Feasibility of Semi-integral to Fully Integral Bridge Abutment Design using FEA Modeling Methods

Presentation by Hugh Rae-Robinson, Bridge Engineer-in-Training with Geometrix Group as part of the ATCx AEC 2025 conference.

GeoMetrix was tasked with creating a Finite Element Model (FEM) using S-Frame to prove the feasibility of the proposed design change to an existing bridge design done in 2007. The original design was a semi-integral abutment bridge with span lengths of 19m-28m-19m on a 30-degree LHF skew with 2 rows of 8-HP310x94 abutment piles. GeoMetrix proposed that the elimination of abutment bearings was possible by changing the semi-integral abutment to a fully integral abutment. The purpose of this change was to decrease the bridge maintenance cost, harmonize the piling equipment to improve construction sequencing, and reduce the amount of material needed for the abutments which would decrease the construction and material costs. With this design change, the abutment piles would be changed to 1 row of 9-HP310x125 piles bending about the weak axis.

In the process of developing the model, lots of model geometry planning was done utilizing AutoCAD to determine the locations of each node and how each shell would connect. The use of S-Frame’s auto-meshing tool was considered but was not utilized due to the complex geometry of the structure. For modeling the girders, the webs were chosen to be shell elements with beam elements used at the top and bottom nodes to represent the flanges. The flanges were modeled as members to reduce the computational time, as well as to decrease the amount of planning needed to create the model. The deck was modeled as shell elements which were attached to the girders by stiff links that had a large stiffness and low mass, to ensure a composite connection between the girders and deck. This was also done to line up the deck nodes to the top of the abutment diaphragm nodes. The abutment diaphragm and wing walls were modeled as shell elements as the wing walls were one of the main areas of interest for this FEM. The wing walls were modeled to have a fixed connection to the abutment diaphragms. To simplify the model as much as possible, the pier piles and abutment piles were modeled as beam elements with their corresponding cross-sectional properties.

Since this is a complex model, GeoMetrix wanted to model the boundary conditions as accurately as possible. To do this, GeoMetrix used P-Y curves for non-linear springs for the abutment piles, wing walls, and the back of the abutment diaphragms. At the bottom of each pile, a vertical restraint was applied to finalize the boundary conditions of the model.