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Experimental evaluation on mechanical performance of OSB webbed parallel strand bamboo I-joist with holes in the web
Construction and Building Materials 101 (2015) 91–98
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Experimental evaluation on mechanical performance of OSB webbed parallel strand bamboo I-joist with holes in the web Guo Chen ⇑, Hai-tao Li, Tao Zhou, Cheng-long Li, Yu-qi Song, Rui Xu College of Civil Engineering, Nanjing Forestry University, Nanjing, Jiangsu Province 210037, China
h i g h l i g h t s Effect of the holes in the web on the OSB webbed parallel strand bamboo I-joists was investigated. The hole height limit for the joists with no obvious strength reduction effect was proposed. The paper addresses the problem of critical clear distance between the two holes in the web.
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Article history: Received 4 May 2015 Received in revised form 12 August 2015 Accepted 12 October 2015
Keywords: Parallel strand bamboo Web opening I shaped joist Mechanical performance Failure mechanism
a b s t r a c t Holes are often made in the web of the OSB webbed bamboo I-joists for the passage of plumbing lines and ventilation systems. A comprehensive study was conducted in order to determine effect of the parameters, such as the size, shape and clear distance of the holes, on the behavior of the joists. A total of 92 OSB webbed bamboo joists were tested to failure to investigate the failure modes and failure mechanism of them. The results showed that holes in the web have considerable effect on the strength reductions of specimens. Holes sharply changes the stress state and reduces the strength evidently due to the high shear stresses appearing in the vicinity of the hole. With the increase of ratio of the hole size to web height, the peak load capacity and stiffness of the joists with holes decreased significantly. A web hole with sharp corners caused greater stress concentrations than the circular hole. Results from the tests on I shaped joists with two circular and two square web holes with a size of 50% web depth showed that the critical clear distance is about 2 times and 2.5 the web hole size respectively. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Bamboos is considered as one of fastest-growing plants in the world, due to a unique rhizome-dependent system [1,2]. And it is principally tropical and subtropical in distribution, especially in India and China. Historically, it is generally used in nonstructural applications, such as furniture, craft, scaffolding and so on [3]. Due to their higher strength/weight ratio than common wood and structural steel, bamboos have prerequisite as a modern structural materials [4]. Exploited moderately and making it rationally will further broaden the scope of bamboo utilization. Although bamboo has been applied to the construction of housing for a long time [5–7], it has many flaws, such as thin-walled hollow, diameter of the bamboo culm and easy to crack when exposure to moisture alternation frequently. More important, untreated bamboo is prone to infestation which reduce its service
⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (G. Chen). http://dx.doi.org/10.1016/j.conbuildmat.2015.10.041 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
life of the tradition bamboo houses. Parallel strand bamboo (PSB) is made in China from Moso bamboo, harvested at 3–5 years old. The PSB is an engineered product made by peeling bamboo culms usually to 2–3 mm thickness and 20–30 mm width strips and gluing them together with urea formaldehyde adhesive [8,9]. Similar to glulam, PSB decreases the flaws by distributing them throughout the structural members, which is considered as a promising engineering material [10]. Recently, much attention were focused on the experimental study and engineering application of PSB. Over the past decade, extensive research has been undertaken on bamboo beams. Glue-laminated bamboo (Glubam) girder was firstly developed by Xiao [11] then it was applied to bamboo bridges and buildings [12]. Sinha [13] found the adhesives have effect on the bending strength of bamboo glulam beams, and have slight impact on the stiffness [13]. The maximum allowable load capacity of the Glubam beams with rectangular section was controlled by stiffness other than strength. Moreover, the fibers of the center height of the beam was not fully utilized. To address this issue, Xiao [11] proposed that CFRP should paste to the bottom of
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the Glubam girder [12]. It is evident that the method of pasting CFRP was helpful to increase the flexural rigidity, while the improvement was limited for peak load capacity. Tomasi [14] and Wei [15] has chosen another reinforcement measure, steel bars were laid in the bottom of the bamboo beams. However, the improvement of the allowable capacity was still limited [14,15]. The rectangular beam becomes somewhat obsolete due to the fact that the ultimate strength of bamboo beams was controlled by the outermost bamboo fibers, while normal stress of the fibers closed to the neutral axis was low. Compared to solid joists, prefabricated I-shaped joists are more efficient for structural use where the web is designed to carry most of the shear capacity, and the flanges provide the moment capacity [16]. Test results provided by Wu [17] indicated that applying an external prestressing steel wire on the bamboo composite I-joist helps to reduce the vertical displacement of the joists [17]. However, the function of external prestressing for improving structural rigidity and peak load capacity of the joists is limited. Li [9] proposed a thin-walled steel webbed bamboo I-shaped beam and better ductility can be obtained [18]. Aschheim [19] found bamboo OSB webs would be preferred as a means of reducing deflection. Also, the allowable load of the beam under the serviceability limit state was only 30% of the ultimate limit states [19]. Oriented strand board(OSB) webbed parallel strand bamboo (PSB) joists, which can be used as an alternative to wood I-joists, were present. However, holes are often made in the web of the OSB webbed PSB joists for the passage of plumbing lines and ventilation systems. Obviously, the holes change the load transfer path in the webs, and then bring many disadvantageous effects to shear strength, stability and deformation performance. More important, the difficulty with addressing the question is the lack of detailed guidance of building codes, and design guides for OSB webbed PSB joists with holes. So far, experimental research on OSB webbed
PSB joists with web holes has not been conducted. Although the ASTM D5055 is governing testing procedure for wood I-joists without web holes and the WIJMA (Wood I Joist Manufacturers Association) created testing specification for establishing shear capacities for prefabricated wood I-joists with holes, OSB webbed bamboo joists was not included.
2. Materials and methods Prefabricated OSB webbed PSB I-joists consist of two components, two PSB flanges and the OSB web, which form a I shaped cross-section. The TolkoÒ OSB panels, 9.5 mm in thickness, are engineered wood products which meet the APA quality system certification. Of particular importance to this study are the shear properties of the web materials. Average shear properties through the thickness as determined by rail shear test method [20] for OSB is 1420 N/mm2. The bending strength and modulus of elasticity of the OSB is 25.1 MPa and 3.56 GPa, respectively. The PSB flanges was 2440 mm in length, 25 mm in width, and 35 mm in thickness, with a modulus of elasticity of 10.2 GPa. The web was nailed and bonded to double flanges with outdoor epoxy adhesive. To avoid uneven adhesive layer, it is particularly important to ensure the coating surface of the members was clean and smooth. Consumption of adhesive between each other was 250 g/m2. After then, the OSB panels were connected to the PSB with 40 mm nails at spacing of 150 mm along the panel edges and the edge distance of nails should not less than 10 mm. According to the suggestions of the adhesive manufacturers, the finished specimens were allowed to cure for two weeks at a relative humidity of (60 ± 5)% and a temperature of (20 ± 2)°C prior to testing. Bearing stiffeners are required at the beam ends and also at points of concentrated loads or reactions. Install bearing stiffeners tight against the bottom flange of the I-joist, leaving 5 mm gap at the top. But the load stiffeners had the opposite installation. For each series configuration, either 3 or 4 identical specimens were tested. For example, the specimens of 24I1 series had 4 replicas, which was marked 24I1A, 24I1B, 24I1C and 24I1D respectively. A parameter study of the influence of beam size, relative hole size with respect to web height, hole shape, clear distance between holes was present. Table 1 shows the different hole configuration tested including 84 joists with holes and 8 joists without holes. The joists were divided into five categories. Among them, the specimen 30SI5 and 30SI6 have a square hole with a corner radius of 25 mm and 15 mm respectively. All of the joists, 2000 mm in span, had hole centrically placed with respect to joist height.
Table 1 Details of joists. Joist #
b t (mm mm)
H (mm)
Hole shape
d/hw (%)
l1 (mm)
l (mm)
Number of tests
I
24I1 30I1
59.5 35 69.5 35
240 300
No hole No hole
0 0
– –
– –
4 4
II
24CI1 24CI2 24CI3 24CI4 30CI1 30CI2 30CI3 30CI4
59.5 35 59.5 35 59.5 35 59.5 35 69.5 35 69.5 35 69.5 35 69.5 35
240 240 240 240 300 300 300 300
Circle Circle Circle Circle Circle Circle Circle Circle
25 50 75 100 25 50 75 100
500 500 500 500 500 500 500 500
– – – – – – – –
3 3 3 3 3 3 3 3
III
24SI1 24SI2 24SI3 24SI4 30SI1 30SI2 30SI3 30SI4 30SI5 30SI6
59.5 35 59.5 35 59.5 35 59.5 35 69.5 35 69.5 35 69.5 35 69.5 35 69.5 35 69.5 35
240 240 240 240 300 300 300 300 300 300
Square Square Square Square Square Square Square Square Square with circular edge Square with circular edge
25 50 75 100 25 50 75 100 50 50
500 500 500 500 500 500 500 500 500 500
– – – – – – – – – –
3 3 3 3 3 3 3 3 3 3
IV
24SI5 24SI6 24SI7 24SI8 24SI9
59.5 35 59.5 35 59.5 35 59.5 35 59.5 35
240 240 240 240 240
Square Square Square Square Square
50 50 50 50 50
330 287.5 245 202.5 160
170 255 340 425 510
3 3 3 3 3
V
30CI5 30CI6 30CI7 30CI8 30CI9
69.5 35 69.5 35 69.5 35 69.5 35 69.5 35
300 300 300 300 300
Circle Circle Circle Circle Circle
50 50 50 50 50
270 212.5 155 97.5 40
230 345 460 575 690
3 3 3 3 3
Note: relative hole size with respect to web height (d/hw).
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(a) Category I (f) Bearing stiffeners
(g) Load stiffeners
Fig. 1 (continued)
(b)Category II
The testing was displacement-controlled until failure. The loading rates were to comply with the ASTM D5055 (2010) requirement to cause failure within 8–15 min. The loads, deflections of the mid-span, settlement of the reactions, strain distribution of the bottom, top and sides of the specimens were recorded by TDS-530 data acquisition system and the sampling frequency was 5 HZ. In order to eliminate the system error and ensure the test equipment operating reliably, pre-loading on the specimens was necessary prior to formal testing. From these measurements, the peak load, stiffness, and load at a deflection of L/250 (L is the span between supports) were selected to evaluate the behavior of the I-joists. Due to the difficulties in computing a composite modulus term, the slope of the load–deflection curve in the linear elastic range of testing was defined as the stiffness term.
3. Experimental results 3.1. Failure mode
(c)Category III
(d)Category IV
(e)Category V Fig. 1. Detail diagram of specimens. Experiments were performed on simply supported joists using center loading, aimed at evaluating the behavior of specimens. The testing apparatus is shown in Fig. 1. The specimens were loaded vertically using a actuator with a capacity of 100 kN with an accuracy of ±0.1 kN. The load was transmitted to a steel plate (200 100 20 mm) which was designed to be large enough to ensure no crushing failures occurred at the top flanges. Three Laser Displacement Sensors (LDS) with an accuracy of ±0.1 mm were used for measuring deformations at mid-span and two supports. So the real deformation of the specimens was the difference between the mid-span deflection and the supports. It is extremely complicated to gain the strain field from testing in a region using strain gauges because a strain gauge measures average strain over a limited region of web. For simplicity, strain gauges were glued on the surface of the corners and numbered in a counterclockwise direction, numbering starts from the upper left corner from 1 to 8.
3.1.1. Reverse and delamination of OSB in the flange The damage of all the joists began with reverse. Different failure modes occurred for varying holes types and size. At the beginning of the experiment, the reference specimens (24I1 and 30I1) without holes showed good ability to deform. As the load increased, the specimens started to reverse and the deformation became obvious gradually. When the vertical load reached to 0.7Pu (Pu was defined as the peak load at failure), the wood flakes of OSB panel in the flanges delaminated with great splitting sound, and the OSB board crack continued widening until the cracks spread along the length of the joists. Eventually a few of nails were pulled out or fractured, significant deformation could be observed. Once the joists reached the ultimate strength, the carrying capacity of the joists lost in an instant. In short, damage of joists began frequently with the twist and the appearance of micro crack in the flanges, no visible damage of web and flanges was investigated when the failure of joists has happened. The behavior of the specimens with smaller holes (d/hw 6 25%) was similar to those joists without holes. Even the strain gauges around the holes remained intact without wrinkles. The effect of web opening on the behavior of the specimens could be ignored. In other words, the hole limit ‘‘d/hw < 0.25” for 240 mm and 300 mm high I-joists was 42.5 mm and 57.5 mm, respectively. Similar results were found by Aicher et al. [21] and Ardalany et al. [22], the hole size limit for Glulam and LVL with no obvious strength reduction effect is 50 mm [21,22]. 3.1.2. Shear failure around web holes Introducing a hole in the web drastically changes the stress state and reduces the strength significantly due to the high parallel to the length of joist tensile stresses and shear stresses appearing in the vicinity of the hole. As the hole size increased, the increasingly negative impacts of holes on the performance of specimens (25% < d/hw 6 75%) would severely reduce shear capacity and decrease stiffness. The present of holes reduce cross-section of
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the web, resulting in a bad shear load transfer. During the testing, the specimens with web holes (25% < d/hw 6 75%) had similar failure characteristic. Take specimens with square holes (25% < d/hw 6 75%) as an example, the square holes would became progressively parallelogram with the load increased. The upper right and lower left corners of the holes turned into an obtuse angle, while the other corners were acute angle. Fig. 2 presented that in
the shear dominant parts of the beams, diagonal parts are going into tension (lower left corner and upper right corner) and compression (upper left corner and lower right corner). Shear failure of the web around the holes was the primary mode of failure for the specimens tested. Unlike the sudden fracture flange failures of the wood joist with web openings [23,24], the PSB flanges of the OSB webbed bamboo joists remained intact. As the load increased
(a) 30I1A
(b) 30CI1D
(c) 24CI2C
(d) 24SI2A
(e) 24CI3B
(f) 30CI3A
(g) 24SI4A
(h) 30CI4C Fig. 2. Typical failure modes of specimens.
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3.2. Load–deflection behavior
Fig. 3. Distribution of failure angle for all tests.
and reached the peak load, cracks formed around the holes with continuous loud noise and gradually propagated towards the flanges. The cracking for the circular holes started at an angle about 40–60°, however, for the square holes the cracking started at the corners of the hole that could be due to stress concentration at the corners. The load which associated with this stage is defined as the ‘‘cracking load”, and represented by Pcr. The readings of strain gauges were about 8000le, when the damage of OSB around the holes occurred. The distribution of failure angles for the specimens with web opening (d/hw > 25%) is plotted in Fig. 3, which has an average of 48.5°. 3.1.3. Web pullout For joists with oversize holes (d/hw > 75%), especially the hole size equal to web height, the rest of the web below and above the hole was not enough to resist shear induced by external load. Cracks formed around the holes with continuous loud noise and gradually propagated towards the flanges. Immediately after the web above and below the hole failed due to shearing, the load was transmitted to adjacent sections of the web. The sudden increase in shear stress along the adjacent sections of the web often caused secondary failures of web pullout or nails shear fracture. Amazingly, the specimens tested herein did not break into two parts, while this phenomenon of breaking was common during the tests of wood joists [25]. The main test results of all the specimens are summarized in Table 2.
The load–deflection curves for joists with and without holes are showed in Fig. 4. The present of holes in the web has significantly reduced the strength of the joists, especially for joists with bigger holes (d/hw > 25%). Obviously, the failure procedure of specimens can be classified into three stages. The joists performed linearly until failure, which was a sudden brittle failure. With regard to joists with smaller holes, it can be seen from the data that an abrupt end of the linear plot. However, the non-linear characteristic of load–deflection curves became obvious at the end before failure would occur as the hole size increased. Webs above and below the hole always work before the plastic hinges emerged at the corners of holes. After then, internal force redistributed around the hole, then the flanges resist the shear stress and moment at the same time. It can be explained that the specimens were able to continue to resist external load induced by actuator after the specimens reached ultimate capacity. According to EC5, the allowable deflection for serviceability is L/250 for floor joists with plasterboard [26]. The corresponding load is represented by PL/250. The ratio of PL/250/Pu decreased gradually as the hole size increased, and the ration was in proportion to the height of web. However, the load carrying capacity of wood I joists or bamboo I-joists under serviceability limit state was less than 30% of the strength limit states (see Fig. 5) 3.3. Initial bending stiffness Due to the difficulties in computing a composite modulus term, the slope of the load–deflection curve in the linear elastic range of testing was defined as the initial bending stiffness term (Fig. 6). As observed above in this paper, the cracking load is less than 50% peak load, in other words, it is reasonable that the initial bending stiffness ke is defined as the slope of the curves, for zeroing of data and 0.4Pu with corresponding deflection. It is evident that the stiffness of specimens with large holes (d/hw > 25%) deteriorated progressively as the applied loads increased. Of course, the higher the joist is, the better the initial bending stiffness is. 3.4. Ductility Ductility is more commonly defined as the ability of member to deform easily after yielding, or as the ability of a member to withstand plastic deformation without breaking. Ductility member
Table 2 Summary of carrying capacity and deflection for specimens. Joist #
Pcr (kN)
Pu (kN)
PL/250 (kN)
Dcr (mm)
Du (mm)
Dy (mm)
PL/250/Pu (%)
l
ke (kN/mm)
24I1 24CI1 24CI2 24CI3 24CI4 24SI1 24SI2 24SI3 24SI4 30I1 30CI1 30CI2 30CI3 30CI4 30SI1 30SI2 30SI3 30SI4
18.09 14.83 12.84 9.38 8.56 13.97 10.37 8.34 5.88 19.20 16.49 14.91 11.61 9.95 17.75 15.33 11.86 8.24
23.19 22.13 19.75 15.13 13.17 21.17 17.00 14.13 10.14 26.66 24.99 23.67 19.03 14.85 24.65 22.55 18.24 13.51
14.91 14.92 11.66 8.98 5.87 13.68 10.79 8.04 6.72 23.26 20.51 16.02 12.91 8.04 19.70 15.56 12.34 7.54
9.27 7.99 8.74 8.31 10.68 8.16 7.69 8.27 7.05 6.58 6.62 7.46 7.84 9.51 6.93 7.89 8.01 8.69
12.08 12.54 14.02 14.92 16.10 12.68 15.08 15.95 16.29 9.72 9.90 12.41 13.15 14.79 10.88 12.60 13.48 15.25
12.31 13.05 14.16 15.22 17.22 13.88 16.88 16.87 18.54 10.54 10.47 12.63 13.19 17.95 11.75 12.90 13.73 17.91
64.3 67.4 59.0 59.4 44.6 64.6 63.5 56.9 66.3 87.2 82.1 67.7 62.6 54.1 79.9 69.0 67.7 55.8
1.02 1.04 1.01 1.02 1.07 1.09 1.12 1.06 1.14 1.08 1.06 1.02 1.00 1.21 1.08 1.02 1.02 1.17
1.88 1.83 1.46 1.14 0.83 1.71 1.35 1.01 0.81 2.91 2.56 2.03 1.61 1.07 2.56 1.95 1.54 0.97
Note: Pcr is the mean cracking load, Pu is the mean failure load, ke is Initial flexural stiffness, l is the ductility.
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Fig. 4. Load versus mid-span displacement curves.
Fig. 5. Influence of relative hole size.
show large deformation and ability to absorb energy in the process before fracture [27]. The ductility factor (l) is defined by the formula Du/Dy, where Du is the displacement at failure and Dy is the yield displacement. However, the specimens tested above has no obvious yield platform and load-descend stage. Dy was defined as the mid-span deflection when load equal to 0.85 times the peak load. The results showed that the ductility of joists increased after reinforced by web stiffeners. As expected, the ductility of specimens had nothing to do with the holes. The coefficient of ductility of all the joists were equal, which coincide with the brittle failure of specimens again.
4. Influential factors 4.1. Hole size and shape The cross section of web was weakened by holes, resulting in reduced stiffness of joists significantly. As shown in Fig. 4, the capacity of the joists appeared to decline as the hole size increased. For joists with smaller holes (d/hw 6 25%), the hole had little influence on the carrying capacity, the drop was less than 10%. For d/h > 25%, the influence of holes on the mechanical performance could not be neglected. Especially, for the case of bigger
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Fig. 6. Comparison of initial bending stiffness.
to the interaction between holes. It appears that the minimum clear distance between circular holes on which no interaction occurs is twice the hole diameter. This finding agrees with the recommendation by Afzal [29]. For square web holes, the critical clear distance between holes with negligible stress interaction appears to be 2.5 times the hole length.
5. Conclusions
Fig. 7. Influence of clear distance between two holes.
hole (d/hw P 50%), the strength of the joists would suffer large losses, and the vertical deformation was larger compared to joists without holes. During the initial testing stage, the joists kept good performance and the shear deformation was small. With increasing load and deformation of the joists, plastic hinges would be formed in the corners of holes, the shear deformation generated by the upper and lower parts of the holes was significant. For example, the strength reduction of the 240 mm high I-joists was 8.7%, 26.7%, 39.1% and 56.3% for opening diameters d/hw equal to 25%, 50%, 75%, 100%, respectively. The DIN 1052 Code (2008) on web holes suggests that rectangular holes should have rounded corners with greater than 15 mm radius [28]. In view of hole shape, experiments were conducted on circle holes, square holes and square holes with rounded corner. Among all the joists with hole, the radius of curve was 15 mm and 25 mm for 30SI5 and 30SI6 respectively. It is not difficult to find that a web hole with sharp corners caused greater stress concentrations than the circular hole. The cracking load of the joists with sharp corners was about 10% lower than the joist with rounded corners. 4.2. Clear distance between two holes Experiments were performed to provide information of the influence of distance between adjacent openings on strength of I-joists. It is obvious that the clear distance between holes have considerable effect on the behavior of joists and the strengths of all double-holes joists are smaller than those with single hole (Fig. 7). For small hole spacing, the reduced strength is clear due
The static test on OSB webbed bamboo joists with web opening conducted in this study has provided an opportunity to study various parameters that influence the behavior of composite joists. Based on the experimental results obtained, the following conclusions can be summarized. (1) The OSB webbed PSB I joists showed characteristic of brittle failure. Once the joists reached the peak load, the carrying capacity of the joists lost suddenly. Unlike the sudden fracture flange failures of the wood joist with web openings, the PSB flanges of the OSB webbed bamboo joists remained intact. (2) Introducing a hole through the joist drastically changes the stress state and reduces the strength significantly. For circular and square web holes, the critical clear distance between holes with negligible stress interaction appears to be 4 times and 5 times the hole radius respectively. For small holes (d/hw 6 25%), the effect of the hole was negligible and did not cause strength reductions. (3) A web hole with sharp corners caused severe stress concentrations than the circular hole. Whether the corners is rounded off has considerable impact on the cracking load, and has effect on the peak load, but not significantly. The strength of joist with square holes improved by 3–7%, once the corners of holes were rounded off. The bigger the radius is, the greater the improvement. It is conservative that the carrying capacity of joist with a square web hole can be treated the same as a circular hole that inscribes the square hole. (4) Maybe the web around the hole was a weak regions. It is necessary to take remediation measures to recover strength and stiffness, such as plywood or galvanized steel sheets were glue-nailed to the both sides of the joists. The effect of the reinforcement is to increase the load bearing capacity by preventing the cracking to occur, and when it occurs, to reduce the brittleness of the failure.
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Acknowledgments The material presented in this paper is based upon work supported by the National Natural Science Foundation of China under Grant No. 51408312, the China Postdoctoral Science Foundation under Grant Nos. 2013M541679 and 2014T70528, the Natural Science Foundation of Jiangsu Province under Grant No. BK20130982, the Postdoctoral Science Foundation of Jiangsu Province under Grant No. 1301017A, and the College Natural Science Foundation of Jiangsu Province under Grant No. 13KJB560008. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the writer(s) and do not necessarily reflect the views of the foundations. The writers gratefully acknowledge Gao-qin Zhang, Nian-qiang Zhou, Long-long Zhao and so on from the Nanjing Forestry University. References [1] Hai-tao Li, Qi-sheng Zhang, Dong-sheng Huang, et al., Compressive performance of laminated bamboo, Compos. B Eng. 54 (2013) 319–328. [2] Juan.F. Correal, Juan.S. Echeverry, Fernando Ramírez, et al., Experimental evaluation of physical and mechanical properties of glued laminated Guadua angustifolia Kunth, Constr. Build. Mater. 73 (2014) 105–112. [3] K.F. Chung, W.K. Yu, Mechanical properties of structural bamboo for bamboo scaffoldings, Eng. Struct. 24 (2002) 429–442. [4] M. Mahdavi, P.L. Clouston, S.R. Arwade, A low-technology approach toward fabrication of laminated bamboo lumber, Constr. Build. Mater. 29 (2012) 257– 262. [5] Shan Bo, Gao Li, Xiao Yan, et al., Experimental research and application of prefabricated bamboo pole house, J. Hunan Univ.: Nat. Sci. 40 (3) (2013) 7–14 (in Chinese). [6] Yan Xiao, Guo Chen, Bo Shan, et al., Research and application of lightweight glue-laminated bamboo frame structure, J. Build. Struct. 31 (6) (2010) 195–203 (in Chinese). [7] Y. Xiao, G. Chen, L. Feng, Experimental studies on roof trusses made of glubam, Mater. Struct. 47 (2014) 1879–1890. [8] Huang Dongsheng, Zhou Aiping, Bian Yuling, Experimental and analytical study on the nonlinear bending of parallel strand bamboo beams, Constr. Build. Mater. 44 (2013) 585–592. [9] Haitao Li, Jing-wen Su, Qisheng Zhang, et al., Mechanical performance of laminated bamboo column under axial compression, Compos. B Eng. 79 (2015) 374–382. [10] Y. Xiao, R.Z. Yang, B. Shan, Production, environmental impact and mechanical properties of glubam[J], Constr. Build. Mater. 44 (6) (2013) 765–773.
[11] Y. Xiao, B. Shan, G. Chen, Q. Zhou, L.Y. She, Development of a new type of glulam-glubam, in: Xiao. Yan (Ed.), Modern Bamboo Structures, CRC, Changsha, China, 2008. [12] Xiao Yan, Zhou Quan, Shan Bo, Design and construction of modern bamboo bridges, J. Bridge Eng. 15 (5) (2010) 533–541. [13] A. Arijit Sinha, Daniel Way, Skyler Mlasko, Structural performance of glued laminated bamboo beams, J. Bridge Eng. 140 (1) (2014) 04013021 (8). [14] Roberto Tomasi, Maria Adelaide Parisi, Maurizio Piazza, Ductile design of glued-laminated timber beams, J. Struct. Eng. 14 (3) (2009) 113–122. [15] Y. Wei, M.Q. Zhou, D.J. Chen, Flexural behavior of glulam bamboo beams reinforced with near surface mounted steel bars, Mater. Res. Innovations 19 (S1) (2015) 98–103. [16] M. Shahidul Islam, Md. Shahnewaz, M. Shahria Alam, Structural capacity of timber I-joist with flange notch: experimental evaluation, Constr. Build. Mater. 79 (2015) 290–300. [17] Wenqing Wu, Experimental analysis of bending resistance of bamboo composite I-shaped beam, J. Bridge Eng. 19 (4) (2014) 04013014. [18] Yushun Li, Wei Shan, Huangying Shen, et al., Bending resistance of I-section bamboo–steel composite beams utilizing adhesive bonding, Thin-Walled Struct. 89 (2015) 17–24. [19] M. Aschheim, L. Gil-Martín, E. Hernández-Montes, Engineered bamboo I-joists, J. Struct. Eng. 136 (12) (2010) 1619–1624. [20] ASTM D3044-94. Standard Test Method for Shear Modulus of Wood-Based Structural Panels. American Society for Testing and Materials, Philadelphia, PA, 2011. [21] S. Aicher, J. Schmidt, S. Brunold, Design of timber beams with holes by means of fracture mechanics, Proc., CIB-W18, Meeting 28, Paper 28-19-4, University of Karlsruhe. [22] M. Ardalany, M. Fragiacomo, B. Deam, A. Buchanan, Design of reinforcement around holes in laminated veneer lumber (LVL) beams. World Conference on Timber Engineering (WCTE 2012), P. Quenneville (Ed.), Auckland, New Zealand, 539–547, 2012. [23] C. Geoffrey Morrissey, David W. Dinehart, W. Gary Dunn, Wood I-joists with excessive web openings: an experimental and analytical investigation, J. Struct. Eng. 135 (6) (2009) 655–665. [24] E.C. Zhu, Z.W. Guan, Buckling of oriented strand board webbed wood I-joists, J. Struct. Eng. 131 (10) (2005) 1629–1636. [25] A.M. Harte, G. Baylor, Structural evaluation of castellated timber I-joists, Eng. Struct. 33 (12) (2011) 3748–3754. [26] British Standards Institution. BS EN 1995-1-1 Eurocode 5: Design of timber structures. Part 1-1: General. Common rules and rules for buildings, 2004. [27] Guo Chen, Qi-sheng Zhang, Dong-sheng Huang, et al., Bending tests of OSB webbed bamboo I-joist, J. Hunan Univ. 42 (5) (2015) 72–79. [28] DIN 1052:2008-12 – design of timber structures – general rules and rules for buildings (in German). [29] Muhammad T. Afzal, Shouyong Lai, Ying H. Chui, Ghulam Pirzada, Experimental evaluation of wood I-joists with web holes, Forest Prod. J. 56 (10) (2006) 26–30.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Experimental evaluation on mechanical performance of OSB webbed parallel strand bamboo I-joist with holes in the web Guo Chen ⇑, Hai-tao Li, Tao Zhou, Cheng-long Li, Yu-qi Song, Rui Xu College of Civil Engineering, Nanjing Forestry University, Nanjing, Jiangsu Province 210037, China
h i g h l i g h t s Effect of the holes in the web on the OSB webbed parallel strand bamboo I-joists was investigated. The hole height limit for the joists with no obvious strength reduction effect was proposed. The paper addresses the problem of critical clear distance between the two holes in the web.
a r t i c l e
i n f o
Article history: Received 4 May 2015 Received in revised form 12 August 2015 Accepted 12 October 2015
Keywords: Parallel strand bamboo Web opening I shaped joist Mechanical performance Failure mechanism
a b s t r a c t Holes are often made in the web of the OSB webbed bamboo I-joists for the passage of plumbing lines and ventilation systems. A comprehensive study was conducted in order to determine effect of the parameters, such as the size, shape and clear distance of the holes, on the behavior of the joists. A total of 92 OSB webbed bamboo joists were tested to failure to investigate the failure modes and failure mechanism of them. The results showed that holes in the web have considerable effect on the strength reductions of specimens. Holes sharply changes the stress state and reduces the strength evidently due to the high shear stresses appearing in the vicinity of the hole. With the increase of ratio of the hole size to web height, the peak load capacity and stiffness of the joists with holes decreased significantly. A web hole with sharp corners caused greater stress concentrations than the circular hole. Results from the tests on I shaped joists with two circular and two square web holes with a size of 50% web depth showed that the critical clear distance is about 2 times and 2.5 the web hole size respectively. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Bamboos is considered as one of fastest-growing plants in the world, due to a unique rhizome-dependent system [1,2]. And it is principally tropical and subtropical in distribution, especially in India and China. Historically, it is generally used in nonstructural applications, such as furniture, craft, scaffolding and so on [3]. Due to their higher strength/weight ratio than common wood and structural steel, bamboos have prerequisite as a modern structural materials [4]. Exploited moderately and making it rationally will further broaden the scope of bamboo utilization. Although bamboo has been applied to the construction of housing for a long time [5–7], it has many flaws, such as thin-walled hollow, diameter of the bamboo culm and easy to crack when exposure to moisture alternation frequently. More important, untreated bamboo is prone to infestation which reduce its service
⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (G. Chen). http://dx.doi.org/10.1016/j.conbuildmat.2015.10.041 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
life of the tradition bamboo houses. Parallel strand bamboo (PSB) is made in China from Moso bamboo, harvested at 3–5 years old. The PSB is an engineered product made by peeling bamboo culms usually to 2–3 mm thickness and 20–30 mm width strips and gluing them together with urea formaldehyde adhesive [8,9]. Similar to glulam, PSB decreases the flaws by distributing them throughout the structural members, which is considered as a promising engineering material [10]. Recently, much attention were focused on the experimental study and engineering application of PSB. Over the past decade, extensive research has been undertaken on bamboo beams. Glue-laminated bamboo (Glubam) girder was firstly developed by Xiao [11] then it was applied to bamboo bridges and buildings [12]. Sinha [13] found the adhesives have effect on the bending strength of bamboo glulam beams, and have slight impact on the stiffness [13]. The maximum allowable load capacity of the Glubam beams with rectangular section was controlled by stiffness other than strength. Moreover, the fibers of the center height of the beam was not fully utilized. To address this issue, Xiao [11] proposed that CFRP should paste to the bottom of
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the Glubam girder [12]. It is evident that the method of pasting CFRP was helpful to increase the flexural rigidity, while the improvement was limited for peak load capacity. Tomasi [14] and Wei [15] has chosen another reinforcement measure, steel bars were laid in the bottom of the bamboo beams. However, the improvement of the allowable capacity was still limited [14,15]. The rectangular beam becomes somewhat obsolete due to the fact that the ultimate strength of bamboo beams was controlled by the outermost bamboo fibers, while normal stress of the fibers closed to the neutral axis was low. Compared to solid joists, prefabricated I-shaped joists are more efficient for structural use where the web is designed to carry most of the shear capacity, and the flanges provide the moment capacity [16]. Test results provided by Wu [17] indicated that applying an external prestressing steel wire on the bamboo composite I-joist helps to reduce the vertical displacement of the joists [17]. However, the function of external prestressing for improving structural rigidity and peak load capacity of the joists is limited. Li [9] proposed a thin-walled steel webbed bamboo I-shaped beam and better ductility can be obtained [18]. Aschheim [19] found bamboo OSB webs would be preferred as a means of reducing deflection. Also, the allowable load of the beam under the serviceability limit state was only 30% of the ultimate limit states [19]. Oriented strand board(OSB) webbed parallel strand bamboo (PSB) joists, which can be used as an alternative to wood I-joists, were present. However, holes are often made in the web of the OSB webbed PSB joists for the passage of plumbing lines and ventilation systems. Obviously, the holes change the load transfer path in the webs, and then bring many disadvantageous effects to shear strength, stability and deformation performance. More important, the difficulty with addressing the question is the lack of detailed guidance of building codes, and design guides for OSB webbed PSB joists with holes. So far, experimental research on OSB webbed
PSB joists with web holes has not been conducted. Although the ASTM D5055 is governing testing procedure for wood I-joists without web holes and the WIJMA (Wood I Joist Manufacturers Association) created testing specification for establishing shear capacities for prefabricated wood I-joists with holes, OSB webbed bamboo joists was not included.
2. Materials and methods Prefabricated OSB webbed PSB I-joists consist of two components, two PSB flanges and the OSB web, which form a I shaped cross-section. The TolkoÒ OSB panels, 9.5 mm in thickness, are engineered wood products which meet the APA quality system certification. Of particular importance to this study are the shear properties of the web materials. Average shear properties through the thickness as determined by rail shear test method [20] for OSB is 1420 N/mm2. The bending strength and modulus of elasticity of the OSB is 25.1 MPa and 3.56 GPa, respectively. The PSB flanges was 2440 mm in length, 25 mm in width, and 35 mm in thickness, with a modulus of elasticity of 10.2 GPa. The web was nailed and bonded to double flanges with outdoor epoxy adhesive. To avoid uneven adhesive layer, it is particularly important to ensure the coating surface of the members was clean and smooth. Consumption of adhesive between each other was 250 g/m2. After then, the OSB panels were connected to the PSB with 40 mm nails at spacing of 150 mm along the panel edges and the edge distance of nails should not less than 10 mm. According to the suggestions of the adhesive manufacturers, the finished specimens were allowed to cure for two weeks at a relative humidity of (60 ± 5)% and a temperature of (20 ± 2)°C prior to testing. Bearing stiffeners are required at the beam ends and also at points of concentrated loads or reactions. Install bearing stiffeners tight against the bottom flange of the I-joist, leaving 5 mm gap at the top. But the load stiffeners had the opposite installation. For each series configuration, either 3 or 4 identical specimens were tested. For example, the specimens of 24I1 series had 4 replicas, which was marked 24I1A, 24I1B, 24I1C and 24I1D respectively. A parameter study of the influence of beam size, relative hole size with respect to web height, hole shape, clear distance between holes was present. Table 1 shows the different hole configuration tested including 84 joists with holes and 8 joists without holes. The joists were divided into five categories. Among them, the specimen 30SI5 and 30SI6 have a square hole with a corner radius of 25 mm and 15 mm respectively. All of the joists, 2000 mm in span, had hole centrically placed with respect to joist height.
Table 1 Details of joists. Joist #
b t (mm mm)
H (mm)
Hole shape
d/hw (%)
l1 (mm)
l (mm)
Number of tests
I
24I1 30I1
59.5 35 69.5 35
240 300
No hole No hole
0 0
– –
– –
4 4
II
24CI1 24CI2 24CI3 24CI4 30CI1 30CI2 30CI3 30CI4
59.5 35 59.5 35 59.5 35 59.5 35 69.5 35 69.5 35 69.5 35 69.5 35
240 240 240 240 300 300 300 300
Circle Circle Circle Circle Circle Circle Circle Circle
25 50 75 100 25 50 75 100
500 500 500 500 500 500 500 500
– – – – – – – –
3 3 3 3 3 3 3 3
III
24SI1 24SI2 24SI3 24SI4 30SI1 30SI2 30SI3 30SI4 30SI5 30SI6
59.5 35 59.5 35 59.5 35 59.5 35 69.5 35 69.5 35 69.5 35 69.5 35 69.5 35 69.5 35
240 240 240 240 300 300 300 300 300 300
Square Square Square Square Square Square Square Square Square with circular edge Square with circular edge
25 50 75 100 25 50 75 100 50 50
500 500 500 500 500 500 500 500 500 500
– – – – – – – – – –
3 3 3 3 3 3 3 3 3 3
IV
24SI5 24SI6 24SI7 24SI8 24SI9
59.5 35 59.5 35 59.5 35 59.5 35 59.5 35
240 240 240 240 240
Square Square Square Square Square
50 50 50 50 50
330 287.5 245 202.5 160
170 255 340 425 510
3 3 3 3 3
V
30CI5 30CI6 30CI7 30CI8 30CI9
69.5 35 69.5 35 69.5 35 69.5 35 69.5 35
300 300 300 300 300
Circle Circle Circle Circle Circle
50 50 50 50 50
270 212.5 155 97.5 40
230 345 460 575 690
3 3 3 3 3
Note: relative hole size with respect to web height (d/hw).
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(a) Category I (f) Bearing stiffeners
(g) Load stiffeners
Fig. 1 (continued)
(b)Category II
The testing was displacement-controlled until failure. The loading rates were to comply with the ASTM D5055 (2010) requirement to cause failure within 8–15 min. The loads, deflections of the mid-span, settlement of the reactions, strain distribution of the bottom, top and sides of the specimens were recorded by TDS-530 data acquisition system and the sampling frequency was 5 HZ. In order to eliminate the system error and ensure the test equipment operating reliably, pre-loading on the specimens was necessary prior to formal testing. From these measurements, the peak load, stiffness, and load at a deflection of L/250 (L is the span between supports) were selected to evaluate the behavior of the I-joists. Due to the difficulties in computing a composite modulus term, the slope of the load–deflection curve in the linear elastic range of testing was defined as the stiffness term.
3. Experimental results 3.1. Failure mode
(c)Category III
(d)Category IV
(e)Category V Fig. 1. Detail diagram of specimens. Experiments were performed on simply supported joists using center loading, aimed at evaluating the behavior of specimens. The testing apparatus is shown in Fig. 1. The specimens were loaded vertically using a actuator with a capacity of 100 kN with an accuracy of ±0.1 kN. The load was transmitted to a steel plate (200 100 20 mm) which was designed to be large enough to ensure no crushing failures occurred at the top flanges. Three Laser Displacement Sensors (LDS) with an accuracy of ±0.1 mm were used for measuring deformations at mid-span and two supports. So the real deformation of the specimens was the difference between the mid-span deflection and the supports. It is extremely complicated to gain the strain field from testing in a region using strain gauges because a strain gauge measures average strain over a limited region of web. For simplicity, strain gauges were glued on the surface of the corners and numbered in a counterclockwise direction, numbering starts from the upper left corner from 1 to 8.
3.1.1. Reverse and delamination of OSB in the flange The damage of all the joists began with reverse. Different failure modes occurred for varying holes types and size. At the beginning of the experiment, the reference specimens (24I1 and 30I1) without holes showed good ability to deform. As the load increased, the specimens started to reverse and the deformation became obvious gradually. When the vertical load reached to 0.7Pu (Pu was defined as the peak load at failure), the wood flakes of OSB panel in the flanges delaminated with great splitting sound, and the OSB board crack continued widening until the cracks spread along the length of the joists. Eventually a few of nails were pulled out or fractured, significant deformation could be observed. Once the joists reached the ultimate strength, the carrying capacity of the joists lost in an instant. In short, damage of joists began frequently with the twist and the appearance of micro crack in the flanges, no visible damage of web and flanges was investigated when the failure of joists has happened. The behavior of the specimens with smaller holes (d/hw 6 25%) was similar to those joists without holes. Even the strain gauges around the holes remained intact without wrinkles. The effect of web opening on the behavior of the specimens could be ignored. In other words, the hole limit ‘‘d/hw < 0.25” for 240 mm and 300 mm high I-joists was 42.5 mm and 57.5 mm, respectively. Similar results were found by Aicher et al. [21] and Ardalany et al. [22], the hole size limit for Glulam and LVL with no obvious strength reduction effect is 50 mm [21,22]. 3.1.2. Shear failure around web holes Introducing a hole in the web drastically changes the stress state and reduces the strength significantly due to the high parallel to the length of joist tensile stresses and shear stresses appearing in the vicinity of the hole. As the hole size increased, the increasingly negative impacts of holes on the performance of specimens (25% < d/hw 6 75%) would severely reduce shear capacity and decrease stiffness. The present of holes reduce cross-section of
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the web, resulting in a bad shear load transfer. During the testing, the specimens with web holes (25% < d/hw 6 75%) had similar failure characteristic. Take specimens with square holes (25% < d/hw 6 75%) as an example, the square holes would became progressively parallelogram with the load increased. The upper right and lower left corners of the holes turned into an obtuse angle, while the other corners were acute angle. Fig. 2 presented that in
the shear dominant parts of the beams, diagonal parts are going into tension (lower left corner and upper right corner) and compression (upper left corner and lower right corner). Shear failure of the web around the holes was the primary mode of failure for the specimens tested. Unlike the sudden fracture flange failures of the wood joist with web openings [23,24], the PSB flanges of the OSB webbed bamboo joists remained intact. As the load increased
(a) 30I1A
(b) 30CI1D
(c) 24CI2C
(d) 24SI2A
(e) 24CI3B
(f) 30CI3A
(g) 24SI4A
(h) 30CI4C Fig. 2. Typical failure modes of specimens.
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3.2. Load–deflection behavior
Fig. 3. Distribution of failure angle for all tests.
and reached the peak load, cracks formed around the holes with continuous loud noise and gradually propagated towards the flanges. The cracking for the circular holes started at an angle about 40–60°, however, for the square holes the cracking started at the corners of the hole that could be due to stress concentration at the corners. The load which associated with this stage is defined as the ‘‘cracking load”, and represented by Pcr. The readings of strain gauges were about 8000le, when the damage of OSB around the holes occurred. The distribution of failure angles for the specimens with web opening (d/hw > 25%) is plotted in Fig. 3, which has an average of 48.5°. 3.1.3. Web pullout For joists with oversize holes (d/hw > 75%), especially the hole size equal to web height, the rest of the web below and above the hole was not enough to resist shear induced by external load. Cracks formed around the holes with continuous loud noise and gradually propagated towards the flanges. Immediately after the web above and below the hole failed due to shearing, the load was transmitted to adjacent sections of the web. The sudden increase in shear stress along the adjacent sections of the web often caused secondary failures of web pullout or nails shear fracture. Amazingly, the specimens tested herein did not break into two parts, while this phenomenon of breaking was common during the tests of wood joists [25]. The main test results of all the specimens are summarized in Table 2.
The load–deflection curves for joists with and without holes are showed in Fig. 4. The present of holes in the web has significantly reduced the strength of the joists, especially for joists with bigger holes (d/hw > 25%). Obviously, the failure procedure of specimens can be classified into three stages. The joists performed linearly until failure, which was a sudden brittle failure. With regard to joists with smaller holes, it can be seen from the data that an abrupt end of the linear plot. However, the non-linear characteristic of load–deflection curves became obvious at the end before failure would occur as the hole size increased. Webs above and below the hole always work before the plastic hinges emerged at the corners of holes. After then, internal force redistributed around the hole, then the flanges resist the shear stress and moment at the same time. It can be explained that the specimens were able to continue to resist external load induced by actuator after the specimens reached ultimate capacity. According to EC5, the allowable deflection for serviceability is L/250 for floor joists with plasterboard [26]. The corresponding load is represented by PL/250. The ratio of PL/250/Pu decreased gradually as the hole size increased, and the ration was in proportion to the height of web. However, the load carrying capacity of wood I joists or bamboo I-joists under serviceability limit state was less than 30% of the strength limit states (see Fig. 5) 3.3. Initial bending stiffness Due to the difficulties in computing a composite modulus term, the slope of the load–deflection curve in the linear elastic range of testing was defined as the initial bending stiffness term (Fig. 6). As observed above in this paper, the cracking load is less than 50% peak load, in other words, it is reasonable that the initial bending stiffness ke is defined as the slope of the curves, for zeroing of data and 0.4Pu with corresponding deflection. It is evident that the stiffness of specimens with large holes (d/hw > 25%) deteriorated progressively as the applied loads increased. Of course, the higher the joist is, the better the initial bending stiffness is. 3.4. Ductility Ductility is more commonly defined as the ability of member to deform easily after yielding, or as the ability of a member to withstand plastic deformation without breaking. Ductility member
Table 2 Summary of carrying capacity and deflection for specimens. Joist #
Pcr (kN)
Pu (kN)
PL/250 (kN)
Dcr (mm)
Du (mm)
Dy (mm)
PL/250/Pu (%)
l
ke (kN/mm)
24I1 24CI1 24CI2 24CI3 24CI4 24SI1 24SI2 24SI3 24SI4 30I1 30CI1 30CI2 30CI3 30CI4 30SI1 30SI2 30SI3 30SI4
18.09 14.83 12.84 9.38 8.56 13.97 10.37 8.34 5.88 19.20 16.49 14.91 11.61 9.95 17.75 15.33 11.86 8.24
23.19 22.13 19.75 15.13 13.17 21.17 17.00 14.13 10.14 26.66 24.99 23.67 19.03 14.85 24.65 22.55 18.24 13.51
14.91 14.92 11.66 8.98 5.87 13.68 10.79 8.04 6.72 23.26 20.51 16.02 12.91 8.04 19.70 15.56 12.34 7.54
9.27 7.99 8.74 8.31 10.68 8.16 7.69 8.27 7.05 6.58 6.62 7.46 7.84 9.51 6.93 7.89 8.01 8.69
12.08 12.54 14.02 14.92 16.10 12.68 15.08 15.95 16.29 9.72 9.90 12.41 13.15 14.79 10.88 12.60 13.48 15.25
12.31 13.05 14.16 15.22 17.22 13.88 16.88 16.87 18.54 10.54 10.47 12.63 13.19 17.95 11.75 12.90 13.73 17.91
64.3 67.4 59.0 59.4 44.6 64.6 63.5 56.9 66.3 87.2 82.1 67.7 62.6 54.1 79.9 69.0 67.7 55.8
1.02 1.04 1.01 1.02 1.07 1.09 1.12 1.06 1.14 1.08 1.06 1.02 1.00 1.21 1.08 1.02 1.02 1.17
1.88 1.83 1.46 1.14 0.83 1.71 1.35 1.01 0.81 2.91 2.56 2.03 1.61 1.07 2.56 1.95 1.54 0.97
Note: Pcr is the mean cracking load, Pu is the mean failure load, ke is Initial flexural stiffness, l is the ductility.
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Fig. 4. Load versus mid-span displacement curves.
Fig. 5. Influence of relative hole size.
show large deformation and ability to absorb energy in the process before fracture [27]. The ductility factor (l) is defined by the formula Du/Dy, where Du is the displacement at failure and Dy is the yield displacement. However, the specimens tested above has no obvious yield platform and load-descend stage. Dy was defined as the mid-span deflection when load equal to 0.85 times the peak load. The results showed that the ductility of joists increased after reinforced by web stiffeners. As expected, the ductility of specimens had nothing to do with the holes. The coefficient of ductility of all the joists were equal, which coincide with the brittle failure of specimens again.
4. Influential factors 4.1. Hole size and shape The cross section of web was weakened by holes, resulting in reduced stiffness of joists significantly. As shown in Fig. 4, the capacity of the joists appeared to decline as the hole size increased. For joists with smaller holes (d/hw 6 25%), the hole had little influence on the carrying capacity, the drop was less than 10%. For d/h > 25%, the influence of holes on the mechanical performance could not be neglected. Especially, for the case of bigger
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97
Fig. 6. Comparison of initial bending stiffness.
to the interaction between holes. It appears that the minimum clear distance between circular holes on which no interaction occurs is twice the hole diameter. This finding agrees with the recommendation by Afzal [29]. For square web holes, the critical clear distance between holes with negligible stress interaction appears to be 2.5 times the hole length.
5. Conclusions
Fig. 7. Influence of clear distance between two holes.
hole (d/hw P 50%), the strength of the joists would suffer large losses, and the vertical deformation was larger compared to joists without holes. During the initial testing stage, the joists kept good performance and the shear deformation was small. With increasing load and deformation of the joists, plastic hinges would be formed in the corners of holes, the shear deformation generated by the upper and lower parts of the holes was significant. For example, the strength reduction of the 240 mm high I-joists was 8.7%, 26.7%, 39.1% and 56.3% for opening diameters d/hw equal to 25%, 50%, 75%, 100%, respectively. The DIN 1052 Code (2008) on web holes suggests that rectangular holes should have rounded corners with greater than 15 mm radius [28]. In view of hole shape, experiments were conducted on circle holes, square holes and square holes with rounded corner. Among all the joists with hole, the radius of curve was 15 mm and 25 mm for 30SI5 and 30SI6 respectively. It is not difficult to find that a web hole with sharp corners caused greater stress concentrations than the circular hole. The cracking load of the joists with sharp corners was about 10% lower than the joist with rounded corners. 4.2. Clear distance between two holes Experiments were performed to provide information of the influence of distance between adjacent openings on strength of I-joists. It is obvious that the clear distance between holes have considerable effect on the behavior of joists and the strengths of all double-holes joists are smaller than those with single hole (Fig. 7). For small hole spacing, the reduced strength is clear due
The static test on OSB webbed bamboo joists with web opening conducted in this study has provided an opportunity to study various parameters that influence the behavior of composite joists. Based on the experimental results obtained, the following conclusions can be summarized. (1) The OSB webbed PSB I joists showed characteristic of brittle failure. Once the joists reached the peak load, the carrying capacity of the joists lost suddenly. Unlike the sudden fracture flange failures of the wood joist with web openings, the PSB flanges of the OSB webbed bamboo joists remained intact. (2) Introducing a hole through the joist drastically changes the stress state and reduces the strength significantly. For circular and square web holes, the critical clear distance between holes with negligible stress interaction appears to be 4 times and 5 times the hole radius respectively. For small holes (d/hw 6 25%), the effect of the hole was negligible and did not cause strength reductions. (3) A web hole with sharp corners caused severe stress concentrations than the circular hole. Whether the corners is rounded off has considerable impact on the cracking load, and has effect on the peak load, but not significantly. The strength of joist with square holes improved by 3–7%, once the corners of holes were rounded off. The bigger the radius is, the greater the improvement. It is conservative that the carrying capacity of joist with a square web hole can be treated the same as a circular hole that inscribes the square hole. (4) Maybe the web around the hole was a weak regions. It is necessary to take remediation measures to recover strength and stiffness, such as plywood or galvanized steel sheets were glue-nailed to the both sides of the joists. The effect of the reinforcement is to increase the load bearing capacity by preventing the cracking to occur, and when it occurs, to reduce the brittleness of the failure.
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Acknowledgments The material presented in this paper is based upon work supported by the National Natural Science Foundation of China under Grant No. 51408312, the China Postdoctoral Science Foundation under Grant Nos. 2013M541679 and 2014T70528, the Natural Science Foundation of Jiangsu Province under Grant No. BK20130982, the Postdoctoral Science Foundation of Jiangsu Province under Grant No. 1301017A, and the College Natural Science Foundation of Jiangsu Province under Grant No. 13KJB560008. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the writer(s) and do not necessarily reflect the views of the foundations. The writers gratefully acknowledge Gao-qin Zhang, Nian-qiang Zhou, Long-long Zhao and so on from the Nanjing Forestry University. References [1] Hai-tao Li, Qi-sheng Zhang, Dong-sheng Huang, et al., Compressive performance of laminated bamboo, Compos. B Eng. 54 (2013) 319–328. [2] Juan.F. Correal, Juan.S. Echeverry, Fernando Ramírez, et al., Experimental evaluation of physical and mechanical properties of glued laminated Guadua angustifolia Kunth, Constr. Build. Mater. 73 (2014) 105–112. [3] K.F. Chung, W.K. Yu, Mechanical properties of structural bamboo for bamboo scaffoldings, Eng. Struct. 24 (2002) 429–442. [4] M. Mahdavi, P.L. Clouston, S.R. Arwade, A low-technology approach toward fabrication of laminated bamboo lumber, Constr. Build. Mater. 29 (2012) 257– 262. [5] Shan Bo, Gao Li, Xiao Yan, et al., Experimental research and application of prefabricated bamboo pole house, J. Hunan Univ.: Nat. Sci. 40 (3) (2013) 7–14 (in Chinese). [6] Yan Xiao, Guo Chen, Bo Shan, et al., Research and application of lightweight glue-laminated bamboo frame structure, J. Build. Struct. 31 (6) (2010) 195–203 (in Chinese). [7] Y. Xiao, G. Chen, L. Feng, Experimental studies on roof trusses made of glubam, Mater. Struct. 47 (2014) 1879–1890. [8] Huang Dongsheng, Zhou Aiping, Bian Yuling, Experimental and analytical study on the nonlinear bending of parallel strand bamboo beams, Constr. Build. Mater. 44 (2013) 585–592. [9] Haitao Li, Jing-wen Su, Qisheng Zhang, et al., Mechanical performance of laminated bamboo column under axial compression, Compos. B Eng. 79 (2015) 374–382. [10] Y. Xiao, R.Z. Yang, B. Shan, Production, environmental impact and mechanical properties of glubam[J], Constr. Build. Mater. 44 (6) (2013) 765–773.
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