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  • 1.  Seismic Design with Class F Soil

    Posted 04-03-2019 10:34 AM
    Edited by Tirza Austin 04-03-2019 10:34 AM
    ​A project in Manhattan (NYC Bldg. Code 2014 - ASCE 7-10) has a Preliminary Soils Report which identifies the site as being underlaid with Type F soils. I've been asked to design a thirteen story concrete building with shear walls for the "worst case" and not wait for the site specific geotechnical report. What is the correct procedure?

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    David Young P.E.,M.ASCE
    Whitman MA
    (781) 447-6988
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  • 2.  RE: Seismic Design with Class F Soil

    Posted 04-04-2019 07:44 AM
    Edited by Tirza Austin 04-04-2019 07:43 AM
    Hi David,
    Former NYC foundation engineer here. Usually in New York City, Site Class F is triggered for liquefaction susceptibility. The code has a conservative and overly simplified screening process that is sometimes misapplied leading to false positives. Given what I know about Manhattan subsurface conditions, the declining state of geotechnical practice, and the owners' and developers' preference of cheap to good, my first guess would be that you have a false positive. So the first thing, I would check is whether the screening is being done correctly.

    If the screening is correct, you next need to evaluate the susceptibility. It appears that the code allows this to be demonstrated by analysis, although I have been involved with projects where a more extensive site-specific analysis was performed, including measurement of shear wave velocity and analysis of site response. This can increase the site class by a couple of steps and I understand pays for itself, at least for mid- and high-rise buildings.

    If that doesn't work, you may be looking at drilled shafts, caisson piles and other substantial foundation elements. For a relatively small building, ground improvement might make more sense. These same foundation types might apply if the Site F is due to organics or soft clay. The specific subsurface conditions matters quite a lot.

    I am not sure what your preliminary report says versus the "site specific" report you expect to see later, but NYC code is particular about subsurface investigations. Conditions can change significantly over relatively short distances. While I have used phase explorations in the past, I would caution you that when I have seen projects try to get a way with using subsurface data for nearby sites (forget desk studies) it has not ended well. It also violates the code.

    I consult on these issues and I am still licensed in New York. If I can be of assistance, please feel free to get in touch.

    Good luck.

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    Richard J. Driscoll M.ASCE
    Lebanon NH
    rjd@...
    www.richardjdriscoll.com
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    Original Message


  • 3.  RE: Seismic Design with Class F Soil

    Posted 04-04-2019 02:04 PM
    Edited by Tirza Austin 04-04-2019 02:03 PM
    David,
    ​
    Practically speaking, there are far too many things unstated for anyone to give solid direction for your problem. I can imagine that the clients position is that they need to move forward with design work in order to stay on schedule. That's an understandable position, but, there has to be a conversation about risk.

    Ensure your client understands what they've asked of you. They may assume it's just a matter of picking the highest value off of a chart that covers worst case. It's clearly not that. If they understand your position, but need to move forward anyway, that may be doable as long as your firm isn't responsible for the risk.

    The risk is that the final report may specify design criteria in excess of what you used and the design may have to change. So, you need to get approval from the client for whatever design criteria you finally move forward with in such a way that it's understood that they are assuming risk for changes if it doesn't work out.

    If you're in a common situation where the Owner is hiring the geotechnical engineer and therefore is responsible for providing you with the design criteria then you should keep it that way. Structure it so that you clearly have approval, at their risk, to move ahead without the final criteria. You don't want to end up owning the costs for redesign because you were the one who made up the criteria. You can suggest criteria, and the resulting outcome, but don't transfer the risk to your firm. Of course it should probably be agreed that you can't issue any sealed designs until the final report is done and you can verify the criteria.

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    Brett King M.ASCE
    GHD Inc.
    Lake Oswego OR
    (971)925-3743
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    Original Message


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