I have two desks, side by side, that just happen to be exactly the same height (God Bless Ikea). For many reasons, I treat them as “one giant desk” due to the way their surfaces line up. I am placing this thing on top of them - Products - IKEA - as a shelf that runs most of their combined length, with one leg on one desk and the other leg on the other desk. It will hold its own bevy of heavy stuff up top and out of the way - a printer, stereo, etc.
I’m concerned that I’m ultimately going to be putting too much weight on one of the desks - one can hold more weight than the other one - but I’m really not sure how to figure this out. Is it as simple as “each desk holds 50% of the weight,” so that you can say that desk A is bearing 25 lbs. and desk B is bearing 25 lbs.? Is it more complicated than that? I don’t really know anything about how weight is distributed from one object to the next.
If Desk A can hold 30 lbs. and desk B can hold 100 lbs, and the object with one leg on each of them is holding 50 lbs. worth of stuff, will desk B’s ability to bear more weight somehow take that weight off of desk A and desk A won’t collapse or otherwise fail?
If you just put the empty shelf on the two desks, then each one will be supporting 50% of the weight. When you start adding other objects, then you need to factor in the weight of the object and it’s position on the shelf, to figure where the “center of gravity” is for the shelf. Wherever that center of gravity is, half of the weight will be on one side and half will be on the other side.
Basically, you want to put your heavier objects on the side supported by the sturdier desk.
Not necessarily. If, for instance, we have most of the weight in two differently-sized lumps (say, one of 10 kilograms and one of 20), with a long but lightweight connecting piece between them, then the center of mass will be closer to the heavy end, but 2/3 of the mass will be on one side of the center and 1/3 will be on the other side.
If we approximate the bookshelf as being supported on two points (probably a good approximation, since the supporting parts of a bookshelf are usually much narrower than the shelves themselves), then the distance from the center of mass of one side times the weight supported on that side will be the same for both. So, for instance, if the CoM is 1/4 of the way from the left side, then the left side will be supporting 3/4 of the total weight.
This is a combination of two simply supported beam-bending problems; if you know the weights (forces) of objects on the top shelf, you can find the reaction loads (forces) on each of it’s legs - these are the forces being applied to the lower desks. You can then find the reaction forces on each of the lower desks.
If you want to try and determine whether the desks will fail, then you’ll be interested in knowing about the bending moments the forces cause and whether those reach the failure stresses of the materials the desks are made of (you can probably google those values). Add in a safety factor and you can load up the desk with a certain amount of confidence.
This isn’t trivial, but it’s not very hard either - first-year engineering classes certainly model this type of problem. There are a ton of assumptions to put into it (how much can you ignore the depth of the beams, whether the materials actually match the reference values you compare to, etc) but you can get a fairly decent first-order solution on paper with a bit of algebra.
What are the desktops made of? You’ll need to determine if they’re a honeycomb material or if they’re solid wood or wood-like material. They may be able to handle say, 400 pounds evenly distributed, but a ten pound static point load (such as if you left the Malm unit on its four rollers) might punch right through the stuff.
I do know that a dropped one pound object can destroy the laminate plastic over paperboard honeycomb material. My hammer dropped onto an Expedit unit resulted in a cartoon-like hammer-shaped hole. :smack:
Slightly related and probably interesting to people interested in the thread:
“The Sagulator helps you design shelves by calculating shelf sag (deflection) given type of shelf material, shelf load, load distribution, dimensions, and method of attachment. You can also specify an edging strip to further stiffen the shelf. See the notes below for usage tips”
Also: what, no Calvin and Hobbes fans?