Vessel Collision Analysis and Design of a Pile Support Fender System: Fid-Bau Portal

Vessel Collision Analysis and Design of a Pile Support Fender System

El-Sawy, Moustafa / Shagin, Vladimir / Elmorsi, M.

Waterway vessel collision is an accidental load case that is an integral component of the structural analysis and design of bridges and other marine structures spanning a navigable waterway. This paper presents the procedure followed during the analysis and design process of a pile supported fender system protecting a new major multi span bridge with several piers in the waterway. The design of any fender system is an iterative process based on energy absorption as per AASHTO specifications. In this case the total kinetic energy for the impact was calculated based on the vessel parameters to be a total of 44,200 kip-ft. There are two approaches used for the design of these fender systems: one is based on strength and the other is based on ductility. In the strength design method, the impact force is resisted while keeping the structural elements within the material elastic range limiting any permanent structural damage and deformation. This results in insignificant system deflections. The other procedure takes advantage of the ductility of the system to absorb the impact energy by allowing material yielding to occur while maintaining global stability. Since the fender system is designed as a sacrificial system, a decision was made to go with the ductility design approach to achieve considerable construction cost savings. A finite element analysis model (FEM) is developed using ANSYS APDL software. In this model, the piles are modeled using 3D beam line elements supported on non-linear spring elements representing the soil (Beam on Winkler foundation). The spring behavior is derived using Ensoft L-PILE analyses. This method of representing the soil is used instead of the computationally demanding solid continuum elements that require sophisticated calibrated material models. The FEM considers the full non-linear stress-strain behavior of steel, P-delta effects, pile-soil interaction, and global buckling of the piles. Local buckling analysis is also performed separately and explained in detail throughout the paper. After validating the single pile model with L-PILE, a full FEM model for the entire fender system is built. Push over analysis is performed and the total energy absorbed is calculated by integrating the area under the load-deflection curve. Different load cases are considered for different impact angles and vessel speeds. The performance acceptance criterion of the system is established to ensure that enough energy is absorbed to avoid any contact between the fender system and the bridge pier under vessel collision loading. To reduce P-delta effects, the final design is comprised of a hollow pre-cast 12 ft x 12 ft box pile cap supported on twelve 48 inch diameter 1.5 inch thick steel piles filled with low grade concrete to prevent the piles’ local buckling. This paper also addresses the structural detailing aspects implemented to ensure satisfactory performance of the system in the post yield stage. Construction challenges associated with the erection of this system is also discussed.