Seismic Loads Due to Soil (Basement)

[Berniedog]

We are designing a commercial building that has a walk-out basement. Are there seismic loads due to the soil behind the walls that we have to take into consideration?

I checked ASCE-7-05 and I could not find anything. Chapter 19 made no sense to me at all.

 

[Doublest]

seismic effect will liquidize the soil which will make the soil active pressure on basement wall increase. The most conservative consideration is to take active Rankin's ratio=1.0 (like water). the actual Rankin's ratio will be somewhere between the active Rankin's ratio without seismic and 1.0.

 

[Larryhd2]

Are you in SDC D, E or F? If so the soils report is required to provide this loading:

From the CBC/IBC: (similarly in ASCE 11.8.3)

1802.2.7 Seismic Design Category D, E or F. Where the
structure is determined to be in Seismic Design Category D,
E or F, in accordance with Section 1613, the soils investigation requirements for Seismic Design Category C, given in Section 1802.2.6, shall be met, in addition to the following.
The investigation shall include:
1. A determination of lateral pressures on basement and
retaining walls due to earthquake motions.

 

[Berniedog]

Doublest:

Is there a code reference for this?

 

[sebivaq]

In my opinion for underground structures it's important to consider the seismic load. In this case the structure will move with the soil, the inercial forces aren't very important. There are various ways of considering this load case:

1) Analytical (models with springs or with forces and springs) - Watch the attach as an example
2) FEM Model (you should model the structure-soil interaction with non-reflecting boundaries), is the most accurate solution but takes more time.

Analytical:
You need to calculate the deflection of a soil column subjected to the EQ, and then assume that the structure has the same deflection because the move together. And then according with the stiffness of the structure you can obtain the member forces.

FEM Model:
You have to model the structure, the soil (3H to each side, 2H deep, where H is the structure height), and the interaction between them. It's very important the boundary conditions that you addopt. A non-reflection boundary is the best solution, because them doesn't reflect the seismic waves. Bye,

Sebastian

 

[Berniedog]

We are in a seismic design category B.

 

[Doublest]

Berniedog:

As I know, there is no such code that directly covers this issue, however, ASCE7-05 session 15.7.9.3.1 (page 171) covers the issue of increased lateral pressure of granular material on tank walls during seismic shaking, I think this is a good reference. Besides, this is an educated logic deduction. I have used it in designing in several of my previous projects.

 

[WillisV]

The answer to your initial question for a SDC "B" Building is: No.

 

[Berniedog]

WillisV:

Can you give me a code reference for that?

Berniedog

 

[Larryhd2]

For those looking for a design procedure with earthquake loading on a retaining wall, the most widely used method is the Mononobe-Okabe method.

Do a google search and you will find good material on it.... as well as here in these forums.

 

[Andrewstructure]

If this is a commercial building, I would consult the geotechnical engineer who did the soils report for the job. It is much better to have him take the liability for you, and also to get his local expertise.