So the question has come up over the last few weeks about PT design and bonded rebar in PT slabs.
The current status of the CTE design philosophy is wrapped up in a FE calc done using RAM Concept that typically aims at the minimum prestress levels required by code. Code requires 125psi min in slabs... we generally aim at 150 to 200psi depending on column layouts, significant on-off spans etc...
As you know, the benefits of maintaining the code min prestress is that you get to "write-off" some of the "speed-bumps" that you'd otherwise run into for normal RC design... such as min bonded rebar spacings, distribution of rebar in beams and slabs, requirements for 2-way slabs and so on. So its important to get those benefits when designing PT. However, it is (more-or-less) common knowledge that our PT designs are generally quite conservative. We have all witnessed the fact that we get tendons cut on our projects (routinely)... we make a big fuss about it and yell at the contractor... the truth is that it almost never causes any heartburn... the slab works fine without that tendon.... WHY?!
The fact of the matter is that we have in the past used any combination of bottom rebar mats on our projects: #4 @18"; #4@24; #5@18; #5 @24, and so on... In many cases with PT design, rebar requirements for the most part are minimal in our PT slabs except for unusual conditions and spans that require "a bit more attention". So it is evident that our use of rebar at the bottom mat is generally VERY conservative, leading to a high steel tonnage in our slabs -which is part of the reason that we can weather the "tendon cuts" so well.
If we were at the other end of the spectrum where we max out the P/A at 4-500 psi with minimal bonded, then i think you would notice a snapped cable more...
All this to say - we should come up with a bottom mat configuration that we're confortable with that still gives us good use of materials and an efficient design...
Since the code ACI[18.12.4] will let us go to 8d or 60" for tendon spacing, and [18.9] spells out some minimum requirements for bonded rebar, we should consider relaxing our "strangle-hold" on the #5 @ 24"oc and ooch a little over toward say... #5 @ 36"oc?
Somebody...?
The current status of the CTE design philosophy is wrapped up in a FE calc done using RAM Concept that typically aims at the minimum prestress levels required by code. Code requires 125psi min in slabs... we generally aim at 150 to 200psi depending on column layouts, significant on-off spans etc...
As you know, the benefits of maintaining the code min prestress is that you get to "write-off" some of the "speed-bumps" that you'd otherwise run into for normal RC design... such as min bonded rebar spacings, distribution of rebar in beams and slabs, requirements for 2-way slabs and so on. So its important to get those benefits when designing PT. However, it is (more-or-less) common knowledge that our PT designs are generally quite conservative. We have all witnessed the fact that we get tendons cut on our projects (routinely)... we make a big fuss about it and yell at the contractor... the truth is that it almost never causes any heartburn... the slab works fine without that tendon.... WHY?!
The fact of the matter is that we have in the past used any combination of bottom rebar mats on our projects: #4 @18"; #4@24; #5@18; #5 @24, and so on... In many cases with PT design, rebar requirements for the most part are minimal in our PT slabs except for unusual conditions and spans that require "a bit more attention". So it is evident that our use of rebar at the bottom mat is generally VERY conservative, leading to a high steel tonnage in our slabs -which is part of the reason that we can weather the "tendon cuts" so well.
If we were at the other end of the spectrum where we max out the P/A at 4-500 psi with minimal bonded, then i think you would notice a snapped cable more...
All this to say - we should come up with a bottom mat configuration that we're confortable with that still gives us good use of materials and an efficient design...
Since the code ACI[18.12.4] will let us go to 8d or 60" for tendon spacing, and [18.9] spells out some minimum requirements for bonded rebar, we should consider relaxing our "strangle-hold" on the #5 @ 24"oc and ooch a little over toward say... #5 @ 36"oc?
Somebody...?
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