Cold-formed trapezoidal sheeting of thin steel plate is a very popular product for building construction. It combines low weight and high strength and is economical in use. Current design rules, which predict sheeting failure for an interior support, do not provide sufficient insight into the sheeting behaviour, and can differ up to 40% in their predictions. To develop a new design rule, this thesis presents new experiments in which first-generation sheeting behaviour is studied for practical situations. The experiments show that after ultimate load, three different post-failure modes arise. Mechanical models have been developed for the three post-failure modes. These models can help to explain why a certain post-failure mode occurs. Finite element models were used to simulate the experiments. Studying stress distributions with finite element simulations, it can be seen that there are only two ultimate failure modes at ultimate load. One of these ultimate failure modes is not relevant for practice. A mechanical model has been developed for the other ultimate failure mode. This model performs as well as the current design rules, and it provides insight into the sheeting behaviour.