The balanced cantilever construction method was first introduced for long span bridges in the 1880s and was first used to build the long span concrete box girder bridge segmentally in the 1950s.While the method represents well-established technology,this paper presents the analysis and design of two box girder bridges that will be constructed by balanced cantilever methods with unbalanced spans.These parallel,620-foot-long,three-span,segmental cast-in-place post-tensioned bridges will serve as approach bridges to the worlds longest floating bridge on Washington State Route 520,the use of transition spans is inevitable to accommodate relatively larger "floating" movements and therefore large reaction forces will be applied directly to the tip of the water side cantilevers.This presents specific challenges to a conventional balanced construction method because of concerns regarding high bending moments in the waterside piers.This paper demonstrates that with special span arrangements and an adjusted balanced cantilever method of construction,pier bending moments can be controlled to acceptable levels.The paper introduces superstructure design and analysis per the AASHTO 2010 Specifications,including the finite element model,longitudinal gravity load analysis,post-tensioning design and optimization,and time-dependent construction analyses throughout the entire design life of the bridges.The paper also discusses the findings of recent research on time-dependent effects that are incorporated in the bridge design.