Voided slabs shear capacity 3

[Gus14]

From my understanding, Voided slabs are essentially waffle slabs with a thin slab at the bottom. However, from most of the designs I have reviewed of voided slabs, I rarely see stirrups provided even at significantly high depth. Even with the voided slab rib depth being between ( 40 - 70 ) cm no stirrups are usually provided. I could understand this for thin slabs with spherically shaped plastic void formers, but for box shaped formers, I have my doubts.

Usually voided slabs are designed as solid slabs with edited properties. However, the shear strength of these ribs near the supporting beams where shear force is at it's maximum seems to get overlooked.

Lets say we provided stirrups at location where required for example, at the ribs ends near the supporting beams. At the other locations is there a code requirement for using minimum shear reinforcement in voided slabs ?

 

[HTURKAK]

AFAIK, Voided Biaxial Slab ( AKA , BUBLE SLAB ) is modelled assuming solid slab with reduced shear and bending stiffness . There are some papers and manuf. recommendations rather than standard.

You may look ;

1-

https://en.wikipedia.org/wiki/Voided_biaxial_slab

2-

https://www.daliform.com/en/construction-solutions/lightweight-slabs-daliform-group/

3- ACI paper by Fanella "Design of Flat Plate Voided Concrete Slab Systems"

4- CRSI "Design Guide for Voided Slabs"

Finally pls search for ( collapse Eindhoven Airport parking garage ),

https://www.bam.com/en/press/press-...lts-known-of-technical-cause-partial-collapse

I would prefer WAFFLE type rather than Voided Biaxial Slab..







I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[dik]

With concrete joist construction, the often have pans that create a flared web for end conditions for shear.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Gus14]

Thank you HTURKAK and dik for replying. Very nice resources will definitely check them out.

I was wrong after all, I though just like beams a minimum shear reinforcement was required in waffle slabs and one way ribbed slabs, and I was trying to apply that logic for voided slabs. But the code does not specify a minimum shear reinforcement for ribbed slabs or waffle slabs. I searched through multiple design examples and they all did not specify a minimum shear reinforcement.

I remember my professor specifically telling me to specify a minimum shear reinforcement in ribbed and waffle slabs, he shall take the fall on this one if I ever meet him again.

 

[Tomfh]

The existing ones I come across in Australia appear to have been designed based on the concrete shear capacity of the ribs. The ribs often step out horizontally as you approach the supports for this reason.

When I’ve designed them I add shear links if the shear loads are getting high, even though as you say you can treat them as a slab and omit the shear steel. In the event of a shear failure a potential plaintiff could likely find someone like your old professor who’d be willing to swear that it’s malpractice to have omitted the ligs.

 

[Gus14]

Thank you Jennifer and Tom for replying,

, it is crucial to consider the shear strength of the ribs near supporting beams and provide additional shear reinforcement as necessary.

Could not agree more

In the event of a shear failure a potential plaintiff could likely find someone like your old professor who’d be willing to swear that it’s malpractice to have omitted the legs.

Yeah, I mean I never saw a building with waffle or ribbed slabs without minimum shear reinforcement in the ribs, it feels like an unwritten rule. Although no minimum is required by my local code.

Another concern I have, is that voided slabs are poured on two stages to ensure concrete fills the lower slab and prevent the molds from floating. But I think there will be a cold joint between the two surfaces. Hence some minimum shear reinforcement should be applied. No studies have been done to address this issue so maybe I am overthinking it.

 

[Tomfh]

Yeah, I mean I never saw a building with waffle or ribbed slabs without minimum shear reinforcement in the ribs, it feels like an unwritten rule.

Here are some real life Australian examples of suspended ribbed slabs with no shear steel.

 

[Gus14]

I just read the CRSI frequently asked question about flat voided concrete slab systems.
They did address the cold joint issue saying the the the weight of the top slab will help. In addition, they rely on the vertical bars of the cage module. However, the void former model I am dealing with does not include vertical bars. So I will specify a single leg stirrup to ensure horizontal shear transfer. Is there a way to verify this though.. ?