I've looked at some of the prior posts on this topic but am still interested in opinions / experiences. I am building an ICF single family residence in western Washington state (very active seismic zone). The engineer I'm using, who is well versed in concrete construction, is recommending 6" vs. the 8" cores I had as part of the original design. Part of the reason for the thicker cores is to achieve the aesthetic of thicker walls. Engineer says just adhere an additional 2" of EPS to the exterior to get a thicker look. Building is roughly 35' tall with 10' below grade. Thanks in advances for your thoughts / experiences.

I would recommend the 8" core for the 10' below grade portion of the walls. That much back-fill is a lot of pressure on the wall and an 8" concrete wall with the appropriate amount of rebar is stronger than a 6" wall in this situation. It would take much more rebar in the 6" wall to be able to adequately support 10' of back-fill. With the extra rebar in the 6" core makes consolidation of the concrete more difficult. 6" should be fine for the above grade portion of the walls.

Don't use the 8" unless engineering dictates it. Walls with 6" cores are plenty thick in appearance at about 12" total. If you really want them to be 2" thicker, adding the rigid foam on the outside is the place to put it and contractors are learning how to add rigid foam around here; at least if it's only 2".

Consolidation with those parameters is all about the mix. Make sure your ICF contractor knows what kind of mud he is ordering and why.

I would use 8" for all the walls. By the PM for below grade the rebar schedule for a 8" wall would mean less rebar and an easier pour. If you continued above ground with 8" it makes it easier to continue the build but I would use Reward blocks or Fox Blocks and use the inserts to increase the r-value and to bring the core to 6" . This still gives access to the webs for siding attachment

California state is suggesting 4" walls. Thickness of the walls are not important. Thick wall means heavy structure. That means more rebar. You need afordable strong enough structure Not the 8" or 10" walls. The engineer will design it.

I'm in Southern California and my ranch house construction project is 1 mile away from the San Andreas fault. I used 4" core walls. I was concerned because there was alot of talk on this forum about using thicker walls in seismic zones with no data whatsoever to back it up. The guy who helped me put up my walls told me that he is bidding on a State of California project and there were studies made that concluded that thinner is better for earthquakes. He said there was a mandate to use 4" walls for all ICF construction done for government (california state) projects. Otherwise he wouldn't use them because he has to ship 4" core blocks in from some state 1000 miles away. I'm not sure if this mandate applies to multi-story structures. Also I have to caution you that I haven't seen the report. Until I do it's hearsay. I have been told my my ICF consultant that there is a report. He was adamant about its existence.   He told me the report was a result of actual stress tests done by some engineering firm.  

4" core ICF below grade with 10' of unbalanced backfill against it? Good luck with that.

4" of concrete is considered bare minimum for structural concrete.

I just don't see the cost savings in going from 6" to 4" concrete cores. Getting consolidation in a 4" core with rebar will be very difficult, not much room in there.

If the theory that less concrete is better in seismic because it's less mass, someone might want to tell that to the engineers who are designing concrete structures with 6" or greater wall and floor cavities. Besides if a 4" concrete wall falls on you, it's bad news whether it's 4", 6" or 8" of concrete.

That is not to say that a 4" core cannot be made strong to resist seismic forces but from everything I read, 4" is the absolute minimum and that is considering 100% consolidation.

I'm finding it very hard to believe that California has "mandated" 4" ICF walls in earthquake zones. I'm pretty sure that goes against IRC which I think allows a minimum of 5-1/2" walls in the higher seismic design categories. And, I followed through the calculations on my recent build and would be surprised if 4" flew for higher seismic. In seismic, it's all about the steel and the wall to footing ties, etc. Very difficult to get that right with 4". There might be a chance if you are talking about roofs and floor membranes, too, i.e. making concrete boxes, but not much otherwise. And, I agree with arkie6 about the 10' of backfill

Until someone produces this "report", talking about it is as silly as building an ICF home without engineering.

All the engineers that I spoke to stated that a 6" concrete core with a moderate to heavy rebar schedule is required for my Seismic Design Category "C". Rebar is key and getting rebar into a 4" cavity is tough and then trying to get concrete to consolidate into that 4" area with heavy rebar is even harder. Then trying to run a vibrator down there during the pour/placement is next to impossible.

If your engineer gives strange shapes to the 4" walls they will be more strong then straight 8" walls. I experinced an earthquake. Think that your house is a stick. An earthquake shakes your house like you are shaking a stick in your hand. The stick will be broken if it is made of a metarial like chipborad(osb). By the way concrete worst than osb. Rebar makes it usable.

I tought too much about the californias 4" walls. I found one point. It is not the being strong. It is using less resources. What happens If all the people starts to build 10" or 12" walls?

I am not saying that use 4" walls. I am saying that find an engineer.

I am writing too much but do you know what i did?

My basement walls are 11'. I used 6" walls with 18" oc 10m horizontal and 8" oc verticall 15m ant it is not in seismic zone.

Sorry.