Three option to Model the elevator pit in safe model 4

[sea2003]

Hi,

I am trying to model elevator pits in Raft in CSI SAFE. the raft foundation thickness is 4000 mm and the PIT Bottom slab is depressed by 4500mm below the Top of RAFT. by that way, we create a raft with a thickness of 8500 mm at the edge of the elevator pit

my question is, which of the following option do you recommend for pit modelling?

1- Using a shell element with a thickness of 8500 mm at the edge of the elevator pit? (refer to attached sketch)
1- Using frame element with thickness 8500 mm and width 4000 mm at the edge of the elevator pit? (refer to attached sketch)
3- ignore the different level and design the raft foundation with a uniform thickness of 4000 mm

 

[dik]

Are you sure 5000 would't be too much?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[rapt]

I would go for 3, constant thickness.

An 8000 deep 4000 wide "shell" element is NOT a shell element!

 

[KootK]

Option 4: call it void space and provide a nominal slab down there designed to do little more than survive. Kinda depends on the ratio of the pit area to the area of the entire raft. In my opinion, your setup has some very deep excavation that's not doing anything much besides accommodating a thick pit floor that probably isn't participating efficiently in flexure anyhow.

 

[KootK]

I imagine this is for your 60 story building sans wind tunnel test? Four car elevator shaft?

 

[sea2003]

I am trying to reduce the raft thickness, but we believe it will not reduce much.


I have already tried to use option 4 but we find that the soil stress getting very high and far exceeds the allowable bearing capacity.
The pit area is a big area.


Yes, you are right.

 

[HTURKAK]

Just for curious, what is the plan size of the bldg and the pit/s ? Apparently the pit depth could be more then storey ht..

will it be unreasonable if i propose, add another basement storey which the elevator access will not be required? so the raft will be flat ?

If there is no other solution, if i were in your shoes, i would provide a haunched transfer and model the raft with SAP 2000 with shell elements .

 

[hotmailbox]

I would use the option 3 in SAFE and detail rebar and the pit like the picture posted above by HTURKAK.

 

[slickdeals]

See Page 16 of this NEHRP guide

https://www.nehrp.gov/pdf/nistgcr12-917-22.pdf

 

[dik]

I'd have thickened the slab to the minimum required for fixed end plate flexure for the bearing pressure to accommodate the pit only...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Enhineyero]

Option 5: I imagine this to be supported by piles? then for the thicknesses, you mentioned I would check/analyse this using Strut-Tie Method. Euler–Bernoulli beam theory mainly applies to span/depth ratio of at least 3.

 

[sea2003]

I totally agree with you if we will use pile, but in our case, we will use raft only

 

[sea2003]

why do you recommend using sap instead of safe software ?, do you think there is a difference in resultant?

 

[HTURKAK]

I am familiar with SAP 2000. Although the software SAFE is also a product of CSI ( the same group) , not sure that the slab with two different levels ( in this case ) could be modelled with safe.

IMO, SAP 2000 more flexible and accurate for such cases ...