Need some feedback. Building a large elliptically shaped “Skygarden” 67′-4″ x 39′-6″
Layed it out onsite using 2 radiuses provided on plans, and everything lined up fairly decent with concrete slab.
Ordered radiuses made up for steel track in 10′ lengths, planning to cut to fit on site.
Steel shop suggested they could make the exact shape and we would just have to put pieces together, no cutting.
OK lets try it…
Steel comes today and the shape fits the overall dimensions but the radiuses are way different, not aligning with concrete at all. This is critical, there is glazing involved.
Here is the question- Given four points (67′-4″ x 39′-6″) set up at true 90 degrees and centered, can more than one ellipse hit all points.
The shape I drew on the floor can be divided into quarters and all will be identical, the same is true for the shape the steel shop made. I am upset that he did not compare his radiuses to mine before fabrication. We are meeting at site in the morning to remedy the situation
Mike
Trust in God, but row away from the rocks.
Replies
"can more than one ellipse hit all points?"
Not if it's a true ellipse. One of you two didn't follow the rules for making a true ellipse. And unfortunately, I think it might have been you.
"Layed it out onsite using 2 radiuses provided on plans"
An ellipse doesn't have a constant radius. The radius is changing as you move around the shape. It sounds like you set up several connecting arches that resemble an ellipse and the metal fabricators created a true ellipse that went through your four points.
Do you know how to use a string and two focal points to make an ellipse?
Do you know how to use a string and two focal points to make an ellipse?
I'm answering my own question...
You have two focal points along the longest axis of the ellipse at +/- 27'3" from the center of the ellipse, stick two spikes in the ground at these two spots. Connect these two points with a string that is 67'4" long. Now when you stretch the string to the ellipse (point P below) and follow its path all the way around, you should always stay right on the ellipse itself. This is a true ellipse.Joe
View Image
http://www.worsleyschool.net/science/files/ellipse/facts.html
I am familiar with the method, but the shape must match what architect drew and the concrete crew formed.
You are right that I used the radiuses, and he used the method you describe.
Maybe the problem started with using the term ellipse, which everyone is doing since it looks like an ellipse.
Is there a clear definition of an ellipse?
Mike
Trust in God, but row away from the rocks.
In the picture below, imagine that the red lines are a single string that connect the two focal points. If you move P around the circle, the string will always stay tight. That is the definition of an ellipse.
Just in case the image didn't show up in the last post, here's a copy.
Yes there is Mike. There is no ambiguity at all:
Ellipse is defined as the locus of points P such that the sum of the distances from P to two fixed points F1, F2 (called foci) are constant. That is, distance[P,F1] + distance[P,F2] = 2 a, where a is a positive constant.
The eccentricity e of an ellipse is defined as e = c/a, where c is half the distance between foci. Eccentricity is a number that describes the degree of roundness of the ellipse. For any ellipse, 0 < e < 1. The smaller the eccentricy, the rounder the ellipse. If e = 0, it is a circle and F1, F2 are coincident.
Another way to describe an ellipse is the intersection of a cylinder and a plane. If you cut round salami at an angle you get elliptical slices. (just added this so you guys won't think I'm a mathematician :))
DG/builder
Thanks for a written definition, I wish I could print the shape I am dealing with.
Two radiuses 49' 6" and the sides are 13' 6" (approximate, I don't have #'s here)
It sure looks like an ellipse, but when I was asked if it was a true ellipse, I clearly stated "I don't know". Fabricator took the radiuses, but made an ellipse.
I just hope he remembers the conversation the way I do. My faxed order is only for radiuses and does not mention ellipse, ellipse was only brought up in conversation.
On site we have been calling it the rugby ball. Mike
Trust in God, but row away from the rocks.
"A plane curve such that the sums of the distances of each point in its periphery from two fixed points, the foci, are equal. It is a conic section formed by the intersection of a right circular cone by a plane that cuts the axis and the surface of the cone. Typical equation: (x²/a²) + (y²/b²) = 1. If a = b the elipse is a circle."
I used the string method for laying out the tops of the windows and doors in my concrete walls. PAHS Designer/Builder- Bury it!
It sounds like someone laid out an oval, which is slightly different than an ellipse. We use the three-nails-and-a-string method to lay out an ellipse. A small ellipse is shape of the hole you cut when a round chimney pipe goes through a sloped roof. Sorry I don'y have the lay out info at hand. But if one of you used a compass to draw the ends or sides of the shape, it is not an ellipse.
The yellow lights go on for me when you said the word radius. An elipse does not use a radius.
It uses a major axis and a minor axis.
Si I'm not sure what happened with whom, but it does not sound on the surface like you understand an elipse. It might be your mistake or it might be abad choice of terminology...I don't know
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I think terminology is the root of the mistake. I wanted the radiused track, just as I had layed out. My shop said the fabricater would come to the site and could make the shape exactly. At our meeting, I gave him the radiuses I needed,and he saw the prints. I was asked if it was a true ellipse and said I did not know, because I did not know the the defination of a true ellipse, but I knew I needed the radiuses we discussed.
The next day he called from his shop and asked me for the width and length and I gave it to him. Aparently he then made an ellipse, and did not use my radiuses or at least compare the two.
The whole problem seems to be the introduction of the term ellipse, which the shape seems to mimic. I should have stuck to my gut and just told my shop to get what I ordered. I also should have directly asked the architect if it was indeed an ellipse.
I am not paying for the steel, but the cost (of a fix) will go on my budget. Lesson learned. Oh well, good thing steal is cheap these days. ; ^ ) Mike
Trust in God, but row away from the rocks.
That makes it all pretty clear that you were building something different formthe shop.You built with radii which indicates to me that you had an oval.He asked for the length and width, which indicates that he was building an elipse baed on major and minor axis.The part that puzzles me is why he did not take the proper information with him when he was there instead of calling back the next day.
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The common way to draw an elipse by hand when drafting is to use two different sized radii. Picture the elipse with the long axis being horizontal. The large radius draws the top and bottom and a shorter radius goes tangent to the two larger radii and through the end point.I'm thinking this is the way it was defined on the drawing for ease of layout on site.Stu
Ruffmike: Water under the bridge now but I'm guessing (as MAsprayfoam is) that you were making something like this Isometric Drawing of a Circle, which is laid out using radii and is different than a true ellipse. Different curves but both can pass through the same points.Joe Bartok
Joe, you are da man!
Good posts all!
blue
Wanted to post this to all, but can't find that option for some reason.
Thanks to everyone. Thanks Joe for the isometric info.
So I guess the shape is elliptical, but not an ellipse. In any case the fabricator agreed he jumped the gun in not making sure we were on the same page and he is making the radiused pieces I originally wanted.
I felt a little bad for him, but he did drive up in a brand new Hummer. ; ^ )
The shape is critical, as it is actually a soffit with segmented glazing below (kerf already in slab) and just clearing structural steel above. The top is also sloped at 7.5 degrees. It will be fun to build. Mike
Trust in God, but row away from the rocks.
Regardless of the definition of an elipse, no one checked and rechecked the shape or the measurements before the steel was fabricated. We all know that no matter what the plan shows, most of the time you have to build to what you have, unless you do everything yourself from start to finish and know, for a fact, that it is correct.
The fact that neither you, nor the steel manufacturer made a template or at least took a measurement for the steel has proved to be a costly mistake. Cost of steel, loss of time, and general confusion are all problems that should have been avoided. In my opinion, the steel manufacturer should foot the bill.
Sure it all could have been avoided, but a problem came up, that led to this post.
It was a mistake on one shape of six ordered. We have been working together for a period of years and this is the only time I can remember a breakdown in information.
It is probably because of our working relationship that he is willing to make things right so easily.
I have never seen a template made for this type of work. In my opinion it was a case of miscommunication with the term ellipse being used. I asked for radii, he went to a true ellipse, if either of us had paid attention (or kept abreast of) our geometry class 30+ years ago, we would have known there is no constant radii in an ellipse. I don't think I will forget that fact again.
Measurements were taken, I drew the thing on the ground. Overall width and length were not part of my measurements because I ordered parts. He should have taken them when he was on site, but it was pouring and muddy. He was given correct measurements over the phone.
In any case it is behind us, and the lesson learned.
Anyone want to build a big elliptical shed? Its 18 gauge steel!! Mike
Trust in God, but row away from the rocks.