Pile as spring below foundation

[bhowmik]

Hi all, good day! Can someone advise me how to model a pile as a (vertical/horizontal) spring below foundation? Specially in ETABS or STAAD model.

[san333]

Hi all, good day! Can someone advise me how to model a pile as a (vertical/horizontal) spring below foundation? Specially in ETABS or STAAD model.

Hi, usually I model the spring constant as a Pile capacity / Allowable displacement. I use this when model a pile in SAFE software.

[oanm2000]

You can obtain the vertical Balast Coeficient (spring coeficient) following the Bowles or Monet procedures, and the horizontal Balast Coeficient following the Tsudik procedure, but you require below the structure the lithological information (SPT, friction, type of soil, etc.). Both geotechnical books youn can download them directly from the Forum (Joseph Bowles, Burt Look)
Another solution is modeling the spring like san333 explain: from the pile capacity / allowable displacement graph
I try to find some example to ilustrate to you the procedure

[aslam]

@san333,

Can u share SAFE file in which u have model piled raft? It will be gratefull to me.
I want to know how u model horizontal and vertical springs for PILE, in addition to vertical spring for raft.

RAFT can be modelled as SLAB. How pile is modelled? As column ? and then applied horizontal and vertical springs to it?

[san333]

@san333,

Can u share SAFE file in which u have model piled raft? It will be gratefull to me.
I want to know how u model horizontal and vertical springs for PILE, in addition to vertical spring for raft.

RAFT can be modelled as SLAB. How pile is modelled? As column ? and then applied horizontal and vertical springs to it?

Hi, I modelled the pile as a vertical spring constant (Column support then choose the spring constant) and the Raft as a slab.
A little bit out of topic. Do you know how the punching shear check work at SAFE. Why sometime I find out the punching shear ratio more than 1. Then, I check using hand calculation it is still okay.
btw, I'm just new engineer with less than 1 year experience. so, need more guidance from the senior here.

[aslam]

While hand calculation generally we assume pressure below raft constant. While in actual case axial loads along with moments are transferred to raft, resulting in non uniform pressure below it. So, punching shear check will be different in both case.

Also, in manual method, we treat raft as rigid. (Raft area= load/SBC & pressure= load/area). While SAFE treat RAFT as flexible. (pressure = ks*displacements).

Hope this will help.

@san333,

Can u share SAFE file in which u have model piled raft? It will be gratefull to me.
I want to know how u model horizontal and vertical springs for PILE, in addition to vertical spring for raft.

RAFT can be modelled as SLAB. How pile is modelled? As column ? and then applied horizontal and vertical springs to it?

Hi, I modelled the pile as a vertical spring constant (Column support then choose the spring constant) and the Raft as a slab.
A little bit out of topic. Do you know how the punching shear check work at SAFE. Why sometime I find out the punching shear ratio more than 1. Then, I check using hand calculation it is still okay.
btw, I'm just new engineer with less than 1 year experience. so, need more guidance from the senior here.

[san333]

@ Aslam.

Thank you for your explanation.
I'm still not understand how SAFE will check the punching shear stress if we only input the spring constant (we never input the pile size). When I learn about punching shear, I need to know the punching shear area (which is for this case is perimeter of the pile multiply by the effective depth of the raft).
Sorry if my question is too basic for you. Sometime I'm just feel curious about this.

For normal slab (flat slab, two way slab), we already input the column size or drop thickness so should be no problem.

Regards,
San333

[bhowmik]

Look, when one model a single pile for horizontal fixity/capacity, it is simple: divide the pile into no of small length, then multiply the perimeter area with subgrade reaction and put the multiplied result as spring at corresponding node. But when you model pile below foundation of a building or bridge, you should model the pile point at foundation as a vertical spring and horizontal spring at the same time. how it can be calculated?

[aslam]

@ Bhowmik, R u student or practising engineer? Where r u from?

Pile resists load by friction (Pf) or end bearing (Pb). You must have both this values along with permissible settlement (s) of pile. You can find Pf and Pb from LOAD-SETTLEMENT graph of pile supplied by geotech consultant.

Pf/s gives horizotal spring value and Pb/s gives vertical one. Which can be used in SAFE.

Hope this will help.

Look, when one model a single pile for horizontal fixity/capacity, it is simple: divide the pile into no of small length, then multiply the perimeter area with subgrade reaction and put the multiplied result as spring at corresponding node. But when you model pile below foundation of a building or bridge, you should model the pile point at foundation as a vertical spring and horizontal spring at the same time. how it can be calculated?

[JIM1975]

You can also use the ALLPILE program in order to find the vertical-horizontal values witch can be put in safe as springs