I am working on an existing building with wide flange steel beam joists at the roof (simple span configuration). They are adding a significant amount of load to the roof. To increase the capacity of the beams, I am adding a steel plate (flat) to the bottom of the beams. Using Enercalc to find the properties of the modified beam I get two values for the section modulus (S). Speaking with another engineer I was told to use the higher value as the added steel will increase the tension capacity of the beam. Looking at my old text book it says that 'c' is the distance from the neutral surface to the extreme fiber and thus would always result in the lower value of 'S'. Please verify which of these is correct and if the second is correct is there ever a situation where you would use the higher value of 'S'.
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