The program is to calc any beam, but I am trying to find out the optium size of a medium strength concrete beam with a Ultimate strength (in the book) of 4ksi. Everything appears to be calculating properly, but I am confused.

As I increase the heigth of the web, the calculated stress lowers. This makes scence as stress is force over area, and there is more area. As I shrink the material in the beam, web, flange, height,etc., the moment of Inertia shrinks and being the devisor, gives a higher stress number.

So what does this mean? I am assuming my calculated stress has to be below the ultimate stress in the book (college Mech of materials book). Is this true? I would expect that increasing the web heigth would increase the strength, and that the calculation gives me the stress at the x-x nutral axis. Also that this calculated number should not exceed 4ksi or the concrete beam will fail.