Engineering Structures 184 (2019) 495–504
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Engineering Structures
journal homepage: www.elsevier.com/locate/engstruct
Combined culm-slat Guadua bamboo trusses
L. Villegas, R. Morán, J.J. García
T
⁎
Escuela de Ingeniería Civil y Geomática, Universidad del Valle, Cali, Colombia
A R T I C LE I N FO
A B S T R A C T
Keywords:
Bamboo light structures
Ring joints
Truss
Floor
Roof
This study pertains to the design and structural testing of trusses assembled with culms and slats of the bamboo
species Guadua angustifolia (GA). The trusses were constructed using a novel design of steel clamps to accomplish
the connections between the top and bottom chord culms and the diagonal and vertical slat elements. The
clamps offer a better alternative to the conventional joining methods that typically lead to longitudinal splitting
of the culms, since the clamps counteract the formation of longitudinal splitting and accommodate themselves to
the geometric irregularities of the culms and slats. The experimental program included the monotonic static tests
of pilot Howe truss prototypes with variations in the clamp joints. Next, four replicates of the proposed design
(3 m long and 0.4 m high weighing 265.4 N) were tested under monotonic static load. The trusses were simply
supported at the ends and loaded at three central joints. The stiffness of the trusses up to 9869 N (1.5 times the
service load for floor use) was 631.3 N/mm (COV 3.6%), and no failure was shown at the proof load of 13,754 N.
This pilot study demonstrates that these trusses are a good candidate to substitute the traditional materials to
support the floors and roofs in low-cost, prefabricated housing projects. However, more experimental studies
have to be accomplished to fully validate this application.
1. Introduction
The alarming data regarding global warming, which is associated
with the high energy required for the production of traditional materials, such as steel and concrete, has motivated the search for nonconventional and renewable materials [1–3]. One of them is Guadua
angustifolia (GA), which is a species of bamboo that grows naturally in
large areas of Central and South America. GA has a high longitudinal
strength as compared to low carbon steel [4], and GA plantations play
an important role in reducing erosion and controlling water cycles [5].
GA culms are cylindrical tubes with transverse diaphragms (natural
nodes) located at variable distances along the length of the culm. The
material is reinforced with strong cellulose fibers immersed in a weak
and flexible lignin matrix [6,7]. The fibers are oriented axially in the
internodes. Therefore, the bamboo mechanical properties are directional, with a high axial strength as compared to mild steel [8,9], but
weak in transverse direction [5,10,11]. Preferential bamboo failure
mode is longitudinal splitting, which is attributed to circumferential
tension and shear stress on the fiber planes [11,12].
Globally, there are many full-culm bamboo structures, such as
scaffolds, trusses, bridges, and houses [7,13–15]. However, bamboo
construction remains a vernacular practice [16]. This might be
attributed in part to the challenges to develop standard connections due
to the tubular shape of the culms, the dimensional variations and the
anisotropy of the material. Bamboo structures usually use coped joints
at the culms ends to increase the area of contact, mortar injection to
improve the transverse strength and through bolts to transmit the load
[17–23].
These joints are prone to longitudinal splitting and their construction requires substantial workforce, increased construction time and
cost and are not amenable to prefabricated processes. Other joints do
not need holes, like the handmade used by Seixas et al. [24] for space
structures, but cannot be easily implemented using prefabricated processes. Modern connections [25–27] are versatile and efficient but require relatively complex and expensive parts to be used in low-cost
projects.
One way to overcome the limitations of using full-bamboo culms is
to produce engineered bamboo, which results from processing the raw
culms into laminated composites, similar to glue-laminated timber
products [28–35]. Engineered bamboo allows the use of standardized
sections, which very much facilitates the construction process with
techniques similar to those used in timber structures [28,36–38]. Unfortunately, engineered bamboo has a high environmental impact as
compared to using raw culms [39].
⁎
Corresponding author.
E-mail addresses: laura.villegas@correounivalle.edu.co (L. Villegas), richard.moran@correounivalle.edu.co (R. Morán),
jose.garcia@correounivalle.edu.co (J.J. García).
https://doi.org/10.1016/j.engstruct.2019.01.114
Received 2 February 2018; Received in revised form 21 January 2019; Accepted 24 January 2019
Available online 01 February 2019
0141-0296/ © 2019 Published by Elsevier Ltd.
Engineering Structures 184 (2019) 495–504
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of 681.1 MPa (COV 32%) and a circumferential tensile strength of
8.0 MPa (COV 26%, n = 10). The tests of slats were conducted under
bending and torsion, according to the protocol proposed in [47]. The
axial modulus obtained was 8787.2 MPa (COV 18%) and the circumferential-axial shear modulus 747.8 MPa (COV 21.4%, n = 10).
These properties are similar to those obtained in previous studies [37]
and are representative of a good quality GA material for construction.
The clamps, curved washers and steel connectors (Fig. 1.b) were
manufactured with ASTM A36 steel by students in the lab using a metal
cutter, a plate bender and a drill press, with a relatively simple and fast
procedure. The width of the GA slats and of the curved washers used to
connect them was 40 mm.
To promote the massive use of raw bamboo, easy and efficient
construction methods have to be devised, for example, by developing
technologies using prefabricated parts, as effectively implemented in
the manufacture of timber houses [40]. Thus, in previous studies, we
proposed the use of metal parts to connect culms and slats with great
versatility [41,42]. First, we developed trusses made of GA slats that
can be used for roof applications [41]. However, buckling of compression members was a concern for the larger spans. Later, we proposed using thin steel clamps to connect bamboo culms and improve
the resistance to longitudinal splitting [42]. Steel clamps can accommodate a wide range of culm sizes, solving the problem of the joints
customization.
Based on the aforementioned joints, the objective of this manuscript
is to propose a bamboo truss design that can potentially be used to
support roofs and floors of low-cost houses using prefabricated processes. Pilot prototypes with variations in the joints were tested for the
development of the proposed design. Next, four replicates of the proposed design were evaluated under static test and simulations.
2.3. Slat joints
Fig. 2 shows a typical joint between slat elements and a culm. The
slat elements were joined to the clamp lugs using 3.2-mm thickness
plates (ASTM A36) that were connected to the slats through 9.5 mm
diameter bolts grade 2 and inserted into 10 mm diameter holes drilled
at 35 mm from the ends of the slats [48]. The bolt was tightened to
apply compression along the thickness of the slat or the radial direction.
The slats were outfitted with curved washers between the bolt head and
the slat and between the nut and the slat to redistribute the pressure
caused when tightening the bolt. For the elements under tension,
curved washers were also inserted between adjacent slats. The curvature of the washers (radius of 62 mm) was nearly matched to that of the
slats.
As a simple means to control the bolt tightening when assembling
this joint, the nut was tightened two turns after the parts came in
contact. With two nut turns, the nominal radial deformation for a 10mm thick slat was 31.8%, which is lower than the failure deformation
of the GA under radial compression [45,49]. The torque to tight the
joint is in the range of 34–48 N-m.
For the slat elements under tension, the critical failure mode is
shearing on the longitudinal fiber planes. Previous experimental studies
[41] indicated that the shear strength in joints with radial compression
was 9.96 MPa. As this joint with radial compression is not covered in
the Colombian standard, as a first approximation to establish preliminary allowable values to check this study’s designs, we used a safety
factor of 3 that yields an allowable shear stress of 3.3 MPa. The safety
factor is equal to that used by Paraskeva et al. [50]; however, we are
fully aware that a more extensive experimental research has to be
carried out to validate this proposal.
2. Materials and methods
2.1. Design specifications and truss geometry
The configuration of the specimens to be tested were that of trusses
intended to support the floor of prefabricated low-cost houses, which is
based on 3 m × 3 m modules. The Colombian Building Standard [43]
was used as a guide to verify the structural performance of the truss. For
GA design, the strength verification is based on allowable stresses.
Assuming that the truss will be used to support floors, a live load of
1766 N/m2 (180 kg/m2) and a dead load of 1373 N/m2 (140 kg/m2)
were considered, which sum to a service load of 3139 N/m2 (320 kg/
m2). The trusses must be oriented parallel to each other to bridge the
3 m × 3 m space. If the separation of the trusses that conform the floor
is 0.75 m, a distributed service load of W = 7063 N corresponds to each
truss.
The joints used in the trusses are not included in the Colombian
standard. Hence, we verified the resistance of the joints of the proposed
truss based on the results of preliminary tests presented in this paper
and others reported elsewhere [41]. The trusses must also comply with
a maximum deflection according to the application. The strength of the
steel components was checked according to the provisions of the Colombian standard [43].
A Howe truss type was chosen, where the top and bottom chords
were composed of GA culms, while each of the diagonal and vertical
elements were formed with longitudinally superimposed GA slats about
40 mm wide (Fig. 1a). Upon the application of vertical loads to the
three upper nodes, the shorter vertical elements are compressed and the
diagonal elements are tensed. Thus, this truss type allows increasing the
buckling load in the vertical members, which might be critical in some
cases [41]. Clamps were used to transmit the load from the chords to
the slat elements. In this joint design (Fig. 1b), both the external load
and the forces from the web elements are applied to the lugs. Hence, the
vertical loads are directly transmitted to the web elements, and the
culms work only under the axial loading.
2.4. Culm joints
The connection to the culms was effect by split clamps formed by
two semi-rings [42,49], as depicted in Fig. 3a. The semi-rings were
manufactured in four sizes to cover a range of culm diameters of
95–135 mm. The clamp was tightened around the culm with bolts (9.5mm diameter, grade 8) inserted into 10-mm diameter holes drilled in
the lugs. The clamps were manufactured with steel plates (ASTM A-36)
of 3.2-mm thickness and 25.4-mm width. This thickness provides good
flexibility for the clamp to deform and follow the geometrical irregularities of the culm.
The clamp applies external pressure to the culm that in turn generates hoop compressive stress. This compression counteracts the formation of longitudinal splitting. The clamp action is similar to the
shrink fit technique widely used to set shafts to wheel hubs in rotatory
machinery that are controlled by radial interference. For the clamp, the
equivalent to circumferential interference (i.e., 2π times the radial interference) is proportional to the difference between the outer circumference of the culm and the curved central length of the semi-rings
[42]. After tightening the bolts, a gap x remains between both the semirings (Fig. 3a). Thus, the circumferential interference Ic, which controls
the clamping force, was calculated as:
2.2. Materials
The bamboo material was obtained from the GA culms between
three and five years of age, with about 12 m in length, growing in the
Eje Cafetero (Colombia, South America) at an altitude of 1500 m. Culms
were supplied by CO2 Bamboo Ltda. (Candelaria, Colombia). The material was borax treated and oven dried as described in literature
[44,45]. The average humidity content was 16.9% (SD 1.6%, n = 5).
Mechanical tests were performed to obtain representative values of
the elastic constant of the GA material and the tensile circumferential
strength. The edge bearing test [46] yielded a circumferential modulus
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Fig. 1. a. Geometrical configuration for the proposed truss. b. Free body diagram of the joint A.
Fig. 2. Joints used to transmit the load from the slat elements to the clamp lugs and critical shear planes for the elements under tensile load.
Ic = Pc − 2La − 2x
(1)
implemented in the pilot trusses described in Section 2.6; both of them
intended to provide redundancy to the joint and avoid early slippage. In
the first variation (Fig. 3b), two drywall screws (4-mm diameter, 25.4mm long) were inserted through the ring of the clamp placed at diametrically opposite positions near the lugs. These screws can easily be
installed after the clamp is tightened. The other option (Fig. 3c) was to
insert a through bolt across the diameter (9.5-mm diameter) into 10mm diameter holes.
where Pc is the perimeter of the culm and La is the interior curved
length of one of the semi-rings. It is recommended to use circumferential interferences that are higher than 10 mm to avoid early slippage
of the clamp and lower than 23 mm to avoid damage of the culm. For
this range of interferences, the torque needed to adjust the clamp is in
the range of 48–68 N m.
Two variations of the aforementioned clamp-culm joint were also
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Fig. 3. a. Simple clamp-culm assembly. b. Clamp-culm with two screws. c. Clamp-culm with a through bolt.
Table 1
Maximum axial load capacity (Fmax) of the three clamp-culm configurations as
shown in Fig. 3. D and t are culm diameter and wall thickness, whereas Ic is the
circumferential interference.
Group
n
D (mm)
(COV)
t (mm)
(COV)
Ic (mm)
(COV)
%H (COV)
Fmax (N)
(COV)
1 (only clamp)
10
2 (2 dw screw)
10
3 (1 through
bolt)
5
110.6
(3.5%)
107.6
(0.3%)
105.5
(1.0%)
10.5
(14.1%)
10.3
(6.3%)
10.1
(8.2%)
17.2
(28.2%)
18.4
(28.5%)
10.4
(1.1%)
14
(6.3%)
14.5
(3.1%)
14.6
(3.2%)
18,275
(18.6%)
18,686
(33.1%)
15,414
(3.3%)
2.5. Experimental set-up and testing procedure
The setup used to test the actual trusses is depicted, schematically
and in actual rendition, in Fig. 5. Three levers, pivoting on radial
bearings at one end and supporting the weight of bricks at the other
end, transmitted the amplified weight to the three central nodes of the
upper chord of the truss. Each lever was made up two steel angles of
50.8-mm × 3.2-mm and weighed approximately 54.9 N. The levers
transmitted the load to the truss through square-thread screws, which
allowed leveling the system before reading the deflections. Small
bronze cylinders were used to reduce friction. The lever mechanical
advantage was 8.6, i.e., each brick of 112.8 N caused an increment of
load of 970.2 N over the specimen. The mechanical advantage of 8.63
of the levers was confirmed by using a load cell that measured the
actual load transmitted to the specimen.
No support was used to prevent the out-of-plane displacement. The
Fig. 4. Test set-up to determine the axial load needed to draw the clamp from
the culm.
Tests, as described in [42], were performed to determine the axial
load capacity of the three clamp-culm configurations (Fig. 3). Briefly,
the test consisted of applying a force to one end of the culm, while the
clamp lugs were supported on a steel pipe (Fig. 4). Table 1 presents the
results of preliminary experiments [42], showing the average of the
maximum axial load for each of the three joint configurations shown in
Fig. 3.
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Fig. 5. Experimental set-up used to test the trusses, a. Levering system, b. Top view.
Fig. 6. Steel brackets used to transmit the load and to support the prototypes.
The vertical displacements were measured at the points shown in
Fig. 7, using four dial gauges with a resolution of 0.0254 mm (0.001″).
Truss stiffness was calculated as the slope of the straight line fitted to
the experimental load-displacements points, where the deflection was
measured with indicator 2 (Fig. 7).
The weight of the levers and the wooden planks was used as the first
load increment. Next, each additional load increment of about 1942 N
was accomplished by placing two bricks on the planks.
During the test of the four replicates of the proposed design, the
maximum load was 13,754 N, which was about twice the service load.
As prescribed in ASTM E73-13, these proof tests were intended to assure that the truss supports a stated load and to determine maximum
bronze cylinders that transmitted the load were visually located to
coincide with the center of the upper chord. U-shaped brackets were
used to support the end clamps and transmit the load to the lugs of the
three upper central clamps (Fig. 6). At a load level of 10,000 N over the
truss, the calculated contribution to the deflections of the steel profiles
supporting the ball bearings was lower than 0.1%, while the contribution of the steel brackets was about 3.1%.
Each load increment was accomplished by adding bricks of 112.8 N
(COV 7.1%) weight over the wooden planks (26.5 N weight) that were
connected to the levers by steel cables. Each load increase was held for
3 min approximately. The total time of one complete test was about
30 min.
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Fig. 7. Positions to measure the vertical displacements marked with red points.
2.7. Description of the proposed design
deflections. In some of the tests of the pilot prototypes, the load was
increased until failure.
The proposed design comprises single culms for the top and bottom
chords (Fig. 8), three-slat elements for the end diagonals and two-slat
elements for the vertical elements and inner diagonals. The conformation with single clamps retrofitted with two dry wall screws were used
in the culm joints. To provide good confinement around the holes of the
diagonal elements under tension, that are under shear stress, curved
washers were inserted between adjacent slats. As the mechanism of
failure of the vertical elements is not shear, only two washers were
used, one at each side of the two-slat element. No restriction about the
clamp location with respect to the natural nodes of the culms is prescribed in this design. Four replicates of the proposed design were built
and tested.
Table 4 presents the culm dimensions of the replicates and the range
of circumferential interferences. With all the materials at hand, a truss
can be assembled in approximately two hours. A preliminary estimation
indicates that the cost of each truss is about US$50, including materials
and workforce. This cost can be accurately calculated in a real production process. The weight of the truss is approximately 19.8 kg (67%)
of GA and 9.7 kg (33%) of steel.
2.6. Description and test results of pilot prototypes
Pilot prototypes with the clamp-culm joints described in Fig. 3 were
developed and tested. These tests were mainly used to assess the facility
of construction and improve the redundancy of the joints and truss
stiffness, which resulted to be a critical factor for design. We next
present a short description of these pilot prototypes. GA elements,
clamps and other connectors were reused in most of the cases.
A description of the joints used in each prototype and a summary of
the results is presented in Table 2. In two of the groups, two clamps
were used to join the culm to the slat elements: one connected to the
slat elements and the other intended to stop the slippage of the connected clamp. In two groups, through bolts were used, which did not
increase the stiffness of the truss. Thus, as a through bolt is relatively
difficult to install, this option was not further considered. One test with
two dry wall screws in the clamps showed a relatively good stiffness of
636 N/mm.
A wide range of 11,811–23,466 N was obtained for the maximum
load. One of the prototypes did not fail at the maximum capacity of the
set up (23,466 N), whereas two others were not loaded to failure to
preserve them for other tests. Eleven specimens failed by clamp slippage, characterized by an important degree of axial displacement of the
clamp (∼1 cm) with respect to the initial position, as depicted in the
first photo of Table 3. Two specimens failed by shear of the end diagonals (second row of Table 3) along the areas shown in Fig. 2. In one
case the upper culm, weakened by a through hole, showed excessive
local deformation near the support (third row of Table 3). In two cases
the upper chord failed by shear (fourth row of Table 3). These culm
failures may be attributed to the propagation of minor fissures caused
during previous tests. No sudden collapse of the truss was observed in
any of the experiments. The maximum load was always higher than the
service load, as shown in the last column of Table 2.
In the trusses with a single clamp without dry wall screws (row 1 of
Table 2) the only mechanism of load support is friction. Thus, they
failed by clamp slippage, two of them at a load of 11,811 N. The insertion of two drywall screws blocks the clamp from the culm and
creates another mechanism of load support complementary to friction,
adding redundancy to the joint.
Based on these experiments, we suggest using at least 30 mm between the clamp and the end of the chords under compression to provide good confinement to the material. This distance should be increased to 50 mm for the chords under tension to prevent the complete
extraction from the culm.
As the dry wall screws add redundancy to the joint and can be easily
installed, the option with a single clamp and two dry wall screws was
finally adopted to build and test four replicates.
3. Theoretical model and structural verification of the proposed
design
The internal forces in the truss might be readily calculated by hand.
However, to have a prediction of the stiffness considering the eccentric
connections between the web elements and the chords, a model was
developed using ANSYS 16.0 (SAS IP, Inc, Chyenne, US). The model is
composed of beam elements and included finite deformations (Fig. 9).
Off-set distances were prescribed to consider the eccentricity of the
joints between the web elements and the chords. A tubular cross section
of 120-mm diameter and 10-mm thick was considered for the culms,
while a rectangular cross section 40-mm wide and 10-mm thick was
considered for all the slats. A Young’s modulus of 9500 MPa was prescribed for GA, according to NSR-10 [43]. Spherical joints were used to
transmit only forces at the ends of the web elements. This model does
not include complex sources of nonlinearities present in the real
structure. However, it provides a reasonably good approximation of the
first part of the load-deflection curve, which is approximately linear.
Three vertical loads W/3 were applied at the central upper nodes
while the ends of the truss were considered to be simply supported. The
maximum compression force (−1.25 W) occurs in the central portion of
the upper chord, while the maximum tension (1.06 W) occurs in the end
diagonals (Fig. 9).
For the service load, Table 5 presents the structural verification of
the design. For the clamp joints, a safety factor was calculated as the
ratio of the mean axial load capacity reported in Table 1 over the load
transmitted by the clamp.
The maximum deflection predicted for the service load of 7063 N is
11.4 mm, which is lower than the admissible deflection of 12.5 mm, as
prescribed by the Colombian Standard [43]. The consideration of the
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Table 2
Summary of the results of the pilot tests.
Type of joint
Truss stiffness range (N/
mm)
Range of maximum load (N)
Type of failure
Range of ratio of maximum load over
service load
Single clamp (n = 4)
416–585
11,811–17,639
Clamp slippage (n = 4)
1.67–2.50
Double clamp (n = 3)
429–600
11,811–23,466
Clamp slippage (n = 1)
Two other did not fail
1.67–3.32
Single clamp, through bolt
(n = 5)
354–493
11,811–21,523
Shear failure (n = 2)
Local deformation end clamp
(n = 1)
Culm failure (n = 2)
1.67–3.05
Double clamp, through
bolt (n = 2)
400–618
19,580, both
Clamp slippage (n = 1)
Shear failure (n = 1)
2.77
Single clamp, two dw
screws (n = 1)
636
13,654
No failure
1.93
3.6%), which is approximately 8.4% higher than that predicted with the
theoretical model (578.3 N/mm). No out-of-plane displacements were
visually noted.
For the service load of 7063 N, the experiments show a maximum
deflection of about 10.5 mm, which is lower than that of the maximum
deflection of 12.5 mm, as prescribed by the Colombian Standard [43]
for floors.
The differences in the experimental curves might be explained by
the inherent variations of elastic constants and dimensions of natural
materials. As the main sources of nonlinearities, i.e., slippage at the
joints, permanent plastic deformation of the clamp lugs and deviations
of the axis of the elements with respect to a straight line, were not
included in the ANSYS model, the theoretical curve is linear, while the
experimental curves show decreasing stiffness with load increments.
The nonlinear effects mainly arise at higher loads levels, which might
off-set distances in the connections of the web elements to the chords
reduces about four times the stiffness as compared to a model with
centric connections.
4. Results and discussion
No replicate of the proposed design showed any sign of failure when
loaded at 13,754 N, which is approximately twice the service load. This
load level supported by the four replicates of the proposed design is
higher than the failure load of two of the examined trusses without
drywall screws (row 1 of Table 2), that showed slippage at load levels of
11,811 N, demonstrating that the drywall screws increase redundancy
of the joint. The load-displacement curves showed reduction of the
stiffness with load increments (Fig. 10). At a load level of 9869 N (approximately 1.4 the service load), the stiffness was 631.3 N/mm (COV
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compared to using laminated bamboo. The slats can be cut from those
culms that do not conform to the geometrical constrains as prescribed
in the construction codes (e.g., conicity) or have substantial longitudinal splitting due to previous treatments (such as immunization,
drying, etc.). Therefore, this system reduces material wasting, and it
could be implemented in prefabricated processes. Additionally, these
trusses provide applications to increment the utilization of a sustainable
non-conventional material like GA at a competitive cost. In turn, more
use of GA in construction might create economic incentives to increase
the plantations. GA forests (guaduales) play a key role in preserving the
environment as they control water cycles, reduce erosion and capture
CO2.
As compared to our previous proposed trusses made only out of GA
slats [41], the construction of the combined culm-slat truss is simpler
and less vulnerable to failure by lateral instability and buckling of the
upper chord under compression, since the tubular shape is well optimized by nature for this application.
In a real application to support floors, the trusses can be transversally connected with the GA slats to conform a fully tridimensional
structure. The transverse connections also prevent out-of-plane displacement and reduce the buckling load of the upper chord. We are in
the process to assemble and test a full scale 3 m × 3 m floor prototype
that we expect to describe in future publications.
One limitation of the study is that the mechanical tests consisted of
monotonic static load, which does not allow detecting the response of
the structure under cyclic forces. In addition, the developed linear
model is only useful to estimate the truss stiffness under service loads
but it has a limited value to understand the influence of many sources of
non-linearity that arise at higher loads. Thus, experimental studies with
the joints have to be carried out to obtain characteristic load-deflection
curves to be input into the computational model to reproduce the
nonlinear behavior of the structure under different loading scenarios.
The span of the prototypes developed in this study was chosen to accommodate a typical modular unit oriented to low-cost housing. More
tests have to be conducted to expand this design to longer trusses that
might be used for other applications, such as pedestrian bridges.
Additionally, more studies have to be performed to analyze the increments of deflections with time.
Table 3
Type of failures observed in the pilot tests.
Type of failure and description
Clamp slippage
Shear failure in two-slat diagonal elements
Excessive local deformation in through-bolt
clamps
Shear failures of the upper culm
explain higher differences in the curves for larger deflections.
This study shows that the combined culm-slat GA trusses could be a
sustainable alternative for a wide variety of applications. However,
other experimental studies need to be conducted with a higher number
of specimens to characterize the joints under various conditions and
geometrical configurations for determining allowable levels of stresses
for design. In addition to reducing the propagation of longitudinal fissures and creating ductile modes of failure, the construction process
with the clamp joints is relatively simple. This is because no complex
parts are required and there is no need to use mortar injection.
This truss system is composed of raw culms and slats. Thus, the
energy consumption to use the material is kept to a minimum, as
5. Conclusions
The trusses proposed can be a sustainable alternative to support
roofs and floors of low-cost housing projects using prefabricated processes. However, more experimental studies have to be conducted to
fully validate the use of these trusses under construction codes. Other
applications of these trusses might be short pedestrian bridges in rural
areas, but more studies on the topic are needed to accomplish good
for longer spans
Two dry
wall screws
Two slats
Three slats
Fig. 8. Representation of the proposed truss.
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Table 4
Range of diameter (D), circumferential interference (Ic) and thickness of the four replicates of the proposed design.
Upper chord
Lower chord
Replicate
1
2
3
4
1
2
3
4
D (mm)
Ic (mm)
t (mm)
119–121
16–22
10.2–10.8
112–118
11–23
9.5–10.4
112–117
10–22
9.9–10.7
110–112
11–23
9.0–9.4
114–119
12–20
10.6–11.2
114–117
10–21
10.1–10.7
110–115
12–18
10.1–10.8
111–112
20–25
9.6–10.0
Fig. 9. Compression and tensile load and elements numeration.
Table 5
Structural verification of the proposed design. The identification of the elements and clamp joints is presented in Fig. 9.
Element ID
Clamp Joint ID
Identification
Internal Force (N)
Admissible load (N)
Criteria to calculate the admissible load
C1
C2
C3
C4
T1
T2
T3
−6639
−3532
−8829
−2331
7847
2543
6639
−10,074
−11,411
−10,074
−11,411
12,474
8316
39,301
Buckling load for columns under provisions of Colombian Norm [43]. Diameter equal to 100 mm and
thickness of 10 mm
Shear in connection. Shear area = 2 * 35 * t * n mm2, n number of slats, t = 9 mm, admissible shear
stress = 3.3 MPa
Tension element, admissible stress of 13.9 MPa
Identification
Force (N)
Safety factor
Criteria to calculate the safety factor
J1
J2
J3
J4
J5
6639
6639
2190
0
0
2.81
2.81
8.53
NA
NA
Mean maximum value of 18,686 N reported in Table 1 for clamp-culm joints retrofitted with two dry
wall screws
Acknowledgments
We thank Jaime Buitrago for his critical review. This research was
financially supported by grant C.I.138-52110517 of the Universidad del
Valle. R. Morán is grateful to Administrative Department of Science,
Technology and Innovation COLCIENCIAS for financial support through
a national doctoral grant.
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