2000 IBC - soil Site Class

[DatG]

I have found that several clients have been complaining about increased costs as a result of seismic design requirements under the 2000 IBC. These increased costs seem to generally occur when you have a soil Site Class of D, E or F. My questions are what can be done to reduce these costs, if anything, and what affect would using a deep foundation system have? Does anyone think a more refined determination of the soil shear wave velocity or Probabilistic Seismic Hazard Analysis would be worth it?

Thank you in advance for any help you can provide

 

[Focht3]

I think the IBC changes reflect the "recent" experience with marginal sites during strong earthquakes - and they are long overdue.

Are the clients you are referring to developers? Architects? Structural engineers? (I'll guess it's a mix of all three, with developers the largest group.)

Can additional data reduce the development costs for some sites? It's possible. But even if it doesn't, the client should recognize the importance of spending their money where it will do the most good - and that a good characterization of the site may significantly reduce the mitigation costs. And reduce the risk of serious damage and/or collapse of the structure during strong ground shaking.

Deep foundations aren't always the answer; I'd guess that my response is "it depends." Each site/structure combination offers its' own challenges, and shallow foundations can perform just as well - even better. Again, there is no "magic answer" here. Does I think a more refined determination of the soil shear wave velocity or Probabilistic Seismic Hazard Analysis would be worth it? Probably, although it might be hard to obtain a "representative" v_s study for some sites.

Anyway, those are my thoughts - anyone else?

 

[DatG]

Focht3, Most of the cost issues have come from the owners with the structural engineers a close second (likey from pressure by the owner).

My reason for the question on the deep foundations was that someone suggested to me the Site Class could be improved (say from a D to C) since the piles were end bearing in better material. Seems to me the Site Class is what it is and shouldn't be altered based on foundation system used, and the effect of the foundation system used would be more appropriately taken into account by the structural engineer.

 

[Focht3]

I agree. The change in Site Class would have to come from some site improvement technique like deep dynamic compaction, surcharging, etc.

 

[TJWATKINSPG]

DatG

This is a pet peeve of mine. Who is sealing these plans? you or the client? If your client wants to meet 2000IBC standards, then do the tests required. One of the problems with using a standard is that very quickly it becomes not a minimum but a target to try to wiggle around. The building codes are in my opinion a clear example of these. builders are always aasking for vsriances. We wind up with 20 year houses, and 50 year bridges. So yes, clients want to spend as little money as possible, but they also will hold your feet to the fire for any failure. I reply that value should be what professionals give. Some times corners can be cut with good experience and judgement. This can be cheaper, it is not the present standard.

Change the standard? (Probabilistic Seismic Hazard Analysis would be worth it?) I am for every bit of improvement. A seismic study detail would improve the standard. Can the industry react in time for this client? probably not.



The truth will set you free. Best of luck. Geodude

 

[RonEbelhar]

It would help if you could clarify what you mean by "increased costs" (professional fees for geotechnical engineering, structural engineering, construction costs, etc?).

The answer(s) depend on the project location, soil/rock profile and the Seismic Use Group / Seismic Design Categories. We have found that essential facilities and structures which represent substantial hazard to human life can easily justify the relatively small costs to accurately determine the Seismic Site Class. Where such facilities are located in Site Class C, D, E or F areas, we have found on several occasions that field shear wave velocity measurements have saved several hundred thousand dollars in structural detailing and construction costs. Larger facilities have seen even larger savings from better soils data. Accurate soil profiling, using CPT, was essential in screening whether it was likely that shear wave velocity testing would yield higher site classifications.

With regard to deep foundations, we typically assume for site class purposes that the soil/rock profile begins at the bottom of the pile cap. The simple explanation is that the deep foundations are typically slender and flexible compared to the free-field motion and thus will not usually significantly reduce the base shear felt at the pile cap (and subsequently transferred to the structure). There are obvious exceptions to this, and in some cases, a full soil-pile-structure interaction analysis is justified.

As geotechnical engineers, we should do our best to educate the owners/structural engineers that it is in their (and our) best interests to get the data necessary to adequately characterize the soil/rock profile for the intended purpose.

 

[Focht3]

[blue]RonEbelhar[/blue]:

Welcome! Glad to see you have joined [blue]Eng-Tips[/blue] - even if you've kept mum since January...


Are you still in the Ohio/Kentucky area?

Post once in awhile - you have a lot to offer on this site. Your experience and education will add a lot to the discussions.

 

[DatG]

Thank you all for your input. It confirms a lot of my own thoughts on the subject.

RonEbelhar: The increased costs are due to added structural requirements. Whether this is due to added engineering fees (Structural) or materials (additional connectors, etc.) I do not know - Probably a little of both.