Force req'd for connection to fail

[craigclick]

Hi - I'm a woodworker, and not an engineer, and would be much obliged if someone could tell me how may lbs/lg would it take for this connection to fail. The force is static. Thanks.

 

[Jboggs]

Thanks for the question, but... you aren't going to get an answer without answering more questions. I'll go first:
1. What is PTR? I found this on Google: "Fiberglass-reinforced plastic (Fiberglass PTR)"
2. I would not trust tapped holes in fiberglass or plastic. VERY weak. Find another way.
A suggestion: Find a way for the screws to engage in metal, not plastic. The screws could go all the way through the tube to engage in tapped holes in a steel plate on the far side of the tube.
Do some research on recommended fabrication and assembly methods for that material.
3. Where are the screw holes located in the angle? The closer to the top of the angle the better..

 

[dvd]

You may have some luck with something like a Lindapter Hollo-Bolt - contact Lindapter and speak with an applications engineer.

 

[Stick.]

More questions/thoughts:

1. Do you have a target for the force you want to hold? I find it's usually easier to approach the problem from that direction than just finding the maximum.
2. What is applying this force? The answer on capacity could change depending on what this thing is holding.
3. How far away from the tube is the force applied?
4. What is the wall thickness of the tube? Even if it's steel tube, just eyeballing it, the wall thickness looks a bit thin for 1/4" dia screws (assuming tapped holes in the tube). You may have to step up the wall thickness, use nuts or a backing plate inside the tube (like Jboggs said), or look at something like Rivnuts.
5. Do you need the countersunk flat head screw or could they be changed out for hex head or socket head screw? I don't love using flat head screws in highly loaded applications. Ease of manufacture is also worse with flat heads since mismatches in hole spacing between the tube and angle can cause issues due to the taper of the countersink.

 

[craigclick]

Thanks for the input. I'll clarify:
1. I thought the nomenclature PTR was universal, but I guess it only applies to Mexico. In this case, it is square tube, A36 steel, with a wall of 1/8". In this size, 2"x2". A thicker wall is not available.
2. The FLD HD screws, 28 TPI, are to keep a clean visual look. The wall is tapped.These are supports for shelves in a high-end residential installation.
3. I have placed the screws above center. Perhaps I should place them higher, closer to the angle.
4. A backing plate inside the square tube is not possible, nor is it possible to place nuts. If you thought it essential, I could place nut-serts to support more thread length, but then I would have to remove material from the aluminum angle, so that they would not stand/off (proud) from the post 1/16". In which case, I would probably consider usine 1/4" thick angle.
3. The shelves are 10' long, and have an internal structure of steel to keep them from sagging under the weight of books. Total weight of shelf, filled with books is apprximately 330 lbs.
4. I've attached another drawing that shows how the shelves are mounted. There will be two angle supports in front off the posts. There will be five anglem supports off the rear wall, each 4" long.
Thanks all for your comments; it's appreciated.

 

[craigclick]

Thanks for the input. I'll clarify:
1. I thought the nomenclature PTR was universal, but I guess it only applies to Mexico. In this case, it is square tube, A36 steel, with a wall of 1/8". In this size, 2"x2". A thicker wall is not available.
2. The FLD HD screws, 28 TPI, are to keep a clean visual look. The wall is tapped.These are supports for shelves in a high-end residential installation.
3. I have placed the screws above center. Perhaps I should place them higher, closer to the angle.
4. A backing plate inside the square tube is not possible, nor is it possible to place nuts. If you thought it essential, I could place nut-serts to support more thread length, but then I would have to remove material from the aluminum angle, so that they would not stand/off (proud) from the post 1/16". In which case, I would probably consider usine 1/4" thick angle.
3. The shelves are 10' long, and have an internal structure of steel to keep them from sagging under the weight of books. Total weight of shelf, filled with books is apprximately 330 lbs.
4. I've attached another drawing that shows how the shelves are mounted. There will be two angle supports in front off the posts. There will be five anglem supports off the rear wall, each 4" long.
Thanks all for your comments; it's appreciated.

 

[LittleInch]

SO if I understand this right, the strange 2" posts are in the outer corner of this bookshelf thing with a very stiff shelf if it's not going to visibly bend with some 150kg of books on it?

My issue would be why the gap between the edge of the shelf and the frame of the post?

Hoving a moment on the angel barcket just increases the pull out force considereably. Having the shelf flush with the surface of te post makes the screws basically in shear where they will be very strong, but having the shelf shy will lead to very high forces on the screws and risk of pull out.

I doubt you can calculate this to any degree of accuracy as so much depends on the exact diameter of the hole and the strength of the screw to create a thread in the steel? 3mm thick isn't a lot of thread and over tighten it once and your hole is buggered.

Think you would be much better off with an oval type hole or rectangular vertical slot and an angle bracket which has a reverse angle on the inside.

Like this. https://www.ironmongerydirect.co.uk...bh73-ijH0I935TwYbyduR3TjSG1uCK1hoC98UQAvD_BwE

 

[craigclick]

SO if I understand this right, the strange 2" posts are in the outer corner of this bookshelf thing with a very stiff shelf if it's not going to visibly bend with some 150kg of books on it?

My issue would be why the gap between the edge of the shelf and the frame of the post?

Hoving a moment on the angel barcket just increases the pull out force considereably. Having the shelf flush with the surface of te post makes the screws basically in shear where they will be very strong, but having the shelf shy will lead to very high forces on the screws and risk of pull out.

I doubt you can calculate this to any degree of accuracy as so much depends on the exact diameter of the hole and the strength of the screw to create a thread in the steel? 3mm thick isn't a lot of thread and over tighten it once and your hole is buggered.

Think you would be much better off with an oval type hole or rectangular vertical slot and an angle bracket which has a reverse angle on the inside.

Like this. https://www.ironmongerydirect.co.uk...bh73-ijH0I935TwYbyduR3TjSG1uCK1hoC98UQAvD_BwE

Thanks for taking the time to respond. The gap is a design element from the architect. The reason I started this post is precisely because of your comment that the 'moment' changes the force from shear, and I wanted to know if it could be calculated. My original idea was to use a T profile with four screws, but I thought perhaps it was overkill, and the architect didn't like it as she wanted a cleaner look. Your comment made me think of another approach; a horizontal slot with a Z bracket. Thanks for your comments.

 

[dgeesaman]

This is a subtly complicated assembly. First, can your flip the bracket to hang? At least then the bending moment will push the bracket into the PTR. The gap makes it much harder to accurately calculate a capacity.

Some will define failure of a bolted component as the moment when the load causes any frictional slippage at all between the bolted parts. At least we mechanical people do. Others will define a failure as when the bracket is physically breaking apart or distorting visibly. It's hard to know for sure just how much friction is happening between the PTR and aluminum. None of my personal design references cover PTR and wood screws so I can't apply the frictional failure criteria as a rough estimate. There is value to purchasing commercial components like this because the supplier has done this work and developed practical working load limits.

Unfortunately one of the things that can make architectural details look fresh and innovative is to make them appear to break the laws of physics a little bit. Since design is generally very practical, that means we are used to seeing practical designs everywhere and that drives architects to imagine highly impractical things. That 'gap' is one such item. You might be wise to hand this to a structural engineer, as they often serve as the go-between for architects and physics.

 

[LittleInch]

Thanks for taking the time to respond. The gap is a design element from the architect. The reason I started this post is precisely because of your comment that the 'moment' changes the force from shear, and I wanted to know if it could be calculated. My original idea was to use a T profile with four screws, but I thought perhaps it was overkill, and the architect didn't like it as she wanted a cleaner look. Your comment made me think of another approach; a horizontal slot with a Z bracket. Thanks for your comments.

I think you can get a very simple and clean block machined with a T shaped pin or pins on the reverse which then slot into keyhole shaped slots in the frame. Very strong and would look "magic" as there is no visible means of fixing.

sort of like this?

 

[mfgenggear]

OP
What is the size and weight of one book.
The roughly calculate the quantity of books.
Weight x quantity.
As an example :One egg weight is very light.
Now multiple x 100 eggs the weight adds up.

I don't need to calculate force or moment for your design. I can tell it is not robust enough.
Take a weight tie a rope to it and hang on the bracket . Keep adding weight till it fails.
The center of book shelve is not robust enough it will yield.

 

[LittleInch]

See post #5.... "The shelves are 10' long, and have an internal structure of steel to keep them from sagging under the weight of books. Total weight of shelf, filled with books is apprximately 330 lbs."

The ability of a thin shelf to not visibly bend with 150kg of books ( two people sitting on it) is ludicrous and typical of an architect who applies no common sense.

This will be used for light decorations only as anything more than about 10kg in the middle will cause it to either buckle and break or visibly bend.

 

[mfgenggear]

thanks Little Inch

OP
look at this style of design

 

[mfgenggear]

also this

 

[TugboatEng]

This is a good candidate for an adhesive mounting if you can swap the aluminum angle out for steel. Adhesives will distribute the load better over the thin tubing. You can keep the screws as they'll facilitate assembly.