Global Structural Performance and Repeated-Loading Effects of Steel–Concrete Composite Bridge Decks with Permanent Flat Steel Bottom Formwork

This study investigates the structural role of permanent flat steel bottom formwork in steel–concrete composite bridge decks from construction to long-term service through global numerical analysis and full-scale testing. A three-dimensional finite element model of a 3 × 40 m continuous composite girder bridge of the Beijing-Taipei (Jingtai) Expressway expansion project compared girders with and without the formwork during girder erection, concrete casting, secondary dead loading, and vehicle loading; a full-scale two-span specimen was tested under staged static loading, 2 million load cycles, and a failure test. The 8 mm formwork substantially reduced upper girder stresses: maximum top-flange compressive and tensile stresses fell by approximately 35% and 38% during erection and by 40% and 39% during casting, with deflection under dead and vehicle loads reduced by up to 3.9%. Increasing plate thickness from 6 to 8 mm clearly improved the response, while 10 mm gave limited further benefit. During repeated loading, cracks propagated, and stiffness degraded, but no visible steel–concrete interfacial slip was observed. After 2 million cycles, tensile strains in the midspan bottom plate and intermediate-support top reinforcement increased by approximately 13.4% and 21%, while initial flexural stiffness and ultimate flexural resistance decreased by approximately 10% and 9.7%, with residual resistance well above service-load demand. It serves as both construction formwork and a permanent structural component, informing design, plate-thickness selection, and durability assessment.