Effective depth Calculation

[LOKSTR]

When we calculate effective depth of a section, do we include allowance for variation in concrete cover or not?

e.g. beam overall depth=24"
clear cover reqd.as per code = 2"
Rebar size = 1"
Cover variation allowed as per ACI is +/- 1/2"

Eff. depth = 24"-2"-1/2"=21.5"

or


Eff. depth = 24"-(2"+0.5")-1/2" = 21.0"

 

[dcarr82775]

I do not include the construction tolerances in design. I would use the 21.5" from your example.

 

[slickdeals]

That being said, if I have 2 layers of steel crossing over each other, I assume the lower "d".

 

[pilesmakesmiles]

Sometimes I do take it into account if the design detail means the compression zone could be compromised in some way (for example the top of a reinforced strip footing where the contractor is likely to allow it to be exposed to rain before it has set).

Normally I don't take account of it.

 

[JAE]

Tolerance in rebar placement is dealt with using the required [φ] factors.

 

[dhengr]

Slick.... with two layers, I certainly might consider both “d’s” and use whichever one to my advantage, and then note that one as the bottom layer. Your approach is certainly conservative and avoids the note or inspection attention at that detail; as long as your ‘assume the lower “d”’ means the smallest “d” and not the bottom, lower “d”. Be careful what you say and how you say it.

 

[slickdeals]

@dh: You are right. I should have said smaller effective depth.

 

[BAretired]

I agree with dcarr and JAE.

BA

 

[hokie66]

Agree with others, but in some cases, I have based my effective depth for slabs on the available height of chairs rather than the specified cover.

 

[rowingengineer]

agree with hokie, use the hieght of steel based on chair height. while the ? factors help for strength, they don't help for deflection.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[rscassar]

Chaired height without account for tolerance. I do this with all aspects of concrete desi gm, there are also variations of compressive strength and e-modulus but the mean value is still the design value adopted.