Stiffening of existing Slab under Compression Loads

[tristan861]

Hello guys:

I have a situation where I have to stiffen an existing slab under compression load of 370 kN (83 kips). that coming from a steel column above, Please see attached sketches.

Im trying to stiffen the slab by adding steel beam beneath it ( 3m length from wall to wall). My understanding is that the steel beam will prevent the slab from bending and will take the shear forces in case mechanical anchors are installed (composite action)?

So,
# 1 I will check the slab bearing capacity using the equation : (0.85 0.65 F'c A1), (f'c = 20 Mpa)
# 2 No need to check shear/punching shear of the slab since expansion anchors will be planted below and shear forces will be transferred to the steel beam.
# 3 Steel beam underneath to be checked for shear, bending, deflection and also flange bearing.
# 4 Mechanical anchors to be checked for shear coming from steel column.

I want to know if my approach is correct and/or if I miss something here(I'm not familiar with concrete design).

Thank you ,

 

[dold]

I wouldn't bother with trying to make the beam act composite with the slab. I sense a deflection compatibility issue.
At ~10ft span i'd just size a beefy compact beam as fully unbraced. Size some web stiffeners at the point load.
Some people in my office might be inclined to core a hole through the slab and place a steel column/stub through the slab and attach the new column to that, just above the existing slab. Essentially to bypass the slab entirely. Still provide some nominal connection to the slab at the point load for lateral stability.
83kip is a fair load. I'd pay closer attention to the steel beam to conc wall connections. Don't skimp on your anchors.

 

[KootK]

1) In general, I'd say that you've got the jist of it.

2) I don't see your anchors doing anything other that generally holding things together and, if necessary, bracing the beam top flange agains lateral torsional buckling. In this situation, I wouldn't normally attempt to use the anchors to make the beam composite with the slab for flexure unless I was desperate for the extra capacity that comes with that.

3) With respect to detailing keep in mind that the underside of the slab is likely not a flat surface. You may need to consider some kind of shimming scheme to bring the beam and slab into contact for load transfer.

4) My preference for something like this is to try to construct a pocket within the wall for the beam to bear if possible. That said, that's a lot of work and, if the numbers work with a bolt on connection, so be it.

5) Your critical bearing condition may be the interface between the top of the steel beam and the underside of the slab. So check the flange for transvers bending as you mentioned. It might aslo be nice to have a few partial depth stiffeners near the pont of load application.

 

[tristan861]

The idea of making the beam composite in my opinion is not to increase the capacity of steel beam. It is to prevent the punching shear in the existing slab. (Depth of slab is only 2/3feet). I dont know if im getting this right.

As for the steel beam to concrete connection I will try the possibilty of making it with through anchors to eliminate the anchorage checks.

 

[KootK]

Coring through the slab and attaching the steel column directly to the steel beam is an attractive alternative.

When you say that your slab is 2/3feet, is that 8" or 24" - 36"?

While I know that many engineers are fans of through bolting, I am not in most cases. In many instances I don't feel that through bolting eliminates anchorage checks. At best, if just increases the volume of your failure frustum(s).

I still don't see how the anchors eliminate punching shear. I would think that your direct bearing upon the steel beam would accomplish that if designed properly. Can you provide a sketch or something to clarify how you see the mechanics of that working?

 

[tristan861]

Hi kootK
Yes, 8''.

Can you provide a sketch or something to clarify how you see the mechanics of that working?




I would simply consider the anchors as additional shear reinforcement (just like stirrups) that would help the concrete to resist the shear cracks that would develop in the slab.

 

[KootK]

I would simply consider the anchors as additional shear reinforcement (just like stirrups) that would help the concrete to resist the shear cracks that would develop in the slab.

Ahh, ok, I see what you're getting a now. In that case, I feel that your anchors need to pass all the way through the slab as shown below in order to be punching shear reinforcemnt. That said, with proper beam & bearing design, I don't feel that there's actually a legitimate punching shear failure mode in this setup to begin with.

 

[tristan861]

I feel that your anchors need to pass all the way through the slab as shown below in order to be punching shear reinforcement


Thanks for the tip



That said, with proper beam & bearing design, I don't feel that there's actually a legitimate punching shear failure mode in this setup to begin with.



If we assume that the steel beam is the bedrock of a footing. wouldn't be the same case?
I mean we usually check punching shear in footing that bearing on a rigid base.so why do you think it is not critical in this case?

 

[KootK]

With a footing, some of the load is carried outside of the area delineated by the first shear crack pattern. For this, I would assume that all of the load needs to be transferred within that area.

 

[dik]

If the foundations can accommodate the load (If they can't, you might consider adding a new footing centered on the new column.), I'd locate a new steel beam centered on the column and secure the new steel beam to the concrete beams on each end using Hilti HAS E55 (you could use the 105ksi stuff), and Hit-Hy 200 Adhesive. Any gap can be dry-packed and I wouldn't use a light beam section, but something stocky. I wouldn't think about securing the beam to the slab except for bracing the beam, ie. no composite action. Neither punching shear nor bearing should not be an issue.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Tomfh]

I agree with Dik. Forget composite. Put a stocky section under the column, anchored each end to the walls/columns. Grout between the beam and slab.

You can predeflect the beam too, to prevent excess settlement prior to the beam engaging.

 

[tristan861]

I appreciate all your answers. Thank you so much

 

[Brad805]

I am with Dold. Keep it simple. It is rare in renovation cases that the material cost is a significant factor. If the section is too difficult to get into place, then I suppose I would look at lighter section.