Hollow Core Slab Opening

[OLDSOUL22]

Hello everyone,

I need some help with a project I'm working on, the project is a remodeling of two condo units on top of each other in a 5-story condominium building, the client bought the unit on top of his old unit (2nd and 3rd floors) for the purpose of joining them vertically for a larger living area. The building was built in 2001, and there aren't any structural drawings available. the ground floor is an open parking garage concrete moment frame (flood zone), and the rest of the stories are CMU walls supported. the floors are made of 8" Hollow Core Slabs and 3.5" to 4" Field topping. Reinforcement of HCS is unknown, we tried investigating with a GPR, However, due to the topping thickness it's hard to tell how many strands or the size of the strands. the HCS are 8' in width.

We need to make an opening of approx. 5'x5' in the 3rd floor for the stair in the location shown in the picture, the dilemma is:

1. the building is 65' in width and there is no way to tell if it's one slab that spans the full width, two slabs or 4 slabs. (Assuming this will be needed for capacity verification).
2. with the reinforcement unknown, how would you go about verifying the HCS to support the spiral stair or the ability to make the opening.

My initial thought was to make a full width cut in one slab (8' x 5') next to a CMU wall and support the other end with a hanger to the adjacent slabs, but I'm not sure how I would go about verifying the capacity of the adjacent slabs to carry the slab I'm cutting without knowing the strand pattern, number, size and pretension stress. Also verifying the 2nd floor HCS can carry the spiral stairs main post / column.

the attached pictures show a plan view of the building with the approximate location of the stairs, a section view and the Hanger detail I mentioned.

Any ideas, suggestions or references would be greatly appreciated.

thank you!

MSc in Structural Engineering, PE

 

[jayrod12]

2001 isn't that long ago, the building authority doesn't have the shop drawings from the hollowcore?

 

[OLDSOUL22]

Unfortunately, the building department lost the bulk of their records during a recent Hurrican.


MSc in Structural Engineering, PE

 

[jayrod12]

Ah, so then this isn't in your home state it would seem. I can't imagine Illinois gets hit with hurricanes often.

You can scan from the underside to get a more accurate read on location and number of strands, since they all are located at the bottom.

Based on the plan you show, it appears the planks only span around 17 feet. Where I am, even the lightest 8" hollowcore with the minimum number of strands would be good for almost 90 PSF of superimposed load. So you'd be close if they were in fact the lightest. The next strand configuration up gets you to 120 PSF.

The plan you show seems odd with that wall at about halfway for some of the planks, are you sure that one is load-bearing?

 

[OLDSOUL22]

Things started slowing down for us around here, so we start picking up work in the sunshine state.
I guess the middle wall was added mainly because of a flight of stairs that connects 4th to 5th floor.

Would you assume the load from the HCS with the opening as a point load at the hanger location to the other slabs? I guess since its filed topped it would be more like a line load.
If I can get away with this assumption, it might work without having to dig deeper. For the stairs loading I think I can do some strengthening from the underside since it a parking garage.


MSc in Structural Engineering, PE

 

[jayrod12]

It would be a bit of a point load, but I'd be assuming that the load can reasonably spread into the adjacent planks a bit such that you could smear the point load over a couple of feet of plank width.

In reality, much of the load that you'd be entertaining through the hanger will actually get transferred to the adjacent planks via the grout joints. But how much that can be quantified is a bit of a difficult thing.

 

[GC_Hopi]

Another way to check for drawings is to contact the precasters that are local to the project site and see if they supplied the job. Generally there are not too many precasters and the economics make it likely that the nearest precaster supplied the job.

 

[OLDSOUL22]

I tried already, got only one negative response. the remainder did not dignify my inquiry with a response.


MSc in Structural Engineering, PE

 

[kipfoot]

I think it's save to assume that these are simple spans.

One way to justify the capacity of planks is to compare the new moment diagram with with the moment envelope from the required uniform loading. If the new moment falls within the envelope, then you'll just need to check the shear.

Another tack is to assume the reinforcing for the least published values. If you're okay with minimum reinforcing, then you're probably good. Keep in mind that you'll have the opportunity to verify your assumptions when the plank is cut.

In my opinion, these ideas will generally work for small openings, like up to one cut plank. If you're cutting two planks you may need to consider supplemental steel beams in this area.

 

[OLDSOUL22]

Thanks for the input, there isn't much published load tables for 8' wide planks, but I managed to find one. After checking the minimum strand pattern available, I believe both flexural and shear capacity would still be fine transferring the loads using a hanger to the adjacent planks, assuming the full width of the adjacent planks contribute to resist the loads.

a second thought came to mind and since I only need the opening to be 5' in width, I might as well keep the remaining 3' and fab steel frame made out of channels to stiffen the opening edges. Assuming the lowest strand qty ((8) 1/2" strands in 8' panel), the flexure and shear capacity of the remaining 3' is good enough for the loads, neglecting any transfer to the adjacent planks.

MSc in Structural Engineering, PE

 

[jayrod12]

If you're confident on the remaining 3 feet of plank, then I don't really see what the channels would be doing for you besides providing a clean edge. Realistically, you'd be causing more damage to the existing plank to install the channel, than any stiffening effect it may provide.

 

[OLDSOUL22]

I see your point; but I feel the level of uncertainty warrants a level of strengthening, maybe I'll think of something less destructive.


MSc in Structural Engineering, PE

 

[jayrod12]

Understood, it is a dubious feeling to cut out over half the width of a plank and go "Yeah, that's ok without anything".

Assuming plank joints are grouted, there would be a fair amount of load transfer between planks down the entire length. The thick topping also lends credence to decent load sharing, especially if it were a reinforced topping.

I just feel that the channel you were showing really wasn't contributing structurally to the overall strength, since it was essentially just hanging off the cut plank with no real span to a undamaged area.

 

[OLDSOUL22]

I assumed the angle to the left side of the opening would be supported by the two longitudinal angles to the top & bottom. I guess I'll sharpen the pencil on the numbers to be 100% sure and I'll disregard, and for peace of mind I'll plan to be on site when they are making the opening to verify the strands pattern and qty.


MSc in Structural Engineering, PE

 

[jayrod12]

I see what your intent was now regarding that steel. But that design would then assume that the two C12 channels were fixed to the plate at the wall right? Could you confidently create that fixed connection you were idealizing? That would be where my concern lies. I'd never try to outrigger/cantilever steel off of a masonry wall.

 

[OLDSOUL22]

my plan was for the channels to be anchored to the edges of the HCS and transfer the load to the plank's edges, and the attachment to wall face plate is to be an auxiliary cnnection.


MSc in Structural Engineering, PE

 

[jayrod12]

Ah, I think you'd be better off to chip away a bit of the topping and create more of a typical hanger style detail. At least from an economics standpoint. But I digress. It's also a far more confident load transfer mechanism.