Definition of Effective Depth of Slab/Footing

[MrFurleyEIT]

Dear all,

I need to seek clarity on the definition of d, effective depth for a slab or a footing. I have always thought that d is the depth from the top (or bottom) of the slab/footing to the center line of reinforcement.

Now, one of my colleagues is saying that d is actually the distance from the top (or bottom) of the slab/footing to the center of tension reinforcement. Golly, who is right?

For a doubly reinforced slab, for instance, with two lawyers of reinforcement at top and at bottom, is d, then the distance between the top (or bottom) to the center line of the two layers of reinforcement (my contention), or is the the distance from the top (or bottom ) of the one bar that is in tension (my colleague's contention.)

Could you please provide reference(s), too, if possible?

 

[csd72]

Look up section 10.0 in ACI318 for the definition of d.
The compression face bars have no relevance for d (although they do have relevance for the moment capacity).

If you have two layers of tension reinforcement then the d is to the centroid of the two tension layers.

csd

 

[PSlem]

Just to add a little more, subtract 2" off the bottom for unevenness and contamination.

 

[Lion06]

It is teh distance from the top/bottom of the slab to the centroid of the tension reinforcement. If you have (2) rows with and equal area of steel in each row then it would be to the centerline of the (2) layers. If, however, you have (2) layers of reinforcement and they don't have equal areas then it will not be to the centerline of the (2) layers. It should be pretty easy to figure out.
I am not following your colleague's contention very clearly. Can you please explain it again? It almost sounds like (based on what I can make out of it) he is taking d as the location of checking strain for a tension/compression controlled section.