hopt
The optimum height hopt is the height at which the concrete is subjected to a compressive strain of 3.5‰ and the steel to a tensile strain of 10‰, in other words the height at which both steel and concrete are used in an optimum way. So if you would choose the beam height would be equal to the hopt, the steel and concrete are optimally utilized.
hmin
The minimum height hmin is the height which just meets the requirements to resist to the loads without needing any compression reinforcement ; in other words, if we should choose a smaller height the beam would collapse by smashing the concrete if we did not foresee compression reinforcement. The steel is just at yield strength.
hopt and hmin are not visible?
hopt and hmin are only calculated and only visible when h < hopt.
bmin
When the cross section does not meet the requirements to resist the shear force, the minimum width at which the concrete compression struts in the classical truss girder analogy of Mörsch just fail to get smashed.
