> For the complete documentation index, see [llms.txt](https://docs.enercalc.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.enercalc.com/sel-faq/using-the-software/calculation-modules/beams2/concrete-beams/effective-depth-shear.md).
# Effective Depth for Shear
Why the effective depth d used for shear can sometimes be reduced when top reinforcement is added to a concrete beam, how to confirm it, and how to control it.
## Why is the effective depth $$d$$ lower than expected?
Under ACI 318, both $$A\_s$$ and $$d$$ refer to tension reinforcement:
ACI 318 variable definitions
The Concrete Beam module uses the neutral-axis location to determine which bars are in tension. Bars below the neutral axis contribute to $$A\_s$$.
On the **General** tab, **Measure effective depth 'd' for shear to centroid of...** defines the tension centroid. The selected method changes the reported $$d$$:
* **Tension reinforcement bar area:** Measures $$d$$ to the centroid of the tension reinforcement area. This default method is easier to check by hand. Every bar below the neutral axis affects $$d$$ in proportion to its area, regardless of its stress.
* **Tension reinforcement resultant force:** Measures the effective depth $$d$$ from the tension face to the centroid of the *resultant tension force*, weighting each bar group by its tension force as determined from strain compatibility with the section's neutral axis. For sections with bars distributed over the depth of the section, the centroid of the tension bar area may differ from the resultant tension force location because bars closer to the neutral axis may be at a stress lower than $$f\_y$$. This option can be more accurate in those situations, and may also provide a more favorable value for $$d$$.
The module then uses $$A\_s$$ and $$d$$ to calculate the reinforcement ratio $$\rho\_w$$ and the concrete shear equations in ACI 318-19 Table 22.5.5.1.
## How do I confirm what the program is doing?
Open **M-V-D Summary** > **Cross Section Values**, then check the neutral-axis location at the affected section. If the neutral axis is above the top reinforcement layer, every bar in the cross section is in tension. The program therefore includes all bar areas in $$A\_s$$, and it measures $$d$$ to the centroid of the full bar group. This can cause a reduction in the calculated value for $$d$$.
Cross Section Values showing the neutral-axis location
## How do I keep top bars from reducing $$d$$ for shear?
Use either of these approaches:
* On the **General** tab, select **Tension reinforcement resultant force**. This method weights each bar group by force, rather than area. Top bars that are in tension but sitting near the neutral axis have smaller strains, stress, and force, and therefore have less influence on $$d$$.
Effective-depth measurement using the tension reinforcement resultant force
* Alternatively, on the **Beam Span Data** tab, set the start and stop extent for each top bar set. Keep top bars only in zones that require negative-flexure reinforcement. Removing unneeded top bars changes which bars contribute to $$d$$ in those zones.
Start and stop extents for top reinforcement bars
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