rib raft foundation (raft foundation combined with beam)

[Groszni]

hello engtips users
i wanna ask what are the considerations of using this ribraft foundation thing?
what are the advantages and disadvantages?
what are some load applied on it?
how do we design the slab and the beam?
thank you.

http://files.engineering.com/getfil...ae-15cb0c0f2f89&file=rift_raft_foundation.jpg

 

[KootK]

Upside down, it's basically the same as an elevated slab with beams. Similar design process and advantages. For the same flexural capacity and stiffness requirements, the beams will require a thinner slab and less concrete. However, the beams will complicate formwork, require a second pour operation, and increase overall system depth and therefore excavation. You'll also require a slab on grade but I usually try to have one anyway with a conventional raft. You'll need to check horizontal shear friction on the beam pour joint but that won't be a big deal.

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.

 

[Groszni]

whats an elevated slab with beam? can you give some pictures to explain? what if the beams are hidden in the raft? whats the purpose of doing such thing like that?

 

[jayrod12]

There would be no point in hiding the beams in the raft. Then it's just a standard raft slab.

 

[KootK]

Elevated slab with beams pic below.

? what if the beams are hidden in the raft? whats the purpose of doing such thing like that?

So the beams project neither below nor above the raft slab concrete? If so then I mostly agree with jayrod: little point to it. The only potential advantages that I could think of would be:

1) it might make for simpler analysis and detailing in some cases.

2) the beams could used to circumvent punching shear issues at the columns.

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.

 

[jayrod12]

If you're making a raft so thick as to "hide" the beams, I have a hard time believing that punching shear would be a real issue.

 

[Groszni]

@KootK and @jayrod12 this is the sketch made by my senior, can you please explain whats going on?
note: sloof is a ground beam

 

[KootK]

This doesn't appear to be a true raft foundation at all. To me, it looks like pad footings at each column with grade beams and integral slab on grade. Possibly suspended, structural slab on grade. Can you share a plan view sketch? How thick is the slab? How far apart are the grade beams?

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.

 

[Groszni]

@kootk this is the plan view of sap2000 i got from him, the spacing between grade beam is 8m, the thickness of slab is 300mm and those grade beams are 1000mm depth, the end span of beam have less amount of reinforced bar than the middle span.

 

[KootK]

Yeah, sounds like pad footings and structural suspended slab on grade to me. It's a whole lot of building. Must be a cool project.

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.

 

[Groszni]

@kootk can you guide me step to step how to design it? its a 3 stories underground basement

 

[rapt]

Groszni,

I would suggest that you approach your senior and tell him/her you are unsure how to approach this design and that you need help. No-one on this site has sufficient information on the project to give the guidance you are asking for. Your senior does, and also needs to know that you are unable to do the project you have been assigned.

 

[KootK]

@kootk can you guide me step to step how to design it? its a 3 stories underground basement

I can certainly guide you through the basic steps of a design based on the assumptions that I've made regarding your project if you wish. That may not be the wisest way to proceed however. As rapt mentioned, proceeding with a design assuming that I know enough about your project to guide you appropriately is a risky proposition. And I very much agree with him that good communication with your senior engineer is of great importance to the success of both this project and your development as an engineer in general.

Consider these questions which I do not presently know the answer to:

1) What's the superstructure here?

2) What are the site geotechnical recommendations?

3) What are those big grade beams for? Are they intended to iron out a differential settlement issues across the site? Are they to provide base rigidity for for a moment frame system (probably not if you're three stories below grade)? Are they simply to hold up the suspended, structural slab on grade?

4) Is the 300 slab really intended to be a suspended, structural slab on grade? If so, why is the soil good enough for pad footings at the columns but not good enough to support a grade supported slab on grade?

These are all questions that will impact your design and can surely be answered by your senior colleague.

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.