Intravenous levosimendan-norepinephrine combination during off-pump coronary artery bypass grafting in a hemodialysis patient with severe myocardial dysfunction

This the case of a 63 year-old man with end-stage renal disease (on chronic hemodialysis), unstable angina and significantly impaired myocardial contractility with low left ventricular ejection fraction, who underwent off-pump one vessel coronary bypass surgery. Combined continuous levosimendan and norepinephrine infusion (at 0.07 μg/kg/min and 0.05 μg/kg/min respectively) started immediately after anesthesia induction and continued for 24 hours. The levosimendan/norepinephrine combination helped maintain an appropriate hemodynamic profile, thereby contributing to uneventful completion of surgery and postoperative hemodynamic stability. Although levosimendan is considered contraindicated in ESRD patients, this case report suggests that combined perioperative levosimendan/norepinephrine administration can be useful in carefully selected hemodialysis patients with impaired myocardial contractility and ongoing myocardial ischemia, who undergo off-pump myocardial revascularization surgery

Collapse risk and residual drift performance of steel buildings using post-tensioned MRFs and viscous dampers in near-fault regions

The potential of post-tensioned self-centering moment-resisting frames (SC-MRFs) and viscous dampers to reduce the collapse risk and improve the residual drift performance of steel buildings in near-fault regions is evaluated. For this purpose, a prototype steel building is designed using different seismic-resistant frames, i.e.: moment-resisting frames (MRFs); MRFs with viscous dampers; SC-MRFs; and SC-MRFs with viscous dampers. The frames are modeled in OpenSees where material and geometrical nonlinearities are taken into account as well as stiffness and strength deterioration. A database of 91 near-fault, pulse-like ground motions with varying pulse periods is used to conduct incremental dynamic analysis (IDA), in which each ground motion is scaled until collapse occurs. The probability of collapse and the probability of exceeding different residual story drift threshold values are calculated as a function of the ground motion intensity and the period of the velocity pulse. The results of IDA are then combined with probabilistic seismic hazard analysis models that account for near-fault directivity to assess and compare the collapse risk and the residual drift performance of the frames. The paper highlights the benefit of combining the post-tensioning and supplemental viscous damping technologies in the near-source. In particular, the SC-MRF with viscous dampers is found to achieve significant reductions in collapse risk and probability of exceedance of residual story drift threshold values compared to the MRF. © 2016 Springer Science+Business Media Dordrech

Self-adaptive approach for optimisation of passive control systems for seismic resistant buildings

The concept of passive control of the seismic response of structures was introduced to improve the performance of structures by increasing their energy dissipation and reduce or eliminate damage in the structural elements. The key task in the design of passive systems is to determine the forces in the control devices (yield/slip or post-tensioning) at each floor, that will result in best performance (e.g. minimum inter-storey drift). This can be achieved by large parametric studies in which both the maximum control force (e.g. at ground level) and the distribution of forces along the height of the structure are varied. Alternatively, optimum forces in the devices can be achieved by semi-active control, where the structure self-adapts to the earthquake. Both solutions are expensive: the first requires hundreds of non-linear response simulations in the design stage; the second needs a system of sensors, controllers and electromechanical devices. Presented here is a new Self Adaptive Optimisation Approach (SAOA) in which the self-optimisation of a semi-active system is used in the design stage and the resulting distribution of control forces is adopted as a passive system. The new approach was evaluated through comparing the simulated dynamic responses of two relatively simple benchmark structures (braced and post-tensioned) with three sets of control forces: (1) passive system with forces obtained in parametric study, (2) semi-active system with self-adapting control forces, and (3) passive system with SAOA-optimized forces. The results show good performance of the SAOA systems, indicating that SAOA offers a simple and effective solution that can replace the existing optimisation approaches for the design of passively controlled earthquake resistant structures. This study presents a novel idea of using the semi-active control as a tool for optimising a passive control system. The passive control systems can be further improved by a larger study in which the semi-active control algorithms are also optimised

Collapse risk evaluation of self-centering steel MRFS with viscous dampers in near-fault regions

This paper presents an evaluation of the seismic collapse risk of self-centering moment-resisting frames (SC-MRFs) with viscous dampers located in near-fault regions. This evaluation is based on the comparison of different designs of a prototype steel building using four lateral load resisting frames: 1) conventional moment-resisting frames (MRFs), 2) MRFs with viscous dampers, 3) SC-MRFs and 4) steel SC-MRFs with viscous dampers. The frames are modeled in OpenSees where material and geometrical nonlinearities are taken into account as well as cyclic strength and stiffness degradation. A database of 91 near-fault, pulselike ground motions with varying pulse periods is employed for the nonlinear dynamic analyses. Collapse resistance of the frames is evaluated through incremental dynamic analysis (IDA). The results of the IDA are combined with probabilistic seismic hazard analysis models that account for near-fault directivity to assess collapse risk of the structures. Results show that the predicted collapse capacity is affected by the pulse period of the near-fault ground motions and highlight that self-centering connections can significantly improve the collapse resistance of conventional MRFs. Finally, it is shown that supplemental damping provides superior collapse resistance for all frames

Collapse risk of self-centering moment resisting frames with viscous dampers in near-fault regions

This paper evaluates the seismic collapse capacity of self-centering momentresisting frames (SC-MRFs) with viscous dampers located in near-fault regions. For this purpose, a prototype steel building was designed using different seismicresistant frames, i.e.: conventional steel moment-resisting frames (MRFs); MRFs with viscous dampers; SC-MRFs; and SC-MRFs with viscous dampers. The frames are modeled in OpenSees where material and geometrical nonlinearities are taken into account as well as cyclic strength and stiffness degradation. A database of 91 near-fault, pulse-like ground motions with varying pulse periods is employed for the nonlinear dynamic analyses. Collapse capacity of the frames is evaluated through incremental dynamic analysis (IDA). The results of the IDA are combined with probabilistic seismic hazard analysis models that account for near-fault directivity to assess collapse risk of the structures. Results show that the predicted collapse capacity is affected by the pulse period of the near-fault ground motions and highlight that SC-MRFs have superior collapse resistance. Finally, it is shown that the use of viscous dampers significantly improves collapse resistance of all types of frames examined herein. © Civil-Comp Press, 2015