How to increase the number of wall openings and windows while meeting the building code’s requirements for lateral force (wind) resistance
So you’ve decided to hire a design professional or design team to help you build your dream home –GREAT! However, during the design process, you’re advised by at least one of the design professionals (probably the Structural Engineer) that there are too many windows on that one wall in the rear of the house that has the beautiful views of the Front Range that you’ve always dreamed about, and that the design required to make this work will be way too costly for your budget – NOT GOOD! So now what?
How do you incorporate those beautiful views into the design of your dream home without going way over budget? Even if the budget could allow the costly engineered solution of a Structural Steel Moment Frame, etc., wouldn’t it be nice to have a little extra cash left to spend on some new furnishings for your new home? Well, you’re in luck! The likely solution to your dilemma is Force Transfer Around Openings (FTAO) wood shear walls. FTAO shear walls are an excellent alternative to the other traditional shear wall design methods because they provide the owner with a more versatile architectural approach during the design process, as well as cost savings coupled with increased structural performance for the building’s lateral resistance. FTAO is one of the three shear wall design methods recognized by the building code for wood-framed shear walls. The other two methods are the “Segmented Method” and the “Perforated Shear Wall Method.” FTAO is one of the two code-recognized design methods for wood shear walls that allows the engineer to utilize a wall opening along with the adjacent full-height wall segments to be designed together as one shear wall. There are many advantages that accompany this method. The two main advantages are:
- Fewer holdowns being required, and
- Being able to define the code limited aspect ratios for the wall piers as just the height of the opening divided by the length of the wall pier (as opposed to the entire height of the wall divided by the length of the wall pier).
This allows for more freedom in the architectural design to include more openings, such as windows and doors, that may otherwise be impossible to include due to having to conform to the requirements of the code with more conventional design methods. FTAO is also much more economical in that the holdown requirements can be substantially reduced or, in many cases, completely eliminated. The main difference, as well as the defining distinction of FTAO compared to other methods, is the addition of continuous straps above and below the openings which allow the forces to be “transferred around the openings.” The design and construction of FTAO shear walls has been very successfully utilized for several decades on the west coast of the United States, which serves as a great testimonial for their effectiveness at not only providing an excellent engineered solution for challenging designs, but also provides a long history of outstanding performance during extreme lateral force events such as earthquakes and high wind events. FTAO shear walls have notably been observed to have fewer issues with cracking of stucco and other wall finishes, which is inherently due to the overall greater stiffness they have as a result of being designed as a large single structural wall element. FTAO shear walls have been recently tested in full-scale models and have been found to be very accurate and reliable when compared to the predominant calculation methods for FTAO walls which are the Thompson technique (developed by my father) and Diekmann technique. In summary, the signature benefits of using FTAO wood shear walls as opposed to more traditional basic shear walls in general are:
- greater flexibility for openings in the architectural design of the building,
- reduced construction cost, and
- providing a stiffer wall that will mitigate stucco cracking.
Author: Mike Thompson P.E., S.E. – LinkedIn