Conduits in a post tension slab

[scottam99]

Does anyone know if there is a formula for what is the maximum size conduit that can be installed in a Post Tension Slab? I have a 7" slab I am working on. I sent an RFI to the Engineer. But I was wondering if there is a generally accepted formula.

 

[SparkyHC]

Oohh...danger, Recently worked with an airport and their parking garage which also had post tension slabs along with other types of structural concrete......
Big liability here I would not recommend assuming anything or using any rules of thumb here...
Inform the structural engineer on the project of exactly what you propose and let him tell you if and exactly how to do it.

 

[benaround]

Scott,

Are you on the ground floor or above ?

 

[infinity]

Not sure if there's a formula but we've worked in post-tension slab buildings before and core drilled holes. The engineer or architect approved the locations and then each location had to be x-rayed before we could drill. I wouldn't drill anything without someone's approval first.

 

[ohmhead]

Does anyone know if there is a formula for what is the maximum size conduit that can be installed in a Post Tension Slab? I have a 7" slab I am working on. I sent an RFI to the Engineer. But I was wondering if there is a generally accepted formula.

Well there is no electrical formula its set by the structual engineer in your specs for the last 35 years of construction most PT decks you must space 2 inches apart conduits and your largest conduit in a slab is min 2 inches from top 2 inches from bottom covered .
Just a example 1 inch to 1 1/4 inch is the largest conduit but ask you engineer in a RFI.

You must use chairs you can run across but not paralleled to pt cables meaning touching them .

There is a set distance on most jobs by structual engineer and just for forum info you will never see pt cable in a ground floor ever only elevated decks .

You can cross them you can lay on them but stay away from columns where there combined .

If you install sleeves for riser conduits do not install touching pt cables they will pop your sleeve out when stressed but tensioning .

When they tighten up the cables its 5000 lbs of pull so if your on top of pt its bad news use chairs save your conduit .

And never hit one with a core drill ! I have

 

[benaround]

and just for forum info you will never see pt cable in a ground floor ever only elevated decks .

ohmhead,

Maybe you have not seen them on ground floor, but, the last two hotel jobs we have done

the ground floor cables were tightened after the pour.

 

[Smart $]

ohmhead,

Maybe you have not seen them on ground floor, but, the last two hotel jobs we have done

the ground floor cables were tightened after the pour.

The definition of ground floor can make a difference here. Typically, ground floor is where ingress and egress is easiest at grade level... but not always (same as ground floor isn't always first floor ). Using the preceding definition does not preclude the fact there may be sub-ground-floor levels.

 

[ohmhead]

Well ok lets correct my statement its slab on grade maybe its just florida but i have never seen post tension cables used on grade level bottom dirt level .

Only on elevated floor decks and weve seen a few jobs so it maybe a florida code issue if your doing it in arizona thats interesting to me ill ask our structual engineer on site why they can and we cant or dont just for my own interest .

Soil in arizona is the same as florida sandy clay ?

How long did they wait to tigthen cables ?

 

[benaround]

ohmhead,

Yes, slab on grade. Soil in Az. is hard as a rock clay, you need to pick before you shovel.

Last thing, I don't remember for sure, but it was a 'few' days before they tightened the

cables after they poured. The cables are inside a plastic cover.

 

[Smart $]

ohmhead,

Yes, slab on grade. Soil in Az. is hard as a rock clay, you need to pick before you shovel.

Last thing, I don't remember for sure, but it was a 'few' days before they tightened the

cables after they poured. The cables are inside a plastic cover.

I thought that was the whole idea behind post-tensioning.

Inside a plastic cover is unbonded. Some tightening is required beforehand simply to get the cables inside the pour, i.e. not laying on the bottom. the actual pre-stress tightening is done after the pour sets.

Can't imagine why anyone would PoTen a slab on grade with conditions as stated.

 

[ohmhead]

Well most jobs we do after a week or so they tension cables i think 5000lbs or 8000lbs and that depends on thickness of slab i seen two weeks on some .

And i have also seen one come out of deck ripping up concrete deck .

Today we must xray deck before we core drill no one can core drill with out it and i think its around $400.00 a pop.

Then after tensioning the tables are pulled and they use a cutting torch on the ends .

Our PT cables are also inside a hard plastic cover its blue or red in color this is so when one breaks they can push another in place without the plastic sleeve that would be bad news .

As to grounding its not done there is no connection of post tension cables to ground thats what i have seen .

 

[benaround]

Can't imagine why anyone would PoTen a slab on grade with conditions as stated.

I'm surprised, I thought you had quite the imagination for some reason. Rather than explain

-ing it to you I'll hit the basic principal, faster, cheaper. Imagine that.

 

[ohmhead]

Well Benaround i ask our structual engineer at the job he said yes they do it in florida but its rare do to our sink holes and soil content moisture acids but its done in housing mostly not so in commercial also something new i learned today .

You should add meaning less weight to a structure also .

 

[benaround]

It's kind of ironic really, because in 43 years of doing this stuff, these two jobs are the

only time I've ever seen it done on the ground slab !! I'll tell you this, once the underslab

is signed off in one or two days tops, it's being poured. Faaaaaassssssttttttt.

 

[ohmhead]

It's kind of ironic really, because in 43 years of doing this stuff, these two jobs are the

only time I've ever seen it done on the ground slab !! I'll tell you this, once the underslab

is signed off in one or two days tops, it's being poured. Faaaaaassssssttttttt.

Well my biggest problem with PT cables is the design they always run them thur our elelectrical rooms and then bitch at us for running 200 conduits into that room and then ask us to space them apart and try and not get to close to the PT cables.

And i tell them -- ==== ======== that electrical panels are 20 inches wide and were going thur bubba.

 

[scottam99]

Post Tension

Post Tension

Our first floor slab will be 8 to 9" thick. I need to be able to put a 2-1/2" Conduit in it if ppossible . I have sent an RFI to engineer no answer yet. My thoughts are treating it like cover requirements 2" of concrete. (Both above and below). I'll still wait for the engineers blessing on it. Also There is a cheaper way than X-ray. Ground penetrating Radar. Only down side is it can't tell the difference between PT cable and Rebar. But if you have a clear area then you're good to go.

 

[ohmhead]

Our first floor slab will be 8 to 9" thick. I need to be able to put a 2-1/2" Conduit in it if ppossible . I have sent an RFI to engineer no answer yet. My thoughts are treating it like cover requirements 2" of concrete. (Both above and below). I'll still wait for the engineers blessing on it. Also There is a cheaper way than X-ray. Ground penetrating Radar. Only down side is it can't tell the difference between PT cable and Rebar. But if you have a clear area then you're good to go.

Well Scottam i spent some time in vero beach we did your power plant refit years ago nice place to live .
If your on the ground floor is it dirt level if it is no problems your below the slab coming up its not a issue or is this a deck at or above grade at first floor running horitzontal .

I dont think they will let you run 2 1/2 in slab its not just the cover its the amount of displaced area of volume in slab and with that size conduit its a lot taken out of the structure.

 

[zappy]

Well there is no electrical formula its set by the structual engineer in your specs for the last 35 years of construction most PT decks you must space 2 inches apart conduits and your largest conduit in a slab is min 2 inches from top 2 inches from bottom covered .
Just a example 1 inch to 1 1/4 inch is the largest conduit but ask you engineer in a RFI.

You must use chairs you can run across but not paralleled to pt cables meaning touching them .

There is a set distance on most jobs by structual engineer and just for forum info you will never see pt cable in a ground floor ever only elevated decks .

You can cross them you can lay on them but stay away from columns where there combined .

If you install sleeves for riser conduits do not install touching pt cables they will pop your sleeve out when stressed but tensioning .

When they tighten up the cables its 5000 lbs of pull so if your on top of pt its bad news use chairs save your conduit .

And never hit one with a core drill ! I have

What happened when you did hit one?

 

[ohmhead]

What happened when you did hit one?

Well we had a conduit broken off by another contractor you know rolled over a stub up so we had to chip up the floor to repair it .

I put a helper on this task told him to be real careful and watch out for the PT stuff in deck .

This was many years ago before people started xray pictures out in the construction field .

He hit it and its under high tension so its like a rubber band it shot out the side of the building like a steel cable flying out in the air part of the cable length the other half didnt move .

So we had to push a new one in with the rod buster which we were friends with thank god for that and he uses a crimp tool to bond the two cables back together and then repull it again to 5000lbs .
Note sometimes they dont push in easy by hand you use a hydraulic ram but we just pushed it in .


You see we didnt charge him for his temp power in his trailer so he did it free !

 

[Smart $]

...Can't imagine why anyone would PoTen a slab on grade with conditions as stated.

I'm surprised, I thought you had quite the imagination for some reason. Rather than explain

-ing it to you I'll hit the basic principal, faster, cheaper. Imagine that.

OK, I'll take that back if you let me

If not, let me restate it this way: With the conditions stated, I can't imagine anyone, in their right mind, would PoTen a slab on grade.

Faster cheaper than what?