Propless or shoreless whole building precast construction

[struggle66]

Hi Good Day Everyone

Here in Singapore authorities are pushing for less manpower whole building precast propless or shoreless construction. Please refer to the photo below.

Have you guys ever encountered this before? Is there better scheme to construct? Will it better to use Hollow-core slab instead of precast single tee and post tension beam in the place of RCB but design as simply supported RCB during construction stage.

Another question!

Here in Singapre when designing the continuos hollow core or single tee slab, designers here are using shorter continous design span as shown below. They claim that beam is so rigid that the width of the beam can be ignored in the precast continuous slab design but I think it is still the same as cast in-situ continous slab.

Thanks in advance!

 

[saadpervez1]

Hollowcore slabs are basically simply supported beams supported on the Ledge only by bearing. Hence the design span is limited to its length only. The cast-in-place topping acts as a diaphragm and minimum reinforcement is provided.

 

[dik]

I've used a similar method to your Section B for several apartment buildings. The ledger is considerably smaller and the HC is placed within 2" of the beam web. Reinforcing is placed in the HC joints, extending a couple of feet on either side of the beam, to provide continuity with the topping. Additional rebar can be placed in the topping if required.

Reinforcing appears to be quite heavy for the pile spans; you might consider the wall arching between the piles with little of no loading going on to the beam. The wall is much stiffer than the beam. Between the piles, any dowels from the beam to the wall are likely in tension.

Dik

 

[struggle67]

saadpervez1,

I agree! but is Hollow core continuous span design possible? I am just wondering since HCS specialists here are doing so!

dik,

Thanks but I couldn't quite understand and imagine your 2nd sentence.

 

[saadpervez1]

Dear aungthu13490,

Continuous span design is possible, the top of the hollowcore slab is hacked and chipped at the support for the required length, the reinforcement is placed and pouring is done. However that process is labor intensive and time consuming completely offsetting the benefits of precast construction.

Providing reinforcement at the top is not possible during casting as far as I know because the extruders are not built that way.

 

[rapt]

The design assuming continuity should be done using the full centre to centre of support span length. Checks on slab negative moment capacity should be made at the normal critical section and at the edge of the beam support.

The beams shown on this drawing are not stiff enough to justify anything else.

 

[struggle67]

Dear saadpervez,

They have at least 75 mm thk structural topping. All the top negative bars are always placed inside this topping.

Hi Rapt,

When can we say, a beam is stiff? "Band beams are not stiff but beams are." Is it engineering definition of stiff?

Critical sections for continuous slab negative moment are at the beam edges for slab with transverse beams and at edges of band & face of column for slab with bands beams. Can I say so? I can't recall what the code says . I think Eurocodes allows us to reduce the support moment for the continuous slab and beam as follow.

BTW, any comment or advice on the construction method. Which method construction have you guys been using for this prop-less construction in Australia or Worldwide.

Thanks

 

[struggle67]

And all the brackets shown in my thread are only for construction stage.

 

[saadpervez1]

Dear aungthu13490,

I agree that negative moment rft is placed in the topping but that is for beams only where you provide stirrups to achieve a homogeneity with the precast section and the cast in place topping.
In case of HCS, that homogeneity is not possible because there is no way you can provide stirrups in the HCS panel.
Honestly, this issue is debated whether topping reinforcement alone can ensure continuity, I have worked in precast industry in 2 countries and the approving authority (or the engineer for that matter), has their own concepts which we as a contractor had to follow. As far as I understand according to my severely limited knowledge and experience, continuity will only be possible with the method I mentioned above.

Continuity calculations for HCS are also not simple because of the absence of singular depth across the section, you have to evaluate based on Moment of Inertia of the section.

In one particular project, we had to chip off the top beams to ensure continuity as well.

 

[struggle66]

Hi saadpervez1,

Sorry for late reply! Thanks for sharing your experience. I think authorities here accept that toping reinforment alone can ensure the continuity.

BTW aungthu 13490 is also my user name. Something went wrongs!