Waffle slab foundation

[Jonathan Monge Cubillo]

HI,

Does anyone knows the New Zealand way to design a waffle slab for foundations under liquefable conditions?
Our company is based in Costa Rica and we´re developing a project in liquable sands, so we are trying to replicate the new zealand way.
I´ve seen companies like Firth Concrete in New Zealand, but the details of mat foundations doesn´t provide stirrups so i´m a little concerned about it. It seem like to little reinforcement.

Is there an exeption to the minimum steel required by the ACI for shear and temperature reinforcement?

 

[dik]

You might want to talk with a reputable geotekkie in New Zealand... He might be able to point you in the direction of a good foundation contractor.

Dik

 

[Backcheckrage]

I design waffle slabs all the time. Even in New Zealand.
In general, they are designed only to offer plate stiffness to a foundation plate. There are design requirements here in NZ that apply to residential houses, but obviously they will apply to similar commercial structures as well.
Generally, the concept is that the waffle slab should be designed to have a certain deflection when soil is "absent". These specific requirements are in the link below. The concept is that when there is an earthquake the foundation is able to act as one and can also be re-levelled with a reasonable amount of effort.

These design requirements are not actual requirements from a material code like ACI or something but rather a strong recommendation and best practice for engineers to use, and to make sure the engineering community is doing the same thing (more or less).

Firth has a lot of typical details but in Costa Rica I imagine that you will use more traditional methods to form the waffles.

See this link as a start:

https://www.building.govt.nz/assets...uilding-houses/canterbury-guidance-part-a.pdf

 

[Jonathan Monge Cubillo]

Thanks Backcheckrage, that´s really helpful:

Actually I used the recomendations on the Canterbury residential code in my designs, because i couln´t find anything like it in our codes. That guide work´s fine for us.

I saw Firth´s typical details for rib raft TC3 and there´s no shear stirrups or ties in the beams. In costa Rica we usually put them even if the concrete can take all the shear, so we´re trying to understand that detailing in order to get a more cost-efficient design. Our construction methods allows us to make it similar to Firth´s or even we can import what we need because it´s a large housing project.
Can you give us any reference about lack of shear ties in some details? in firth´s documents says the concrete takes all the shear stresses, wich is true in my case, but I´m worried about the liquefaction uncertainity conditions and possible moments when there´s a lack of support.


Best regards,