The connection detail makes an assumption; there is no moment. This isn’t strictly true, and is the point where it all begins to fall down (literally).
Over the last year I’ve been learning some real practical engineering lessons from the most unlikely of sources: my IKEA Austmarka wardrobe. Brought from that nightmarish trip, which has become the modern day ritual for all people moving into an unfurnished place; I estimate that this modular masterpiece survived for about three months before it needed some engineering assistance.
My engineering lesson came at two in the morning when Austmarka decided that supporting the clothes rail was something of an optional duty. After a week of playing ‘if I just add one more shirt will the whole rail come down’-buckaroo, one of the supporting clips sheared-off. It was at this point that I began my life as a forensic engineer.
As my Leonardo’ific sketches show, the support for the clothes rail is a plastic cup that slots, using two shear-pins, into the wardrobe wall. Making the assumption that I do not store my anti-gravity clothing in this cupboard, the clothes rail is propped vertically by the cup, and restrained from making a lateral escape by the wall itself. The weight of the rail (and the hanging clothing) is then transferred through shear by the two pins, which sit in small holes along the wardrobe wall.
Something that irked me about the detailing of the support was that the second pin could have been prevented from disengaging so easily.
The connection detail makes an assumption; there is no moment. This isn’t strictly true, and is the point where it all begins to fall down (literally). Our ‘cup’ is essentially a bearing-plinth/pad-stone, and, as a (very) quick rule-of thumb, this means a semi-triangular distribution of load with the equivalent ‘point’ vertical force being about two-thirds along the triangle. That means there is eccentricity; and with eccentricity comes moments.
Just to make things worse for us, the tolerances on the rail are surprisingly high; if the rail is flush with one side, the length only extends about half-way along the other support. A little bit of statics shows us that the moment increases the shear through the pin connections by the ratio of effective rail eccentricity to about a-half to one-thirds of the pin-length. In our case that more than trebles the shear through the pin; which is beyond even a civil engineering factor of safety.
Actually, the above statement is a bit of a conservative simplification, because the top pin would add additional restraint. This complication, however, never arose, as the tolerance around the top-pin’s hole was enough to let it beat a hasty retreat from the socket, out into a world free of constraint. This left our bottom pin to deal with exactly that conservative simplification on its own; at which point, it gave up on life and sheared straight through.
Whilst I would still suggest the more obvious solutions of having IKEA design taking into account the worst-case geometric effects of their tolerances; something that irked me about the detailing of the support was that the second pin could have been prevented from disengaging so easily by simply placing the cup at the top, instead of the bottom. The bar would then act as a prop, allowing the increased shear to have at least been shared across both pins.
For anyone that came here looking to fix their wardrobe, we’ve found some good mileage in using 1p coins as shims to ensure the clothes bar short-tolerance is distributed evenly between both supports, and then nailing through the top-pin to hold the assembly in place. Possibly there is something in using steel rings, or modelling clay, to create a distribution plate/pad and reduce the effective eccentricity of the rail; but life is a little short.