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Ring action stability on roofs
- Thread starter TRPAM
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I can see from your sketch that it's trapezoidal in section and also that there is a ring of beams supporting something on top.
My question was to do with the shape of the ring. Is it square, rectangular, circular, elliptical or something else? No one can design the ring without knowing its shape.
The red colored material shown in section, I assume, is concrete. If so, it may be adequate to resist lateral forces without relying on "ring action". You need to describe the structure in a good deal more detail than you have done to date if you expect any useful responses.
I suggest you provide a preliminary roof framing plan to give us an idea of the opening size and shape as well as the kind of stiffness to be expected in the structure surrounding the trapezoidal thing.
My question was to do with the shape of the ring. Is it square, rectangular, circular, elliptical or something else? No one can design the ring without knowing its shape.
The red colored material shown in section, I assume, is concrete. If so, it may be adequate to resist lateral forces without relying on "ring action". You need to describe the structure in a good deal more detail than you have done to date if you expect any useful responses.
I suggest you provide a preliminary roof framing plan to give us an idea of the opening size and shape as well as the kind of stiffness to be expected in the structure surrounding the trapezoidal thing.
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TRPAM said:
There are two rings. The one on the bottom is "made of beams", but is not a compression ring. It is a tension ring, with bending about its vertical axis from lateral forces shown by the red arrows. The one on top is a compression ring, loaded by forces shown by blue arrows.
Offhand, I don't know of a book which would be of any help.
The only thing that makes a ring significantly different from other structural elements is the stability aspect of it. And that would include both the in plane stability of the individual rings as well as the overall stability of the system in which the ring(s) will be used. With respect to the latter issue, stability of your system looks dubious per the sketch below. As far as a reference goes, I'd start with the book show below.
In the olden days, there were a lot of grain silos with roofs similar to that.
Design would generally have been low-tech, using circular ring formulas (bending and buckling) in Roark, treating rafters as beam/columns.
And rafters may have been attached to, or assumed to be supported by, the roof plate.
Buckling such as KootK illustrates just likely not addressed.
One reason for lack of information here is that for an overall geometry similar to that, you can have a lot of variation in details. Is that ring circular or octagonal or N-sided? Are the rafters attached with moment connections or pinned? In the grain-silo cases, the roof plate would be very good at keeping one side of the compression ring from rising, but wouldn't do much to keep the other side from dropping- which complicates an analysis.
Unequal loading due to wind and unbalanced snow would likely be most critical cases.
Design would generally have been low-tech, using circular ring formulas (bending and buckling) in Roark, treating rafters as beam/columns.
And rafters may have been attached to, or assumed to be supported by, the roof plate.
Buckling such as KootK illustrates just likely not addressed.
One reason for lack of information here is that for an overall geometry similar to that, you can have a lot of variation in details. Is that ring circular or octagonal or N-sided? Are the rafters attached with moment connections or pinned? In the grain-silo cases, the roof plate would be very good at keeping one side of the compression ring from rising, but wouldn't do much to keep the other side from dropping- which complicates an analysis.
Unequal loading due to wind and unbalanced snow would likely be most critical cases.
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thanks for the recommendation!
the shape of the mechanism you have described is exactly what I was thinking. but i think that it works as a three dimensional structures and that's what's keeps the stability.
I think you are right about the Unequal loading due to wind.
but what about all the one story houses with triangulate roof ? the connection between the wall and roof is of a pin connection. and there is no diaphragms to transfer the load to "shear wall".
if wind exerted on the roof from one side, it can theoretically make the roof collapse.
the shape of the mechanism you have described is exactly what I was thinking. but i think that it works as a three dimensional structures and that's what's keeps the stability.
I think you are right about the Unequal loading due to wind.
but what about all the one story houses with triangulate roof ? the connection between the wall and roof is of a pin connection. and there is no diaphragms to transfer the load to "shear wall".
if wind exerted on the roof from one side, it can theoretically make the roof collapse.
Phew! Well just as well we live in a work with three spatial dimensions. I don't know about you. But I've never ever seen a two dimensional structure in the flesh.TRPAM said:
TRPAM. Are we talking about real structures or a text book structures? Because if we are talking about the latter you have wasted many people's time.
In this same 2D theoretical world you are proposing for you house, there seems no way for a theoretical 2D stick man to enter the house. Which to be honest is a far bigger problem.TRPAM said:
Yet again, we can be thankful that our world has a third dimension.
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TRPAM said:
Sure. If you repeated the post and strut radially, and then made a diaphragm of the exterior with structural sheathing or discrete bracing, you'd basically have a dome with a hole in the top. Some of those have proven themselves to be very stable.
TRPAM said:
No, there absolutely are horizontal diaphragms on these structures that stabilize them via connection to the shear walls. That's precisely how they work. In some parts of the world, that's the roof sheathing. In others, it's the ceiling.
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ok,then structures of this kind (in the video) must have a horizontal diaphragms ?
Be careful throwing that stuff around. Human909 is a contributing member of this forum and we don't really tolerate disrespect. If anything, I agree with his responses to you. You seem to be stuck looking at things only 2-dimensional which honestly to me seems like you're a student trying to learn the basics.TRPAM said:
I'll admit, you appear to have the right mind for structural however if your stuck on stability of structures this early. Most people don't care about lateral stability, even some structural engineers I know only have suspect knowledge.
Anyway, I believe KootK has done an adequate job, as always, of answering your query.
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The plan shape of the ring makes a difference. The sloping diaphragms of the truncated pyramids shown below provide stability to the compression ring, but the rectangular ring has a much larger bending moment to resist than the square one. Apart from that, there seems to be no particular stability issue with regard to compression ring beams.
If you do not want to rely on sheathing, the sloping diaphragms can be braced like a truss.
If you do not want to rely on sheathing, the sloping diaphragms can be braced like a truss.
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