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.
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
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
* * *
\ | /
\ | /
*-----*-----*
/ | \
/ | \
* * *
Otherwise, you end up with very high localized plate moments.
RE: mat foundation on piles using FEM software
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
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
same logic: moment is at the face of column/pile
RE: mat foundation on piles using FEM software
RE: mat foundation on piles using FEM software
RE: mat foundation on piles using FEM software
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
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
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
RE: mat foundation on piles using FEM software
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
RE: mat foundation on piles using FEM software
RE: mat foundation on piles using FEM software
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
Not disagreeing with you guys, just trying to generate discussion.
RE: mat foundation on piles using FEM software
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: mat foundation on piles using FEM software
say the pile is 12"x12"
RE: mat foundation on piles using FEM software
RE: mat foundation on piles using FEM software
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
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
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
Otherwise you can find it by the yieldline theory (plastic design)
RE: mat foundation on piles using FEM software
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
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
Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it