two way concrete slab middle strip

[mck26]

I have an existing two way concrete flat slab built in 1950s. Owner wants to bring the structure to current code for new occupancy. Stress levels work for flexure (shear is a different story), however the bottom rebar splice requirements have changed. I understand the column strip bottom steel must be continuous and I plan to add FRP or something to make that happen. Do I need to also make the middle strip bottom steel continuous since the ACI requires a 6" lap (effectively doing nothing). The old code used in original design has, consequently, a 6" gap in those bottom bars.

 

[KootK]

In my opinion, it would be excessively onerous to add, and possibly fireproof, FRP just to satisfy the detailing requirements of the modern integrity steel provisions. I'd make a play for a waiver of those requirements, either from the client or the AHJ, whomever is mandading the upgrade.

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.

 

[rapt]

Mck26,

Are you sure about the capacity in the 2 directions. There was a long period in the USA where they did not design for full load carried in both directions for slat slabs. I am not sure when the practice changed but I thought it was around the 50's.

Though I agree that it is nice for middle strip bottom steel to be continuous as there is not much negative moment there and it could easily go into tension, I doubt that it would be required for Integrity requirements. I would think that only column strip reinforcement especially in the shear head zone would need to be continuous for that.

 

[Ingenuity]

There was a long period in the USA where they did not design for full load carried in both directions for slat slabs. I am not sure when the practice changed but I thought it was around the 50's.

Assuming we are both talking about the same thing, whereby the total static moment for a flat slab/plate panel was less than WL/8, then it was later than the 1950's.

Here is an extract from ACI-318 in 1963:

I think this was permissible until the 1971 edition.

 

[rapt]

Ingenuity,

Yes, that is what I meant, but I thought they joined the real world and recognised the theory of statics a little earlier than that.

There were lots of older flat slabs in the USA that were very under-designed, and I doubt that the steel was elastic enough in those days to allow membrane action to save them!

Looking at the allowed L/D ratios, they must have had deflection problems as well!

 

[Ingenuity]

Some of the very early (like 1905) flat slab systems (C.A.P. Turner, for example), before these slabs were codified, had significant issues:

With resulting deflections:

 

[mck26]

It was originally designed using the working stress method with an fs = 20 ksi. The steel used however has an fy = 40 ksi. Using strength design method in current ACI with Fy = 40, it works for flexure however punching shear fails. Maybe we will waiver to get exemption from certain requirements, but it is a strict client/AHJ.

 

[Ingenuity]

mck26,

Since your flat slab was constructed after the 1950's it is probably okay, but worth checking bond and anchorage, especially if justifying the as-built capacity based upon USD, for an original design by WSD.

From CRSI Engineering Data Report #48 'Evaluation of Reinforcing Bars in Old Reinforced concrete Structures'
:

 

[Ingenuity]

By way of follow up with regards to my post above re ACI-318 1971 and the total static moment. I checked one of my old references, and the 1963 code was the last issue where less than 100% of statics was code permissible.

From: THE DESIGN OF FLAT SLAB STRUCUTRES - AN HISTORICAL SURVEY by K.A. Faulkes: