Transfer column on a thickned slab 2

[slickdeals]

Folks,
One of my colleagues is designing a building where a column is offset by 24" from the column below (architects !!!!). This is an edge condition. They don't want a transfer beam and will only allow a thickened slab beam (20" thick).
Column below is a rectangular column (24x48) and column above is circular 18" diameter.

I was suggesting the use of an embedded steel beam to transfer the shear at the interface. The axial load from the column above is of the magnitude of 700-750K (ultimate).

Has anyone had a similar situation to deal with and if so, what details did you use?

 

[BAretired]

I have changed the sketch a bit. The top bars have been extended to develop in bond. The bottom bars have been removed from the sketch as they do not play a role in the strut and tie model

With the assumed strut dimensions, it seems to me that the structure is quite doable.

BA

 

[csd72]

Hokie66,

Trusses dont fail in shear they fail in axial load.

at a macro level you may think of it as shear but at a detailing level it is axial compression strut.

 

[paddingtongreen]

Are the adjacent columns concentric?

I wonder if all of the moment has to be taken at this junction or if the adjacent columns could be designed to prop/tie the top of the 18" dia. column in place.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[csd72]

paddingtongreen,

If you treat it as a beam there will be a small uplift on the next column. The column load is vertical so there is no need for a horizontal component (except locally with the strut and tie)

 

[hokie66]

csd72,

Beam shear, better described as diagonal tension, is always dealt with by a combination of compression struts and tension ties which can be thought of as analogous to a truss, where the tension is taken by stirrups.

others,

The strut under the 18" column is confined by a 60" wide beam. It's not going to fail by splitting or crushing.

BA,

There is not a unique strut-tie model for this, but I disagree that the top corner is critical. I would just use the column to column strut, and provide bottom ties to resist the horizonal component. Anchorage of bottom bars at the lower right corner is the most important issue, IMHO.

 

[BAretired]

hokie,

That is the way I first looked at it too, hence why I initially showed bottom bars anchored at the right end of the beam. But the problem is the 'L' shaped bars from top into the column have to be anchored at the bend point and this means we need a second compression strut to handle the anchor force.

I should say that strut and tie model is not something I have used extensively in my practice, so I may be missing something.

BA

 

[Tomfh]

Beam shear, better described as diagonal tension, is always dealt with by a combination of compression struts and tension ties which can be thought of as analogous to a truss, where the tension is taken by stirrups.

Well said.

The strut under the 18" column is confined by a 60" wide beam. It's not going to fail by splitting or crushing.

I think you should still check strut stresses and check they are below code requirements.

 

[paddingtongreen]

I'm sorry but I still can't see how this joint can be separated from the adjacent columns and joints, i.e. from the frame as a whole. An eccentric moment is introduced, even if the slab beam set up is strong enough it will not be rigid, it will rotate and the upper column will try to sway from the vertical, but the upper beams, supported by the other columns, will push back, relieving some of the moment at the joint.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

paddingtongreen,

I agree with you, but to consider the eccentric column acting on the column without benefit of the beam is conservative. The beam will carry some of the eccentric moment, but we don't know its span, so cannot determine relative stiffness of the beam vs the column.

Even in the absence of the beam, the strut and tie model can be made to work.

BA

 

[rapt]

BAretired,

I was not disagreeing with your model, only pointing out that there is a 3rd dimension to it, in relation to my disagreement with Hokie66.

The struts have a width and this is dependant on the applied load width and the support width. My commnet was that the width of the compression strut from the column above will be controlled by the width of the column above and is not related to the width of the band beam. This means that compression stress in the strut cannot be based on the 60" width, tather it is based on the width of the applied load above and that this will control the strut tie design.

 

[hokie66]

I agree, except that the stress field can spread as it goes down to the wider column. Therefore, the width of the bottom horizontal tie need not be restricted to the width of the upper column. This is similar to strut-tie design of a pile cap, where rarely is strut confinement reinforcement necessary, and where anchorage of the bottom bars is the most important consideration.

Remember, though, that we have only talked about transferring the column load. The band has other loading, but we have not been advised how much.

 

[BAretired]

In my last sketch, I considered the struts to fan out to 8" x 48" which resulted in a maximum factored compressive stress of 2.58 ksi for a factored load of 700k.

As hokie stated in an earlier post, the beam reaction should be included. And as paddingtongreen suggested, the fixed end moment for the beam, including the upper column load should be used in the frame analysis.

BA

 

[hokie66]

I might design this with the beam support both fixed and pinned, as it doesn't hurt to be conservative with a situation like this.