basement wall failure

[PSUengineer1]

I have a nominal 10-inch CMU basement wall that is tipped inward as a result of inadequate lateral support provided at the top of the wall.

The basement wall was built just before IRC was written (1999).

IRC provides a guide for good construction practices regarding the attachment of the wood sill plate to the top of the basement wall. I was unable to safely get my eyes on this connection for the basement wall is moved inward by approx. 4 inches.

DOES IRC PROVIDE RECOMMENDATIONS FOR BLOCKING between top of wall and first interior joist WHERE JOISTS RUN PARALLEL TO BASEMENT WALL?

thank you.

 

[JAE]

Yes

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[PSUengineer1]

@JAE: what section of IRC for blocking? Section R430.1.6 of 2000 IRC is for anchor bolt requirements. Please let me know what section for blocking requirements. Thanks.

 

[NorthCivil]

I don't know what the IRC recommendations are, but my standard spec is for blocking @ 24" oc for the first 3 joist cavities.

Of course, if you are going to spec anything, what you should do is first calculate the strength requirements from first principles. Calculate the line load at the top of the foundation wall, determine what sort of spec you will need for diaghram strength.... Etc etc.

 

[XR250]

The IRC is very deficient in my opinion on bracing at the top of basement walls. If you have over 6 ft. of backfill, you need lots more nails and anchor bolts. Also, if the joists are parallel, check the uplift at the end of the in-board block. Say your you have a 2x10 floor system and a reaction of 400 plf at the top of the wall. The uplift at the third joist bay (if you do as NorthCivil does) is 400 x 9.25/48 = 77 plf. That could be enough to put a nice hump in the floor (I have never seen it happen and I do the same thing he does). Lots of things about basement don't compute, but they always seem to perform better than they should. I think the equiv. hydrostatic pressure in practice is much less than the design pressure.

 

[NorthCivil]

"I think the equiv. hydrostatic pressure in practice is much less than the design pressure."

Most definitely. But with all things Geo related, we have to be conservative. Photos of foundations backfilled before the floor plate was built do show that these forces are real however.

I always wondered how a rim board, sitting on top of a still plate, fixed only with toe nails, manages to restrain the lateral thrust on the foundation wall. That connection is way too fragile IMO but always seems to work.

My preferred detail is to bolt the rim board to the side of the concrete wall, with the sheathing running over the top of the concrete wall and floor framing. Some contractors like to let the rimboard into the wall during concrete forming, others bolt on afterwards. Either is fine with me.

 

[HouseBoy]

NEARLY ALL of the basement wall failures I have seen that are the result of unbalances soil pressure and not some other mechanism, occur where the wall is long and straight.
When I analyze a wall supported horizontally on the bottom and two sides with the top unsupported, anything over about 16 to 18 feet long is vulnerable and most real bad looking walls are in the 30 ft and longer range. This leads me to expect that horizontal spanning is often at work since many portions of the foundation wall are shorter lengths (measured horizontally). Vertical spanning seems to be at work for the long portions and they require more attention.

I agree that the sill plate anchorage is a weak link. I often spec Simpson FWANZ along with 32" AB spacing.

NC - I'd like to see what that detail looks like and how often you see that in practice?
Does that require that the concrete placement height is greater than it would be if the sill and joists are bearing on top of the wall? Just curious.

 

[NorthCivil]

House boy

Here is the detail - it's my standard and contractors rarely complain or have a problem. Maybe it's normal in my area?

I never thought about the excess concrete required- I will give some thought to using that Simpson clip with a lower wall.

Though, if you have a long run of wall, over 18 feet like you mention, it might be easier on the contractor to use my detail rather than forming buttresses - depending on what is going on in your floor framing of course.

 

[HouseBoy]

NC,
Very nice detail. That's what I THOUGHT you were writing but .... around here (Greater Cincinnati area) everyone seems to get all worked up over the cost increase associated with that extra foot (approx.) of wall forms that will be needed.
Do you usually have 9 ft or 10 ft ailing heights (or greater?)
9 ft form height is common for a custom house but it's a up-charge over 8 ft forms.
Sometimes we see 10 ft form heights but for whatever reason, the cost increase is a big turn-off.
Thanks for sharing.

 

[Tmoose]

Hi NC,

What is the detail of the Microlams end bearing? On sill sealer or pressure treated at a lower elevation?
I didn't think regular Microlams are to be placed against or on concrete.

https://www.techsupport.weyerhaeuse...697034-Can-Microllam-LVL-be-pressure-treated-

regards,

Dan T

 

[NorthCivil]

Houseboy - I work mainly on custom homes, 9ft finished height for the basement is typical. I haven't gotten much pushback for my detail yet, will have to ask the contractors about pricing. Most of the forms I see are reusable plastic forms that I understand the contractors typically rent.

TMoose - the LVL ledger is either set into the forms (as shown in the detail i posted) or bolted on afterwards. Either way, no wood, engineered or not, is to be in direct contact with concrete. Something must separate. When contractors set in the ledger as in my detail, typically they will wrap the LVL with blueskin on the portions where it's in contact with the concrete.