Quake Action On Soils-request for basic info 4

[IJR]

Most of us who havent had time to go through soil dynamics are having trouble understanding how seismic waves respond to soil stiffness(class of soil)

So any ancyclopaedic info will get us to start

1) A building 30ft by 30ft supported by piles 20ft deep. Will seismic action govern the base of the piles or of the building?

2) The 30ft by 30ft chunk of soil is a sea in a big fish. I mean there are miles of un-piled soil around the building. Does seismic waves ignore these miles of mud as far as the building is concerned?

Sorry if this seems not serious a question, but no harm learning.

regs
IJR

 

[Qshake]

Another example of a basic question with complex answers.

Using the code, the seismic base shear is only calculated to the base of the building. Hence if your building is located in a mild seismic zone your questions extends beyond the practical aspect and into academia.

Soil-Structure Interaction is a very difficult concept to grasp as noted by the literature on the subject and lack of guidance in any one direction. Having said that, there are many good papers out on SSI, but few that are of practical significance. For example, SSI involves non-linear modeling and yet when analyzing the structure, structural engineers, until recent times, were limited to only linear springs. And I'm sure that the approximation of the behavior doesn't begin there. Thus each and every step following rigorus geophysical modelling loses resolution, if you will, by the time soil is accounted for in the actual structual software.

Also note that the building and foundation will actually introduce impedence in the wave system. Yet another not-so-well defined subject. Add to this the near-field (close to the faulting source) and far-field (removed from the fault source) behavior and you begin to see just how complex this subject is.

Moreover, the foundation, if embedded in a stratigraphy that is prone to liquifaction will be more likely controlled by displacement rather than forces.

Two sources I recommend on this topic are Geotechnical Earthquake Engineering by Steven Kramer (Univ of Washington) and Soil-Structure Interaction published by ASCE edited by Shamsher Prakash. Keep in mind it may take more to understand the whole ball of wax!

 

[IJR]

Thanx for the time and the clarity, Qshake

regs
IJR

 

[dimitrios]

Hello IJR,


although I am not familiar with the structural behavior of buildings I cannot help but shearing some thoughts that I have had on occasions when dealing with similar problems when dealing with embedded structures, as tunnels for example.

Building codes (as I know for Greece and some other countries) deal with providing a surface acceleration together with some factors to enhance or diminue it according to the depth of foundation, quality of the soil, percentage of the structure ability to deform plastically etc. This acceleration is used at the base of the building. Starting from the base the building can vibrate free and therefore subjected to deformations.

The case with partial or total embeddement of the structure is a bit difference. In this case if the soil moves so does the structure as a whole and at the limit there would be no deformations expected to the structure. This is more true as the wave propagation velocity in the soil/rock becomes higher.

What is of major importance is the distribution of the wave motion and not an acceleration. It is the difference of movement in the various levels in the soil that will couse problems to any embedded structure, apart from the influence of any ovelying surface building. Also due to the dumping ability of the soil usually only the first and second vibration modes play a significant role.

So, I think, that the piles will be subjected to the wave motion field assumed up to the base of the building with additional shear stresses at the connecting points. The forced vibration of the piles cannot be different from that of the soil, as you also point out, the piles cannot change the soil response.

Now, what is the distribution of the motion in the soil, I think it is not a simple matter. I have seen structural studies starting with a rock level they consider motionless and solve the problem upwards, but I haven;t been able to understand how in this case there is an earthquake if the rock doesn;t move (apart from any Rayleigh waves etc. of course).

Sorry, if I have been too simple in my reasoning and perhaphs these points have been solved already.

Regards

 

[IJR]

dimitrios

very fine points in your response,thanx.

As for "stationary rock-swaying building approach", it could have something to do with one of the fundamental principles in structural dynamics- that the effect of ground movement on a building is roughly the same as that of a "fixed ground on a moving building".

every word of yours is taken into consideration to needy ones like me, dimitrios. please feel free to comment.

IJR