topic Re: Raft Slab on ground springs - concrete modification values - Eurocode 2 in Robot Structural Analysis Forum

Raft Slab on ground springs - concrete modification values - Eurocode 2

KillianPMG — Fri, 06 Oct 2017 08:34:50 GMT

Good Morning,

I am currently modelling a large concrete raft slab supporting stiff clay.

Modeled in Robot using panels and setting parameters of foundation elasticity using Kz.

Do i need to modify the concrete E and G values by modification factor of 0.2 to act as cracked.

The SLS and ULS behaviour of The raft varies substantially if between no modification factors (robot default) and 0.2 modification factor.

Thaks

Killian

Re: Raft Slab on ground springs - concrete modification values - Eurocode 2

mustafahesenow — Sat, 07 Oct 2017 08:41:43 GMT

Hi @KillianPMG

You should make reduction but not in this way.

You have to account the cracking section and to do that you have to Calculate I effective base on your reinforcement design of the raft .

To do this one do this steps :

1-Design the raft for none reduced raft and check how much need reinforcement top and bottom .

2-Calculate M cracked

3-Calculate I effective base on M cracked and Ma (maximum service moment .

4-Reduce the moment of inertia of the raft by ratio I (eff)/Ig

5-Re calculate the model and finalize the design.

this is based on ACI code and I think Euro code has similar way.

Re: Raft Slab on ground springs - concrete modification values - Eurocode 2

Refaat — Sat, 07 Oct 2017 13:49:54 GMT

Hi@ KillianPMG

Do i need to modify the concrete E and G values by modification factor of 0.2 to act as cracked.

Please could you attach the Euro code reference for this reduction (modification factor 0.2) ?

Refaat

Re: Raft Slab on ground springs - concrete modification values - Eurocode 2

KillianPMG — Mon, 09 Oct 2017 18:54:47 GMT

Hi Refaat,

I do not have a eurocode reference, this guidance was based on other documents.

“How to design reinforced concrete flat slabs using finite element analysis” by O Brooker, The Concrete Centre
Technical Report (TR) 58
CIRIA Report 110 (2nd edition)

Killian

Re: Raft Slab on ground springs - concrete modification values - Eurocode 2

KillianPMG — Mon, 09 Oct 2017 18:58:33 GMT

Thanks Mustafa,

Just to confirm I understand:

Run analysis on using uncracked proporties
determine RC requirements
Determine M cracked and determine if section is cracked
Determine I cracked
Then reduced moment of inertia based on the ratio of I un-cracked to I cracked
Then re run analysis

Having done this calculation it confirms that modification factor of 0.2 in this case is reasonable.

Re: Raft Slab on ground springs - concrete modification values - Eurocode 2

mustafahesenow — Tue, 10 Oct 2017 05:32:19 GMT

Hi @KillianPMG

All above correct except this

Then reduced moment of inertia based on the ratio of I un-cracked to I cracked

When Ma < M cracked the section no cracked and no reduction to I

When Ma > M cracked you need to calculate I effective as per ACI code this is the formula to calculate it

I g : Moment of inertia of the concrete section without reduction

Then Reduce the I of the section by ratio I eff/ I g

Re: Raft Slab on ground springs - concrete modification values - Eurocode 2

mustafahesenow — Tue, 10 Oct 2017 05:36:41 GMT

Hi @KillianPMG

0.2 for beam is very little its not reasonable I think this because you reduce the stiffness by I cracked / Ig un cracked instead by reducing it by

I eff / Ig un cracked

I Cracked <= I eff <= Ig un cracked

Re: Raft Slab on ground springs - concrete modification values - Eurocode 2

Refaat — Tue, 10 Oct 2017 15:43:51 GMT

Hi @ KillianPMG

This the first time I hear about considering (Raft foundation) as cracked section since its (substructure element). Usually we use the stiffness reduction for superstructure element as recommended in EC8 4.3.1(7) in case if the structure exposed to seismic loads. (Pic)

I think all your mentioned references related to superstructure elements not for substructure elements as (raft foundation).

I am not sure if the method mentioned by Eng.Mustaf can be applicable for substructure elements rested on soil since its related to calculation long term deflection for superstructure elements RC beams ,RC slabs ACI318M-08 (Clause 9.5.2.3) .

So, you have to take care if you want to deal with raft foundation slab as cracked section. unless there is something ignorance.

Refaat

Re: Raft Slab on ground springs - concrete modification values - Eurocode 2

mustafahesenow — Wed, 11 Oct 2017 06:21:56 GMT

Hi @Refaat

The calculation I presented up is applicable for all sub or super structural element.

Any member subjected to bending It has to be modeled with stiffness reduction to reflect the cracking section.

Re: Raft Slab on ground springs - concrete modification values - Eurocode 2

Refaat — Wed, 11 Oct 2017 10:07:11 GMT

Hi @mustaf

Thank you a lot for your illustrate.

Actually, I was suspected because usually for raft foundation slabs we consider only the elastic raft deformation (without stiffness reduction)
to be used for the expected (maximum settlement & differential settlement).

As in the attached shot the rigidity mat factor Kr is calculated based on full section stiffness.

(Foundation Analysis and design – Joseph E.Bowles – Fifth Edition- page 541) .

Refaat

Re: Raft Slab on ground springs - concrete modification values - Eurocode 2

mustafahesenow — Wed, 11 Oct 2017 10:15:04 GMT

Hi @Refaat

Believe me this very simplified and in any section subjected to bending you should do the reduction even in raft case

Because the rigidity will be based on cracking section all software and even robot shows you the stiffness factor after calculating the reinforcement .

this not only for showing parameters not important is to reduce the stiffness also to consider the exact case and its related moment redistribution .