Seismic Upgrade of Steel Frame Buildings by Using Damped Braces

Abstract

Supplementary energy dissipation has proved to be an effective way of protecting structures from the disastrous effects of earthquakes and has been used in the last decades both in new and in existing constructions. In this regard, various procedures for the design of the damping system for the seismic retrofit of buildings have been formulated over the years, mainly focused on reinforced concrete (RC) constructions, which represent the largest part of the existing stock in many seismic-prone countries. The study deals with the assessment of a displacement-based design procedure for proportioning the damping system recently proposed in the literature for RC framed buildings, with the goal of establishing a good practice for the application of the procedure to steel buildings as well. The method was applied to three case-study frames, regular in plan and in elevation, which were assumed as being representative of old structures designed without consideration of seismic requirements. The retrofit was performed by using chevron braces equipped with dampers with an elastic-perfectly plastic behavior. The method aimed at defining the properties of the dampers to achieve a target performance in terms of the maximum lateral deflection for a specific level of seismic intensity. The effectiveness and reliability of the proposed procedure was eventually assessed by evaluating the seismic performance of the upgraded steel structures in static and dynamic non-linear analyses.

Assessment of Non-Linear Analyses of RC Buildings Retrofitted with Hysteretic Dampers According to the Italian Building Code

“…A T 1 topography, and soil type B is considered for Benevento resulting in a PGA equal to 2.94 m/s 2 , while T 1 topography and soil type C are assumed for Tramutola for a PGA = 3.41 m/s 2 . The retrofit design is carried out applying a procedure developed by the authors of work [1,41]. This methodology is implemented in the ADRS space, where the structural system composed of the RC frame and the hysteretic dampers is modelled through an equivalent SDOF system, characterized by its secant stiffness and equivalent viscous damping ratio.…”

“…In this study, the two structures are retrofitted by using hysteretic steel dampers installed into diagonal steel braces (Figure 7), which is a common configuration for frame buildings [1,5,6,[43][44][45]]. An elastic-perfectly plastic constitutive behavior is assumed for the dampers, similarly to other studies [1,6,41,43,44]. Four units on each floor in both X and Z directions are installed in the external bays according to the layout shown in Figure 7.…”

“…The mechanical properties of each diagonal steel brace and the encased hysteretic steel damper form an in-series system can be expressed as a combination of the properties of the linear elastic brace (B) and of the elasto-plastic damper (D), as shown in Figure 8, where K B and K D are the elastic stiffnesses of the brace and of the damper, V y is the maximum force of the damper, d y, d bd and d 2 are the yield displacement, the design seismic displacement calculated at LLS and the ultimate displacement of the damper at CLS, respectively. Damper ductility µ D , defined as the ratio between d bd and yield deflection d y , for standard steel hysteretic dampers typically ranges between 4 and 16 [1,15,41,43]. The properties of the single damped brace (DB), in terms of stiffness K DB , ductility µ DB , and damping capacity ξ DB , are determined through the Equations ( 6)-( 8), where ratio K B /K D between the stiffnesses of the steel brace and the damper should be taken ≥2 in order to guarantee that the largest part of the deformation of the story is concentrated in the damper [45,46].…”

“…The case study structures are modelled as 3D buildings in the OpenSees software program version 3.6.0 [47,48], adopting the modelling procedure presented in [49] and validated in [1,5,6,41,43]. Beams and columns are implemented through the forceBeamColumn element object [50], assigning a non-linear behavior to the external sections and a linear elastic behavior to the internal section of the element, as shown in Figure 9.…”

Analytical study on seismic strengthening of existing reinforced concrete buildings by implementation of energy absorbers

“…2 Energy dissipation system in a structure [18] There are several energy dissipation devices have been studied by scientist and some of them are applied in newly designed and strengthened buildings. These devices are Metallic Yielding Dampers [19,20,21], Energy Dissipative Bracing (EDB) [22,13,23,24], Hysteretic Energy Dissipative Bracing (HEDB) [25,26] , Dissipative Energy Device Based on The Plasticity of Metals [15], Seismic Design of RC Braced Frames with Metallic Fuses [27,28], Saw Type Seismic Energy Dissipaters [29,30], Re-centering Energy Dissipative Braces [31] and Adaptive Hysteretic Dampers [32], Impulsive Semi-Active Mass Dampers [33], Damped Braces [34], bolt connected buckling-restrained braces [35] [36] and visco-elastic dampers [37].…”