6" basement wall 2

[Prestressed Guy]

Question. Is it permissible to design a restrained basement wall 6” thick as long as it meets the requirements of rational design per chapter 10? Lateral loads will be the at-rest loads.

ACI 318-08 section 14.5.3.2 limits the thickness of exterior basement (restrained) walls to 7½” thickness when designing by the 14.5 - Empirical Design Method. For a wall designed under 14.4 – Walls designed as compression members, I can find no such limitations.

 

[vandede427]

there's another guy asking about a 4" wall. you two should email each other.

 

[frv]

same guy

 

[BAretired]

Are you talking about cast-in-place walls? If so, how do you ensure the reinforcement remains in the correct location during the pour?

BA

 

[vandede427]

well, then he should already have his email address.

 

[RacingAZ]

He might just be a new designer starting out on his career so let's try to educate instead of mocking.

Hayden, it's typical to see at least 8" thick wall in basements and I normally start my design there. Even if I can make the numbers to calc up to 6" wall, I would not use it for my basement walls. BA mentioned 1 issue with the rebars. A 6" wall would not have a lot of tolerance with regards to the rebar layout + you'll have 3" concrete cover requirement at the retaining side for concrete cast against and permanently exposed to dirt. Then you'll also have the issue of requiring shoring at the top of the wall when the floor joists are not yet in place.

FYI, your calcs show active pressure value that is normally used for unrestrained design. For restrained retaining wall design, active pressure is normally higher, maybe in the 60 psf/ft or more range. Check your soils report.

 

[Prestressed Guy]

Thanks for the reply RacingAZ.

Sorry, not a Noob. I am a registered P.E., S.E. with 18 years in practice. For 7 years I did nothing but precast/prestress design. I just live in the boon-docks now so I don’t have a local sounding board.

I agree that the biggest challenge is rebar placement. In a plant cast situation, there is total control of rebar location which is why they have smaller cover requirements. I have designed miles of 4” above grade wall and 6" retaining wall in a precast plant designed under the PCI handbook. This Standard allows things that cannot be done under ACI. In a standard wood form I also use 8” due to concern for rebar placement.

I am looking at ICF and this gives me back the control of the reinforcement location because of the tie system See attached pdf. I make conformation of the bar location a special inspection item. If the horizontal bars are snapped into the correct holders and the vert is between them the bar WILL stay in the correct location.

I am 95% sure this is not a problem but it never hurts to sound out other engineers.

 

[concretemasonry]

The ICFs and the ties between the exterior foam faces provide the spacing guides and hold rebar in place.

With ICFs, the problem is proper vibration (not over vibrated) and placement of the concrete. This is really critical near corners and around/under openings. Unfortunately, the forms are never seen, so the poor placement can go unknown.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[msquared48]

Depending on what ICF system you are using, they may or may not have load tables for lateral soil loads on basement walls.

I have dealt with these animals a few times over the years and heard many claims. I would stay away from the suppliers that just wave their hands saying their product has been used many times in similar situations when they cannot provide you the documentation to prove it. I've seen that and backed away real fast.

I believe that the code requires 8" walls for two story structures, but there is an exception for TESTED and proven systems.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

There is nothing fundamentally wrong with using a 6" wall thickness. The problem with insulated concrete forms is that inspection is difficult, both before and after the pour.

In one case I recall, the foam face was removed only to find several cold joints in the wall. The contractor was using concrete left over from other pours to fill the wall forms.

Honeycombing or voids in the concrete cannot easily be detected in ICF systems.

BA

 

[Prestressed Guy]

I do a lot of ICF design. There are only a few block manufactures that approve internal vibration and fewer still that recommend it. I have a short list of block manufacturers that I will approve and they must show that their forms can withstand the pressure of internal vibration. I also spec a mix design that has very good flow properties. It is a bit more expensive but is vastly easier to properly place.

I will only use a block that has good test data behind it but do not use any of their load tables. All designs are specific to the actual loads and comply with ACI 318-08 rather than their tables.

Msquared48, you mention an 8” requirement but I am unaware of one. Do you know what Standard that would be in. I have seen local jurisdictions that have a standard basement design that is required unless the wall is designed by a registered Professional Engineer. That is the main thing I am looking for. I know that I can get these walls to work from a øMn > Mu basis. I have not found anywhere a Standard says “THOU SHALT NOT BUILT A BASEMENT WALL LESS THAN x INCHES THICK!!

Can anyone think of a minimum thickness provision. As I said, the only mention on thickness that I can find is Empirical design and section 14.2 trumps that. I have designed sandwich composite panels that were 22’ tall where 60’ roof double tees were bearing on the inner 3½” wythe but that was with prestress so it is a different animal.

 

[Prestressed Guy]

BAretired. I agree with this point which is why I require continuous special inspection during concrete placement. If there is doubt about the consolidation of the wall, you can do non-destructive testing with thermal imaging just like with CMU. It shows every void.

 

[boo1]

Haydenwse what mix design do you use?

 

[Prestressed Guy]

As I stated in my other thread concerning 4” above grade walls the mix I use has a high percentage fly ash, 3/8” pea gravel, super-p and a viscosity modifier. It flows very well with a slump of about 7”. It gets a 28 day strength upwards of 5,000 psi but is a little lower than expected at 7 days after placement due to the fly ash. This is usually not a problem with ICF forms because you do not need to strip them. I require that the bracing remain in-place until they get a 2,500 psi cylinder break, which is usually around day 3-4. I had one project that had large welded connections at the top of the wall that needed to be done two days after placement so we used conventional concrete in the top two feet to get a minimum of 3,000 psi behind the welded embed.

Think of this mix as a thick SCC. Internal vibration with a ¾” pencil vibrator in this mix yields pretty dense concrete.

 

[BAretired]

The attached page from the National Building Code of Canada (NBC) indicates that 140 mm (5.5") is acceptable for unsupported basement heights up to 3.0m (9.84').

I don't think there is anything wrong with your approach.

BA

 

[concretemasonry]

The is a big difference in form suppliers.

Some of the ICFs suppliers use the ICFs as a part of their sales when they are not making meat trays for Walmart, Sams Club or Costco. Others have exhaustive technical support, specifications, installation guides, test reports and design tables to be used as guides, but a good engineer should be able to see where they are conservative for the uniformed. A good design can be done with the information available for specific applications.

One site I have used frequently is "Reward" ICF (no plug intended) and there are several other very professional brands that are also around.

The good systems were quickly controlled by the more progressive suppliers (real building materials dealers or major concrete suppliers) and the followers just tried to copy as close as possible without enfringing on patents, but tried to make them slightly cheaper to buy into the market late.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[vandede427]

I can't find anything that limits a reinforced CIP or ICF wall.

IBC 1909.6.1 limits a structural plain concrete basement wall to 7.5" but that's not your situation.

IRC has a lot of prescriptive stuff on ICF walls. Table R404.4(1) takes a 5.5" ICF basement wall up to 10'-0", but you're not bound by this if you use ACI. IRC R404.4 refers ICF design to ACI if not using their tables.

ACI doesn't seem to limit you, so I think you're good with the 4" as crazy as it might seem.

 

[COEngineeer]

Are you sure you are specifying the correct one? I am pretty sure the 6" one is meant for walls above the grade instead of using 2x6 walls. You hide all your electrical inside the foam after concrete cured. You can do a transition from 8 or 10 thick basement wall to 6" above grade wall.

The one only project I had (3 years ago and I was only in residential market for 1.5 years) all exterior walls made out of ICF. Not only foundation walls. So you probably pick the exterior wall ICF product, not foundation.

Never, but never question engineer's judgment

 

[BAretired]

COEngineer,

Have a look at the attachment in my last post.

BA

 

[Prestressed Guy]

As far as my designs go the ICF is just a form and all walls are calculated per ACI 318. I use the 6” walls below grade until the area if reinforcement starts to get too high and then change to 8”.

I am planning to start using 4” walls for above grade work as long as I have enough wall to keep the in-plane seismic shear forces to less than 1 square root of f`c. After that I will go up to 6” to avoid the need to kick in the special reinforcement requirements.