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Concrete Beam Discrepancy Between Moment Check and Steel Area Check

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Why does the Concrete Beam module say to "Check As Min Limits" when it also says phiMn > Mu?

 

The calculation of phiMn is done with a very detailed dedicated solver that considers the location of the neutral axis under the applied load and the beneficial effects of compression steel, if any exists.  

 

The minimum steel calculation looks at minimum steel, (4/3) of minimum steel, and the flexural steel required to satisfy the basic formula of phiMn = phi As Fy (d - a/2).  In other words, the As Required column in MVD Summary > LRFD Shears & Moments is conservatively ignoring the effects of any compression reinforcing.

This difference means that it is possible in some instances to see a result that shows that the section has the necessary moment capacity, but appears to fail the reinforcing area requirements.  In these situations, it's necessary to apply engineering judgment to verify that the design meets minimum steel requirements for ductility.

 

 

Why does the Concrete Beam module report a different area of steel required or a different reinforcing ratio required as compared to hand calcs based on first principles?

 

In order to handle complex conditions like compression reinforcing, multiple layers of reinforcing, and shapes other than simple rectangles, ENERCALC uses a powerful reinforced concrete section solver that evaluates the strain diagram for the cross section.  The strains can then be related to stresses, which dictate a moment capacity.

 

Because the program uses this very precise solver to arrive at strengths, the required reinforcing ratio that it calculates will almost always be different than values from a simple hand calculation based on the common assumptions like the depth of the compression block and the magnitude of the concrete stress.