Does an IRC Design Work for Most Residences?
Originally published by: Steve M. Sylvest, P.E. — July 18, 2019
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I read your article The Structural Design Process of a Building with interest since it relates to some observations I have made for some time. There is a wide disparity in design and execution of the structural portion of structures, particularly residential, in this region. Some are reasonable, but many are not. Yet they get built and permitted and pass inspection.
In the immediate area, many residential projects largely do not qualify to be designed per the IRC prescriptive standards (are at least some key portions do not since exceeding the limits). Many must be mimicking construction details they have seen and deem to be adequate. Some are obviously far from good mechanics (e.g. hinged tall walls, lack of adequate shear or braced wall lines, connections not consistent with load path, etc.). The permitting and inspection process does not seem to be at a level to distinguish when structural elements are outside of typical IRC provisions.
A majority of the residential projects are designed by Building Designers, though a smaller number by Architects. Few have structural engineers involved. The range of structural information on the design documents (of the ones I have seen) range from zero to more often just a collection of standard details based on IRC conventional framing, with little or no specifics. A small minority actually provided a viable level of specific information to tell the contractor what to do. Most leave it to the framer to do what he deems is reasonable. The inspectors must have a few hot-button feature to look for, but otherwise must not be too aware.
Very few residential (1 and 2-family) structures in this region use CM components (e.g. roof or floor trusses or wall panels). Many use Engineered Wood Products (EWP). These are typically designed (for gravity loading) and supplied through a distributor. The process is similar to a performance specification leading to deferred submittal, but most often without any design engineer input at the design stage nor any review of the submittals. For gravity loading, this process usually works well. A couple of things are usually missing. One is any consideration to lateral loading paths in the building and the other is a design professional in responsible charge to confirm the members, load paths, and connections all are consistent with the rest of the structure. So, the final result is a structure with a few well-engineered EWP products (for gravity loading), and some portions of the structure (almost) in line with IRC, but the remaining is just whatever the particular framer deemed adequate (similar to what he is used to seeing).
Several things work against making meaningful changes. Most builders, even high-end ones and builders desiring quality results, do not realize there is a gap (or wide range of results getting delivered). Likewise, the buyer is unaware. Permitting and inspection is not attuned enough to discriminate. Nobody is interested in adding more cost. The already –completed projects are still standing (and working for the most part, as far as they know).
A while back, I watched a video about SBCA’s participation in the Central Iowa Code Consortium, which, caught my attention as a way to gradually educate the various stake-holders and increase code-compliance consistency.
Steve M. Sylvest, P.E.
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