CMU bearing wall removal

[structSU10]

I have a condition where there are existing long span bar joists (span ~50') roof bearing on a CFMF wall that was built no top of an HCP on block bearing wall building. The CFMF wall bears on what was the old parapet of the block building. The section below shows the condition, although this architectural section shows the plank the wrong way - it bears on the wall in question (span ~22').

The renovation for this building calls for the CFMF to remain and the block wall to be demolished. I think I need to keep at least the bond beam below the plank, as the parapet has reinforcing that extends down into the bond beam, but I don't know that I feel great with 8" of block left below the plank to bridge between needle beams prior to the permanent support installation. What are others thoughts on this? Do I need more block to remain? Architect wants to keep as much head room as possible of course.

Roof dead load ~20 psf, if they have to do this in winter, an additional 35 psf snow. plank loads are ~100 psf dead load, plus construction live loads.

 

[KootK]

With just the bond beam, you've got four courses of masonry to distribute the loads coming in from the upper story. And both the bond beam and, to a degree, the precast will act as ties and improve matters further. So, for that component of load, a reasonable 4-5' needle beam spacing seems feasible. The load coming in from the precast, however, might need some additional study. If you'll consider it carried by only the masonry below, then the bond beam alone needs to distribute the precast weight to the needle beams. If, as I'd expect, there is continuous vertical reinforcing from the base of the block walls to the top of the parapet, then you may be able to drag the hollow core load back up into the parapet and again claim to have three or four courses of block helping your cause.

Any chance it would be feasible to just shore the precast a little inboard of the block wall? The parapet remnant may need a little extra stabilizing under that arrangement. I'll get into that below.

Even in the completed state, I believe that there may be a stability problem here. I imagine the remnants of the block wall (parapet + a course or two) wanting to rotate about the lateral connection to the precast plank. Perhaps you can do something above the plank and on the interior side to address this by creating a more convincing moment connection between the plank and parapet. I really kinda wish that the original renovation removed the block parapet and took the cold formed walls down to the precast plank.

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.

 

[structSU10]

for what its worth, the existing parapet detail is this:

Not that I feel great about it, but the dowels between the plank and the parapet may help for something. Although I want to make some kind of positive attachment of the beam to the block for this.

I wasn't feeling good about just shoring on one side of this wall, making the plank cantilever to support the construction above. If that seems feasible thought, it might make things a little easier.

 

[hokie66]

What is the permanent support? Steel moment frame?

 

[structSU10]

Yes, a steel frame.