What's exerted force?
August 23, 2006 4:54 PM Subscribe
Calculation of Force Filter: How do I determine the exerted force at the end of a (small) lever? Yes, there's
I am making a sculpture consisting of coffee cups mounted on short rods. The rod will go into a drywall anchor that is rated for fifty pounds. The rods will not be more four inches long. Am I safe?
The catch: there are going to be LOTS of these cups (about 100), spread out over a 18 inch by 12 foot section of wall. I think I'm safe on an individual level, but I'm concerned about the total load, and possible effect of having that many anchors on a wall. This is beyond a simple lever-calculation, I realize, so I consider any answers to this part to be gravy!
I am making a sculpture consisting of coffee cups mounted on short rods. The rod will go into a drywall anchor that is rated for fifty pounds. The rods will not be more four inches long. Am I safe?
The catch: there are going to be LOTS of these cups (about 100), spread out over a 18 inch by 12 foot section of wall. I think I'm safe on an individual level, but I'm concerned about the total load, and possible effect of having that many anchors on a wall. This is beyond a simple lever-calculation, I realize, so I consider any answers to this part to be gravy!
How the bending moment (torque)* is dealt with at the wall anchor principally depends on the dimensions of the wall anchor. The number of cups is effectively irrelevant--the density of the cups (# cups/square inch) is what's important. It sounds like you'll have 100 cups on 18 sq.ft, which works out to a whole lot of square inches per cup. In my opinion, this means you don't really need to worry about the interactions between cups on the wall, provided the wall is well-supported (I'm assuming drywall on wooden stud construction).
What's left is how well the individual anchors in the wall handle the bending moment. To know for sure, you'd be best off experimenting with a single rod installed in a representative fashion. I could pull out a whole bunch of math if you were willing to pay me and provide me with all sorts of information, but experimentation would be faster and cheaper.
* Physicists call it torque and engineers call it bending moment. It's the same thing: force times lever arm.
posted by cardboard at 5:44 PM on August 23, 2006
What's left is how well the individual anchors in the wall handle the bending moment. To know for sure, you'd be best off experimenting with a single rod installed in a representative fashion. I could pull out a whole bunch of math if you were willing to pay me and provide me with all sorts of information, but experimentation would be faster and cheaper.
* Physicists call it torque and engineers call it bending moment. It's the same thing: force times lever arm.
posted by cardboard at 5:44 PM on August 23, 2006
To a rough approximation, if you consider the back of the drywall to be the pivot point, and the drywall is half an inch thick, then the first downwards at the front of the drywall is 4.5/0.5 = 9 times that of the actual weight of the cup.
(though the important thing is the distance of the centre of gravity of the cup from the wall, which you don't mention, not the length of the rod)
posted by cillit bang at 6:05 PM on August 23, 2006
(though the important thing is the distance of the centre of gravity of the cup from the wall, which you don't mention, not the length of the rod)
posted by cillit bang at 6:05 PM on August 23, 2006
Run a test on a much smaller section first.
Will the cups be disturbed at all, or are they protected from any form of bumping? It is unexpected jolts that will cause the most load.
Is this pre existing wall, or is it being erected special for the sculpture?
As long as the anchors are spaced out, lets say by at least 6" in all directions (you could probably get away with less than 4" actually - do a test!), the total quantity won't really matter to the drywall. Eventually you will weigh it down enough that the support structure holding the drywall up will be too stressed.. If the wall is built to home construction standards, I don't think you will ever get to that point.
posted by Chuckles at 7:43 PM on August 23, 2006
Will the cups be disturbed at all, or are they protected from any form of bumping? It is unexpected jolts that will cause the most load.
Is this pre existing wall, or is it being erected special for the sculpture?
As long as the anchors are spaced out, lets say by at least 6" in all directions (you could probably get away with less than 4" actually - do a test!), the total quantity won't really matter to the drywall. Eventually you will weigh it down enough that the support structure holding the drywall up will be too stressed.. If the wall is built to home construction standards, I don't think you will ever get to that point.
posted by Chuckles at 7:43 PM on August 23, 2006
Mounting it to the studs instead of using drywall anchors should solve your problem.
Do you actually want to know how to calculate the force or would you prefer "I'm doing X with Y, will it work?" If you want the latter, then you need to provide more specifics, ie how many cups? how many mounting rods? what's the arrangement of fasteners?
I think the suggestion of mounting it all to a piece of plywood, then attaching the plywood to the wall is the best. Drywall anchors are designed to support a downward force and will most likely pull out if subjected to a moment arm.
posted by electroboy at 7:26 AM on August 24, 2006
Do you actually want to know how to calculate the force or would you prefer "I'm doing X with Y, will it work?" If you want the latter, then you need to provide more specifics, ie how many cups? how many mounting rods? what's the arrangement of fasteners?
I think the suggestion of mounting it all to a piece of plywood, then attaching the plywood to the wall is the best. Drywall anchors are designed to support a downward force and will most likely pull out if subjected to a moment arm.
posted by electroboy at 7:26 AM on August 24, 2006
Response by poster: Thanks for all the tips!
I have already done some experimentation, and what I have seen so far doesn't cause me any alarm. My concern was for potential things I might be overlooking, hence the request for math-type assistance. When in doubt, fall back on the numbers. (at least then, if something goes wrong, you have something vaguely concrete to point to!)
If you are interested in more specifics, here goes:
The cups (basic cafe/diner style mugs) are pierced through the side, so they stick out horizontally from the wall. The placement on the wall should appear random, with the cups being placed at various intervals and distances from the wall (within that 4 inch range). The wall itself is a soffit wall that curves above a coffee bar (about a 45 degree arc), and will be out of reach of all but the most determined leapers. The drywall itself is probably 5/8ths inch board, over metal studs. the studs are spaced about every 8 inches, so there are *plenty* of attachments points for the drywall.
Unfortunately, one of the stipulations of the deal was that I not place any board or mounting surface on the wall. I am required to go straight into the drywall, with as small a visible attachment as possible. The anchors are basic metal screw-in anchors. "StudSolver" is the brand name, and I got them at Home Depot. They are designed to go into studs as well as drywall, so they are kind of a specialty. I can't plan on hitting only the studs (that wouldn't look very random!), but I need to be ready for it.
I have tried simulating these circumstances, with a piece of scrap drywall and all the hardware, and everything seemed to hold just fine. Connection seemed solid (after 24 hours in the corner of the wood shop where I work), and they was no visible bulging or signs of failure on the drywall.
So, I think I'm covered! Thank you all again, and if you are in Milwaukee, you will be able to see this creation at the Whole Foods opening in sept/oct...
posted by schwap23 at 7:41 PM on August 24, 2006
I have already done some experimentation, and what I have seen so far doesn't cause me any alarm. My concern was for potential things I might be overlooking, hence the request for math-type assistance. When in doubt, fall back on the numbers. (at least then, if something goes wrong, you have something vaguely concrete to point to!)
If you are interested in more specifics, here goes:
The cups (basic cafe/diner style mugs) are pierced through the side, so they stick out horizontally from the wall. The placement on the wall should appear random, with the cups being placed at various intervals and distances from the wall (within that 4 inch range). The wall itself is a soffit wall that curves above a coffee bar (about a 45 degree arc), and will be out of reach of all but the most determined leapers. The drywall itself is probably 5/8ths inch board, over metal studs. the studs are spaced about every 8 inches, so there are *plenty* of attachments points for the drywall.
Unfortunately, one of the stipulations of the deal was that I not place any board or mounting surface on the wall. I am required to go straight into the drywall, with as small a visible attachment as possible. The anchors are basic metal screw-in anchors. "StudSolver" is the brand name, and I got them at Home Depot. They are designed to go into studs as well as drywall, so they are kind of a specialty. I can't plan on hitting only the studs (that wouldn't look very random!), but I need to be ready for it.
I have tried simulating these circumstances, with a piece of scrap drywall and all the hardware, and everything seemed to hold just fine. Connection seemed solid (after 24 hours in the corner of the wood shop where I work), and they was no visible bulging or signs of failure on the drywall.
So, I think I'm covered! Thank you all again, and if you are in Milwaukee, you will be able to see this creation at the Whole Foods opening in sept/oct...
posted by schwap23 at 7:41 PM on August 24, 2006
This thread is closed to new comments.
Regarding the wall as a whole: if each individual cup is securely held, then the strain on the wall is no more than if they were attached directly to the wall. This is a bit of a simplification but it's going to be true because the size of the wall that is covered is much larger than the size of the lever.
I'd like to propose a practical solution: mount a piece of painted plywood or fiberboard (1/2" or even 3/8" would probably be fine) to the wall that is larger than the area you wish to cover, and attach this through the studs, which are the strongest part of the wall. Attach the levers that the cups will be on to this board, and the connection will be much stronger than a drywall anchor, plus the load will be supported by the studs. Also, this would allow you to do the installation beforehand, on the board, and just attach the whole contraption wherever it is that it's going to go. This would work well if the wall is flat, but if you want to mount to a curved surface you'll probably have to do it another way, or perhaps use a few layers of thin (1/8") material like hardboard or something.
posted by RustyBrooks at 5:28 PM on August 23, 2006