Design and Installation of a 20 x 40ft Tubular Steel Frame Shade Pergola with Redwood Vanes
by Joe Mehaffey and Kristi Mehaffey MSME/PE
We have a client who had a wood frame pergola at his home for about 14 years. Unfortunately, the “weatherproof and waterproof” glue lam beam proved to be completely misrepresented. At the end of 14 years, the glue in the glue lam beam had failed and the frame was sagging badly. He wanted to replace the pergola frame with “something more permanent”. The original Doric columns included a 4″ x 4″ x 1/2″ wall steel tube inside. This type construction was used originally so as to allow for the possibility of adding a full roof at a later time. This philosophy was continued in the new design. We decided on a painted steel frame design which should last perhaps 50 years. Since we wanted to use all of the original Redwood Vanes, the new design had to be adjusted to use the existing vanes without cutting where possible. This caused us to make minor adjustments to some of the post connections and dimensions so proper alignment could be maintained. Actual lengths of the wood vanes in this pergola are 115 inches in three bays and 82.25 inches in the fourth bay. The photo below shows the finished all steel pergola frame with the redwood vanes installed.
Note: In NO WAY does the following article pretend to be a “step by step” guide to allow a person not skilled in handling steel and/or large timbers to erect this project! The erection of the steel frame requires significant skills and the steel beams are heavy and if mishandled, could be deadly. This article is intended as a guide to a skilled craftsman in how to design and erect this kind of pergola project. There is no warranty given or intended that this will work for any particular project.
Figure 1: Finished steel pergola frame installed and equipped with redwood vanes shows the dramatic sun shading provided
The Client notes that the outside surface of the vanes get pretty warm in bright sunlight and the upward movement of air through the vanes keeps a bit of air moving under the pergola thus creating more comfort than you would have with (say) a closed canopy, awning, or solid roof.
Figure 2: Another view of the finished pergola system from the South
The design process was as follows:
Step 1) Locate all of the 6 existing 4×4 steel posts within the Doric columns relative to each other on a master drawing for alignment purposes.
Step 2) Make a master sketch drawing of the posts and pergola showing accurate spacing and dimensions for all of the members. This drawing is shown below in figure 2.
Figure 2: Drawing of the basic frame showing all critical dimensions and notes to the steelwork designer on how the parts fit together.
Don’t worry that you cannot read the above sketch well. The detailed drawings following are complete and easy to read. Working sketches are often “sketches”!
Photo by Sara Sadeghloo on Unsplash