Hollow Core Slab - Staircase opening

[CovertShear]

Hello everyone.

I am designing an opening in a existing hollow core slab (HCS).
Opening dimensions are 2,0 x 2,0 m (see attachment). There are no drawings for the structure, height of the section approximated 220 mm.

Luckily the owner of the property is open to installing a new steel frame support (i.e. I am not considering using a trimmer).

I have designed a similar opening in "HCS" in the past, but that one was a bit smaller.

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My questions are :

1) Preloading of the new steel frame:

I browsed around this site for similar problems and found partial solutions that suggested preloading the supporting frame.

How would I got about doing that in reality, and should I in this case?

Whats the thinking process behind preloading a support frame?

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2) Tensions in upper surface of HCS:

Due to the existence of a new steel support, especially near the stiff column, tension peaks jump up in the upper surface of HCS.

Is this indeed just a local peak result, or something to consider. Should one reinforce the top surface of the HCS with steel plates?

Method used: I approximated the section of a hollow core panel with a set of orthogonal 2D plates.
The grout in between panels is defined as a plate with zero bending stiffness (Iy=0 / Iz=0)
Defined a spring support in the place of steel frame to align with real life deflection.
Defined external loads.
Ran the analysis.

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I hope my questions are not too exhausting.
Thanks in advance.

 

[KootK]

1) I am sometimes a precast hollow core engineer.

2) Generally speaking, I feel that both FEM and preloading are too fancy for this situation.

3) When one preloads a beam, the goal is usually to encourage the beam to pickup load without the existing structure having to move a bunch in order for that to happen. You're beam deflection is likely to be so small here that this probably wouldn't be feasible. If anything, you'd want to jack up the plank prior to installing the beam and column.

4) The top side tension perpendicular to the planks and the potential cracking is a real thing. There isn't much that can realistically be done about that, however, unless you can recast the topping with some rebar in it which would be a nightmare. If it's any consolation, new builds often have the same issue at plan jobs and the "problem" never seems to amount to much.

5) While your FEM is overkill in my opinion, I'll give you full marks for having pulled it off in a way that respects the one way character of the planks. That's pretty cool.

 

[CovertShear]

@KootK

2) Generally speaking, I feel that both FEM and preloading are too fancy for this situation.

- I had a feeling this would be the case, but it's good to hear it from others, especially from a hollow core engineer.

3) When one preloads a beam, the goal is usually to encourage the beam to pickup load without the existing structure having to move a bunch in order for that to happen. You're beam deflection is likely to be so small here that this probably wouldn't be feasible. If anything, you'd want to jack up the plank prior to installing the beam and column.

- I see. Theoretically in a different case, would that mean jacking up the near wall end of the steel beam, and fixing it to a plate in a state of stress?
Jacking up the plank, hmm I haven't thought of that, interesting.

4) The top side tension perpendicular to the planks and the potential cracking is a real thing. There isn't much that can realistically be done about that, however, unless you can recast the topping with some rebar in it which would be a nightmare. If it's any consolation, new builds often have the same issue at plan jobs and the "problem" never seems to amount to much.

- I was thinking maybe I could bolt through the section few steel plates on the upper surface, but that seems a bit too "barbaric".

5) While your FEM is overkill in my opinion, I'll give you full marks for having pulled it off in a way that respects the one way character of the planks. That's pretty cool.

- Thank you, I appreciate it. I went through few iterations to get there