This last week, after spending several sleepless nights wondering how I was going to write about architects and cut-drawings, I finally had a little “eureka” that brought me to my keyboard. What I am always harping on are the structural differences between SIPs and sticks. For most traditional building this doesn’t come into play, but for many structures it does and the importance of understanding exactly how your structure is working is essential to have in your head before you start laying out the pieces of your SIP-puzzle, which is actually what your cut-drawings are. So let’s back up a little for some residential structural history…..
Structure was handled by the rules; these became imbedded in our codes. In many instances these rules-of-thumb worked against good sense and efficiency. For example, lintel schedules with their accompanying jack stud requirements (doubled for spans 8 feet and over) would be utilized regardless of whether or not the opening was in the non-loadbearing gable end, how wide the building was (what were the PLF floor or roof loads on that lintel) or if the structure was more than one story high.
The theoretical base for the stick structure was, again, independent and redundant frames or “bents” every 16 inches. Stability was given by the triangulation of the diagonal sheathing. Although in the real world of the actual physical performance of these structures there is considerable integrated response to imposed loads –- a dynamic, indeterminate behavior – practical considerations demand a static structural analysis that is easy to understand and perform. This whole indeterminate/determinate approximation gets worse when plywood sheathing was introduced after World War 2. The superior rigidity of the plywood actually makes the whole assembly behave more like a true shear diaphragm than an assembly of stick components, but again, true structural analyses for these structures rather than simple approximations would turn away all but the most sophisticated of structural engineers.
SIPs also succumb to the regimen of approximation that makes inexpensive buildings possible. In truth, SIPs create shell structures with true shear diaphragms, dispersing point loads throughout the entire structure and behaving in an integrated indeterminate fashion. If we look to nature for structural design models, we see that stick construction is more like us with our skeletons of individual component bones. SIPs are more like lobsters or turtles with their exoskeletons; hard external shells with no internal structure of separate elements.
Perhaps it is for this reason that we find such resistance in the building community to changing over to SIPs; sub-consciously we identify with stick structures more readily because of our own skeletons! Seriously, we do appreciate and warm to buildings that express how they work. That’s why so many are attracted to barns. The huge timber beams and posts are a living lesson in structure. In seeing how humans conceived of such a structure and created it leaving the chisel marks of the process, almost like teeth marks, viscerally connects us to them. This is called “transparency” in architecture-speak. SIPS are more like the igloo domicile creations of the Inuit. They are, in fact, true shell structures. Once built of hard-pack snow blocks, the interior develops a coating of ice from condensation while the exterior develops a coating from natural precipitation or is helped along with water applied by the builder. Windows are cut out with no need for lintels and severe wind loads are easily dispersed throughout the entire shape down to the ground. The structure is capable of sustaining astonishing loads without difficulty – all from water vapor!
Understanding all this enables an architect or engineer to make much more efficient use of the mechanical abilities of SIPs. Walls may be analyzed as box beams allowing for large spans and cantilevers where stick structures must be supplemented with additional structural members, in many instances with steel. In addition to checking all openings, the designer must layout the panels, not just for economy, but for structural integrity. For example, it may or may not be O.K. to have a horizontal panel joint between the first and second floor windows. If there is a second floor “bump out,” should the floor panel be inset or overlap the wall panels? Which way is best to overlap the corners? Should the panels be laid out horizontally or vertically? or a mix of both? What will the roof-to-wall detail be? the ridge detail? Will it be necessary to have structural splines or will the usual 3” x 5/8” plywood splines be sufficient? If not, what size structural splines at 4 foot centers…and whoops!, there go our 8 foot wide “jumbo” panels!
For many conventional buildings, say a 2 story colonial, most of the above may not even surface as issues, but for some more exciting designs the correct resolution of these issues is the difference between a sound structure or something I don’t want to think about!
All of this information must clearly be shown on the cut drawings.
Again, if the design is unique and exploits a particular SIP configuration, it is probably best that the cut drawings be developed by the architect or engineer. Should the design be a simple and small “monopoly box,” then let the work be done by the SIP company.
There is one additional advantage to the client in having an independent architect or engineer develop the SIP cut drawings, and that is that they belong to the owner who may ship out multiple copies for competitive bidding. It has always seemed to me that if one forms an early partnership in the project with a SIP company who translates the architectural drawings into cut drawings, one is obligated to follow through with that company and their work cannot be utilized to secure bids from competitive outfits.
To sum up, cut drawings represent the structure. Who is the best person to create them? The owner should be the judge for his project after counseling with his consultant and a competent representative of the SIP manufacturing world.