In concrete design, one of the most basic concepts is the determination of effective depth, d. This is one of the first things we all learned in our concrete design class. But just like with so many other code provisions, something simple can come into question very quickly. Here is a recent question that we received on the determination of effective depth, d.

Q: This is related to tolerances on the effective depth, d, for concrete beam design. The question is, does the design engineer need to explicitly consider that ACI 318-14 Table 26.6.2.1(a) permits a tolerance of 0.5 inch on the effective depth, d? In other words, should the design engineer reduce his or her theoretical effective depth by an additional 0.5 inch to account for tolerances in construction? I always assumed that design methodology and assumptions in ACI 318 would consider these tolerances inherently. For example, if the tolerance is permitted to be 0.5 inch on d, then the beam may have a strength reduced by say 5% and still be okay. I have assumed that there was enough conservatism in the φ-factor, material overstrength, etc that it need not be considered by the design engineer in determining d.

In reading ACI 318-14 Section R26.6.2.1, it states ”This provision is included in the design information section because tolerances on d should be considered in member design”. That threw me off. That suggests that designers should reduce their d in their calculations by these tolerances. Is that how it should be applied?

A: (in Dr. Ghosh’s words) I’m totally with you. I always understood that design was supposed to be done with nominal dimensions. This is how I was taught. This is how I have taught it. This is how I have always done it. I frankly have never seen it done any other way. But my sample size is obviously not large. Maybe there are smart, conservative engineers out there who explicitly account for tolerances in design. I wouldn’t know. However, d is not the only item with a tolerance on it. Design is liable to get awfully complicated if we needed to consider tolerances on everything. Would we always even know which way is conservative? It seems to me to be an impractical proposition.

Until this problem can be fixed by ACI Committee 318, our advice to designers would be to ignore the aforementioned ACI 318-14 Chapter 26 commentary. Chapter 26 is supposed to be the chapter on construction aspects. This is not where a designer is supposed to look for design guidance.