Single foundation in existing floor slab

  • Erstellt am 2022-02-05 12:31:32

daytona

2022-02-05 12:31:32
  • #1
Hello everyone,

according to the existing structural analysis, I would like to implement the following and have a few questions regarding practical execution: The existing 2-layer reinforced Q188 slab (C20/25), 20cm thick, is to be cut at 1.5x05,m and supplemented by a new single-layer reinforced individual footing (C25/30), Q335 (bottom, closely meshed), also 20cm thick. A steel column is to be placed on it.

Quite a lot of existing reinforcement remained standing during the demolition.
-> Would you use this as the so-called connecting reinforcement for the new footing? This is not explicitly mentioned in the structural analysis.
-> Would you "decouple" the new individual footing from the existing slab or cast it directly onto it?

Thank you very much and best regards
 

Jann St

2022-02-10 14:48:50
  • #2
Hello,


It depends on how it was designed. If the new foundation is supposed to transfer its load to the existing slab, i.e., acts monolithically, then you have to connect it.
If it was designed as an isolated footing that does not transfer its loads to the slab, you have to decouple it (e.g., due to differential settlement, etc.).
If you decouple it, you must consequently also separate the reinforcement.



Almost the same answer as above. However, with an addition:

If the slab was designed as monolithic, you must ensure that the forces can be transferred via the reinforcement from the old to the new slab. For this, so-called lap lengths must be observed. In your case, with 1.05 m x 0.50 m length, it depends on the direction whether it is sufficient. The structural engineer must comment on this.

Furthermore, the structural engineer must tell you whether additional connecting reinforcement has to be installed there, e.g., into the old slab (bonded connection).

And to put it briefly:

All these questions must be answered by the structural engineer who designed it.

Best regards, Jann
 

daytona

2022-02-10 19:16:02
  • #3
Hello Jann,

thanks for your answers, even though honestly, I would have wished for more concrete statements ;-).

But just for understanding: What if I can no longer contact the creator or the structural engineer? Isn't it still possible to derive the assumptions or the resulting construction measures (answers to my questions) from the existing structural analysis? For example, from the specified standards, terms, numbers, or even the sketch or the symbolism (standardized?)
Since I am an engineer myself, albeit not a civil engineer, this should be possible in the structural analysis - as an officially recognized document, right?
And if not, what would be the lesser evil, to decouple or not?

Thanks and regards
 

Jann St

2022-02-14 10:55:54
  • #4
Hi,

if you can no longer contact him, you could ask another structural engineer regarding the statics. He can derive the answer from the existing calculation.
I understand that you hoped for a concrete answer, but it’s not that simple here.

Which option is better is difficult to say.

Briefly, the reason why I refrain from making any statement:
Assuming the support is calculated in such a way that it transfers the loads to the adjacent base plate and thus the shear force is transferred to the plate. Additionally, this transfer increases the surface load of the load on the base, and a lower soil pressure would be sufficient.
Now, however, you decouple the foundation from the plate –
Consequence: the shear force is not transferred to the plate; as a result, the failure mode "punching" can occur and the support pushes through the foundation with the load above it moving downward.
Another failure mode could be excessive soil pressure, causing the support along with the foundation to be pressed further into the ground and resulting in settlements.

All this can happen but does not have to. However, since no one wants to take responsibility for this case, no one can just give you a statement "just like that."

Therefore, my advice for your case:

Take the documents and find a structural engineer whom you officially commission and also pay for it.
 

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