Flat Slab Reinforcement Details 5

[damo74]

Hi folks. I'm designing an r.c flat slab and I need drop down panels over my r.c columns to prevent punching shear. Has anyone any suggestions how I should detail my steel reinforcing bars? Should I lap the bottom bars in the shear head with the top reinforcement in the slab or the bottom reinforcement in the slab?

 

[JedClampett]

Is there a reason you can't make your drop panel deep enough to avoid shearhead reinforcing? I've never used shearhead reinforcing. I think it would cause a congested reinforcing region just where you need it least.

 

[rapt]

If you are putting in drop panels to avoid punching shear problems, why do you need shear head reinforcement? Make the drop panel deep enough to not require punching shear reinforcement.

Is this a real drop panel (minimum L/3 in each direction) which can be used for flexure and deflection control or a small local thickening for punching shear only?

The bottom slab reinfrocement I would not normally step at a drop panel unless compression reinfrocement is needed at that column. I would normally continue the bottom reinfrocement through at the slab soffit level unless it is really required at minimum cover to the depth of the drop panel for some reason..

 

[damo74]

JedClampett; rapt,

Apologies for not making myself clear. Once I've inserted the shear heads I do not require punching shear reinforcement. I would like to place bottom bars in the bottom of the shear head. Any advice on whether I should lap these bars with the top or bottom mat in the slab? What logic should be used?

 

[JAE]

damo74 - when you say "shear head" are you referring to the drop panel itself? Or are you referring to a steel grid connected to the column to prevent punching shear? The reason I ask is that you said:

Once I've inserted the shear heads I do not require punching shear reinforcement

Shear heads ARE punching shear reinforcement.

 

[rapt]

I think you are mixing everyone up by calling it a shear head. The concrete shapes used at columns are
Capitals
Drop Caps
Drop Panels

The arrangement of steel sections added to increase the punching is a shear head.

If you want to place reinforcement in the bottom of the drop panel, the slab bottom reinforceemnt should extend a development length into the drop panel and the drop panel bottom reinforcement should extend a lap length past this slab bottom reinforcement which normally would mean that it would need to cog at the top and lap partially with the top reinforcement. This reinforcement would improve the punching capacity and ductility of the slab at the column seven though the codes do not quantify this punching improvement.

You have still not said if it is a true drop panel or simply a shear thickening that is sometimes called a Drop Cap.

 

[damo74]

Apologies once again. I think we use slightly different terminology in Ireland.

Here, drop panels can be known as shear heads. These are local thickenings in the slab around the tops of columns. For instance, in my case, my slab is a standard 400mm thick. However, this increases to 700mm thick around the tops of columns for a distance of 1.5m square.

Hope this makes sense. rapt has answered my last question with regards to the bottom reinforcement in the drop panel.

Thanks to all for the great tips by the way.

 

[JAE]

Most cases in the USA, for drop panels (shear heads) is not to reinforce them at all. The bottom steel of the flat slab itself extends to the column centerline and doesn't drop or sag into the drop panel.