Design Parametric Analysis for Plastic Neutral Axis of PSC Composite Girder Bridges Using High-Strength Concrete

In this paper, a parametric analysis of the design is conducted for a pre-stressed concrete (PSC) composite girder bridge utilizing high-strength concrete, focusing on the study of design approaches to meet the required plastic neutral axis depth. The current road bridge design standard, known as the limit state design method, serves as the basis for this study. The design variables encompass the compressive strength of the PSC girder concrete, the initial allowable jacking force of the tendon, strand strength, and the design compressive strength of the concrete bridge deck, along with the longitudinal reinforcement ratio. For the parametric analysis, standard cross-sections of the Korea Expressway Corporation EX girder, with a span of 50 m, were selected as the PSC girder sections. The process involved designing PSC composite girder bridges that satisfy the serviceability limit state, followed by an evaluation of whether the regulation on the plastic neutral axis depth is satisfied during the ultimate limit state. To address the time-dependent factors during construction stages, the study applies the age-adjusted effective modulus method, which considers the age of concrete. The design flexural strength is calculated by applying the strain compatibility analysis method, using the stress-strain curves of the prestressing tendon and concrete for cross-section design. Based on the results of the parametric analysis, it was confirmed that the plastic neutral axis depth increased as the PSC girder concrete design compressive strength increased, acting as a detrimental factor in complying with design regulations for the PSC composite girder bridge. As a method to meet the design criteria for the plastic neutral axis depth, the study evaluates that increasing the allowable tensile stress developed at the lower surface of the PSC girder under service load conditions is more effective than enhancing the design compressive strength of the concrete deck.