Resistance tests

Evaluated culms belong to four species of bamboo plants grown in the Huasteca region of the state of San Luis Potosi, Mexico: Bambusa oldhamii Munro, Bambusa vulgaris var. vulgaris Schrad, Bambusa vulgaris ‘Striata’and Gigantochloa verticillata Willd. The collection site belongs to the experimental agricultural field of the National Institute of Forestry, Agriculture and Livestock Research (INIFAP), located in the municipality of Huehuetlán, San Luis Potosí, México, located at 21º 28’ 43” north latitude and 98º 58’ 30’ west longitude with an altitude of 92 m. In the Köppen’s classification the climate is called Cfga (humid temperate climate without dry season, average annual temperature of 23 °C and annual precipitation of 1 800 mm) (^{Pineda, 2005}). Plant materials were established 10 years ago from the state of Veracruz, México.

Three culms per plant of three years of age were collected, with heights of more than 12 m and thicknesses between 8-9 cm. These were cut to the first six meters from the base to facilitate transportation, the apical part was discarded and allowed to air in the shade and room temperature.

The resistance properties were determined 160 days after collection of the culms. For this, the culms were previously divided into three sections of equal size: basal, median and superior (2 m each).

Resistance tests were performed for compression flexion and tension. These properties are required for the design of structures such as greenhouses. Due to the lack of a specific construction standard with bamboo in Mexico, it was taken as reference the Colombian technical standard NTC-5525 2007, test methods to determine the physical and mechanical properties of Guadua angustifolia Kunth., of the Colombian Institute of Technical and Certification Standards, based on norm ^{ISO 22157 (2004)} Bamboo: determination of physical and mechanical properties.

The test specimens for the compressive strength test were prepared according to the aforementioned Colombian standard NTC 5525, without knots, with a length equal to the external diameter and its ends with flat surfaces and at right angles to the length. The probe was positioned in such a way that the center of the press head was located in the center of the cross section of the specimen.

The specimens for the flexural strength tests were mounted with a length between the supports centers of 30 times the outside diameter, with a variable number of knots per specimen. Mounting culms for bending had four points, applying the load of the centers of the frames of the charging device at two points in the middle third of the distance between the supports. Stubbing culms was avoided using a plate and metal support of 15 cm long.

The test specimens for tensile strength tests parallel to the fibers were prepared with a total length of 60 cm, in the middle of which was left a test portion of a rectangular cross-section measuring 2.5 cm wide and 10 cm length independently of wall thickness. For mounting the specimen in the testing machine a distance of 30 cm between the end catchers was established.

Each test was repeated three times for each section of the culm (low, medium and high) for each species, giving a total of 36 measurements.The mechanical tests were performed in the mechanics laboratory of soils and materials of the Faculty of Engineering of the Autonomous University of San Luis Potosí. The compression tests were carried out on an AUTOMAX 5 press and the bending and tensioning tests on a Forney universal test machine.

The obtained data were statistically analyzed by analysis of variance and comparison of means to see if there are differences between sections of the culms (low, medium and high), as well as among the four tested species, the Tukey’s test with a confidence level of 5% was used. Both were conducted with the experimental designs software FAUANL package Version 1.1 (^{Olivares, 2012}).

Greenhouse agronomic and structural design

The prototype was thought to be built in the center of protected agriculture from the Faculty of Agriculture of the Autonomous University of San Luis Potosí, México.The location is 320 km east of the collection site (Huehuetlán, SLP), with geographic coordinates 22° 09’ 00” north latitude and 100° 59’ 00” west longitude with a height of 1 874 masl. Due to the semiarid climate of the place, the greenhouse design was based on this climate. For this purpose the literature was consulted to determine the desirable characteristics of a greenhouse for semiarid áreas in México: ^{Matallana and Montero (1995)}; ^{Castellanos (2004)}; ^{Sánchez del Castillo (2007)}; ^{Muñoz (2009)}. The characteristics to define for the agronomic greenhouse design were: shape, height, ventilation, live and dead loads for tutoring, cover features and physical barrier to pests.

Based on the resistance values and the agronomic design defined above, and the maximum wind speed (110 km h^-1), ideas of basic structural frameworks were proposed in order to favor such conditions. These models were captured using the STAAD.Pro^®V8i software, which is widely used in three-dimensional structural analysis designed with an open architecture for static and dynamic analysis of concrete, steel, aluminum, and wood structures, in bridges, buildings, towers, stadiums, etc. (^{Zertuche and Hernández, 2014}). Using this software the behavior of structures under extreme conditions were simulated and maximum efforts were calculated at different critical points of the structure to verify that they were within safety margins. Finally the best model was chosen for its simplicity and structural stability.

The design of the foundation was based on the Building Regulations for the Distrito Federal (^{GODF, 2004}), which specifies the design strength to limit state failure and specified resistance factors to both the structure and its foundation resist shear forces, twisting moments of mezzanine and earthquake-induced turning moments combined with those corresponding to other stresses and affected by the corresponding load factor.

Estimation of involved CO ₂

CO₂ emission of that would be generated by steel production for the construction of a greenhouse with dimensions and characteristics equivalent to those of the bamboo prototype was estimated. Based on the recommendation of the Mexican standard for the design and construction of greenhouses NMX-E-255-CNCP (^{DOF, 2008}), which mentions the compliance of 5 to 6 kg of steel per m² of the greenhouse. Furthermore CO₂ emissions of steel manufacture were considered, which is 1 700 kg CO₂ for 1 000 kg Steel (^{WSA, 2008}).

Likewise, CO₂ sequestered in biomass of bamboo culms was estimated by calculating the total of the structure once built dry weight. Three representative samples of each species were previously oven dried with forced air at 65°C to constant weight. The weight obtained was multiplied by the factor of 0.5 whereas 50% of the plant biomass is composed of carbón (^{Brown, 2002}; ^{Castañeda et al., 2005}).

Agronomic greenhouse design

The criteria for agronomic design were made taking into account the recommendations of ^{Matallana and Montero (1995)}; ^{Castellanos (2004)}; ^{Sánchez del Castillo (2007)}; ^{Muñoz (2009)}. Table 1 shows the criteria.

Table 1 Criteria of agronomic design for the prototype of bamboo greenhouse. 

Greenhouse structural design

The desing proposed for the basic structural framework is shown in Figure 2, which arises from STAAD.Pro^®V8i software. The design integrates the features and specifications resulting from the resistance of culms of B. oldhami and agronomic design. Each frame is supported on four columns with a 3 m gap between them, each consisting of two bamboo culms. The geometric shape is two-water with an inclination angle of 18.5°. The three sections were structured, at its top (3 to 4.5 m) in a triangular shape in order to increase the resistance and stability of the greenhouse.

Figure 2 Detail of the basic structural framework emerged from STAAD.Pro®V8i software. 

Joints and joint elements of the structural frame are illustrated in Figure 3. The main connection piece between culms is the threaded rod of galvanized steel 3/8 with 3/8 washers welded at one end and fastened with a screw (under “T”), or in some cases only with screws at both ends. For 90° joints a “fish mouth” type cut was held. For joints smaller than 90° a “slant” type cut was made. The stops are bamboo pieces of 8 cm thick that are used to connect bamboo pieces with 45° angle. Figure 3 also shows in a perspective of 90°rotation the detail of the joints of the columns carrying two culms, there is a transverse connection at the height of the channel in the first column, and two in the second column to join a crossbar and a spar.

Figure 3 Complete prototype model of bamboo greenhouse showing details of the connections. 

The basic structural framework can be repeated indefinitely to extend the greenhouse (if desired), longitudinally, transversely or both. In this paper, for experimental purposes, the prototype was defined in longitudinal shape at 15 m. For this, seven structural frames were required, which were joined at the top by nine 15-meter stiles. The total area of greenhouse prototype is 135 m^-2 with a 3 m height at the cannel and 4.5 m total height (Figure 4). For the placement of the plastic of the greenhouse, a profile and fastener wire were placed in zig-zag form in the outer contours that joined to the culms by “butterfly” type screws.

Figure 4 Overview of bamboo greenhouse prototype emerged from STAAD.Pro®V8i software. 

The arrangement of the pads for the greenhouse foundation is shown in Figure 5, with a total of 28 elements. The foundation model has a depth of 55 cm, which is calculated according to the depth where the solid subsoil was found where the greenhouse construction is planned. In each pad, it is projected to place two bamboo poles mounted on a number 4 rod with a concrete filling for fixing and clamping.

Figure 5 Detail of the design and location of the pads. 

Environmental benefit of using bamboo

The procedure used to calculate the carbon released in steelmaking is shown in Table 2, where the use of 5 to 6 kg of steel per m² of greenhouse is considered, according to the norm NMX-E-255-CNCP (^{DOF, 2008}). In this case it is estimated a total of 1 147.5 kg of CO₂ emissions by the steel used in a greenhouse with the dimensions of the bamboo prototype. On the other hand, the procedure used to calculate the carbon sequestrated in the bamboo biomass is shown in Table 3.

Table 2 Calculation memory of CO2 emissions released into the atmosphere during the production of steel. 

Table 3 Calculation memory of CO2 sequestered in structural bamboo biomass. 

The estimated value is 463.8 kg of C and 1 699.5 kg of CO₂. Summing up the amount of CO₂ that would remain fixed in the structural bamboo biomass plus that which would stop being dumped into the atmosphere during steel production for an equivalent structure, the environmental benefit of using a bamboo structure instead of a steel one can be estimated in 2 847 kg of CO₂.

The foregoing without considering other environmental benefits not evaluated in this work, such as CO₂ which remain fixed in the ground where bamboo grows, amount that according to ^{FAO (2002)}, might be significant. The bamboos not only store carbon in foliar biomass, but also contribute to the existing ground biomass through their root systems and the contribution of waste to the soil (branches and leaves) and thus, soils are the largest carbon reservoir of the terrestrial cycle of this element.

45 months after the design and construction of the greenhouse prototype with bamboo structure, this has remained stable and firm. The maintenance work has been reduced to adjustments of fastening screws and varnishing with castor oil. It is considered that the plastic cover has contributed to reduce the wear of the culms by avoiding contact with rainwater and reduce the incidence of UV rays, both considered agents with degradation power of natural materials.