Posted By oldsam on 12 Oct 2011 09:51 PM
I'm in the process of building a icf house. My basement is done. I'm now close to the pour of the main floor and was thinking of perhaps doing the gable ends as well. That would be around 21 feet of concrete at the peek. Should I split the pour into two pours? I would feel a little more comfortable with it. How about bracing. Any recomendations on bracing at that heigth? I don't think the bracing that I plan on renting goes that high. Also one more question. My current wall is 6" thick. Would it be wise to transition to a 4" wall for the gable ends? Is it true it is actually stronger when compared to the 6" wall with the same amount of rebar. Speaking of rebar I have been using number 4 grade 60 spaced every 16 inches with a horizontal every block level. Is that adaqute for the gable end as well? I do appreciate any responces.
Oldsam, My preference is to go all the way up. I prefer the closed attic and if you do the closed attic with a conventional 2x6 gables that will be a thermal weak spot in your envelope. Don't forget that the R-value of your cavity insulation is not the wall R-value due to the conduction of the studs. I recently did a large house where some of these areas were were double stud walls. If you go with wood gables you can get similar thermal performance out of double stud without giving up too much strength.
I regularly pour 21 feet peeks in one pour. If you are not comfortable with that then you should stick with two pours. There's nothing wrong with that as long as you maintain appropriate overlap of rebar into the next pour -- usually 40-48 diameters depending on engineer. If you don't have 16' stiff backs or better you might also want to stick with two pours. The only issue really is two pump trucks and two pour days. You can double up (and stagger) shorter stiff backs but you may not want to try that if you aren't used to it.
There is no truth to the 4" wall being even as strong as a 6" wall with the same rebar. In fact due to clutter in the wall it is difficult to get enourgh larger steel in a 4" wall to make it as strong as a 6" wall with less steel. I would stick with 6" all the way up. That said, even a four inch ICF wall is a few times stronger than a 2x6 wall so you don't really need to worry about strength should you decide to go with it. That said if you go 6" to 4" you almost definately want to break it into two pours because pouring a tall wall into a 4" and then a 6" can be problematic or at least "advanced". Most ICF blocks (either 4" or 6" ) cost the same so your only real consideration is concrete cost.
Your rebar spacing if fine. With that spacing your several times stronger than wood - double stud or not. That said, it may be a good idea to run it by an engineer, particularly if you are in a Hurricane or tornado zone. The engineer might change your verticals to #5 for tornadoes and hurricane resistance.
If you're near Texas I can loan you a few 16' stiff backs. Regards.