Question about umbrella roof shape

[chicopee]

Is the shape spherical since its radius "r" is minimal at 0.8D and maximum at 1.2 D? if not which shape is it following. Looking at pictures on the net, an umbrella roof seems to follow either an ellipsoidal or parabloid shape.

 

[IFRs]

If you are looking at API 650, my interpretation is that the plates for an umbrella roof are curved in one direction only and flat in the other. It looks like a standard consumer umbrella, with curvature in the radial direction and flats in the circumferential direction. When viewed through a section that cuts the roof in elevation, it is a circular shape and it has a distinct and measurable radius. It is neither ellipsoidal nor parabolic. It is circular, has only one radius and this radius is fixed between the minimum and maximum values you noted above. In this manner API has narrowed down the possible values for the roof radius to a relatively small range so their design equations are valid. However, that is not to say that ellipsoidal or parabolic shapes are not possible, economic or hard to fabricate - just that API 650 does not have a cookbook design for anything other than ubmrella (singly curved) or dome (doubly curved) roofs with a circular or spherical shape.

 

[chicopee]

I have never seen an umbrella roof in the Northeast, that Ct, MA, Vt, NY and RI since the '60's when I got involved with the N.B.,so where is the prevalence of such roofs? I thank for your input as I never realized that there were flatnesses in umbrella roofs.

 

[IFRs]

I see them a lot in NJ terminals on smaller chemical tanks, and I've built a few in refineries in NJ.

 

[chicopee]

Good to know as I will be driving thru NJ soon, so I'll keep an eye for them.

 

[Geoff13]

Umbrella rafters would be a maximum of about 7' apart. Thus the flat area isn't very different from a true dome. Seen from a distance (say an adjacent road) it would likely be invisible.

This site has a couple of umbrella roof photos.

Some umbrella roof pics