Below is a greenhouse cross-section for our third and largest greenhouse. This diagram shows the typical configuration of each rib and how it relates to the purlins, ridge piece, joists, diagonal braces and foundation.
As shown in the diagram above, the ridge piece and purlins are spaced about the same distance from one another and the foundation so there is evenly spaced strength holding the "rib cage" together. The purlins and the ridge piece run the length of the building and are tied in with each "rib".
Diagonal bracing ties in the ridge piece with the lower purlins. A joist runs horizontally across the width of the structure and ties in the upper purlins. The two diagonal braces and the joists are then tied together to help form a more rigid shell for the building.
The greenhouse cross-section also shows the location of large turnbuckles and a set of "dead men" (earth anchors) made of power pole sections sunk deep into the earth. This design is used pull the structure down onto it's foundation of concrete form stakes to resist our 100mph winds out here on the pairie.
In hindsight, a better method of aligning and anchoring the structure to the ground is to use end clamps to tie in each of the "rib" ends to a purlin that runs the length of the structure at ground level. The ground level purlin is set on a shallow trench filled with rock and gravel to promote drainage, and long screw-in ground anchors are used with jaw and jaw (bolt-closed end) turnbuckles.
This foundation design is not shown in the greenhouse cross-section, but is offerred as the preferred method of assembling and anchoring the structure for ease and strength. Details about this foundation method are presented in the separate fabrication and installation instructions (ready for release in 2009).
Alternate foundation designs can be employed to suit your needs for "rib" spacing. As examples, ribs can be attached to framing boards for smaller designs, and moderate size greenhouses can make use of anchor tubing pounded or cemented into place so it will accept the swaged and unswaged ends of the "ribs".
Regardless of how you decide to anchor your structure, you'll be "staying the course" with respect to frugal living if you build it yourself.
Done with Greenhouse Cross-Section, take me back to Build a Greenhouse.