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Smart Energy Dissipation: Damped Outriggers for Tall Buildings under Strong Earthquakes
The use of outriggers in tall buildings is a common practice to reduce response under dynamic loading. Viscous dampers have been implemented between the outrigger and the perimeter columns, to reduce vibrations without increasing the stiffness of the structure. This damped outrigger concept has been implemented for reducing vibrations produced by strong winds. However, its behaviour under strong earthquakes has been not yet properly investigated. Strong earthquakes introduces larger amount of energy into the building’s structure, compared to moderate earthquakes or strong winds. In tall buildings, such seismic energy is dissipated by several mechanisms including bending deformation of the core, friction between structural and nonstructural components, and eventually, damage.
This research focuses on the capability of tall buildings equipped with damped outriggers to undergo large deformations without damage. In other words, when the ground motion increases due to strong earthquakes, the dampers can be assumed to be the main source of energy dissipation whilst the host structure displays an elastic behaviour. These investigations are based on the assessment of both the energy demands due to large-earthquake induced motion and the energy capacity of the system, i.e. the energy capacity of the main components, namely core, outriggers, perimeter columns and dampers. The objective of this research is to determine if the energy dissipated by hysteresis can be fully replaced by energy dissipated through the action of passive dampers.
This research is based on finite element (FE) models developed in Diana-FEA software. These analytical models consider the use of nonlinear settings throughout almost the whole FE model. The numerical investigations on passive damped outriggers are based on master Matlab scripts, which run combined parametric analysis within Diana.
Parametric analyses – Chapter 4
This chapter answers the question: Which parameters influence the distribution of seismic input energy through a tall building structure equipped with damped outriggers?
The numerical investigations focus on the aspects of the modelling and the structural parameters influencing the behaviour of tall building equipped with fixed and viscous damped outriggers. This chapter also provides a parametric study to assess the distribution of seismic energy in tall buildings equipped with viscous damped outriggers, i.e. with outriggers that have one or more viscous dampers installed between their ends and the perimeter columns. The aim of this explorative study is to determine which parameters influence (a) the structural response and (b) the distribution of seismic input energy through the building structure. First, this chapter describes a parametric study that addresses the influence of natural period of the building, position of the outriggers, damping coefficient, and stiffness coreto- outrigger and core-to-columns ratios in the control performance of the outrigger structures. Indirectly, it provides the basis for exploring which strategies will extend the elastic response threshold of a tall building equipped with viscous dampers and subjected to strong earthquake ground motions. The optimization of these parameters define pseudo-optimal configurations, which are further are assessed in terms of response reduction, namely displacement, acceleration, base shear, base moment and stress distribution; and, in terms of energy distributions. The strategy to assess the distribution of earthquake energy in tall buildings equipped with viscous damped outriggers and subjected to strong motions is based on the numerical study of 60-storey buildings equipped with conventional and damped outriggers, respectively. Secondly, this chapter describes the inter-dependency between structural properties of tall buildings equipped with damped outriggers and ground motion characteristics, which is examined under small, moderate, strong, and severe levels of the 1940 El Centro earthquake record.
Single passive damped outrigger system – Chapter 5
This chapter provides answers to the questions: How such energy is eventually dissipated by both the host structure and the viscous dampers? To which extent can hysteretic energy be completely overcome by the energy dissipated by the action of dampers?
The objective of the study presented in this chapter is to determine if the energy dissipated by hysteresis (damage) can be fully replaced by energy dissipated through the action of passive viscous dampers. More precisely, the goal is to determine whether it is correct to assume that main structural components will remain elastic during the entire strong earthquake response of a tall building, as well as which parameters mainly affect the response of damped outrigger structures and how such influence is exerted. In order to determine to which extent the use of viscously damped outriggers would avoid damage, both the host structure’s hysteretic behaviour and the dampers’ performance need to be evaluated in parallel. First, the time-history responses of fixed and damped outrigger structures, subjected to different levels of peak ground accelerations (PGA) of a suite of eight earthquake records, are obtained using 2D finite element (FE) models. Using these results, the nonlinear behaviour of the outrigger system with and without viscous dampers is examined under small, moderate, strong and severe long-period earthquakes to assess the hysteretic energy distribution through the core and outriggers. Next, the distribution of seismic energy in the structures is assessed by means of the damping-to-input (ED/EI), dampers’ dampingto- input (EDAMPERS/EI), and hysteretic-to-input (EH/EI) energy ratios; the concept of optimal configuration is therefore discussed in terms of reducing the hysteresis energy ratio of the structure. This assessment gives insights on which strategies will extend the elastic response threshold of a tall building equipped with viscous dampers and subjected to strong earthquake ground motions. The results show that, as the ground motion becomes stronger, viscous dampers effectively reduce the potential of damage in the structure if compared to conventional outriggers. However, the use of dampers cannot entirely prevent damage under critical excitations.
Double conventional and damped outrigger system – Chapter 6
This chapter answers the question: Which strategies will extend the elastic response threshold of a tall building equipped with viscous dampers and subjected to strong earthquake ground motions?
The use of a set of outriggers equipped with oil viscous dampers increases the damping ratio of tall buildings in about 6-10%, depending on the loading conditions. However, if a single damped outrigger structure is designed for an optimal damping ratio, could this ratio still be increased by the addition of another set of outriggers? Should this additional set be equipped with dampers too? In order to answer these questions, several double damped outrigger configurations for tall buildings are investigated and compared to an optimally designed single damped outrigger, located at elevation 0.7 of the total building’s height (h). Using free vibration analyses, double outrigger configurations increasing damping up to a ratio equal to the single-based optimal are identified. Next, selected configurations are subjected to small, moderate, strong, and severe earthquake levels of eight ground motions to compare their capability for dissipating energy and thus avoiding damage under critical excitations. Last, a simplified economic analysis highlights the advantages of each optimal configuration in terms of steel reinforcement savings versus damper cost. The results show that combining a damped outrigger at 0.5 h with a conventional outrigger at 0.7 h is more effective in reducing hysteretic energy ratios and economically viable if compared to a single damped outrigger solution.
From the parametric analyses using FE models with conventional and damped outrigger systems, under free vibration, it is concluded that optimal damping coefficient Cd and optimal location λ have a major influence in the optimal damping ratio ζ. This optimal damping ratio may not necessarily imply a significant reduction in the overall response of the outrigger structure. Nevertheless, when λ and Cd approximate to the optimal values, the effect of ρctc may imply an overall ζ increase in 7%. This suggests that if required damper sizes are not available, a modification in the ratio ρctc will help to increase the overall damping ratio. It should be noted that such increase occurs only if ρctc decreases.
Complementary modification of the stiffness ratios may help to improve the effect of the viscous damped outriggers in the reduction of response of the building. For example, both λ and ρctc exert their influence by modifying the building’s natural frequency. The fact that ρcto does modifies the response but not the frequency, suggests that its influence is closely related to the effect of the viscous dampers. None of the parameters under discussion, namely λ, ρctc and ρcto, have any influence on the frequency shift of the damped outrigger, when λ<0.6. Frequency shifts become more significant as the outrigger approaches the roof.
From the numerical analyses under El Centro earthquake, it is concluded that when the outrigger is flexible (ρcto =4), EI is comparatively large under all earthquake levels except by severe. This condition is not affected by the value of ρctc. Under severe earthquakes, the use of a rigid outrigger (ρcto =1) implies larger amount of input energy in the system. This shift may be the result of large damping forces being linearly amplified by the high velocities of the severe motions.
From all the parametric analyses, it is concluded that regardless the optimal Cd'λ < 0.4 has less effect on improving the overall damping ratio of the building, if compared to values of λ >= 0.4. This suggests that optimal λ is somewhere between 0.4 and 0.9. Nevertheless, the optimal damping varies with the mode, so no single outrigger location will lead to reduce the response of all the modes to its minimum.
From the numerical analyses using FE models with conventional and viscous damped outrigger systems, subjected to four levels of ground motions, it was concluded that as the ground motion becomes stronger, viscous dampers effectively reduce the potential of damage in the structure if compared to conventional outriggers. The results confirm that increasing dynamic stiffness by using dampers is more effective than simply increasing stiffness by adding outriggers to reduce the overall response of core structures. The use of dampers in the outrigger seems to be effective in reducing both kinetic and strain energies, which also explains the overall decrease in the accelerations.
In addition, the use of viscous damped outriggers under optimal design conditions, reduces the overturning moments and stresses of the main components of the system, i.e. core, outriggers and perimeter columns, under strong earthquakes –if compared to a conventional outrigger.
From the numerical analyses using FE models with conventional and viscous damped outrigger systems, subjected to four levels of ground motions, it was concluded that inter-storey drifts, peak accelerations and base shear are not substantially reduced with the addition of viscous dampers to the outriggers. These results reinforce the conclusion that no optimal configuration can be considered optimal for reducing all structural responses.
From the numerical analyses using FE models with conventional and damped outrigger systems, subjected to four levels of ground motions, it was concluded that damped outriggers cannot reduce completely the structural damage under critical earthquakes because the peak EH/EI usually precedes the peak Edampers/EI. On the other hand, since dampers increase the dissipative action of energy by damping, the energy that must be absorbed by hysteresis of the structure is reduced.
Hysteretic energy is concentrated in the core, whose damage is provoked by the overpass of the tensile strength. Hence, the core is the main dissipative source of both damping and hysteretic energy. With the addition of viscous dampers the outrigger has a minor load-bearing role. The main advantage of adding viscous dampers to the outriggers is the overall reduction of stress in the members, thus increasing ductility in the structure.
From the analyses of several configurations of double damped and combined fixed+damped outrigger systems described in Chapter 6, under free vibration, it is concluded that only a double set of damped outriggers and the combined damped and fixed outriggers (attaching viscous dampers in the lower set of outriggers) display larger increase of ζ than the 8% of the single damped outrigger. Optimized ζ of the former two are 8.8 and 8.6%, respectively.
Despite this increase of ζ, double and combined outrigger solutions do not present further reduction of peak inter-storey drifts when compared with the single configuration. This seems to suggest that configurations with optimal ζ might not be further optimized for inter-storey drifts reductions. From these results it is not possible to conclude which configuration seems to be the optimal to reduce the overall structural response.
From the analyses of optimal double set of damped outriggers and the combined damped and fixed outriggers, subjected to eight different ground motions, it is concluded that these configurations reduced the hysteretic energy ratio (EH/EI). In addition, the double damped outrigger is more effective for reducing the damage in the structure when subjected to strong and severe earthquake levels. However, such reduction in the hysteretic energy provided by the supplemental damping is not significant.
From all the time-history analyses using a set of eight earthquake records, it can be concluded that viscous damper outrigger structures exhibit a comparatively improved performance if the use of two outriggers matches the predominance of the 2nd mode of vibration, given by the ground motion frequency.
From the simplified economic analyses of optimal double set of damped outriggers and the combined damped and fixed outriggers, it is concluded that the extra costs due to the double damped are about 50% more expensive than the single damped solution. This is valid within the framework given by the Cd values involved in these optimal designs, and assuming the building costs mostly influenced by the amount of reinforcement steel and viscous dampers. To the contrary, the additional costs due to the combined damped and fixed solutions are about 16% cheaper than the single damped solution.
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