Area of steel per m run of concrete slab

[cyphos168]

Hi all,

I have a problem I would like to have better understanding of. Its a flat slab design supported on columns. I am using Protastructure to analyse the slab by FE method. The result is showing the contour of the top hogging reinforcement near the column.

Here is a snapshot of the top reinforcement.

How should I tackle this in terms of providing reinforcement to the slab? There is a high concentration of steel required at two corners while the rest of the area only require minimal reinforcement. Would you apply the maximum steel required based on this design for the column strip? Thanks in advance.

Al

 

[cyphos168]

Ok. I have been reading some topic on similar subject and the method of averaging the peak moment was mentioned several time. I till read through some of the resources mentioned.

 

[HTURKAK]

I suppose 1300 mm2 per meter ... Not sure how you did you calculate this amount but 1300 mm2 / m is not high for flat slab..

Apparently the snap shows M11 or M22 at joints and not the averaged amount per meter..

You may post more detailed info... plan of the slab ( spans , loading..) to get better responds.

 

[cyphos168]

Hi HTURKAK,

Thanks for the response. I will post more info shortly but would like to just say the contour you see is the contour of top steel area in direction 1 or 2. Area of steel isn't that much different. Suffice to say the area of 1300mm2 per m run is the area of steel required for that corner. Not Moment. Moment we are looking at around 60kNm or thereabout. I have gone through some resources as advised in this forum and picked up a few pointers on this issue. Getting a better understanding of how to apply the steel over the column strips.

Additional info. This is a design of a food processing warehouse ground slab (200mm thick) supported on pile caps. For the purpose of the design I just modeled the structure as 200mm flat slab supported by pile cap size columns. Span lx=4.25m ly=5.25m. Dead load=1kN/m2 and Live load=10kN/m2.

Al

 

[jayrod12]

The one concern I would have about how you modelled it is if moment is going into your pile cap. If it is, ensure that the pile cap, and therefore the pile, have the capacity for the moment. If not, adjust your stiffness sharing to have the slab to pilecap joint act as pinned.

I would agree that 1300mm2 over a pile/column isn't unruly by any stretch. That's essentially 6-15m bars meaning 15m @ 200, I'm more of a fan of 15m @ 150 in that scenario, but that's also because often my regular top mat is at 300 o/c so just halve the spacing over the supports with additional steel. 15m @ 150 gives you 1333mm2 per metre.

 

[WARose]

Whenever I've done a slab by FEA, I've never trusted the computer to generate reinforcement....I typically rely on it for the forces and I make judgment calls as to the distribution. (When it comes to concentration.) A good justification for that is: I've rarely seen FEA software that will combine the orthogonal & twisting moments properly. (But I am not familiar with "Protastructure".)

Just eyeballing this case.....I don't know how "fine" your mesh is....but if it is giving you really high concentrations, you may want to have smaller plate elements. A lot of times when I do a slab, I have the elements at 1'x1'. It just makes reading the output easier (and inputting loads). After that, some judgement comes into play.