Pile Spring Linear Constant piles - concrete core pile cap design

[JohnnySm]

Dear All,

I have posted something similar previously.

Currently modelling a 12 no pile cap supporting a concrete core, piles will be terminated in stiff clay. As we know in order to design the full extent of the rebar top and bottom for the pile cap and understand a more realistic distribution of loading through each of the piles i have to model the piles with vertical spring linear constants.

Without access to a geotechnical specialist how would i develop a constant that is conservative? i have been told previously to assume a 10mm settlement against the SWL of the piles? but that seems silly as in order to attain my SWL for the piles i need the spring linear constant to model the distribution and value of load passing through each pile (higher stiffness less sharing, lowere stiffness more sharing between adjacent piles)? one relies on the other, chicken or egg situation?

I have been asked to design the foundations so as to max the pile loads at 350kn. for feasbility purposes. would it be conservative to assume 350kn/0.01m stiffness equaling 35,000kn/m constant for the model? using this value the results from the model show what seems to be an acceptable sharing of loads through each of the piles and does not require unrealistic amounts of hogging moment (top rebar) reinforcement.

Im not worried about the rebar for the pile cap, i can always add more bars to cope however i need the loads per pile for the pile ga and schedule.the constant i have pulled out of thin air is what governs, i have no real base to my assumption?

any help please!!!

JS

 

[WARose]

Without access to a geotechnical specialist how would i develop a constant that is conservative?

You can't. And if something goes wrong.....you won't be able to defend the design. Doing piles without a geotech is just unheard of in my experience. It's one thing if we are talking about a couple of footings that someone uses a presumptive value of (allowable) bearing pressure......but piles? Not good. IIRC, the code pretty much mandates it.

I have been asked to design the foundations so as to max the pile loads at 350kn. for feasbility purposes. would it be conservative to assume 350kn/0.01m stiffness equaling 35,000kn/m constant for the model? using this value the results from the model show what seems to be an acceptable sharing of loads through each of the piles and does not require unrealistic amounts of hogging moment (top rebar) reinforcement.

For the purposes of estimating the developed moments in the slab/pile cap (for static, vertical loads).....I typically don't use spring constants. I'll just represent the point of pile support as being fixed/pinned. As far as settlement goes, in many cases I will do an enforced displacement (at several different points) to get an idea of what kind of moments are developed from that. (Usually they are significant......although highly localized.) And just superimpose that on top of the required moment caused by the vertical loads on top of the cap.

 

[JohnnySm]

WARose,

Thanks for the reply i have since forwarded my concern to my manager.

JS