Horizontal earthquake coefficient

[harpoon]

Hi

I am in Ontario Canada and the structural has asked me to provide a kh for lateral earth pressure design. My question is what is the procedure (step by step) for getting the kh value and if there are any good references. I have done a search on this forum and looked at the Mononube-Okabe method in the CFEM which says kh=ah/g where ah is the peak ground acceleration in the horizontal direction and g is the gravity accleration.

My question is does this account for wall movement? How do I get the horizontal peak acceleration from the peak acceleration provided in the Ontario Building Code and is there an adjustment to be made to the peak acceleration due to soil conditions?

The CHDBC gives a similar equation which says that kh=0.5(PGA/g).

Please can anyone shed light on what they are using in practice.

 

[structian]

Great thread Harpoon. I would like to know the answer as well. I have been doing earthquake related structural assessments lately, so it would be handy.

 

[muuddfun]

Harpoon

There is no concensus on what the value of Kh should be. We here in California use anything from 1/3 of pga up to full pga for Kh. There is a wide spread in the values people use depending on the type of wall, the site conditions, the conservatism warranted, and any other factors the geotech feels should be considered. We recently had a code change that required giving earthquake loads for retaining walls much more frequently than used to be done. There is a general feeling that from experience in earthquakes most retaining walls have performed very well, and that the numbers you get from the equations such as M-O or the Seed-Whitman approximation, or Woods equations, or any other method are usualy quite conservative. That is why people put an arbitrary reduction factor into the value of Kh to use in these equations to make a more resonable assumption of the earthpressures involved. There is not a lot of back up in the literature however for these reduction, that is why i said they are more arbitrary, and that is why the lack of consensus exists. Using a Kh at 50% of pga would be fair.

Here is a presentation I previously got off the internet, that goes a bit more into detail on some of the issues related to retaining walls and siesmic forces.

 

[msucog]

see the following thread

http://www.eng-tips.com/viewthread.cfm?qid=213054

in particular there is a very recent report nchrp 611 that discusses the topic (links included in thread above). it discusses the reductions and briefly mentions when the different reductions may be more/less appropriate.

 

[moe333]

See the following report:

http://www.ce.berkeley.edu/~lalatik/Final Report.pdf