Swept Blend Fails

[ctc]

I am trying to make a hip roof, the edge (eave) of which in plan follows an elliptical path. There is 1 high point (ridge) to the roof at the center of the elliptical shape in plan. (Think of a circus tent with 1 pole without the sagging of the material.)

I thought that this could be done using the swept blend.

I would create this in 4 pieces, one for each quarter of the roof as defined by the longitudinal and transverse axes. The path would be the eave in plan and the 2 profiles would each be sketched parellel to each axis.

Unfortunately Revit returns the error that the swept blend cannot be created.

At first I thought the problem was that, at the "ridge", each of the 2 profiles shared the same location in space. So I experimented by moving the ridge, in each profile, closer and closer to the eave. The swept blend continued to fail until the "ridge" was within approximately 1/3rd the distance from the eave. Then the swept blend worked. Unfortunately I was left with a big elliptical hole in my roof.

I tried this as both a mass and an in-place family. Both failed.

Any ideas?

[sfaust]

would it work to just create a blend that is eliptical at the base and a very small elipse at the top?

[AP23]

I have also encountered problems with the swept blend. About 95 % of the time I get the “swept blend cannot be created” message. In further testing it appears that Revit just can’t handle certain path and profiles angles, certain profile sizes and if the difference between the profiles is too big or the path is too small. So as we are accustomed with Revit even with the swept blend you will still have to rely on other software packages.

[brenehan]

“When creating a sweep, be sure the profile is small enough to sweep around the arc or corner without the resulting geometry intersecting itself.”

If you follow this rule most of your sweeps will work. Basically, sweeps can not double back on them selves. Before you create the geometry try to understand how it works in your head first. If it still can not make the sweep, start if off in little sections, add a bit more as it works. You will find your own genometry error.

[ctc]

would it work to just create a blend that is eliptical at the base and a very small elipse at the top?

This workaround approximates the roof. For schematic design it's ok. Thanks.

[ctc]

“When creating a sweep, be sure the profile is small enough to sweep around the arc or corner without the resulting geometry intersecting itself.”

In this example I see no reason why the sweep would intersect itself unless non-intuitive rotation occurs.

[patricks]

When you create a swept blend, Revit has to interpolate the sweep profile in between the two profiles you drew. At certain points along your path, it may be that some parts of that interpolated shape intersects with other interpolated shapes at other points along the path.

What's probably happening is the rear vertical edge of your sweep profile at the end of the ellipse is crossing over itself, since a true ellipse does not have a constant radius.

If you made the sweep pieces using arcs to approximate an ellipse, it would probably work.

[Steve_Stafford]

By overlapping itself, imagine two square lengths of wood, like 2x2x24 in size.
Place them end to end, then begin to rotate them so that they create an angle. When the angle becomes small enough between them that the surface of the one you are rotating touches the surface of the other Revit will begin to complain. You can mock this up in a family pretty easily to see. You can generate it in any sweep. If the end of one segment begins to overlap or touch the previous segment you'll get the error.

If you can do what you are doing with a regular sweep without an error and can reproduce the error with the swept blend I suggest you report it to support so they can sort it out.