Impact of link-slabs on the design of precast pre-stressed bridge girders

Abstract

Precast, pre-stressed girders offer a cost-effective solution for the design of bridge structures in the medium span range (span length between 12 m and 50 m). The girders are layed out as simply supported and connected by a cast-in-situ topping reinforced concrete slab. The cast-in-situ slab provides continuity between the spans over the open joint dimension between the girder edges. This short distance of the slab over the open joint is labeled as the link-slab and is usually neglected in the design. This report investigates the impact of the stiffness of the link-slabs on the design of precast, pre-stressed girders. Although its stiffness is relatively small compared to the precast, pre-stressed girder, its short length gives it rigidity which induces negative moments. If the link-slab is not designed to resist these negative moments, it will crack before it redistributes the negative moments to the precast girders which causes deterioration problems near the crack location. The study develops closed form solutions that can be used to determine the magnitude of the flexural moments induced. Due to the complexity of the equations, a parametric study is performed for standard AASHTO precast, pre-stressed girders in the medium span range and charts are developed to allow predicting the magnitude of the induced moments as a function of the parameters. Recommendations are finally made and practical solutions presented that allow the link-slab to be properly designed. The two computer softwars Wolfram Mathematica and LEAP CONSPAN are used for this purpose and the charts are plotted using Excel spreadsheets.