High Tunnel Research: Parts of a High Tunnel
- [Instructor] Welcome to Penn State's High Tunnel Research Facility.
Today, we are going to familiarize you with the parts of a high tunnel, specifically, the collars, bows, purlins, trusses, crossbraces, hipboards, track, and baseboards.
Let's begin with the collars.
The collars are used to provide stability and anchor the tunnel structure to the ground.
Some high tunnel providers refer to the collars as anchor posts or ground posts.
They are metal tubes typically two to three feet in length with holes drilled through on one end of the pipe.
Eventually, bolts will be inserted through these holes to connect to other parts of the structure.
Usually 2/3 of the collar is hammered into the ground.
This process is repeated for all required collars of the high tunnel.
The number of collars differs depending on the length of the tunnel and the span between the collars.
Our collars are placed four feet apart.
However, your installation instructions may require different spacing.
Next, let's look at the bows.
The bows are also known as ribs or hoops.
The bows make up the frame of the high tunnel and are inserted into the collars.
They are available in a wide variety of sizes depending on the company's options and the grower's needs.
Steel is the most common material from which bows are made.
There are two types of shapes available: gothic or peaked and round or hooped.
A gothic shape is recommended in northern regions as snow is more likely to slide off quickly.
Growers are encouraged to make sure the tunnel shape and bow strength is appropriate for their region as tunnel collapse from snow load or damage from high winds are common and costly problems.
Next, let's look at the purlins, trusses, and corner crossbraces.
The purlins provide structure and support between the bows.
Each purlin is made up of multiple pieces depending on the tunnel length and is attached based on installation instructions.
Trusses or cross ties are attached to the bows to strengthen the tunnel against wind and snow.
They are more necessary in wider tunnels where a greater span exists.
Generally, they are not used in tunnels that are 17 feet wide or less.
However, this may vary depending on the bow strength and the potential snow loads in your area.
Both trusses and purlins can be used to assist in crop production within the tunnels.
The corner crossbraces should be placed as recommended by the installation instructions to keep the tunnel parts square with each other and to prevent the tunnel from shifting in the wind.
Next, let's look at the hipboards, track, and baseboards of a high tunnel.
The hip boards provide the backing for the metal track used to attach the plastic covering to the tunnel.
Please note that the hipboards are optional depending on the type of track being used.
Track can only be attached directly to the bows without the hipboard if the track base is sufficiently rigid.
Track is available in a cap style where the cap is snapped into a base pinching the plastic between them or in a wired style using wiggle wire or zigzag wire to attach the plastic.
Some high tunnel providers have pre-drilled holes in the bows for hipboards and track.
If no holes are present, the grower will need to use pipe straps during hipboard or track attachment.
The hipboard or track height determines how far the roll-up side walls can be vented.
If placed too low, hot air will be trapped in the upper portion of the tunnel and venting will be less effective.
This will result in plants and workers that are too warm.
The baseboards are wide, thick, and heavy pressure-treated boards attached to the collars to provide a barrier against wind, snow, and small animals.
It also further strengthens the structure of the tunnel.
They should be attached so the ends of the boards do not meet at the bows, as this makes it possible to ease the installation and reduce the boards warping over time.
Frequently Asked Questions