adding new slab to the existing slab + inverted beam 1

[mats12]

Im dealing with a reconstruction.

English is not my 1st language so I think its best to see attached pictures.

Id like to add a new concrete slab + inverted beam to the existing slab.

Should I approach new slab + inverted beam as one element - "L beam"? But in that case i think I shouldnt make connection to the existing slab.

Im not sure how to model this and if I should connect new slab with existing slab.

 

[Teguci]

If you lock the new slab and beam in with the existing, I am concerned that the existing slab will not be able to take the new negative bending over the existing wall and will need to be reinforced on the top (left side of sketch). I like the idea of transferring shear across the new joint, but eliminating any moment transfer (right side). I show a compressible pad, but shrinkage alone may be enough to allow the hinge to perform with just the construction joint. I'd also detail the slab to beam connection so that the beam is rectangular and has a horizontal neutral axis for bending - allow the slab to rotate at the beam slab interface.

 

[KootK]

I'd make that beam very stiff such that it's anticipated deflection was minimal. Then I'd leave everything connected together and let come what may. It appears that you have access to as much beam depth as you need and a stiff beam probably suits the new bearing wall and the stuff supported by it anyhow.

Other alternatives that I like less.

1) Cantilever the new slab from the existing but don't connect it to the new beam. Let the new slab and the new beam move relative to one another. That may have impacts for finishes and the building envelope however.

2) Cast the new slab a while after the new beam and bearing wall are in place. That should eliminate most of the subsequent movement.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

3) If your new wall is masonry or concrete or anything else that would form a deep beam once is has set, shore the concrete beam until the wall is in place and cured and let the beam mostly hang from the wall with next to no deflection.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[greznik91]

I'd make that beam very stiff such that it's anticipated deflection was minimal.

I dont really understand what do you accomplish with that? Can someone elaborate please?


It appears that you have access to as much beam depth as you need

Maximum height of the beam can be 118 cm. Span is 500 cm.

I was thinkin more about dimensions like 30/50 cm.

 

[KootK]

The main issue with your design appears to be the cracking that would result from relative displacement between the new beam and the existing slab. A stiff beam reduces relative deflection and therefore alleviates cracking problems.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Teguci]

I'm with you greznik, whether the beam is stiff or not, you will still get negative bending over the existing wall. If you have rugs ready, you can quickly place them on the crack that forms and you might not get a call back from the client who paid you to anticipate that kind of problem. At the least, detail a construction joint with a tooled edge so that everyone knows this crack wasn't an engineering mistake.

 

[KootK]

There's a cold joint, and therefore an apparent crack, no matter what. It already exists. So the only design parameter of concern is the size of that crack. And the size of that crack will be linearly related to the deflection, and therefore stiffness, of the beam. So the choices are:

1) Shallow beam and larger crack or;
2) Deeper beam and smaller crack.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.