1. Introduction
Agricultural facilities can be generally defined as the facilities that are used to improve the environment of cultivating and/or breeding animals and/or plants for enhancing the qualities and/or quantities of agricultural products as revealed in the technical guidelines issued by the European Commission Joint Research Centre [
1]. Among the various agricultural facilities, greenhouses have an over three-hundred-year-long history and have been evolved with technologies to satisfy the needs of modern agriculture. It is known that the basic function of a greenhouse is to provide a well-controlled indoor environment for optimal growth and productivity of the target crops. In addition, the greenhouse should be capable of protecting the target crops from frequent natural hazards and pest intrusion. Therefore, the design and construction of a greenhouse are involved with several important factors. Von Elsner et al. [
2] made a comprehensive overview on the main factors influencing greenhouse design and the observations of the factors from several European countries. Moreover, in an accompanying paper [
3], the characteristics of several greenhouse designs adopted in European countries were demonstrated and their advantages and disadvantages were discussed. Villagran et al. [
4] investigated the spatial distribution of temperature and relative humidity inside a greenhouse built in the Colombian Caribbean. Flores-Velazquez et al. [
5] used a validated computational fluid dynamic model to improve the design of the greenhouse mechanical ventilation system in three postulated configurations. López-Cruz et al. [
6] made a review on the development and analysis of dynamical mathematical models of greenhouse climate and addressed the importance of dynamic models in understanding, optimizing, and controlling of the greenhouse system.
Greenhouse structures are generally constructed using thin-walled steel or aluminum frames covered with glass or plastic cladding. Owing to the characteristic of lightweight, wind load is the major concern for the structural design of greenhouses, especially in the typhoon-prone regions [
7,
8]. Emekli et al. [
9] conducted numerical analyses to examine the safety of five selected types of greenhouse structures under design wind loads in the Mediterranean region of Turkey. Saltuk [
10] used the finite element analysis program SAP2000 [
11] to design and analyze a sample glass-covered gable-roofed greenhouse in Antalya province, Turkey. Indore et al. [
7] used finite element method to investigate the member forces of some common greenhouse frames under load combinations in India and suggested some revisions to the specifications. These studies indicated that appropriate structural analysis and design are necessary for the safety of greenhouses under wind loads.
Taiwan is located at a natural hazard prone region. In addition to seismic attacks, periodic typhoons and torrential rain are even more common in general. In recent years, due to significant climate change, strong wind and heavy rain become the most potential factors for agricultural disasters, especially for high-valued flowers and vegetables. Hence, it is important to develop hazard-resistant gardening facilities to increase the agricultural resilience. The Council of Agriculture (COA) of Taiwan has been promoting the application of greenhouse structures for pest protection and disaster mitigation. It has been over three decades in the history of agricultural greenhouse in Taiwan [
12]. Originally, according to the Taiwan Building Code, design and construction of greenhouse should be performed by licensed architects. Several developed countries have adopted more flexible design codes for greenhouses to enhance their agricultural competitiveness [
13,
14,
15]. Hence, the COA has proposed an alternate approach to reduce the cost for the design and construction of greenhouses. That is the architectural commitment is no longer mandatory if the greenhouse is designed and constructed as one of the standard types issued by the COA. There are nine standard types of greenhouse structures and six of them (designated as UTP, UBP, VTP, VBP, WTG, and SP) were designed with a wind-resistance level of Beaufort Scale 11 [
16]. The rest (designated as UP, LT, and LTP) were proposed as simple plastic film or net greenhouses and no specific wind-resistance level was prescribed. Nevertheless, the common greenhouse structures made of galvanized steel are usually expensive as compared with the crops for general farmers, so most of them cannot afford it. Therefore, it will be beneficial to those farmers if the price of greenhouses could be moderately or significantly reduced. For examples, Pack and Mehta [
17] proposed an affordable greenhouse to reduce the costs for east Africa. Saglam et al. [
18] proposed a prefabricated simple greenhouse structure with plastic columns and a series of novel model units that permits an inexperienced person to quickly and easily construct.
In fact, there are many disposable plastic products in daily life. According to the statistics publicized on the Recycling Fund Management Board of Environmental Protection Administration (EPA) in Taiwan (
https://recycle.epa.gov.tw/ConvenienceServices /Downloads accessed on 30 September 2021), around 80% of those disposable plastic products are recycled every year, including Polyethylene terephthalate (PET), Polyethylene (PE), Polypropylene (PP), Polystyene (PS), and Polyvinylchloride (PVC). It is a very stable recycled rate. If the recycled plastics could be reproduced through appropriate process and manufacture, they may be a suitable alternative material for simple greenhouses. Hence, a composite plastic made of recycled PET, Nylon, and glass fiber reinforced Nylon was produced and applied to the structural frames of a prototype simple greenhouse. An appropriate section was determined for the composite plastic frame members and used in the structural design of the prototype simple greenhouse. Numerical comparisons for the performances of the simple greenhouse models made of conventional galvanized steel and the composite plastic under static wind load were carried out. After the performance evaluation, the composite plastic structural members and associated fasteners were manufactured and the prototype simple greenhouse was constructed. On-site free vibration tests of the prototype greenhouse were conducted to estimate its fundamental vibration period and structural damping.
4. Conclusions
A composite plastic material made of recycled PET, Nylon, and glass fiber reinforced Nylon was proposed for application to a simple greenhouse structure. The composite plastic material has a lower elastic modulus and yield strength as compared with conventional galvanized steel. Therefore, comparisons of member strengths and sectional rigidities were conducted to determine the appropriate section dimensions for the simple greenhouse structures. Performances of two greenhouse models constructed with the conventional SSC400 galvanized steel pipes and the composite plastic 田-sections under equivalent static wind loads were evaluated. The analysis results revealed that under the specified Beaufort Scale 11 wind load, both the galvanized steel and the composite plastic greenhouse models had comparable peak responses. However, with a significantly larger flexural strength for the designed 田-sections, the composite plastic greenhouse model was more resistant to the design wind load.
Production of the composite plastic members and custom-made fasteners were completed and a prototype greenhouse structure was constructed. On-site free vibration tests were conducted to validate the design assumptions and assembly procedure. The test results show that the prototype greenhouse had similar vibration period to that predicted from the numerical model. Moreover, the composite plastic greenhouse can have an average damping ratio of 6.2%, which is larger than the conventional assumed 2% damping for the galvanized steel greenhouse. Therefore, the composite plastic greenhouse can have better vibration mitigation under wind excitations. However, because the applied wind load may vary with the structural dimensions, such as structural height and width, the sectional and structural dimensions should be adhered to the current recommendations. Further studies should be conducted if the proposed composite plastic members would be applied to different structural types of greenhouses.