Revelation about structural analysis
During our peer review meeting last Saturday, Yadu pointed out that we have been focused on the deflection over the cantilevered tabletop and neglecting the compliance in the bracket used to attach the desktop to the slider in a single-column design. This was something I had not thought about previously, but I pointed out that it would be relatively easy to stiffen up the bracket using gussets or heavier gauge material.
After checking my error budget to take this into account, however, I realized that this was a bigger problem than I thought. My friends who are building standalone desks will likely be fine because a standalone desk gives you a large amount of clearance below the tabletop to add stiffening members. I, on the other hand, need to keep the structure below the tabletop as thin as possible so that the “sit” position is not excessively elevated above the original height of the supporting table. I estimated that I could get away with elevating the work surface in the “sit” position by at most 2.5 inches, considering the amount of vertical adjustment allowed by the typical office chair.
Change of direction: dual-column design
The limited space below the tabletop means I would have to either resort to unsightly gussets above the work surface to make my bracket sufficiently stiff, or use comically thick angle stock to make my bracket. A quick calculation showed that a 2 inch-wide bracket made our of steel would have to be 7 mm-thick just to be stiff enough for me to blow the entire 4 mm of allowable load-induced deflection on that one member. At this point, I decided that the chunky hardware required to realize the single-column cantilevered desk design would obviate the visual lightness that made such a design appealing in the first place. Therefore, I will pick up my fall-back twin column design from this point on.
Error budget and CAD
Most of this week was spent on populating and refining my error budget for the dual-column design. In a way, I am glad I pursued the single-column design initially — this helped me gain some very valuable experience preparing error budgets. Especially interesting was figuring out how to approximate a closed-loop structure in the error budget spreadsheet.
Based on the error budget, which of course is still a work in-progress as I work on better ways to model interface stiffnesses, the twin-column design looks set to satisfy my total error allowance of 5 mm (80:20 split between load-induced and geometric). In fact, I have been able to cut back on the size of many structural members, which improves the appearance of the desk and allows me to build using actual furniture-grade boards instead of heavy structural timber.