RELIABILITY ANALYSIS OF A PRESTRESSED CONCRETE BEAM

The Reliability Analysis of a Prestressed Concrete Beam (PCB) was presented using First Order Reliability Method and Eurocode 2 procedures to carry out the analysis. The results show that the safety of the PCBin bending decreasedfrom 2.9 to 1.0 and 3.1 to 2.6 as prestress force and the depth from the extreme compressive fiber to the neutral axis of the beam increased from 20kN to 100kN and 150mm to 350mm respectively, therefore the PCB is safer at low prestress force and depth to the bottom layer of the beam. The safety of the PCB increased from 1.52 to 2.5, 1.52 to 2.5 and 0.1 to 2.0 respectively as depth to the neutral axis, area of the compressive reinforcement and eccentricity increased from 150mm to 350mm, 800mm2 to 1200mm2 and 100mm to 300mm respectively. The safety of the prestressed concrete beam remained constant at 0.1 with eccentricity of 100mm, 0.8 with eccentricity of 150mm, 1.3 with eccentricity of 200mm, 1.7 with eccentricity of 250mm and 1.95 with eccentricity of 300mmas effective widths and load ratios of the beam increased from 100mm to 300mm and 0.2 to 1.0 respectively in shear. Increase in dead load and span resulted to a corresponding increase in the safety of the beam in deflection. The best value of eccentricity for a reliable prestressed beam is within the range of 200mm to 300mm. It was also observed that the variation of depth to the bottom layer and prestressing force of the prestressed beam in bending are at equilibrium at 60kN and the safety index is 1.83. Also, the target safety indices considering bending, shear and deflection failure criteria of the prestressed beam were obtained to be 2.01, 1.526 and 4.716 respectively.

Prestressed concrete beam is one of the most widely used construction element in bridge projects around the world. It has rent itself to an enormous array of structural applications, including buildings, bridges, nuclear power vessels, Television towers and offshore drilling platform (Antonie,2004).

Prestressing involves inducing compressive stresses in the zone which will tend to become tensile under external loads. Thesecompressive stresses neutralize the tensile stresses so that no resultant tension exists (or in only very small values). Cracking is therefore eliminated under working load and all of the concrete may be assumed effective in carrying load. Therefore lighter sections may be used to carry a given bending moment and over much longer spans than reinforced concrete.