometers fixed at the position of load points, reactions and mid-span, where five strain gauges lo-cated throughout the depth. All the measured data were record simultaneously by the static strain measurement system.

Some parameters of small beams were listed in Table 6, such as cross-section dimension, span, rein-forcement-rate and so on. Through Comparison with three types of beams, the quantitative reinforce ef-fects of FRP was obtained accurately. The results showed that load carrying ability of FRP reinforced beams were higher than non-reinforced beams with a range from 2.95%- 28.77%, and larger thickness re-sulted in more increase of ultimate load. Further tests are still underway in this testing series. T able 6. Test results of small volume beams

Beam number

Cross-section dimen-

sion(mmmm)

Span / mm

Rein-force- ment

rate/ %

type

Fmax /kN

S1, 2, 3 56112 2016 27.12 S4,S5 56112 2016 H* 13.95

S6 56112 2016 0.1 28 S7 56112 2016 0.5 32

S8,9,10 84160 2240 73 S11S12 84160 2240 H* 28.5

S13 84160 2240 0.21 74 S14 84160 2240 0.35 78 S15 84160 2240 0.69 94

* Load is perpendicular to the plane of bamboo sheet Figure 2. Load-deflection curve of small volume beam.

4 CONCLUSION

Loading tests were conducted on glue-laminated bamboo (GluBam) beams with or without CFRP strengthening. Some observations can be made

through the preliminary examination of the testing results.

Longer finger-length and bolt reinforcements con-tributed to increase of the initial stiffness, load car-rying capacity and deformability of glubam beams.

Through the analysis on FRP reinforced glubam beam, it was found that FRP reinforcement could ef-fectively improve the specimen stiffness. Appropri-ate FRP thickness should be chosen while enhancing the load carrying capacity of glubam beam, in order to avoid over-reinforcement.

Since different batches of specimens were pro-duced in different period and the process and gluing environment is not the same, resulting in their vari-ous characteristics in bending, it is advised that the same process and quality control means should be applied to ensure uniform performance in structure.

Due to the presence of finger-zone which may re-duce the load bearing capacity of glubam beams, the joint effect factor J was adopted in the calculation of load bearing capacity of components. However, more studies are required to improve the accuracy of prediction. Therefore, more researches and studies are needed to be done in order to improve the factor of J in future.

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1 INSTRUCTIONS2 EXPERIMENT DESIGN3 RESULT AND ANALYSIS3.1 Experimental results3.2 Analysis of experimental results3.2.1 Results of 3.5m span beam tests3.2.2 Results of 4.5m span beam tests3.2.3 Results of 4m span beam tests3.2.4 Small cross-section beam

4 CONCLUSIONPREFERENCES