In the last couple of decades, architects and builders have started an architectural fad in residential construction: the eyebrow arch. Such arches are commonly installed in homes with masonry veneers. Eyebrow arches are graceful, stylish, appealing, and inherently unstable.
The aqueducts built by the Romans have classic arches shaped like the top half of a circle. They can be supported on columns, but the upper section is always semicircular. Such arches are quite stable, and have been in use for thousands of years.
If you ever go to a children’s science museum, you might see an interactive exhibit where the kids can make an arch out of loose blocks. If they set it up carefully, it will not fall over. This demonstrates the stability of classic arches.
After using classic arches for a millennium or so, builders started incorporating gothic arches in buildings. Gothic arches are narrower than classic arches but are also fairly stable.
At the bottom ends of any arch, two types of forces are in play. The weight of the arch pushes down on each leg, and also pushes to each side. If left unrestrained, the arch would spread or splay. The amount of push to each side is dependent upon the shape of the arch. Both gothic and classic arches push mostly straight down on each leg and very slightly to each side.
After thousands of years of progress, the development of the scientific method, and tremendous improvements in technology, architects started using eyebrow arches. Such arches are wide and shallow. For this shape, it is easy to imagine how the legs push very strongly to the sides. The wider an arch is in comparison to its height, the more likely it is to spread and collapse.
To make this more real, imagine standing on a gym floor on the day you forgot your tennis shoes and wearing only socks. The farther apart your feet, the more they try to slide to the sides. Arches behave the same way. If you had to stand for a long time, would you prefer to stand like a classic arch, a gothic arch or an eyebrow arch? The pain felt from standing in an eyebrow arch stance is a useful illustration and comparison to the stress applied to the legs of these arches.
Let’s pretend that you are in the eyebrow stance but some friends come to stand on each side of you, blocking your feet from sliding. Suddenly, the pain is gone. Your feet push against the other people, and you no longer have to use your leg muscles to keep from falling over.
Arches are just like that. If an eyebrow arch is installed with an adequate structure on each side of it, it is kept from splaying, its strain is reduced, and it can last a long time. With no such structures, its “feet” spread slowly apart. Eventually, it pulls a leg muscle, and falls over.
In a bridge, for example, the structure which keeps the arch from splaying is called an abutment.
In residential construction where an eyebrow arch is in the center of a wall, the walls on each side of the arch act as abutments. In that case, there is no problem with such an arch. Unfortunately, many builders install eyebrow arches with one end supported by a column. Such an unspecialized column has almost no strength to resist the sideways load from the arch, so the top of the column tilts to one side. The arch, deprived of support, fails.
Homeowners and builders might claim that eyebrow arch failures are due to settling, earthquake, tornado, or a number of other perils. While an earthquake certainly could make the problem evident, the actual failure is often due to the nature of the arch.
Eyebrow arches are not a bad idea as part of a building, but they must be restrained to hold up well. Unless your builder understands the concept of eyebrow arch instability, and your architect has the arch checked by an engineer, it may be safer to try another, more stable style.