Finite element analysis of the axial folding bridge

Abstract

The U.S. Army needs a Heavy Dry Support Bridge (HDSB) that can support the Military Load Class (MLC) 96 Heavy Equipment Transporter (HET) tractor trailer carrying an M-I Tank. An existing Axial Folding Bridge (AFB), which was originally designed for MLC 70 loading, has been proposed for load upgrade to demonstrate the feasibility of achieving MLC 96 capacity. This report reviews the upgrade, a finite element model of the bridge, and the actual testing of the bridge to verify the model.

The bridge was modeled using the Structural Analysis and Design/Integrated Structural Design System (STAAD-III/ISDS) finite element analysis software. The analysis demonstrated that the existing bridge could be upgraded by simply bolting flat, thin plate elen1ents to the bottom chord of the structure using existing pin plate connector bolt holes and longer bolts. These plates effectively stiffen the bridge structure and maintain a stress level for all elements of the bridge that is no higher for the MLC 96 loading than was experienced for the unreinforced MLC 70 base structure. The modification added 208.7 kg per center section, or an increase in weight of 4.6%, for an increase in load capacity of 37%.

A working load static test was conducted on a modified bridge. Four hydraulic actuators applied incremental loading on the bridge. Strain gauge and deflection data were recorded and compared to the analytical model. The model proved to be more flexible than the bridge, but the data showed good correlation in terms of stress ratios between various members. Stress levels were acceptable for the given loading, demonstrating that MLC 96 had successfully been achieved on the bridge.