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Performance of connection system used in lightweight glubam shear wall
Construction and Building Materials 206 (2019) 419–431
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Performance of connection system used in lightweight glubam shear wall R. Wang a, S.Q. Wei b, Z. Li b,⇑, Y. Xiao c,d a
College of Civil Engineering, Hunan University, Changsha, Hunan, China College of Civil Engineering, Nanjing Tech University, Nanjing, Jiangsu, China c Zhejiang Univ.-Univ. of Illinois Institute (ZJUI), Zhejiang University, Haining, Zhejiang, China d Sonny Astani Department of Civil & Environmental Engineering, University of Southern California, CA, USA b
h i g h l i g h t s Structural-used glued laminated bamboo – GluBam and lightweight glubam structures are introduced. Performance of connection systems used in lightweight glubam shear walls are presented. A simple numerical model of lightweight glubam shear wall can be used as a fast design tool is created based on test information of connection system.
a r t i c l e
i n f o
Article history: Received 3 December 2018 Received in revised form 9 February 2019 Accepted 13 February 2019
Keywords: Laminated bamboo Glubam Nail connection Hold-down connection Shear wall Mechanical parameters
a b s t r a c t Industrial-produced laminated bamboo panel is a new type of eco-friendly material used for glubam structures in construction industry. This type of panel can be further cold-glued with different orientation combinations to form various types of glubam structural elements and to meet different structural and architectural requirements. The lightweight glubam frame structure is one of the popular forms of glubam structures which can be produced in factory and installed on site with few labor forces and transportation equipment, making it potentially possible to be widely used in urban or rural areas. Like the lightweight wood-frame structures, the governing structural element determining the lateral resistance capacity of lightweight glubam structures is the frame shear wall, as well as that the shear wall’s lateral resistance strength is determined through the metal connections used. Thus, this study is focused on the performance of connection system used in lightweight glubam shear walls made with thick strip plybamboo boards, the connection system includes panel-frame nail connections, frame-frame connections and hold-down devices, respectively. Yield moment and curves of nails used in lightweight glubam structures were experimentally obtained according to ASTM F1575 standard. The monotonic and cyclic loading tests of panel-frame connections and frame-frame connections with different nail connectors were performed following ASTM D1761 standard. Mechanical parameters (such as elastic stiffness ke, maximum force Fmax, ultimate displacement Du) are obtained. Tests of hold-down connection with two different capacities under monotonic and cyclic loading are also presented in this research. A finite element model of lightweight shear wall based on metal connections information is validated through previous research in literature, and is used to predict the performance of lightweight frame shear walls with various combination of connections and studs, to find a suitable type of glubam or timber-glubam hybrid shear wall corresponding to certain load bearing and deformation requirements. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Bamboo, which is abundant in China, is one of the environmentally friendly construction materials that can be used in residential buildings and industrial structures. However, the natural geomet⇑ Corresponding author. E-mail address: [email protected] (Z. Li). https://doi.org/10.1016/j.conbuildmat.2019.02.081 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
rical shape of bamboo culms makes it difficult to meet the requirements for industrialized production. Thus, a new type of bamboo material, with a trademark of GluBamÒ, has been studied since 2005 by the last author’s research team, with a relatively systematic database established [1]. Glubam is structural used glued laminated bamboo, which is made by industrialized bamboo panel products, like laminated bamboo or ply-bamboo panels. This type of bamboo panel products can be further cold-glued with different
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orientation combinations to form various types of glubam structural elements and to meet various structural and architectural requirements. Several research studies on lightweight glubam structures were initiated ten years ago, including tests and analysis of various structural elements, such as lightweight shear walls, roofs and diaphragms [2–5]. Demonstrated projects were also executed in China and Africa, with good performance record for more than 9 years [6]. A typical prototype of two-story lightweight glubam frame construction is shown in Fig. 1, consisting of four main structural parts, which are a concrete foundation (sometimes concrete block masonry), floor, shear wall and roof systems, respectively. A lightweight glubam frame building can be constructed using the socalled platform construction. The floor of each story is constructed of joists covered with wood-based or ply-bamboo panels. The floor is then used as a construction platform for stacking materials and for erecting the walls of the upper story. The walls are connected to the foundation or the lower story walls through the floor using anchoring devices, typically made with steel bolts. The roof system, usually prefabricated trusses, is attached to the top beam of shear walls with metal connections. Similar to lightweight timber structures, a glubam frame shear wall is the main structural element resisting lateral forces. Thus, experimental research on its overall performance and panelframe connections is an important subject. Existing research [2] indicates that lightweight glubam shear wall has higher strength but limited ductile performance than wood-frame shear wall when using the same nail connections. Thus, in order to obtain a wellbalanced strength and ductility performance of lightweight glubam shear wall, metal connections used in three parts of such structural element were experimentally researched and reported in this paper. The performance of shear wall was modelled based on the test information, and a set of cost-benefit connections was chosen to install the lightweight glubam shear wall with predicted performance. 2. Materials and manufacture of glubam shear walls 2.1. Basic physical and mechanical proprieties As shown in Fig. 2, two types of ply-bamboo boards can be used to make glubam frame elements and sheathing panels of shear wall: thick strip ply-bamboo board laminated by bamboo strips of section about 5–8 mm thick and about 20 mm wide, and thin strip ply-bamboo board laminated by bamboo strips of about 2 mm thick. The direction of the strips (also the direction of bamboo fibers) in thick strip ply-bamboo are normally all along longitudinal direction, while the bidirectional configuration of strips is typical for thin strip ply-bamboo, with the ratio of longitudinal and transverse strips variable [7,8]. The main mechanical properties, including embedment strength with different nails along strip (x direction as denoted in this study) direction, of thick strip ply-bamboo boards used in this research are given in Table 1 [7]. 2.2. Production and metal connections used in glubam shear walls Existing research demonstrated that ply-bamboo boards can be used to construct lightweight frame shear walls, along with suitable metal connections [2,9]. In this research, the 40 mm thick strip ply-bamboo panels were used for the frame of lightweight glubam shear walls, due to the conventional 40 mm 90 mm section size of studs and beams. The glubam shear wall frame can be assembled by studs and top & bottom beams which can be obtained through simply saw-cut from the boards as shown in Fig. 3. The 8 mm thin strip ply-bamboo panel with equal amount of bamboo strips for both directions was used as sheathings panel of shear walls. Similar to wood-frame shear walls, the prefabrication of glubam shear walls are based on three types of connections, which are panel-frame connections, frameframe connections and hold-down connections, respectively. Frame-frame connectors, normally nails, are used to hold the horizontal glubam pieces (top and bottom beams) and vertical pieces (studs) together to form a frame. The frame is then sheathed on one or both sides with sheathing panels (e.g. Orient Strand Boards, Plywood, Ply-bamboo sheets etc.,) by panel-frame connectors, commonly dowel type fasteners (e.g. nails, screws, staples). Hold-down devices can limit global overturning under lateral force and facilitate shear transfer.
Fig. 1. Lightweight glubam frame building.
3. Experimental research on connections The testing matrix of connection system is provided in Table 2. Three types of nails were used in the testing program: 1) ST nails (or T shape steel nails) which can be driven by air-driven gun, 2) common nails and 3) high strength nails, respectively. Three different types of nails were selected for panel-frame connection tests, and two loading configurations were conducted according to the angle between the loading direction and the fiber direction of glubam frame. Two types of nails with the same length were used in frame-frame connection tests. Two types of hold-down connections with the same hold-down device but different capacities of bottom anchor bars were tested in this research. All connections tested in this research were intended to be based on real construction practice and totally 76 tests were finished with the aim of establishing a relative comprehensive database for the further modelling and design purpose. 3.1. Test setup and loading protocol 3.1.1. Yield moment of nails The capacity of panel-frame connections is determined by the bending strength of the fastener and the embedment strength of panel & frame under the fastener [10]. Bending yield moment of nails were determined through center-point bending test method [11] in this research. Fig. 4 shows the test apparatus. The length between nail bearing points sbp was selected in accordance with ASTM F1575-03 [11]. The load was applied to the nail at the center between the two bearing points, and the rate of loading was 2 mm/ min. 3.1.2. Lateral resistance of nail connections and hold-down connections As shown in Fig. 5a and b, steel jigs were designed to apply monotonic and cyclic load on panel-frame nail connections. Two different types of panel-frame connections were tested under monotonic and cyclic loading: panel-stud and panel-beam connections, representative of connections loading parallel to bamboo strip direction of glubam frame and perpendicular to bamboo strip
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z Thickness
y x Main bamboo
Zero bamboo fiber direction
fiber direction
z Thickness
y x Main bamboo
Less bamboo fiber direction
(a)
fiber direction
(b)
Fig. 2. Engineered ply-bamboo boards with different strips: (a) thick strip ply-bamboo board, (b) thin strip ply-bamboo board.
Table 1 Mechanical properties of thick strip ply-bamboo panels. Property Tension strength Compression strength
Static bending
Main fiber direction ft,x Main fiber direction fc,x Zero fiber direction fc,y Thickness direction fc,z Bending around z-axis Bending around y-axis
Shear strength Embedment strength (along x-axis as in Fig. 2)
MOE MOR MOE MOR
Parallel to glue line sx,z ST 46 nail 50 mm high strength nail 60 mm common nail
Mean (MPa)
SD (MPa)
COV (%)
85.0 73.0 24.8 25.0 11,200 119.5 10,500 108.9 16.9 73.38 82.95 78.97
12.8 3.4 0.9 1.4 757.7 8.7 1021.1 9.2 2.9 4.51 3.89 2.56
15.1 4.6 3.5 5.5 6.4 7.2 9.3 8.4 9.3 6.1 4.7 3.2
Note: SD: standard deviation; COV: coefficient of variance; MOE: modulus of elasticity; MOR: modulus of rupture.
Application Panel-frame connections
Type of connections ST 46 nails 50 mm high strength nails 60 mm common nails
Frame-frame
80 mm high strength nails
connections
80 mm common nails
Hold-down connections
12 mm screw bars
Fig. 3. Production of glubam shear walls and connection systems.
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Table 2 Test matrix of connection system. Bending yield moment of nails Fastener type
Number of specimens
ST 46 nails 50 mm high strength nails 60 mm common nails 80 mm high strength nails 80 mm common nails
10
Panel-frame connections Fastener type
Sheathing
Loading direction
Number of specimens
ST 46 nails
8 mm plybamboo sheet
1-panel-stud 2-panel-beam 1 2 1 2
3 Monotonic + 5 cyclic
50 mm high strength nails 60 mm common nails Total number of test specimen
48
Frame-frame connections Fastener type
Number of specimens
80 mm high strength nails 80 mm common nails Total number of test specimen
3 Monotonic + 5 cyclic 16
Hold-down connections Fastener type
Number of specimens
12 mm screw bars with 15 & 30 kN capacity Total number of test specimen
3 Monotonic + 3 cyclic 12
d
Load
sbp/2
sbp L
sbp/2
Fig. 4. Center-point bending test for nails.
Fig. 6. Test setup for hold-down connection.
direction, respectively. The steel jigs used for frame-frame connections are the same as panel-beam connections as shown in Fig. 5c. One nail was used to connect the glubam beam and stud. Fig. 6 depicts the test setup of hold-down connections in vertical direction. One hold-down device was used for each connection specimen on one side of glubam frame. The hold-down device is detached from the base to avoid local brittle failure of glubam stud [2]. Monotonic tests of connections were performed under deformation control with a loading rate of 2.5 mm/min for nail connections, and 1 mm/min for hold-down connections, in accordance with the ASTM D1761 standard [12]. The consortium of universities for research in earthquake engineering (CUREE)-Caltech
Fig. 5. Test setup for nail connections used in glubam shear wall: (a) panel-stud connection, (b) panel-beam connection, (c) frame-frame connections.
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Fig. 7. Loading protocol of connection system: (a) nail connection, (b) hold-down connection.
strength nails and ST nails have much higher yield strength than common nails.
Fig. 8. Failure modes of nails under bending tests. (a) ST 46 nails, (b) 50 mm high strength nails, (c) 60 mm common nails, (d) 80 mm high strength nails, (e) 80 mm common nails.
loading protocol [13] was adopted for the cyclic tests with a loading rate of 15 mm/min for nail connections and 3 mm/min for hold-down connections, as shown in Fig. 7. The reference deformation D was taken as 0.6 time of displacement, Dm, which was the displacement corresponding to 80% of the maximum force Fmax. In the cases where the load did not drop to 0.8Fmax, the failure displacement should be used as the monotonic deformation Dm. The program of cyclic loading consists of four parts: the first part was 6 cycles at 0.05D peak displacement; the next was 7 cycles at 0.075 and 0.1D; then, 4 cycles at 0.2 and 0.3D; the number of cycles in the final part was 3 of amplitudes 0.4, 0.7, 1.0, 2.0D, respectively.
3.2. Test results and discussions 3.2.1. Yield moment of nails Fig. 8 shows the failure modes of nails under bending tests. Corresponding moment-deformation curves of different nails are given in Fig. 9. Brittle failure can be found for some ST nails and high strength nails in accordance with Fig. 8, meanwhile relatively good ductile performance can be noticed for common nails. The bending moment M is calculated as Psbp/4, where P is test load, sbp is bearing point spacing. The yield load Py is determined by the 5% offset method that consists: first, fitting a straight line to the initial linear part of load-deformation curve, then offsetting the line by 0.05 times of nail diameter d, and the intersection of load-deformation curve and the offset line is yield load Py. If the offset line does not intersect the curve, then the maximum load Fmax shall be selected as the yield load. The nominal bending yield strength can be calculated as My/S, where My is bending yield moment, and S is effective plastic section modulus for full plastic hinge. For nails, S = d3/6, where d is nail diameter. Table 3 describes the average results of nail bending tests. It can be noticed that high
3.2.2. Lateral resistance of nail connections Table 4 summarized the failure modes of panel-frame connections. The ultimate failure modes of panel-frame connections using ST 46 nail were brittle failure of nail and nail head pulled through of sheathing panel in both panel-stud (parallel to bamboo fiber direction) and panel-beam (perpendicular to bamboo fiber) directions under monotonic loading, whereas in cyclic tests, only was brittle failure of nails. For panel-frame connections using 50 mm high strength nail in monotonic tests, the main damage pattern was nail head pulled through ply-bamboo sheet, however, in few cases in panel-beam direction, yielding failure and withdrawal of nail was observed. In cyclic tests of panel-stud direction, the main failure modes of high strength nail connections were nail head pulled through sheathing panel and brittle failure of nail, whereas in panel-beam direction, only was brittle failure. For 60 mm common nail connections loaded in both directions, the most frequent failure mode was nail head pulled through sheathing panel in monotonic tests, and fatigue failure of nail in cyclic tests. In few cases in panel-stud direction, it was observed yielding and withdrawal of the 60 mm common nail. Fig. 10 summarizes the average load-displacement curves obtained from monotonic and cyclic tests. The envelope curves of hysteretic curves in both positive and negative loading directions were obtained and compared in Fig. 11. The test results show that the angle between the loading direction and the fiber direction of glubam frame member influenced the ultimate strength and ductility of the connections. The bearing capacity of panel-stud connections was slightly lower than that in panel-beam connection tests, but the deformability of panel-stud connections was better. For nail connectors, the bearing capacity and deformability of 50 mm high strength nail and 60 mm common nail connections are shown to outperform those of ST nail connections. For frame-frame connections using 80 mm high-strength nail, the failure mode was bending of nail and split of glubam frame under both monotonic and cyclic loading, as shown in Fig. 12. The main failure mode of frame-frame connections using 80 mm common nail was yielding of nail in monotonic tests and fatigue failure of nail in cyclic tests. Figs. 13 and 14 show the force-displacement curves obtained from frame-frame connection tests. It can be noticed that the bearing capacity of frame-frame connections using high-strength nails was higher than those using common nails. For common nail connections, due to different failure modes, the ultimate deformation in monotonic test was much larger than that under cyclic loading.
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Fig. 9. Deformation-moment curves of nail connections in lightweight glubam shear wall: (a) ST 46 nails, (b) 50 mm high strength nails, (c) 60 mm common nails, (d) 80 mm high strength nails, (e) 80 mm common nails, (f) mean deformation-moment curves of different nails.
Table 3 Nail bending test results. No.
Nail type
Diameter (mm)
Length (mm)
Yield moment (kNmm)
Yield strength (GPa)
1 2 3 4 5
ST 46 nail 50 mm high strength nail 60 mm common nail 80 mm high strength nail 80 mm common nail
2.00 3.36 2.76 4.11 3.47
46.21 50.05 60.32 79.82 79.31
2.01 12.50 2.70 22.52 4.09
1.50 1.97 0.77 1.94 0.59
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For high-strength nail connections, the backbone curve obtained from cyclic test was close to the monotonic one. Table 5 shows the key parameters obtained and calculated from monotonic and cyclic tests for nail connections. The elastic stiffness is ke = Pe/De, where Pe is equal to 0.4Pmax, and De is the corresponding displacement. The maximum force Pmax is obtained from the test, and Dp, max is the corresponding displacement. The ultimate force Pu is the load degraded to the value equal to 0.8Pmax, however in those cases where the force does not drop to 0.8Pmax, then the force when the specimen fails is used as Pu, and Du is the corresponding displacement. The data obtained from test results indicate that the elastic stiffness of panel-stud connections was higher than that of panel-beam connections when using the same nail connector, and the stiffness obtained from cyclic tests was higher than that in monotonic tests. Based on the results of nail bending and nail connection tests, the high strength nail has higher yield strength and relatively good energy dissipation capacity, it is more suitable for glubam shear wall than the common nail, as the glubam material has higher strength than timber. 3.2.3. Hold-down connections Based on existing research by the authors [2], since bamboo is a natural high-strength and brittle material compared with conventional construction materials such as steel or timber, thus ductile connections are required to induce the deformability to glubam structural members. Recent studies [14,15] on cross laminated timber (CLT) shear walls show that the hold-down connection is one of the most important component improving the ductile performance of the walls. Since CLT panels are fairly stiff in comparison with their connections, thus cannot dissipate significant amount of energy when subjected to lateral force, and the ductile performance of CLT shear walls depends mostly on the metal connections [14,15]. For lightweight wood-frame shear walls, the ductile performance also can be influenced by hold-down devices [16– 18]. For a typical 2.44 2.44-m lightweight shear wall, using holddown devices can significantly increase the stiffness and energy dissipation capacity in comparison with the wall without holddown devices when there is no vertical load applied. When a certain vertical load applied (around 10–30 kN/m), the lateral performance of the wall with hold-down devices is similar to that of wall without hold-down devices. Thus, for the experimental research aspect, the lateral tests of shear wall with hold-down device can also be used to describe the performance of shear wall in the ground level of the building (as shown in Fig. 1). The hold-down device designed for lightweight glubam shear wall using ST 46 nails and 50 mm long high-strength nails shown as Fig. 15. The anchor bar used in hold-down tests was 12 mm screw bar. For the restrictions in experimental conditions, particularly due to the limitation of bamboo shear strength, two types of hold-down connections with weakened anchor bars were designed to check the performance of shear walls using hold-down connections with different bearing capacities. The diameter of anchor bars in center part were weakened to 6 mm for 15 kN capacity, and 8.2 mm for 30 kN capacity, respectively. Two 80 mm high strength nails were used to connect glubam frame as frame-frame connection. Fig. 16 describes the failure modes of hold-down connection between glubam frame. Due to the weakening of screw bar, the main failure mode was tension failure of anchor bar, as well as withdrawal of nails. The hold-down device was virtually undamaged. Only one specimen of 30 kN hold-down connection experienced shear failure of glubam frame during the test, as shown in Fig. 16(b). This type of shear failure is because of the limitation of test load condition for specimen and in the actual situation such damage does not occur.
The load-deformation response of hold-down connections is shown in Fig. 17. The ultimate bearing capacity of these two types of hold-down connections are 17.6 kN, and 30.9 kN respectively. The experimental results can be used in the finite element modeling of lateral performance of glubam shear walls. 4. Modelling of shear wall based on connection information Based on mechanical information of the connection system obtained above, the performance of lightweight shear wall can be modelled thorough a relatively simple model as introduced in [19–21]. Thus, a numerical model of lightweight glubam shear wall is established in the finite element program SAP 2000. The model wall is 2.4 m high and 1.2/2.4 m wide, with one-side sheathed 8 mm ply-bamboo panel connected by nails, spaced at 150 mm, to 40 90-mm framing. Studs are spaced at 400 mm center-tocenter. Frame elements are used to model glubam framing members. Sheathing panels are modelled using shell elements, with only membrane actions considered. A multilinear plastic link element is used to model nail connections and hold-down connections. The force-displacement relationship curves of the link element are derived from experiment data in accordance with the loading direction of connections in local coordinates. The ST46 nails and 50 mm high-strength nails are selected as sheathing-to-framing connectors. ST nails, although with lower strength and ductility performance, can be easily installed by airdriven guns and greatly reduce the time of prefabrication of shear walls. Constructing shear walls with high-strength nails can cost more time, because a hole smaller than the nail diameter needs to be drilled in advance in the sheathing panel, then the nail is hammered into it to connect the sheathing to framing. However, the strength and ductility of such nail connections are better than those of ST nails, which can satisfy higher performance requirements of shear walls in seismic design areas. Therefore, the performance of a glubam shear wall with different nails is required to be known with engineering application reasons. In other words, the design of light weight glubam shear wall can always be a selection and combination of panel-frame connections. The FE model given in this research, along with the connection test database, can be a useful tool for shear wall capacity estimation and design. Table 6 provides a list of shear walls with various combination of connections and studs modelled in this research. The shear wall model 1–4 are used to compare with the experiment results given in [9] as to demonstrate the model accuracy. Model 1–4 are woodframe shear walls with ply-bamboo sheathing panels. Model 1–3 are 1.2 2.4-m walls with different nails or nail spacing. Model Table 4 Failure modes of panel-frame connections. Nail type
Loading direction
Loading type
Failure mode
ST 46 nail
Panel-stud/ Panel-beam
Monotonic
50 mm high strength nail
Panel-stud
Cyclic Monotonic Cyclic
Panel-beam
Monotonic
Panel-stud
Cyclic Monotonic
Brittle failure of nail Pull through of nail Brittle failure of nail Pull through of nail Pull through of nail Brittle failure of nail Pull through of nail Yielding & withdrawal of nail Brittle failure of nail Pull through of nail Yielding & withdrawal of nail Fatigue failure of nail Pull through of nail Fatigue failure of nail
60 mm common nail
Panel-beam
Cyclic Monotonic Cyclic
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Fig. 10. Lateral resistance of panel-frame connections: (a) panel-stud connection used ST 46 nail, (b) panel-beam connection used ST 46 nail, (c) panel-stud connection used 50 mm high strength nail, (d) panel-beam connection used 50 mm high strength nail, (e) panel-stud connection used 60 mm common nail, (d) panel-beam connection used 60 mm common nail.
4 is 2.4 2.4-m shear wall with 50 mm common nails. Details of nail and hold-down connections for Model 1–4 shear walls can be found in [10,22]. Model 5–9 are designed to check the influence of the capacity of hold-down connections and vertical load on glubam shear walls. Model 5–8 are walls without vertical load applied, and Model 9 is wall with vertical load applied to the top beam but without hold-down device. Model 9 simulates the wall
in the ground level of the building as shown in Fig. 1, and the value of vertical load is calculated in accordance with the weight of superstructure. Model 10 is intended to provide comparison for wall with different nail connections. Model 11–12 are designed to predict the performance of shear walls with different nail connectors and framing types in one wall, in order to find suitable connection types to achieve a good balance between 1) the bearing
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Fig. 11. Envelope curves of panel-frame connections: (a) panel-stud connections, (b) panel-beam connections.
Fig. 12. Failure modes of 80 mm high strength nail connections.
Fig. 14. Envelope curves of frame-frame connections.
Fig. 13. Lateral resistance of frame-frame connections: (a) 80 mm high strength nail, (b) 80 mm common nail.
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Table 5 Test results of nail connections. Nail type
Loading direction
Loading type
ke (kN/mm)
Pmax (kN)
DP,
ST 46 nail
Panel-stud
Monotonic Cyclic Monotonic Cyclic Monotonic Cyclic Monotonic Cyclic Monotonic Cyclic Monotonic Cyclic Monotonic Cyclic Monotonic Cyclic
0.82 1.15 0.60 0.97 1.45 1.64 0.96 1.08 0.70 1.25 0.45 0.78 0.34 1.57 0.55 0.93
1.60 1.20 1.52 2.02 2.51 3.19 4.22 3.10 2.82 2.48 2.53 3.18 3.09 3.49 2.53 2.50
8.33 6.11 8.58 9.00 20.00 7.74 13.75 9.30 15.16 7.99 14.03 12.37 16.92 13.20 19.79 9.00
Panel-beam 50 mm high strength nail
Panel-stud Panel-beam
60 mm common nail
Panel-stud Panel-beam
80 mm high strength nail
Frame-frame
80 mm common nail
Frame-frame
Fig. 15. Details of hold-down device.
max
(mm)
Pu (kN)
Du (mm)
1.28 – 1.24 – 2.01 – 3.38 – 2.26 – 2.02 – 3.07 – 2.02 –
13.16 – 11.48 – 25.06 – 23.22 – 17.96 – 16.40 – 22.03 – 31.88 –
capacity, 2) ductility requirement and 3) prefabrication time and total cost. Model 11 is glubam shear wall with panel-frame connections using 50 mm high strength nails along the edge spacing 150 mm and ST 46 nails within the panel spacing 150 mm. Model 12 is hybrid shear wall, the top & bottom beam and two end studs were made by glubam panels, and other frame members were made by SPF (Spruce-Pine-Fir). Fig. 18 shows the results obtained from finite element analysis of wood-frame shear walls (wall Model No. 1–4) in comparison with the test results [9], and an acceptable agreement can be found. The model results are slightly higher (about 5%) than the corresponding test ones, which might be due to the sheathing panels assumed to be a rigid panel without local buckling. Fig. 19 shows the simulation results of glubam shear walls with different connections. For wall with ST46 nails (wall Model No. 5–9), the lateral stiffness and bearing capacity can be significantly increased by
Fig. 16. Failure modes of hold-down connection: (a) tension failure, (b) shear failure.
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Fig. 17. Load-deformation response of hold-down connections: (a) 15 kN hold-down connection, (b) 30 kN hold-down connection.
Table 6 Different types of lightweight glubam shear wall modelled based on connection test information. No.
Frame type
Panel type
Wall size
Connection details
Nail spacing
Hold-down or vertical load
1 2 3 4 5 6 7 8 9 10 11
SPF
8 mm ply-bamboo sheets
1.2 2.4-m
50 mm common nails 50 mm staple nails 50 mm staple nails 50 mm common nails ST 46 nails
150 mm/300 mm 150 mm/300 mm 150 mm 150 mm/300 mm 150 mm
50 kN hold-down
12
Thick-strip ply-bamboo (outer)/SPF (inner)
2.4 2.4-m Thick-strip ply-bamboo
50 mm high strength nails 50 mm high strength nails (along the edge)/ST 46 nails (within the panel) ST 46 nails
N 15 kN hold-down 30 kN hold-down 50 kN hold-down 7.5 kN/m 50 kN hold-down 50 kN hold-down
50 kN hold-down
Fig. 18. Finite element simulation results of wood-frame shear walls along with test results: (a) 50 mm common nails, (b) 50 mm staple nails.
using hold-down device and applying vertical load. Hold-down connections with different capacities (wall Model No. 6–8) have little effect on the lateral performance of the glubam shear wall.
The bearing capacity of shear wall with vertical load of 7.5 kN/m (wall Model No. 9) is lower, relative to that of wall with holddown device (wall Model No. 6–8). Thus, it is suggested that when
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Fig. 19. Finite element simulation results of glubam shear walls.
Fig. 20. Force-displacement curves for shear walls with different connection systems.
the superstructure has a light weight or for one-story lightweight glubam structures, it is better to install hold-down device on the walls in the ground level. The hybrid glubam-wood shear wall (wall Model No.12) has similar stiffness and slightly lower bearing capacity when compared to glubam wall (wall Model No. 8), but the application of such hybrid wall can reduce the weight and cost of shear wall. Glubam shear walls using high strength nails (wall Model No. 10) have higher stiffness and strength, and the application of ST46 nails within panels (wall Model No. 11) does not lower the strength obviously and can reduce the prefabrication time. To compare the simulation results with other similar structural system, two types of lightweight wood-frame shear wall experimental results are selected from literature [9,23]. Table 7 describes the details of the shear walls. The frame of test shear walls (T1-2) was made by SPF, and the sheathing panels were 9.5 mm OSB (Oriented Strand Board) panels for T1 and 8 mm ply-bamboo panels for T2. The panel-frame connections used in T1-2 were 50 mm long nails with different materials. The nail spacing for T1-2 were 150 mm along the edge and 300 mm within the panel. All the test shear walls were one-side sheathed and full anchored to the basement using hold-down devices. Because of the similar anchor system, simulation results of wall Model 8, 10–12 are chosen to compare with experimental results. Fig. 20 shows the force-displacement curves of wood-frame and glubam shear walls. The initial stiffness of glubam shear walls (M8,10–12) is close to wood-frame shear wall T1 using similar length of panel-frame nails, and slightly lower than that of wall T2 using the same panels and similar length of nails. The maximum load of glubam shear walls is much higher than that of wood-frame shear walls, while the deformability was less. Based on the comparison, it is indicated that with similar panel-frame connections and anchor system, the bearing capacity of glubam shear walls is higher than that of timber walls, but the ductility capacity is lower.
5. Conclusions An experimental program on mechanical behaviors of connections for glubam shear wall was conducted in this research, with the aim to provide an overall understanding of such connection system, as well as creating a database for future building information modelling (BIM) construction. Connection system of glubam structures should be designed to achieve a good balance between strength and ductility performance. The performance of lightweight shear walls is essentially governed by the characteristics of panel-frame, frame-frame and hold-down connections, especially panel-frame connections. In the testing program, bending yield strength of nails, which is an important parameter affects the performance of nail connections, was studied. The results show that bending yield strength of high-strength nail and ST nail is higher than that of common nail. For panel-frame connections, test results indicate that the angle between the loading direction and the bamboo strip direction of glubam frame influences the strength and ductility performance. The main failure modes are different according to different nail connectors and loading types. For frame-frame connections, the ultimate strength and energy dissipation capacity of high strength nail connection are better than those of common nail connections. Hold-down connection with two different capacities were tested. Based on the test results of connection systems, a simple finite element model which can be used as a simple selection and desin tool of such shear walls was created. The lateral response of shear walls with various combination of nail connectors and frame types are predicted. Simulation results show that the hold-down device and a certain vertical load can increase the stiffness and bearing capacity of shear walls. Glubam shear wall using high-strength nails had better lateral performance than the wall using ST46 nails.
Table 7 Shear wall for comparison. No.
Frame type
Panel type
Wall size
Nails
Nail spacing
Reference
T1 T2 M8 M10 M11
SPF
9.5 mm OSB 8 mm ply-bamboo
2.4 2.4-m
50 mm spiral nails 50 mm common nails ST 46 nails 50 mm high strength nails 50 mm high strength nails (along the edge)/ST 46 nails (within the panel) ST 46 nails
150 mm/300 mm
[23] [9] This paper
M12
Thick-strip glubam
Thick-strip ply-bamboo (outer)/ SPF (inner)
150 mm
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On the premise of not lowering the strength of shear wall, using hybrid walls and ST nails within panels could reduce the cost and the time of prefabrication. A comparative study between test results of wood-frame shear walls and glubam simulation results reveals that glubam shear walls have higher bearing capacity but lower ductility capacity when using similar connection system. Conflict of interest There is no conflict of interest. Acknowledgement This work was supported by National Natural Science Foundation of China [grant numbers 51608262, 51678296]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.conbuildmat.2019.02.081. References [1] Y. Xiao, B. Shan, R.Z. Yang, et al., Glue laminated bamboo (GluBam) for structural applications, Mater. Joints Timber Struct., RILEM (2014) 589–601. [2] R. Wang, Y. Xiao, Z. Li, Lateral loading performance of lightweight glubam shear walls, J. Struct. Eng. (2017) 143(6). [3] Y. Xiao, G. Chen, L. Feng, Experimental studies on roof trusses made of glubam, Mater. Struct. 47 (11) (2013) 1879–1890. [4] G. Chen, Experimental Study and Engineering Application of Light Frame Bamboo Structure [Ph D. thesis], Changsha: Hunan University, 2011 (in Chinese). [5] Y. Xiao, G. Chen, B. Shan, et al., Two-by-four House Construction using Laminated Bamboos, World Conference on Timber Engineering, Riva del Carda, Tento, Italy, 2010. [6] Y. Xiao, B. Shan, Glubam structures, China Architecture & Building Press, Beijing, 2013 (in Chinese).
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[7] Z. Li, G.S. Yang, Q. Zhou, et al., Bending performance of glubam beams made with different processes, Adv. Struct. Eng. (2018). [8] Y. Xiao, L. Feng, X.H. Lv, et al., Experimental studies of glubam columns under axial loads, Ind. Constr. 45 (4) (2015) 13–17 (in Chinese). [9] Y. Xiao, Z. Li, R. Wang, Lateral loading behaviors of lightweight wood-frame shear walls with ply-bamboo sheathing panels, J. Struct. Eng. 141 (3) (2015) B4014004. [10] Z. Li, Y. Xiao, R. Wang, et al., Studies of nail connectors used in wood frame shear walls with ply-bamboo sheathing panels, J. Mater. Civ. Eng. 27 (7) (2015) 04014216. [11] ASTM, ASTM F 1575-2003 Determining Bending Yield Moment of Nails, ASTM International, West Conshohocken, PA, 2003. [12] ASTM, ASTM D1761-12 Standard Test Methods for Mechanical Fasteners in Wood, ASTM International, West Conshohocken, PA, 2012. [13] H. Krawinkler, F. Parisi, L. Ibarra, et al., Development of a Testing Protocol for Wood Frame Structures, CUREE Publication No. W-02, CUREE, Richmond, CA, 2000. [14] I. Gavric, M. Fragiacomo, A. Ceccotti, Cyclic behaviour of typical metal connectors for cross-laminated (CLT) structures, Mater. Struct. 48 (6) (2014) 1841–1857. [15] F. Benedetti, V. Rosales, A. Opazo, Cyclic Testing and Simulation of Hold-down Connections in Radiata Pine CLT Shear Walls, World Conference on Timber Engineering, Vienna, Austria, 2016. [16] D.J. Lebeda, R. Gupta, D.V. Rosowsky, et al., Effect of hold-down misplacement on strength and stiffness of wood shear walls, Pract. Period. Struct. Des. Constr. 10 (2) (2005) 79–87. [17] A.R. Johnston, P.K. Dean, H.W. Shenton, Effects of vertical load and hold-down anchors on the cyclic response of wood framed shear walls, J. Struct. Eng. 132 (9) (2006) 1426–1434. [18] F. Germano, G. Metelli, E. Giuriani, Experimental results on the role of sheathing-to-frame and base connections of a European timber framed shear wall, Constr. Build. Mater. 80 (2015) 315–328. [19] B. Folz, A. Filiatrault, Cyclic analysis of wood shear walls, J. Struct. Eng. 127 (4) (2001) 433–441. [20] J.P. Judd, F.S. Fonseca, Analytical model for sheathing-to-framing connections in wood shear walls and diaphragms, J. Struct. Eng. 131 (2) (2005) 345–352. [21] R. Wang, Y. Xiao, G. Monti, et al. Seismic fragility of glubam shear walls, OpenSees Days-2nd Italian Conference, Salerno, Italy, 2015. [22] R. Wang, Connections of Shear Walls in Modern Bamboo-wood Structurestheoretical Analysis and Experimental Study [Master thesis], Hunan University, Changsha, China, 2013 (in Chinese). [23] J. Durham, F. Lam, H.G.L. Prion, Seismic resistance of wood shear walls with large OSB panels, J. Struct. Eng. 127 (12) (2001) 1460–1466.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Performance of connection system used in lightweight glubam shear wall R. Wang a, S.Q. Wei b, Z. Li b,⇑, Y. Xiao c,d a
College of Civil Engineering, Hunan University, Changsha, Hunan, China College of Civil Engineering, Nanjing Tech University, Nanjing, Jiangsu, China c Zhejiang Univ.-Univ. of Illinois Institute (ZJUI), Zhejiang University, Haining, Zhejiang, China d Sonny Astani Department of Civil & Environmental Engineering, University of Southern California, CA, USA b
h i g h l i g h t s Structural-used glued laminated bamboo – GluBam and lightweight glubam structures are introduced. Performance of connection systems used in lightweight glubam shear walls are presented. A simple numerical model of lightweight glubam shear wall can be used as a fast design tool is created based on test information of connection system.
a r t i c l e
i n f o
Article history: Received 3 December 2018 Received in revised form 9 February 2019 Accepted 13 February 2019
Keywords: Laminated bamboo Glubam Nail connection Hold-down connection Shear wall Mechanical parameters
a b s t r a c t Industrial-produced laminated bamboo panel is a new type of eco-friendly material used for glubam structures in construction industry. This type of panel can be further cold-glued with different orientation combinations to form various types of glubam structural elements and to meet different structural and architectural requirements. The lightweight glubam frame structure is one of the popular forms of glubam structures which can be produced in factory and installed on site with few labor forces and transportation equipment, making it potentially possible to be widely used in urban or rural areas. Like the lightweight wood-frame structures, the governing structural element determining the lateral resistance capacity of lightweight glubam structures is the frame shear wall, as well as that the shear wall’s lateral resistance strength is determined through the metal connections used. Thus, this study is focused on the performance of connection system used in lightweight glubam shear walls made with thick strip plybamboo boards, the connection system includes panel-frame nail connections, frame-frame connections and hold-down devices, respectively. Yield moment and curves of nails used in lightweight glubam structures were experimentally obtained according to ASTM F1575 standard. The monotonic and cyclic loading tests of panel-frame connections and frame-frame connections with different nail connectors were performed following ASTM D1761 standard. Mechanical parameters (such as elastic stiffness ke, maximum force Fmax, ultimate displacement Du) are obtained. Tests of hold-down connection with two different capacities under monotonic and cyclic loading are also presented in this research. A finite element model of lightweight shear wall based on metal connections information is validated through previous research in literature, and is used to predict the performance of lightweight frame shear walls with various combination of connections and studs, to find a suitable type of glubam or timber-glubam hybrid shear wall corresponding to certain load bearing and deformation requirements. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Bamboo, which is abundant in China, is one of the environmentally friendly construction materials that can be used in residential buildings and industrial structures. However, the natural geomet⇑ Corresponding author. E-mail address: [email protected] (Z. Li). https://doi.org/10.1016/j.conbuildmat.2019.02.081 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
rical shape of bamboo culms makes it difficult to meet the requirements for industrialized production. Thus, a new type of bamboo material, with a trademark of GluBamÒ, has been studied since 2005 by the last author’s research team, with a relatively systematic database established [1]. Glubam is structural used glued laminated bamboo, which is made by industrialized bamboo panel products, like laminated bamboo or ply-bamboo panels. This type of bamboo panel products can be further cold-glued with different
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orientation combinations to form various types of glubam structural elements and to meet various structural and architectural requirements. Several research studies on lightweight glubam structures were initiated ten years ago, including tests and analysis of various structural elements, such as lightweight shear walls, roofs and diaphragms [2–5]. Demonstrated projects were also executed in China and Africa, with good performance record for more than 9 years [6]. A typical prototype of two-story lightweight glubam frame construction is shown in Fig. 1, consisting of four main structural parts, which are a concrete foundation (sometimes concrete block masonry), floor, shear wall and roof systems, respectively. A lightweight glubam frame building can be constructed using the socalled platform construction. The floor of each story is constructed of joists covered with wood-based or ply-bamboo panels. The floor is then used as a construction platform for stacking materials and for erecting the walls of the upper story. The walls are connected to the foundation or the lower story walls through the floor using anchoring devices, typically made with steel bolts. The roof system, usually prefabricated trusses, is attached to the top beam of shear walls with metal connections. Similar to lightweight timber structures, a glubam frame shear wall is the main structural element resisting lateral forces. Thus, experimental research on its overall performance and panelframe connections is an important subject. Existing research [2] indicates that lightweight glubam shear wall has higher strength but limited ductile performance than wood-frame shear wall when using the same nail connections. Thus, in order to obtain a wellbalanced strength and ductility performance of lightweight glubam shear wall, metal connections used in three parts of such structural element were experimentally researched and reported in this paper. The performance of shear wall was modelled based on the test information, and a set of cost-benefit connections was chosen to install the lightweight glubam shear wall with predicted performance. 2. Materials and manufacture of glubam shear walls 2.1. Basic physical and mechanical proprieties As shown in Fig. 2, two types of ply-bamboo boards can be used to make glubam frame elements and sheathing panels of shear wall: thick strip ply-bamboo board laminated by bamboo strips of section about 5–8 mm thick and about 20 mm wide, and thin strip ply-bamboo board laminated by bamboo strips of about 2 mm thick. The direction of the strips (also the direction of bamboo fibers) in thick strip ply-bamboo are normally all along longitudinal direction, while the bidirectional configuration of strips is typical for thin strip ply-bamboo, with the ratio of longitudinal and transverse strips variable [7,8]. The main mechanical properties, including embedment strength with different nails along strip (x direction as denoted in this study) direction, of thick strip ply-bamboo boards used in this research are given in Table 1 [7]. 2.2. Production and metal connections used in glubam shear walls Existing research demonstrated that ply-bamboo boards can be used to construct lightweight frame shear walls, along with suitable metal connections [2,9]. In this research, the 40 mm thick strip ply-bamboo panels were used for the frame of lightweight glubam shear walls, due to the conventional 40 mm 90 mm section size of studs and beams. The glubam shear wall frame can be assembled by studs and top & bottom beams which can be obtained through simply saw-cut from the boards as shown in Fig. 3. The 8 mm thin strip ply-bamboo panel with equal amount of bamboo strips for both directions was used as sheathings panel of shear walls. Similar to wood-frame shear walls, the prefabrication of glubam shear walls are based on three types of connections, which are panel-frame connections, frameframe connections and hold-down connections, respectively. Frame-frame connectors, normally nails, are used to hold the horizontal glubam pieces (top and bottom beams) and vertical pieces (studs) together to form a frame. The frame is then sheathed on one or both sides with sheathing panels (e.g. Orient Strand Boards, Plywood, Ply-bamboo sheets etc.,) by panel-frame connectors, commonly dowel type fasteners (e.g. nails, screws, staples). Hold-down devices can limit global overturning under lateral force and facilitate shear transfer.
Fig. 1. Lightweight glubam frame building.
3. Experimental research on connections The testing matrix of connection system is provided in Table 2. Three types of nails were used in the testing program: 1) ST nails (or T shape steel nails) which can be driven by air-driven gun, 2) common nails and 3) high strength nails, respectively. Three different types of nails were selected for panel-frame connection tests, and two loading configurations were conducted according to the angle between the loading direction and the fiber direction of glubam frame. Two types of nails with the same length were used in frame-frame connection tests. Two types of hold-down connections with the same hold-down device but different capacities of bottom anchor bars were tested in this research. All connections tested in this research were intended to be based on real construction practice and totally 76 tests were finished with the aim of establishing a relative comprehensive database for the further modelling and design purpose. 3.1. Test setup and loading protocol 3.1.1. Yield moment of nails The capacity of panel-frame connections is determined by the bending strength of the fastener and the embedment strength of panel & frame under the fastener [10]. Bending yield moment of nails were determined through center-point bending test method [11] in this research. Fig. 4 shows the test apparatus. The length between nail bearing points sbp was selected in accordance with ASTM F1575-03 [11]. The load was applied to the nail at the center between the two bearing points, and the rate of loading was 2 mm/ min. 3.1.2. Lateral resistance of nail connections and hold-down connections As shown in Fig. 5a and b, steel jigs were designed to apply monotonic and cyclic load on panel-frame nail connections. Two different types of panel-frame connections were tested under monotonic and cyclic loading: panel-stud and panel-beam connections, representative of connections loading parallel to bamboo strip direction of glubam frame and perpendicular to bamboo strip
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z Thickness
y x Main bamboo
Zero bamboo fiber direction
fiber direction
z Thickness
y x Main bamboo
Less bamboo fiber direction
(a)
fiber direction
(b)
Fig. 2. Engineered ply-bamboo boards with different strips: (a) thick strip ply-bamboo board, (b) thin strip ply-bamboo board.
Table 1 Mechanical properties of thick strip ply-bamboo panels. Property Tension strength Compression strength
Static bending
Main fiber direction ft,x Main fiber direction fc,x Zero fiber direction fc,y Thickness direction fc,z Bending around z-axis Bending around y-axis
Shear strength Embedment strength (along x-axis as in Fig. 2)
MOE MOR MOE MOR
Parallel to glue line sx,z ST 46 nail 50 mm high strength nail 60 mm common nail
Mean (MPa)
SD (MPa)
COV (%)
85.0 73.0 24.8 25.0 11,200 119.5 10,500 108.9 16.9 73.38 82.95 78.97
12.8 3.4 0.9 1.4 757.7 8.7 1021.1 9.2 2.9 4.51 3.89 2.56
15.1 4.6 3.5 5.5 6.4 7.2 9.3 8.4 9.3 6.1 4.7 3.2
Note: SD: standard deviation; COV: coefficient of variance; MOE: modulus of elasticity; MOR: modulus of rupture.
Application Panel-frame connections
Type of connections ST 46 nails 50 mm high strength nails 60 mm common nails
Frame-frame
80 mm high strength nails
connections
80 mm common nails
Hold-down connections
12 mm screw bars
Fig. 3. Production of glubam shear walls and connection systems.
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Table 2 Test matrix of connection system. Bending yield moment of nails Fastener type
Number of specimens
ST 46 nails 50 mm high strength nails 60 mm common nails 80 mm high strength nails 80 mm common nails
10
Panel-frame connections Fastener type
Sheathing
Loading direction
Number of specimens
ST 46 nails
8 mm plybamboo sheet
1-panel-stud 2-panel-beam 1 2 1 2
3 Monotonic + 5 cyclic
50 mm high strength nails 60 mm common nails Total number of test specimen
48
Frame-frame connections Fastener type
Number of specimens
80 mm high strength nails 80 mm common nails Total number of test specimen
3 Monotonic + 5 cyclic 16
Hold-down connections Fastener type
Number of specimens
12 mm screw bars with 15 & 30 kN capacity Total number of test specimen
3 Monotonic + 3 cyclic 12
d
Load
sbp/2
sbp L
sbp/2
Fig. 4. Center-point bending test for nails.
Fig. 6. Test setup for hold-down connection.
direction, respectively. The steel jigs used for frame-frame connections are the same as panel-beam connections as shown in Fig. 5c. One nail was used to connect the glubam beam and stud. Fig. 6 depicts the test setup of hold-down connections in vertical direction. One hold-down device was used for each connection specimen on one side of glubam frame. The hold-down device is detached from the base to avoid local brittle failure of glubam stud [2]. Monotonic tests of connections were performed under deformation control with a loading rate of 2.5 mm/min for nail connections, and 1 mm/min for hold-down connections, in accordance with the ASTM D1761 standard [12]. The consortium of universities for research in earthquake engineering (CUREE)-Caltech
Fig. 5. Test setup for nail connections used in glubam shear wall: (a) panel-stud connection, (b) panel-beam connection, (c) frame-frame connections.
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Fig. 7. Loading protocol of connection system: (a) nail connection, (b) hold-down connection.
strength nails and ST nails have much higher yield strength than common nails.
Fig. 8. Failure modes of nails under bending tests. (a) ST 46 nails, (b) 50 mm high strength nails, (c) 60 mm common nails, (d) 80 mm high strength nails, (e) 80 mm common nails.
loading protocol [13] was adopted for the cyclic tests with a loading rate of 15 mm/min for nail connections and 3 mm/min for hold-down connections, as shown in Fig. 7. The reference deformation D was taken as 0.6 time of displacement, Dm, which was the displacement corresponding to 80% of the maximum force Fmax. In the cases where the load did not drop to 0.8Fmax, the failure displacement should be used as the monotonic deformation Dm. The program of cyclic loading consists of four parts: the first part was 6 cycles at 0.05D peak displacement; the next was 7 cycles at 0.075 and 0.1D; then, 4 cycles at 0.2 and 0.3D; the number of cycles in the final part was 3 of amplitudes 0.4, 0.7, 1.0, 2.0D, respectively.
3.2. Test results and discussions 3.2.1. Yield moment of nails Fig. 8 shows the failure modes of nails under bending tests. Corresponding moment-deformation curves of different nails are given in Fig. 9. Brittle failure can be found for some ST nails and high strength nails in accordance with Fig. 8, meanwhile relatively good ductile performance can be noticed for common nails. The bending moment M is calculated as Psbp/4, where P is test load, sbp is bearing point spacing. The yield load Py is determined by the 5% offset method that consists: first, fitting a straight line to the initial linear part of load-deformation curve, then offsetting the line by 0.05 times of nail diameter d, and the intersection of load-deformation curve and the offset line is yield load Py. If the offset line does not intersect the curve, then the maximum load Fmax shall be selected as the yield load. The nominal bending yield strength can be calculated as My/S, where My is bending yield moment, and S is effective plastic section modulus for full plastic hinge. For nails, S = d3/6, where d is nail diameter. Table 3 describes the average results of nail bending tests. It can be noticed that high
3.2.2. Lateral resistance of nail connections Table 4 summarized the failure modes of panel-frame connections. The ultimate failure modes of panel-frame connections using ST 46 nail were brittle failure of nail and nail head pulled through of sheathing panel in both panel-stud (parallel to bamboo fiber direction) and panel-beam (perpendicular to bamboo fiber) directions under monotonic loading, whereas in cyclic tests, only was brittle failure of nails. For panel-frame connections using 50 mm high strength nail in monotonic tests, the main damage pattern was nail head pulled through ply-bamboo sheet, however, in few cases in panel-beam direction, yielding failure and withdrawal of nail was observed. In cyclic tests of panel-stud direction, the main failure modes of high strength nail connections were nail head pulled through sheathing panel and brittle failure of nail, whereas in panel-beam direction, only was brittle failure. For 60 mm common nail connections loaded in both directions, the most frequent failure mode was nail head pulled through sheathing panel in monotonic tests, and fatigue failure of nail in cyclic tests. In few cases in panel-stud direction, it was observed yielding and withdrawal of the 60 mm common nail. Fig. 10 summarizes the average load-displacement curves obtained from monotonic and cyclic tests. The envelope curves of hysteretic curves in both positive and negative loading directions were obtained and compared in Fig. 11. The test results show that the angle between the loading direction and the fiber direction of glubam frame member influenced the ultimate strength and ductility of the connections. The bearing capacity of panel-stud connections was slightly lower than that in panel-beam connection tests, but the deformability of panel-stud connections was better. For nail connectors, the bearing capacity and deformability of 50 mm high strength nail and 60 mm common nail connections are shown to outperform those of ST nail connections. For frame-frame connections using 80 mm high-strength nail, the failure mode was bending of nail and split of glubam frame under both monotonic and cyclic loading, as shown in Fig. 12. The main failure mode of frame-frame connections using 80 mm common nail was yielding of nail in monotonic tests and fatigue failure of nail in cyclic tests. Figs. 13 and 14 show the force-displacement curves obtained from frame-frame connection tests. It can be noticed that the bearing capacity of frame-frame connections using high-strength nails was higher than those using common nails. For common nail connections, due to different failure modes, the ultimate deformation in monotonic test was much larger than that under cyclic loading.
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Fig. 9. Deformation-moment curves of nail connections in lightweight glubam shear wall: (a) ST 46 nails, (b) 50 mm high strength nails, (c) 60 mm common nails, (d) 80 mm high strength nails, (e) 80 mm common nails, (f) mean deformation-moment curves of different nails.
Table 3 Nail bending test results. No.
Nail type
Diameter (mm)
Length (mm)
Yield moment (kNmm)
Yield strength (GPa)
1 2 3 4 5
ST 46 nail 50 mm high strength nail 60 mm common nail 80 mm high strength nail 80 mm common nail
2.00 3.36 2.76 4.11 3.47
46.21 50.05 60.32 79.82 79.31
2.01 12.50 2.70 22.52 4.09
1.50 1.97 0.77 1.94 0.59
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For high-strength nail connections, the backbone curve obtained from cyclic test was close to the monotonic one. Table 5 shows the key parameters obtained and calculated from monotonic and cyclic tests for nail connections. The elastic stiffness is ke = Pe/De, where Pe is equal to 0.4Pmax, and De is the corresponding displacement. The maximum force Pmax is obtained from the test, and Dp, max is the corresponding displacement. The ultimate force Pu is the load degraded to the value equal to 0.8Pmax, however in those cases where the force does not drop to 0.8Pmax, then the force when the specimen fails is used as Pu, and Du is the corresponding displacement. The data obtained from test results indicate that the elastic stiffness of panel-stud connections was higher than that of panel-beam connections when using the same nail connector, and the stiffness obtained from cyclic tests was higher than that in monotonic tests. Based on the results of nail bending and nail connection tests, the high strength nail has higher yield strength and relatively good energy dissipation capacity, it is more suitable for glubam shear wall than the common nail, as the glubam material has higher strength than timber. 3.2.3. Hold-down connections Based on existing research by the authors [2], since bamboo is a natural high-strength and brittle material compared with conventional construction materials such as steel or timber, thus ductile connections are required to induce the deformability to glubam structural members. Recent studies [14,15] on cross laminated timber (CLT) shear walls show that the hold-down connection is one of the most important component improving the ductile performance of the walls. Since CLT panels are fairly stiff in comparison with their connections, thus cannot dissipate significant amount of energy when subjected to lateral force, and the ductile performance of CLT shear walls depends mostly on the metal connections [14,15]. For lightweight wood-frame shear walls, the ductile performance also can be influenced by hold-down devices [16– 18]. For a typical 2.44 2.44-m lightweight shear wall, using holddown devices can significantly increase the stiffness and energy dissipation capacity in comparison with the wall without holddown devices when there is no vertical load applied. When a certain vertical load applied (around 10–30 kN/m), the lateral performance of the wall with hold-down devices is similar to that of wall without hold-down devices. Thus, for the experimental research aspect, the lateral tests of shear wall with hold-down device can also be used to describe the performance of shear wall in the ground level of the building (as shown in Fig. 1). The hold-down device designed for lightweight glubam shear wall using ST 46 nails and 50 mm long high-strength nails shown as Fig. 15. The anchor bar used in hold-down tests was 12 mm screw bar. For the restrictions in experimental conditions, particularly due to the limitation of bamboo shear strength, two types of hold-down connections with weakened anchor bars were designed to check the performance of shear walls using hold-down connections with different bearing capacities. The diameter of anchor bars in center part were weakened to 6 mm for 15 kN capacity, and 8.2 mm for 30 kN capacity, respectively. Two 80 mm high strength nails were used to connect glubam frame as frame-frame connection. Fig. 16 describes the failure modes of hold-down connection between glubam frame. Due to the weakening of screw bar, the main failure mode was tension failure of anchor bar, as well as withdrawal of nails. The hold-down device was virtually undamaged. Only one specimen of 30 kN hold-down connection experienced shear failure of glubam frame during the test, as shown in Fig. 16(b). This type of shear failure is because of the limitation of test load condition for specimen and in the actual situation such damage does not occur.
The load-deformation response of hold-down connections is shown in Fig. 17. The ultimate bearing capacity of these two types of hold-down connections are 17.6 kN, and 30.9 kN respectively. The experimental results can be used in the finite element modeling of lateral performance of glubam shear walls. 4. Modelling of shear wall based on connection information Based on mechanical information of the connection system obtained above, the performance of lightweight shear wall can be modelled thorough a relatively simple model as introduced in [19–21]. Thus, a numerical model of lightweight glubam shear wall is established in the finite element program SAP 2000. The model wall is 2.4 m high and 1.2/2.4 m wide, with one-side sheathed 8 mm ply-bamboo panel connected by nails, spaced at 150 mm, to 40 90-mm framing. Studs are spaced at 400 mm center-tocenter. Frame elements are used to model glubam framing members. Sheathing panels are modelled using shell elements, with only membrane actions considered. A multilinear plastic link element is used to model nail connections and hold-down connections. The force-displacement relationship curves of the link element are derived from experiment data in accordance with the loading direction of connections in local coordinates. The ST46 nails and 50 mm high-strength nails are selected as sheathing-to-framing connectors. ST nails, although with lower strength and ductility performance, can be easily installed by airdriven guns and greatly reduce the time of prefabrication of shear walls. Constructing shear walls with high-strength nails can cost more time, because a hole smaller than the nail diameter needs to be drilled in advance in the sheathing panel, then the nail is hammered into it to connect the sheathing to framing. However, the strength and ductility of such nail connections are better than those of ST nails, which can satisfy higher performance requirements of shear walls in seismic design areas. Therefore, the performance of a glubam shear wall with different nails is required to be known with engineering application reasons. In other words, the design of light weight glubam shear wall can always be a selection and combination of panel-frame connections. The FE model given in this research, along with the connection test database, can be a useful tool for shear wall capacity estimation and design. Table 6 provides a list of shear walls with various combination of connections and studs modelled in this research. The shear wall model 1–4 are used to compare with the experiment results given in [9] as to demonstrate the model accuracy. Model 1–4 are woodframe shear walls with ply-bamboo sheathing panels. Model 1–3 are 1.2 2.4-m walls with different nails or nail spacing. Model Table 4 Failure modes of panel-frame connections. Nail type
Loading direction
Loading type
Failure mode
ST 46 nail
Panel-stud/ Panel-beam
Monotonic
50 mm high strength nail
Panel-stud
Cyclic Monotonic Cyclic
Panel-beam
Monotonic
Panel-stud
Cyclic Monotonic
Brittle failure of nail Pull through of nail Brittle failure of nail Pull through of nail Pull through of nail Brittle failure of nail Pull through of nail Yielding & withdrawal of nail Brittle failure of nail Pull through of nail Yielding & withdrawal of nail Fatigue failure of nail Pull through of nail Fatigue failure of nail
60 mm common nail
Panel-beam
Cyclic Monotonic Cyclic
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Fig. 10. Lateral resistance of panel-frame connections: (a) panel-stud connection used ST 46 nail, (b) panel-beam connection used ST 46 nail, (c) panel-stud connection used 50 mm high strength nail, (d) panel-beam connection used 50 mm high strength nail, (e) panel-stud connection used 60 mm common nail, (d) panel-beam connection used 60 mm common nail.
4 is 2.4 2.4-m shear wall with 50 mm common nails. Details of nail and hold-down connections for Model 1–4 shear walls can be found in [10,22]. Model 5–9 are designed to check the influence of the capacity of hold-down connections and vertical load on glubam shear walls. Model 5–8 are walls without vertical load applied, and Model 9 is wall with vertical load applied to the top beam but without hold-down device. Model 9 simulates the wall
in the ground level of the building as shown in Fig. 1, and the value of vertical load is calculated in accordance with the weight of superstructure. Model 10 is intended to provide comparison for wall with different nail connections. Model 11–12 are designed to predict the performance of shear walls with different nail connectors and framing types in one wall, in order to find suitable connection types to achieve a good balance between 1) the bearing
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Fig. 11. Envelope curves of panel-frame connections: (a) panel-stud connections, (b) panel-beam connections.
Fig. 12. Failure modes of 80 mm high strength nail connections.
Fig. 14. Envelope curves of frame-frame connections.
Fig. 13. Lateral resistance of frame-frame connections: (a) 80 mm high strength nail, (b) 80 mm common nail.
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Table 5 Test results of nail connections. Nail type
Loading direction
Loading type
ke (kN/mm)
Pmax (kN)
DP,
ST 46 nail
Panel-stud
Monotonic Cyclic Monotonic Cyclic Monotonic Cyclic Monotonic Cyclic Monotonic Cyclic Monotonic Cyclic Monotonic Cyclic Monotonic Cyclic
0.82 1.15 0.60 0.97 1.45 1.64 0.96 1.08 0.70 1.25 0.45 0.78 0.34 1.57 0.55 0.93
1.60 1.20 1.52 2.02 2.51 3.19 4.22 3.10 2.82 2.48 2.53 3.18 3.09 3.49 2.53 2.50
8.33 6.11 8.58 9.00 20.00 7.74 13.75 9.30 15.16 7.99 14.03 12.37 16.92 13.20 19.79 9.00
Panel-beam 50 mm high strength nail
Panel-stud Panel-beam
60 mm common nail
Panel-stud Panel-beam
80 mm high strength nail
Frame-frame
80 mm common nail
Frame-frame
Fig. 15. Details of hold-down device.
max
(mm)
Pu (kN)
Du (mm)
1.28 – 1.24 – 2.01 – 3.38 – 2.26 – 2.02 – 3.07 – 2.02 –
13.16 – 11.48 – 25.06 – 23.22 – 17.96 – 16.40 – 22.03 – 31.88 –
capacity, 2) ductility requirement and 3) prefabrication time and total cost. Model 11 is glubam shear wall with panel-frame connections using 50 mm high strength nails along the edge spacing 150 mm and ST 46 nails within the panel spacing 150 mm. Model 12 is hybrid shear wall, the top & bottom beam and two end studs were made by glubam panels, and other frame members were made by SPF (Spruce-Pine-Fir). Fig. 18 shows the results obtained from finite element analysis of wood-frame shear walls (wall Model No. 1–4) in comparison with the test results [9], and an acceptable agreement can be found. The model results are slightly higher (about 5%) than the corresponding test ones, which might be due to the sheathing panels assumed to be a rigid panel without local buckling. Fig. 19 shows the simulation results of glubam shear walls with different connections. For wall with ST46 nails (wall Model No. 5–9), the lateral stiffness and bearing capacity can be significantly increased by
Fig. 16. Failure modes of hold-down connection: (a) tension failure, (b) shear failure.
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Fig. 17. Load-deformation response of hold-down connections: (a) 15 kN hold-down connection, (b) 30 kN hold-down connection.
Table 6 Different types of lightweight glubam shear wall modelled based on connection test information. No.
Frame type
Panel type
Wall size
Connection details
Nail spacing
Hold-down or vertical load
1 2 3 4 5 6 7 8 9 10 11
SPF
8 mm ply-bamboo sheets
1.2 2.4-m
50 mm common nails 50 mm staple nails 50 mm staple nails 50 mm common nails ST 46 nails
150 mm/300 mm 150 mm/300 mm 150 mm 150 mm/300 mm 150 mm
50 kN hold-down
12
Thick-strip ply-bamboo (outer)/SPF (inner)
2.4 2.4-m Thick-strip ply-bamboo
50 mm high strength nails 50 mm high strength nails (along the edge)/ST 46 nails (within the panel) ST 46 nails
N 15 kN hold-down 30 kN hold-down 50 kN hold-down 7.5 kN/m 50 kN hold-down 50 kN hold-down
50 kN hold-down
Fig. 18. Finite element simulation results of wood-frame shear walls along with test results: (a) 50 mm common nails, (b) 50 mm staple nails.
using hold-down device and applying vertical load. Hold-down connections with different capacities (wall Model No. 6–8) have little effect on the lateral performance of the glubam shear wall.
The bearing capacity of shear wall with vertical load of 7.5 kN/m (wall Model No. 9) is lower, relative to that of wall with holddown device (wall Model No. 6–8). Thus, it is suggested that when
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Fig. 19. Finite element simulation results of glubam shear walls.
Fig. 20. Force-displacement curves for shear walls with different connection systems.
the superstructure has a light weight or for one-story lightweight glubam structures, it is better to install hold-down device on the walls in the ground level. The hybrid glubam-wood shear wall (wall Model No.12) has similar stiffness and slightly lower bearing capacity when compared to glubam wall (wall Model No. 8), but the application of such hybrid wall can reduce the weight and cost of shear wall. Glubam shear walls using high strength nails (wall Model No. 10) have higher stiffness and strength, and the application of ST46 nails within panels (wall Model No. 11) does not lower the strength obviously and can reduce the prefabrication time. To compare the simulation results with other similar structural system, two types of lightweight wood-frame shear wall experimental results are selected from literature [9,23]. Table 7 describes the details of the shear walls. The frame of test shear walls (T1-2) was made by SPF, and the sheathing panels were 9.5 mm OSB (Oriented Strand Board) panels for T1 and 8 mm ply-bamboo panels for T2. The panel-frame connections used in T1-2 were 50 mm long nails with different materials. The nail spacing for T1-2 were 150 mm along the edge and 300 mm within the panel. All the test shear walls were one-side sheathed and full anchored to the basement using hold-down devices. Because of the similar anchor system, simulation results of wall Model 8, 10–12 are chosen to compare with experimental results. Fig. 20 shows the force-displacement curves of wood-frame and glubam shear walls. The initial stiffness of glubam shear walls (M8,10–12) is close to wood-frame shear wall T1 using similar length of panel-frame nails, and slightly lower than that of wall T2 using the same panels and similar length of nails. The maximum load of glubam shear walls is much higher than that of wood-frame shear walls, while the deformability was less. Based on the comparison, it is indicated that with similar panel-frame connections and anchor system, the bearing capacity of glubam shear walls is higher than that of timber walls, but the ductility capacity is lower.
5. Conclusions An experimental program on mechanical behaviors of connections for glubam shear wall was conducted in this research, with the aim to provide an overall understanding of such connection system, as well as creating a database for future building information modelling (BIM) construction. Connection system of glubam structures should be designed to achieve a good balance between strength and ductility performance. The performance of lightweight shear walls is essentially governed by the characteristics of panel-frame, frame-frame and hold-down connections, especially panel-frame connections. In the testing program, bending yield strength of nails, which is an important parameter affects the performance of nail connections, was studied. The results show that bending yield strength of high-strength nail and ST nail is higher than that of common nail. For panel-frame connections, test results indicate that the angle between the loading direction and the bamboo strip direction of glubam frame influences the strength and ductility performance. The main failure modes are different according to different nail connectors and loading types. For frame-frame connections, the ultimate strength and energy dissipation capacity of high strength nail connection are better than those of common nail connections. Hold-down connection with two different capacities were tested. Based on the test results of connection systems, a simple finite element model which can be used as a simple selection and desin tool of such shear walls was created. The lateral response of shear walls with various combination of nail connectors and frame types are predicted. Simulation results show that the hold-down device and a certain vertical load can increase the stiffness and bearing capacity of shear walls. Glubam shear wall using high-strength nails had better lateral performance than the wall using ST46 nails.
Table 7 Shear wall for comparison. No.
Frame type
Panel type
Wall size
Nails
Nail spacing
Reference
T1 T2 M8 M10 M11
SPF
9.5 mm OSB 8 mm ply-bamboo
2.4 2.4-m
50 mm spiral nails 50 mm common nails ST 46 nails 50 mm high strength nails 50 mm high strength nails (along the edge)/ST 46 nails (within the panel) ST 46 nails
150 mm/300 mm
[23] [9] This paper
M12
Thick-strip glubam
Thick-strip ply-bamboo (outer)/ SPF (inner)
150 mm
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On the premise of not lowering the strength of shear wall, using hybrid walls and ST nails within panels could reduce the cost and the time of prefabrication. A comparative study between test results of wood-frame shear walls and glubam simulation results reveals that glubam shear walls have higher bearing capacity but lower ductility capacity when using similar connection system. Conflict of interest There is no conflict of interest. Acknowledgement This work was supported by National Natural Science Foundation of China [grant numbers 51608262, 51678296]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.conbuildmat.2019.02.081. References [1] Y. Xiao, B. Shan, R.Z. Yang, et al., Glue laminated bamboo (GluBam) for structural applications, Mater. Joints Timber Struct., RILEM (2014) 589–601. [2] R. Wang, Y. Xiao, Z. Li, Lateral loading performance of lightweight glubam shear walls, J. Struct. Eng. (2017) 143(6). [3] Y. Xiao, G. Chen, L. Feng, Experimental studies on roof trusses made of glubam, Mater. Struct. 47 (11) (2013) 1879–1890. [4] G. Chen, Experimental Study and Engineering Application of Light Frame Bamboo Structure [Ph D. thesis], Changsha: Hunan University, 2011 (in Chinese). [5] Y. Xiao, G. Chen, B. Shan, et al., Two-by-four House Construction using Laminated Bamboos, World Conference on Timber Engineering, Riva del Carda, Tento, Italy, 2010. [6] Y. Xiao, B. Shan, Glubam structures, China Architecture & Building Press, Beijing, 2013 (in Chinese).
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[7] Z. Li, G.S. Yang, Q. Zhou, et al., Bending performance of glubam beams made with different processes, Adv. Struct. Eng. (2018). [8] Y. Xiao, L. Feng, X.H. Lv, et al., Experimental studies of glubam columns under axial loads, Ind. Constr. 45 (4) (2015) 13–17 (in Chinese). [9] Y. Xiao, Z. Li, R. Wang, Lateral loading behaviors of lightweight wood-frame shear walls with ply-bamboo sheathing panels, J. Struct. Eng. 141 (3) (2015) B4014004. [10] Z. Li, Y. Xiao, R. Wang, et al., Studies of nail connectors used in wood frame shear walls with ply-bamboo sheathing panels, J. Mater. Civ. Eng. 27 (7) (2015) 04014216. [11] ASTM, ASTM F 1575-2003 Determining Bending Yield Moment of Nails, ASTM International, West Conshohocken, PA, 2003. [12] ASTM, ASTM D1761-12 Standard Test Methods for Mechanical Fasteners in Wood, ASTM International, West Conshohocken, PA, 2012. [13] H. Krawinkler, F. Parisi, L. Ibarra, et al., Development of a Testing Protocol for Wood Frame Structures, CUREE Publication No. W-02, CUREE, Richmond, CA, 2000. [14] I. Gavric, M. Fragiacomo, A. Ceccotti, Cyclic behaviour of typical metal connectors for cross-laminated (CLT) structures, Mater. Struct. 48 (6) (2014) 1841–1857. [15] F. Benedetti, V. Rosales, A. Opazo, Cyclic Testing and Simulation of Hold-down Connections in Radiata Pine CLT Shear Walls, World Conference on Timber Engineering, Vienna, Austria, 2016. [16] D.J. Lebeda, R. Gupta, D.V. Rosowsky, et al., Effect of hold-down misplacement on strength and stiffness of wood shear walls, Pract. Period. Struct. Des. Constr. 10 (2) (2005) 79–87. [17] A.R. Johnston, P.K. Dean, H.W. Shenton, Effects of vertical load and hold-down anchors on the cyclic response of wood framed shear walls, J. Struct. Eng. 132 (9) (2006) 1426–1434. [18] F. Germano, G. Metelli, E. Giuriani, Experimental results on the role of sheathing-to-frame and base connections of a European timber framed shear wall, Constr. Build. Mater. 80 (2015) 315–328. [19] B. Folz, A. Filiatrault, Cyclic analysis of wood shear walls, J. Struct. Eng. 127 (4) (2001) 433–441. [20] J.P. Judd, F.S. Fonseca, Analytical model for sheathing-to-framing connections in wood shear walls and diaphragms, J. Struct. Eng. 131 (2) (2005) 345–352. [21] R. Wang, Y. Xiao, G. Monti, et al. Seismic fragility of glubam shear walls, OpenSees Days-2nd Italian Conference, Salerno, Italy, 2015. [22] R. Wang, Connections of Shear Walls in Modern Bamboo-wood Structurestheoretical Analysis and Experimental Study [Master thesis], Hunan University, Changsha, China, 2013 (in Chinese). [23] J. Durham, F. Lam, H.G.L. Prion, Seismic resistance of wood shear walls with large OSB panels, J. Struct. Eng. 127 (12) (2001) 1460–1466.