Site classification for seismic design in Foundation design. 4

[Chunil]

I want to ask you how to classify site for seismic design ?
For example, if a spread footing is planned to under beneath ground level then site classification should be defined including all ground material from ground level to 30m depth below?
Or just be classified with soil just from the bottom of footing (not from ground level)?

 

[r13]

The article that came with the photo didn't say the magic word - where the 30m/100' should be taken, but kind of indicative/suggestive.

 

[BridgeSmith]

The 100ft begins at the ground surface. It determines the amount of lateral ground movement the structure is likely to have to sustain during the design seismic event (and therefore the required ductility of the structure). Solid rock moves far less with the same energy than softer soils.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[EngMan40]

The depth to bottom of footing for shallow foundation or bottom of pile cap for deep foundations should be discounted from the total 100 ft. The site specific response spectrum is calculated to bottom of footing level anyway. The bottom of footing is where a structure transfers its loads to and receives seismic loading from.

 

[BridgeSmith]

I have to respectfully disagree, EngMann. My understanding of the use of the 100' is to estimate the magnitude of the ground motions, which are not affected by the presence of the foundation, nor does the depth of the foundation significantly affect the magnitude of the displacement the above-ground portion of the structure experiences. Seismic loads are transferred by the soil to the substructure up to the ground surface.

The addition of this consideration was the result of analyzing the damage from the 1985 'Mexico City' earthquake, which was considerably worse than anticipated. The ground motions were found to be much larger than anticipated, due to type of material (silt) that underlies the city. The cities to the west, closer to the epicenter, fared far better because they sat on massive rock formations that barely moved, even though the energy was greater.

It's not the magnitude of the quake, so much as the magnitude of lateral ground movement (and the inertia of the structure), that determines the seismic forces. Foundations in the ground, mostly move with the ground, so their depth matters little to the seismic forces the structure experiences.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Rod,

I think that's depending on what is the ground refers to for the shallow foundations - is it the excavated ground (usually bottom of footing), or the finished ground after backfill? I am still on the side that a footing is mostly influenced by motion direct beneath, and the surrounding fill/soil just flows/moves with the footing. It shouldn't cost much to investigate a few feet deeper, does it?

 

[BridgeSmith]

The 100 ft is rather arbitrary, but because of what it's used to indicate (the aforementioned magnitude of the ground motion), we have concluded that it should be 100 ft from the average ground elevation at the site, and also evaluated using the typical soil conditions for the site as well - the typical native material around the excavation, irrespective of the foundation backfill.

In our area, that would typically be the conservative approach on both counts, since the density and strength of the material from, say, 100 ft to 110 ft would be greater than that of the top 10 ft. So, the average soil properties from 0 ft to 100ft would typically be worse than the average soil properties between 10ft and 110 ft.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[EngMan40]

It takes judgement on a case by case basis, like a lot of things in geotechnical engineering, to assign a seismic site class for the entire site that contains heterogeneous materials and multilevel basement levels. I can't worry about 1 ft of loose fill few feet below ground some 5 ft or so away from the basement wall to influence my site class more than 1 ft of loose soil located below bottom of footing that I have to deal with and may potentially remove below footing but keep below slab (just as an example).

Just be ready to be able to justify your conclusion if someone raise a question.

 

[geotechguy1]

If you have a site where the distinction actually makes a difference for the site classification, would you not err towards the more conservative side anyway?

 

[BridgeSmith]

If you have a site where the distinction actually makes a difference for the site classification, would you not err towards the more conservative side anyway?

I agree. That's what we decided in the DOT bridge design section, so we go from the ground surface.

Rod Smith, P.E., The artist formerly known as HotRod10