@staticmethod
def k639(breadth_ratio):
    """Evaluate the k-factor for Eq. (6.39), as implied by
        Table 6.1, on the basis of the ratio of the breadth of
        the side parallel to the eccentricity over the breadth of
        the other side.

        The k-factor represents the portion of unbalanced moment
        that is balanced by an additional shear-stress distribution
        along the critical perimeter.

        :param float breadth_ratio:
        :rtype: float
        :return: The value of the k-factor.
        """
    if breadth_ratio <= 0.5:
        return 0.45
    elif breadth_ratio <= 1.0:
        return 0.45 + 0.3 * (breadth_ratio - 0.5)
    elif breadth_ratio <= 3.0:
        return 0.6 + 0.1 * (breadth_ratio - 1.0)
    else:
        return 0.8
