$11\mathrm{.}6$ Moment Modification or $C_m$ Factors:

D An entirely different situation is considered in Fig. $11\mathrm{.}5,$ where the end

moments tend to bend the member in reverse curvature.

The initial maximum moment occurs at one of the ends, and we shouldn't

increase it by a value $P_{nt}$ that occurs some distance out in the column,

because we will be overdoing the moment magnification.

D The purpose of the modification factor is to modify or reduce the magnified

moment when the variation of the moments in the column is such $B_1$ that is

made too large.

If we didn't use a modification factor, we would end up with the same total

moments in the columns of both Figs. $11\mathrm{.}4$ and $11\mathrm{.}5,$ assuming the same

dimensions and initial moments and load.

Modification factors are based on the rotational restraint at the member ends

and on the moment gradients in the members.