Design method and experimental investigation of two-stage prestressed precast continuous beams

Excessive prestress will lead to significant negative deflection and even precast prestressed beam unit cracking during production and transportation. Meanwhile, many rebars must be installed to effectively control the occurrence of cracks at the beam ends during the service stage, resulting in difficulties during the construction process. To effectively distribute the initial tension of prestressed strands in precast prestressed beams, this paper proposed two-stage prestressed precast continuous beam (TPPCB) to balance the load during construction and service stages by tensioning different prestressed strands. This study conducted a comparative test between the TPPCB and cast-in-place prestressed continuous beam (CPCB) during the construction stage and under vertical loading conditions. The test results indicated that, compared to the specimen CPCB, the specimen TPPCB exhibited a lesser negative deflection following the completion of the construction stage. When the specimen TPPCB is designed according to the mechanical model of two-stage tension prestress, its mechanical performance can be similar to those of the specimen CPCB in both construction and service stages. The crack range at the top of the middle support beam was relatively smaller in specimen TPPCB than in specimen CPCB. Furthermore, the crack width at the top of the middle support beam in TPPCB is significantly greater when compared to specimen CPCB. Compared to the specimen CPCB, the mechanical performance of the specimen TPPCB resembled those of a two-span simply supported beam once the penetrating cracks were formed. Due to the concentrated and significant cracks at the middle support section of specimen TPPCB, the premature failure of this region led to the structural failure of the continuous beam. Consequently, the specimen TPPCB exhibited stronger deformation capabilities. When the specimen TPPCB was subjected to dead and live loads, the compressive stress experienced by the fibers at the top of the concrete in the midspan of precast prestressed beam unit was the highest, making it the critical and vulnerable section. The high compressive stress in the top concrete of the midspan section concrete of the precast prestressed beam unit resulted in a vulnerability of specimen TPPCB to failure due to midspan section concrete crushing during service stage or extreme conditions. Design equations and design processes are presented according to the design goal for specimen TPPCB.