placement of Flat slab extra negative moment over column 2

[Steel Inspector]

Hello,
in beams, extra negative reinforcement at columns would be placed under the main upper reinforcement of the beam. in flat slabs however, the general practice ( at least in this part of the world ), is to place the extra negative reinforcement at columns over the slab top mesh. what happened in this case is that we would have to pour more concrete to provide concrete cover for the extra reinforcement steel. a very common example would be that we have an extra negative moment at columns consisting of a 20 mm diameter bar on both directions. if this extra steel is placed over the main slab top mesh, that means that we need to pour extra 4 cm of concrete to provide concrete cover for the extra steel. ofcourse this means an increase in slab thickness of 4 cm that is not accounted for during design. my question is. why not instead of that put the extra negative moment reinforcement under the top main mesh? just like in beams? wouldn't that be structurally correct?

 

[dik]

It's been a while since I've done a flat plate or flat slab... but a similar approach here. With RJC we used to proportion the top steel into two parts with one part uniformly distributed and the other part concentrated... this distribution was based on model testing by the late Per Christoffersen. In addition for narrow rectangular columns I used to chuck a couple of #8 or 25M into the bottom, centred on the column.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Steel Inspector]

" other part concentrated... "
the concentrated part, where would you place it? over or below the distributed part?

 

[hokie66]

Place all the bars in two layers, whether 'main' or 'extra'. You don't want extra layers in a flat slab.

 

[rapt]

Ask thew designer to detail it correctly, then you will know what he has assumed in the design.

Agree with Hokie on where it "should" be designed to be.

 

[dik]

In the same level and concentrated over approx half the width of the distributed part... there were formulae for this distribution which have sunk into the grey matter, and maybe lost forever. The bot 25Ms were located in a separate layer at the bottom.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

Correct... and if memory serves, part of the distribution was based on the column width.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Steel Inspector]

hookie66 ,
I am not the designer, but wouldn't that be uneconomical, i mean if their main mesh is 14mm and their extra was 20mm, converting that to one single layer would probably mean a mesh of 22mm? isn't that costly?


anyway i am just trying to find a solution for the placement of steel as it is without changing the design, changing the design is you might say bureaucratically difficult.

 

[Steel Inspector]

rept:
same answer, bureaucratic difficulties will get in the way.
i am on my own.

 

[dik]

Reinforcing is with rebar... not mesh...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[hokie66]

The use of the term 'mesh' is indeed confusing. Perhaps a better description of the slab would help. Thickness? Dimensions of drop panels? Main reinforcement both directios? Extra bars both directions?

 

[Settingsun]

You probably don't need a design change, just a clarification from the designer. As stated above, the additional bars generally go between the 'main mesh' bars. No additional layers, no increase in concrete thickness, and no change of bar size from what was shown on the drawings.

 

[Steel Inspector]

this is an example.
in the plan attached, slab thickness is 25cm. slab reinforcement consists of two parts.
the first part is a main top and bottom mesh( two layers ) that covers the whole slab. the main bottom mesh consists of 5 bars of 14 mm dia for every meter on both directions. the main bottom mesh consists of 5 bars of 12 mm dia for every meter on both directions.

the second pat is an extra reinforcement to account for high stress concentrations, at column locations in the top layer, and at mid-span in the bottom layer. now, my question is about the placement of the extra reinforcement at column locations in the top layer. what is commonly done here is that it will be placed above the layer ( main top mesh ), but the top mesh is already placed with only 20 mm concrete cover from the top surface of the slab, so adding extra steel of 20 mm dia both directions on top of it and providing minimum of 20 mm concrete cover for that extra reinforcement means that we will exceed the slab boundaries by 40 mm. so the slab thickness at site will be 29cm instead of 25cm as per the drawings. which i can only assume to be incorrect, as this increase in slab thickness at site will result in an increase in dead weight that is unaccounted for in design. why not instead of that just place the extra reinforcement at column locations under the top mesh ( top layer ), just like we do in beams where any extra negative reinforcement at support is just layed in a second layer under the main top beam reinforcement. at the end, beams and slabs are both flexural members.

 

[hokie66]

As indicated above, place the 'extra' bars in the same layer as the 'main' bars running in that direction. You have 'extra' bottom bars as well as 'extra' top bars. One layer in each direction, both bottom and top.

 

[Settingsun]

The main bars are black and the additional bars are red.

 

[Retrograde]

what is commonly done here is that it will be placed above the layer

That might be common where you are - but it is wrong.

 

[Steel Inspector]

hookie66, steveh49:
Thank you, i got it. but that still would increase the slab by 1 ~ 2 cm. that increase is OK?

Retrograde:
thank you.

 

[hokie66]

I cannot see why the added bars would change your slab thickness. The bottom bars will be chaired off the form, so no change. There may be a slight change in the position of the top bars if the chair size is not figured precisely, but hey, you are not building a piece of furniture.

 

[Steel Inspector]

hookie66,
correct. thank you very much

 

[Trenno]

Agree with Steve's sketch. A typical mat spacing and additional bars spacing of 200mm usually works well (i.e. 100mm between red and black bars).