def VD_531(delta_P, delta_S, n, FA = 0):
    """
    5.3.1 Load factor and additional bolt load up to
          the opening limit
    ---
    delta_P  : Elastic resiliance of the clamped parts 
               for concentric loading
    delta_S  : Elastic resiliance of the bolt
    n        : Load introduction factor
    ---
    FA : Axial load in the axial axis of the bolt
    ---
    """
    
    # For the theorical case of the load induction in
    # the bolt heat and nut bearing areas
    Phi_K = delta_P / (delta_S + delta_P)   # (5.3/4)
    
    # Load factor in the case of of concentric loading
    # and clamping
    Phi_n = n * Phi_K                       # (5.3/6)
    
    # Axial additional bolt load
    FSA = Phi_n * FA                        # (5.3/7)
    #
    return Phi_K , Phi_n, FSA
    #