Second Floor Concrete Slab Analysis

[CTW]

Attached is the first floor plan for an existing two story building. The second floor is 5.5" thick reinforced concrete. The building is around 40 years old. No drawings exist so we had the reinforcing located in the slab, then certain areas chipped out to reveal the rebar. The rebar is #3 @ 10" east to west and #5 @ 12" north to south. I've been trying to rationalize why someone would design a slab with #3 bars running parallel to the short span and #5 parallel to the long span, but since given up on that. Trying to come up with a justifiable live load capacity is stumping me. I've considered a one way slab in the short span direction and yield line theory, but neither one is giving me anything that I feel confident in. I'm now going back through some old references I have from Westergaard and Park/Gamble.

Does anyone have any suggestions regarding how they would approach this?

 

[SethGuthrie]

At 5'1/2" total thickness it makes a big difference which bars are in the outer layer. Maybe using small bar sizes gives you a better capacity since "d" is a little more? Regardless, I would promote modern methods (e.g. full finite element analysis) and current codes to establish a safe allowable load.

 

[Archie264]

I wonder if the contractor confused the directions of the steel? If it's run the other way it seems the numbers might work in bending, though it doesn't meet minimum slab thickness criteria. I don't know which direction you take it from here, but that's my only guess on how the steel got placed the way it did.

 

[msquared48]

Are there any concrete beams/purlins under the slab that you have not shown, in addition to the CMU bearing walls? And I have to assume that they all are bearing walls.

Could the #5's be in periodic beam strips within the slab to which the #3's span?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[CTW]

I've been wondering too if the contractor confused the steel direction. The #3's are on the bottom with d = 4.3125". There are no other beams or purlins under the slab. All CMU walls are bearing. There are no ribs or anything to indicate periodic beam strips. The underside of the slab is flat with sprayed on insulation. I'm curious if there are cracks on the bottom of the slab, but with the building being 40 years old, the insulation can't be disturbed due to the possibility of asbestos.

I've thought of using FEM to evaluate the slab, but I'm trying to make some sense of the original design first so that I can check the computer output. Someone had to design the slab using hand calcs, so I'm trying to figure out the assumptions used. At least I hope someone designed the slab, although it's not looking that way.

Very good questions. Please keep them coming.

 

[BAretired]

If the insulation is sprayed on asbestos, it should be removed for health reasons. Then you could examine the underside of slab for cracks and perhaps add steel beams in selected locations.


BA

 

[Archie264]

CTW,

When I did a quick check of slab with the #5 bars running the other direction it easily worked for residential loading...but for that pesky minimum slab thickness requirement.

If I had to guess how it came to be oriented the other direction I can all too easily envision a scenario wherein the field personnel took it upon themselves to "correct" the engineer's "obvious mistake" of calling for the large bars to run the short direction and the small bars to run the long direction. That's considering the slab as a whole, of course, not how it actually spans. I would give the field personnel more benefit of the doubt if I didn't have to experience that type of thing on a regular basis...

 

[BAretired]

It seems to me that the slab is spanning N-S in some locations, E-W in others and both ways in one corner. There should be top steel over each bearing wall. If not, significant cracks could be expected on the top surface.

BA

 

[hokie66]

The worst area is the section north of the stair opening, as it is essentially a single span. Perhaps that is why the designer used heavier reinforcement running north-south. Like BA, I hope there is a top mat as well as the bottom bars.

 

[hawkaz]

Did you locate all the reinforcing in the slab, or just a portion of it? I would check if they beefed up reinforcing peridically to create slab beams inside the 5 1/2" depth. If this is what they did, I would expect to see additional rebar at the interior wall corners running e/w, and perhaps another set in the middle.

 

[concretemasonry]

I have seen similar, but larger buildings (apartments, motels, etc.) in CA that had the slab poured floor by floor continuously and the design was based on continuity of the slab (2 way slab and not 1 way strips)over the interior walls, which distributes the load differently as long as there is continuity. - It is more of a "global" design without assumption of the behavior of the assumed strips. It is tough to spot on those type of structures because of the finishing. Some even had the bond beams poured/filled when the cells were grouted and then topped off with concrete instead of grout and poured to unify the concrete/grout portion of the structure.

Could that fit into what you have there?

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[CTW]

Hopefully the attached sketch will clarify the reinforcing directions. We didn't locate all of the reinforcing but located it in four large areas. There was nothing found that indicated beams or a top mat of reinforcing.

concretemasonry-
That seems to fit what I have here.

 

[mtu1972]

I agree with those above that thought the field guys decided that they know better. Another thing to be concerned about - 40 years ago was the time when the transition from 40 to 60 kisi was taking place. I recall numerous projects where #5's and above were 60 ksi while #3's and #4's were 40 ksi. Depended what the rebar fabricator was keeping in stock.

gjc