mat foundation on piles using FEM software 1

[westheimer1234]

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.

 

[paddingtongreen]

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.

 

[structurallyyours]

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.

 

[cessna98j]

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.

 

[chrede]

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)

 

[rowingengineer]

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

 

[chrede]

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?

 

[rowingengineer]

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