def yield_displacement_wall(phi_y, heights, max_height):
    """
    The yield displacement of a concrete wall. Eq. 6 (Sullivan et al. 2010).
    :param phi_y: yield curvature
    :param heights: height
    :return:
    """
    return phi_y * heights ** 2 / 2 + phi_y * heights ** 3 / max_height
