Evaluation of OSB webbed laminated bamboo lumber box-shaped joists with a circular web hole

Journal Pre-proof Evaluation of OSB webbed laminated bamboo lumber box-shaped joists with a circular web hole Guo Chen, Jing Wu, Hao Jiang, Tong Zhou, Xiang Li, Yunfei Yu PII:

S2352-7102(19)31541-4

DOI:

https://doi.org/10.1016/j.jobe.2019.101129

Reference:

JOBE 101129

To appear in:

Journal of Building Engineering

Received Date: 10 August 2019 Revised Date:

4 December 2019

Accepted Date: 13 December 2019

Please cite this article as: G. Chen, J. Wu, H. Jiang, T. Zhou, X. Li, Y. Yu, Evaluation of OSB webbed laminated bamboo lumber box-shaped joists with a circular web hole, Journal of Building Engineering (2020), doi: https://doi.org/10.1016/j.jobe.2019.101129. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Author Statement Guo Chen: Supervision, Writing- Original draft preparation, Writing- Reviewing and Editing. Jing Wu: Data curation. Hao Jiang: Investigation. Tong Zhou: Investigation. Xiang Li: Investigation. Yunfei Yu: Software, Validation.

Evaluation of OSB webbed laminated bamboo lumber box-shaped joists with a circular web hole Guo Chen, Jing Wu, Hao Jiang, Tong Zhou, Xiang Li, Yunfei Yu College of Civil Engineering, Nanjing Forestry University, Nanjing 210037, China;

Guo Chen; e-mail: [email protected]

Abstract: Openings are frequently required in the web of oriented strand board (OSB) webbed laminated bamboo lumber (LBL) box-shaped joists for passage of service ducts, plumbing, and wiring. A total of 105 joists with a circular web hole were tested and compared with 7 joists without a web opening (control joists). It was found that for smaller circular holes (D/h w ≤30%), the effect of the hole was negligible and did not cause strength and stiffness reductions. The unreinforced joists with larger web holes (D/h w >30%) failed in a brittle and sudden shear mode, resulting in a reduction in strength. Reinforcing of joists with an opening to stop or prevent crack formation or propagation could be accomplished by using one of three methods including collar shaped oriented strand board patches (C-OSB), two “U” shaped oriented strand board patches (TU-OSB), or collar shaped steel patches (C-SP). Results showed that the C-OSB worked most effectively and returned 2 of 5 series of joists to a strength and stiffness equivalent to the “no hole” condition. The mechanical performance and deformation properties of joists with a web opening could be improved by C-SP, but the improvement was only a small percentage of the total load-carrying capacity. TU-OSB worked effectively for a limited opening diameter (D/h w ≤50%), and was easier to install without removing the service pipes and less expensive than the C-OSB or C-SP options. Good correlation was obtained between experimental results and numerical simulations. Keywords: Laminated Bamboo Lumber (LBL); Oriented Strand Board (OSB); Web Opening; Box-Joist; Reinforcement

1. Introduction Wood I joist is an engineered product, which was successfully introduced into the fields of civil engineering in the 1970s as a substitute for timber beam with rectangular section [1]. The reasons for the popularity for the wood I joists include light weight, ease of installation, a variety of lengths and design options, the possibility of drilling holes through their web, and better environmental impact [2]. Usually, the flanges can be made of laminated veneer lumber or solid wood and the web is typically made from wafer board, plywood or oriented strand board (OSB). There is abundant evidence that OSB is thought to be the most cost-effective web materials [3]. With the environment of the world deteriorating and the forest coverage being dropped rapidly [4], large scale deforestation is severely restricted. Therefore, many countries are facing the difficulty of wood shortages due to over-exploitation of their wood resources over the last few decades, particularly in China. Bamboo is regarded as one of fast-growing plants, widespread in Africa, South America and Southeast Asia, and thus to provide adequate raw materials for the development of bamboo structure

[5-9]. As a sustainable green building material, increasing attention has been paid on the application of laminated bamboo lumber (LBL) in civil engineering, which is made from bamboo [10-15]. Obviously, the effective utilization of engineered bamboo as a substitute to wood will help to mitigate the pressures on the ever-shrinking forests resources and thus, facilitate the conservation of the global environment [16]. Holes in the web of wood joists, usually circular or square shaped, are often introduced to allow passage of services (such as ventilation and plumbing), because of the headroom clearance limitation in wood structure [17]. The flanges and webs of wood I-joists are often designed to carry moment and shear forces, respectively, and the stresses between the flanges and web are transmitted through the flange-web glue line and nails. Therefore, introducing opening in the web of wood joist makes the stress distribution more complicated around the opening and changes the failure modes of the joist, resulting to drastic reduction in capacity for the case of opening located close to the supports and may cause joists to fail in premature sudden and brittle shear failure. In the past decades, a series of experimental researches on structural performance of wood I-joists with opening in the web has been carried out. Extensive previous research evaluated the failure modes and capacity reduction of wood I-joists with opening in the web [18-21]. It was demonstrated that joists with web openings exhibited a loss of stiffness as openings located in shear dominant regions, but little effect on the bending strength of moment critical I-joists. Pirzada et al. [19] studied the influence of the web hole on the performance of wood I-joists and found the typical failure mode was that the OSB fractured from tension zones around the opening. The joist eventually collapsed after the cracks reached the flanges, which is not conducive to protect personnel and property, thus further study and solution are needed urgently. Previous studies showed that the presence of holes in the web had adverse effect on the stress state of joist, therefore could cause a decrease of strengthen drastically. To change this unfavorable situation, several reinforcement techniques were developed, such as attaching steel plates, plywood plates and FRP sheets by adhesive or nails. Morrissey et al. [20] tested the performance of wood I joists with and without openings in the webs and found the steel angles retrofits were an effective way to enhance the strength and stiffness of joists. Hindman et al. [22] found that the I-joists with the web hole and the double-sided cold formed steel reinforcers retained the I-joist strength and 95.1% of the stiffness of the original I-joist. Ardalany et al. [23] used plywood and thin steel plate for reinforcement of laminated veneer lumber beams with holes and concluded that plywood worked more effectively for reinforcing laminated-veneer lumber (LVL) beams. Polocoser et al. [24] has chosen other reinforcement methods, such as OSB collar, U-shaped OSB and side laminated-strand lumber (LSL). Results showed that the OSB collar reinforcement technique was more effective in returning strength than the side LSL technique. The flange material of wood I-joist is typically laminated veneer lumber or solid sawn lumber, which is made of slow-growing wood [1]. Replacing wood by laminated bamboo lumber (LBL) plays an important role in mitigating a contradiction of timber supply and demands, natural forest preservation and improvement of ecological environment. Due to higher strength to weight ratio,

lower cost and locally available, OSB webbed LBL box-shaped joist is introduced, which had potential to replace wood I-joist as roof and floor systems. Cutting web destroys the integrity of the joists, which is very dangerous and may result in catastrophic failure. To address these issues, the joists reinforced by collar shaped oriented strand board patches (C-OSB), two “U” shaped oriented strand board patches (TU-OSB) and collar shaped steel patches (C-SP) around the web opening were tested, respectively. The failure modes, strength and stiffness of joists with/without holes were investigated to evaluate the reinforcement effectiveness of different methods.

2. Materials and Methods 2.1 Materials Laminated bamboo lumber (LBL) is an engineered bamboo product made by peeling bamboo culms usually to 4 mm thickness and 5-8 mm width strips and bonding them together with urea formaldehyde adhesive [25-27]. During the production process of LBL, the natural flaws such as nodes, cracks, irregular shape and thin-walled hollow existing in raw bamboo are spread at random over the strips, causing its strength to be uniform [28-30]. Typically, LBL exhibits less of a tendency to expand or contract under the influence of heat or cold respectively, which means excellent dimensional stability in response to shifting climate conditions [31]. The oriented strand board (OSB), 9.5 mm in thickness, is a multi-layer material made from fast growing Poplar [32]. Both of the C-OSB, TU-OSB and web were manufactured by OSB panels. The tensile strength, compressive strength, modulus of elasticity (MOE), shear properties of OSB was 12.1MPa, 13.6MPa, 3253MPa and 1420N/mm 2 , respectively. The internal bond strength, moisture content, and density of OSB was 0.43MPa, 6.6%, and 580kg/m 3 , respectively. The C-SP, 1 mm in thickness, was made of galvanized steel sheet by cutting a circular hole in the middle. The yield strength and MOE of C-SP is 220MPa and 205GPa, which was provided by the manufacture. The smooth shank nails used in this test are cold-formed from a steel wire. The average bending yield strength of nails obtained by 5% off-set method is 705.2 Mpa.

2.2 Unreinforced joists with/without web opening Prefabricated OSB webbed LBL composite joists with a box-shaped cross-section were introduced [33], which consisted of LBL flanges and OSB webs, as shown in Fig.1. And the dimensions of the flanges were 2440 mm long by 30mm wide by 35mm thick, with a modulus of elasticity (MOE) of 10.24GPa. Two 240mm×9.5mm webs were attached to flanges with phenol formaldehyde resin (Yijiayi New Material Technology Co., Ltd., Jiangsu Province) and smooth-shank nails (2.1mm diameter×38mm long) spaced at 150 mm on center. Due to the LBL flanges is relative weak perpendicular to grain, the shear plug generated by the nails was likely to cause cracking [34]. So, the webs were connected to the flanges with nails at spacing of 100 mm along the web edges and the edge distance of nails was 10 mm. Install bearing stiffeners tight against the bottom flange of the joists [35], leaving 5 mm gap at the top (Fig.1(a)). But the load stiffeners had the opposite installation (Fig.1 (b)). The dimension of bearing stiffeners was identical to load stiffeners, and the

width, thickness and length of stiffeners was 30mm, 35mm and 165mm, respectively. For maximum adhesive bond strength, the liquid adhesive must “wet” the coating surface, flow over and permeate into the surface of OSB webs and LBL flanges. According to the suggestion of manufactures, the consumption of adhesive between each other was 250g/m 2 . During the gluing process, a pressure was applied with the aid of nails. Finally, the finished specimens were kept in laboratory room at (20±2) and (65±5) % relative humidity, before being tested after two weeks.

No gap 35

5mm gap

49

240

49

(a) Bearing stiffener

LDS

170

OSB 9.5 Stiffener Nail

35

170

No gap

240

5mm gap OSB 9.5 Stiffener Nail

35

LBL 35

LBL

(b) Load stiffener

Lateral support

Lateral support Actuator

Strain gauge 4

3 Opening

1 Bearing stiffener l1

2 D

Load stiffener

LDS l2

Bearing stiffener 0.5L

L

(c) OSB webbed LBL box-shaped joists

(d) Photo Fig.1 Test setup (dimensions in mm)

Openings are often made for the passage of ventilation, service ducts and wiring, most of which

are circular. Previous study showed that up to 25 percent of web removal had less influence on the capacity and stiffness of wood I-joists [18, 30]. Therefore, larger opening sizes were considered in this study. To fully understand the mechanical behavior and failure mode of joists with web opening, the varying ratio of circular hole diameter to web height (D/h w =30%, 50% and 75%) and at different locations (250 mm, 500 mm, 750 mm and 850 mm from the left support) were chosen. For D/h w =30%, 50% and 75%, the nominal diameter of circular opening was 51 mm, 85 mm and 127.5 mm, respectively.

2.3 Reinforcement joists with web opening The specimens were divided into three different series (“A”, “B” and “C”). Among which, control joists (series “A1”) had no opening in the web. Series “B” was cut circular hole in the webs, which was common practice in joists to accommodate service pipes. Series “C” was retrofitted by double-sided C-OSB, TU-OSB or C-SP, which had a same size circular hole in the webs. Therefore, the joists with same section sizes as Series “B” were retrofitted by double-sided C-OSB (9.5mm in thickness), TU-OSB (9.5mm in thickness) or C-SP sheets to evaluate the impact of the reinforcement on capacity, stiffness and failure modes. And some structural measures of reinforcement were adopted as suggested [24, 30]. The minimum length of the reinforcement (L r ) on either side of the opening was twice as much as hole diameter (D) and the height of the reinforcement (H r ) was equal to the joist height, as shown in Fig.2. For the TU-OSB reinforcement technique, the minimum gap between the left and right piece was 2~3mm. If not, the two pieces were forced between the flanges; it was possible to cause additional stress on the web-to-flange connection, resulting in delamination. A couple of reinforcements located on either side of the webs were attached to the joist by adhesive and the consumption of adhesive between each other was 250g/m 2 . Then the reinforcements were connected to OSB webs with 50 mm common nails along the panel edges, which provided uniform pressure for the cure of adhesive. The reinforced joists were allowed to cure for two weeks at a temperature (20±2)℃ and a relative humidity (60±5)° prior to testing. Nail

LBL flange

LBL flange

C-OSB or C-SP

(a) Joist reinforced by C-OSB and C-SP

Nail

LBL flange

LBL flange

TU-OSB

(b) Joist reinforced by TU-OSB

(c) C-OSB

(d) C-SP

(e) TU-OSB

Fig.2 Details of web reinforcer Details about the testing matrix and the different joist configurations are given in Tab. 1. In this testing program, a total of 112 joists with five or seven replicates were tested. For example, the specimens of A1 series had 7 replicas, which are designated as A1-1, A1-2, A1-3, A1-4, A1-5, A1-6 A1-7, respectively. Among which, 105 joists were fabricated with openings through the web whilst the others had a solid web. The OSB patches used for the subsequent retrofit was the same as the webs. Joists with a length of 2.4m are often used in the kitchen and toilet floor. Circular holes are frequently required in the web of joists for the passage of water supply and drainage pipes. All joists measured 2000 mm between supports (L=2000 mm) and 240 mm in height (H=240 mm). The C-OSB, TU-OSB and C-SP patches were designed to reinforce the joists with web opening. Sometimes the web of joist may be drilled through by the sub-contractor without permission, then the service pipes are installed in place. A common practice is to remove the joists and replaced by a new one, leading to rising costs of remove and installation. So this may not be a cost-effective solution [17]. Obviously, C-OSB and C-SP were no longer appropriate for this case, so TU-OSB patches were designed to retrofit the joists without removing service pipes. Tab. 1 Details of box-shaped joists Opening location l 1 Joist

Series

Reinforcement

Unreinforced

L r ×H r

(mm)

(mm×mm)

D/h w

(mm) Control joist

Diameter D

Quantity of specimens

A1

-

-

-

-

-

7

B1

250

-

30%

51

-

5

B2

250

-

50%

85

-

5

B3

250

-

75%

127.5

B4

500

-

50%

85

-

5

B5

850

-

50%

85

-

5

B6

500

-

75%

127.5

-

5

5

Joists with opening

C1

250

C-OSB

50%

85

170×240

5

C2

250

TU-OSB

50%

85

170×240

5

C3

250

C-SP

50%

85

170×240

5

C4

250

C-OSB

75%

127.5

255×240

5

C5

250

TU-OSB

75%

127.5

255×240

5

C6

250

C-SP

75%

127.5

255×240

5

C7

500

C-OSB

50%

85

170×240

5

C8

500

TU-OSB

50%

85

170×240

5

C9

500

C-SP

50%

85

170×240

5

C10

850

C-OSB

50%

85

255×240

5

C11

850

TU-OSB

50%

85

255×240

5

C12

850

C-SP

50%

85

255×240

5

C13

500

C-OSB

75%

127.5

255×240

5

C14

500

TU-OSB

75%

127.5

255×240

5

C15

500

C-SP

75%

127.5

255×240

5

Reinforced joists

2.4 Methods The three-point bending experiments were conducted to investigate the mechanical performance and failure modes of simply supported joists using a 100kN hydraulic actuator (Fig. 1(d)). Lateral restraints were applied at the ends of specimens by two pairs of steel supports to prevent rollover. Three Laser Displacement Sensors (LDS) with an accuracy of ±0.1mm were used for continuously monitoring the vertical deformations at mid-span and supports of specimens. All the experimental data was fed to a data acquisition system at a frequency of 10 Hz. To ensure apparatus operating well, pre-loading is necessary before formal testing. Then the specimens should be horizontally and vertically leveled again. The actuator load cell, strain gauges and displacement transducer readings were reset to zero, representing the initial state. The formal loading of specimens was implemented with a displacement control method, according to ASTM D5055-13 [36] requirements with a time of 6~10 min for each joist until failure. The maximum load, stiffness, load at a mid-span deflection of L/360 (5.56 mm) and L/180 (11.11 mm) were chosen to evaluate the joists reinforcement techniques.

3 Experimental observations and results All the joists show similar failure behavior and load-displacement responses, therefore, only the

average load-displacement curves are presented in Fig.3. In case of control joists (series A1), the load-displacement curves of 7 replicates at load-ascending stage are averaged. However, it is hard to calculate the average curve when the load drop is occurring. For simplicity, one of the measured curves of specimens at descending stage is selected as the average descending curve of series A1. The load-displacement curves of joists are linear up to approximately 90% of the ultimate load carrying capacity. Subsequently, the load-displacement curves displays non-linear characteristics until reach the ultimate load carrying capacity, followed by an abrupt and rapid decrease in capacity. Then the observed failure modes in control joists, unreinforced joists with web opening, and reinforced joists with web opening, were detailed which were all different depending on the opening size, opening location and reinforcement scheme. 35

21 14

21 14 7

7 0

C1 C4 C7 C10 C13

28

Load(kN)

28

Load(kN)

35

A1 B1 B2 B3 B4 B5 B6

0

5

10

15

20

25

30

0

35

0

5

(a) Unreinforced joists 35

35

14

25

30

35

21 14 7

7 0

20

C3 C6 C9 C12 C15

28

Load(kN)

Load(kN)

21

15

(b) Joists reinforced by C-OSB

C2 C5 C8 C11 C14

28

10

Mid-span displacement(mm)

Mid-span displacement(mm)

0

5

10

15

20

25

30

35

0

0

5

(c) Joists reinforced by TU-OSB

10

15

20

25

30

35

Mid-span displacement(mm)

Mid-span displacement(mm)

(d) Joists reinforced by C-SP

Fig.3 Average load-displacement curves

3.1 Control joists (solid web) Series A1 represents the control joists without web opening. The load-displacement of curves of seven control joists without hole in the web are plotted in Fig.3 (a). As expected, seven control joists without holes behave almost elastically until reaching the ultimate load carrying capacity, followed by abruptly shear failure in the mid-span of the web. As shown in Fig.4 (a), the failure of these specimens started from shear buckling of the web adjacent to the load point (mid-span), however, the ultimate failure was attributed to a mixture of shear buckling of the webs and separation of OSB in

flanges. The ultimate failure condition is defined as the load at 80% of peak load on the descending portion of the load-displacement curve. Surprisingly, no whole fracture of flanges was observed, which was helpful for protecting personnel from injury or damage. However, this failure phenomenon was very common in the tests of wood I joists [19-20] and bamboo beams [37]. It was observed that only one joist failed at a low load compared to the other control joists, due to the de-bonding of the web-flange joint (Fig.4 (b)).

(a) Web shear failure

(b) De-bonding of web-flange joint

Fig.4 Typical failure modes of control joists without opening

3.2 Joists with web opening and without reinforcement A total of thirty joists with opening were tested with different hole configurations. The presence of web opening has detrimental effect on the mechanical performance of joists, and thereby changes the capacity, stiffness and failure modes of joists. Different failure modes occurred for varying hole sizes and location. The mechanical performance of specimens with smaller opening (D/h w ≤30%) was similar to those joists without opening. The failure was initiated mostly due to the web shear failure at mid-span, as shown in Fig.5 (a). Some of specimens failed in de-bonding and nail withdrawal, which was investigated in the web-flange interface (Fig.5 (b)). Nonetheless, failures in which the nails pulled through the OSB webs occurred, but were seldom observed, as shown in Fig.5 (c). As the tests progressed, failures in the form of nail heads embedding into and pulling out from the OSB webs were also observed (Fig.5 (d)). Introducing a larger opening (D/h w ≥50%) in the web brought a significant reduction in shear capacity and consequently the flange shear capacity became important. The rest of the web below and above the hole was not enough to resist shear induced by external load. The presence of large opening markedly affected the flow of stresses in the webs and the stress field at the opening edge varied from tension to compression. Due to the removal of material from the web, the reduction in shear capacity was relatively large. The cracks firstly appeared almost simultaneously at the two tension corners of opening and developed diagonally towards the flanges, accompanied with noises. Immediately after the cracks reached the flanges, the web above and below the opening could not enough to resist the shear stress then the shear was transmitted to adjacent sections of the webs and flanges. A sudden increase of combination stress in flanges and webs usually caused the secondary failures of de-bonding between them and nails pulled out of the OSB webs, as shown in Fig.5 (e).

Surprisingly, no visible damage in flanges was observed when the failure of joists has happened. The presence of holes reduced cross-section of the web, resulting in a bad shear load transfer. During the testing, the specimens with large holes had similar failure characteristic. With the increase of load, the circular holes gradually became elliptical. Cracking noise was firstly heard at 10.47kN in specimen “B3-2”. Fig.6 shows the strain readings from the strain gauge 1 to 4, diagonal parts are going into tension (lower left corner and upper right corner) and compression (upper left corner and lower right corner), which is coincide with the experimental phenomena (Fig.5 (f)). Cracks formed around the holes with continuous noise and gradually propagated towards the flanges, indicating peak load is reaching soon.

(a) web shear failure

(b) Delamination of OSB in flange

(d) Nail withdrawal

(e) Nail pull through

(c) Debonding of web-flange joint

(f) Web shear failure around opening

Fig.5 Typical failure modes of unreinforced joists with opening 15

Load (kN)

12 9 6

Gauge1 1 Gauge Gauge Gauge2 2 Gauge 3 Gauge 3 Gauge 4

3

Gauge 4

0 -4

-3

-2

-1

0

1

2

3

4

Strain (×103 µε) Fig.6 Load-strain curves around the circular opening in specimen B3-2

3.3 Reinforced joists Three different reinforcement schemes were considered, including TU-OSB, C-OSB and C-SP. Observed failure modes in reinforced joists were different depending on the reinforcement scheme. The joists reinforced by OSB plates behaved similarly. The fracture line at mid-span, as shown in Fig.7 (a), was caused by critical tensile stress after buckling. However, some of specimens failed in de-bonding and nail withdrawal, which was investigated in the web-flange interface, as shown in Fig.7 (b). For joists with opening reinforced by C-SP, tests showed that the C-SP yielded and then OSB cracks formed in the tension concentration zones around the opening. The application of C-SP patches to the web opening could improve the tensile strength significantly, but the improvement in the compressive strength was relatively small. A few nails used to connect the web and reinforcers were pulled out, but the adhesive layers between the webs and reinforcers remained intact, which could be considered to have successfully transferred stresses and acted as one piece with the web around the boundary of the opening. 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.

(a) Shear failure at web opening

(b) Combined compression flange buckling and de-bonding of web-flange joint Fig.7 Typical failure modes of reinforced joists

The U-OSB either prevented the shear cracks initiation or stopped the web shear cracks around the opening from propagating, and thus the effect of opening on the mechanical performance of the joists was negligible. Most of the joists still failed in shear diagonally, which was followed by nail pull through or withdrawal. In the case of C-OSB and TU-OSB on both sides, crack propagation was prevented. In the tests, the failures of specimens (D/h w =50%) with web opening reinforced by C-OSB were mainly governed by mid-span failures in the high moment area of the specimens. Also, the opening locations to the supports from 250mm to 850mm were considered and it was found that no noticeable failure modes changed for specimens. The specimens (B2 and B4) failed in shear failure at the opening location and the rest of the specimens (B5 and B6) failed in combined flexure and shear. The phenomenon of crack initiation and propagation in reinforced joists was similar to the unreinforced joists with larger web opening (D/h w =75%).

4 Discussion

The average and coefficient of variation (COV) of peak load P u , cracking load P cr , stiffness K e , and load at serviceability conditions (P L/180 and P L/360 ) are presented in Tab.2. Prior to failure, no cracking was visible but noises were heard. It is believed that there must have been micro-cracks that developed in the joists before the macro ones seen at failure [38]. The load associated with this stage is defined as the “cracking load”, and represented by P cr . As illustrated in Fig.8, K e is defined as the slope the secant between 10-40 percent of the peak load [39]. The serviceability limit states concerned the deformation property of the floor/roof systems, which is closely related to the comfort of personnel. P L/180 is load at D=L/180 is the serviceability condition for flooring & roofing system [40]subjected to total load (dead load + live load + snow load), P L/360 is load at D=L/360 is the serviceability condition for flooring & roofing system based on live load [41]. The effective of three methods of reinforcement (C-OSB, TU-OSB and C-SP) are considered, in order to restore the full capacity and stiffness of a joist with opening to the control joist (series A1). Load Pu

0.4Pu

α

0.1Pu 0

tanα=Ke

Displacement

Fig.8

Definition of initial stiffness

Tab. 2 Test results of joists Pu

COV

P L/ 360

COV

P L/ 180

COV

P cr

COV

Joist

COV K e (kN/mm)

(kN)

(%)

(kN)

(%)

(kN)

(%)

(kN)

(%)

(%)

A1

28.03

11.0

6.16

9.6

13.85

10.5

25.03

10.4

1.37

10.9

B1

27.30

10.9

5.75

11.2

12.98

9.9

23.95

10.1

1.32

11.0

B2

21.17

8.6

5.51

7.8

12.62

10.6

15.22

9.9

1.22

10.0

B3

18.68

10.2

4.72

10.4

11.12

11.4

11.17

11.1

1.07

10.8

B4

22.89

8.9

5.25

8.3

12.89

8.6

15.43

8.7

1.23

8.7

B5

19.35

12.1

4.69

11.0

11.91

10.2

12.90

10.9

1.16

13.7

B6

18.89

10.5

4.22

9.6

9.75

11.3

12.35

11.4

0.93

12.3

C1

28.63

11.9

6.51

8.9

13.90

12.4

18.49

9.8

1.32

13.3

C2

25.91

8.4

6.31

7.9

13.21

9.5

17.25

10.4

1.31

10.4

C3

25.19

8.1

5.77

8.0

12.83

9.3

16.19

10.1

1.27

10.3

C4

22.38

9.6

6.35

10.5

13.71

10.6

18.16

9.8

1.30

12.2

C5

20.41

7.4

5.71

8.7

12.92

8.3

16.14

9.6

1.25

9.4

C6

19.98

10.2

5.82

12.4

12.65

11.3

14.09

11.5

1.20

12.9

C7

28.82

11.3

6.59

10.5

14.75

10.1

19.92

10.8

1.45

14.2

C8

27.43

9.4

5.95

10.0

13.01

8.7

17.53

9.8

1.34

11.7

C9

24.79

11.4

5.58

10.5

13.29

12.5

12.68

10.1

1.31

13.1

C10

25.90

9.9

6.13

8.6

13.75

9.8

16.05

10.9

1.36

12.1

C11

23.83

8.7

5.87

9.1

12.34

10.1

14.93

9.6

1.27

10.4

C12

22.89

10.8

5.98

10.2

12.92

11.2

13.13

10.4

1.23

12.7

C13

23.70

9.9

6.08

8.3

13.23

9.7

15.10

10.2

1.28

9.8

C14

21.51

9.6

4.89

10.6

11.52

10.7

13.20

11.0

1.17

10.8

C15

19.99

10.1

4.48

9.2

10.89

9.5

11.40

9.5

1.12

11.1

4.1 Diameter of the hole The control joists (Series A1) without web opening had a peak load of 28.03kN and Series B1, B2 and B3 with varying hole size (D/h w =30%, 50% and 75%) had a capacity of 27.30kN, 21.17kN and 18.68kN, indicating a decrease of load carrying capacity by 3%, 24% and 33%, respectively compared to the control joists (Fig. 9). With regards to the size of openings, 30 percent of the web height is the demarcation line for the terms “small” and “large”. For smaller opening (D/h w ≤30%), the impact of web opening was negligible and did not cause a reduction in load carrying capacity, and so no reinforcement was required. Therefore, the opening limit "D/h w ≤0.3" for 240 mm high OSB webbed LBL box-shaped joists was 42.5mm. Similar results were also reported that the limit of 50 mm diameter opening for LVL beams with no obvious strength reduction [23]. Introducing a larger opening (D/h w ≥50%) in the web could cause a significant reduction in capacity and consequently changed the failure mechanism to crack initiation and propagation around opening. The load at the ultimate limit states of joists with opening (Series B3) even decreased up to 67% in comparison with the control joists (Series A1). Cracking around the opening is a sign of ultimate failure of joists about to happen and the average ratio of P cr /P u is 59.8%~89.3%. The cracking load decreases with the increasing of the diameter of circular web hole, especially for bigger hole (D/h w >30%). As shown in Tab.2, the hole diameter in the web has little effect on the stiffness of joists. The

average stiffness of Series A1, B1, B2 and B3 was 1.37kN/mm, 1.32kN/mm, 1.22kN/mm and 1.07kN/mm, respectively. Even a web removal of up to 75% of web height, the reduction of stiffness was about 22% for Series B3 (D/h w =75%) in comparison to Series A1.

Fig. 9 Load versus diameter of circular hole (l 1 =250 mm)

4.2 Location of the hole Based on the test results above, a circular hole in the web reduces the load carrying capacity of a joist, which is depended on the hole size and location. Fig. 10 shows the relationship between load and location of the hole in the box-shaped joists. Series B2, B4 and B5 had a web opening with the same dimension (D/h w =50%) at a different distance from the mid-span, and the peak load was 21.17kN, 22.89kN and 19.35kN, respectively. Usually, the stiffness criterion is a predominant controlling factor in the design and use of longer span wood/bamboo beams, rather than the load carrying capacity [42-46]. In Fig.11, circular hole were of the same size, i.e. half of the web height (D/h w =50%). The joists investigated were under constant shear and circular hole at different locations were subject to different values of moment but the same shear. Therefore, the hole location has little effect on the stiffness of joists, regardless of the opening size and position along the length of joist if the opening was located at least 250 mm away from the supports and concentrated load. There was very little difference in the stiffness of joists reinforced by C-OSB, TU-OSB and C-SP. As expected, the design of OSB webbed LBL box joists is determined by the stiffness, rather than the load carrying capacity.

Fig. 10 Load versus opening location (D/h w =50%)

Fig. 11 Stiffness versus opening location (D/h w =50%)

4.3 Retrofit type A couple of C-OSB attached to the joists could effectively restrained shear cracks around the circular hole from propagating towards the flanges, thus helping to prevent shear failure and improve the capacity of joists. Compared to unreinforced joists with opening, the biggest improvement of load carrying capacity of joists reinforced by C-OSB was 35% (Fig.12). Among the three retrofit methods, the C-OSB reinforcement worked most effectively for opening diameter up to 75% of the web depth and returned 2 of 5 series of joists to a capacity equivalent to the “no hole” condition. However, the TU-OSB and C-SP could effectively improve the capacity of joists with opening diameter less than 50% of the web depth. For joists with web opening (D/h w =75%), the average capacity improvement of joists reinforced by C-OSB, TU-OSB and C-SP were 25%, 14% and 6%, respectively compared to that of the Series B6. It was demonstrated that the C-OSB worked most effectively, the C-OSB was shown to be the most effective to return the strength, TU-OSB was the middle and the C-SP was the worst.

25

Load (kN)

+26%

+35%

-24%

+20% -18%

-31%

-33%

20

30 Control +22%

25

Load (kN)

30 Control

Control Unreinforced C-OSB +34% +25% -33%

15

+20% -18%

-24%

+9% -33%

20

Control Unreinforced TU-OSB +23%

-31%

+14% -33%

15

3

10

10

5

5 A1

B2 C1 B3 C4 B4 C7 B5 C10 B6 C13

3

A1

(a) Joists reinforced by C-OSB

(b) Joists reinforced by TU-OSB

Control Unreinforced C-SP

30 Control +19%

Load (kN)

25

-24%

20

B2 C2 B3 C5 B4 C8 B5 C11 B6 C14

+8% -18%

+7% -33%

+18%

-31%

+6% -33%

15 10

3

5 A1

B2 C3 B3 C6 B4 C9 B5 C12 B6 C15

(c) Joists reinforced by C-SP Fig.12 Comparison of load at ultimate limit state (negative sign means capacity decrease in % compared to control joists

without web opening, and positive sign means improvement after reinforcement)

The average load improvement of Series C13, Series C14 and Series C15 was 36%, 18% and 12% for roof & flooring system subjected to total load (∆=L/180) and 44%, 16% and 6% for roof & flooring system subjected to live load (∆=L/360), respectively compared to those of the load at serviceability conditions of unreinforced Series B6. The experimental results indicated that the load at serviceability conditions of joists with web opening could be improved by three different reinforcement techniques, among which, the C-OSB was most effective, TU-OSB less and C-SP least. 4.4 Prediction of load carrying capacity Many models have been suggested to predict the load carrying capacity of wood I-joists with web opening [30, 38]. Shahnewaz et al. [30] studied the effect of size and location of web openings on wood I-joists and proposed empirical formulas to calculate the capacity of I-Joists with openings. Zhu et al. [38] conducts a study on OSB webbed timber I-beams with circular and square opening, whilst the location of opening has little effect on the reduction of capacity and developed empirical formulas to predict the capacity of joists with web openings. Based on the experimental results, a regression analysis was performed to develop models to estimate the capacity of unreinforced and reinforced box-shaped joists with a circular web hole. As mentioned above, for small circular web hole (D/h w ≤30%), the effect of the hole was negligible and did not cause strength reductions. The reduced capacity due to the presence of circular opening in the web can be determined as follows: For unreinforced joists with web opening (D/h w >30%):

D Fur = 33.05 − 19.16    hw 

(1)

D Fr = 32.9 − 15.3    hw 

(2)

For reinforced joists:

Where, F e is the capacity of unreinforced joists with opening (D/h w ≤30%), F ur and F r is the capacity of unreinforced and reinforced joists, respectively.

(a) Unreinforced joists

(b) Reinforced joists

Fig.13 Comparison of test results and theoretical results

The predictions using these models are compared against the test results in Fig. 13. In case of reinforced joists, the Zhu’s model significantly over-predicts the capacity of joists with small web opening

(D/h w ≤30%), but appropriate for joists with larger opening (D/h w >30%). In case of reinforced joists, Zhu’s model under-estimates the capacity and the deviations between experimental and theoretical model results increase with increasing ratio of diameter of web height (D/h w ) . The Shahnewaz ’s model remarkably over-predicts the capacity of unreinforced and reinforced joists. The proposed approach (equation 1 and 2) are quite suitable with the experimental results.

5 Conclusions In this paper, the test results of OSB webbed laminated bamboo lumber box-shaped joists in the presence of a circular web hole are presented. The observed failure modes are reported, and the reinforcement effect on the joists are evaluated. With regards to the size of openings, 30 percent of the web height is the demarcation line for the terms “small” and “large”. For smaller holes(D/h w ≤30%), the detrimental impact on performance of joists was negligible and did not cause strength reductions. The unreinforced joists with larger opening in the web (D/h w >30%) failed in a brittle and sudden shear mode, resulting to the reduction in strength. The mechanical behavior of reinforced joists could be improved remarkably and failed in web buckling, de-bonding of web-flange joint and nail withdrawal. No catastrophic collapse of box-shaped joists was observed, while this phenomenon is common during the tests of wood joists and bamboo beams. The load carrying capacity of joists decreases linearly with opening size, whilst location of opening has little effect on the reduction of capacity. No significant change was found in stiffness, regardless of the opening size and position along the length of unreinforced joist if the opening was located at least 250mm away from the supports and concentrated load. C-OSB worked most effectively for retrofitting joists with web holes to a strength and stiffness equivalent to the “no hole” condition, as it could effectively prevent the cracks developing prematurely. The mechanical performance and deformation properties of joists with a circular hole could be improved by C-SP reinforcer, but the improvement was only a small percentage of the total load-carrying capacity. TU-OSB worked effective for the limited opening diameter (D/h w ≤50%), but was easier to install

without removing the service pipes and less expensive than the C-OSB and C-SP. The proposed formulation is proved to be a good method for predict the capacity of box-shaped joists with varying configurations. Further study on more joists with other geometrical and loading conditions is needed to provide more information for engineering applications of box-shaped joists.

Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51408312), the Natural Science Foundation of Jiangsu Province (Grant No. BK20130982), and R&D Project of Ministry of Housing and Urban–Rural Development of People’s Republic of China (Grant No. 2018-K5-003).

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steel

bars.

Materials

https://doi.org/10.1179/1432891715Z.0000000001377

Research

Innovations

19:

98-103.

1.

The oriented strand board (OSB) webbed laminated bamboo lumber (LBL) box shaped joist was introduced.

2.

Effect of the web openings on the failure modes and mechanical performance of joists was investigated.

3.

The opening height limit for the joists with no obvious strength and stiffness reduction was suggested.

4.

Reinforcing of joists with opening to stop or prevent crack formation or propagation could be accomplished by using C-OSB, TU-OSB and C-SP.

Conflict of interest The authors declare that they have no conflict of interest.