Supporting Existing Concrete Slab with New Beam

[psychedomination]

Hi there,

I'm working on a residential project where the client would like to remove a wall, which is holding up a reinforced concrete slab and wall above.

See below :

I've carried out the structural analysis and the bending moment is relatively high due to the slab spans and the wall above(160kNm).

I'm wondering what the best approach for this would be. Should I specify a concrete beam or a steel beam? Which would be the easiest to construct? A reinforced concrete beam will need be about 450mm deep, which may be a negative from an aesthetic perspective. I can probably get a shallower steel section to work but the house is CMU and reinforced concrete so the steel beam would need to be concealed regardless.

I assume either option would require a 2" minimum gap for dry pack after?

Just checking to see if someone has a more cost-effective alternative that they would be willing to discuss.

I'd appreciate any advice really.

 

[TLHS]

What's the wall above made of?

 

[KootK]

Much depends on market forces in your region but my money is on the steel beam option with a gap for dry pack grout or some other means of tolerance accommodation. Mobilizing concrete strikes me for a one off beam in a closed space.

I would not be shy about making this a pretty stiff beam. Unless the beam is post tensioned etc, the upper structure will, of course, settle in as much as the beam deflects when the shoring is removed and the beam assumes the existing load. The block wall above will be sensitive to cracking at small magnitudes of beam deflection.

 

[BridgeSmith]

Without access from above, getting the concrete in for a CIP concrete beam would be very challenging, and therefore likely a much more expensive option than a steel or precast concrete beam.

You may or may not need it, but providing temporary shoring for the floor (and the wall above) would seem like a prudent move.

Be sure you have adequate foundation capacity/distribution for the load at the ends of the beam.

 

[psychedomination]

Thanks for the replies so far.

@TLHS It is either hollow CMU block or solid limestone block.

@KootK Very helpful points. Where I'm from, for residential projects the contractors tend to mix concrete on site. Although, I never really trust the quality to be very high, so I tend to use 20MPa concrete as a design starting point. I'll take your point about beam stiffness and make it stiff enough to reduce deflections as much as possible. The only thing with the steel beam is the end connections. I'll likely need to design a padstone. For steel beams on CMU walls - is it best practice to have a baseplate anchored to the padstone with slotted holes to allow for temperature movement (or is this a waste of money if uplift isn't an issue?). Or alternatively, have the steel beam just rest on the padstone without being anchored to it?

@BridgeSmith Fair point: The constructability of the concrete beam might be quite difficult due to it being near to the slab above. I'll definitely specify that temporary shoring of the floor/wall above will be required. However, the temporary works design/responsibility will fall on the contractor.

Yes, the foundations for this project are a bit of a concern for me. The architectural backgrounds show that the foundation for this beam will rest on an existing 12" thick water tank wall. They don't have any drawings for the existing structure, so I am planning on putting a disclaimer in the drawing set stating that no demolition of the existing wall will be allowed until they do investigative work on the existing foundations (demolish the a portion of the grade slab to either reveal an existing strip footing or water tank wall). When they reveal the foundation conditions of the existing wall, I would come to site to review and determine the adequacy (not sure how this lack of as-built information is normally dealt with).

 

[TLHS]

Yeah, deflection control is going to be a big deal. Something like this likely should stay below around L/600 and maybe a hard limit of 1/4" regardless of span. Given that it's existing, you might want to be even more stringent because people are more likely to notice minor signs of distress on something existing than if it were always there.

I might try to insert myself somewhat in the shoring. At a minimum I'd write requirements for deflection and strength of the shoring. It's potentially as tricky a problem as the permanent installation depending on what the restrictions are on the shoring.

 

[Enable]

I'll cast another vote for a stiff steel beam with drypack. With limited access from above the contractor will be unlikely to vibrate the concrete sufficiently well, which would push you to a self-consolidating concrete solution. And based on your remark that contractors site-mix their concrete in your local, that is an absolute no-go for me.