def _get_v1(self, imt):
        """
        Calculates Bradley's V1 term. Equation 2 (page 1814) and 6 (page 1816)
        based on SA period
        """

        if imt == PGA():
            v1 = 1800.

        else:
            T = imt.period
            v1a = np.clip((1130 * (T / 0.75)**-0.11), 1130, np.inf)
            v1 = np.clip(v1a, -np.inf, 1800.)

        return v1