The concrete slab (?) supporting the swimming pool (≈ 10 x 10m x 1m) of my building has clearly a 'belly':

concrete slab belly

I'm not sure whether this was built this way or evolved over time, however, my best guess is that after 25 years the heavy load of the swimming pool deformed the structure and even the pipes that goes right under it.



  • What is the technical term for this?
  • Is this a problem or not? Why?
  • Any action should be taken in order to prevent an accident?

2 Answers 2

  • What is the technical term for this?

Bending downwards is called sagging. (Technically "sagging" isn't moving downwards; it's bending in a shape that looks like the mouth on a smiley face).

One thing you'll see with concrete structures is creep. This is where under constant load, the deflection increases over time.

  • Is this a problem or not? Why?

Sagging deflection is inevitable. If you put a load on something it will move. Whether this movement is big enough to be visible or not it another question, but it will move.

When designing something we often check for two different failures: ultimate strength and deflection. (Note: There are plenty of other things we check for, this is a simplification.) Ultimate strength check = is it going to fall down. Deflection check = is it going to deflect so much that it scares people.

When assessing (i.e. checking) an existing structure, we generally only do ultimate strength checks; we don't worry so much about whether people are worried or not...

So, your structure has clearly failed the deflection limits, as it's scared you! But that's no indication as to whether or not it has exceeded its ultimate strength limits, so its no indication of whether there's actually a problem or not.

  • Any actions should be done in order to prevent an accident?

Get a structural engineer to assess it. You've said in comments that the structure has been inspected due to this deformation - as long as that inspection has been carried out by a suitably experienced/qualified engineer, then you have no need to worry. I have no idea of how you satisfy yourself of their qualifications in your country.

  • $\begingroup$ Hi @AndyT I came to this SE looking for Civil Engineers who might be able to help us answer this question, on the brand new Materials Stack Exchange: materials.stackexchange.com/q/493/5 $\endgroup$ May 10, 2020 at 16:29

That will fail eventually, concrete supports compression easily but not tension (something Barnes Wallis used to break the dams).

Get this inspected by a professional.

One wonders if the situation has been exacerbated by increasing the level of water in the pool - is the level what the designer originally designed for?

Not only that was it correctly built to specification? Many situations exist where what the designer designed, the builder changed in terms of dimensions, quality (of mix for example) the reinforcing etc

If that is your car, I would move it away...

So, one possibility is to drain it, cover it over and turn it into a dance hall or presentation / conference room.

  • $\begingroup$ > "is the level what the designer originally designed for?" The swimming pool is being used according to the specifications. However, I'm not sure whether is the designed or builder specifications. $\endgroup$
    – Mark Messa
    Oct 6, 2019 at 12:14
  • $\begingroup$ > "was it correctly built to specification?" I'm not sure. To confirm that, most likely it would require comparing the drawings dimensions with the real structure. Considering that the building passed all the inspections, my best guess is that this is not a problem. $\endgroup$
    – Mark Messa
    Oct 6, 2019 at 12:19
  • $\begingroup$ > "quality (of mix for example) the reinforcing" Considering that there are cases which the building collapsed due the builder used beach sand, I would not exclude this possibility. How can I check that? $\endgroup$
    – Mark Messa
    Oct 6, 2019 at 12:23
  • $\begingroup$ > "Get this inspected by a professional." This building passed all its inspections. However, I've never seen a convincing argument explaining why this deformation is not an issue. $\endgroup$
    – Mark Messa
    Oct 6, 2019 at 12:26
  • 1
    $\begingroup$ @MarkMessa The building passed all its inspections 20 years ago during its construction and at final sign off. Get this change inspected NOW to find out what it means. As for your "best guess" what type of paper are you writing that on? $\endgroup$
    – Solar Mike
    Oct 6, 2019 at 12:30

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