Prefabricated roof elements: knowledge and experience accumulated in over twenty years

Roof elements are a highly technological prefabricated product of Italian industry. Though a large variety of shapes are offered on the market, almost all production is characterized by thin-webbed open cross-section profiles. In the last twenty years, both the design and the technological aspects have evolved the production of these structural elements. A wide experimental experience on their structural behaviour has highlighted the effectiveness of beam theory in the prediction of serviceability and ultimate limit states, but also a significant interaction between the longitudinal and the transversal bending at failure. The interaction is due to second order effects, and becomes not negligible when the longitudinal bending curvature approaches the onset of yielding in the pre-stressed reinforcement. Owing to the open cross-section and to the reduced thickness of the inclined wings, a shape loss can anticipate the longitudinal bending collapse. A simple design model is proposed to predict this collapse. An extensive experimental investigation was also carried out on the use of Steel Fibre Reinforced Concrete to substitute the diffused reinforcement in prefabricated long-span covering roof elements. In the paper the mechanical behaviour up to failure of a traditional R/C element with that of SFRC elements with reference to a shed shape are discussed. The SFRC elements are characterized by two types of fibre and two concrete matrix strengths.