Loading on PT slab before grouting 2

[struggle66]

Hi all,

Main contractor want to operate a mobile crane on our Post-tensioned slab before grouting.

(1) I'm gonna check the slab as unbonded with lower concrete grade to match the actual strength later.
(2) What about the min As? The strands were counted as min As in the first place. Anyway the software will calculate min As for me .
(3) I will check punching shear. Will ask them to provide steel plates.

In here structural consultants or QP, they want the grout compression strength to be the same as the slab/beams concrete grade. But in the code CP 65, the requirement is only 27 MPa. Is there any particular reason to be the same? Anyway the grout is normally in the tension zone.

Thanks

 

[KootK]

This sounds like a reasonable approach to me.

(2) What about the min As?

In most respects, I think that you're still satisfying the intent of As_min.

Is there any particular reason to be the same?

I'm not familiar with the documents you've referenced but don't see why the grout must match the slab strength. The grout's main job is to transfer bond stress between the cables and the ducts. Development length calculations should reflect the actual grout strength however.


The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Ingenuity]

I too am not familiar with your code documents, and you do not state if this is a PT flat slab, plate, 2-way slab, slab/beam.

If this is a banded-uniform tendon flat plate design (which I doubt as it is grouted and therefore likely outside the USA) be careful about distributing large random-placement point loads in these type of systems.

Does the PT slab have additional BONDED rebar for flexure - like a partial prestress design?

A_s min. is usually codified as minimum BONDED reinforcement, so if your A_p (area of prestress) qualifies as A_s min. then until the tendons are grouted A_p does not satisfy A_s min.

Also, the stress in the prestress at ultimate flexural capacity will NOT be based upon strain compatibility, so you will have to check f_ps based upon empirical formulas, like those obtained in ACI 318, for example.

Like KootK stated, grout is for bond transfer and also corrosion protection - seldom is grout sampling and testing done on building-type structures, so this seems overkill to have f^'_{c grout}=f^'_{c conc}.

Make sure the contractor giving you the mobile crane loading info gets you axle/wheel loads (whilst crane is traversing the slab), and also the maximum outrigger loadings (under calculated lift conditions).

 

[msquared48]

Make sure there are no outrigger loads allowed as this will raise serious shear issues.

Mike McCann, PE, SE (WA)

 

[Ingenuity]

Yep, this photo is from Sydney, Australia. Bonded PT band beam and slab system (not yet grouted). Outrigger punched through slab, then see-sawed for about an hour, then crane tipped...FAIL

Crane was only 9 days old (probably about the same age as the slab!). Crane purchase price was AU$4M. Shoring/back props are way less expensive.

No one hurt, except egos and pride.

 

[Ingenuity]

The 200mm thick PT slab (composite metal deck as soffit formwork) was shored (twin SuperSlim HD props) but the outrigger placement vs shoring placement was the straw...

 

[KootK]

Sweet pics Inginuity. I've saved those to my hard drive to use for scaring contractors witless in the future.

I know that you do a lot more PT than I do so I'll take your word for it on the minimum As business. For the sake of my own education, can you explain to me what the issue with that is? My understanding is that un-bonded PT can serve as T&S reinforcement. And strength concerns should be obviated by the fact that OP is doing a separate, un-bonded design check.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Ingenuity]

KootK,

For UNbonded PT:

R18.9.1 — Some bonded reinforcement is required by the
code in members prestressed with unbonded tendons to ensure
flexural performance at ultimate member strength, rather than
as a tied arch, and to limit crack width and spacing at service
load when concrete tensile stresses exceed the modulus of rupture.
Providing the minimum bonded reinforcement as stipulated
in 18.9 helps to ensure adequate performance.

It is usually more economic to provide a PT framing solution with rebar too, rather than a "fully" prestressed design.

 

[struggle66]

Hi Ingenuity
I will get the info from the contractor.

For my own education and knowledge, how can I compute the outrigger load and pad?

I saw one formula before in this forum.

Load at each outrigger foot = 0.6 x (crane weight + lifted load)/area of each outrigger. Is it accurate enough?

BTW, nice photo I've saved them too.

Hi KootK/Ingenuity
I also don't understand why they want fc'grout = fc'slab. Maybe they don't know about PT and just trying to play safe.

In Singapore, we are using CP65. If I am not wrong, it is exactly the same as BS 8110.

Thanks for your enlightening replies.

 

[asixth]

Cranes manufacturers will normally provide you with max outrigger loads. For a crane that size I reckon it would be at least 300kN.

Needed to do a better job marking out the megashores. Even just moving the crane across the slab without adequate backpropping would have been risky.

And there would be no need for a crane that size if it was to erect steelwork. I reckon it would have been there to lift the precast panels.