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.

 

[rowingengineer]

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

 

[cessna98j]

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.

 

[westheimer1234]

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..

 

[cessna98j]

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.

 

[westheimer1234]

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

 

[westheimer1234]

and NOT the one directly above spring support

 

[cntw1953]

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

 

[rowingengineer]

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

 

[westheimer1234]

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.

 

[rowingengineer]

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

 

[cntw1953]

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.

 

[westheimer1234]

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

 

[cessna98j]

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.

 

[westheimer1234]

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

 

[structurallyyours]

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.

 

[cessna98j]

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.

 

[paddingtongreen]

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.

 

[westheimer1234]

padding how do you average the peak moment?

say the pile is 12"x12"

 

[cntw1953]

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