A structural design is proposed for a fault tolerant steer-by-wire (SBW) system consisting of a principal and a standby SBW unit together with a manual backup. A fault tree is constructed for a top event, "complete loss of steering". Minimal cut sets are derived accordingly. Operation rules are derived for degraded system states with partial failures before causing a minimal cut set. A transition diagram is depicted to summarize the rules. Low level SBW control rules are also proposed for sensor failures of monitoring vehicle states. Experiments using driving simulator are performed to examine the operation and control rules from a safety point of view, indicating 1) fault tolerant drivability for steady driving including constant radius turns, and 2) potential vehicle unstabilities during unsteady pylon slalom driving when the SBW is switched into a manual steering.