Heavy Moment Connections Due to Gravity Loads

[jandlo]

I am working on a 5 over 1 mixed-use building with a composite steel podium. the 5 stories above are CFMF. The bearing walls align with the column lines. The resulting gravity loads are very large and the beams are quite big: up to W40x211. The upper floor diaphragms span laterally to the CMU stair towers, but the podium level has moment frame for the lateral support of only the podium level. So the lateral forces in the moment frames are not large. However, because we can't use flexible moment connections, the moment contribution from the gravity load is huge: Moments of up to 1,500 k-ft (factored LRFD).

Is this a common theme with this type of podium construction? I feel like these moment connections are going to be significantly over-designed. Thoughts?

 

[EZBuilding]

Are your columns able to resist 1,500 kip*ft of moment in addition to it's axial demand?

Removing the moment connection to the column and reviewing the diaphragm to ensure it is able to distribute the loads to the CMU shear walls seems like a preferred design method.

 

[jandlo]

Yes, the columns are heavy, but they work. I am trying to avoid contributing anymore lateral load to the stair towers than what's already there. Plus, it's a long span for that diaphragm so I would be a little concerned about the deflection with the cumulative lateral loads at that level.

 

[KootK]

Some ideas:

1) I live load reduce the bejeezus out of my transfer decks.

2) Flange plated moment connections and only install the top plates once most of the DL is in place.

3) Add another story of steel framing at the second floor. That way, you can do the gravity transfer at the second floor where you have simple shear connections rather than at the main floor where you've got the sway frames.

 

[jandlo]

I appreciate the suggestions KootK.

1. I have already reduced the live loads to the maximum extent.

2. I don't see how that could be done considering the top plate would need to be installed before placing the podium slab.

3. We're too far down the road in design to do this and it wouldn't work with the proforma/budget.

 

[KootK]

2) I was thinking that one might block off the pour over the top flange plate (6' x 2'maybe) and fill that back in at the last possible moment. In your case, you might also need a temporary, 6' framed opening in the CFM at the same location.