Demo and support partial concrete rib with steel beam 1

[SleeplessEngineer]

Hello
I'm very new to any renovation works and very confused about, probably, a simple thing. So, I'd appreciate proper guidance.

We are adding one floor (wood frame) on top of concrete roof( rib system). The building was originally 1 story structure. We need a opening for stair that will cut through 3 concrete ribs. The client doesn't want any concrete works. So we need to support these by steel beams. The span is only 24' and so designing steel beams is not an issue. But I'm not sure how to transfer the load from ribs to the steel beam. I don't think we can just let the rib beam bear on the steel beam, that would reduce the height clearance (which seems to be pretty low already for this old building). Is there any way to connect steel beams at the same level using some anchors to the rib or something else??

The structure appears to be at least 50 years old (architect actually attached a photo of the building dated 1937!) and since it was also originally intended for roof but we intend to use it as floor, need to be very careful.

Thanks for reading this far!

 

[phamENG]

Any way you can adjust the size of the cut for the stair so you preserve a rib on either side? That would probably help. Then design a steel header between the ribs to support the cut rib. Otherwise, I'd use a rectangular HSS with a continuous plate or angle with gussets as needed. It would likely require a column or two to support the frame.

 

[SleeplessEngineer]

Thanks phamENG for your comment!

Any way you can adjust the size of the cut for the stair so you preserve a rib on either side?

Unfortunately, the clear spacing b/w ribs are only 2.5' and the architect wants much bigger opening.
My bad, I didn't give enough info to picture the situation. Pls see the below plan.

The opening size is approx, 10'x8'. the rib span 22'.I've sketched my original plan (adding a steel beam across the ribs and catching that using two steel beams running from wall to wall. ( I need to check if wall can bear that load later, but I think it can considering it's small area)
Then I realized that the ribs (14" below the ceiling, exposed) already too low, and I can't just run a beam underneath the ribs. I'm sure this situation would have been encountered before several times, but I could not find a proper detail anywhere.

Then design a steel header between the ribs to support the cut rib.

Unfortunately that would be too low - height clearance issue.

Otherwise, I'd use a rectangular HSS with a continuous plate or angle with gussets as needed. It would likely require a column or two to support the frame.

This is promising. I will work on this and see if it can work. Thank you so much!

 

[phamENG]

By between the ribs I meant within the height of the ribs, so you wouldn't lose any floor to ceiling clearance. You'll be doing something similar with your sketch, but it'll be connecting to the two steel beams running across the room rather than the existing ribs.

Good luck.