Capacity of Existing Waffle Slab 2

[Structural Matt]

I have a building with a waffle slab roof. The owner wants to add stuff on the roof so I need to find the capacity of the current system. I have the existing drawings (from 1980) that give me all my info, but I'm at a complete loss as to how to find the capacity.

Do I follow Chapter 8.8 of ACI and treat this as a two-way joist system? That seems reasonable, but the only things that section gives me are the minimum width of ribs be 4 in, overall depth of ribs not exceed 3.5 time the minimum width, and clear spacing between ribs not exceed 30 in. Ok, so I do meet those criteria with my waffle slab... now what??

 

[jayrod12]

Aren't these essentially two way slabs with a lower dead load?

 

[Structural Matt]

I guess. But to analyze it as a simple flat slab I would need to convert the existing waffle slab into an equivalently strong flat slab. Any ideas how to do that?

 

[human909]

Sounds like you have everything you need to determine the slabs bearing capacity. If you need a soil report then get that otherwise make an assumption about bearing capacity of the soil.

The other item you might want is the slab capacity between the ribs. for that you need slab thickness. Either investigate this by hand excavation from the side or a pilot drill hole to determine the slab depth.

 

[Structural Matt]

This is an elevated roof slab...

 

[KootK]

I would start by getting your hands on one of the CRSI manuals that has a chapter on waffle slab design. Ideally, the version that was current in 1980. That will serve as an excellent primer with respect to what's important and how the original structure was likely designed. Then compare new and old code stuff as need be. The older manuals are fairly gettable online.

Waffle slabs are great. I rejoice every time an existing structure that I have to look at turns out to be one.

 

[Structural Matt]

Koot to the rescue again. That should help a lot

 

[JLNJ]

I'm not sure how often they were printed, but I have the CRSI 1975 version here at my desk if you need a screenshot or two.

 

[Structural Matt]

Thanks but I did find a book around our office. You all never cease to amaze me