Demonstrating the importance of Seismic Site Class

[VAStrEngr]

Does anyone have knowledge of a reference that explains in laymans terms the implications of selecting the proper seismic site class?

Background: I am trying to educate other disciplines in my company about the implications of not including a seismic site study in a geotechnical investigation. Because it is not included, I usually assume a site class D which seems to push it into a Seismic Design Category D. However this assumption obviously results in a more beefy/expensive structure because of the increased factors, detailing requirements, etc depending on the material being used.

I would like to provide some documentation that spending the extra up front in a geotechnical investigation might be/might not be justified when considering the implications down the road. Short of actually taking a case study project and entirely reworking it for two different site classes, I am hoping there is something else out there already compiled.

 

[JAE]

I don't know of any comparative studies of this but typically your local geotechnical engineers should have a good understanding of the local geology and tell you what would be the appropriate site class that would be expected.

If you still insist on proving that a few 100 ft. deep borings could be helpful - get a proposal from a geotech to do the necessary number of 100 ft deep holes and provide a site class - then you ONLY have to prove that the geotech cost is less than the benefit of a lesser site class... assuming that you do have the potential to get to a C or lower.

 

[Brad805]

I assume you are in the US. I know our last Canadian code update follows the US code fairly closely, but I know there are still some differences. I have not read any studies like what you are looking for, so I am going to try to explain in simple terms what this definition is used to calculate. The site class is a signifcant item, assuming a value should only be done for small projects (say less than $1mil).

The Site class is used to pick out the appropriate Fa and Fv values which have a direct impact upon the design spectral acceleration (S(T)). Look at the forumlas for S(T) and you will see how critical this value can be. Also, take a look at the total seismic shear load (V) forumalas and you will see the impact of the S(T) value.

The value you put in your report has a considerably impact upon the seismic forces. Lets say the total seimic force on a structure is X for a Site Class C, if you change to a D, or E, the force could potentially increase to 1.3*X or 2.1*X respecitively (Canadian Code). This explanation is simplified greatly, but it gives you a general description of the impact of the site class definition.

I had one project that started with a poor geotechical report that failed to define this value properly. The EOR suggested a value to assume, so our client got started with their shop drawings. After the first geotech was replaced it turned out to be a site class "F" and my client (Precast Concrete Fab shop) lost the project. This error resulted in significant additional costs and time lost.

Brad