Inclusion of Ground Floor Suspended Slab for RSA to EC8

[LMass94]

Hi all, long time lurker first time poster.

I'm designing a portal frame to EC8 with a piled foundation and a suspended slab acting as a diaphragm, spanning between pile caps / ground beams. Piles are modelled as linear springs.

Per EC8-1 4.3.3.2, ELF base shear is calculated using m, defined as "the total mass of the building, above the foundation or above the top of a rigid basement, computed in accordance with 3.2.4(2)"

This implies that effective seismic weight is taken above foundations, the caveat being that this only applies to ELF. EC8-1 4.3.3.3 gives no such recommendation for RSA.

I would imagine that a suspended slab, which will provide a sizable portion of the total system mass, will introduce considerable seismic inertia at top of foundation level. EC8 seems intentionally vague on the subject, so my question is ultimately: do I need to consider the mass of foundations / ground floor slab in the effective seismic weight for RSA? And bonus point, is anyone aware of any guidance notes that confirm this which can be used to justify this approach?

Many thanks in advance.

 

[HTURKAK]

My suggestion would be;

- if the subject building meets the conditions given in 4.3.3.2, follow ( 4.3.3.2 ) Lateral force method of analysis,

- you may perform RSA and you do not need to consider the mass of foundations / ground floor slab..

- If you want to add the found. and ground slab , i would think that , you want to increase the fundamental period of the bldg , to get the advantage of lower spectral acc. coeff.

- You may approach with two models .. first is without considering found . and slab and the columns fixed at found. level ( if RC frame ) and second , adding the effect of soil interaction ( piles modelled FEM frame elements with horizontal springs , and the rc slab as suspended slab)

- If you add the found . system , you will notice that ,the upper portion will not significantly affected by contributions higher modes and lower portion will be affected by higher modes. You shall choose CQC combination ..

- If you post the structure , and provide some descriptive data , you may get better responds..











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

 

[Zando]

It depends on your soil profile and structure. It's important to remember that the design actions originate from ground accelerations.
Typically I'd expect a damp-proof membrane under the slab which will drastically reduce the friction/damping of the soil below it, so your piles would be expected to transfer the seismic impulses to the slab. Let's say you have weak soil for the first 5m or so and then stiffer soil beneath that, then your base shear is probably acting somewhere around that interface in which case the slab likely has a noticeable influence. Conversely if you have very stiff soil then that fulcrum is going to be a lot closer to the surface in which case it likely doesn't have a great influence.

 

[LMass94]

Thank you both for your insights.

HTURKAK, I will review the possibility of using ELF, which would preclude consideration of the GF slab. Good idea to produce 2 models (with / without GF in seismic weight). Plenty to explore! Unfortunately I am unable to share details of the model but your advice has been useful regardless.

Zando, really useful advice regarding soil stiffness, I will review the GIR and see what the case may be. The site is part infill, so the soil is not going to be stiff. Hence I imagine we will be considering the mass of GF slab as you suggest.