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[siwalak]

Dear all.

I have question about stiffness factor in appendix C of IS 2911-2010.
I have a 3D model with pile and need to apply lateral spring constant to determine lateral force and moment in pile.

I have suggested to use IS standard but I have some confusion how to determine spring constant.

Appendix C, Section C2.1 provide K= p/y = nh z (sand)
and C2.2 provide K= p/y = k1*0.3/(1.5*B) (clay)

when I want to assign point spring in finite element analysis this value shall be multiply by length (tributary length of node) and pile width which will be
point spring = K * B * L
or only multiply by length (tributary length of node)
point spring = K * L

Greatly appreciated for your help.

[vikram.jeet]

Though not computer savy,,

I think tributary area has to be taken

Say if pile is of D   m dia  
The peripheral length = pi*D

If you are putting N springs along circum ference

And spacing of spring s along pile length = s

Then it to be multiplied with tributary area
Value = K. s.( Pi D) /N

Just my view,, but Other eminent Group members can say correctly - those working on routine basis

[JVCSNL]

As per the definition, the value of p is per unit length of pile and hence, the tributary length shall be considered.  You can compare units on each side and that will help further.  

The pile lateral stiffness is a complex phenomena and depends of thickness of layers, pile property and soil properties of each layer.  Due to this fact, there will be soil spring for each layer.  The effective pile stiffness will be calculated by combining springs of all layers for these all springs in parallel.  This is a complex task and there are software(s) to determine lateral capacity of pile for a desired displacement.  

For a structural engineer, the lateral pile capacity for a permissible value of deflection is given in soil report.  Theoretically, the stiffness of pile shall be pile capacity divided by the permissible displacement.  Usually this approach is conservative (resulting in lower lateral stiffness) and a more practical approach would be to use the load-displacement curve of pile lateral test.  The slope of this curve (identify a straight line from this graph) will be lateral stiffness of pile.  Note that during pile load test, the displacement will be for an actual applied load magnitude (without factor of safety considered) and the same will reflect true pile stiffness.  Per my experience, stiffness from test results are higher (generally due to lesser displacement than theoretically estimated) and one can use appropriate correction to account of uncertainties in soil property in areas other than test locations.  

Further, note that stiffness of a pile is for a single pile (whether theoretically calculated from IS 2911 or test procedure) and effect of group of pile is yet not considered.  If pile spacing are larger than defined in soil report, each pile will behave independently of nearby piles. However, if pile capacities are not reported based on group effect (usually geotechnical reports indicate min. spacing between piles), then the group factor shall be applied for both lateral capacity and stiffness.  

Hope this helps.  

Best Regards,

Jignesh V Chokshi

[siwalak]

Many thanks for all help.


To
JVCSNL:
Someone tell me as you said but I not sure because I'm not familiar with IS code and found in some manual must be multiple by pile width and tributary length.


Many thanks for the confirmation.