A prestressed TT beam had been designed considering the ultimate limit state of bending, and considering a temperature distribution given by a graph which was valid for a section with a similar shape. When tested in a furnace, it proved to have a fire resistance of only 79 minutes instead of the 2 hours foreseen by the calculation. An investigation was made, with the help of numerical modelling, to explain the reasons of the failure and to design a new specimen. Some modifications were introduced to increase the resistance to bending forces, to shear forces and to the slip of the tendons. A modified specimen behaved satisfactory in a second test. The formula proposed in Eurocode 2 for the shear resistance of reinforced concrete elements at ambiant temperature has been adapted for the elevated temperature situation and its application allowed to accurately predict the failure mode observed in the fiist test.