International Journal of Earthquake and Impact Engineering (7 papers in press)
Optimal Damper Allocation in Shear Buildings with Tuned Mass Dampers and Viscous Dampers
by Baki Ozturk, Ersin Aydin, Huseyin Cetin
Abstract: The use of viscous dampers (VD) together with a tuned mass damper (TMD) to protect building structures is not much common according to the available knowledge in the literature. In this study, a hybrid passive optimal control method is proposed to find the optimal damping coefficients of VDs and a TMD in shear building structures. The stiffness and the mass of TMD are fixed by using the empirical equations. The damping coefficients of TMD and VDs are taken as design variables in the optimization problem. Cost function is defined as the sum of damping coefficients of the dampers and to be minimized to find optimal damping coefficients under a target added damping ratio and considering lower and upper bounds of each one of the damping coefficient of the added dampers. In first stage of the proposed algorithm, a simple optimization problem to find the optimal damping coefficients is resolved by using some numerical optimization methods such as Differential Evolution, Nelder Mead and Simulated Annealing. In the second stage, the optimal design obtained in the first stage is tested by performing linear time history analyses under a design earthquake whether attained to a specified response level of the structure. The target structural response is defined with achieving the allowable level of all interstorey drift ratios (IDR). The proposed method is applied to a six-storey shear building model with VD&TMD. The variation of the upper limits of damping coefficients, the variation of storey mass and stiffness are also investigated in the numerical examples. The investigated method is effective to find the optimal damper allocation in the mixed passive control system.
Keywords: Tuned mass dampers; Optimal dampers; target damping ratio; added dampers; optimal passive control; interstorey drift ratio.
Investigating the effects of smoothing on the performance of earthquake hazard maps
by Edward M. Brooks, Seth Stein, Bruce D. Spencer
Abstract: In recent years, it has become clear that the actual performance of earthquake hazard maps often differs from that ideally expected, for reasons that are unclear. As a result, we are exploring map behavior to learn more about how they actually perform. We take an empirical approach of asking what maps do, rather than what they should ideally do. Here, we explore whether less detailed probabilistic hazard maps might perform better by assessing how smoothing Japan's national earthquake hazard maps affects their fit to a 510-year record of shaking. As measured by the fractional exceedance metric implicit in such probabilistic hazard maps -- that the predicted ground motion should be exceeded only at a specific fraction of the sites simple smoothing over progressively larger areas improves the maps' performance such that in the limit a uniform map performs best. However, using the squared misfit between maximum observed shaking and that predicted as a metric, map performance improves up to a ~75-150 km smoothing window, and then decreases with further smoothing, such that a uniform map performs worse than the unsmoothed map. Because the maps were made by using other data and models to try to predict future earthquake shaking, rather than by fitting past shaking data, this result is probably not an artifact of hindcasting rather than forecasting. It suggests that the probabilistic hazard models and the resulting maps may be over-parameterized, in that including too high a level of detail to describe past and future earthquakes may lower the maps' ability to predict future shaking. Hence to forecast future hazard, the goal should be not to build the most detailed model, but instead one that is robust or stable in the sense that the forecast is not unduly affected when the earth does not behave exactly as expected.
Keywords: earthquake hazard maps; probabilistic seismic hazard analysis; smoothing; metrics; japan; earthquake; hindcasting; parameterization;.
Effect of vertical ground motion on the response of structures isolated with friction pendulum isolators
by Huseyin Cilsalar, Michael C. Constantinou
Abstract: Structures isolated with friction pendulum isolators have their instantaneous stiffness and friction force dependent on vertical load. As such they are affected by vertical ground motion. This paper presents a study to investigate the vertical earthquake effects on the base shear force and isolator displacement in structures equipped with friction pendulum isolators and with or without added viscous damping devices in the isolation system. Analyses are carried out for a simple rigid structure and results are presented for values of the ratio of the base shear and isolator displacement calculated with the effect of the vertical ground motion and divided by the same quantity when the vertical ground motion effect is disregarded. The effects of two-component horizontal ground motion are also studied. The results may be used to modify the Equivalent Lateral Force method of ASCE 7 for seismic isolated structures in order to account for the vertical earthquake effects.
Keywords: vertical ground motion; friction pendulum isolators; dampers; seismic isolation.
General evaluation method of seismic resistance of residential house under multiple consecutive severe ground motions with high intensity
by Yudai Ogawa, Kotaro Kojima, Izuru Takewaki
Abstract: Two severe ground shakings attacked some specific regions in Kumamoto Prefecture in Japan on April 14 and April 16 of 2016 with the seismic intensity 7 (the highest level in Japan Metheorological Agency (JMA) scale; approximately X-XII in Mercalli scale). In the seismic design codes of many countries, it is usually supposed that a building experience severe earthquake ground shaking with such level once in its working period. In this paper, a general method is proposed on the evaluation of the seismic resistance of damaged residential houses under multiple consecutive severe ground motions with high intensity. The method can be used for the analysis and design of buildings under multiple consecutive severe ground motions. As in the previous paper, the present paper adopts an impulse as a representative of near-fault ground motion and multiple separated impulses are used as the repetition of intensive ground shakings with high seismic intensity. A building collapse scenario (collapse limit in terms of zero restoring force with P-delta effect) under multiple repeated severe ground shakings is provided and an energy balance law is used for the response evaluation. The reliability of the proposed theories is examined through numerical analysis using recorded ground motions.
Keywords: repeated earthquake ground motions; upgrade of seismic resistance; residential house; near-fault ground motion; elastic-plastic response; energy approach; collapse.
Dual hysteretic damper system effective for broader class of earthquake ground motions
by Takuma Shiomi, Kohei Fujita, Masaaki Tsuji, Izuru Takewaki
Abstract: This paper aims to develop a Dual Hysteretic Damper (DHD) effective for a broader class of ground motions, which includes two level-oriented hysteretic dampers and a gap element. To reveal the influence of DHD parameters on the earthquake structure response, a closed-form solution is used of the maximum response of a single-degree-of-freedom system with DHD under the critical double impulse. An energy balance approach plays a central role in the derivation of such closed-form solution. A design process of DHD in a multi-degree-of-freedom system is proposed which is based on the sensitivity analysis. The transformation of earthquake ground motions into the double impulse overcomes the difficulty in the emergence of sensitive response variation to design parameters and the closed-form expression makes the proposed system efficient. It is demonstrated that DHD is effective both for small and large-amplitude input motions and the proposed system is applicable to recorded ground motions approximately. It is also verified through the comparison with the designs obtained by the Monte Carlo simulation and the genetic algorithm (GA) that the proposed design method is more effective and efficient than conventional methods.
Keywords: damping; hysteretic damper; dual use; double impulse; gap element; optimal design.
Evaluating Seismic Response Modification Factor of Steel Frames with Different Bracing Systems
by Yousef Al-Qaryouti, Besan Alagawani
Abstract: This paper aims to evaluate seismic response modification factor, overall ductility factor, and overstrength factor of ordinary moment steel frames with different concentrically bracing systems (X-bracing, V-bracing, and inverted V-bracing). The effect of bracing type and number of stories have been considered in the course of this study. Such factors effects on seismic parameters are not considered in seismic codes such as International Building Code (IBC) and European standards (EN). Linear time history analysis has been performed using multiple earthquake records selected to include the variability in ground motion characteristics for four-, eight-, twelve-, and sixteen-story steel frame buildings. Nonlinear pushover analysis was then carried out by assigning nonlinear material behaviour and plastic hinges to steel elements according to code. It has found that each bracing system has a unique behaviour as well as different seismic parameters than other types, which are not differentiated in seismic codes such as IBC and EN. Also, it was found that the overall ductility factor, decreased with increasing the number of stories, while response modification factor (except for the inverted V-braced system) and overstrength factor increased with increasing the number of stories.
Keywords: earthquake engineering; seismic behaviour factors; response modification factor; ductility factor; overstrength factor; moment steel frames; steel bracing system; static pushover analysis; time history analysis; plastic hinges.
Fragility curves for steel-concrete hybrid tall buildings
by Huanjun Jiang, Yanfeng Duan, Huan Zuo
Abstract: Fragility curves of steel-concrete hybrid tall buildings designed according to the current Chinese seismic design code were derived by analytical methods, taking into account the uncertainty of earthquake ground motions. Totally, 45 analytical models were analyzed considering all combinations of three design parameters, i.e., seismic protection intensity, site soil type and design group. On the basis of a large number of nonlinear time history analyses, fragility curves were derived reflecting exceeding probabilities corresponding to each performance level. The influence of different engineering demand parameters (EDPs) and design parameters on fragility curves was analyzed. The result showed that fragility curves based on different EDPs varied significantly, indicating that different structural components played different roles in seismic performance of the structure. While seismic protection intensity had little impact on seismic vulnerability, structures tended to be more vulnerable on the site with softer site soils and longer characteristic periods.
Keywords: steel-concrete hybrid tall buildings; fragility curves; seismic protection intensity; site soil type; design group; engineering demand parameters; fragility curves; exceeding probability; performance-based seismic design; performance objective; incremental dynamic analysis.