Trickle Down Arch Design

Our latest design challenge is a brick arch with recessed accent courses that tie in to the recessed accent courses of the running bond of the building.  Here is a building in Baltimore with a similar style arch.

What complicates the matter further is the 32" soffitt and the question of how we will prefabricate the arch.  When designing a normal radial brick arch the measurements that we need to determine are the circumference, angle, length of the outside arc, and brick size.  

circumference x angle  =  length

360    

But in this case the masonry opening arc is half round and the outside arc is elliptical.  Also we are limited by the size of brick because the joints of the accent course must line up with the running bond.  Here is where it gets tricky.  Instead of starting with the arch, I started with the running bond and accent course, and started at the top of the arch.  To pass the eye test the arch cannot be too tall in the center, but also the 2 1/4" brick size must line up to the same number of courses in the arch.  The only way to get the desired effect was to start with 3 5/8" closure size brick towards the top of the arch, then work our way smaller towards the bottom.  This is how we came up with the ellipse.

I am no math wiz, so if anyone knows a better way to reverse engineer this thing send me a message.  What has me mixed up is the length of the elliptical arc combined with the size limit of the radial segments.  

Ft. Myer

I had never heard of Ft. Myer before Randy of LDC masonry called me to talk about a building he is renovating there. The base is nestled right next to Arlington National Cemetery. We met across from the chapel on base at 8 am a few days before veterans day. I was waiting in my truck for Randy to arrive while I watched a military funeral procession happening in front of the chapel. I felt a deep sense of gratitude for the life I have as I watched the funeral of some anonymous soldier. The army funeral rituals were so deliberate and sorrowful. I was moved by the immense size of Arlington Cemetery and I felt heavyhearted the rest of the day.

The arches we were trying to match were made of standard size brick and are 8 brick courses tall. They arch is double wythe with a 3" rise in the center.



I designed the arch radially so that the bed joints will not be horizontal but would correspond with the 3" center rise. This also keeps the bonded soffit effect of the arch and keeps all the small pieces at the top of the arch.



The resulting arch has a 75 degree skew back angle. This is structurally sound and self supporting because the width at the top of the jack arch is 56" and the masonry opening is 44.625". There is some simple geometry involved in designing a radial arch out of brick. The key is to maximize the thickness of the 2.25" brick. If we have a masonry opening and rise in inches we can get the radius...if we have the radius we can get the circumference(2πr)...Take the circ x ANG divided by 360 = length of arc. When you have the length of the arch in inches you can determine bond, brick size and joint angle.

193" - 5J o/s flemish bond

This is my first attempt at a blog, so if you are reading this thank you for your patience. The purpose of writing this is to share the mathematics and geometry that goes into designing and prefabricating brick arches. I design the arches in AutoCAD for the Archway System but simple geometry is the foundation of my design work.

We have a job that calls for a 193" masonry opening brick jack arch made from oversize brick, with a flemish bond, 5 courses tall (15.625"). Here is how I made the full scale template.

-The bottom measures 193"
-The jack arch skew back angle is 70
-The height is 15.625"
-This gives me a top measurement of 204.374"
-Take top measurement and divide by brick size + mortar joint size and then round that number up to the nearest odd number = number of brick in the arch

- 204.374/(2.75 + .375) = 65.4

-round up to nearest odd number = 67 brick
-mortar joints x mortar joint width = total mortar joint width to take out
-traditional colonial architecture has mortar joints of 3/8"
-an arch with 67 bricks has 66 mortar joints

- 66 x .375" = 24.75"

-(top measurement - total mortar joints) / brick = brick width at top of arch

-(204.374 - 24.75) / 67 = 2.681"

-Repeat this with the bottom width to find the brick width at the bottom of the arch

-(193 - 24.75) / 67 = 2.511"

Now I know the 193" jack arch has 67 brick courses. It also has consistent .375" joints throughout. The brick need to be cut down to 2.681" at the top of the jack arch and cut down to 2.511" at the bottom of the jack arch. These numbers are put into AutoCAD and the jack arch begins to take shape. Next is the flemish bond bed joints. To get 15.625" height for a jack arch made from brick that are 7.625" long, the arch flemish bond needs to be 3 piece-3 piece. I decided on 6" stretcher brick, 2.875" plug brick for the flemish course, and a 4.4375" bat brick for the next course. Hopefully the shop drawings makes the bond easier to understand. Now I have what I need to plot a full scale jack arch so our saw men can cut the brick shapes and our bricklayers can prefabricate the arch. Call me if you have questions: Justin Wethington 301-499-2222.