Distribution of peak support moments in a slab with irregular supports

[Aakalim103]

I am designing a slab for blast loads. The slab is supported on wall on the edges and two column and the middle span is unsupported. I have made a FEM model is the RFEM software and the moment results are attached for the the x direction. As can be seen, I am having very high moments at the supports and slab corners.

The software allows me to define a average region which averages the moments across the defined area. I have marked the areas where I am thinking of defining the average regions (in blue). The software also allows me to average the moments along the x-direction or y-direction. Now my question is that in what direction should I average the moments? Should I average (or distribute) the x-direction moments in the x-direction or the y-direction?

 

[r13]

Can you release rotation restrain of the corner nodes in both directions, and see the results. Another way is to increase size of the elements around the corners, if the mesh was too fine.

 

[KootK]

If you're going to redistribute moments, I don't see any logic in not doing that in both directions. You're only ever redistributing one dimensionally, perpendicular to the design strip. It is possible to redistribute moments longitudinally with a strip but, to my knowledge, that isn't something that software will do for you and isn't something that's common for slab design nowadays.

Over what width you should distribute your moments is another question. I've been getting mixed messages about this in recent years. In the past, we did significant redistribution and I still deem that a highly rational approach. That said, I've seen recent ACI documents suggesting that no redistribution should be assumed to minimize serviceability cracking. I feel that's a bit impractical and that it fails to take account of one of the truly great features of reinforced concrete. Given the nature of blast loading, I would have no qualms about applying significant moment distribution to such a design.

 

[Aakalim103]

@r13, I am not sure if the software will allow me to do that. I have line supports at the edges and I will have to release the moments for the whole line support. The slab is actually supported on walls and I have given some partial fixity to the line supports to simulate the wall supports.

 

[KootK]

With walls, I feel that there is often some inaccuracy in the modelling in that, usually, the software is assuming perfectly rigid hold down between the slab and the walls at all locations. This basically leads to the slabs kind of cantilevering from the wall ends and corners with nearly rigid back spans (less slab twist). If the true flexibility inherent in these situations were modelled more accurately, I suspect that you'd see the moments come down some.

 

[r13]

In most FEM program, you can manipulate joint connectivity to change stress flow, but I don't know yours. The reason for the high stress is mainly due to stress concentration, under which the corner is likely cracked, and the stress redistributed to the surrounding elements. Did you try to adjust the element, it might help to relieve the stress in those element (similar to assign distribution width).

 

[Aakalim103]

Thank you Kootk for your reply. I understand that both x direction and y-direction moments will be redistributed. But what I am confused about is in what direction should I redistribute the x-direction moment. Judging by your comment about distributing perpendicular to the strip, I believe I should distribute the x-direction moment in the y-direction. My next question would have been the width of the distribution strip, but since serviceability is not important in my case, I guess I can have fairly wide strips.

As for the walls, you are right that software will usually consider the wall and slab connection as perfectly fixed. But RFEM allows to model partial fixity for the wall supports so they are not completely fixed.

 

[KootK]

Judging by your comment about distributing perpendicular to the strip, I believe I should distribute the x-direction moment in the y-direction.

That sounds about right. It's a lateral distribution across your design strip(s).

 

[molibden]

Do a mesh refinement in areas of singularity and apply an average region with diameter around 50-75 cm. I do this all the time. These corners will always have singularities, I wouldn't worry about them so much.

 

[r13]

Agree the above.

 

[rapt]

Aakalim103

I wouldn't try to fiddle the model at all as others have suggested. Concentrations like those are standard in flat slabs.

The normal column/middle strip guidance in codes is a good starting point. This can be adapted to your irregular support layout fairly easily looking at support strips and span strips.

One thing you need to consider is that the design moments are not Mx and My. You also need to combine in Mxy using Wood Armer Method or some other in both directions.

 

[Aakalim103]

Yes I ended up making the column strips and middle strips and averaged the moments across those strips and it gave me a reasonably good moment distribution.

As for the design moments, the software has an option of giving the design moments and I design based on those (I am not sure if it uses the Wood and Armer method or something else), but for checking the basic moment distribution, I use the Mx and My moments. I find it easier that way.

Anyways, thank you all for your help!

 

[MSUK90]

Besides the recommendation by others, ACI 318 suggests to provide the reinforcement for moment carried through flexure by column in a particular effective width of slab. That should always be checked when distributing reinforcement. Do everyone practice that?

 

[KootK]

Sure, that's the 1.5d either side, right? I sometimes see that extended to 2.0d.

 

[MSUK90]

Sure, that's the 1.5d either side, right? I sometimes see that extended to 2.0d.

It's 1.5h(full slab depth), but that doesn't make much difference I believe.