Site Class F evaluation 1

[EBF]

I am a structural engineer working on a small commercial building on Long Island, NY. We had assumed Site Class D for the seismic coefficients since we did not have a geotech report. The owner provided some soil borings (no geotech report, just borings) that were done about 8 years prior for a building that is directly adjacent to our site. The borings indicate there is a 15 ft thick layer of organic peat about 10 ft below grade. Looking at the IBC, this would fall under Site Class F. We have told the owner they need to get a geotech engineer to evaluate this and give recommendations. They are refusing because they said they just built the adjacent building without it (they apparently assumed Site Class D). My question is, how much of an amplification effect can we expect this to have on the seismic loads? Are we being sticklers for the Code given that we are in a low seismic area (Ss~0.3 and S1~0.1) and wind loads are probably going to govern anyway? Thoughts...

 

[Okiryu]

I remember that ASCE 7-10 allows for using other classes than F if the structure has a period less than 0.5 seconds. Look at ASCE 7-10, if your building is not too tall you may be able to use class E (also If I remember correctly, class F requires a specific site seismic analysis per ASCE 7-10).

 

[EBF]

I thought that exception was only for liquefiable soils. We still have the issue of the organic material.

 

[GeoPaveTraffic]

Even if you design based on Site Class D, you still have 15 feet of of peat just below your building. Depending on the consistency of the peat, that could cause quite a few problems for the building. How is your design going to react to several inches of differential settlement? Could happen with a peat layer that thick located at that depth.

Mike Lambert

 

[JAE]

Site class for seismic involves the top 100 ft. of soil below your structure - so even with 15 ft. of peat, you could have much better materials below that would significantly affect the site class designation.

You need to have the geotechnical engineer provide an opinion on the 100' based seismic site class - it could well be that it is still D.

However, as GeoPaveTraffic suggests, the peat may be a serious issue for your foundation design with regards to settlements...a separate issue from the seismic site class.

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[moe333]

With 15 feet of peat, your site class will likely be E. Look at ASCE7-10.

 

[Okiryu]

Looking at ASCE 7-10, yes, that exception appears to be only for liquefiable soils. Also as the OP mentioned, with 15 ft of peat the site would fall under Site Class F. Without the support from the owner to hire a geotechnical engineer it makes things more complicated. If the owner will allow for at least some confirmation borings... Also, just a thought: does the code allow for upgrading the site class after soils improvements are done(inclusions, cement treatment or other methods)?...improvements for that layer which may result in an upgrade of the site class may be more economical than doing a site seismic response analysis and the consequent design/construction...

 

[SlideRuleEra]

Present the Owner with a cost estimate to avoid the soils problem by driving piling. That may convince them that hiring a geotech is not unreasonable. I'm not joking or being "cute", we often used this technique to justify unexpected costs.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[EBF]

The owner has gotten another engineer to design piles for the building, so we are not responsible for the foundation design. The issue we are left to deal with is the amplification factors for the seismic load calculations.

 

[JAE]

At least in our area we don't usually (ever?) see geotechnical engineers hired to do 100 ft. deep borings to determine the site class. As I mentioned above, the top 15 feet of whatever you have may not be enough to significantly affect your site class.

Usually, the local geotechnical engineers have historical boring data for deep (100 ft.) borings or they have knowledge of the local geology of your region to establish a site class.

So if you hired a geotech in our area it would be really only for asking their opinion/knowledge of the local conditions for the top 100 ft. of material.

The site class is there to adjust the base seismic demand to local overall geological conditions from site class B values which represents how the seismic energy might be magnified by those overall (i.e. deep) soil conditions. Again, the top 15 feet may not matter much to the energy delivered from 1 mile deep.


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[Okiryu]

Question to the structural engineers here: what are the differences in construction costs for say a 2-story RC office building in a highly seismic area between a site class D and site classes C and B. I understand that there may be several parameters involved here, but if I am just curious to know the rough differences in costs...

 

[cegeoguy]

We often use several different geophysical tests to determine shear wave velocities and seismic site class down to 100'. Completing a ReMi survey will require no bore holes and take about an hour to run the test. Not very expensive if someone in your area has the equipment. This could be a quick way to check the site class if that is all you are needing.

 

[JAE]

Thanks for that info cegeoguy!


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