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mat foundation on piles using FEM software

mat foundation on piles using FEM software

mat foundation on piles using FEM software

(OP)
i'm still confuse which plate element best to use for the moment for rebar design.

is it plate right on the point of concentrated loads say there is a pedestal (i find this overly conservative even if i spread the concentrated loads by adding fictitious diagonal "columns")
or plate next to it?
same goes with piles as spring support, i use the plate next to the spring support.  

RE: mat foundation on piles using FEM software

westheimer1234,
 Not knowing you level or experience, but if you are new to FEA i would recommended designing the concrete structure by simpler methods such as the IDEALIZED FRAME METHOD and using the FAE as a learning experience.

FEA design using plate analysis with point loads can get tricky. The numerical analysis used causes singularise at supports and point load positions.  You need to resist the urge to design for the peak bending moments; instead you need to spread these across strips as defined by your code or other. Would recommend you talk to an experienced user in your office about your particular situation.  

would recommend you read chapter 4 of Finite element design of concrete structures: practical problems and their ... By Guenter Axel Rombach and also How to design reinforced concrete flat slabs using Finite Element Analysis by the concrete centre (uk).
 

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

RE: mat foundation on piles using FEM software

When I've modeled a mat / pile foundation using FEA, I would typically use rigid links to help spread out the point load to surrounding nodes.  For example if I was applying a point load from a 24" column to a joint using 12" plates, I would draw a rigid link to each adjacent node.  This helped to spread out the load and more realistically model the load distribution.  

      *     *     *
        \   |   /
          \ | /
      *-----*-----*
          / | \
        /   |   \
      *     *     *


Otherwise, you end up with very high localized plate moments.

RE: mat foundation on piles using FEM software

(OP)
rowing, i got the book.. i'll read it.. so far i am not understanding it =)

cessna,
that's what i exactly do..
the question is after doing that do you now consider the plate moment under the "spread" loads or the plate moments right next to it..
the logic i am using is you design the footing moment at the face of the column per aci..

RE: mat foundation on piles using FEM software

I usually design based on the highest moment (Mmax + Mxy), whether it be under the pedestal or not.  If it is under the pedestal and then tapers off quickly I will just add additional reinforcement locally at the pedestal to handle the higher moment.

Same with the pile supports.  If you think about it, the concrete directly above the pile will likely have the highest negative moment - so the moment here can't be neglected.


 

RE: mat foundation on piles using FEM software

(OP)
i do the same thing on pile support. i consider the plate moment right next to the spring support and the one directly above spring support.
same logic: moment is at the face of column/pile

 

RE: mat foundation on piles using FEM software

(OP)
and NOT the one directly above spring support

RE: mat foundation on piles using FEM software

What is the rigidity of the rigid link in your model?  

RE: mat foundation on piles using FEM software

cessna98j & westheimer1234,
I may not understand you discussion here but you don't design for the peak moments. You design for average moments, averaged over a strip. These can be the same strip as used in the idealized frame method. This is due to the micro cracking reliving the slab at the support and redistributing to the surrounding areas.  While designing for the peak moments won't give you a slab that will fail it will give you more steel than you need.  I attach two pages out of the second reference I recommended for your review.

cntw1953
Normally rigid links are a magnitude 10 or more on stiffness of the material being modelled, if you are doing by hand, if not the computer will give you a even higher stiffness. I don't like using the rigid link myself, I prefer to thicken the slab at the supports, sure it is a bit more conservative, but it removes a complexity in the modelling.


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

RE: mat foundation on piles using FEM software

(OP)
Hahaha easy on me. Not everyone is an expert with FEA.

Rowin, believe it or not I have seen so may designs using my or cessna for mat foundation design. We don't do  "strip" just whatever maximum plate moment and use that for footing rebar design.

RE: mat foundation on piles using FEM software

Sorry wasn't trying to give you a hard time, your method is conservative as long as you check punching shear using the moment transferred to the support, you should have a design that will be well above the required design criteria. Seems strange to me to want to use FEA then design the slab to be heavy. But I see it done every day, must be because the graphical output look nice.  

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

RE: mat foundation on piles using FEM software

My question orientated from a project long time ago. One of my colleague (Ph.D) tested the rigid link feature in STAADIII, the results varied with varying rigidity for the very same model. Since then, I have avoided it completely, but still kind of courios on how others handle that, and was there any mistake we made at then.

RE: mat foundation on piles using FEM software

(OP)
Thanks rowin for the effort
honestly though cessna was the one who understand I'm doing unfortunately he just said what he does but didn't explain to me why it is the better approach and not mine  

RE: mat foundation on piles using FEM software

westheimer - Not quite sure what you mean.  From what I understand you are using the same method that I use, the only difference being that I use the higher moment if it exists in the plates directly below the pedestal / above the pile.  While it may be a bit conservative, it seems to me that the highest moments could occur in these areas so I use additional reinforcement locally to accommodate if necessary.

 

RE: mat foundation on piles using FEM software

(OP)
what i meant is i dont consider plates under the pedestal only plates around the pedestal because bending starts at the FACE of column/pedestal

RE: mat foundation on piles using FEM software

westheimer1234
I agree with you
the practice thich we follow is just neglect plates which fall inside pedestal or pile crosssection this care we take while designing.
This can be safely justified because at pedesatal ans pile cap junction the c/s area of plate is very high. One have to consider the height of pedestal in calclulation.

RE: mat foundation on piles using FEM software

westheimer / structurallyyours - I can see your rationale for ignoring the plate moments directly beneath the pedestal or above the pile cap.  It seems to me though that there can be times where the highest moment occurs directly above a pile for example.  If you simplify this to a continuous beam running over a support, wouldn't the highest negative moment be directly above the support (centerline of the pile)?  

Not disagreeing with you guys, just trying to generate discussion.   

RE: mat foundation on piles using FEM software

cessna98j, the peak moment would be averaged for the whole width over the line of piles.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: mat foundation on piles using FEM software

(OP)
padding how do you average the peak moment?

say the pile is 12"x12"

RE: mat foundation on piles using FEM software

I think Pad has the same idea as rowing pointed out on his Jan. 6 (20:45) post.

RE: mat foundation on piles using FEM software

Suppose that the pilecap is on a grid of piles, take one row and calculate the total moment across that row and divide it by the width of the cap, to yield the design moment for each foot of width.

I think it is what rowing was getting at, I just wanted to help with slightly different words. I think we were both thinking that you were going to take the peak moment and resist it with the narrow strip over the pile.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: mat foundation on piles using FEM software

cessna98j
We cannot compare beam elements with plates / shell elements in FEM software. The shape functions for beam element in FEM software are exact and for shell / plate there is approximation. If we apply concentrated load at single point then there is condition of singularity and you can see high stresses developed at that point.

Another way so resolve above issue is to convert force from pedestal in terms of uniform pressure and apply it on mat foundation.

Also regarding neglecting plate elements below the pedestal I would like to give you example regarding beams where we neglect beam. When size of column is larger then to economize beam it is advisable to consider moment at the face of the column eventhough we have modelled beam up to centre line of the beam. This practise will help in saving good amount of steel when column sizes are large.

RE: mat foundation on piles using FEM software

structurallyyours - I can see how that makes sense, it looks like I've got some studying to do on FEA for these scenarios.

rowing - thanks for posting that little FEA article, it made for some good reading.

Thanks to all for your insight and discussion, I know I learned something new from it.

RE: mat foundation on piles using FEM software

A professor told me, that I could find the design moment by doing this: Md = ((Mpeak / 3) + Mpeak) x ½  (elastic design)

Otherwise you can find it by the yieldline theory (plastic design)

 

RE: mat foundation on piles using FEM software

chrede,
sorry to say but your formula is wrong, and we don't have enough time to go into how you can't use plastic anaylsis with FEA, as FEA is elastic anaylsis. you ask why is formula wrong; the peak moment will change due with the meshing size.  

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

RE: mat foundation on piles using FEM software

Rowingengineer

I know that yieldline is a platic theory, I was just trying to come up with an alternative.. !

You are right about the meshsize. If the meshsize is small you will get a higher peakmoment. What meshsize will you suggest in top of a column?

RE: mat foundation on piles using FEM software

A good rule of thumb for starting mesh sizes is the lesser of span/10 or 1000mm.  

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

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