My main concern about EC8 is how do

[kellez]

My main concern about EC8 is how do we apply the safety factors when designing a retaining wall for seismic loads. Its not clear in the code.

There are 6 scenarios below, anyone knows which apply?

1) do we apply the same safety factors as we do in EC-7 and also add up the seismic action without a safety factor??
2) do we apply the same safety factors as in EC-7 and also apply a safety factor to the seismic load
3) do we apply some of the safety factors as in EC-7 and also apply the seismic action without a safety factor?
4) do we apply some of the safety factors as in EC-7 and also apply the seismic action with a safety factor?
5) we do not apply any safety factor from EC-7 and we just add up the seismic action without safety factor
6) we do not apply any safety factor from EC-7 and we just add up the seismic action with a safety factor

 

[kellez]

Ok, probably most of you guys do not use Eurocodes, could someone please direct me on what does the ACI code suggests for the seismic design of retaining walls? How does ACI apply the safety factors?

 

[HTURKAK]

Dear kellez (Civil/Environmental)(OP),

I am familiar with EC's.. You are expected to look. Eurocode 8: Design of structures for earthquake resistance Part 5: Foundations, retaining structures and geotechnical aspects. At Annex A, you will find simplified method for the design force acting on the retaining structure from the land-ward side. Part 5 may not cover your specific design situation in details and its proper use may require experience and engineering judgment.Assuming your case as design of gravity retaining wall, the steps you should follow are,

1 ) Calculate the design force as per Annex A using the formula,

Ed = (1/2)* γ* (1 ± kv) K⋅H2 + Ews + Ewd K is calculated from Mononobe-Okabe formula .

You are expected to use reduced soil parameters ( γcu =1,4 ,γτcy =1,25,γqu =1,4 ,γφ’ =1,25)

2) For stability , ( overturning and sliding ) load factors are in general 1.0 (for G and Q).. and γ1 for Ed could be 1.0 or greater as per importance of structure...
3)For strength, Rd > Ed ... Rd shall be calculated using material partial factors ..( for steel 1.15 and for concrete 1.5...) However, for seismic ,the use partial factor=1 is allowed..but I will not recommend..

If your question is a real case, please publish the sketch, to receive better comments..

 

[kellez]

Thanks a lot, my next question now is,

If i apply factors of safety to soil parameters as you said then i need to apply factors to:

1)angle of friction of base soil, which then decreases the resistance to sliding.
2)angle of friction of backfill, which then increases the total horizontal force applied on the wall.

Am i supposed to apply both of the above in the same load combination or do i have to make two different combinations?

tomorrow i will post my sketch, thanks alot

 

[HTURKAK]

Dear kellez (Civil/Environmental)(OP);

The answer to your both question is yes...You are expected to calculate the friction resistance,overturning moment and sliding force with reduced cohesion (Cu with 1.4) and friction angle (φ’ with 1.25)...
With reduced cohesion and friction angle, the sliding resistance of retaining wall will reduce, the active pressure will increase..

You are expected to apply both of the above in the same load combination..

 

[HTURKAK]

Just for clarification: My comment is valid for cantilever gravity retaining wall. In the case of underground structure,you should consider two combinations for design approach 1 (DA1). Partial factors will vary for favorable and unfavorable case..

 

[kellez]

Hi, below is the picture of the table you posted above. I am familiar with this table which is from Eurocode 7.
As you can see, the table refers to only one Shearing Resistance (tanf), it doesnt say if it is Favourable or Unfavourable as in the case of Permanent Actions.
What i am trying to say is that in the case of a retaining wall we have two types of soil, 1) the base soil, which is resisting sliding and 2) the backfill which is causes Sliding.
In your first post you said i should apply the partial factor to both of the soils in the same combination, ok i do understand that, it also produces a safer design but are we sure
this is what the code suggests?

I will be asked a lot of questions for my design therefore i want to be able to justify everything.