Summer 2019

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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C O N C R E T E B R I D G E T E C H N O L O G Y 36 | ASPIRE Summer 2019 by Dr. Michael Davidson, Henry T. Bollmann, and Dr. Gary R. Consolazio, Bridge Software Institute, University of Florida Dynamic Effects of Superstructure Mass during Barge Collisions with Bridges During barge-bridge collision events, the superstructure mass of concrete bridges can influence internal forces generated throughout underlying substructures. This article presents a method for dynamically quantifying collision forces and structural demands and applies that method to the analysis of an in-service coastal bridge. The method and tools described here can help engineers in designing concrete bridges that span navigable waterways. Modeling Barge- Bridge Impact The St. George Island Bridge is a coastal bridge spanning Apalachicola Bay in nor thwes tern Florida. The sou thern channel pier supports adjacent 250 ft and 257.5 ft spans. The post-tensioned Florida bulb-tee girders vary in depth from 6.5 ft at drop-in locations to 12 ft above the pier. Both the girders and the 47.1-ft-wide deck are continuous across the pier cap. The five evenly spaced girders each rest on two elastomeric bearings that straddle a cast-in-place shear pin. For an impac t scenario using this structure, a finite-element model of the southern channel pier and adjacent spans (that is, "one-pier, two-span" model) is developed using FB-MultiPier software. In this type of model, all bridge portions more than a span length away from the impacted pier are simplified as springs and lumped masses (spine model), which are positioned at the far ends of the spans. Spring and mass quantities are automatically computed by the program based on a larger bridge model that includes several piers and spans. The simplified "one-pier, two-span" approach allows collision-related design forces to be efficiently computed within a few minutes using ordinary computers, and the accuracy of this method has been verified against more-complex multiple- pier, multiple-span bridge models. 1 Th e c h a n n e l p i e r c o n t a i n s t w o 6-ft-diameter pier columns spaced 30 ft apart, which are braced mid-height by a 6-ft-deep shear strut. The 52-ft-long columns are supported at the waterline by a 6.5-ft-thick pile cap. The foundation consists of 4.5-ft-diameter prestressed concrete cylinder piles (14 battered and one plumb) with 10-ft-long reinforced- concrete pile-top plugs. Each of the 62.6-ft-long piles extends through medium-dense to dense sand, and the pile tips bear upon a weathered limestone layer. Based on waterway traffic local to Apalachicola Bay, a representative collision scenario is formed. It consists of a 3666 ton barge/tug traveling at 6.6 knots (11.1 ft/sec) and striking the 28-ft-wide waterline footing head-on. FB-MultiPier is used to analyze the collision scenario by specifying vessel weight, impac t velocity, and impact location, and allowing the software to automatically assign a nonlinear stiffness to represent the impacting barge bow. Both impact forces and bridge internal demands (shears and moments) are computed using this analysis approach, which has been validated against Elevation of channel pier and adjacent spans used in model for barge-bridge collision scenario. All Figures: Bridge Software Institute. Typical section of superstructure.

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