Concrete ceiling over basement 3

[JAG-E]

EDIT - I want to reply to the individuals that replied to my original post. How is that done??

I am not sure how to research this floor construction. Single Family Home with basement, built about 1954, by local college professor. No plans, no one knows construction of floor. The walls are cinder block. The exposed basement ceiling is a layer of concrete over cinder block, 21"L, 7.5" tall, 6" thick. The the blocks are on side, with holes horizontal to provide passages for in-floor heating pipes. There is no sign of rebar. There are no beams or joists. According to owner when carpets were replaced the floor is a concrete slab viewed from first floor. The rebar in the photo is just loose in the hole of the cinder block, and may have been forgotten during construction. So other portions of concrete may have rebar. The reason I was asked to look at this, the owners, older couple, intends to remove an old cast iron tub for a shower pan for easy access. The shower company plans to wreck the tub, remove piecemeal, to minimize cosmetic damage around the tub and easier removal. But they are afraid they will weaken this concrete floor. There is one access point where the construction is visible and that is under the subject tub. Can anyone shed light on this type of construction? The only thing I can think of would be to build the walls, create a temporary floor at the same height of the top of the walls, place the cinder blocks on the temporary wall, extending over the top of the basement inner walls, run the piping through the blocks, and then pour the cement. When cement is cured, remove temporary floor. If this method has a name it would provide a start. I am not sure how to up loads more than one photo. Maybe I can comment to my own post and add photos. Thanks Joe.

 

[JAG-E]

@Eng16080

[I wouldn't do anything until you understand how the floor actually works]

That is where I am now. Dox type structure, I did not know existed until now, so now I know what to look for.

 

[JAG-E]

@XR250
Recommending another method for tub removal, I think is a given, even if the floor is sounds and done to best practices for this type of contraction.

 

[JAG-E]

@LittleInch
If the construction is a Dox Plank or variant, propping ("scaffold boards", if I understand this correctly) between the intended end supports, will/may cause a negative moment on the plank. The literature warns against this. IF the floor is what we think this is, IF it was done correctly, IF that is true I will then need to consider the moment reversal. But owner is out of town. When they return I will take a second look with a better understanding.

 

[lexpatrie]

If you mean propping it mid span, that could create a negative moment on the top side, sure, but if there's no capacity for that, I would expect it to turn into effectively two shorter simple spans, both with tension on the bottom. If the continuity moment can't develop, an alternate force distribution must arise (or, I suppose, collapse).

Is the rebar in the hole actually loose? I kind of expect that now that we discuss it further, because that's where the bottom face shell, if you would, has been removed. Rebar may have been cut elsewhere along the span for the plumbing. It seems like it's functioning similar to one of those precast headers you sometimes see on older buildings.

Oldcastle Elematic Hollowcore Plank, Technical Data Guide, 2019

If we don't consider that "plank" effective, its hanging via friction and whatever mechanical connection (flooring?) is present. It's against the wall, too, so it may have partial bearing on the wall as well. You could perhaps entertain grouting it, or FRP provided you knew anything about the compressive strength of the system and the cross section. Otherwise it's pretty woolly.

 

[LittleInch]

What I meant was a more general distributed support, hence the planks, not a set of point loads, with the supports basically providing minimal up lift, but providing support from vertical loads / vibrations and providing some level of support to prevent collapse.

So a set of boards maybe 8ft long running in line with the "planks" over most of the width of the room.

But I guess you have to report to the client that you they have a non standard floor and hence you can't do any calculations or give any further guidance without a lot more physical investigation. Even then it may not be possible to determine maximum loading or longevity. So long as there is no visible distress or cracking then it probably has a very low risk of collapse, but the whole floor may in reality be being held up by the concrete slab on top with its rather weedy looking bit of mesh reinforcement?

This post is very interesting - please let us know how you get on or if you can poke a camera down the hole with the re-bar in.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JAG-E]

@LittleInch
I think we are on the same track. More of a deflection limiting/preventing device. This means of communication does have its limitations vs a phone let alone in person conversation. I will let the group know how it turns out.
Thanks.

 

[JAG-E]

@LittleInch
@lexpatrie
@Eng16080
@XR250
@bookowski
The rest of the story:
The floor looks like a Dox Plank but I cannot locate any rebar. How it was install???
There is wire mesh in the concrete topping.

A different contactor than the shower pan installer, who has worked with this customer, will remove the cast iron tub with a grinding blade. Noise and dust abatement discussed and planned for. Object, no shock or vibration loads. The contactor will measure the height of the basement ceiling/bath floor with a laser: before work starts, after tub removed, after shower pan installed, and after a few weeks of use. If any sag results the owner knows the contactor will install supports. Other rooms have has the cinder blocks plastered and painted for cosmetics, and no cracking or movement is visible. This may sound like an overkill interior decorating project but it is a medical issue. The couple are up in years. The husband wears an oxygen tube and needs a cane to get around. So a no step walk in shower is needed, not just a style fad.

Thanks for the help.

Joe