Reinforcing Concrete Joists - Options Available?

[randymch]

Hi guys

Some advice or suggestions would be much appreciated regarding some building works prior to my kitchen installation...

We live in a typical 1960s ex-council semi-detached house in Leeds. The intention is to completely redevelop the kitchen area. The project is going to be a MAJOR strip-back before we start installation. This will include the knocking down of some internal ("slightly load-bearing") brick walls.

Having stripped the ceiling plasterwork away we can easily see the main joists running across the entire room. They are concrete and measure approx 3 inches wide by 8 inches high - I estimate they are about 12 feet total length.

One of the walls we are planning to knock down runs perpendicular to the direction of the joists and is four feet into the room and therefore four feet away from the ends of the joist where the connect with the outside of the house.

Several neighbours in the street have done this exact same job without major issue but a couple have complained that when the internal wall is taken down some of their concrete joists have cracked a little or sagged slightly (none have ever come down as I presume they are steel rebar strengthened).

My question is this:

Are there any solutions available which would allow me to add extra rigidity/strength to these concrete joists prior to me removing the internal wall beneath them?

I wish to eliminate any possibility of sagging due to the expensive tiled and heated floor in the bathroom above.

Cheers in advance for any help or suggestions.

Regards

Kyle, Leeds

 

[Jackrae]

Sounds like internal wall was load-bearing.
Cracked and sagging concrete has effectively failed and I'd consider it no longer safe since it is obviously just the re-bar holding things together.

 

[randymch]

Hi Jackrae

I would be inclined to agree except that there are some areas of the kitchen where the exact same joists run the full width of the room without any internal walls underneath them.

I suppose theoretically, those neighbours who did suffer issues may have simply gone about the demolition too heavy handed? I would have thought a load of sledge-hammering mihgt cause this type of cracking, especially bearing in mind the age of the joists. I plan to carefully deconstruct the internal wall from top to bottom whilst also shoring-up until 100% happy.

My main query still holds - are there any ways of reinforcing/bracing these concrete joists... I'm thinking along the lines of additional metalwork along their sides?

 

[dann09]

The short wall might be just that milli or two higher than the outer wall, and the joists would have historically rested on the short wall, not the outer wall. Remove the short wall and the joists will drop that milli or two. Only a possibility.

Why not carefully remove say, one brick from under the joist that's "centred" on the short wall.
Then pack up to a few milli below the joist and watch and wait? If the joist sags it will only have a few milli to drop. Anything more and the grout or tile might crack in the bathroom.

How do you know if the joists that cracked in your neighbour's were the short wall joists? Likewise, for the "load of sledge hammering" theory?

Any bracing etc. will take you into Civil Engineering and it's associated expense.
However, others might have cheaper bracing solutions.

 

[Deluks]

Add timber joists in between each concrete one to pick up the load? You don't mention spacings, but an 8x3 timber every 600mm ish will carry it off easily.
You should probably give up the idea of the tiles not cracking if you are going to mess about with the floor they are sitting on.