Concrete Building Aspect Ratio in Seismic D2
I am planning an ICF house for a seismic D2 zone. Ultimately, I think an engineer will need to approve the design, but I want to try and understand as much of it as I can and to minimize changes that an engineer might request.
I have gotten quite far down the road with the design only to recently discover the PCA document, PCA100-2017 Prescriptive Design of Exterior Concrete Walls for One- and Two-Family Dwellings. Right off the bat, in Section 1.2.2, they state that the building shall be rectangular with a maximum aspect ratio of 2:1. They give no examples of what I guess, to them, is obvious. To me, it is not as clear. I have attached a simple outline of the exterior concrete surface of the walls for the floor plan I have currently in mind.
My first question has to do with rectangular shape. My design is not exactly straight up rectangles. Would this shape be outside the scope of this document due to shape?
My next question has to do with aspect ratio. To my understanding, the important ratio ought to be A/C. This is 2.06, which means I need to increase dimension C by only about a foot in order to bring it down to 2.0. But maybe I am wrong, and the important ratio is B/C, which is 2.54. To get this down to a value of 2.0 would require major changes to the design. Which ratio is the correct ratio to consider, A/C or B/C?
Thanks!
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Replies
gerrha,
Using the prescriptive seismic designs in building codes is a trade off. It allows you to forgo employing an engineer, but it restricts the design. Before worrying about possible changes to your plans, I would run it by an engineer, who I very much doubt will require any.
A lot will depend on whether or not your building department will issue permits WITHOUT using a registered engineer. Our local building department will NOT accept the prescriptive method for building without an engineering stamp, specific to your build. Depending on the size/complexity of the home, you are probably looking at $4k-$8k in engineering costs.
My suggestion is to sit down with your building department and go over what you are trying to do. They will let you know pretty quickly whether or not you can use the prescriptive method or if you need to hire an engineer and have the plans stamped.