Post Tension Brick Jack Arch

We are currently prefabricating 8 foot long brick jack arches for a project in Philadelphia that have no angle iron support under the brickwork. Oh the joys of design-build. Actually it has given us the chance to try a system that Ray Wethington invented, of using post tension #4 epoxy coated rebar through he length of the arch.

Post-tensioned is the descriptive term for a method of applying compression after pouring concrete and the curing process. 99% of the brick jack arches we prefabricate are set on steel angle irons, and it is the steel that carries the load of the weight above the masonry opening. We do make structural jack arches but code usually dictates steel reinforcement unless a historical variance is given.

For this job we are drilling a 1" hole into the brick before we prefab the arch. Then we are inserting the rebar that has a hook on one end and threaded on the other end. We then tighten a nut on the rebar threaded end giving the brick jack arch post tension strength. The dramatic effect is 8 foot straight spans with brick and joints visible from below.

Brick Arches of the Roman Colosseum

Walking up to the Colosseum through the modern streets of Rome with my wife Brigid made me feel alive. I felt proud to work for a company that designs brick arches because there is no greater proof of masonry’s mastery than in Rome. I also felt insignificant in the physical and historical scope of the Colosseum. The stories of thousands of people who had died and cheered on the spot I was standing are what movies are made of.

The exterior of the Colosseum is made from white travertine stone that was quarried from the local hillside and held together with iron clamps. I was surprised to learn that all of the interior was brick and mortar. I we walked through the arcades and hallways, the tour guide gave a short talk on the significance of Roman technological breakthroughs in the field of masonry. The Romans discovered that masonry strength was not compromised by adding arches to span openings. The strength of the masonry structure increases as weight is dispersed above an arch.

The structural brick arches of the seating areas held up as many as 80,000 people as well as the weight of the masonry above it. The arena itself has a brick underground structure that allowed for trap doors with people and animals below like in the movie Gladiator. The brick jack arches of the arena and ground level doors were brilliantly designed and built.

Using four foot long brick, the Romans constructed the jack arch for functionality. The jack arches that supported the arena floor have a 77 degree skew but the jack arches under the massive seating area are built with a 70 degree skew to support more weight above it. What a marvel of engineering.

Cast Stone Arch

Chuck of Oswald Masonry placed an order for jack arches, sills, and radial arches made of cast stone for a 5 unit townhouse. Cast stone or precast is a concrete product that essentially is man made limestone. We fabricate cast stone in our shop using raw materials in a mix of approximately 70% sand, 20% portland cement, 10% water, and less than 1% admixture of superplasticizer and water reducer. Here are pictures of the stone starting at the CAD design.

full scale paper template

template made into a wooden mold


We use the dry tamp method to make our cast stone at Archway System. Dry tamp uses zero slump concrete (the consistency of pancake mix if you only used half the milk the recipe called for). We make a custom mold of what the finished stone is going to look like and add the mix into the mold. We then use a vibrating rammer to tamp the mix into the mold face down. Epoxy coated rebar is inserted in the middle of the stone for reinforcement and then more mix is added and tamped until the desired thickness is achieved. The mold is then flipped onto a table and the mold is removed.

stone flipped out of mold



The stone is then "pointed up" by hand to fix any imperfections that happened when removing the mold. Then the mold is put back together and the process repeated all day long.

5 cast stone arches


Cast stone cures in warm temperature and high humidity on these tables and is about 95% cured after 7 days. The arches are rock solid after a few days and the brick layer installs the cast stone above the windows.

Maple Lawn in Howard County Maryland/Oswald Masonry

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.

48" 4J O/S 1/4" joint 3/4 bond

We often get jobs that are either renovations or additions, and the architect and masonry contractor want the new brickwork to match the existing building. Brick arches built decades ago were made without autoCAD and designed with form following function. What we pay close attention to when we need to match an old brick arch are: structure, bond, skew, striking tool, and joint width. Here is picture a bricklayer sent us.



He said he needs 16 jack arches with a 48" M.O.(masonry opening) for an addition to a YMCA on the Eastern shore. The picture he sent gives us enough info to go ahead with the design. It looks like the building is flemish bond but the four course tall jack arch is 3/4 bond but closer to 1/2 bond. This is common in older buildings because bricklayers did not usually build jack arches this height with flemish bond, too many little pieces. They wanted to use the full length of the brick and still achieve 4 course height (12 1/2"). It also looks like the jack arch joint is tighter and slick joint and the rest of the building is grapevine joint and 3/8". We decide to design the jack arch with 1/4" joints, a 70 degree skew, and 3/4 bond.



We were sent oversize handmade brick from the bricklayer to build the jack arches. The length of the handmade brick is 8". When we took the 8" brick and stood it up as a stretcher, then leaned it 70 degrees, we figured the max height of our flemish bond stretcher in the jack arch to be 7". The old YMCA jack arch was probably made from brick longer than 8" because the 3/4 bond looks closer to a 1/2 bond. Since handmade is a long as these brick get nowadays without ordering special slugs, we proceed to match the bond as best we can. If the jack arch is 12 1/2" tall with 1/4" joint it leaves us with a 5 1/4" bat. We then designed the jack arch the same way we always do here at Archway System, with a georgian style, double struck consistent joint.

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.