Sameh S. F. Mehanny

The type of impact model (Linear Spring, Hertz, HertzDamp, Full Rebound, etc.) and its computational simulation would affect the seismic fragility of precast beam bridges with a continuity slab and inverted T cap beam (buffer) at piers. Current design consensus emphasizes the critical role of the size of the gap between the super- and sub-structure as well as the bearings’ mechanical characteristics. However, modelling the pounding behavior is yet simplified, hampered, or even fully ignored for this type of bridges. A three-span precast beam bridge equipped with a buffer-gap-elastomeric bearings system serving as hybrid seismic isolation scheme is modelled in OpenSees, with different gap sizes and simulating five different pounding models. Seismic fragility curves are developed for three performance Limit States (namely, Operational, Life Safety, and Collapse). Results show that HertzDamp model mimics cogent behavior that captures pounding-related energy dissipation, as should be. On the other hand, Linear Spring impact element either exhibits unrealistic behavior in case ‘large’ stiffness is prescribed or offers a trade-off between reasonable representation of pounding forces and computational simplicity if enhanced through considering suitable ‘reduced’ elastic stiffness extracted from a bilinear idealization of a pushover curve of the bridge. Gap sizes also strongly affect the failure mechanism; smaller gaps mobilize extreme column ductility demands under large seismic events, whereas larger gaps tend to cause abutment failure at comparatively lower seismic demands. Overall, this study establishes curated evidence-based guidance for the selection of suitable pounding models for precast beams bridges coupled with various gap size scenarios.

The effect of the exposed length of piles supporting bridges crossing waterways on the spread of inelasticity along the pile shaft compared with the extent of plasticity in the body of the column, and hence on the precedence of the formation of plastic hinges in the substructure system is of paramount importance. Plastic hinges may form ideally and preferably at the bottom of the column just above the pile cap, or undesirably either at the pile’s head or at any other section within the pile be it along its free/exposed length immersed in water or its embedded part in soil. A set of bridges are designed according to Eurocode for multiple configurations with various relative stiffness between the column and the group of piles featuring partly exposed shaft. Pushover as well as time history inelastic analyses under a set of ten earthquake records are performed accounting for soil-structure interaction. Results demonstrate that it is likely in some cases characterized by a remarkable increase in the flexural stiffness of the column relative to the group of piles to have spread of inelasticity and plastic hinges forming in piles prior to the column. This is undesirable for a robust, reliable and resilient seismic design as devised by universal design standards for bridges on piled foundation. While there is no clear consensus on the most effective corrective measure to such undesirable response, a few proposed remedial design actions have been formulated in order to preclude (or at least improve the behavior for the case featuring) plastic hinges deplorably forming in the piles prior to the base of the column.

The 2.7 kilometers double track elevated viaducts of Doha metro green line, currently under construction, consist of cast-in-situ and precast segmental simply supported spans ranging from 20 to 35 m and continuous cast-insitu two and three spans (30-57 m), (50-51-44 m) and (37-68-37 m) U-trough decks. The non-typical configuration of the continuous span arrangements was imposed to the designer by existing underneath utilities and infrastructures. In order to ensure the passenger comfort and traffic safety by preventing track instability during train operation for this special landmark project, performing a dynamic analysis was vital. The dynamic analysis focuses on the vertical accelerations and vertical displacements as well as lateral frequencies of vibration. All simply supported spans as well as the continuous spans of the project are considered. The real train of the project composed of 6 vehicles with a total length of 120m and with actual axle loads (maximum axle load of 160 kN with 4 axles per vehicle) is adopted in the dynamic analysis. The analysis is carried out using both direct time integration of the equation of motion and modal time history analysis for different train speeds ranging from 60 km/hr to the maximum permissible speed along the metro line (160 km/hr) with the maximum operating speed ranging from 100 to 130 km/hr. For each train speed the maximum vertical acceleration and the maximum vertical deflection are monitored using the CSI bridge software and are compared to the allowable values given in EN 1991-2 and EN 1990-Annex 2. According to relevant Euronorm requirements, the vertical accelerations and the vertical deflections were found acceptable for all segments of the elevated viaduct.

The seismic design and response prediction of irregular bridges supported on piers of unequal heights—a commonly adopted solution when crossing steep-sided valleys—represent a particularly challenging problem that is yet to be effectively addressed by seismic design code provisions worldwide. From a force-based design perspective, shorter piers are subjected to increased ductility demand, and consequently damage tends to localize in these relatively stiff piers at increasing seismic hazard levels. This paper presents an investigation of the seismic response of a few schemes of a three-span case-study continuous bridge (commonly encountered in practice), featuring two unequal piers with relative heights of 0.5, 0.64, 0.75, and 0.86, respectively. Static pushover and time history (under incrementally scaled-up actual records) nonlinear inelastic analyses are performed using OpenSees to check the validity of Eurocode 8 (EC8) and recently proposed AASHTO-LRFD provisions for regular seismic behavior of...

A multi‐level seismic vulnerability assessment of reinforced concrete moment frame buildings located in moderate seismic zones (0.25g) is performed on a set of ductile versions of low‐ to mid‐rise two‐dimensional moment frames. The study is illustrated through application to comparative trial designs of two (4‐ and 8‐story) buildings adopting both space‐ and perimeter‐framed approaches. All frames are dimensioned as per the emerging version of the seismic design code in Egypt. These new seismic provisions are in line with current European norms for seismic design of buildings. Code‐compliant designs (CCD), as well as a proposed modified code design relaxing design drift demands for the investigated buildings, are examined to test their effectiveness and reliability. Applying nonlinear inelastic incremental dynamic analyses, fragility curves (FC) for the frames are developed corresponding to various code‐specified performance levels. Code preset lower and upper bounds on design acceleration and drift, respectively, are also addressed along with their implications, if imposed, on the frames seismic performance and vulnerability. Annual spectral acceleration hazard curves for the case study frames are also generated. Estimates for mean annual frequency (MAF) of exceeding various performance levels are then computed through an integration process of the data resulting from the FC with the site hazard curves. The study demonstrates that the proposed design procedure relaxing design drift demands delivers more economic building designs relative to CCDs, yet without risking the global safety of the structure. The relaxed design technique suggested herein, even though scoring higher, as expected by intuition, MAF of exceeding various code‐limiting performance levels expressed in terms of interstory drift ratios, still guarantees a reasonably acceptable actual margin against violating code limits for such levels. Copyright © 2010 John Wiley & Sons, Ltd.