Two-centered and four-centered arches share something in common—a pointed peak. It’s not surprising that both are commonly found in Gothic and Gothic-inspired architecture. But a three-centered arch—sometimes called a ‘basket-handle arch’ or ‘Anse de panier’—closely resembles an ellipse, which puts it in a field of its own.
Be sure to read Part 1 of this series on arches: Circular-Based Arches
This depressed type of arch, like the Segmental and Drop arch, can be used when the design requires the rise—or height—of the arch to be reduced. While segmental and ‘elliptical shaped’ arches both share a rounded top, the elliptical variation provides the benefit of a clean vertical transition, and respects traditional design principles.
A true ellipse is the shape created by making a diagonal section-cut through a cone or cylinder. The ellipse has two focal points and a constantly changing arc radius.
It can be difficult to determine if an arch is a true ellipse, or just one composed of simple tangent arcs, swung from three centers. Either way, elliptically shaped arches are more commonly found in traditional homes based on colonial styles—though their use depends more upon the skill of the architects, millwrights, and finish carpenters.
I’ve heard some carpenters say (and I won’t mention any names!) that the popularity of segmental arches—sometimes one of the most boring and ugly forms of architecture—results more from a lack of knowledge and technique than from an understanding of classical forms—both Gothic and Colonial.
These carpenters believe that elliptical arches—or, at the very least, three-centered arches—are far more attractive, but that the technique is beyond the skill of most contemporary carpenters. I don’t necessarily agree. I don’t think the segmental arch should be completely avoided.
In the first part of this series, I shared some images of segmented arches gone wrong. But, when designed and executed properly, a segmented arch forms a pleasing and handsome frame, as long as the arches (the rise, the radius, the span) are nearly identical in size. But, if the openings have variable spans, a three-centered arch is a better answer!
At this point, I can’t help but mention Gary Striegler’s article in JLC about building an arched passage door. I’m including a PDF of that article here. It’s a critical part of this study, both because it will help readers form a better understanding of complex arches (arches with more than two centers, and elliptical arches), and because Gary’s article provides techniques for constructing a three-centered arch, which is much easier than milling elliptical molding! In fact, mill shops often use a similar technique to create their elliptical moldings, sometimes using five or more centers to create a more accurate elliptical shape.
Another example of where a three-centered arch is easier on the carpenter, as opposed to a true ellipse, is in a coffered passageway. The curved panels of the head only require two different radii. In the photo to the right, you can see that the panels across the top share the same curvature, and panels with a tighter radius are used as the arch terminates on each side.
Getting back to the purpose of this article—how do we layout this pseudo ellipse? Well…it all depends on what you are given to work with. Although being involved at the planning stages is ideal, most of the time it’s not a reality.
Hopefully, the following Quick Reference Guides will help you deal with any ‘curve’ you’re thrown.
The Classic Three-Centered Arch
This layout is for the classic three-centered arch. You only need to know the required width or span of the arch. The rise of the arch will be determined by proportion only.
|Download the Quick Reference Guide for The Classic Three-Centered Arch|
Three-Centered Arches with a Known Height & Width
This layout is used when you must fit an arch within a predetermined height and width.
|Download the Quick Reference Guide for Three-Centered Arches w/Known Height & Width
Three-Centered Arches with Known Radii
This layout is used for creating a three-centered arch when the two radii to be used are predetermined. This is the situation used in Gary Striegler’s article.
|Download the Quick Reference Guide for Three-Centered Arches w/Known Radii
Keep an eye out for the last part in this series, on Four-Centered Arches!