Increasing the permissible live load on old reinforced concrete flat slab
Increasing the permissible live load on old reinforced concrete flat slab
(OP)
A colleague has asked me to review his idea for increasing the permissible live load on an old (1950's era) flat plate slab supported on concrete columns with capitals, from the original design live load of 100 psf, to a new design live load of 250 psf for warehouse type loading. The building is 6 stories high and only the first suspended floor needs to have its live load increased in this manner. His proposal is to:
- place a reinforced bonded topping to act compositely with the old flat plate slab;
- add carbon fibre strip reinforcement on the bottom of the old slab (presumably fireproofed);
- add drilled-in grouted vertical reinforcement to increase the shear capacity.
He says he has checked the columns and footings and they work for the heavier loading.
Any comment?
- place a reinforced bonded topping to act compositely with the old flat plate slab;
- add carbon fibre strip reinforcement on the bottom of the old slab (presumably fireproofed);
- add drilled-in grouted vertical reinforcement to increase the shear capacity.
He says he has checked the columns and footings and they work for the heavier loading.
Any comment?






RE: Increasing the permissible live load on old reinforced concrete flat slab
RE: Increasing the permissible live load on old reinforced concrete flat slab
RE: Increasing the permissible live load on old reinforced concrete flat slab
Maybe Seismic, im a bit rusty on how storage loads play into lateral loads, if at all.
RE: Increasing the permissible live load on old reinforced concrete flat slab
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RE: Increasing the permissible live load on old reinforced concrete flat slab
RE: Increasing the permissible live load on old reinforced concrete flat slab
With the drilled in shear reinforcement:
1) I would expect that would only be necessary near columns for punching shear and;
2) Make sure that you're happy with the tie anchorage situation for those drilled in vertical bars. This might be effective: Link
This is probably a minor point but, if the topping will be thick, it might be worth looking at the behavior of the existing slab while it's loaded up with wet topping concrete.
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.
RE: Increasing the permissible live load on old reinforced concrete flat slab
RE: Increasing the permissible live load on old reinforced concrete flat slab
RE: Increasing the permissible live load on old reinforced concrete flat slab
Mike McCann, PE, SE (WA)
RE: Increasing the permissible live load on old reinforced concrete flat slab
Thanks again.
RE: Increasing the permissible live load on old reinforced concrete flat slab
RE: Increasing the permissible live load on old reinforced concrete flat slab
RE: Increasing the permissible live load on old reinforced concrete flat slab
RE: Increasing the permissible live load on old reinforced concrete flat slab
I disagree. In the ultimate state, as the rebar yields, the compression zone will move up into the added topping. If all of the compression block fits within the topping then you've got the same tensile force that you had originally but effective on a larger moment arm. And that means increased moment capacity. If the compression block does not fit within the new topping (unlikely for slabs), then you have a more complicated state of stress in the portion of the compression block that overlaps the original slab which would require a more detailed analysis.
The bigger issue with this method, in my mind, would be predicting deflections given that some strain would be baked in the caked when the topping is applied. This may not be a scenario where deflections are particularly important anyhow, however.
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.
RE: Increasing the permissible live load on old reinforced concrete flat slab
you are correct in the sense that when the steel begins to yield, it will get a higher strength than the yield strength number(stress-strain curve), HOWEVER, we do not design with the ultimate strength values, which is what you would be in turn doing. the theory used, for safety reasons, is generally that when the reinforcing begins to yield, its strength is reduced. With that in mind, all you would be doing is shrinking your compression block above once it begins to yield.
direct response to your quite, you do not have the same tensile strength after yielding, therefore it can not be done that way according to (Canadian)design codes. Maybe it is different in the US, as i believe you guys design off of the ultimate yield strength
RE: Increasing the permissible live load on old reinforced concrete flat slab
Negative moment reinforcement can be increased as required and would be placed in the new topping. In order to achieve good bond, the top of the existing slab would need to be roughened by chipping the surface. Drilling through the columns would be desirable, but the chances of hitting column reinforcement are pretty high so this may not be practical.
BA
RE: Increasing the permissible live load on old reinforced concrete flat slab
Good report here:
http://www.see.ed.ac.uk/FIRESEAT/DFT_PDF/10_FRP.pd...
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Increasing the permissible live load on old reinforced concrete flat slab
RE: Increasing the permissible live load on old reinforced concrete flat slab
You misread my intent. I was not proposing the use of ultimate rebar strengths. I was assuming the use of our standard bilinear stress strain model in which Fy is attained and maintained for some time.
I practice in Canada so am subject to the same engineering dogma as you. Rebar yielding sometimes represents the limit of usable strength by not always. Sometimes, as I believe to be the case in this scenario, rebar yielding simply represents the beginning of the stress redistribution to required to reach a new state of stable equilibrium.
It is commonly accepted that placing a structurally bonded topping over an in place precast plank will increase the capacity of that system. Fundamentally, I fail to see how this situation would be any different.
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.