Load Rating Complex Structures

Load rating is the determination of the live load carrying capacity of a structure.  This technique provides information regarding the safety of the travelling public over bridges, allows for the safe passage of goods, and produces valuable information to maintain an infrastructure asset. Our highly skilled and talented Structural Engineers recently completed two separate projects that involved the load rating evaluation of complex structures.

As a subconsultant under our FDOT D6 Districtwide Load Rating contract, BPA performed the Load and Resistance Factor Rating (LRFR) evaluations of two precast segmental bridge structures for the Florida Department of Transportation (FDOT). Built in the 1980’s, these are considered “first generation segmental” bridges. Both structures were erected using the balanced cantilever method and are post-tensioned longitudinally and transversely with internal tendons, making them distinctive structural systems with unique post-tensioning features.

The Turnpike NB Ramp over I-75, known as Ramp I, is a 2,124 ft long, eleven-span single cell box girder. Ramp I is the first curved precast segmental in the State and uniquely features a quarter-span flexural hinge. The second bridge is Ramp D, located on the north side of the I-75/SR 826 interchange. This is a 396 ft long, three-span, twin single-cell box girder bridge connected with a longitudinal cast-in-place closure.

Due to the structures’ unique structural characteristics, refined analysis of the live load carrying capacity was required, as well as consideration of posting avoidance measures. The structural analysis involved discretizing the top and bottom slab as well as webs in shell finite elements, in addition to imposing essential boundary conditions. Further, the structural model considered all relevant deformations, material properties, curved geometry, time-dependent effects, step-by-step erection sequence, and analyzed their multiple statistical systems, typical of these bridges. The rating was conducted in the longitudinal and transverse directions considering the design, legal, and permit trucks, as well as emergency vehicles. The structural model accounted for the effects of floating lanes, lane permutations, and emergency vehicles mixed with legal trucks for posting avoidance. Service and strength limit ratings were also generated.

Under our current Miami-Dade Transit (MDT) Bridge Inspection and Structural Analysis Services contract, our structures group carried out selective LRFR analyses of the 23,233 ft long Miami Metromover system. The Miami Metromover’s Brickell, Downtown and Omni Loops service the Downtown Miami, Brickell, Park West, and Arts & Entertainment District neighborhoods within the City of Miami. The Metromover carries passengers traveling on an elevated guideway, which is predominately a double-track structure comprising 244 bridge units having 626 spans with on-line stations in between.

A distinctive feature of these loops is that the guideway consists of nonredundant (fracture critical) superstructures made of a dual, parallel steel girder system that carries the Metromover cars. The tires of the cars sit directly within 9-inch thick, 2-ft wide concrete pads which act compositely with the steel girders. Another particular characteristic of the guideway system is that the supporting girders were fabricated with a rotated cross section along its length to accommodate super-elevations of up to 10%. The structural system also includes discretely placed cross frames and a horizontal truss connected to the main girders along the guideway. For these long and unique bridge structures, our engineers developed project specific load rating criteria that considers service and strength rating limit states.

A live load carrying capacity evaluation was performed on approximately 10% of the spans representative of the structural features within the entire system. The selection criteria were based on span length, number of continuous spans, structure curvature, girder type (rolled shape or plate girder), and steel grade. The selected bridge units cover all relevant structural features within the system that will allow us to infer the robustness and performance of the structural system. Ultimately, the evaluation determined that all units selected possess adequate load carrying capacity.

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