High Tunnel Green House

The University of Idaho's high tunnel greenhouse team, Stand Your Ground, is tasked with designing a greenhouse for harsh environmental conditions. Currently there have been failures in greenhouses due to strong gusts of wind that cause structural damage and this project is to design a greenhouse to prevent such damage.

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 (minimum of 60 mph)
 * Withstand snow loads of the area (20 psf according to snow maps)
 * 10 year 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

Design Concept Development
 Initial Phase 

The initial phase of the design was documenting and collecting data about the site and environment. The information collected can be found under environment conditions in design data. This data concluded that the site rarely experienced wind speeds above 75 but to have a safety guideline for the greenhouse the ASCE 7-10 building code was used, requiring that the structure survive a gust of 105 mph. Using tables and equations based on the site conditions equated to a force of about 28 psf from wind loads. Using snow charts provided by the university of Idaho, the snow loads equated to be less than 20 psf on the structure.

 Design Phase 

The design phase was testing the idea of adding windbreaks to the end walls to prevent them from failing. The data collected during this phase can be located under the wind breaker section of the design data. It was determined that a greenhouse with a windbreaker at the ends would help insure that a large percentage of the wind would deflect off the ends and prevent them from blowing in. Using a design matrix it was determined that a Gothic style wind break would have the most benefits to achieving the clients needs. This was followed by a second wind tunnel test as well as a CFD analysis to determine how the forces would act on the structure and how much force was to be expected. Using the data generated from these tests a structure was created to pass the 105 mph wind speed requirement.

 Final Phase 

This phase is creating and documenting how to assemble and build the greenhouse. This process mainly involves attempting to breakdown the greenhouse and reassemble it virtually so that the client can build the greenhouse physically. The results of this phase are posted in the design documents under greenhouse instruction manual.

Design Data
 Greenhouse Design Types 

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

Environmental Conditions
 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.

It is not required in Idaho that a greenhouse meet ASCE standards but they will be used in this project as a safety guideline. The greenhouse is a risk category I building so all values of velocity will be using the 105 (mph) value.

 Wind Loads 


 *  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.

 Wind Tunnel Test Models 

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.

Second Wind Tunnel Test
After the final design model was selected, a more accurate wind tunnel model based off the developing design was created to get more accurate results. This test was not only to test head on winds but winds hitting along the flat face of the wind break. This test showed that our design, with a shorter bow, was less efficient than our original test which had a steep angled bow. The test showed that the windbreak design reduced about 30% of the force against the greenhouse whether or not it came from head on or if the wind hit its broadside.

 

CFD Analysis
A CFD analysis was performed on the windbreak greenhouse design to better understand the distribution of forces. The results of these tests were then used as a comparison against the flat face of a standard greenhouse. What was found was a majority of the force was concentrated at the bow of the windbreak during head on wind, as can be seen in the first picture below. When the wind was shifted to meet the broadside of the windbreak the force concentrated itself along the upper edge of the flat front of the windbreak, as can be seen in the second picture. Both these results showed a reduced amount of force along the greenhouse when compared to the flat face. Using the 105 mph wind speed as a safety guideline, the maximum force during head on wind resulted in 31.6 psf and the broad sides max was about 32 psf. The flat face of a standard greenhouse did the worst with a maximum of about 45.3 psf.

 

Structural Design
Using the results of the CFD analysis a frame structure was created that could withstand the 105 mph wind force. At first, welded members were used to hold the frame together but the client preferred not to have to weld the frame. This brought about the idea of using brackets and to support the frame members. The galvanized steel brackets each had an axial load rating of 2000 lb with a 2:1 safety factor. Using these brackets and the CFD results the frame structure below was developed. Using the maximum stress state it was found that the structure passed the ASCE 7-10 safety guideline imposed on the design with a safety factor in excess of 1.

 

Final Design
Using the information gathered and generated resulted in the final product shown below. Although not required for the Idaho area, the wind tunnel can withstand the ASCE 7-10 wind load requirement of 105 mph and can withstand the areas average snow load. The wind tunnel only has a windbreak on one side as the client did not wish to have a windbreak on a side that rarely receives wind. This allowed us to accommodate the opening for the skid-steer the client asked for on the non-windbreak side. This design meets all the requirements the client set down and fell well under budget. Final results can be seen below.

 

Stand Your Ground Team
Together we make up Stand Your Ground

Design Documents








 Technical Presentation 





 Expo Poster