Post-Tensioned Podium Slab

[strguy11]

I am designing a Post-Tensioned Podium slab (concrete slab with 4-story wood construction ontop).

When designing a typical non-podium slab, i typically design the PT system to balance about 50 - 80% of the dead load of the slab's self weight. For a podium structure, how do you handle this with all of the "superimposed DL" i.e DL of the structure above?

Do most of you design for a higher percentage say 125% of balancing and check slab lift off and cracking at top surface? Or are people still only load balancing for the slab DL? I saw a thread that seems to suggest this, but i cant find much more information anywhere.

It doesnt seem correct to wait to stress until some of the building above has been installed. Shoring would need to remain in place longer.

Thoughts??

 

[rapt]

No, for the stage stressing option shoring does not need to be in place longer. You just need to design the slab for the different stages of loading.

The options are to stress higher initially and put extra reinforcement for crack control at the tension face at transfer. This reinforcement is then possibly wasted under the final design condition.

Or do stage stressing and limit the transfer stresses at each stage. It is still possible that some extra reinforcement will be required for transfer depending on how you have organised the stressing stages.

 

[Ingenuity]

We are currently doing investigation work on a podium slab that is designed for 4 levels of residential in light-gauge cold-formed steel framing.

The banded tendons have stage 1 stressing of 80%, and balance of 20% at completion of level 3. P/A is 210 psi under full prestress.

The uniform tendons are 100% stressed with no staging. Interestingly, the P/A for the uniform tendons is 120 psi.

Slab is 12". Top and bottom mat of rebar, but it is 'light'.

Slab cracked significantly over every bay, midspan and adjacent to columns.

If you go the staged stressing route in your design, make sure you specify that ALL tendons, when stressed, be cut and finished immediately upon approval of elongations/stressing report for stage 1 stressing. Do NOT wait for the balance of the stage 2 tendons to be stressed before cutting and finishing the stressing tails.

If water get into the tendons it causes big problems! If you catch it early it is emulsified grease, if you catch it too late and you have corrosion!

 

[KootK]

@Igenuity: was the corrosion the cause of the cracking?

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.

 

[strguy11]

What is your opinion so far on the cause of the cracking?

 

[Ingenuity]

Was the corrosion the cause of the cracking?

No.

The structure is approx 5 years old, but never placed "in-service" - the upper framing was not constructed due to economy downturn a few years back.

The stressing tails were not cut off and therefore water was able to enter the tendons via the stressing-end anchorages given the 5 year exposure.

Fortunately, the water was not caused corrosion to the strand to date, however, we took grease samples and tested them for water content and they exceed the ACI/PTI specifications for PT coating/grease (0.1% by weight). There is evidence of emulsification of the grease - color change with no translucency, some change in texture, and general thinning of the grease.

What is your opinion so far on the cause of the cracking?

As is typical of these type of structures, it is multi-caused. Restraint-to-shrinkage due to perimeter basement-type walls accounts for a significant portion of the cracking in some areas. Other causes are the distribution of mild-steel flexural reinforcement that does not reflect the elastic distribution of bending moments by significant magnitudes. Couple these actions with UNBonded tendons and the cracks are wide and not well distributed - effectively the opposite to what we try an achieve with RC slabs - small crack widths that are well distributed.

The mid-panel cracking leaks like a sieve. 120 psi P/A is way too low a level of prestress for any suspended PT structure unless it has well distributed flexural reinforcement.

The cracking, coupled with the columns being over-poured by 3" in some cases will require more than 20 shear caps to be installed for punching shear!

Oh yeah, and voids at stressing-end anchorages...the contractor used HILTI HY-150 epoxy to 'inject' fill the voids and went ahead and stressed the tendons!

...and big voids to fixed-end anchorages:

...and fractures to castings due to displacement during stressing because of voids:

New owner is not happy!

 

[dcarr82775]

The ones I have done only had 3 stories, so the 4th could be the tipping point. We balanced more than the weight of the slab for sure, but not so much that required added reinforcing for that condition.