REINFORCING NEEDED FOR SMALL CONCRETE VAULTS
Drawing The Big Idea 213d
My goal here is to present a starting point for calculating vaults. A look at what has worked in the past and what engineers want to see specified on the first plans submitted. My goal is NOT to specify any particular roof for your site-- I can't do that.
The Bottom Line
If a concrete roof is under 16 ft diameter and in a vaulted shape (20-30% rise), for the roof membrane, use:
Two layers of #6666WW (6" X 6 gauge, welded wire--driveway mesh) in a 2 1/2" (7cm) pour of 3,500 psi (220 Kg/c2) concrete for the roof membrane.
This should work in vaults up to 18 ft (5.5M) wide and aller vaults in earthquake zones.
This is a great ferro cement membrane and represents the price and strength to beat. Most small vaults can get by with a lot less steel and with weaker concrete but this membrane should be easy to get stamped anywhere in the world.
The Bottom Line
If a concrete roof is under 16 ft diameter and in a vaulted shape (20-30% rise), for the roof membrane, use:
Two layers of #6666WW (6" X 6 gauge, welded wire--driveway mesh) in a 2 1/2" (7cm) pour of 3,500 psi (220 Kg/c2) concrete for the roof membrane.
This should work in vaults up to 18 ft (5.5M) wide and aller vaults in earthquake zones.
This is a great ferro cement membrane and represents the price and strength to beat. Most small vaults can get by with a lot less steel and with weaker concrete but this membrane should be easy to get stamped anywhere in the world.
REINFORCING RULES OF THUMB FOR SMALL CONCRETE VAULTS
based on welded wire mesh-- this is just for the roof membrane
based on welded wire mesh-- this is just for the roof membrane
One layer #66 10 10 WW,+ 3" thick, 1,200 psi light weight concrete
One layer #66WW, + 4", 1,200 psi concrete One layer #66WW, +2 1/2" 3,500 psi Two layers #66WW, + 2 1/2" 3,500 psi |
vaults under 12 ft
12 ft wide BV or 14 ft dome 14 ft BV or 16 ft dome 16 ft BV in seismic area, 20 ft underground BV (no traffic), or 24 ft dome |
Here in Mexico I have built over 40 vaults (12-16 ft wide) using typically 4" of 1,200 psi light weight concrete with one layer of #6666WW. No problems yet and I'm going for the 400 year guarantee.
For larger vaults (16-20 ft) I use one layer of #6666WW in 2 1/2" of 2000 psi for the structural layer and then another 3" of 1,000 psi for the insulation layer.
For edge beams I typically use rectangular beams; 4" X 8" stirrups, 6" on center with 4 each #3 (3/8") rebar-- poured to 6" X10" with 3,500psi.
"A Pattern Language" has calculations for a 16 X 20 ft. vault using 600 psi concrete and basically NO iron reinforcing-- WOW !!, this is impressive because we are talking about California They unfortunately, don't address the problem of transferring lateral loads and the need for shear walls.
For larger vaults (16-20 ft) I use one layer of #6666WW in 2 1/2" of 2000 psi for the structural layer and then another 3" of 1,000 psi for the insulation layer.
For edge beams I typically use rectangular beams; 4" X 8" stirrups, 6" on center with 4 each #3 (3/8") rebar-- poured to 6" X10" with 3,500psi.
"A Pattern Language" has calculations for a 16 X 20 ft. vault using 600 psi concrete and basically NO iron reinforcing-- WOW !!, this is impressive because we are talking about California They unfortunately, don't address the problem of transferring lateral loads and the need for shear walls.
RIBS AND BUTRESSES - Another old idea
Buttresses are as old as stone wall construction. I'd like to use thin Ferro Cement buttresses on the exterior walls. They are like ribs that extend to the ground in a catenary shape. This helps open up the interior space since there is the possibility to minimize interior shear walls. See A and B vaults |
REINFORCING CONSIDERATIONS FOR SMALL BARREL VAULTS
Calculations for large vaults are a lot like rocket science. It is best to go to college for several years and have a good computer program. Small vaults are much easier. Below is some of my thinking. Keep in mind I missed the "several years of college" part, but I HAVE built over 40 vaults that have a 400 year guarantee.
Calculations for large vaults are a lot like rocket science. It is best to go to college for several years and have a good computer program. Small vaults are much easier. Below is some of my thinking. Keep in mind I missed the "several years of college" part, but I HAVE built over 40 vaults that have a 400 year guarantee.
IDEAS FOR BARREL SHELLS
preliminary design http://www.ketchum.org/-milo/design.html see:barrel shells
- A barrel shell acts as a beam in the long direction and as an arch in the curved area. The bending moments in small vaults are typically minimal. The curved area (membrane) is easier to calculate than the side beams.
- Ideal membrane reinforcing is " a pattern of diagonal tension bars". (note: diagonal tension usually equals shear in BV's) For small vaults I recommend one or two layers of welded wire mesh acting as diagonal tension bars. This isn't ideal but close. My membrane have good shear strength.
- Side and edge beams (or buttresses) are required on all BV's. The edge beams for vaults over 6M diameter need to be quite large ( and engineered). Long exterior walls will need larger beams than those on top of walls associated with small, interior vaults. Buttresses and ribs can possibly take the place of edge beams.
- The edge spans of the shell should be supported approximately every 10 ft on center by intermediate, vertical columns.The shell at the edge should be reinforced with two layers of bars.
- For these horizontal "bond beams"at the edge, I typically use rectangular beams; 4" X 8" (10 X15 cm) closed stirrups, 6" on center with 4 each #3 (longitudinal) rebar-- poured to 6" X10" (15 X 25 cm) with 3,500 psi. concrete. These are small beams but should do the job for a vault 4.5 M wide, in a moderate earthquake.
- End beams-- On a 4.5 M span I typically use 2 each, #4 (1/2") re-bar in a 3" X 6" pour---when I can pour a horizontal beam. This almost never happens due to windows and doorways in the end wall. In this case I pour a curved beam above the doorway with the same specs as the edge beams. These are known as end frames or stiffeners and act to help shear wall action in the end walls. Thickening the shell over the end wall and over shear walls is also a good idea.
preliminary design http://www.ketchum.org/-milo/design.html see:barrel shells
SHEAR WALLS
Shear walls are vertical, interior and exterior walls that are vital to the structural integrity of a vaulted building. They act to transfer the lateral loads to the foundation -- so the walls don't fall over with the vaulted roof in one piece. It's important to understand how they work. Please see this link.
Shear walls are vertical, interior and exterior walls that are vital to the structural integrity of a vaulted building. They act to transfer the lateral loads to the foundation -- so the walls don't fall over with the vaulted roof in one piece. It's important to understand how they work. Please see this link.
A WORD ON DOMED ROOFS
Here I am talking mainly about monolithic (one piece, not geodesic) domes that are less than 24 ft in diameter Domes are incredibly strong and stiff. For example "The top of a concrete dome in the shape of a half sphere, 100 feet in diameter and only 3 inches thick, acted upon by a very heavy load of snow and it's own dead load, deflects less than one tenth of an inch. The ratio of span to deflection is twelve thousand and should be compared to an acceptable ratio of between 300 and 800 for a bending structure such as a beam". This same dome has a ratio of span to thickness of 400-- the ratio for an eggshell is 30. Bending Disturbance Near the Bottom Edge One thing to keep in mind with domes is that the lower, bottom portion of the shell is more prone to bending forces and should therefore receive more reinforcing. For a 20 ft dome I'd add another layer (making 3 total) of #66WW to the lower 3 feet of the dome. This dome will also require a stiffener ring (bond beam, edge beam). I'd use the same specs as for the edge beam of a large BV. Thermal conditions (heating and cooling of the dome) can produce displacement that is three times that of a heavy snow load, so in reality you are designing mostly for thermal expansion. |