Effective moment of inertia for predicting deflections of prestressed UHPC beams

The bridging mechanism of steel fibers in ultra-high performance concrete (UHPC) significantly enhances the post-cracking stiffness of prestressed UHPC beams. Despite this beneficial effect, most existing design guidelines neglect the contribution of steel fibers when determining service deflections. This paper proposes a mechanics-driven yet practical approach for predicting the instantaneous deflection of prestressed UHPC beams under service loads. Firstly, a validated moment-curvature procedure is developed to explicitly capture the steel fiber bridging effect in analyzing the tension-stiffening behavior of beams at service loads. The analytical results reveal that prestressed UHPC sections exhibit an almost fully sustained tension-stiffening response. Subsequently, a curvature integration-based method is employed to derive design equations for the effective moment of inertia of prestressed UHPC beams. Then, a critical section-based effective moment of inertia is recommended for design purpose. Finally, the proposed approach is evaluated by comparing its predictions with experimental data from 38 prestressed UHPC beams. The mean values of experimental-to-predicted deflection ratio for two selected service load levels are 0.914 and 0.952, with the corresponding standard deviations being 0.173 and 0.198, respectively.