And more floating shelves

Status
Not open for further replies.

chris_goris

Chris
Senior User
LOL
I knew you would not take the bait.
You believe what you believe, without being able to prove your point.
Ok, if you want me to look at it, ill need more information.... how wide is your shelf that supported 200#? the vertical height of your support youre screwing to the wall. How many studs did you get screws into?. where were the load placed? you said 50# bags of what? were they 10" wide?
 

Willemjm

Willem
Corporate Member
Ok, if you want me to look at it, ill need more information.... how wide is your shelf that supported 200#? the vertical height of your support youre screwing to the wall. How many studs did you get screws into?. where were the load placed? you said 50# bags of what? were they 10" wide?
Chris, don’t bother, I am just yanking your chain.

But if you insist, the calculation would be based on the weakest part of the structure.

A cantilever beam with a uniformly distributed load of 200 lbs, 36” wide 10” deep.

The weakest point is where the rear slat screws against the wall 4/4 (7/8” actual) 1 7/8” wide 34” long. Six 3” cabinet screws two each going into each stud, being three studs. Drywall in between stud and frame. Frame is hard Maple.

That is all I would need to do the calculation.
A static uniformly distributed load of 200lbs is well within the scope of the structure.
 

chris_goris

Chris
Senior User
Chris, don’t bother, I am just yanking your chain.

But if you insist, the calculation would be based on the weakest part of the structure.

A cantilever beam with a uniformly distributed load of 200 lbs, 36” wide 10” deep.

The weakest point is where the rear slat screws against the wall 4/4 (7/8” actual) 1 7/8” wide 34” long. Six 3” cabinet screws two each going into each stud, being three studs. Drywall in between stud and frame. Frame is hard Maple.

That is all I would need to do the calculation.
A static uniformly distributed load of 200lbs is well within the scope of the structure.
The weakest part of the structure is the corner butt joint and the interrim dado joints. The wall "cleat" may stay intact and on the wall while the rest of the structural joints fail. The rotational forces at the wall on the entire structure is 1000 in/#. (based on 200# @ 5" from the wall) and for that joint to survive that, the screws (3) need to counteract (t o stay in equilibrium) at a force of 220# at each of the top 3 screws ( the lower 2 are inconsequential until the uppers fail and then the force increases on the lower exponentially ) . Your photo shows the structure made of all plywood , not 4/4 you mention above. And the end corners are have a screw in them ?.
 

Willemjm

Willem
Corporate Member
The weakest part of the structure is the corner butt joint and the interrim dado joints. The wall "cleat" may stay intact and on the wall while the rest of the structural joints fail. The rotational forces at the wall on the entire structure is 1000 in/#. (based on 200# @ 5" from the wall) and for that joint to survive that, the screws (3) need to counteract (t o stay in equilibrium) at a force of 220# at each of the top 3 screws ( the lower 2 are inconsequential until the uppers fail and then the force increases on the lower exponentially ) . Your photo shows the structure made of all plywood , not 4/4 you mention above. And the end corners are have a screw in them ?.
If you read my posts, notice I switched away from a Plywood to Hard Maple. Less work, less cost, that is how the ones in this thread were built.

There are 4 butt joints, two are pocket screwed (2 screws) two are dados and screwed. They are all glued. That would handle in excess of 700lbs.

200lbs 5 inches from the wall is a point load. We are not looking at a point load-and the moment (leverage) that causes. We are looking at a uniformly distributed load. You will tear up the dry wall long before those screws fail.
 
Last edited:

chris_goris

Chris
Senior User
If you read my posts, notice I switched away from a Plywood to Hard Maple. Less work, less cost, that is how the ones in this thread were built.

There are 4 butt joints, two are pocket screwed (2 screws) two are dados and screwed. They are all glued. That would handle in excess of 700lbs.
All I see here is plywood in the picture, calling for a 200# load supporting ability..... they may handle 700# if the weight were hanging straight down from the supports, not out at 90 degrees.... Although Im not sure how you arrived at 700#. I guess I dont know what youre building really, it seems to keep changing. Like I said , good luck with it.
 

Attachments

Willemjm

Willem
Corporate Member
All I see here is plywood in the picture, calling for a 200# load supporting ability..... they may handle 700# if the weight were hanging straight down from the supports, not out at 90 degrees.... Although Im not sure how you arrived at 700#. I guess I dont know what youre building really, it seems to keep changing. Like I said , good luck with it.
Here is a clue.
Uniformly supported load of 200lbs. Over 36” wide by 10” deep gives us a load of 200/(36x10) = 0.56.
So that shelf carries a little more than 8 ounces per square inch.
Now if you twist my arm, I will do the entire structure calculation for you, with explanations.
But you would have to be really nice!!
 

chris_goris

Chris
Senior User
Here is a clue.
Uniformly supported load of 200lbs. Over 36” wide by 10” deep gives us a load of 200/(36x10) = 0.56.
So that shelf carries a little more than 8 ounces per square inch.
Now if you twist my arm, I will do the entire structure calculation for you, with explanations.
But you would have to be really nice!!
youre OBVIOUSLY not an engineer, the load calculation has nothing to do with pounds per sq in... sorry, stick to woodwork and hope the shelves stay on the wall.
 

Willemjm

Willem
Corporate Member
youre OBVIOUSLY not an engineer, the load calculation has nothing to do with pounds per sq in... sorry, stick to woodwork and hope the shelves stay on the wall.
Nice, I got a personal attack against my resume.
Any point in continuing this, or is the importance to be right more important than what is right?
 

Willemjm

Willem
Corporate Member
I for one would really like to know who is correct here. It seems like the fail point would be from the screws fastened into the stud, so wouldn't the structural cals need to be for the screws?

Connection Calculator
Chris's approach did not take into account the length of shelf, so his approach would yield the same for a 1 mile long beam with hundreds of screws compared to the 36 inch beam in question each supporting only 200lbs
Keeping things simple, here are the calculations and I will round numbers for the purpose of simplicity
Lets say the cleat was 2" high, the shelf is 10" deep. We have three sets of screws, each set to its individual stud.
The cleat has one screw 1/4" from the top and one screw 1/4" from the bottom.

0 = the distributed load x the distance from the wall - the force of the top screw x 1 1/2" - the force of the bottom screw x 1/2"

0 = 200/3sets of screws x 5" distance from the wall - F x 1 1/2 - F x 1/2

it follows 333 = 2 1/2F
F = 133

So it tells us roughly that each cabinet screw needs to hold 133 lbs if we put 200lbs on the shelf

KCMA rates a maximum weight for an overhead cabinet as 600 lbs and some of those are held by no more than 4 cabinet screws. The bending moment would be much less than the shelf example due the height of the cabinet, but we still have the shear stress. I have seen some cabinet screw data saying they are rated for 75lbs and some data saying their holding force is 100lbs per inch of spruce penetration, so who knows. I have also seen some tests done on pocket screw joints failing at around 700lbs, who knows

All I know Chris thought it was funny that a shelf held 200lbs without issues, and a few people including the customer witnessed the test. He seems to be a bit of a bear, and he knows EVERYTHING, but that is OK

The key with floating shelves is flexing. As long as they are stiff enough, the customers love them.

Over and out.
 
Last edited:

chris_goris

Chris
Senior User
Chris's approach did not take into account the length of shelf, so his approach would yield the same for a 1 mile long beam with hundreds of screws compared to the 36 inch beam in question each supporting only 200lbs
Keeping things simple, here are the calculations and I will round numbers for the purpose of simplicity
Lets say the cleat was 2" high, the shelf is 10" deep. We have three sets of screws, each set to its individual stud.
The cleat has one screw 1/4" from the top and one screw 1/4" from the bottom.

0 = the distributed load x the distance from the wall - the force of the top screw x 1 1/2" - the force of the bottom screw x 1/2"

0 = 200/3sets of screws x 5" distance from the wall - F x 1 1/2 - F x 1/2

it follows 333 = 2 1/2F
F = 133

So it tells us roughly that each cabinet screw needs to hold 133 lbs if we put 200lbs on the shelf

KCMA rates a maximum weight for an overhead cabinet as 600 lbs and some of those are held by no more than 4 cabinet screws. The bending moment would be much less than the shelf example due the height of the cabinet, but we still have the shear stress. I have seen some cabinet screw data saying they are rated for 75lbs and some data saying their holding force is 100lbs per inch of spruce penetration, so who knows. I have also seen some tests done on pocket screw joints failing at around 700lbs, who knows

All I know Chris thought it was funny that a shelf held 200lbs without issues, and a few people including the customer witnessed the test. He seems to be a bit of a bear, and he knows EVERYTHING, but that is OK

The key with floating shelves is flexing. As long as they are stiff enough, the customers love them.

Over and out.
Again, thats not the right formula, thats why I asked about a free body diagram, there is a torsional load at the wall (moment arm) that causes the high stresses at the wall cleat, like I mentioned before, the reactive load required is 200+ lbs required at each of 3 fasteners. Sheer stresses and screw strengths are not an issue here.
but dont take my word for it, even steel brackets arent handling the weight youre talking about. this was the strongest one I found. You claim to have placed 200# weight on that shelf, I cant imagine myself attempting to stand on it!. Maybe an elf on a shelf!.
 

Willemjm

Willem
Corporate Member
Again, thats not the right formula, thats why I asked about a free body diagram, there is a torsional load at the wall (moment arm) that causes the high stresses at the wall cleat, like I mentioned before, the reactive load required is 200+ lbs required at each of 3 fasteners. Sheer stresses and screw strengths are not an issue here.
but dont take my word for it, even steel brackets arent handling the weight youre talking about. this was the strongest one I found. You claim to have placed 200# weight on that shelf, I cant imagine myself attempting to stand on it!. Maybe an elf on a shelf!.
Let’s agree to disagree that calculation takes all the moments into consideration. Torsion is the wrong term, it refers to stresses in a round body.
If you stand on that shelf, the load would not be uniform and it would be dynamic not static.
But hey, surf YouTube a bit, there is a guy sitting on his shelf and the design is pretty close to mine. Perhaps also tell him his video is fake.

Interesting though that you say a shelf 500 ft long with screws every 16”supporting a distributed load of 200 lbs will make no difference to the load terms of the screws?
Most Engineers believe if the load stays at 200 lbs and the shelf length approaches infinity, the load on the infinite amount of screws would be negligible.
 
Last edited:

chris_goris

Chris
Senior User
Let’s agree to disagree that calculation takes all the moments into consideration. Torsion is the wrong term, it refers to stresses in a round body.
If you stand on that shelf, the load would not be uniform and it would be dynamic not static.
But hey, surf YouTube a bit, there is a guy sitting on his shelf and the design is pretty close to mine. Perhaps also tell him his video is fake.

Interesting though that you say a shelf 500 ft long with screws every 16”supporting a distributed load of 200 lbs will make no difference to the load terms of the screws?
Most Engineers believe if the load stays at 200 lbs and the shelf length approaches infinity, the load on the infinite amount of screws would be negligible.
Again, you obviously dont understand simple statics equations, the moment arm creates a torsional load at the wall twisting your structure, so yes, torsion is the correct term. And torsion is measured in force units, like ft/lbs, measuring motion, where youre claiming its about stresses and we havent even begun to touch on stresses here. Stresses are internal material properties that determine its inherent strength, based on deformations caused by external forces.
When did I say a 500 foot long shelf did anything? my calculation did include the length, it had 3 (plus 3 below that like I said, that do nothing) screws into 3 studs, as you claimed your install was. Thats all the length support you have for your 200# load. And I f I stood on the shelf and didnt move it would be a static load, just like your 4 sacks of whatever you used that were 50# each. send the link to this video, you've both defied all engineering principles by using an inferior material (wood) and achieving better results than steel somehow.
 

jpaul

John
Senior User
Willem, very nice work, thanks for sharing, everything.
Chris, you ruined a very nice post about some very nice woodworking, on a woodworking forum. Thanks for that.
 
Status
Not open for further replies.

Our Sponsors

LATEST FOR SALE LISTINGS

Top