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

[KillianPMG]

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

[mustafahesenow]

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.

[Refaat]

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

[KillianPMG]

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

[KillianPMG]

Thanks Mustafa,

Just to confirm I  understand:

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

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

[mustafahesenow]

Hi @KillianPMG

All above correct except this 

1. 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 

[mustafahesenow]

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 

[Refaat]

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

[mustafahesenow]

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.

[Refaat]

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

[mustafahesenow]

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 .