Increasing the permissible live load on old reinforced concrete flat slab

[ajk1]

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?

 

[JStructsteel]

If the numbers work and he can verify the existing slab reinforcing/strength, and economical, go for it. Make sure you check serviceability.

 

[ajk1]

Is it that simple?

 

[JStructsteel]

Check Shear, moment, deflection, column and footing capacity. That simple. Making all the numbers work with a bonded topping, Carbon fiber is the hard part, but if the calcs look good, not sure there is anything else to check.

Maybe Seismic, im a bit rusty on how storage loads play into lateral loads, if at all.

 

[JAE]

I'd sure check the bonding on the two slabs. If there's any loss of bond (bond can be highly variable) then the system won't work as intended.



Check out Eng-Tips Forum's Policies here:
faq731-376

 

[sandman21]

The increase in load could trigger a seismic lateral element review. Change in occupancy?

 

[KootK]

I would question whether it's truly necessary to both increase depth and add the carbon fiber. From a cost perspective, it sure would be nice if you could get away with just one of those.

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.

 

[hokie66]

I would also want to drill some top bars through the columns.

 

[Brad805]

I too would try to complete the repair from the top or the bottom if possible. Are they planning to replace the stair case? If not, adding depth around this area could be a challenge if the arch wants a small ramp.

 

[msquared48]

Seismic in the 1950's was a lot different than now, especially considering concrete buildings. This could trigger a total seismic review of the building, even if it is only at the second story.

Mike McCann, PE, SE (WA)

 

[ajk1]

Thanks everyone for the good comments. I will forward them to the designer. I too worry about the seismic requirements. I will forward Kootk's reference about Hilti as well. A very good reference of which I was not aware.

Thanks again.

 

[sandman21]

Seismic review will depend on what has been done, if it is a change in occupancy which places the building into a higher risk category. If this occurs the building will need to comply with new code structural requirement or show equivalence (good luck). Generally if your load increase stays below 10% and the D/C ratios stay below as well, a review of the building is not needed. An increase greater will require an ASCE 41 review of the building.

 

[ajk1]

To Sandman21 - ok, thanks for the information. This has to meet the Ontario Building Code so I am not sure if the requirements are similar to ASCE or not, but I will point it out to the project manager if he has not already somehow taken the seismic requirements into account.

 

[gagne73]

Kootk, just adding depth to the top wont do it. it will already be governed by yielding steel on the tension side (most likely), so adding in your compression zones, means you need to add to your tension zones as well. it would be fine however in the areas of negative moment

 

[KootK]

Kootk, just adding depth to the top wont do it. it will already be governed by yielding steel on the tension side (most likely), so adding in your compression zones, means you need to add to your tension zones as well.

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.

 

[gagne73]

kootk,

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

 

[BAretired]

A warehouse loading of 250 psf is a substantial increase in live load from 100 psf. The slab must be designed for fully unbalanced load, so the existing positive reinforcement will be working hard to carry that plus the increase in dead load from slab thickening. If the existing reinforcement is known of course, this can easily be checked.

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

 

[IDS]

If you are relying on carbon fibre for increased ultimate load capacity then you need to check the fire requirements. Carbon fibre reinforcement is likely to have effectively zero strength under standard fire design conditions.

Good report here:

http://www.see.ed.ac.uk/FIRESEAT/DFT_PDF/10_FRP.pdf

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[TLHS]

Since it's an industrial occupancy and you seem to have access from the bottom, I'd probably look at cutting spans for the amount of load improvement that you need if the costs work out. Throw a grid of steel beams under there or something. It's less fiddly and easier to verify that it's actually been done adequately. This may not be workable, though, depending on what your top reinforcement looks like.

 

[KootK]

@gagne73:

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.