Key Insights Into Eye and Oculus Openings in Historic Masonry

Here in the blog section of our website, this past week, we looked at several examples of bullseye and oculus openings in historic masonry. Today, for this week’s blog article here on our website, we’re looking at a very similar topic of Bullseye and Oculus type openings in historic masonry facades, but the big difference is that this week I’m looking at these type of openings in ashlar stone facade buildings. Last week we looked at this type of openings in rough cut stone and brick facades. Overall, the concept is very similar but here it’s used differently in an architectural context. As well, the styles of these buildings are vastly different.

The oculi window shown in the picture below looks similar in many ways, to the bullseye fenestration openings that we looked at last week. However, there are several differences in the masonry. If you look closely at the masonry surrounding opening itself, you’ll see that unlike the windows and openings we looked at this past week, this particular masonry has no voussoir bricks or stone masonry units. Instead, based on the ratio and the comparative size of the stone to the opening, the stone blocks themselves are supporting the masonry above the opening without a header. 

In typical conditions, the masonry headers for an ocular ot bullseye windows are generally continuous around the entire circle of the opening. A window opening header, in the case of a rectilinear or square window, just exists at the top of the window, not at the sides or the sill at the bottom.

These round openings are different though because they generally require header type masonry elements to continue around the entirety of the opening. Here though, the pictures above and below show an exception. Here because the opening is relatively small in comparison to the size of the masonry units, those masonry units themselves are self-supporting to allow the opening without compromising the strength of the load above the opening. This comes down to basic structural bearing and support principles.

In the majority of the most common masonry construction, rectangular or rectilinear openings typically are built with a row or arch of angled bricks or stones to create a load-bearing arch above the opening. The individual units inna configuration like this are called voussoirs.

These voussoirs are often arranged in a curved or segmental pattern, with each unit angled, often increasingly out the jamb or springer sides of the opening, so that the compressive load from above are transferred through the arch and down to the jamb or springers on either side of the opening. The keystone, often the centermost of the voussoir units, locks the assembly together and provides an additional degree of structural stability. This configuration allows the masonry wall above to be supported while creating an opening space, most often used for windows or doors.

The physics of bearing load transition in an oculus or bullseye opening, works a bit differently. Unlike a rectilinear or even Roman arch opening where forces are primarily centered above the header, a circular opening has force applied around the entirety of its circumference. This generally requires a continuous ring of voussoirs that forms what is effectively a 360-degree arch. Each voussoir in a typical circular arrangement may be cut, like a rubbed gauged bricks in some historic brick masonry openings. The positioning allows the masonry units to transfer compressive forces to its neighbors while holding the geometric form of that circle.

Back, a few paragraphs above, we used the word “require”. However, the pictures above show a unique exception because here the openings do not have a circular header of any type and do not have voussoir masonry elements.  So if you’re understanding the details so far I’m paying attention you should be wondering how these openings, in these specific photographs above get around this so-called “requirement”.

Technically, from a structural engineering or a physics perspective, there’s a loophole around this requirement because in a header, for an opening for a door window or just for fenestration, a header is only required when an opening is larger or position in a way that it is larger than more than one masonry unit.  

Here the openings are placed to be centered at the perpend (vertical) joints of the stones above. That size and position or placement allows the masonry units above the opening to support themselves and the masonry further above continuously without the use of an added header arrangement. 

The opening shown in the picture below, placed above the rectilinear window openings, look very similar but if you look very closely as you can see in the next picture below, they actually do have voussoir masonry elements that are set together in a locking sort of compressive configuration.

A closer view follows in the next picture. You can see that each of these elements are individually cut to shape.

We can Help

Our company focuses on historic restoration more than modern building upkeep, maintenance, and construction, but our company understands both types of construction very well and a full picture well-rounded approach is needed in any niche in the construction industry.  Although we focus on historic restoration, repointing, tuckpointing and historic brick repair, our company also has technical knowledge and competencies in the areas of modern and contemporary construction as well as we become one of the leaders in that area of the market today.  Understanding both historic and modern or contemporary construction is useful because both aspects help understand the challenges and potential solutions for challenges in building science and construction.

We can help with a variety of historic masonry restoration needs and upkeep, from modest tuckpointing and or repointing to complicated and extensive historic masonry restoration. Infinity Design Solutions is a historic restoration specialist contractor specializing in both historic masonry restoration such as tuckpointing our repointing, and brick repair. If you have questions about the architectural details or facade of your historic building in Washington DC, reach out and say hello and if we can help we’ll be glad to assist you. You can email us or call us on the telephone at the following link: contact us here.