Slab results comparison

[mar2805]

Guys I have a question on slab forces.
I made 2 egsamples of the same slab with the same load cases. Only difference is that the slab on the left is supported on 20 cm thick walls and slab on the right is supported on beams that are supported on columns.
If you look a the pictures for the Mx and My moment diagrams you can clearly see the diference in the values.
The results of the slab with beams are clear to me. Where the internal beams meet the section is very stiff, so the momets are greater (-8,30kNm/m for the Mx and -27,80 kNm/m for the My)
As we move thowards the mid of the span the moment decreases since the slab and beam will deform and because of that it will attract smaller forces.
Now please look the egsmaple on the left, slab supported on walls. Why are maximum moment in the slab at the mid span of the walls and not in the intersection of the walls? (My max=-13,70kNm/m) I dont get this?! Shoudnt this be the same case as with beams?! The maximum negative moments at intersection of the walls?!

http://files.engineering.com/getfile.aspx?folder=70109887-a03b-42ed-9d92-3bf330f956f6&file=Mx.jpg

http://files.engineering.com/getfile.aspx?folder=e52b4d5d-1497-47b9-a2b1-454de52fce87&file=My.jpg

 

[paddingtongreen]

It is correct.

A flat slab under a uniform load tries to "dish". On the walls, the corners try to lift, you may find negative reaction there at the corners. When the beam deflects, it reduces the support in the center so it increases it toward the corner.

Visualize the moment diagram for a middle strip as a continuous beam over a support. Then visualize the effect when the center support dflects a small amount and combine the these diagrams. Play with these images and it should become clear. Do it for strips in different locations.



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

 

[paddingtongreen]

It is correct.

A flat slab under a uniform load tries to "dish". On the walls, the corners try to lift, you may find negative reaction there at the corners. When the beam deflects, it reduces the support in the center so it increases it toward the corner.

Visualize the moment diagram for strips in various locations as continuous beams over interior supports. Then visualize the effect when the center supports deflect a small amount and combine the these diagrams. Play with these images and it should become clear.


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

 

[asixth]

The results look correct. For the slab supported by walls, the load spans to the wall favoring the span in the X-direction because this is the shortest and stiffest load path. Once the load reaches the wall, it can travel down to the foundations.

For the slab supported by the beams and columns, again the load will span towards the stiffest load path, this being the column strips which you have modeled as beams. Despite the X-span being the shorter span, it might not be the most direct load path because beam running along the interior column grid is stiffer, so load will want to span onto this beam, increasing the moment in the Y-direction.

What program are you using to model the structure. Is it a general purpose FE program or a FE program that is specific for RC concrete design and how is Mxy distributed.

The FE contours look consistent with the moments that you would get from an equivalent frame analysis. The positive moment in the X and Y direction is fairly consistent across the strip.

What do the magnitudes represent, are they the values of the moments to design the middle/column strips or are they peak stresses?

Designing for moments is only 20% of the equation. How well does this program calculate deformations and long-term deformations of the structure?

 

[miecz]

Why are maximum moment in the slab at the mid span of the walls and not in the intersection of the walls?

Think of horizontal strips of slab. When a strip is far away from the horizontal wall, the strip acts as a continuous beam. As the strips near the horizontal wall, more and more of the load is caried by that horizontal wall. The strips carry less load in the horizontal direction, and the slab moments over the vertical wall go down.

 

[mar2805]

Its clearer now. Thanx!
But one thing thats I dont still fully get,if you look at my pictures and if you look at the positive bending moment diagrams you can see that I have very lagre positive bending momet under the beams?!
I dont get this?! I mean I understand the max postive moment somwhere in the middle of the bay, but under the beam (support)*!?

 

[asixth]

Maybe if you look at the deflections diagram and visualize the curvatures it will make more sense. If the slab has a positive curvature and produces a "dish" like deformation then the structure will be experiencing positive moment bending.

 

[miecz]

When I look at the plots, I see negative slab moments over the beams. For Mx: -0.84, -3.44, -3.16, -3.44, -0.84. For My: -16.31 and -19.56. Am I missing something?

 

[mar2805]

@asixth
Yes youre right the slab together with beams is deflecting and is producing a dish. But I would never even imagine positive moments to appear there and to be ohnest not that big!
I tried changing beam cross section from 20/40 to 20/50, those postive moments under the beam are getting smaller and smaller wich I guess is logical since the beam will become stiffer it will not let slab to have to big deflection, wich will also result in smaller negative moments over the support. Very nice ;-)
@miecz
Yes you are seeing right. Why do you ask this?

 

[mar2805]

One more thing that bothers me.
There are numerous situations where RC frames have masonry walls in them.
Beams that make frame are usualy poured over the masonry walls and are far stiffer then ordinary beam, since they cannot deflect because of the masonry wall beneath them.
My dilema here is, how to model this walls?
I mean if you look at my pictures attached in the first post you can clearly see different behaviour of slab thats supported on walls and bemas, wich results in different internal forces.
The problem here is that everyone I asked always say the same thing:"Ignore masonry walls,model them as line loads on beams".
No way this will represent thru behaviour of the structure!

 

[miecz]

mar2805

I misunderstood your question.

 

[paddingtongreen]

mar2805, I question your statement that the beams are placed after the masonry wall is in place. I've never seen it done that way, the formwork contractor need space for the formwork, the masonry is fill in.

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

 

[mar2805]

@paddingtongreen
Yes this is done in my country very often for object up to 4 storeys. How would you model this? Maybe override beams stiffnes and make it stiffer?!
For your sitiation when the walls are filled after yes, this is clean situation of modeliimg masonry walls only as line loads.

Someone here asked aboth Mxy distribution. Is this so called taking Wood and Armer effect into account when calculating required reinforcing area?
The torrsion in plates has an act of lowering the positive moments in the middle of bays but because of it their is uplifing effect present in the corrners.

 

[mar2805]

Guys anyone?!
One this also. I saw that a lot of people are describing slab to walll connections to be pinned?
Should this be modeled as fixxied connection if the story walls keep runing up?

 

[rowingengineer]

It is very hard to get a slab connection to wall to work at 100% (when i say hard almost impossible), That is why a lot of engineers will model a pin type connections. see below I have attached some threads that discuss this in more detail that you may wish to look over. Wood armer is also discussed, wood armer is one of the methods commonly used to take Mxy into account to do reinforcement design from finite element modeling.

thread507-254697
thread507-235070

as for the post regarding beams over masonry walls, Depends on what you are doing with the masonry wall in my opinion. Are you using this as a shear wall and reinforcing accordingly? Or is this just infill wall poured at the same time at the beam? is to act with the beam to form a shear web? The modeling should reflect the situation. I also assume your in a low seismic area?

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

 

[mar2805]

argggh
Guys Im having a bit of a problem visualing all this in my head :-(
First of all, what boders me is the usage of elastic FEM analysis that we use in day to day programs and the forces that derive from it.
We are designing structures to their limit state.
What does that mean in tearms for concrete structures?
For me thats compleatly ignoring concrete section thats in tension because it has cracked and calculating requried reinforcement area in tension that will ensure structure ductile failure.
FEM is an elastic method that doesnt take cracking into account, and this is where it gets messy for me. If it doesnt take the cracks into account why do we take these forces into design. They are far greater then the cracked section may even take!
Second of all, since we are talking limit state here and assumuing section has cracked, it must also rotate and cannot be fixed anymore, right?! If it rotates then redistribution of moments will appear meaning that the negative moments derived from elastic FEM analysis are egsaturated and that the positive moments are smaller.
Rotation and redistribution will only be possible if the section has abbilty to act ductil wich is ensured by limiting the postion of the neutral axsis.
So what now?!"!("!!)/ How can this all stand?!?!
Designing with forces derived from elastic analysis on the section thats fully cracked and doesnt act elasticly anymore?! How can you even tell that the eg. moment that you got from the analysis will ever be even possible in a section? eg. edge corners in flat slabs usualy have ultra high moment peaks.
Another thing I read here is reducing slab beams torrsional stifness to minimum. o.k. If I design a member to its limit state wich is a section fully cracked then its logical to reduce its stiffnes to minimum since for experiments have showed that even for smallest cracks the torsional stiffnes for beams reduces significant. So doing this is no mistake. What we are basicly doing here is disabling beams to transfer load from slab via torrsion, right?
So this will mean that we have increased Mx an My bending moments in a slab member since there will be no negative moments on the slab beam connection. Beam has no torsional stiffnes so it will freely rotate around its normal to cross section face wich wont couse any negative moments in the slab.
But why slab-wall connection as pinned?! Doesnt wall have a greater stifnees then a beam?
Im looking this as a beam to column connection wich is fixed...and this might be my problem!
Guys please help me to visual this in another way and please comment what I said here.

p.s. another question how to model beam to wall conection then? Als0 pinned*! This woule be maddnes?! A beam ending in a wall that has a lenght of 2 m in the direction of the beam, would be zillion times stiffer! But what when a beam is ending in a wall like a flat slab rests on a column?! Pinned,fixed Im starting to halucinate now ;-)

 

[rowingengineer]

mar2805,
Would be a good idea to reply to the forums that are discussing the issues, but i will say a little here.

Part 1. The plastic v's elastic analysis:
You are right to be concerned about using a FEA mode for a plastic type material. For instance you have a column in a flat plate design, the elastic analysis will show you that about 70% of the peak moments are located very close tot eh column. But now you need to use your code interpretation of columns and middle strips to distribute this high moment over. thus that your reo is not located only over the columns.

Part 2 The plastic v's elastic analysis
your program to be able to give you correct defelctions needs to take into account the concrete behavior. thus the cracking will cause higher deflections. Creep and shrinkage also need to be taken into account. need to look at your FEA program very closely to see that all these are ticked off.

Last part, some rambling here couldn't follow all that well.
torsion, generally if you have a slab each side for the beam the slabs will ensure that you still have a negative moment.

wall to beam/slab connection: need to look at the detailing of the connection to the wall. generally the detailing limits the effectiveness of the wall connection.

recommend you invest in some books that discuss these topics more in depth. take a look at this thread for good books thread507-251364



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

 

[mar2805]

Part 1.
The distribution of this peak moment is due to fact that the section at the column face will probably crack and that the rest of the slab will have to carry the load (lets say a strip of 1m width)

I also have these peak moments present in a two way slab supported with beams that lie on columns. Right above the intersection of eg. 4 beams that are supported on a column. there is a peak moment also. Do you think it would be wise to distribute this moment also?

Slab resting on beams will ensure negative moments?! Not so shure if you right here. Negative moments will appear only if the beam torrsional stiffnes is not ignored. We are talking edge beams here ofcourse!

Lots of book there. Can you give me few on the fly that adress this problems.
Thanx a lot

 

[rowingengineer]

mar2805,
1. What part of the world are you based, this will help in giving some direction in your design. There are rules for determining the 1m, not just saying 1m because it sounds good.

How are you intending for your beams to work? T, L or rectangle?

Edge beams, here it your choice in how to model, you can assume stiffness and take moment by torsion, but you need to ensure that the cracking moment is not exceeded for deflection and strength. I would always set my beam stiffness to zero.


Books again where in the world are you?






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

 

[mar2805]

1. Europe. I said 1m just for an simple egsample. I know my code and this is not what it says. We have to calculate the effective width of the column strip.
But what I wanted you to comment is my statement "The distribution of this peak moment is due to fact that the section at the column face will probably crack and that the rest of the slab will have to carry the load". I guess this statment stands.

2. The beams are acting like a T beams in the slab. Edge beams are acting like L shape with slab

3. As for torrsion you said "but you need to ensure that the cracking moment is not exceeded for deflection and strength"
Can you explain this a bit to me:-(
How do you know your cracking momet?!