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.
