Experimental Studies on Glue-laminated Bamboo ...

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Keywords: glue-laminated bamboo; glubam; trusses; stiffness; strength. Abstract. ... In ancient China, wood is the most popular engineering material. Chinese ...
Advanced Materials Research Vols. 639-640 (2013) pp 757-762 Online available since 2013/Jan/11 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.639-640.757

Experimental Studies on Glue-laminated Bamboo Trusses Guo Chen 1,a, Yan Xiao 2,3,b and Bo Shan 2,c 1

College of Civil Engineering, Nanjing Forestry University, Nanjing, 210037, Jiangsu, China 2 Institute of Modern Bamboo, Timber and Composite Structures (IBTCS), College of Civil Engineering, Hunan University, Changsha, 410082, Hunan, China 3 Dept. of Civil Engineering, University of South California, CA90089, USA)

a

[email protected](corresponding author), [email protected], [email protected]

Keywords: glue-laminated bamboo; glubam; trusses; stiffness; strength

Abstract. Prefabricated light-frame bamboo trusses are widely used in light-frame construction as the main vertical load-carrying elements in roof and floor systems because of their long-span capabilities, ease of use, versatile configuration, rapid installation, and competitive cost. This paper presents experimental studies on glue-laminated bamboo trusses. Six full-scale model trusses with two types of configuration and sizes were tested to failure under gradually increased vertical load. The failure of the model trusses was caused by lateral buckling of the top chords. Tests show that the modern glue-laminated bamboo trusses have adequate stiffness and strength. Introduction In ancient China, wood is the most popular engineering material. Chinese ancient timber structure has a long history, as early as 3500 years ago, the mortise and tenon joint has been the predominant type in ancient wooden structure. For example, the Yingxian Wood Tower which located in Datong Basin seismic zone has experienced several times of strong earthquake and war destruction without collapse since completion. This has fully demonstrated the Chinese ancient timber structure excellent seismic performance. Light wood structures originated in North America, was born in the mid-19th century, dating back over 100 years of history. From solid wood to the composite wood, wooden structure has been reached the extent can replace steel in some areas. Wenchuan earthquake caused huge economic losses and heavy casualties caused by the concrete and masonry structures collapse. It sounded the alarm to the researchers; that is to say, how to minimize earthquake losses is priority in the future design of construction. However, light wood structure shows the excellent performance during the earthquake. For example, in the Northridge earthquake, most of the wood houses only deformed slightly without collapsed, even if the house was forward a few meters or separated from the foundation (Lee 1994). Overview of bamboo resources At present, there are more than 40 genera bamboo plants and bamboo forest area of 7.2 million hectares in China. The production of moso bamboo accounts for 90% of the world, an area of about 300 million hectares, so bamboo is known as the "second forest" in the world. Compared with wood, bamboo has the advantage of higher yield, higher strength/weight ratio, shorter growth cycle, etc. (Sumardi 2007). Prefabricated light-frame bamboo trusses are widely used in light-frame construction as the main vertical load-carrying elements in roof and floor systems because of their long-span capabilities, ease of use, versatile configuration, rapid installation, and competitive cost(Fig. 1). All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 221.181.145.133, Nanjing Forestry University, Nanjing, China-26/03/13,08:57:54)

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Roof systems used in residential construction have an outstanding record of structural performance. The objective of this study is to characterize the strength and stiffness performance of two light-frame bamboo truss configurations. The main basic material used in manufacturing the structural elements was bamboo plywood with a dimension of 2,440 mm long, 1,220 mm long and 28 mm thick. The panels were further made into various laminated structural elements using the glubam technology developed by Xiao et al. (2007, 2008 and 2009).

(a) (b) (c) Fig.1 Engineering Application of Bamboo Trusses (a) Mobile bamboo houses in Sichuan after Earthquake (b) Bamboo villa in Hunan university(c) Demonstration bamboo house in Black Bamboo Park Experimental Programs The objective of this study is to characterize the strength and stiffness performance of two light weight-frame truss configurations. Six full-size trusses were tested with two replicate per test type. Six trusses of two configurations were tested to 1.25 times their design load to determine their stiffness characteristics and then the undamaged trusses were tested to failure. Table 1 provides the details of the model trusses. Both truss configurations were designed for top chord load only. Table 1-Description of Bamboo Truss specimens Truss number

top chords(mm)

BT11, BT12, BT13 BT21, BT22, BT23

28×150 56×140

bottom chords(mm) 28×150 56×120

webs(mm)

span(mm)

Slope

56×90

5000 6000

3:12 6:12

Test Setup The bamboo plywood was visually graded to meet the requirements and marked with an identification number. The test bamboo trusses were constructed in the laboratory using glubam technology. All three trusses were geometrically identical for the same slope truss. The bamboo panels used to fabricate the trusses were obtained from the same bundle acquired from a building products supplier. Figure 2 shows the schematic of truss specimens, as well as the loading conditions applied. The specimens were evaluated in a horizontal position using a test facility. Three pairs of steel frame were used to prevent any out-of-plane movement of the trusses during testing. Furthermore, all of the deflection readings (∆d and ∆u) were measured directly from the truss with a reference to the foundation by centimeter. Prior to testing, the specimens were stored in the laboratory for at least two weeks to allow for bamboo relaxation around the bolts. Load was applied at the nodes of top chords by weights.

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TC1

759

TC2

TC2 TC1

TC3

WEB1

BC1

WEB2

BC1

Fig.2 (a) BT1 specimens

TC4

WEB3 BC2

Fig.2 (b) BT2 specimens

Fig.2 Bamboo truss specimens (a) BT1 specimens (b) BT2 specimens Test Results and Discussion Truss Stiffness. Presented in Fig.3 are the load-deflection curves for model trusses. For BT1 trusses, it was observed in both tests that obvious deformation did not occur until the 8th loading step. The results of the BT2 trusses in the tests have linear deflection performance up to twice the design load and obvious deviations from linearity up to three times the design load. The conclusion is similar to the wood truss tests (Wolfe 1986). With the experiments carried out, the deformation of the BT1 trusses became more and more significant than the BT2 trusses. The following values, summarized in Table2, were obtained from the specimens tests.

Truss BT11 BT12 BT13 Average BT21 BT22 BT23 Average

Table2. Main test results Design load ratio 3.2 3.5 3.4 3.4 1.7 2.1 2.0 2.0

Pult (kN) 8.4 9.1 8.8 8.8 36.0 44.3 42.2 40.8

∆d (mm) 14.16 14.66 17.76 15.53 7.06 7.46 4.79 6.44

∆u (mm) 43.71 55.49 57.91 52.37 27.61 34.26 27.52 29.79

10

50

8

40

6

30

Load(kN)

Load(kN)

The test results (Table 2) show slightly dispersion. It was noticed that the 3/12 sloped truss had a maximum ultimate load capacity of 9.10kN, and the 6/12 sloped truss had a maximum ultimate load capacity of 44.3kN.

4

2

20

10

BT11

BT21

BT12

BT22

BT13

0

0

10

20

30

deflection(mm)

40

50

BT23

60

0

0

10

20

deflection(mm)

(a)BT1 (b)BT2 Fig.3.Load-deflection plots for mid-span

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40

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Truss Strength. Table 2 provides a summary of ultimate loads, and gives failure for each truss tested to destruction. These trusses failed during the 5-minute holding period between load applications. Of the 6 trusses tested to failure, all the trusses failed in the top chord, as shown in Fig.4. All these failures were due to compression stresses. When compressive load on the remaining effective section exceeded the threshold required causing it to buckle, it suddenly failed by brittle fracture.

(a) (b) Fig.4. Failure of top chord buckling (a) BT1 (b) BT2 Chord strain behavior. Fig.5 shows the average load-chord strain curves from the tests for bamboo trusses BT1 and BT2, respectively. The load-chord strain curves were exhibited very similar for the three specimens with slight variances, before loaded to twice of the design capacity. During the subsequently loading, the top chords (TC1) and the webs (TC4) show obviously dispersion. This is mainly because top chord twisted, which resulted in internal force redistribution, therefore the force of the webs increasing rapidly, during the tests. Because the top chords did not twist all the time, there was good agreement between the three specimens. With the testing carried out, the reading of centimeters more time would be spent. When loaded to 9th step need 10min to require the stable reading of the centimeters. Once the top chord twisted, the truss would be failed rapidly. During the test, no other failure was observed. Thereafter, the failure of the top chord (TC1) progressed in the form of twist and buckling, resulting in a gradual reduction of load carrying capacity. Upon continued loading, the lower chord (BT1) and webs (WEB1) twisted obviously, but did not destroy. The dominant failure mode of the bamboo trusses was the bucking of the top chords.

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(b)

(c) (d) Fig.5. Load versus strain relationship (a) TC1 chord of bamboo BT1, (b) BC1 chord of bamboo BT1, (c) TC1 chord of bamboo BT2, (d) BC1 chord of bamboo BT2. Conclusions From the results of this study, the following conclusions can be drawn. These test results demonstrate truss performance characteristics which will be useful in modeling the stiffness and strength of light-frame wood trusses to be used in tests to evaluate a full-scale roof system model. (1) The trusses were appropriately designed and had enough reserve capacity. In terms of bamboo truss, results of this study suggest that current allowable stress design method for wood truss is conservative. The bamboo truss is an economy, reliable and environmentally friendly structural system. The weakest link of the wood truss is the metal plate connector (MPC), which destroyed firstly. However, the weakness link of the bamboo truss is the top chords, as shown in Fig.4. The top chords twisted and then yielded suddenly. (2) During the tests of the wood truss, the truss failed by brittle fracture. The failure of the metal plate connector will result in global collapse of the wood truss. Contrary to previous wood truss, the test results showed that a substantial increase in ductility and ultimate load when compared to the wood truss. The dominant failure mode was the bucking of the top chords. The failure of the top chords won’t result in global collapse of the truss, which will reduce more the casualties and property loss, compared to the wood truss during the accident. (3) There are many factors which have significant influence on the ultimate load of the bamboo truss, for instance, the property of the bamboo material, the connector, manufacturing process and so on. Modulus of elasticity exhibits a spatial variation in bamboo. Because the elastic critical buckling load is a function of E, the variation of critical buckling load for unbraced members is in part due to the variation in E between members, as well as variation in E within individual pieces (Leichti 2002).

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Acknowledgements The projects described in this paper were conducted at the Institute of Bamboo, Timber and Composite Structures (IBTCS), MOE Key Laboratory of Building Safety and Energy Efficiency, the Hunan University. Supports to various phases of the projects are provided by the Program for Changjiang Scholars and Innovative Research Team Project by the Ministry of Education of China (Project No. IRT0619), National Natural Science Foundation (Project No. 50938002), Blue Moon Fund and the Advanced Bamboo and Timber Technologies, Ltd. The project described in this paper also funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors warmly thank all the sponsors and collaborators. References [1] Chen, G.; Xiao, Y.; Shan, B. and She, L.Y. Design and construction of a two-story modern bamboo house, Modern bamboo structures, (2007)215-221 [2] Lee, A.W.C., Bai, X.S. and Peralta, P.N. Selected physical and mechanical properties of giant timber bamboo grown in South California, Forest Products Journal, 44(1994), 40-47. [3] Leichti, R. T-Bracing for stability of compression webs in wood trusses, J. Struct. Eng., 128(3),(2002) 374-381. [4] Sumardi, I., Ono, K. and Suzuki, S. Effect of board density and layer structure on the mechanical properties of bamboo oriented strandboard, Journal of wood science, 53 (6), (2007).510-515. [5] Van der Lugt, P., Van den Dobbelsteen, A.A.J.F. and Janssen, J.J.A. An environmental, economic and practical assessment of bamboo as a building material for supporting structures, Construction and Building Materials, 20(2006) 648–656. [6] Wolfe, R.W., and LaBissoniere, T.G. . Structural performance of light-frame roof assemblies. Ⅱ.conventional truss assemblies, Rep.FPL-RP-499, U.S.Department of Agriculture, Forest Products Lab., Madison, Wis.(1991) [7] Xiao, Y.; Shan, B.; Chen, G.; Zhou, Q. and She, L.Y . Development of A New Type of Glulam, Modern bamboo structures, (2008)41-47. [8] Xiao, Y.; Zhou, Q.; and Shan, B. Design and Construction of Modern Bamboo Bridges, Journal of Bridge Engineering, 15(5), 2010, pp. 533-541.

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Experimental Studies on Glue-Laminated Bamboo Trusses 10.4028/www.scientific.net/AMR.639-640.757