Whole Lotta Shakin' Goin' On 2

I just finished reading an article with the above title in the latest "Building Design & Construction" Magazine (June 2003 issue, p. 52, also on the web at

http://www.bdcmag.com).

It gives a summary of the IBC 2000 code changes regarding seismic design requirements under the new code.

One of the biggest changes is the effect that the soil classification has on seismic design requirements. Areas of the country that previously had no seismic requirements may now be affected, under certain soil conditions, simply because of the type of soil!

I'm wondering if the geotechs are now providing the Seismic Design Category for a site in their soils reports.

 

[Longhorn2]

It depends on where you are geographically and if the project includes a critical structure like a hospital or school.

 

[kenvlach]

Here's a link to the article:

http://www.bdcmag.com/magazine/articles/b03f052.asp

 

[kmerl]

I think it is a simple site characterization that engineers tend to forget about. I would be glad to see some increased empahsis placed on information that has been traditionally left up to geologists. After all geotechnics and geology are closely related, arn't they?

Keithe J. Merl

 

[BigH]

You don't forget about it if you are in Vancouver!

 

[jheidt2543]

The interesting thing about the new code requirements and definitions is that they can effect a design change here in the upper midwest US. This area does not have a history of seismic activity, yet, based on ONLY the soil classification, the IBC 2000 can require the building to be bumped into the seismic design provisions. So, theoretically at least, all past projects in the area are underdesigned on this basis.

It just seems to rub me the wrong way. It would be understandable if there was an ancient fault line discovered nearby.

 

[BigH]

I know you are up in the Wisconsin area, but there is a seismic zone in Dayton Ohio area - my dad, in Indiana near New Castle was probably the only one in the whole area with seismic insurance!!
[cheers}

 

[jheidt2543]

BigH,

You are correct, there is a seismic area in the midwest and it is a powerful one, but not a real active one. The area is centered on the southern tip of Illinois. But, luckily it doesn't extend into Wisconsin so most of my work is not affected. I do have to consider it when working in northern Illinois however.

Hope you had a great holiday season, although I suppose Christmas in India is an unusual experiance. Best wishes.

 

[jas2]

We routinely provide the IBC, NEHRP seismic site classification because the information is valuable to our clients (even though many have not yet recognized this). The problem is that assigning the classification requires judgment of conditions to 30 meter depth. Most geotechnical projects do not include 30m borings due to cost - especially if SPT data is developed all the way down. We solve this problem by measuring the Shear-wave velocity at the surface using geophysics. The cost is small and the benefit large. The correct classification allows the design to match the site, saving money for large or small projects built on good ground and increasing safety when conditions are poor.

We used to rely on deep data from well logs, geologic maps, etc. to estimate seismic site class; however, since we have started measuring S-wave profiles it has become obvious that conditions change far more rapidly than one would imagine and onsite mesurement, if you can do it, is far better than more general area information.

 

[mrwgeo]

JAS2, I'm curious, what method are you using to obtain deep shear wave velocities from surface measurements?

 

[BigH]

jheidt2543 - thanks. We "cheated" and went to Bangkok!

 

[jas2]

mrwgeo:
Sorry for the delay, I have been away from the office - out in the field measuring shear waves.

We measure refraction microtremors generated by ambient background 'noise' such as trucks on a highway or anything that vibrates the ground a bit. The data is recorded for relatively long duration (about 20 seconds) then processed to create a dispersion of the surface wave spectrum to develop the typical dispersion curve similar to MASW or SASW. We then build an earth model of velocity and depth through a trial and error process to match the observed dispersion. The method is known as ReMi - developed by a fellow at the University of Nevada (Reno) - Dr. John Louie and marketed by Optim Software also at Reno. All you need is a good engineering seismograph (at least 24 channels) and a set of low frequency receivers (4.5 Hz.) and away you go. Of course there are pitfalls and a learning curve but the process can resolve some interesting geology to considerable depth (100 meters and more).

The beauty is the method's use of ambient noise rather than a timed source allowing success at noisy sites where other methods are difficult or impossible. It works, we have duplicated data at sites with plethora of more traditional down-hole measurments for way less cost and much less effort. Cross-hole shear measurements are better at delineating thin beds but ReMi will identify velocity reversal and moderately thin beds if not too deep and most definately will get you into the correct NEHRP classification. Optim Software has information at their website (Optimsoftware.com)

 

[genomty]

jheidt2543:

I understand your concern. Look I'm not a seimic engineer, however I will give you an example that may help.

Here in Mexico, the country is divided in 4 seismic Zones, A through D, D zone is the area with the highest seismic intensity (above 0.7 G )and A is obviuosly the area with the lowest seismic activity, where it is not expected that an acceleration higher than 0.1 G could occur, in fact within the A zone there are no records of seismic activity. Additionally 3 soil types in combination with the seisnic zone give you the parameters to be used to compute the ground accelertion, which are based in the soil consistency/relative density.

So, now, tell me in which area do you think Mexico city is located? You could believe that is in the D zone because of the 1985 earthquake. Due to this earthquake many buildings like hospitals and hotels were reduced to piles of debris in just a minute fraction. Well, Mexico City is located within the B zone. So, what happend back in 1985, it is simple, it was foult of the soils, the ground acceleration was amplified by the very soft/loose and wet soils conforming the subsurface of the city, remember it was a lake,and ground acceleration reach magnitudes similar to the D Zone.

Apparently, that effect was not completly considered when those buildings were designed and constructed. Imagine a table with a bowl full of jelly at the center of it, then put some glasses on the table surrounding the bowl and then put some glasses in the bow, now suddenly shake the table, you can guess the effect. well that was what occur.

Since then Seismic desing in Mexico City became more complex than in other country regions and nowadays you do not design a building to be constructed in Hermosillo, Sonora (within the B zone)like you will desing it in Mexico City or Tijuana B.C. (within D Zone).

So, that is why the importance of the soil and structure type in the sismic category of a site.

Regards

Geno

 

[jheidt2543]

genomty,

Thank you for a very descriptive example and I can see the need to consider all the relavent factors when designing in seismic areas. I guess my surprise or consternation with the new code is that, in areas of no prior earthquake history, you can be forced to design to an elevated seismic criteria based solely on the soil classification.

Adoption of the IBC 2000 code has made a number of changes in the way things are done in the areas I work in. I guess I'll have to think it through some more and just get used to it!