Reinforcing A Basement Wall With Pilasters?

[psychedomination]

Hi there,

I am a graduate engineer and am working on a basement project. I am dealing with a retaining wall that was recently constructed by a contractor. The wall is a 6” masonry concrete block wall with T12s at 16” centres that was built up to the underside of the upper floor slab. The height of the wall from the top to the bottom of the foundation is about 9’.

I am assuming that the dead load from the upper floor can not be considered to stabilise the wall because when the 6” wall was constructed it did not have a proper bearing interface at the connection to take the dead load.

With this said I analysed the wall as a standard cantilever retaining wall. I do not have much knowledge in masonry design but seeing as concrete blocks were used and the cells were filled with concrete I analysed the wall like a standard concrete retaining wall. I found that the footing was too small for the overturning moment and the vertical steel reinforcement in the wall stem was inadequate.

A fix that I am proposing is to construct pilasters spaced at ~1m (3’) against the 6” wall. These pilasters would take the overturning moment and the bending in the stem and would cause the 6” wall to span horizontally between the pilasters instead of vertically between the floor slabs. As for connecting the pilaster to the existing wall I am thinking of anchoring the pilaster ties into the 6” wall using an epoxy adhesive. The pilaster ties would end up being U shaped bars doweled and epoxied into the 6” wall.

Is a spacing of 1m too small? I am thinking that this can be increased, perhaps to a limit of 6’ depending on the wall capacity (also needs to be small enough to ensure the wall mainly spans between the pilasters). To determine this would I just find out what the distance a 6” concrete slab can span before needing horizontal reinforcement (the 6” wall only has the vertical bars so it has no contribution from horizontal bars)?

Also can it be safe to assume and design these pilasters to act as columns that will take the full overturning moment and bending moment from the vertical direction that the wall was originally meant to take?

Also as shown in the sketch the external wall is resting on some compacted sand (quite hard material, needs a jack hammer or something to break it). I assume I will need to take the vertical load from the external wall on that piece of sandy material as a surcharge acting on the 6” wall also when determining the bending moment in the stem and overturning moment? Please correct me if I am wrong.

I am trying to completely avoid demolishing the existing wall and would prefer to reinforce it some way. The architect is fine with the addition of pilasters.

Any ideas or feedback/advice for this method would be greatly appreciated.

 

[XR250]

I would ditch the masonry pilaster idea and install steel soldier beams that can be bolted to both the upper and lower slabs. 36" sounds like a reasonable spacing. I usually check these things by using the lateral pressure 2/3 of the way down (6 ft down in you case) and check the flexural tension in the wall spanning 3 ft.

 

[oldestguy]

Can you show a diagram with what loads and pressures you used to evaluate the situation? Will all the existing former wall load transfer onto any pilasters? When this was dug out, did the exposed earth under the old wall stand "easily". If so, perhaps the new wall horizontal load can be taken as active pressure below that old wall elevation. An assumption for high internal friction angle there might be OK. Of course with a "belt and suspenders" approach also put all that vertical load on the new work.

 

[psychedomination]

I would ditch the masonry pilaster idea and install steel soldier beams that can be bolted to both the upper and lower slabs. 36" sounds like a reasonable spacing. I usually check these things by using the lateral pressure 2/3 of the way down (6 ft down in you case) and check the flexural tension in the wall spanning 3 ft.

Thanks for the advice XR250, the only issue would be from an aesthetic point of view. I assume that those soldier piles would need to be encased in concrete to look halfway decent. If thats done, what is the difference to using the steel beam as reinforcement opposed to a pilaster with steel rebar doweled into the footing and into the existing wall?

Can you show a diagram with what loads and pressures you used to evaluate the situation?

Hi oldestguy, I couldn’t find the calcs for this specific scenario but found some calc checks I did in the same basement for a similar wall that was 8” instead of the 6” wall. I have attached the loading and pressure for that example as it would be the same for this one.

Will all the existing former wall load transfer onto any pilasters?

Only if the wall deflects inward pushing load onto the pilasters. So I will assume so for design purposes. Or is this too conservative?

When this was dug out, did the exposed earth under the old wall stand "easily".

Yes the sand was very firm and compacted. It was almost rock like. It's not like you can put your hand in and take it out. You would need some form of tool to break it up such as a jack hammer.

If so, perhaps the new wall horizontal load can be taken as active pressure below that old wall elevation. An assumption for high internal friction angle there might be OK.

This is what I am somewhat struggling with at the moment. Do I assume that the newer 6" wall is retaining only the earth underneath the old existing wall or do I assume that it retains the earth all the way to the top? If it only retains the earth underneath the older existing wall there may not be the need for pilasters. Although I would assume that I would also need to consider the surcharge from the old existing wall in my calculations as well?

Of course with a "belt and suspenders" approach also put all that vertical load on the new work.

Just for clarity you mean to put all of the vertical load from the soil onto the pilasters, correct?

 

[psychedomination]

Also for clarity I have attached a photo what the existing wall with the sand underneath looks like in another area of the basement. I should note that the house is about 150 years old, so was made with about 12" thick exterior limestone walls that did not have any steel reinforcement in them.

 

[XR250]

Thanks for the advice XR250, the only issue would be from an aesthetic point of view. I assume that those soldier piles would need to be encased in concrete to look halfway decent. If thats done, what is the difference to using the steel beam as reinforcement opposed to a pilaster with steel rebar doweled into the footing and into the existing wall?

I think they look fine, but it is not my call. Have you asked your client? I agree that encasing them in concrete would not make sense as you might as well make them out of reinforced concrete. Not sure how you would cast them all the way up, however.
From a physics standpoint, what ever you use would be most effective dumping its load out into the upper and lower slabs.
Soldier beams are likely your most cost and structurally effective option here unless steel is not readily used or available.

 

[oldestguy]

A note here on using this site. A common problem with new posters is that they think they know just what or where the answer should be. Giving ALL the possibly related info early on is needed. You did pretty well, but now to hear it is a house with thick stone walls, there is a significant load from those walls and perhaps other parts. That's one reason for providing something of estimated loads on the area you are concerned with. Another piece of info that might help deals with that "sand". Roughly where in the world is this site and what info is there about geology or soil conditions? This may be somewhat weathered sandstone, not a typical soil.

Off hand let's look at that wall loading and where it goes. I would question transferring the wall loading to an interior new foundation via a slab.. I even question that you would want to depend on the new block wall and pilasters as being able to take on that load, but have you checked that the slab or other floor can transfer that? It would require a detailed study of the upper floor slab and how that load gets down to a new foundation. It may be totally OK, My guess would be that the upper floor slab is not under the stone wall and that all that load is on the shallow depth old wall. At this point, I'd also look at underpinning the old wall. It may take demolishing the new 6" wall. A detailed look at that "dense sand" may show you don't need any new wall. What is the angle of fiction for that material? Maybe better, what is the unconfined comprehensive strength?

Another thought. Maybe "bite the bullet" and contact an experience4d structural and geotech engineer. It is somewhat likely that what comes from this WEB room is not your best step now.