Segmental Wall-Seismic loading 3

[fargofarmer]

I am designing a 10 feet tall segmental block retaining wall in South Carolina. The project is located near Charleston(Dorchester County). Do I need to consider seismic loading in the design for this area? I don't have any geotech report. I couldn't find anything about it in the SC building/residential code. Does anybody have any idea?
(I read somewhere that it is not required to consider seimic loading if the retaining wall height is less than 20 feet. I think this is especially for some counties in California. )

Thank you very much for your help and time.

Fargo Farmer

 

[Qshake]

Most definitely. Huge 7.1 earthquake hit the Charleston area in 1886 so the area definitely has a history of seismicity. You should expect to find iso-seismal graphs for the area to help with the acceleration coefficient.

That said, I would lean on your geotechs to provide the information and let the structural engineer of record decide, base code, or proximity of the retaining wall to the public and or damage implications for the client decide whether to apply them or not.

I've not heard of a height requirement though I think most would say that less than 5' is not significant and likely as small block modular wall.

Again, as to applying the seismic load, you need to evaluate the code IBC or similar code and discuss with the client, unless it's on public land. Like many structures that are not frequented by the public or large numbers of people, the requiremnts can be relaxed if life safety is not an issue. However, if the wall supports a building corner and it later settles after a small tremor the owner isn't going to be happy.

There are some good references for seismic design of retaining walls. Among those are

Soil Dynamics by Shamshar Prakash.
Principles of Foundation Engineering by Das.

The examples aren't weighted down with theory and basically come down to applying a lateral pressure of the weight of the wall times the accel coeff. (for mass inertia of the wall) and to apply that resultant at 1/2 the height. There is also a Kae, which is the active earth pressure for the earthquake load case. You'll also need to consider bearing pressure and what might happen due to ground motion - should be discussed by your geotech.

good luck.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[fargofarmer]

Qshake,

Thank you very much for your suggestions. Unfortunately, we don't have any information regarding original geotechnical engineer or his report. But, I will try to get more information from client regarding structural engineer of record. The proposed wall is located behind an existing house.

I used SR wall method and I got ground acceleration from AASHTO 2007 Seismic loading CD. The software database gave 0.31g for that area. Again, it is not a site specific coefficient. It's a general value for that area. Anyhow, thanks for your time. I have Das's foundation engineering book. I will refer that for calculation portion.

Thanks
Fargo
Farmer

 

[ItsOnlyDirt]

Ah you structural engineers.....

Dont forget if the soil/wall system can take some permanent deflection (like most segmental walls can) your peak ground acceleration is likley to be very conservative. As a pointer values of between 03.-0.6 of the PGA are typical.

Cheers

 

[fargofarmer]

Yes. I came to know that typical dispacement is 3 in, right?

Do we need to reduce the PGA value? How can we calculate that?

What are the methods can be used for seismic loading other M-O analysis if PGA is greater than 0.4g?

Thanks

 

[moe333]

Do a search for papers by Sitar, Atik, Lew on recent recommendations for lateral earth pressure. Also look at NCHRP 611.

 

[ItsOnlyDirt]

try this page also

http://www.geostructsparks.com/mononobeokabe.htm

Happy calc-ing

 

[fargofarmer]

I searched for Sitar and Atik's paper. But, I couldn't find it. If anybody has acopy of it, could you please post it here?

 

[moe333]

Check PEER.berkeley.edu

A relevant paper is attached

 

[ItsOnlyDirt]

Theres lots of references for reducing PGA.

Here's a few in the attachment

Cheers

 

[msucog]

keep in mind that reducing PGA is appropriate only if additional displacement can be tolerated by the wall designer. i always suggest nchrp 611

http://www.trb.org/Main/Public/Blurbs/160387.aspx

 

[moe333]

I'm not going to get into the discussion of using full PGA as a k value "again". I'll just say do so if you want to be extremely conservative and leave it at that. Search previous threads on the topic and read the links provided here for more background.