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# How to roughly equate Seismic Design Category with Richter Scale Magnitude

## How to roughly equate Seismic Design Category with Richter Scale Magnitude

(OP)
Is anyone aware of any documentation that roughly equates Richter Scale Magnitudes with Seismic Design Categories? I have a client that wants to ensure his building will meet a particular Richter scale factor (7.0). I've tried explaining, to no avail, that the code (ASCE 7) uses a different approach toward the design criteria used in the structural design to resist seismic forces. I appreciate anything you can share on this topic.

### RE: How to roughly equate Seismic Design Category with Richter Scale Magnitude

Well, first off I don't think that "Richter" scales are even used anymore. The magnitude X events you read about are a different system I believe than what Mr. Richter originally created. (terminology so no real big deal of course).

(see page 2-24 of this link: Fema document on magnitude

There is this: http://earthquake.usgs.gov/learn/topics/mag_vs_int...

Table 2 on page 58 of this: http://c.ymcdn.com/sites/www.nibs.org/resource/res...

However, the magnitude of a quake is, I believe, at the quake center.
A building's response to a seismic event depends on the distance from the epicenter (attenuation), the depth of the epicenter, the type of soils between them, the type of soils the building is supported on, etc.

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### RE: How to roughly equate Seismic Design Category with Richter Scale Magnitude

(OP)
Thank you. I agree that the question doesn't really make sense in the context of what we do as structural engineers. However, I found out quickly that it was difficult to answer such a seemingly innocent question posed by a non-engineer. I did find through another colleague that USGS has a correlation table of accelerations with the Mercalli scale and another chart that correlates Mercalli with Richter. From all of this I am able to loosely tie Richter with anticipated accelerations (PGA) at the site.

### RE: How to roughly equate Seismic Design Category with Richter Scale Magnitude

Would that be just for that site?

If you put a table together - I'd like to see it - might be handy to have on Eng-Tips for other's future reference.

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### RE: How to roughly equate Seismic Design Category with Richter Scale Magnitude

Now that this question of yours has marinated in me for a while - one thing I might recommend is to be careful how you present this information.

Your client will hear from you: "Your building is designed for SDC X, therefore your building is designed against a magnitude Y event."

However, in actuality our seismic design criteria is focused on life safety, not building resistance/survival. The codes allow the buildings to undergo inelastic behavior without losing stability. Your client's building may be designed for a Magnitude Y event under code but when a Magnitude Y event (or even perhaps a smaller one) occurs, and their building is damaged to the extent that it must be torn down, they may come back at you with lawyers.

Just be clear on how you present it to them so there's no misunderstanding.

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### RE: How to roughly equate Seismic Design Category with Richter Scale Magnitude

(OP)
Yes, totally agree. Thanks for the thoughts and discussion.

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