Novel adaptive hysteretic damper for enhanced seismic protection of braced buildings

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Highlights

  • Experimental development of an innovative steel damper featuring an adaptive hysteretic behaviour.

  • Proposal and validation of a dedicated linear equivalent design procedure.

  • Application of the design procedure to a real case-study hospital.

  • Structural protection target for severe ULS earthquakes.

  • Enhanced protection of non-structural components under minor SLS earthquakes.

Abstract

Traditional bilinear hysteretic dampers (BHDs) are installed in buildings to enhance their seismic performance by increasing the effective stiffness and damping and are usually designed to guarantee the structural safety for severe ultimate limit state seismic events. As a negative consequence, only minimal damping is provided during weak but more frequent serviceability limit state earthquakes since BHDs mainly operate in their elastic regime. This can cause high peak floor accelerations (PFAs) that are detrimental for sensitive non-structural components (NSCs), like electric network, elevators, false ceilings, and computers, whose integrity is crucial in high-technological buildings (e.g. hospitals, and emergency centers). In order to improve this unacceptable situation, a novel adaptive hysteretic damper (AHD) has been developed by the authors. The AHD can modulate its effective damping and stiffness to the intensity (e.g. peak ground acceleration) of the occurring earthquake leading to: (i) reduced PFAs and enhanced NSCs protection for minor earthquakes; (ii) not impaired structural safety under major severe events. In this paper the force-displacement response of the AHD is experimentally assessed and a simple linear equivalent design method for braced buildings implementing this innovative technology is proposed and validated through comparison with non-linear time history analyses. The procedure is then exploited to design the seismic-retrofit intervention of a real case-study hospital and the enhanced seismic performances, compared to those offered by conventional BHDs, are quantified.

Introduction

Among antiseismic hardwares, traditional Bilinear Hysteretic Dampers (BHDs) were introduced in the ‘70s of the last century as a viable and effective solution to mitigate the effects of strong earthquakes on braced frames [1,2]. Indeed, BHDs are usually inserted along buildings diagonal braces in order to dissipate the seismic energy through the plastic deformation of sacrificial mild steel components. This ensure a stable force-displacement (F-d) response that is not, or only marginally, affected by velocity and is conventionally described by means of an elasto-plastic model with post-yielding hardening.

Over the last thirty years, several BHDs were developed and introduced in the market. BHDs plastically deformed under axial loads [3], shear loads [4], bending [5,6], and torsion [7] have been proposed and implemented worldwide for the seismic retrofit of schools [[8], [9], [10]] and for the protection of hospitals [11] profiting of their lower cost in comparison to other technologies (e.g. fluid viscous dampers).

Among BHDs, the Shear Hysteretic Panels (SHPs) consist of a steel plate with equally spaced openings delimiting a number of dissipative laminae. The relative motion between the bottom and the upper edge of the SHP induces the shear deformation of the laminae, first in the elastic and then in the plastic regime, resulting in the typical bilinear F-d behaviour. Despite since the early ‘90 the response of honeycomb [12], rectangular [13], and hourglass [14] shear hysteretic laminae have been investigated, a widespread diffusion of SHPs among popular anti-seismic technologies has been so far constrained by the unsatisfactory strength against out of plane buckling problems.

Performance Based Design (PBD) procedures were firstly formulated by Priestely in the early 80's [15] and have been nowadays endorsed in the most advanced design codes and guidelines like the ASCE 7–16 [16] and the Eurocode 8 [17]. In particular the latter establishes two different seismic scenarios and the relevant performance requirements for the design of new structures as well as for the retrofitting of existing ones:

  • 1)

    under the severe “Ultimate Limit State” (ULS) scenario, featuring 10% probability of exceedance over the reference period VR=50 years, the structures shall be capable to withstand the dynamic seismic loads without any global or local collapse preserving as well a residual lateral strength after the quake. This requirement is usually referred as “structural safety requirement” or “no-collapse requirement”;

  • 2)

    the “Serviceability Limit State” (SLS) scenario provides less intense but more frequent earthquakes characterized by 10% probability of exceedance over the lower reference period VR=10 years. In the aftermath of the SLS seismic scenario, the structures are demanded to remain fully-operational (“damage limitation requirement”).

In order to attain the “Damage limitation requirement” target, besides the absence of structural damages, both “acceleration-sensitive”, and “drift-sensitive” non-structural components (NSCs) should be protected against detrimental high peak floor accelerations (PFAs) and interstorey-drifts since their integrity is crucial in high-technological buildings (e.g. hospitals, and emergency centers) [18]. Due to the huge number of NSCs and the wide variety of possible installation layouts, an exhaustive definition of relevant failure thresholds is still not available. However, a simple approach to the scope is the HAZUS method [[19], [20], [21]] that is based on the definition fragility curves. These functions provide the statistical distribution of the probability P that the seismic demand D exceeds the capacity C (strength) of the NSC. Given the intensity of the seismic event IM, and assuming the log-normal distribution ε for the random variable C, the probability of failure P(C<D|D) is:P(C<D|D)=0IM1xβ2πe12(log(x/Cm)2β2)dx=φ(log(IM/Cm)β)being Cm=1 the median value of the capacity (C=Cmε), β the standard deviation, and φ the standard normal cumulative distribution function. The HAZUS method establishes four damage levels: (1) slight damage; (2) moderate damage; (3) extensive damage; and (4) complete damage. A list of moderate-extensive capacity limits for NSCs most common in high-technological buildings is reported in Table 1 and is based on a wide literature survey [18] that has been recently enlarged in Ref. [22]. With respect to the original list, some “displacement-sensitive” components are here also added. Indeed, it is well known that components such as storage racks, which are often tall, and heavily loaded, parapets, and chimneys could suffer extensive failure when experiencing large displacements [[23], [24], [25]]. However, due to the lack of dedicated statistical investigations, the “on-site expert judgment” is the approach suggested here for the definition of the median capacity of these components.

Although approaches more refined than the HAZUS have been recently proposed, their application is not always practical since requiring the knowledge of the natural frequency of each NSC to account for its dynamic interaction with the supporting structure by means of “floor response spectrum” analyses [23,26]. Despite the increasing number of shake table tests conducted in the last years on single NSCs [[27], [28], [29]], as well as on small or full scale structures with integrated plants and architectural elements [[30], [31], [32], [33]], this information is often missing.

The seismic response of several different layouts of frames braced through hysteretic dampers has been recently investigated through shaking table tests carried out at the University of Basilicata, Italy [41]. Maximum interstorey drifts and peak floor accelerations have been compared to those recorded for the bare frame. On one hand, the introduction of the hysteretic braces allowed a significant reduction of the frame lateral deformations while, on the other hand, caused a huge increase (up to 300%) of PFAs. Similar outcomes are also described in Ref. [42] where high PFAs, induced by the introduction of hysteretic braces for seismic-retrofit purpose, were proven to be detrimental for “acceleration-sensitive” NSCs. Enhancements in traditional seismic design methods and new approaches based on low damage methodologies and cost-efficient technologies are hence needed [43,44]. Nevertheless, in common design practice antiseismic devices are designed accounting for the “structural safety requirement” at the ULS only [[45], [46], [47], [48], [49], [50]]. In order to improve this unacceptable situation, the novel Adaptive Hysteretic Damper (AHD), capable to modulate its effective damping and stiffness based on the intensity level of the ongoing earthquake, is presented in this paper. The analytical model ruling the two-stage hysteretic behavior of the AHD is illustrated and its accuracy, as well as and the reliability of the device under multiple seismic-excitations, is assessed through displacement-controlled experimental tests carried out on a 2.4 MN prototype. Eventually a simple linear equivalent design method (based on iterative response spectrum analyses) for braced frames implementing AHDs is developed and applied for the seismic-retrofit of a real case-study building. The advantages warranted by the new proposed technology are argued and compared to the performances offered by more conventional BHDs.

Section snippets

Novel adaptive hysteretic damper (AHD)

Like in conventional SHPs, the dissipative core of the novel AHDs provides a number of shear hysteretic laminae featured by an optimized hourglass shape. In addition, in order to increase its strength against lateral buckling, the laminae does not lie on a single plane but are arranged to form a squared hollow section (Fig. 1-a). Tension and compression loads are clockwise alternatively transmitted to the corners of the pipe section inducing the shear deformation of the laminae (Fig. 1-b).

Experimental tests

The hysteretic response of the AHD, with a particular focus on its robustness under repeated cyclic motions, was investigated through displacement-controlled tests carried out at the Eucentre Lab. (Pavia, Italy) [51]. A full-scale prototype of AHD prototype, made of common structural steel S355, was manufactured by Maurer SE [52] according to the following design parameters: (1) minor hysteretic core featuring Fy,D1=450kN, kel,D1=80kN/mm, kpl,D1=6.0kN/mm (2) width of the hole in the

Design procedure for buildings braced through AHDs

A design procedure dedicated to braced frames implementing the novel AHDs is proposed in Section 4.1 assuming, as starting condition, the a-priori knowledge of the structural properties of the bare frame. This circumstance is always representative of retrofitting projects while, in case of new buildings, could be attained through a preliminary design of the bare frame taking into account, for instance, the gravitational loads only. The final layout, including the dissipative braces with the

A case-study

In this Section, the proposed design procedure is applied to a real case-study building and the beneficial effects offered by the AHD, compared to traditional bilinear hysteretic dampers (BHDs), are quantified. The “Piastra building” of the hospital of Lamezia Terme, a small city located in a high seismic area in southern Italy, is selected as case-study. Despite the whole structure is composed of three independent frames interspersed by seismic joints, for sake of simplicity, only block C has

Conclusions

The Adaptive Hysteretic Damper (AHD), a novel anti-seismic device that is capable to modulate its effective stiffness and damping depending on the intensity (e.g. PGA) of the occurring earthquake, has been presented in this paper. The AHD comprises a minor and a major hysteretic cores that are arranged in series and are mutually linked through a “gap-connector”. Compared to conventional bilinear hysteric dampers (BHDs), the AHD offers: (i) not impaired structural safety under severe ULS events;

Credit author statement

E.G: Investigation; Methodology; Writing – review & editing. IS.C: Investigation; Methodology. J.D: Investigation; Methodology. P.D: Supervision. F.W: Supervision; Investigation. A.T: Supervision; Writing – review & editing

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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