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# Non linear analisys

## Non linear analisys

(OP)
Hi, i'm new here and i don't know if somebody can help me...
I must model a bridge with soil-structure interaction in account, i have read some works of Mylonakis, Gazetas ecc...
They model the interaction with springs and dashpot, calcolated by foundation and soil characteristic.Does this system work for non-linear analisys?
Thanks for any reply and suggestion

### RE: Non linear analisys

Yes it will. What is a dashpot?

### RE: Non linear analisys

(OP)
it's a viscous damper. Thank for your reply. can you suggest me a metod to determinate the springs stiffness? My model is a bridge with pile foundation

### RE: Non linear analisys

Have the geotechnical engineer provide a modulus of subgrade reaction which can be converted to a lateral spring stiffness.

Other than this I guess a spring stiffness as being the stiffness which allows about a 1/4" movement at the top of the pile, bit dodgy but it should give subgrade reactions and pile moments and shears in the right ballpark.

### RE: Non linear analisys

Hold on a minute....

A linear spring and a non-linear analysis...I don't believe this combination can replicate all bridge behavior and so would like to add this caveat.

If you're structure is a small bridge, say 200-300', and is bracketed by end fill and the superstructure is integral with the abutments, a linear spring may work with a non-linear analysis.  However, if the bridge is quite large and has expansion joints inbetween the abutments then the answer is no.

A linear spring has no mathematical concept of compression or tension.  Obvioulsy when the abutment is compressing the backfill (this is usually the definition of the spring constant) you can rely on the spring value but when the abutment is moving away from the backfill, the piles are the dominant resistance.  At best this is bi-linear behavior and that doesn't include if you have an open seat abutment where a gap must close first before engaging the backfill.

You should investigate tension and compression elements if your software allows.  Of course, you should do more researching to make sure your specific case is treated properly.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

### RE: Non linear analisys

(OP)
In fact this is my question...Can a linear springs give right resoults with a time-history non linear analysis? Some new work of Gazetas and Wolf (soil-structure interaction analysis in time domain) model the ground with springs and damper with linear formolation, but i'm still confused

### RE: Non linear analisys

Is the Gazetas and Wolf work applicable to NLTH or do they use linear elastic response spectrum?

I've read many papers by Gazetas in the past regarding soil modeling and it's my understanding such linear elastic spring models are used with linear elastic analysis.

Please see the paper on non-linear soil structure interaction reference by Earth Mechanics/CALTRANS engineers: ANOOSH SHAMSABADI AND LIPING YAN.  The non-linear spring is depicted for different directions and composition.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

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