Barajas Valdés, Ulíses

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    Methodologies for the analysis and load rating of prestressed and preflex bridge superstructures
    (2009) Barajas Valdés, Ulíses; Portela Gauthier, Genock; College of Engineering; López Rodríguez, Ricardo R.; Suárez, Luis E.; Department of Civil Engineering; Schröder, Eduardo C.
    Bridge superstructures consisting of prestressed, post-tensioned, and preflex systems are evaluated with load rating methodologies adopted by private and government entities, establishing possible restrictions. Based the importance bestowed in these structures to properly withstand service loads, this study considers the following tasks: i) description of the analysis and load rating methods of prestressed beams based on AASHTO requirements; ii) develops a methodology for the analysis and load rating of preflex beams; and iii) present a parametric study of prestressed and preflex bridges with four different span lengths. The results show that AASHTO distribution factors are conservative in comparison to the values obtained from three-dimensional finite element models. In terms of the comparison between both systems (pre-tensioned and preflex), the preflex systems provide more slender sections (in the order of 62%) than prestressed concrete AASHTO I-type sections. Contrarily, live load deflections in the pre-tensioned systems were smaller (in the order of 8%), but for no case AASHTO limits were exceeded. The load rating values obtained from the methodologies considered in this study show that for both systems, the LRFR philosophy results more critical to determine special conditions.
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    Study of light weight, high strength, multifunctional composite material: Aluminum reinforced with AIB2 particles
    (2017) Barajas Valdés, Ulíses; Suárez, O. Marcelo; College of Engineering; Suárez, Luis E.; López Rodríguez, Ricardo R.; Portela Gauthier, Genock; Department of Civil Engineering; Rodríguez Solís, Rafael A.
    Aluminum, its alloys and composites are widely used in many modern life applications, such as microelectromechanical systems. In this research, an aluminum matrix composite reinforced with AlB2 particles was studied in bulk and in thin films. Bulk samples were manufactured by centrifugal casting the composite with different levels of boron forming AlB2 particles. The effect of embedded particles on the aluminum matrix was evaluated using Brinell hardness and nanoindentation. Brinell hardness increased from 250 MPa to 450 MPa as a result of particle content. The nanoindentation technique showed that the mechanical improvement is a result of the combined action between the two phases present. Thin films were made from pure aluminum and the Al-4wt. %B (Al-4B) composite using sputtering magnetron by radio frequency at different discharge power on glass substrates and silicon wafers (100). Nanoindentation revealed a higher modulus of elasticity, hardness and adhesion in the Al-4B films. The surface morphology of the films was studied using atomic force microscopy. The surface of the aluminum films presented a greater roughness and grain size with respect to the composite films. Additionally, in composite films the hillock formation was significantly reduced. The structural evaluation of the material via x-ray diffraction exposed in the aluminum samples a larger lattice strain. In addition, temperature effects were studied in samples deposited at 200 watts of sputtering power via hot stage tests using nanoindentation and x-ray diffraction. The materials exhibited softening by heating effect that expands the structure reducing mechanical properties. Additionally, the samples presented material annealing as result of the temperature cycle used in the tests. This was reflected in the mechanical behavior and the crystal structure.