MS3.3 - Bridge Dynamics

As the increasing requirement in nonlinear time history analysis of structures under bi-directional ground motions, the demand for the employment of RotD100 response spectrum-compatible bi-directional ground motions also increases. However, only spectrum-compatible bi-directional ground motions cannot guarantee a correct evaluation of the structural seismic response, but suitable seismic inputs. The orientation angle (OA) of bi-directional ground motions is a new concept and affects the nonlinear response of structures. And it will be preferable in the seismic design that if the concerned feature of bi-directional ground motions can be specified in a controllable manner. Thus, firstly, a new algorithm is proposed for generating RotD100 response spectrum-compatible bi-directional ground motions that OA remains constant regardless of natural period. This algorithm is based on the complex-discrete-Fourier-transform (CDFT), which takes the advantage of controlling the azimuth angle (AA) of ellipse at each natural periods in the frequency domain. The proposed algorithm is validated using a target RotD100 response spectrum and a suite of bi-directional accelerograms. It is shown that the performance of the proposed algorithm is satisfactory and its application is straightforward in the relative fields of earthquake engineering. In the second part, the strong motion characteristics of the RotD100 spectrum-compatible bi-directional ground motions using the proposed method are compared with a suite of bi-directional ground motions that were directly matched to be spectrum-compatible using a conventional method by modifying two horizontal components simultaneously. In the third part, the effect of the OA property of bi-directional ground motions on the nonlinear response of bridge piers involving a progressive damage process is investigated. As a representative example, idealized single column steel bridge piers with different geometric configurations were constructed using fiber model in OpenSees. The property of steel was modelled by a bi-linear hysteretic model with kinematic hardening and isotropic hardening and a modified rainflow cycle counter has been implemented to account for the effects of low cycle fatigue. Incremental Dynamic Analysis (IDA) was conducted with the two suites of bi-directional ground motions applied to the piers through 360° at an angle increment of 9°. The Intensity Measure (IM) was defined as the peak ground acceleration (PGA). The structural response differences under the two suites of bi-directional ground motions were discussed in hysteresis curves, IDA curves and the fragility curves, etc. It is shown that the difference in seismic demanding between the two suites bi-directional ground motions becomes significant as PGA increases. The seismic demands of bi-directional ground motions with OA remains constant is more seismic demanding when the piers are subjected to strong earthquakes (i.e., PGA is high). This sheds light on the importance of a reasonable suite of bi-directional ground motions in designing new bridges or retrofitting old bridges.

The behavior of irregular bridges is more complex than that of regular systems, which is why some standards propose procedures or modification factors in the design of these structures are considered. Bridges can be irregular in their substructure (due to changes in stiffness and resistance of their elements) or in their superstructure (in curved and skew systems, or by changes in stiffness of their elements). Bridges with an irregular substructure in urban areas are generally defined to save other communication routes, so they usually have a longitudinal slope in their deck, which produces variation in the pier height. To understand the behavior of bridges with irregular substructure due to changes in their slope, six irregular models of reinforced concrete are analyzed, with slopes between 1° and 6°, which could be a maximum extreme value. The response of these models is compared with the response of a regular bridge with the same pier height, with a horizontal RC slab deck and AASTHO-type girders. All the models considered were designed to be located in three characteristic zones of Mexico City, defined as soft, intermediate and almost rigid soils. In the design, a real design truck and characteristic design spectra of the bridge location areas were assumed. From designs procedures, is defined the necessary information to elaborate models of the structures in the SAP-2000 program, considering that the structures are embedded. For each model, the dynamic characteristics of the bridges and their maximum responses are defined, in the form of displacements and mechanical elements in the piers. Also, normalized differences between the responses of irregular and regular systems are defined; these responses show the influence of every irregular condition. The analysis of the dynamic response of the bridges under study helps to understand how their periods and modal shapes change for different conditions of irregularity, which broadens the knowledge of these systems for a better design. The analysis of the maximum responses helps to understand the influence of different irregularity conditions, which could ultimately be considered in design standards. The obtained results are also compared with ones defined in highway bridges without slope but with changes in the length of their piers.

Steel trusses have often been used as medium-to-long-span railway-over-bridge structures due to their rapid installation and reliable performance. However, fatigue reduces the service life of these steel bridges. Rapid stress reversals are generated in steel bridges due to the high-speed movement of trains. The damage is further intensified with aging of the structure, due to corrosion. This coupled action of corrosion and rapid stress reversals increase the stress concentration in steel bridges, especially in the vicinity of existing initial defects. Thus, the focus of the current study is to develop a relationship between the residual fatigue life and the coupled deterioration due to corrosion and incremental freight speed, for varying rail loads, provided in standard railway design guidelines. For the present study, the 63m railway-over-bridge structure modeled in professional design and analysis software is subjected to eleven 25t rail loads taken from Research Designs and Standards Organisation (RDSO) guidelines. The rainflow-counting algorithm has been used to convert critical member's stress histories into an equivalent set of varying amplitude stress reversals. The obtained equivalent stress histories are applied to the critical member along with optimally simulated welds of the steel bridges exclusively modeled using ANSYS. From the numerical results obtained, the residual fatigue life is calculated in terms of number of equivalent sets of varying stress reversal that the simulated member can withstand before failure.