When developing safety-critical systems, performing dependability analyses such as computing the reliability is of utmost importance.In the safety standard IEC61508, Markov processes are suggested forquantifying the reliability. However, real-world systems can not always beaccurately modeled as a Markov process. Semi-Markov Processes (SMPs)generalizes Markov processes to allow for more accurate models. It hasbeen previously suggested that a intuitive modeling approach of semiMarkov processes is to assign a timer to each possible transition. Thesetimers race to first reach zero which triggers the corresponding transition. However, some situations such as non-perfect diagnostic procedures cannot be modeled with these transition timers. As the first, andmain contribution, the theory of modeling SMPs with transition timers isextended with branching transitions, i.e. transitions with several possibleoutput states. The second contribution is tool support for dependabilityanalyses of SMPs modeled with branching transitions. A use case example of an automotive steering system modeled as an SMP with transitiontimers and with branching transitions is considered and analyzed.