An important consideration while engineering and sizing the wall members for a screen enclosure is the provision of lateral bracing members, such as chair rails, girts, cables, K‐braces, etc. While these members do not contribute directly to the strengthening of a pool enclosure under gravity loads, they are a crucial part of the load path for lateral loads such as wind.
In this article, we intend to answer a few general questions about these secondary wall members and their ideal location/profile.
Are chair rails necessary?
Chair rails, while not primary structural members themselves, significantly stiffen up the primary structural members against the applied loads. Columns are more prone to failure due to sway and buckling without chair rails. Chair rails create linkage between the columns and stabilize the cage walls so that the structure resists sway better as a system.
Consider a 24’‐0” long X 13’‐0” wide X 12’‐0” eave height host‐attached screen enclosure located in a partly crowded area with a basic wind speed of 150 mph.
Is it better to have a chair rail or a girt?
For the purpose of this article:
Chair rail = Secondary wall member close to the bottom of the structure.
Girt = Secondary wall member close to the top of the structure.
Structurally speaking, it is better to provide stiffening close to the host, i.e., rails are more efficient
at the bottom of the wall as compared to the top. Consider the structure from the previous
example.
What other methods can help stiffen a screen wall?
Addition of other lateral bracing members, such as cables or K‐braces, can help stiffen a wall
against wind. Keep in mind that a cable only provides resistance in one direction, so a pair of
cables must be provided for structural stability. K‐braces, on the other hand, provide resistance
in both directions, but are still provided in pairs for structural symmetry and aesthetics.
K‐braces reduce the lateral drift by about 83%
K‐braces reduce the stress by about 46%.
What is the ideal direction for running a K‐brace in a wall with a chair rail and a girt?
The AAF Guide to Aluminum Construction in High Wind Areas (2020) recommends Option 1.
At Florida Engineering, we specialize in the design and analysis of aluminum frame structures. With our
wealth of highly qualified and experienced engineers, we work towards providing you with the most
economical structures that are structurally sound and code‐compliant, by exploring different bracing
options to meet your requirements.
For a free quote, email us at contact@fleng.com or call 941-391-5980.
References
- 2020 Florida Building Code, 7th edition.
- AAF Guide to Aluminum Construction in High Wind Areas (2020).
