B Understanding Stress Transfer in Pin-Tied Stone and Wall Interfaces

[new6ton]

If you have a pin tied to a string carrying a stone in the wall. And you put epoxy to the stone and wall attaching them. How is the stress transferred from the pin to the interface between stone and wall? What method is used to model the transfer function?

 

[berkeman]

How is the stress transferred from the pin to the interface between stone and wall?

They have a heartfelt conversation, expressing their feelings to one another.

J/K. You really need to attach a sketch of this situation, and show your FBD please. Thanks.

 

[new6ton]

Imagine you put glue between block and wall. As the glue hardens, how do you model the string tension slowly diminishing and the weight or tension transferred to the glue? Or what is the best technical language to describe it?

 

[new6ton]

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Imagine you put glue between block and wall. As the glue hardens, how do you model the string tension slowly diminishing and the weight or tension transferred to the glue? Or what is the best technical language to describe it?

In the analysis. Let us treat the string as fixed and not flexible. This is because when you glue the block and it got stuck to the wall with the string in tension. It would remain in tension (imagine a rubber band). So treat it as string made of iron (or something which doesn't stretch like a chain). Now I want to know what happens when the glue/epoxy hardens slowly. Does the string slowly lose weight it supporting in proportional to how the glue hardens? How is the stress transfer function of such dynamics?

 

[berkeman]

In the analysis. Let us treat the string as fixed and not flexible. This is because when you glue the block and it got stuck to the wall with the string in tension. It would remain in tension (imagine a rubber band). So treat it as string made of iron (or something which doesn't stretch like a chain). Now I want to know what happens when the glue/epoxy hardens slowly. Does the string slowly lose weight it supporting in proportional to how the glue hardens? How is the stress transfer function of such dynamics?

Stated like that, the answer is No. Why would it? The position of the mass does not change during the hardening of the glue.

 

[new6ton]

Stated like that, the answer is No. Why would it? The position of the mass does not change during the hardening of the glue.

But when the glue hardens. The block is stuck to the wall and no longer has weight to the string. So why would the string still feel weight, it's like the block is weightless already because it is fully epoxied to the wall.

 

[Ibix]

But when the glue hardens. The block is stuck to the wall and no longer has weight to the string. So why would the string still feel weight, it's like the block is weightless already because it is fully epoxied to the wall.

The weight stretched the string and the string is still stretched, so it still has tension in it.

If you think of the epoxy as supporting the weight of the mass then you have to ask why the string is stretched. The weight isn't stretching it any more, so what is? The answer is that the epoxy is not permitting it to shorten. The force required to keep it stretched is equal to the weight of the mass - so in total the force on the epoxy is zero.

It seems easier to say that gluing the epoxy to the wall changes nothing. But I don't think it's wrong to say that the epoxy is both supporting the weight and stretching the string, and that the two forces are equal and opposite.

 

[new6ton]

The weight stretched the string and the string is still stretched, so it still has tension in it.

If you think of the epoxy as supporting the weight of the mass then you have to ask why the string is stretched. The weight isn't stretching it any more, so what is? The answer is that the epoxy is not permitting it to shorten. The force required to keep it stretched is equal to the weight of the mass - so in total the force on the epoxy is zero.

It seems easier to say that gluing the epoxy to the wall changes nothing. But I don't think it's wrong to say that the epoxy is both supporting the weight and stretching the string, and that the two forces are equal and opposite.

How would it change the analysis if the string is a thick steel cable?

 

[Ibix]

How would it change the analysis if the string is a thick steel cable?

I would replace each instance of the word "string" in my post with the words "thick steel cable".

 

[new6ton]

I would replace each instance of the word "string" in my post with the words "thick steel cable".

Are you saying there would be same stress in the steel cable only the capacity is much more?

 

[Ibix]

Are you saying there would be same stress in the steel cable only the capacity is much more?

Stress is a force per unit area. The force (the weight of the mass) would be the same for the string or the cable (neglecting the weight of the string/cable), but if your cable is "thick" then the area would be much larger than that of the string and the stress would be much lower. If the cable had the same cross-sectional area as the string then the stress would be the same, yes.

The strain (the extension divided by the natural length) of the cable would probably be much lower than that of the string because the cable has more tensile strength. But that strength is not infinite, so the cable would stretch very slightly and my analysis in post #7 remains the same.

 

[Nugatory]

Are you saying there would be same stress in the steel cable only the capacity is much more?

And the amount by which the cable stretches when supporting a given weight is correspondingly less.

 

[new6ton]

Stress is a force per unit area. The force (the weight of the mass) would be the same for the string or the cable (neglecting the weight of the string/cable), but if your cable is "thick" then the area would be much larger than that of the string and the stress would be much lower. If the cable had the same cross-sectional area as the string then the stress would be the same, yes.

The strain (the extension divided by the natural length) of the cable would probably be much lower than that of the string because the cable has more tensile strength. But that strength is not infinite, so the cable would stretch very slightly and my analysis in post #7 remains the same.

Imagine you are carrying a person with a rope over the bridge with your arms. Then a boat passed by and the person is glued to side of the yacht. You can no longer feel the weight. I guess this occurs when you lower your arms. But if your don't lower your arms, then the weight is the same even if the person is glued to the side of the yacht?

 

[Ibix]

Imagine you are carrying a person with a rope over the bridge with your arms. Then a boat passed by and the person is glued to side of the yacht. You can no longer feel the weight. I guess this occurs when you lower your arms. But if your don't lower your arms, then the weight is the same even if the person is glued to the side of the yacht?

This is the same scenario as above, with added complexity in the form of a potentially moving boat and human muscle power instead of a static rigid system. In principle the answer is the same; in practice there are a lot of uncontrolled factors that might well mask that. Why are you asking?

 

[new6ton]

This is the same scenario as above, with added complexity in the form of a potentially moving boat and human muscle power instead of a static rigid system. In principle the answer is the same; in practice there are a lot of uncontrolled factors that might well mask that. Why are you asking?

I have some paintings each hanging on string on a nail. I thought by gluing the paintings frame to the wall, it can relieve some stress in the nail. So it can't. So need to replace the strings and nail. Lol.

 

[A.T.]

I thought by gluing the paintings frame to the wall, it can relieve some stress in the nail. So it can't. So need to replace the strings and nail.

Gluing picture frames to the wall was your preferred solution compared to replacing the string and nail?

 

[new6ton]

Gluing picture frames to the wall was your preferred solution compared to replacing the string and nail?

Because the nail is a special screw and vintage and it would damage the concrete if it would be pulled out and replaced. But since glueing the painting frame at the lower side only and partially won't make the wall screw and screw share the load. Then will just pull out the vintage/screw nail and fix the wall.

 

[Nugatory]

I have some paintings each hanging on string on a nail. I thought by gluing the paintings frame to the wall, it can relieve some stress in the nail. So it can't. So need to replace the strings and nail. Lol.

Ah - if that’s what you’re trying to do, you’ve been asking the wrong question all along.

In fact, gluing the frame to the wall will do exactly what you want (provided that the adhesive is strong and stable enough to hold the picture)... even though it will not reduce the stress on the nail. If the nail/wall connection does fail, the glue will stop the picture from moving down far enough to rip the nail out of the wall.

 

[new6ton]

Ah - if that’s what you’re trying to do, you’ve been asking the wrong question all along.

In fact, gluing the frame to the wall will do exactly what you want (provided that the adhesive is strong and stable enough to hold the picture)... even though it will not reduce the stress on the nail. If the nail/wall connection does fail, the glue will stop the picture from moving down far enough to rip the nail out of the wall.

You mean the slightly failed nail/wall connection sharing half of the load and the epoxy to wall sharing half the load. This was why I was asking what kind of model to describe this stress transfer between them so can compute how to use minimal epoxy for stress and load sharing. This is for academic understanding and opportunity to know the principles involved.