High Tunnel Green House

This project is to create a greenhouse that can withstand strong winded areas in the northwest. This project specifically targets Buhl, located near Twin Falls Idaho.

Stand Your Ground
Stand your ground is the design team in charge of the project and it is composed of:

Sponsor
The University of Idaho extension program is the sponsor of this project and Tony McCammon, of the University of Idaho extension program, is the liaison for what the extension program desires for the project.

Problem Statement
Given the high wind speeds and average snow loads in the Buhl, Idaho area, the University of Idaho extension's high tunnel greenhouses have been failing. Our project is to design the ideal high tunnel greenhouse to survive the area's environmental conditions.

Interview with Tony McCammon (summarized)

 * The site is located on the north west side of a pond
 * The design is to be a high tunnel greenhouse with rolling up sides to allow for passive ventilation in the summer
 * There has been a history of snow load damage and the end walls blowing in due to wind loads
 * The project is to see if a better design for high tunnel greenhouses can be achieved for high wind speeds and snow loads
 * The minimum specs are for 35 MPH sustained winds with 65 MPH gusts.
 * The design team is free to redesign the greenhouse to whatever is necessary
 * The plastic sheeting for the outside of the greenhouse must last a minimum of 5 years but a 10 year life span is expected if it is possible.
 * The structural frame must last a minimum of 10 years.
 * The orientation of the greenhouse is fixed with winds coming from the north west.

Design Goals and Specifications

 * Withstand high wind speeds
 * Withstand heavy snow loads
 * Long lifetime
 * Door to fit a small skid-steer loader
 * Roll up sides for passive air ventilation
 * Must fit within a 24 x 40 area

Deliverables

 * Design analysis
 * Cost analysis
 * Prototype
 * Final Design
 * Possible final model

Initial Phase
Collecting data on greenhouse types and environmental conditions, the results of these can be found in Desing Data as: Location Wind Speeds, Wind Loads, Snow Loads, and GreenHouse Design Types. The greenhouse design types have been limited to the two most popular types, Gothic and the Quonset, for the purpose of time limitations.

Design Phase
The design phase determined that a greenhouse with a windbreaker at the ends would help insure that most of the wind will deflect of the ends and prevent them from blowing in. It was also the Gothic style had the most benefits to achieving the clients needs.

Current Phase
The current design phase right now is beginning a scaled model of the final design and to construct the windbreaker that will fit on the ends of the green houses

Greenhouse Design Types
The two pictures above demonstrate the two most popular design styles for greenhouses.

Location Wind Speeds

 * Latitude: 42.3900
 * Longitude: -114.4439


 * ASCE 7-10 Wind Speeds
 * (3-sec peak gust MPH*):


 * Risk Category I: 105
 * Risk Category II: 115
 * Risk Category III-IV: 120
 * MRI** 10 Year: 76
 * MRI** 25 Year: 84
 * MRI** 50 Year: 90
 * MRI** 100 Year: 96


 * ASCE 7-05: 90
 * ASCE 7-93: 70

Wind speed information is Provided by the Applied Technology Council.

The greenhouse is a risk category I building so all values of velocity will be using the 105 (mph) value.

ASCE 7-10 Velocity Pressure

 * qz10= 00256.0V*Kz*Kzt*Kd*V^2
 * where:
 * qz10 = ASCE 7-10 velocity pressure evaluated at mean roof height (psf)
 * Kz = velocity pressure exposure coefficient
 * Kzt = topographic factor
 * Kd = wind directionality factor
 * V = basic wind speed (mph) from ASCE 7-10 maps referred to as ultimate wind speed maps in 2012 IBC.

Assuming all correction factors K=1 for exposure C and a mean roof height that will be < 30ft and head on wind:
 * qz10=28.224 (psf). This would be assuming a worst case scenario for the greenhouse.

ASCE 7-10 data is provided by the American Wood Council

Velocity Pressure using tables
Using the ASCE 7-10 Table 27.6-1 for exposure C and assuming the maximum height of around 20ft, the wind pressure is read to be between 23-27 (psf). This value will be slightly higher then expected values as the table ends at 110 (mph) instead of the wind velocity provided by the Applied Technology Council of 105 (mph).

Snow Loads
Using the interactive snow load map provided by the University of Idaho, the worst case snow load can be assumed to be about 20 (psf).

Wind Tunnel Test
The University of Idaho Extension office would like a high tunnel greenhouse design to withstand the extreme weather conditions in the Buhl, Idaho area. This experiment was performed to determine if a style of high tunnel end-type and wind speed plays a significant role in drag force on a high tunnel greenhouse. To perform this analysis, four models were created and tested inside of a wind tunnel at two different wind speeds, 30 and 60 mph. The models were gothic and Quonset styles with and without windbreak features. The data collected from this experiment was then used in a factorial analysis. This analysis showed that there were significant differences in drag force caused by each wind speed, model types, and their interaction. An additional ANOVA and Tukey test was conducted with the 60 mph data, due to design parameters, to determine statistically significant differences between model types. This testing showed that each type was statistically different. The two models with the least mean force were the gothic and Quonset with windbreaks. The Quonset, with a windbreaker, had the lowest mean of 4.4 N and the Gothic, with a windbreaker, had a mean of 5.9 N. The Gothic with a windbreaker was chosen for the final design due to the small practical difference between the drag force means, its ease of construction, and snow shedding abilities. For more information look at the document Wind Tunnel Report.

Decision Matrix for Final Model
Through external research, it was found that the gothic style has better qualities for shedding snow loads. The feasibility of construction is also an important factor in design selection. These factors were then rated where a higher number meant that the design was better in a particular area than one with a lower score in the same area. The wind data generated lead to the results in the wind load section. The numbers were based off of practical differences, such as the gothic and Quonset without windbreakers were practically better than a square end but were not greatly different between each other and the windbreaker models were better than those without, but also not greatly different from each other. The weighing total was based off of what was considered important for the client, where wind load and snow sheading abilities were considered of most importance and speculated cost and ease of construction were secondary, space was of minor concern for the client. The following table, Table 6, summarizes the selection criteria for the recommended design.

The matrix shows that the research leads to the conclusion that the gothic style with a windbreak would be the most beneficial design for the client’s application. The ease of construction, snow load shedding capabilities, and the significant drag force reduction on the end walls are the main reasons for this decision. The Quonset windbreak design’s advantage of drag force reduction does not outweigh its difficulty of construction and snow shedding capability. Therefore the project will investigate the best methods to build, attach, and accommodate a gothic style high tunnel with a windbreak to meet the client’s specifications.

Design Documents






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