Flat slabs limited "edge column" moment transfer
Flat slabs limited "edge column" moment transfer
(OP)
Hi!
I have a question on this subject.
When calulating this problem by hand, you divide flat slab into 2 strips, one is the column strip and the second is the middle strip.
When calculating negative moments at the "edge column" we had a limitating moment trasfer to column that was expressed as Mt=0,18*b*d^2*fck. This expression limited the slab section of becoming over-reinforced.
Now, if I calculated this using quivalent frame method, I had an option of reducing negative edge moments by not more then 50%, if calculated moments where higher then this, but then I had to increase the postivive momets in the span by the same precentage..
Im now using structural analysis program that uses FEM.
My question is, should I also limit the values of Momet transfer to edge column, and also reduce -moment/increse +momets if necesary or not?
Thank you
I have a question on this subject.
When calulating this problem by hand, you divide flat slab into 2 strips, one is the column strip and the second is the middle strip.
When calculating negative moments at the "edge column" we had a limitating moment trasfer to column that was expressed as Mt=0,18*b*d^2*fck. This expression limited the slab section of becoming over-reinforced.
Now, if I calculated this using quivalent frame method, I had an option of reducing negative edge moments by not more then 50%, if calculated moments where higher then this, but then I had to increase the postivive momets in the span by the same precentage..
Im now using structural analysis program that uses FEM.
My question is, should I also limit the values of Momet transfer to edge column, and also reduce -moment/increse +momets if necesary or not?
Thank you






RE: Flat slabs limited "edge column" moment transfer
I've never seen that before.
ACI's approximate method works well by hand.
RE: Flat slabs limited "edge column" moment transfer
DO you have something familiar in ACI?
RE: Flat slabs limited "edge column" moment transfer
Is you slab thickness constant?
RE: Flat slabs limited "edge column" moment transfer
But you got me wrong.
Does your ACI code limits the value of the moment that can be transfered from slab to column?
RE: Flat slabs limited "edge column" moment transfer
Can you correct the issue by increasing the slab by 1/2" or maybe even an inch?
RE: Flat slabs limited "edge column" moment transfer
Although depending on your shear, you could theoretically transfer all moment into a column.
RE: Flat slabs limited "edge column" moment transfer
If this is the case, then to increase moment transfer you need to provide spandrel beams along the edge to transfer moment to the column via torsion.
RE: Flat slabs limited "edge column" moment transfer
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You will need to register to get the downloads, but this should help.
RE: Flat slabs limited "edge column" moment transfer
Basicly, what this expresion is doing is limiting your neutral axis depth to 0,5d insuring that the stell in your section (slab) would yiled.
If this is not satisfied, your slab would probably fail due punching mechanism.
But my question is, should I check what is the value of Mt, after my software provides me with the results of bendnig moment isocurves, to insure that the punching shear restrictions are satisfied???
eg. look at the picture,
http:/
I have value of Mx=55,75kNm/m in the slab at the end column.
Mt for concrete strenght of 25Mpa and column 400x400mm, and slab thickness 200mm (d=180mm) is:
Mt=0,18*25*180^2*(400+400)= 116,64kNm/m wich is more then 55,75, so its O.K.!
ughhh I compllicated things now
RE: Flat slabs limited "edge column" moment transfer
I modeled a simple structure, flats slabs thicknes 20 cm, rectangular column array 6,00 m spans, column 40/40 cm and storey hight 3,00 m.
Loads where: self-weight and 10kN/m^2 on slab deck.
Heres the structure:
http://img206.imageshack.us/img206/3079/s12tp4.jpg
and the rusults:
http://img142.imageshack.us/img142/2566/s32tk6.jpg
As you can see in the picture, Mx at the end column is the biggest one in the slab*!*!=!(&#
Is this normal?
I thought it should be the smallest one?
Can some one use this egsample and recalculate it in thier software.
RE: Flat slabs limited "edge column" moment transfer
You need to look at the links and follow the examples step by step, I have not used eurocodes yet(that is a pleasure that I will have soon though!). signing up is free and there is really great guidance there.
a few points I would suggest:
1. check that you are allowed to reduce the end moment by the 50% according to the code, just because the software allows it doesnt mean that it is okay.
2. compare the bending to your Mt.
3. if the bending exceed Mt than you will need to design a spandrel beam as mentioned above to transfer the moment into the column.
RE: Flat slabs limited "edge column" moment transfer
Thanx for the response. I downloaded the "worked egsamples" from the link you gave me, and Im looking at the "flat slab" egsample...and yes they are using the expresion for Mt. that limits the value.
But whats wrong is that they didnt even calculate the moments at the end column, only at the interior column.
They used the Mt expresson and used that value as the "end moment".
I think this isnt such a great idea, expecialy if you look at my bending moment results that I got from computer analysis, wich clearly show that the bending moments at end columns are the greatests.
What do you think?
RE: Flat slabs limited "edge column" moment transfer
The traditional method is to use beams along the edge of the slab that frame into the columns(called spandrel beams) these are then designed to take the end moment via torsion into the column.
If you have no room for spandrel beams then the thickness of the slab needs to be sufficient to transfer this load, this is often the limiting criteria.
Alternatively you can look at proprietary shear reinforcement, but I dont know how this fits in with either bending moment issues or eurocodes.
RE: Flat slabs limited "edge column" moment transfer
A friend of mine did the same egsample and his values of momets were very different expecialy at the end column.
Please
I described all the values few posts earlier.
RE: Flat slabs limited "edge column" moment transfer
RE: Flat slabs limited "edge column" moment transfer
RE: Flat slabs limited "edge column" moment transfer
Check that you have not provided a fixed moment support at the intersection of the columns and the slab.
RE: Flat slabs limited "edge column" moment transfer
I modeled fixed connection with slab and column.
How elese would you get a bending moment in the column?
RE: Flat slabs limited "edge column" moment transfer
To help me get a better idea, what are your spans, sections, live loads and moments?
RE: Flat slabs limited "edge column" moment transfer
Loads where: self-weight and 10kN/m^2 on slab deck.
Heres the structure:
http://img206.imageshack.us/img206/3079/s12tp4.jpg
and the rusults:
http://img142.imageshack.us/img142/2566/s32tk6.jpg
RE: Flat slabs limited "edge column" moment transfer
Also, I dont know the specifics of your analysis package, but I am sure that the slab dosnt stop half way across the column as shown. Model the slab edge as a cantilever to get out to the actual slab edge.
You should get advice from a more experienced person in your office. Dont be afraid to ask, we all have to learn.
RE: Flat slabs limited "edge column" moment transfer
http://img245.imageshack.us/img245/3011/dfkx5.jpg
RE: Flat slabs limited "edge column" moment transfer
Also, have you allowed for cladding loads along the slab edge? I would recommend you take a closer look at this before you finalise your slab design.
Good luck.
RE: Flat slabs limited "edge column" moment transfer
Sorry my english is kinnda limited
RE: Flat slabs limited "edge column" moment transfer
RE: Flat slabs limited "edge column" moment transfer