30 foot high round concrete (icf) tower with scissor trusses

[scherry]

I am working on a residential structure that is not much like a residential structure. The Owner wants to use ICF for this part of it. I have researched ICF and given the hype and lack of hard info, I will design it as though it were regular concrete, since it is thirty feet tall. The tower is 36 feet in diameter, and there is a cantilevered steel framed balcony at 10-6" above finished floor. It extends around about half of the circumference. I am framing it into the "silo" walls. About one third of the circumference is laterally unsupported to the top of the wall. It is clad in 4" limestone. (I have managed to get that load off of the exterior face mostly). I am using scissor trusses for the roof framing to facilitate the architectural domed ceiling. I have heard the term ring beam. I have no clue where to start. I have generated the loads on the tower and designed the roof framing, and I know that these trusses, if not properly detailed, will transmit a thrust load to the wall. I am concerned about the ultimate quality of the icf wall, and the unsupported height, although I am not in earthquake country or hurricane country. I do not think it will wind up being a slender wall, I am estimating 12" thickness at the base. Any hints, thoughts, design aid suggestions?

 

[dicksewerrat]

Ring beams are ussually standard structural steel forms, w, ells, boxes, that have been bent to some predetermined radius. I have used them to provide support for tunnels under construction.

 

[dik]

I'm not sure if it was the ACI or the PCA that had information on the design of circular silos... Publications were about 20 years or older...

12" seems a tad thick, but you do the numbers and be comfortable.

Done a few ICF walls and one of the concerns I have is that they use a fairly high slump for the mix (LaFarge, up in these environs has an actual 'wall mix' for ICF... other suppliers likely do; it's proprietry and getting info on it is like pulling teeth). I have no idea where shrinkage cracks are going to occur (likely at window and door openings) and can only guess how wide they are and what effect they have on the structure... might be a bigger issue with your silo.

 

[ANDY121]

what is icf....

 

[dik]

Insulated Concrete Form... did my first one 30 years ago with a product called 'Foam Form', but they've become more common in the last 5 years...

 

[dik]

Andy121:

Sorry, I should have been a little more descriptive...

ICF is a manufactured 'foam building block'. The block consists of two face surfaces constructed of an insulating foam that are used to contain concrete. The blocks interlock with the blocks below and those adjacent. Concrete is pumped or poured between the two surfaces. The blocks are integral units with a spacer that is used to support reinforcing and to keep the two faces from being pushed out by the wet concrete. They are stacked like lego blocks to make a wall; all that is required are some temporary vertical members located outside the foam faces to provide stability for the overall wall as it is poured. The foam block walls can be built up 8' or more, reinforcing placed and the void between is filled with concrete producing a reinforced concrete wall that is insulated on each side.

The void between the faces is generally 6" or 8" to provide the equivalent of a 6" or 8" concrete wall. I've encountered walls as thin as 5-1/2" and there may be others.

Concrete used for filling them is usually a fairly high slump concrete and may use some form of superplasticizer to make it more fluid.

There are about 20 different manufacturers (and the list is almost growing daily).