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Experimental study of an unsymmetrical prefabricated hybrid steel-bamboo roof truss
Engineering Structures 201 (2019) 109781
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Experimental study of an unsymmetrical prefabricated hybrid steel-bamboo roof truss
T
Zhi Lia, , Tao Lia, Can Wanga, Xiaozhou Hea, Yan Xiaob,c ⁎
a
College of Civil Engineering, Nanjing Tech University, Nanjing, Jiangsu 211816 China Zhejiang Univ.-Univ. of Illinois Institute (ZJUI), Zhejiang University, Haining, Zhejiang 314400, China c Sonny Astani Department of Civil & Environmental Engineering, University of Southern California, Los Angeles, CA 90089, USA b
ARTICLE INFO
ABSTRACT
Keywords: BIM-based design Bamboo Prefabricated Hybrid truss
A 12 m hybrid unsymmetrical steel-glued laminated bamboo (glubam) truss used in a roof construction was designed with building information model (BIM) platform software. The hybrid truss composed of glued laminated bamboo (glubam) as top chords and web members, and steel pipes used as bottom chords. Based on such design outcomes, the manufacture, transportation, prefabrication and installation details, with corresponding advantages, are reported. Experiments of glubam chords and connections were performed. The behaviors of glubam chords along with its load-displacement/strain curves, are given in this research. Experimental results on full-scale truss indicate that it can fulfill the mechanical requirements with respect to deflection limitations and loading bearing capacity. The performance of the truss with end-connection change and under unsymmetrical loading conditions was also experimentally studied and discussed in this research. The prefabricated hybrid truss obtained a relatively right balance between structural requirements and economic benefits, thanks to easy manufacture, transportation, prefabrication, and installation.
1. Introduction and background Timber is a popular material used for truss structure for a long period. Researches related to it, including historic retrofit structures, large-span structures, innovative design methods, and many other aspects. Two historic timber collar roof trusses were surveyed and assessed using non-destructive methods and evaluated structurally by full-scale load-carrying tests made at the laboratory by Branco et al. [1]. Andersen and Dietsch [2] discussed the robustness of large-span timber roof structures, based on findings from failures of two roof structures. Villar et al. [3] presented the optimization by genetic algorithms of the members and joints of heavy timber trusses fastened by mechanical joints with dowels and metal plates. Meanwhile, due to the inherent similarities between timber and round bamboo, roof or space truss made with round bamboo culms has been studied by some researchers. Villegas et al. [4] reported the design and mechanical performance of trusses assembled with culms and slats of Guadua bamboo, which is rich in South America. A space truss made with round bamboo and plastic joints was experimentally researched by Albermani et al. [5]. Besides round raw bamboo made for truss structures mentioned above, engineered bamboo panels can also be used as a structural material with even better cost-benefit performances as discussed in
⁎
[6–11]. These researches indicate that bamboo has very high strengthweight ratio thus can be effectively used for large-span structures with smaller sections compared with wood, such ply-bamboo panels can also be further cold-laminated similar to cross-laminated timber (CLT), to form glubam structural elements and structures with various architectural shapes. Since 2009, Xiao et al. [6] established a relatively comprehensive research database for this type of glubam structures, including mechanical properties, structural elements, and proof construction projects. Based on such engineered bamboo products, glubam, several innovative kinds of research concerning glubam trusses have been conducted recently. Six conventional Howe triangle roof trusses, with a span of 6 m, made with glubam were experimentally researched by Xiao et al. [12]. Two types of 20 m long large-span glubam roof truss with conventionally triangle shape was researched by Xie and Xiao [13]. A hybrid glubam-steel space truss system along with a case-study construction, a rain-shed canopy, was researched by Wu and Xiao [14]. In this research, to the aim of including engineered bamboo as an effective engineered material for large-span structures, a 12 m span hybrid truss was designed with glubam used as the top chords and web members. With 30/2 steel tube (steel tube with 30 mm in diameter and the thickness of steel is 2 mm) used as bottom chords according to the architectural requirements on the lighting system. Such trusses were
Corresponding author. E-mail address: [email protected] (Z. Li).
https://doi.org/10.1016/j.engstruct.2019.109781 Received 3 April 2019; Received in revised form 5 September 2019; Accepted 8 October 2019 0141-0296/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. BIM-based design information management strategy of construction with glubam hybrid truss.
used as structural elements of a roof in a resort construction project located in a mountainous area of Jiangsu, China, with a land area of 32 m × 12 m, as shown in Fig. 1. The steel frame spacing along the length is 4 m, and the span is 12 m. Following the architectural design, the elevation difference between the two ends of the truss is 0.69 m, whereas the value is 3.48 m between the top of the truss and the lower end of it. Thus, the ratio between height and span H/L is 0.29. The horizontal projection length of each parallelogram element, such as the horizontal distance between points 1 and 2 as shown in Fig. 10, is 1 m; in other words, the prefabricated truss can be easily changed to fit different roof span and elevation differences. Compared with the engineered bamboo trusses reported in previous researches [12–16], several advantages can be noticed for the prefabricated hybrid truss studied herein. (1) All glubam elements were made with standard
30 mm thickness ply-bamboo panels (1.2 m × 2.4 m); thus the price of bamboo members is less than the corresponding elements reported in [14] and [15,16]. (2) The length of all elements are less than 1050 mm, thus, without requiring specific transportation and installation equipment as reported in [13], the 12 m span truss can be transported and installed with small equipment and hand tools. (3) The span and elevation difference of the truss can be changed to meet different conditions. 2. Materials and design of the truss With the fast development of industrial bamboo products in recent ten years, now there are two types of commercial available laminated bamboo panels can be effectively used for glubam structures, which are
Table 1 Main properties of thick strip laminated bamboo (in MPa) used in the truss.
ft,x = 119.3 MPa
fm,xz = 104.6 MPa Em,xz = 8682 MPa
Tension
Static bending
σ = 22.0 MPa
fc,x = 57.7 MPa
σ = 7.5 MPa σ = 764 MPa
τyx = 8.3 MPa
Compression
Shear
σ = 4.7 MPa
σ = 1.4 MPa
Embedment strength fh,xz = 65.0 MPa σ = 2.2 MPa
Notes: The coordinate system can be found in Fig. 2 and 3; σ is standard deviation; for bending strength fm,xz, the stress is acting on a plane normal to the x-axis, z is the loading direction; for shear strength τyx, the shear plane is xoz, along the x-axis; for embedment strength fh,xz, the load is acting on a plane normal to the x-axis, the direction of fastener is y-axis, and the value was obtained by 12 mm steel bolt. 2
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z
40
12 0
y
x
90
Fig. 2. Embedment curves of ply-bamboo with 12 mm steel bolts. Table 2 Software used in this research based on the BIM platform.
12 0
30
x
Design tools
Conceptual design Architectural design Structural design Assembly and joint design
SketchUp/ Autodesk® Revit Autodesk® Revit SolidWorks
Communications and building information between the architect, structural engineer, and manufacturer effectively and frictionless transferred thanks to Building Information Model (BIM) based software used during the truss design, as illustrated in Table 2. The BIM-based design information management plan for this project is given in Fig. 1. However, it must be noted that the information transmission still to be realized is denoted as the dashed line in the figure. Up to now, in this information management plan, design information created by various software is yet to be instantly exchanged and shared in an open information platform, due to the lack of internet-based building information platform, lack of auto integration of various software files and limitations on fast-wireless communication technology. More discussions on this BIM-based building information management are beyond the topic of this paper; thus, the following content will be only focused on the truss, as well as robust benefits obtained from this practical BIM-based model, the hybrid roof truss including three elements: glubam members, steel chords and metal connection system. Design files of all elements in the truss can be saved as stl. format, then connected with a 3D printer for model research and further optimal design. Assembly performance of the truss, which is very important for prefabricated structures, was checked through computer software (SolidWorks) as well as a 1:3 3D print model, as shown in Fig. 4(b). In
z 30
Steps
y
Fig. 3. Compression test of laminated bamboo used for glubam.
thick-strip unidirectional laminated bamboo panels [6] and thin-strip cross-laminated bamboo panels [17], respectively. 30 mm thick-strip based laminated bamboo panels were used for glubam elements in this research. Based on the existing research by the first author, the basic mechanical properties of such thick strip laminated bamboo are given in Table 1. Embedment tests of such panel in its bamboo strip direction under 12 mm steel bolts were also performed according to ASTM code [18]. The embedment curves are given in Fig. 2. Compression test results of laminated bamboo used for glubam along its bamboo strip direction are given in Fig. 3. The moisture content of the glubam is around 8.5% in this research.
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(a) 12 m span hybrid truss and its connection system
(b) The 3D Print model of the truss Fig. 4. 12 m span prefabricated hybrid truss: (a) 12 m span hybrid truss and its connection system, (b) The 3D print model of the truss.
Fig. 5. Production of glubam elements used in the hybrid truss: (a) cold lamination, (b) element cut, (c) hole drill, (d) notch open.
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Fig. 6. Test specimen and setup of glubam chords (in mm): (a) top chord, (b) web member, (c) test setup.
conclusion, all building information of the truss (not whole project as indicated from Fig. 1) used in this research can be frictionless transferred between architect and engineers, as well as software and hardware. Based on the models mentioned above, if no errors or conflicts noticed for the assemblage of the truss. Then, as shown in Fig. 5, glubam structural elements for the truss were be produced: (1) two 30 mm thick ply-bamboo panels were cold laminated with compression pressure about 3 MPa for more than 48 h. (2) the 60 mm thick glubam panels were cut to be elements with a height of 60 mm and 120 mm. (3) 12.5 mm holes were drilled for bolts connections. (4) 6 mm U-shaped notches were opened for the installation of steel joint plates into the glubam members, edges of the elements were also cut according to its position in the truss. All glubam truss elements can be produced with conventional wood or CNC machines and finished by two workers in three hours. Steel joints and bottom chords were made in a steel company by Q235 steel (fy = 235 MPa). Assembly of this truss can be finished in four hours by three workers with hand tools, as shown in Fig. 4. The transportation of all elements can be finished by a small truck, with 5 ton carrying ability, indicating that the truss can be easily transferred to the construction place where only rural roads available. Meanwhile, it is convenient for manufacturing and transportation because of the limited types of truss elements. The length of chords can be changed to adopt different elevation differences. The bottom chords made by steel tube can meet the requirements of wire layout in the tube, for lighting systems. The design of end-joint plates considered the connection with the column as shown as connection type a in Fig. 4(a).
3. Load-carrying test of the truss For the hybrid truss studied in this research, all elements were assumed to be under axial compression or tension force during the structural design; meanwhile, test on glubam members and the corresponding connection region were performed, for the further modeling & design purpose. 3.1. Test on chords and connection region 3.1.1. Test on glubam members In order to obtain the behavior of glubam compression members with consideration of the influence of end metal connections, the test specimens were designed as shown in Fig. 6. Three tests were performed for each group. The calculated slenderness ratio ( = l ox / i x ) of top chords & web members were about 55, in which lox is the effective **length, i x = I A is the radius of gyration, I is the second moment of inertia, and A is the area of the cross-section. All the chords were calculated as lattice column members with 120 × 25-mm and 60 × 25-mm sections. According to the effective length method (ELM) suggested in GB500052017 [19], the mean compressive capacity of such column was estimated by the following equation: (1)
Nc = fc A0
coefficient is estimated as 1/1 + ( 80 Compression test results of laminated bamboo used herein are shown in Fig. 3; thus, 57.7 MPa is
)2 .
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z y x
(a)
Web member
Top chord
(b)
(c)
(d)
Fig. 7. Load-displacement (strain) curves and failure modes of glubam chords: (a) load-displacement curves of top chords and web members, (b) load-strain curves of web and top chords, (c) failure of web members, (d) failure of top chords.
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used as the mean value of compression strength in Eq. (1). Based on the information given above, the calculated bearing capacities are 233 kN and 116 kN for top chords and web members, respectively. The mean load-displacement curves of glubam chords under compression force are given in Fig. 7(a). For web members with a section of 60 × 60-mm, due to the differences of failure modes, and relatively large variance of F- curves in y-direction for all three tests, all recorded curves in this direction, denoted as y-1, y-2 and y-3 are given herein. Typical load-strain curves of web member & top chord are given in Fig. 7(b). Stain gauges numbered 3 and 7 were glued along the coldlamination lines, as illustrated in Fig. 7(b). It can be noted that for web members, the differences among recorded deformations of strain gauges 2/3/4, and 6/7/8 are relatively smaller, than these for top chords when the applied load is less than 50 kN. The load-strain curves, along with the recorded failure modes, as shown in Fig. 7(c) and (d), validated the reliability of cold-lamination steps to produce glubam members, as no apparent failure was noticed along the secondary gluelines in all tests performed in this research. All the failures concentrate in the joint’s region, due to local bucking plus embedment failure. Therefore, the moment M and force N in the middle height of the chords can be estimated as:
M=
1 6
+ N= 1 2
1
5
2 5
ab2 E x
Earthquake Engineering (CUREE) [31]. Scaling of the protocol was defined with respect to a reference displacement. The reference displacement for cyclic tests was defined as 60% of the ultimate displacement at which the load dropped to 80% of the maximum load during monotonic tests. In the case where the load did not drop to 80% of the maximum load, the failure displacement should be used as an ultimate displacement of monotonic tests. The program of cyclic loading consists of four parts: the first part was 6 cycles at 0.05 times of reference displacement; the next was 7 cycles at 0.075 and 0.1 times of peak displacement; then, 4 cycles at 0.2 and 0.3 times of peak displacement; the number of cycles in the final part was 3 of amplitudes 0.4, 0.7, 1.0, 2.0 times of peak displacement, respectively. Load and displacement information was recorded by an MTS test machine at a rate of 10 Hz for the cyclic tests. The loading speed was set at 2.5 mm/ min and 7.5 mm/min in the monotonic and cyclic loading tests, respectively. Load-displacement curves of the tested connection, as well as corresponding failure models, are given in Fig. 9. The ultimate tension bearing force is about 60 kN and 90 kN for the web member’s connection and the top one. Glubam elements were torn out accompanied by a considerable split sound, and the crack in the bolt line extended rapidly to the end part of the connections when the failure occurred in the test. Although the tensile strength of laminated bamboo is about two times than the compression strength, as given in Table 1. It is noted that in the connection test, the bearing capacity of top chord connections under compression is slightly higher than the connections under tension.
(2)
ab E x
(3)
3.2. Full-scale test on hybrid truss
where Ex is the modulus of elasticity of glubam under compression, a is the width of the section, b is the height of the section. 1 and 5 are measured strains, as shown in Fig. 7(b).
Test setup of the truss is shown in Fig. 10(a). In this test, steel endsupports connected with the end connection of the truss with four screws, as shown in Fig. 10(b), as such rigid connection is typical of that planned for use in the real construction, due to deflection control reasons. Loading on the truss applied by using iron weight with cables and bags, as shown in Fig. 10(c). The displacement of the loading points, from 1 to 11, is measured by LVDT (Linear Variable Differential Transformer) displacement sensors as shown in Fig. 10(d). The strains of all steel and glubam elements were measured with strain gauges, as shown in Fig. 10(d). The gauge resistance is 120 ± 1 , with gauge sensitivity factor of 2.08 ± 1%. The size of gauges is 7 × 4.5-mm and 55 × 5-mm, for steel and glubam elements, respectively. All these sensors and gauges linked were with one real-time data acquisition system. Based on the geometry information given by Fig. 1, the distance among the trusses is two meters. Meanwhile, the characteristic value of the live load on the roof was 0.5 kN/m2, and the dead load of the hybrid truss was 0.83 kN/m. Therefore, the calculated reference value for loading test Pk, which means the sum of the characteristic value of live load Lk and dead load Dk, was estimated as 2 kN for all eleven loading points as shown in Fig. 10, based on the calculation method suggested in GB/T 50329-2012 [32]. Thus, 200 kg of steel blocks were chosen as the corresponding reference loading mass for each joint. Meanwhile, deflection limitation of the roof truss is l/240 = 50 mm according to Chinese timber design code [19] when Pk applied to it. According to the truss-loading methods suggested in ASTM E73-83 [33] and GB/T 50329-2012 [32], three steps loading procedure was adopted in this research. Such a loading program including three parts, as shown in Fig. 11. T1 is the pre-loading phase, and the aim is to check the condition of the test apparatus and measurement devices. T2 is the standard loading phase, and the aim is to understand the deflection performance of the truss under service load level. T3 is the failure loading step, and the aim is to estimate the ultimate bearing capacity of the truss. In this research, 275 kg/point was chosen as the max static loading mass due to lab limitations. After three loading phases, as shown in Fig. 11(a) finished, if no visible damage noticed for the truss, then, it will be further loaded under following conditions, denoted as
3.1.2. Test on the connection region Existing research on timber truss [20] indicated that the joint region influenced the overall performance of truss structures a lot; meanwhile, optimal design of joints can improve the overall performance of truss structures effectively[12,21–27], as the cost of a truss largely depends on the number of joints. Thus, experimental tests on steel plate-glubam chords connection region of the hybrid truss were performed in this research, in order to obtain joint mechanical information for modeling and optimal design purpose. The test specimen and setup are shown in Fig. 8. Based on the embedment test results given in Fig. 2 and methods suggested by Eurocode 5 [28], the ultimate bearing capacity of the glubam joint connection [29,30] can be estimated as (Eq. (8.11) in EN 1995-1[28]):
fh t1 d Fv = min fh t1 d
2+
4My fh dt12
2.3 My fh t1 d +
1 + Fa x 4 Fa x 4
(4)
where fh is the embedment strength of glubam, which is 65 MPa in this research; t1 is the smaller of the thickness of the glubam side member, which is 25 mm herein; d is the diameter of screws, which is 12 mm; My is the yield moment of the bolts, can be estimated as 0.3 fu d 2.6 , with ultimate tension strength fu = 1000 MPa; Fax is the withdrawal capacity of the fastener, which is zero in this research as only the head part of the high-strength bolts have threads and predrill holes. Thus, the calculated bearing capacity is 39 kN and 58.5 kN for web member connections and top chords connections, respectively. In order to have a relatively comprehensive understanding on mechanical performances of such engineered bamboo joints, three monotonic tests, and one cyclic test were performed for the two types of connection used herein, which are web members connection and top chords connection, respectively. The tests were performed according to the protocol given by the Consortium of Universities for Research in 7
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Fig. 8. Test specimen and setup of joints.
Fig. 9. Load-displacement curves of joints region and failure modes.
the T4 loading phase, shown in Fig. 11(b): (1) unload each loading points to be 100 kg; (2) to change one of the end-joints to be a hinged one by removing two front screws at the end-joint; (3) unsymmetrical loading tests. Real-time displacement and strain values will be recorded for the truss under such conditions.
applied to it. The max displacement measured during the test was 11 mm of point 4 when 275 kg was applied on all eleven loading points. The largest vertical deflection of the truss when Pk applied on it is 6.5 mm, while the deflection limitation is 50 mm. Small deflection increment less than 1 mm, was recorded when Pk applied on it for more than 24 h. Test results obtained in this research indicated that the fastprefabricated hybrid truss fulfills the mechanical requirements of a typical roof span when the truss distance along the length is two meters, whereas the common space for roof-truss used in lightweight timber structures is around 600 mm due to out-of-plane stability reasons [34].
4. Test results A trial test of the truss without any bracing system was performed before the formal test, in order to understand the stability of the truss during transportation and prefabrication. Out-of-plane failure was noticed at the joint 14, as shown in Fig. 12, when 100 kg mass was applied on all loading points. After this test, the bent steel plate was changed, as well as bracing system was used. During the test, in total 11 × 275 kg = 3.025 ton iron mass was
4.1. Load-displacement & strain curves Typical load-displacement curves, as well as strain curves of the chords, are shown in Fig. 13. Load-displacement curves of point 1, 7, 8
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6
7 8
5
9
4
10
3 2
19
1
18
0
11
20 21
17
23
16
24
15 13
12
22
25
14
(a)
(b)
(c)
(d)
Fig. 10. Test setup of 12 m hybrid truss: (a) test setup, (b) end-joint, (c) loading points and bracing, (d) steel loading weight, displacement sensors and strain gauges.
and 4 under T2 loading phase are given in Fig. 13(a). Small deflections, less than 1 mm, were noticed for points near the end connections, such as points 1 and 11, during the test. The largest deflection of the truss was measured at point 4, which is near the middle of 7 m span part of the truss. The deflection of point 7, which is in the highest part of the truss, is about 4 mm when 200 kg mass applied on all eleven joints. The max increment of deflection is about 1 mm when 2.2 ton iron mass applied on it for more than 24 h, as illustrated in Fig. 13(a). Load-displacement curves, along with the deflection shape of the truss under T3 loading phase are given in Fig. 13(b) and (c). The calculated stiffness of the truss under T3 loading phase is 0.28 kN/mm and 0.47 kN/mm, based on values obtained for point 4 and 7 of the truss. Load-strain curves of steel bottom chords No. 24-25 and No. 16-17, glubam top chord No. 3-4 under T2 and T3 loading phase are given in Fig. 13(d). No visible damage and stiffness degradation noticed during the test. Test results obtained herein indicated that the truss can fulfill the mechanical requirements of the truss, with respect to deflection limitations and loading bearing capacity, under design loads.
4.2. Truss behavior with end-connection change and unsymmetrical loading The design of end-connection is important to the robustness of the truss under dynamic loading conditions [26], such as blast loading phase [35,36]. Thus, the performance of truss under end-connections change and unsymmetrical loading is experimentally studied in this research, as illustrated in Fig. 11(b). After removal of the screws, the remaining structure of the roof truss survived with a re-balanced state at about 800 s. As shown in Fig. 14(a), the balanced configuration was very similar to the progressive collapse-resistant behavior of steel planar trusses, studied by Zhao et al. [37]. The compressive force in bottom chord 24-25 reduced and then changed to tensile force to accommodate a local force equilibrium. After removing screws, obvious change of measured strain values from −170 με to 1000 με was recorded for steel bottom chord No. 24–25, which is close to the endconnections with removed screws, and then slowly descended to equilibrium. At the same time, there was no significant change in other bottom chords. In addition, bottom chord 24-25 was also the key element in preventing the remaining structure from progressive collapse, 9
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Mass (kg) 275 60
200 30
120
30 30
30 30
30
T1
30 30
30
30
30 30
60
24 h
30
30 30
30
30
30 30
30
T2
30
Time (min) T3
(a)
removal of screws
50 kg/joint for 1 to 6 &100 kg/joint for 7 to 11
100 kg/joint for 7 to 11
50 kg/joint for 7 to 11 (b)
Fig. 11. Loading programme for the hybrid truss: (a) T1-T3 loading phases, (b) T4: change of end-joints and unsymmetrical loading.
measured strain values are shown in Fig. 14(c). Load-displacement values of all loading points, except point 3 due to sensor problem during the test, are given in Fig. 14(d). About 1 mm increment of deflection for point 4, which is the location with the most considerable deflection value of the truss during the test, was recorded when the end-support connection changed. Test results indicated that (1) the end-connections applied in real construction can be assumed as a rigid one; (2) the compression force of steel members close to ending support connections are not induce out-of-plane buckling under service load level; (3) the truss has enough robustness concerning end-connection change and unbalanced loading conditions. 5. Conclusion and discussion Building Information Model (BIM) based software used in this research help the design of the truss by terms of instant 3D modeling, construction materials calculation, joint, and prefabrication design. Engineering information between architect and engineers transferred effectively and smoothly, with the help of such software. Based on such design outcomes, the transportation, assembly, and installation of the truss can be conducted with limited labor force and common equipment. However, it also should be noted that there is still no platform or software system can fulfill all requirements up to now, even for a small construction project conducted in this research. Cooperation between various design tools still required for real construction practices.
Fig. 12. Out-of-plane failure of unbracing truss.
where the tensile force in this member was significantly larger than the others. Meanwhile, limited changes in strain values were recorded for glubam members, as indicated in Fig. 14(b). Unsymmetrical loading test was performed after the end-connection of the truss changed,
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Fig. 13. Deflection curves and measured strain: (a) load-displacement curves of typical points under T2, (b) load-displacement curves under T3, (c) displacement of different points during T3, (d) strain curves of typical chords under T2 and T3.
Tests on glubam compression members and connections were conducted in this research before the full-scale truss test, due to limited understanding of such bamboo-based engineering material. The main failure mode of glubam compression members with metal connectors was buckling along the weak axis (Fig. 7d), and the ultimate compression bearing capacity can be estimated and designed as lattice elements through effective length method (ELM). The failure mode of glubam connections was the tension failure at the end of glubam chord (Fig. 9). Experimental results indicated that the bearing capacity of glubam members under compression is more significant than that of connection regions. Meanwhile, the load-bearing performance of glubam members and joints can be improved by using latticed compression members with larger second moment of inertia value (I), optimal-designed steel joints, screw-reinforced connection regions [38] and effectively bracing system, respectively.
Full-scale test results of the hybrid truss indicate that it can fulfill the deflection limitations and bearing capacity requirements. The bracing system at the bottom part of the truss, along with the purlin system located at the top of the truss, is suggested to be applied, to avoid outof-plane failure. The robustness of the hybrid truss experimentally studied preliminarily, under end-connection change and unbalanced load conditions. The bottom chords near the end-support connections are suggested to be changed to glubam chords, with screw reinforcement [38]. Based on the test results, it can be concluded that the design of the truss obtained a relatively right balance between structural performance and economic benefits, thanks to easy manufacture, transportation, prefabrication, and installation when compared with the previous studies on bamboo truss structures [12–14]. Experimental researches reported herein will be used for next-step modeling research and improved design.
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Fig. 14. Truss performance with end-joint change and unsymmetrical loading: (a) real-time strain values of steel bottom chords, (b) strain values for top glubam and bottom steel chords with end-joint change, (c) strain values for top glubam and bottom steel chords under unsymmetrical loading, (d) displacement of different points during T4.
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Acknowledgement
trusses. Appl Mech Mater 2016;847:485–91. [17] Li Z, Yang GS, Zhou Q, Shan B, Xiao Y. Bending performance of glubam beams made with different processes. Adv Struct Eng 2018;22:535–46. [18] ASTM. Standard test method for evaluating dowel-bearing strength of wood and wood-based products. West Conshohocken, PA: ASTM International; 2013. [19] Ministry of Housing and Urban-Rural Development of the People's Republic of China (MOHURD). Code for design of timber structures. Beijing: China Architecture & Building Press; 2017. [20] Vallée T, Tannert T, Hehl S. Experimental and numerical investigations on full-scale adhesively bonded timber trusses. Mater Struct 2011;44:1745–58. [21] Mathieson C, Clifton GC, Lim JBP. Novel pin-jointed connection for cold-formed steel trusses. J Constr Steel Res 2016;116:173–82. [22] Šilih S, Premrov M, Kravanja S. Optimum design of plane timber trusses considering joint flexibility. Eng Struct 2005;27:145–54. [23] Azad SK. Monitored convergence curve: a new framework for metaheuristic structural optimization algorithms. Struct Multidiscip Optim 2019;60:481–99. [24] Bybordiani M, Azad SK. Optimum design of steel braced frames considering dynamic soil-structure interaction. Struct Multidiscip Optim 2019;60:1123–37. [25] Azad SK. Seeding the initial population with feasible solutions in metaheuristic optimization of steel trusses. Eng Optim 2017;50:89–105. [26] Azad SK, Bybordiani M, Azad SK, Jawad FK. Simultaneous size and geometry optimization of steel trusses under dynamic excitations. Struct Multidiscipl Optim 2018;58:2545–63. [27] Azad SK. Enhanced hybrid metaheuristic algorithms for optimal sizing of steel truss structures with numerous discrete variables. Struct Multidiscip Optim 2017;55:2159–80. [28] European Committee for Satandardization. Eurocode 5: Design of timber structures -part 1–1: general-common rules and rules for buildings. Brussels: European Committee for Satandardization; 2004. [29] Li Z, Xiao Y, Wang R, Monti G. Studies of nail connectors used in wood frame shear walls with ply-bamboo sheathing panels. J Mater Civ Eng 2015;27. [30] Wang R, Wei SQ, Li Z, Xiao Y. Performance of connection system used in lightweight glubam shear wall. Constr Build Mater 2019;206:419–31. [31] ASTM. Standard test methods for cyclic (re-versed) load test for shear resistance of vertical elements of the lateral force resisting systems for buildings. West Conshohocken, PA: ASTM International; 2012. [32] Ministry of Housing and Urban-Rural Development of the People's Republic of China (MOHURD). Standard for test methods of timber structures. Beijing: China Architecture & Building Press; 2012. [33] ASTM. Standard practice for static load testing of truss assemblies. West Conshohocken, PA: ASTM International; 2002. [34] Song X, Lam F, Huang H, He M. Stability capacity of metal plate connected wood truss assemblies. J Struct Eng 2010;136:723–30. [35] Bondok DH, Salim HA. Failure capacities of cold-formed steel roof trusses endconnections. Thin-Walled Struct 2017;121:57–66. [36] Miyachi K, Nakamura S, Manda A. Progressive collapse analysis of steel truss bridges and evaluation of ductility. J Constr Steel Res 2012;78:192–200. [37] Zhao X, Yan S, Chen Y, Xu Z, Lu Y. Experimental study on progressive collapseresistant behavior of planar trusses. Eng Struct 2017;135:104–16. [38] Dietsch P, Brandner R. Self-tapping screws and threaded rods as reinforcement for structural timber elements – a state-of-the-art report. Constr Build Mater 2015;97:78–89.
We thank Mr. Dongyu Kim, Mr. Gang Li, and Dr. Rui Wang for their architectural design and drawing of this project. This research work was supported by the National Natural Science Foundation of China [grant numbers 51678296, 51608262, 51878343]. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.engstruct.2019.109781. References [1] Branco JM, Sousa HS, Tsakanika E. Non-destructive assessment, full-scale loadcarrying tests and local interventions on two historic timber collar roof trusses. Eng Struct 2017;140:209–24. [2] Munch-Andersen J, Dietsch P. Robustness of large-span timber roof structures — two examples. Eng Struct 2011;33:3113–7. [3] Villar JR, Vidal P, Fernández MS, Guaita M. Genetic algorithm optimisation of heavy timber trusses with dowel joints according to Eurocode 5. Biosyst Eng 2016;144:115–32. [4] Villegas L, Morán R, García JJ. Combined culm-slat Guadua bamboo trusses. Eng Struct 2019;184:495–504. [5] Albermani F, Goh GY, Chan SL. Lightweight bamboo double layer grid system. Eng Struct 2007;29:1499–506. [6] Xiao Y, Yang RZ, Shan B. Production, environmental impact and mechanical properties of glubam. Constr Build Mater 2013;44:765–73. [7] Sharma B, Gatóo A, Bock M, Ramag M. Engineered bamboo for structural applications. Constr Build Mater 2015;81:66–73. [8] Correal J, Ramirez F, Gonzalez S, Camacho J. Structural behavior of glued laminated Guadua bamboo as a construction material. World Conf Timber Eng Trentino Italy 2010. [9] Sharma B, Bauer H, Schickhofer G, Ramag M. Mechanical characterisation of structural laminated bamboo. Struct Build 2016;170:1–15. [10] Correal JF, Echeverry JS, Ramírez F, Yamín LE. Experimental evaluation of physical and mechanical properties of Glued Laminated Guadua angustifolia Kunth. Constr Build Mater 2014;73:105–12. [11] Luna P, Takeuchi C, Cordón E. Mechanical behavior of glued laminated pressed bamboo Guadua using different adhesives and environmental conditions. Key Eng Mater 2014;600:57–68. [12] Xiao Y, Chen G, Feng L. Experimental studies on roof trusses made of glubam. Mater Struct 2013;47:1879–90. [13] Xie Q, Xiao Y. Experimental study on large-span glubam roof truss (in Chinese). J Build Struct 2016;37:47–53. [14] Wu Y, Xiao Y. Steel and glubam hybrid space truss. Eng Struct 2018;171:140–53. [15] Mancini M, Caruso S, Li Z, Marnetto R, Monti G. A long span hybrid truss for a sport building. Appl Mech Mater 2016;847:479–84. [16] Caruso S, Wang R, Li Z, Marnetto R, Monti G. Highly standardized long-span hybrid
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Engineering Structures journal homepage: www.elsevier.com/locate/engstruct
Experimental study of an unsymmetrical prefabricated hybrid steel-bamboo roof truss
T
Zhi Lia, , Tao Lia, Can Wanga, Xiaozhou Hea, Yan Xiaob,c ⁎
a
College of Civil Engineering, Nanjing Tech University, Nanjing, Jiangsu 211816 China Zhejiang Univ.-Univ. of Illinois Institute (ZJUI), Zhejiang University, Haining, Zhejiang 314400, China c Sonny Astani Department of Civil & Environmental Engineering, University of Southern California, Los Angeles, CA 90089, USA b
ARTICLE INFO
ABSTRACT
Keywords: BIM-based design Bamboo Prefabricated Hybrid truss
A 12 m hybrid unsymmetrical steel-glued laminated bamboo (glubam) truss used in a roof construction was designed with building information model (BIM) platform software. The hybrid truss composed of glued laminated bamboo (glubam) as top chords and web members, and steel pipes used as bottom chords. Based on such design outcomes, the manufacture, transportation, prefabrication and installation details, with corresponding advantages, are reported. Experiments of glubam chords and connections were performed. The behaviors of glubam chords along with its load-displacement/strain curves, are given in this research. Experimental results on full-scale truss indicate that it can fulfill the mechanical requirements with respect to deflection limitations and loading bearing capacity. The performance of the truss with end-connection change and under unsymmetrical loading conditions was also experimentally studied and discussed in this research. The prefabricated hybrid truss obtained a relatively right balance between structural requirements and economic benefits, thanks to easy manufacture, transportation, prefabrication, and installation.
1. Introduction and background Timber is a popular material used for truss structure for a long period. Researches related to it, including historic retrofit structures, large-span structures, innovative design methods, and many other aspects. Two historic timber collar roof trusses were surveyed and assessed using non-destructive methods and evaluated structurally by full-scale load-carrying tests made at the laboratory by Branco et al. [1]. Andersen and Dietsch [2] discussed the robustness of large-span timber roof structures, based on findings from failures of two roof structures. Villar et al. [3] presented the optimization by genetic algorithms of the members and joints of heavy timber trusses fastened by mechanical joints with dowels and metal plates. Meanwhile, due to the inherent similarities between timber and round bamboo, roof or space truss made with round bamboo culms has been studied by some researchers. Villegas et al. [4] reported the design and mechanical performance of trusses assembled with culms and slats of Guadua bamboo, which is rich in South America. A space truss made with round bamboo and plastic joints was experimentally researched by Albermani et al. [5]. Besides round raw bamboo made for truss structures mentioned above, engineered bamboo panels can also be used as a structural material with even better cost-benefit performances as discussed in
⁎
[6–11]. These researches indicate that bamboo has very high strengthweight ratio thus can be effectively used for large-span structures with smaller sections compared with wood, such ply-bamboo panels can also be further cold-laminated similar to cross-laminated timber (CLT), to form glubam structural elements and structures with various architectural shapes. Since 2009, Xiao et al. [6] established a relatively comprehensive research database for this type of glubam structures, including mechanical properties, structural elements, and proof construction projects. Based on such engineered bamboo products, glubam, several innovative kinds of research concerning glubam trusses have been conducted recently. Six conventional Howe triangle roof trusses, with a span of 6 m, made with glubam were experimentally researched by Xiao et al. [12]. Two types of 20 m long large-span glubam roof truss with conventionally triangle shape was researched by Xie and Xiao [13]. A hybrid glubam-steel space truss system along with a case-study construction, a rain-shed canopy, was researched by Wu and Xiao [14]. In this research, to the aim of including engineered bamboo as an effective engineered material for large-span structures, a 12 m span hybrid truss was designed with glubam used as the top chords and web members. With 30/2 steel tube (steel tube with 30 mm in diameter and the thickness of steel is 2 mm) used as bottom chords according to the architectural requirements on the lighting system. Such trusses were
Corresponding author. E-mail address: [email protected] (Z. Li).
https://doi.org/10.1016/j.engstruct.2019.109781 Received 3 April 2019; Received in revised form 5 September 2019; Accepted 8 October 2019 0141-0296/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. BIM-based design information management strategy of construction with glubam hybrid truss.
used as structural elements of a roof in a resort construction project located in a mountainous area of Jiangsu, China, with a land area of 32 m × 12 m, as shown in Fig. 1. The steel frame spacing along the length is 4 m, and the span is 12 m. Following the architectural design, the elevation difference between the two ends of the truss is 0.69 m, whereas the value is 3.48 m between the top of the truss and the lower end of it. Thus, the ratio between height and span H/L is 0.29. The horizontal projection length of each parallelogram element, such as the horizontal distance between points 1 and 2 as shown in Fig. 10, is 1 m; in other words, the prefabricated truss can be easily changed to fit different roof span and elevation differences. Compared with the engineered bamboo trusses reported in previous researches [12–16], several advantages can be noticed for the prefabricated hybrid truss studied herein. (1) All glubam elements were made with standard
30 mm thickness ply-bamboo panels (1.2 m × 2.4 m); thus the price of bamboo members is less than the corresponding elements reported in [14] and [15,16]. (2) The length of all elements are less than 1050 mm, thus, without requiring specific transportation and installation equipment as reported in [13], the 12 m span truss can be transported and installed with small equipment and hand tools. (3) The span and elevation difference of the truss can be changed to meet different conditions. 2. Materials and design of the truss With the fast development of industrial bamboo products in recent ten years, now there are two types of commercial available laminated bamboo panels can be effectively used for glubam structures, which are
Table 1 Main properties of thick strip laminated bamboo (in MPa) used in the truss.
ft,x = 119.3 MPa
fm,xz = 104.6 MPa Em,xz = 8682 MPa
Tension
Static bending
σ = 22.0 MPa
fc,x = 57.7 MPa
σ = 7.5 MPa σ = 764 MPa
τyx = 8.3 MPa
Compression
Shear
σ = 4.7 MPa
σ = 1.4 MPa
Embedment strength fh,xz = 65.0 MPa σ = 2.2 MPa
Notes: The coordinate system can be found in Fig. 2 and 3; σ is standard deviation; for bending strength fm,xz, the stress is acting on a plane normal to the x-axis, z is the loading direction; for shear strength τyx, the shear plane is xoz, along the x-axis; for embedment strength fh,xz, the load is acting on a plane normal to the x-axis, the direction of fastener is y-axis, and the value was obtained by 12 mm steel bolt. 2
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z
40
12 0
y
x
90
Fig. 2. Embedment curves of ply-bamboo with 12 mm steel bolts. Table 2 Software used in this research based on the BIM platform.
12 0
30
x
Design tools
Conceptual design Architectural design Structural design Assembly and joint design
SketchUp/ Autodesk® Revit Autodesk® Revit SolidWorks
Communications and building information between the architect, structural engineer, and manufacturer effectively and frictionless transferred thanks to Building Information Model (BIM) based software used during the truss design, as illustrated in Table 2. The BIM-based design information management plan for this project is given in Fig. 1. However, it must be noted that the information transmission still to be realized is denoted as the dashed line in the figure. Up to now, in this information management plan, design information created by various software is yet to be instantly exchanged and shared in an open information platform, due to the lack of internet-based building information platform, lack of auto integration of various software files and limitations on fast-wireless communication technology. More discussions on this BIM-based building information management are beyond the topic of this paper; thus, the following content will be only focused on the truss, as well as robust benefits obtained from this practical BIM-based model, the hybrid roof truss including three elements: glubam members, steel chords and metal connection system. Design files of all elements in the truss can be saved as stl. format, then connected with a 3D printer for model research and further optimal design. Assembly performance of the truss, which is very important for prefabricated structures, was checked through computer software (SolidWorks) as well as a 1:3 3D print model, as shown in Fig. 4(b). In
z 30
Steps
y
Fig. 3. Compression test of laminated bamboo used for glubam.
thick-strip unidirectional laminated bamboo panels [6] and thin-strip cross-laminated bamboo panels [17], respectively. 30 mm thick-strip based laminated bamboo panels were used for glubam elements in this research. Based on the existing research by the first author, the basic mechanical properties of such thick strip laminated bamboo are given in Table 1. Embedment tests of such panel in its bamboo strip direction under 12 mm steel bolts were also performed according to ASTM code [18]. The embedment curves are given in Fig. 2. Compression test results of laminated bamboo used for glubam along its bamboo strip direction are given in Fig. 3. The moisture content of the glubam is around 8.5% in this research.
3
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(a) 12 m span hybrid truss and its connection system
(b) The 3D Print model of the truss Fig. 4. 12 m span prefabricated hybrid truss: (a) 12 m span hybrid truss and its connection system, (b) The 3D print model of the truss.
Fig. 5. Production of glubam elements used in the hybrid truss: (a) cold lamination, (b) element cut, (c) hole drill, (d) notch open.
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Fig. 6. Test specimen and setup of glubam chords (in mm): (a) top chord, (b) web member, (c) test setup.
conclusion, all building information of the truss (not whole project as indicated from Fig. 1) used in this research can be frictionless transferred between architect and engineers, as well as software and hardware. Based on the models mentioned above, if no errors or conflicts noticed for the assemblage of the truss. Then, as shown in Fig. 5, glubam structural elements for the truss were be produced: (1) two 30 mm thick ply-bamboo panels were cold laminated with compression pressure about 3 MPa for more than 48 h. (2) the 60 mm thick glubam panels were cut to be elements with a height of 60 mm and 120 mm. (3) 12.5 mm holes were drilled for bolts connections. (4) 6 mm U-shaped notches were opened for the installation of steel joint plates into the glubam members, edges of the elements were also cut according to its position in the truss. All glubam truss elements can be produced with conventional wood or CNC machines and finished by two workers in three hours. Steel joints and bottom chords were made in a steel company by Q235 steel (fy = 235 MPa). Assembly of this truss can be finished in four hours by three workers with hand tools, as shown in Fig. 4. The transportation of all elements can be finished by a small truck, with 5 ton carrying ability, indicating that the truss can be easily transferred to the construction place where only rural roads available. Meanwhile, it is convenient for manufacturing and transportation because of the limited types of truss elements. The length of chords can be changed to adopt different elevation differences. The bottom chords made by steel tube can meet the requirements of wire layout in the tube, for lighting systems. The design of end-joint plates considered the connection with the column as shown as connection type a in Fig. 4(a).
3. Load-carrying test of the truss For the hybrid truss studied in this research, all elements were assumed to be under axial compression or tension force during the structural design; meanwhile, test on glubam members and the corresponding connection region were performed, for the further modeling & design purpose. 3.1. Test on chords and connection region 3.1.1. Test on glubam members In order to obtain the behavior of glubam compression members with consideration of the influence of end metal connections, the test specimens were designed as shown in Fig. 6. Three tests were performed for each group. The calculated slenderness ratio ( = l ox / i x ) of top chords & web members were about 55, in which lox is the effective **length, i x = I A is the radius of gyration, I is the second moment of inertia, and A is the area of the cross-section. All the chords were calculated as lattice column members with 120 × 25-mm and 60 × 25-mm sections. According to the effective length method (ELM) suggested in GB500052017 [19], the mean compressive capacity of such column was estimated by the following equation: (1)
Nc = fc A0
coefficient is estimated as 1/1 + ( 80 Compression test results of laminated bamboo used herein are shown in Fig. 3; thus, 57.7 MPa is
)2 .
5
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z y x
(a)
Web member
Top chord
(b)
(c)
(d)
Fig. 7. Load-displacement (strain) curves and failure modes of glubam chords: (a) load-displacement curves of top chords and web members, (b) load-strain curves of web and top chords, (c) failure of web members, (d) failure of top chords.
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used as the mean value of compression strength in Eq. (1). Based on the information given above, the calculated bearing capacities are 233 kN and 116 kN for top chords and web members, respectively. The mean load-displacement curves of glubam chords under compression force are given in Fig. 7(a). For web members with a section of 60 × 60-mm, due to the differences of failure modes, and relatively large variance of F- curves in y-direction for all three tests, all recorded curves in this direction, denoted as y-1, y-2 and y-3 are given herein. Typical load-strain curves of web member & top chord are given in Fig. 7(b). Stain gauges numbered 3 and 7 were glued along the coldlamination lines, as illustrated in Fig. 7(b). It can be noted that for web members, the differences among recorded deformations of strain gauges 2/3/4, and 6/7/8 are relatively smaller, than these for top chords when the applied load is less than 50 kN. The load-strain curves, along with the recorded failure modes, as shown in Fig. 7(c) and (d), validated the reliability of cold-lamination steps to produce glubam members, as no apparent failure was noticed along the secondary gluelines in all tests performed in this research. All the failures concentrate in the joint’s region, due to local bucking plus embedment failure. Therefore, the moment M and force N in the middle height of the chords can be estimated as:
M=
1 6
+ N= 1 2
1
5
2 5
ab2 E x
Earthquake Engineering (CUREE) [31]. Scaling of the protocol was defined with respect to a reference displacement. The reference displacement for cyclic tests was defined as 60% of the ultimate displacement at which the load dropped to 80% of the maximum load during monotonic tests. In the case where the load did not drop to 80% of the maximum load, the failure displacement should be used as an ultimate displacement of monotonic tests. The program of cyclic loading consists of four parts: the first part was 6 cycles at 0.05 times of reference displacement; the next was 7 cycles at 0.075 and 0.1 times of peak displacement; then, 4 cycles at 0.2 and 0.3 times of peak displacement; the number of cycles in the final part was 3 of amplitudes 0.4, 0.7, 1.0, 2.0 times of peak displacement, respectively. Load and displacement information was recorded by an MTS test machine at a rate of 10 Hz for the cyclic tests. The loading speed was set at 2.5 mm/ min and 7.5 mm/min in the monotonic and cyclic loading tests, respectively. Load-displacement curves of the tested connection, as well as corresponding failure models, are given in Fig. 9. The ultimate tension bearing force is about 60 kN and 90 kN for the web member’s connection and the top one. Glubam elements were torn out accompanied by a considerable split sound, and the crack in the bolt line extended rapidly to the end part of the connections when the failure occurred in the test. Although the tensile strength of laminated bamboo is about two times than the compression strength, as given in Table 1. It is noted that in the connection test, the bearing capacity of top chord connections under compression is slightly higher than the connections under tension.
(2)
ab E x
(3)
3.2. Full-scale test on hybrid truss
where Ex is the modulus of elasticity of glubam under compression, a is the width of the section, b is the height of the section. 1 and 5 are measured strains, as shown in Fig. 7(b).
Test setup of the truss is shown in Fig. 10(a). In this test, steel endsupports connected with the end connection of the truss with four screws, as shown in Fig. 10(b), as such rigid connection is typical of that planned for use in the real construction, due to deflection control reasons. Loading on the truss applied by using iron weight with cables and bags, as shown in Fig. 10(c). The displacement of the loading points, from 1 to 11, is measured by LVDT (Linear Variable Differential Transformer) displacement sensors as shown in Fig. 10(d). The strains of all steel and glubam elements were measured with strain gauges, as shown in Fig. 10(d). The gauge resistance is 120 ± 1 , with gauge sensitivity factor of 2.08 ± 1%. The size of gauges is 7 × 4.5-mm and 55 × 5-mm, for steel and glubam elements, respectively. All these sensors and gauges linked were with one real-time data acquisition system. Based on the geometry information given by Fig. 1, the distance among the trusses is two meters. Meanwhile, the characteristic value of the live load on the roof was 0.5 kN/m2, and the dead load of the hybrid truss was 0.83 kN/m. Therefore, the calculated reference value for loading test Pk, which means the sum of the characteristic value of live load Lk and dead load Dk, was estimated as 2 kN for all eleven loading points as shown in Fig. 10, based on the calculation method suggested in GB/T 50329-2012 [32]. Thus, 200 kg of steel blocks were chosen as the corresponding reference loading mass for each joint. Meanwhile, deflection limitation of the roof truss is l/240 = 50 mm according to Chinese timber design code [19] when Pk applied to it. According to the truss-loading methods suggested in ASTM E73-83 [33] and GB/T 50329-2012 [32], three steps loading procedure was adopted in this research. Such a loading program including three parts, as shown in Fig. 11. T1 is the pre-loading phase, and the aim is to check the condition of the test apparatus and measurement devices. T2 is the standard loading phase, and the aim is to understand the deflection performance of the truss under service load level. T3 is the failure loading step, and the aim is to estimate the ultimate bearing capacity of the truss. In this research, 275 kg/point was chosen as the max static loading mass due to lab limitations. After three loading phases, as shown in Fig. 11(a) finished, if no visible damage noticed for the truss, then, it will be further loaded under following conditions, denoted as
3.1.2. Test on the connection region Existing research on timber truss [20] indicated that the joint region influenced the overall performance of truss structures a lot; meanwhile, optimal design of joints can improve the overall performance of truss structures effectively[12,21–27], as the cost of a truss largely depends on the number of joints. Thus, experimental tests on steel plate-glubam chords connection region of the hybrid truss were performed in this research, in order to obtain joint mechanical information for modeling and optimal design purpose. The test specimen and setup are shown in Fig. 8. Based on the embedment test results given in Fig. 2 and methods suggested by Eurocode 5 [28], the ultimate bearing capacity of the glubam joint connection [29,30] can be estimated as (Eq. (8.11) in EN 1995-1[28]):
fh t1 d Fv = min fh t1 d
2+
4My fh dt12
2.3 My fh t1 d +
1 + Fa x 4 Fa x 4
(4)
where fh is the embedment strength of glubam, which is 65 MPa in this research; t1 is the smaller of the thickness of the glubam side member, which is 25 mm herein; d is the diameter of screws, which is 12 mm; My is the yield moment of the bolts, can be estimated as 0.3 fu d 2.6 , with ultimate tension strength fu = 1000 MPa; Fax is the withdrawal capacity of the fastener, which is zero in this research as only the head part of the high-strength bolts have threads and predrill holes. Thus, the calculated bearing capacity is 39 kN and 58.5 kN for web member connections and top chords connections, respectively. In order to have a relatively comprehensive understanding on mechanical performances of such engineered bamboo joints, three monotonic tests, and one cyclic test were performed for the two types of connection used herein, which are web members connection and top chords connection, respectively. The tests were performed according to the protocol given by the Consortium of Universities for Research in 7
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Fig. 8. Test specimen and setup of joints.
Fig. 9. Load-displacement curves of joints region and failure modes.
the T4 loading phase, shown in Fig. 11(b): (1) unload each loading points to be 100 kg; (2) to change one of the end-joints to be a hinged one by removing two front screws at the end-joint; (3) unsymmetrical loading tests. Real-time displacement and strain values will be recorded for the truss under such conditions.
applied to it. The max displacement measured during the test was 11 mm of point 4 when 275 kg was applied on all eleven loading points. The largest vertical deflection of the truss when Pk applied on it is 6.5 mm, while the deflection limitation is 50 mm. Small deflection increment less than 1 mm, was recorded when Pk applied on it for more than 24 h. Test results obtained in this research indicated that the fastprefabricated hybrid truss fulfills the mechanical requirements of a typical roof span when the truss distance along the length is two meters, whereas the common space for roof-truss used in lightweight timber structures is around 600 mm due to out-of-plane stability reasons [34].
4. Test results A trial test of the truss without any bracing system was performed before the formal test, in order to understand the stability of the truss during transportation and prefabrication. Out-of-plane failure was noticed at the joint 14, as shown in Fig. 12, when 100 kg mass was applied on all loading points. After this test, the bent steel plate was changed, as well as bracing system was used. During the test, in total 11 × 275 kg = 3.025 ton iron mass was
4.1. Load-displacement & strain curves Typical load-displacement curves, as well as strain curves of the chords, are shown in Fig. 13. Load-displacement curves of point 1, 7, 8
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6
7 8
5
9
4
10
3 2
19
1
18
0
11
20 21
17
23
16
24
15 13
12
22
25
14
(a)
(b)
(c)
(d)
Fig. 10. Test setup of 12 m hybrid truss: (a) test setup, (b) end-joint, (c) loading points and bracing, (d) steel loading weight, displacement sensors and strain gauges.
and 4 under T2 loading phase are given in Fig. 13(a). Small deflections, less than 1 mm, were noticed for points near the end connections, such as points 1 and 11, during the test. The largest deflection of the truss was measured at point 4, which is near the middle of 7 m span part of the truss. The deflection of point 7, which is in the highest part of the truss, is about 4 mm when 200 kg mass applied on all eleven joints. The max increment of deflection is about 1 mm when 2.2 ton iron mass applied on it for more than 24 h, as illustrated in Fig. 13(a). Load-displacement curves, along with the deflection shape of the truss under T3 loading phase are given in Fig. 13(b) and (c). The calculated stiffness of the truss under T3 loading phase is 0.28 kN/mm and 0.47 kN/mm, based on values obtained for point 4 and 7 of the truss. Load-strain curves of steel bottom chords No. 24-25 and No. 16-17, glubam top chord No. 3-4 under T2 and T3 loading phase are given in Fig. 13(d). No visible damage and stiffness degradation noticed during the test. Test results obtained herein indicated that the truss can fulfill the mechanical requirements of the truss, with respect to deflection limitations and loading bearing capacity, under design loads.
4.2. Truss behavior with end-connection change and unsymmetrical loading The design of end-connection is important to the robustness of the truss under dynamic loading conditions [26], such as blast loading phase [35,36]. Thus, the performance of truss under end-connections change and unsymmetrical loading is experimentally studied in this research, as illustrated in Fig. 11(b). After removal of the screws, the remaining structure of the roof truss survived with a re-balanced state at about 800 s. As shown in Fig. 14(a), the balanced configuration was very similar to the progressive collapse-resistant behavior of steel planar trusses, studied by Zhao et al. [37]. The compressive force in bottom chord 24-25 reduced and then changed to tensile force to accommodate a local force equilibrium. After removing screws, obvious change of measured strain values from −170 με to 1000 με was recorded for steel bottom chord No. 24–25, which is close to the endconnections with removed screws, and then slowly descended to equilibrium. At the same time, there was no significant change in other bottom chords. In addition, bottom chord 24-25 was also the key element in preventing the remaining structure from progressive collapse, 9
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Mass (kg) 275 60
200 30
120
30 30
30 30
30
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30 30
30
30
30 30
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30
30 30
30
30
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(a)
removal of screws
50 kg/joint for 1 to 6 &100 kg/joint for 7 to 11
100 kg/joint for 7 to 11
50 kg/joint for 7 to 11 (b)
Fig. 11. Loading programme for the hybrid truss: (a) T1-T3 loading phases, (b) T4: change of end-joints and unsymmetrical loading.
measured strain values are shown in Fig. 14(c). Load-displacement values of all loading points, except point 3 due to sensor problem during the test, are given in Fig. 14(d). About 1 mm increment of deflection for point 4, which is the location with the most considerable deflection value of the truss during the test, was recorded when the end-support connection changed. Test results indicated that (1) the end-connections applied in real construction can be assumed as a rigid one; (2) the compression force of steel members close to ending support connections are not induce out-of-plane buckling under service load level; (3) the truss has enough robustness concerning end-connection change and unbalanced loading conditions. 5. Conclusion and discussion Building Information Model (BIM) based software used in this research help the design of the truss by terms of instant 3D modeling, construction materials calculation, joint, and prefabrication design. Engineering information between architect and engineers transferred effectively and smoothly, with the help of such software. Based on such design outcomes, the transportation, assembly, and installation of the truss can be conducted with limited labor force and common equipment. However, it also should be noted that there is still no platform or software system can fulfill all requirements up to now, even for a small construction project conducted in this research. Cooperation between various design tools still required for real construction practices.
Fig. 12. Out-of-plane failure of unbracing truss.
where the tensile force in this member was significantly larger than the others. Meanwhile, limited changes in strain values were recorded for glubam members, as indicated in Fig. 14(b). Unsymmetrical loading test was performed after the end-connection of the truss changed,
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Fig. 13. Deflection curves and measured strain: (a) load-displacement curves of typical points under T2, (b) load-displacement curves under T3, (c) displacement of different points during T3, (d) strain curves of typical chords under T2 and T3.
Tests on glubam compression members and connections were conducted in this research before the full-scale truss test, due to limited understanding of such bamboo-based engineering material. The main failure mode of glubam compression members with metal connectors was buckling along the weak axis (Fig. 7d), and the ultimate compression bearing capacity can be estimated and designed as lattice elements through effective length method (ELM). The failure mode of glubam connections was the tension failure at the end of glubam chord (Fig. 9). Experimental results indicated that the bearing capacity of glubam members under compression is more significant than that of connection regions. Meanwhile, the load-bearing performance of glubam members and joints can be improved by using latticed compression members with larger second moment of inertia value (I), optimal-designed steel joints, screw-reinforced connection regions [38] and effectively bracing system, respectively.
Full-scale test results of the hybrid truss indicate that it can fulfill the deflection limitations and bearing capacity requirements. The bracing system at the bottom part of the truss, along with the purlin system located at the top of the truss, is suggested to be applied, to avoid outof-plane failure. The robustness of the hybrid truss experimentally studied preliminarily, under end-connection change and unbalanced load conditions. The bottom chords near the end-support connections are suggested to be changed to glubam chords, with screw reinforcement [38]. Based on the test results, it can be concluded that the design of the truss obtained a relatively right balance between structural performance and economic benefits, thanks to easy manufacture, transportation, prefabrication, and installation when compared with the previous studies on bamboo truss structures [12–14]. Experimental researches reported herein will be used for next-step modeling research and improved design.
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Fig. 14. Truss performance with end-joint change and unsymmetrical loading: (a) real-time strain values of steel bottom chords, (b) strain values for top glubam and bottom steel chords with end-joint change, (c) strain values for top glubam and bottom steel chords under unsymmetrical loading, (d) displacement of different points during T4.
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Acknowledgement
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