Restrained Basement Wall Design

[AZengineer]

I know a lot of questions have been posted on this topic, but I still am unable to see a popular consensus:

I have a very long rectangular building with a full basement (10' ceiling). There are no intermediate walls perpendicular to the long direction. The floor trusses run parallel to the long dimension of the building, with steel beams spaced at about 25' o.c.

Since the basement is backfilled on all sides, I would like to design the walls as restrained masonry walls, and the contractor would obviously like to avoid the extra expense of a cantilevered wall. The geo report specifies the at-rest pressure as 63 pcf, which gives a lateral reaction of roughly 1000 plf into the floor sheathing.

I cannot justify the load transfer at an interior sheathing panel joint. I am able to justify the load transfer through the bearing plate/ledger into the sheathing via blocking, but that's where I run into problems. The APA recommends a 1/8" gap between sheathing panels, which means that 1000 plf will have to be transferred across this joint by the sheathing nails/screws. I am also slightly concerned about plywood buckling under axial load, since the tabulated values in the NDS assume all edges are restrained, whereas in my case the floor trusses are spaced at 24" o.c.

I have seen many designs where a 10' basement wall is designed as a restrained wall, and many engineers extend blocking several truss bays to get the load into the sheathing, using the IBC diaphragm shear values - which is not correct - this is not a diaphragm, rather an axially loaded plywood floor. Nevertheless, this design has been employed on many local projects (Phoenix, AZ), and seemingly without problems.

Any suggestions on how to justify this? I would like to avoid putting blocking in all the way across the building, as this is not a realistic solution when shrinkage and workmanship are taken into account.

 

[hokie66]

No joy from me. I would either use a concrete floor or cantilever the walls.

 

[JAE]

Well...Hmmm...not sure that I'd ever use a cantilevered wall. I've seen a case where a large amount of earth backfill deflected a wood diaphragm laterally such that the building experienced quite a bit of cracking, deformation and distress.

But I don't agree fully with the description that the plywood is an axially loaded "member". If you simply run blocking perpendicular to the exterior wall and extend it far enough so that the plywood-to-blocking nailing is adequate for the transfer of the top-of-wall lateral thrust, then you are OK for strength. The blocking can be spaced at close enough intervals to keep the length of the blocking to a reasonable amount.

You can have fewer lines of blocking, but the blocking would have to be longer.

You are then simply trying to create a case where the diaphragm is receiving load similar to the case where the joists are perpendicular to the wall.

You must also check for stiffness of the diaphragm as well.

 

[Galambos]

I believe the IBC has an exception for basement walls supported by timber diaphragms to be designed by a lower at-rest earth pressure. couldnt tell you where it is...but it might be in the soils chapter.

 

[PSlem]

I recall pressures were 20 PSF for sand, 30 for silt and 40 for clay. I think the charts went to 10'walls in the 2006 code. They also had a minimum building depth to length and a large increase in number of anchor bolts. I asked counties about these last two and they ignore.

 

[msquared48]

With the pressure at 63 psf, I see this as full hydrostatic pressure - no drainage I would also think you will have problems with 10 feet of hydrostatic pressure on the basement slab, to include the overall uplift on the total structure.

Personally, I would go with the cantilever wall - it would make all your other concerns with respect to the plywood floor "diaphragm" go bye-bye. However, I would use CIP concrete, not masonry block.

Mike McCann
MMC Engineering

 

[strguy11]

If you design it as a restrained wall, make sure you tell the contractor that he will need shoring, or he will have to wait to backfill the wall until the floor diaphragm is installed. If he cant wait, the cost of the shoring, may outweigh the added cost of designing as a cantilever.

 

[hokie66]

As the OP said, this is not a diaphragm in the sense that he is trying to span from end to end. He is just trying to strut across to the earth on the other side for resistance. He has a VERY long building. He wants to resist at rest pressure on one side with passive pressure on the other, and vice versa.

Mike,

63 psf equivalent hydraulic pressure is what you would expect for at rest pressure with no water involved. If there is water, it would be higher, and then you would have to consider uplift.

AZeng,

If you want to persist with the floor bracing the wall and don't want to use a concrete floor, I would not rely on the plywood for axial loading. I would decrease the beam spacings so they act as combined bending/compression members, with the top of the wall designed to span horizontally between the beams. And there is nothing wrong with doing the walls in reinforced concrete masonry. Remember the waterproofing on the outside.

 

[msquared48]

You're right Hokie. Nevertheless, I would still check where the maximum high water table is to see if there is ever any uplift on the slab.

To me, pressures that high means that the soil could be more porus, lowering the intergranular friction and allowing the water to infiltrate more quickly. I'd check it out.

Mike McCann
MMC Engineering

 

[DaveAtkins]

I think you could justify 1000 plf across the joint in the plywood. It might take a lot of nails. And I wouldn't worry about plywood buckling--the plywood is nailed to every truss, so its unbraced length should only be 16".

If you are still uncomfortable with that, you could design the diaphragm to span 25' between beams, and design the beams for the axial force. You would have two diaphragms in each 25' span, one taking the lateral pressure on one side, and the other taking the lateral pressure on the other side.

DaveAtkins

 

[AZengineer]

This project is in Phoenix, AZ - hydrostatic pressure from water table height is of zero concern. The geotechnical report has stated what to use for the at-rest pressure, and that's what I will use... I'm not looking for a way to reduce it, or second guess the geotech's analysis.


"But I don't agree fully with the description that the plywood is an axially loaded "member". If you simply run blocking perpendicular to the exterior wall and extend it far enough so that the plywood-to-blocking nailing is adequate for the transfer of the top-of-wall lateral thrust, then you are OK for strength. The blocking can be spaced at close enough intervals to keep the length of the blocking to a reasonable amount."


JAE, how is this not an axially loaded member? If you take a section of the floor at the middle of the building, your FBD will have 1000 plf on both sides (see attachment). As I stated, I have already designed the blocking so that the lateral thrust is adequately transfered into the sheathing. But as Hokie stated, this is NOT a diaphragm!

Imagine if this were a tunnel, and not a house, so there were no shear walls. The structure will still work because the two retaining walls are still resisting each other's lateral force via the floor, which is indeed an axially loaded "member". If the plywood was replaced by a series of steel pipes, I think you would agree that these were axially loaded members (i.e. what you would use in a braced excavation). There is no difference here - the plywood is a continuous wall brace. Calling the floor a "diaphragm" in this case, and analyzing it as such is eroneous - there is no shear transfer, just axial load.

In regards to spanning diaphragms between the steel beams - I had considered this, but I don't think it's a good idea. First, you will have a ton of horizontal reinforcing at the top of the wall (and I'm not sure you could even get a horizontal span of 25' in a 12" masonry wall). Second, it will be difficult to design a connection for a masonry wall that will adequately resist 25k lateral load. It might be done, but to me it just looks like a bad idea from the outset. Third, since this is such a long building, it will need control joints in the masonry wall, and I intend to put them between each steel beam. Obviously this will be problematic if I'm using the steel beams as compression struts and spanning the masonry horizontally.

Dave, how would you obtain the capacity for the nails? I am guessing that 10d nails at 2" o.c. will be in the right ballpark.

 

[DaveAtkins]

You can calculate the capacity of a nail using the NDS. They have values in their tables for side member thickness as thin as 1/2".

DaveAtkins

 

[miecz]

I think you need to change something to make this work. If you can't use a concrete floor or cantilevered walls, how about concrete walls? You may be able to span the 25 feet to the cross beams with concrete. If you have to use masonry, could you reduce the spacing of the cross beams? I just don't see the plywood carrying that magnatude of compression. The trusses don't offer enough stiffness to keep the deck from buckling. How wide is this building?

 

[AZengineer]

The building is 36' wide, and about 150' long.

I would think that the bracing force will be far less than the gravity loads that the trusses will be designed for (20D + 40L). I think the plywood would buckle between the trusses before any sort of truss failure occured. Plus, the plywood will be 7/8" thick, 24/48 span rating.

The contractor owns a masonry company, so is pressing hard not to use concrete (I already suggested to use concrete but he was not impressed!) Another option I was think of is using two layers of plywood and staggering the joints. This would decrease the load in each of the two layers of plywood and would reduce the load at the panel joint to a sufficient level. Thoughts?

 

[miecz]

I agree that the trusses are strong enough to provide lateral support to the deck, but I'm not sure they are stiff enough. Unless you can count on partitions above stiffening the deck, I imagine the whole floor buckling as a unit. Someone named Engesser studied stability of compression members with discrete elastic supports. The Engesser formula calculates the required stiffess of intermediate supports to prevent global buckling.

 

[JAE]

AZEngineer - sorry - I was imagining earth on one side of the building only (a walk out basement) and the floor acting as a diaphragm spanning from end-to-end of the building - didn't realize it was 150 feet long.

What hokie said:
"If you want to persist with the floor bracing the wall and don't want to use a concrete floor, I would not rely on the plywood for axial loading.  I would decrease the beam spacings so they act as combined bending/compression members, with the top of the wall designed to span horizontally between the beams.  And there is nothing wrong with doing the walls in reinforced concrete masonry."

is correct in my view. You can try to get a wider bond beam at the top of the wall, heavily reinforced, to span the 25 feet between steel beams, or you can cap the top of the masonry wall off with a concrete beam which could be cast such that its inside face is on plane with the inside face of the masonry wall below, and its outside face extends into the dirt beyond the outside face of the masonry wall below.

Thus you have sort of a concrete beam laid sideways reinforced to span the 25 feet and take all the lateral earth load into the steel beams which can easily be designed to take the axial load to the other side.

You'd satisfy the builder by using masonry walls and yet still use concrete to span.

 

[hokie66]

Two layers of plywood should work. Since the beams are steel, you might also consider providing a steel waler between the beams to take the top wall reaction to the beams and to redistribute the force on the other side to the soil. May prove more economical than another layer of plywood.

 

[AZengineer]

Good ideas with the concrete or steel beam spanning between the steel beams. I'd probably opt for a steel waler in this case as the finished grade is a little lower than top of wall in some areas. However, I still like the double plywood idea. Numbers calc out, and there is a little redundancy with joints occuring at two locations instead of one. Another idea I had was reducing the truss depth by 1.5" and running 2x4 or LVL struts all the way accross at 24" o.c. Not sure if this would be any cheaper than 2 plywood layers...

 

[ronster]

Design the wall as a cantilevered wall with a restrained top (a beam with one end fixed and the other pinned). This most closely represents the actual condition and will help reduce your forces at the top of the wall and also will allow for a much smaller footing with less reinforcement then a similar cantilevered retaining wall.

 

[sundale]

I have used a plywood diaphragm as a compression member ("soldier brace") a few times with a full basement condition with no problems/subpoenas so far. The caveats are:
1. Full basement (e.g. no unbalanced dirt forces)
2. Fully blocked diaphragm
3. Dimension lumber joists that have face mounted hangers on a ledger board
4. Dimension lumber blocking perpendicular to joists spaced 1'o.c. for 1st joist space, 2' o.c. for next joist space, and then the 4' o.c. for the blocked diaphragm thereafter.
5. Added nailing into the joists or blocking rows to justify the 1/3 overall dirt force load path into the sheathing. I visualize this like a drag strut/collector loaded in reverse...

I cannot recall for sure, but I think APA might have some guidance about compression loading; I know they have some good stuff for tension loading. With wood floor trusses spanning the long dimension, however, you are not getting any "help" axially from the trusses like my dimensional lumber scheme which is a more "residential" concept; 36' x 150' is a commercial building structurally if not per the occupancy category.

There are thousands of houses with a full basement performing just fine with the load path being some BS toenails through the joists into the sole plate with BS 1/2" anchor bolts at 4' o.c. There is really no load path parallel to the joists... I have fixed a few such full basement houses with helical pier tiebacks too, but 99.9% of these work in practice, if not numerically. This kind of "reality vs. calculation" issue makes me wonder if what we do as SE's makes any sense, sometimes.

If you are (justifiably) nervous about this load path for large permanent lateral forces through a wood diaphragm (not temporary seismic or wind that may not even occur), given the AZ statute of repose for PE liability = infinity, then I would simply pour a concrete floor with steel beams. This is probably more "value received" to the building owner than a cantilever CMU retaining wall (stiffer stronger quieter floor system with less structural depth). Mechanical and plumbing might be more problematic with such a framing scheme though.

Also, 63 psf is a pretty high "at rest" EFP lateral earth pressure; this would reflect a fairly active clay in my locality. When given high numbers like this, I have sometimes asked the geotechnical engineer for a lower "at rest" value for a sand backfill zone to reduce the lateral earth force on the structure.