Seismic retrofitting with buckling restrained braces: Application to an existing non-ductile RC framed building

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Abstract

This paper assesses the seismic performance of typical reinforced concrete (RC) existing framed structures designed for gravity loads only. The sample two-storey structural system exhibits high vulnerability, i.e. low lateral resistance and limited translation ductility; hence an effective strategy scheme for seismic retrofitting was deemed necessary. Such a scheme comprises buckling restrained braces (BRBs) placed along the perimeter frames of the multi-storey building. The adopted design approach assumes that the global response of the inelastic framed structure is the sum of the elastic frame (primary system) and the system comprising perimeter diagonal braces (secondary system); the latter braces absorb and dissipate a large amount of hysteretic energy under earthquake ground motions. Comprehensive nonlinear static (pushover) and dynamic (response history) analyses were carried out for both the as-built and retrofitted structures to investigate the efficiency of the adopted intervention strategy. A set of seven code-compliant natural earthquake records was selected and employed to perform inelastic response history analyses at serviceability (operational and damageability limit states, OLS and DLS) and ultimate limit states (life safety and collapse prevention limit states, LSLS and CPLS). Both global and local lateral displacements are notably reduced after the seismic retrofit of the existing system. In the as-built structure, the damage is primarily concentrated at the second floor (storey mechanism); the computed interstorey drifts are 2.43% at CPLS and 1.92% at LSLS for modal distribution of lateral forces. Conversely, for the retrofitted system, the estimated values of interstorey drifts (d/h) are halved; the maximum d/h are 0.84% at CPLS (along the Y-direction) and 0.65% at LSLS (yet along the Y-direction). The values of the global overstrength Ω vary between 2.14 and 2.54 for the retrofitted structure; similarly, the translation ductility μΔ-values range between 2.07 and 2.36. The response factor (R- or q-factor) is on average equal to 5.0. It is also found that, for the braced frame, under moderate-to-high magnitude earthquakes, the average period elongation is about 30%, while for the existing building the elongation is negligible (lower than 5%). The inelastic response of the existing structure is extremely limited. Conversely, BRBs are effective to enhance the ductility and energy dissipation of the sample as-built structural system. Extensive nonlinear dynamic analyses showed that more than 60% of input seismic energy is dissipated by the BRBs at ultimate limit states. The estimated maximum axial ductility of the braces is about 10; the latter value of translation ductility is compliant with BRBs available on the market. At DLS, the latter devices exhibit an elastic behaviour. It can thus be concluded that, under moderate and high magnitude earthquakes, the damage is concentrated in the added dampers and the response of the existing RC framed structure (bare frame) is chiefly elastic.

Introduction

Framed systems have been extensively used for building structures in earthquake-prone regions because of their seismic performance (e.g. [1], [2], [3], [4], [5]; among many others). However, a number of existing reinforced concrete (RC) framed building structures were designed for gravity loads only and hence do not possess adequate lateral stiffness and resistance; seismic details are also lacking as observed during surveys carried out in the aftermath of recent earthquakes worldwide (Fig. 1).

It is, therefore, of paramount importance to retrofit such existing framed buildings and enhance their seismic performance. A number of intervention schemes, either traditional or innovative, are available (e.g. [6], [7]; among many others), as shown pictorially in Fig. 2.

Existing framed structures may be suitably retrofitted by using diagonal braces, either traditional steel or innovative. Braced systems exhibit high lateral stiffness and strength under moderate-to-large magnitude earthquakes. The most common structural configurations for lateral-resisting systems are concentrically brace frames (CBFs), which possess a lateral stiffness significantly higher than that of unbraced frames, e.g. moment resisting frames. Nevertheless, due to buckling of the metal compression members and material softening due to the Bauschinger effect, the hysteretic behaviour of CBFs with traditional steel braces is unreliable. Alternatively, buckling-restrained braces (BRBs) may be employed as diagonal braces in seismic retrofitting of steel and RC frames designed for gravity loads only. Such braces exhibit compressive strength, which is about 10–15% greater than tensile; the global buckling is inhibited (e.g., [8], [9], [10]). Frames with BRBs are being used for new and existing structures worldwide (e.g. [11], [12], [13] among many others), especially for damage controlled structures as shown pictorially in Fig. 3 and initially formulated by Wada et al. [14].

The global response of the inelastic structural system can be assumed as the sum of the elastic frame (also termed primary structural system) and the system formed by the diagonal braces (secondary system) that absorbs and dissipates a large amount of hysteretic energy under earthquake ground motion.

The primary system is capable of withstanding vertical loads and behaves elastically under earthquake loads. The secondary system includes the dissipative members and is thus designed to damp the seismic lateral actions and deformations. Dissipative members, such as BRBs, may be installed in the exterior frames of multi-storey buildings and can be thus easily replaced in the aftermath of a devastating earthquake. Primary and secondary systems act as a parallel system; the lateral deformation of the structure as a whole corresponds to the deformation of both primary and secondary systems. Fig. 4 compares the earthquake response of a traditional frame and damage controlled structural system. The response is expressed in terms of cyclic action–deformation relationships. When controlled damage strategy is adopted, the primary structure shows a linear elastic response under both moderate and high magnitude earthquakes. The energy dissipation is localized merely in the diagonal braces acting as dampers. Conversely, traditional framed systems dissipate seismic energy either within all members of the structure or in the beams, if the capacity design rules are employed.

This paper assesses the seismic structural performance of a typical RC framed school building retrofitted with BRBs. The results of comprehensive nonlinear (static and dynamic) analyses showed that the use of BRBs is extremely cost-efficient. Notwithstanding, the design of such structural components is not straightforward. A step-by-step procedure, compliant with the performance-based (force- and displacement-based) framework, is outlined hereafter. A brief discussion of the pros and cons of the use of the BRBs is presented in the next paragraph.

Section snippets

Buckling restrained braces (BRBs)

The disadvantages of traditionally braced frames may be prevented whether or not the occurrence of buckling for the metallic braces in compression is inhibited, e.g. using buckling restrained braces (BRBs). The energy dissipation capacity of a traditional brace is limited by the occurrence of buckling and hence stiffness reduction and strength degradation may occur. Conversely, buckling restrained braces exhibit large and stable hysteretic dissipation even at large amplitudes.

Buckling

Seismic retrofitting strategy

The design of new and existing structures with hysteretic buckling restrained braces generally comprises the following:

  • the estimation of the optimum parameters for the dissipative braces by using simplified methods;

  • the application of capacity design checks for all members of the structure under the expected ultimate force induced by the dissipative braces, e.g. the yielding force of the BRBs;

  • the verification of the design performance, preferably through nonlinear response history analyses.

Geometry

The sample RC existing framed building is located near Naples, in South of Italy; the framed structure was built in the early 1960s and it was designed for gravity loads only. The plan layout of the building comprises two T-shaped blocks (termed Building A and Building B) and a connecting rectangular block (named Building C) as pictorially shown in Fig. 5. Buildings A and B are used for classrooms, while Building C is a sport hall. The total area of the building is about 1400 sqm; the area of

Earthquake input characterization

The construction site of the sample framed building is located in a moderate seismicity zone. The soil foundation can be classified as ground type B, according to the classification implemented in the recently issued national seismic standards [17], which is also compliant with the European code provisions [16]. The available soil profiles of the site have shown that the local geology includes deposits of very dense sand and gravel with several tens of metres in thickness, characterized by a

Analytical structural model

Refined three-dimensional (3D) finite element (FE) models were employed to discretize the sample framed as-built and retrofitted structures and analyze the earthquake response. Bare frames were modelled as 3D assemblages of beam members. Shear deformability of beams and columns was also included in the structural model. Panel zone strengths and deformations were not considered. Fig. 10 displays the FE models utilized for the response analyses of the buildings. Such FE models employ a refined

Structural assessment

The seismic performance of the existing and retrofitted structures was assessed through linear and nonlinear analyses, i.e. eigenvalue analysis, nonlinear static analysis and nonlinear dynamic analysis. The results of the performed analyses are further discussed hereafter.

Conclusions

The present work focuses on the seismic performance assessment of typical reinforced concrete (RC) existing building structures designed for gravity loads only. A refined fibre-based three-dimensional finite element model was implemented to assess the nonlinear earthquake response of a sample non-ductile RC multi-storey building. The existing two-storey framed structure exhibits high vulnerability, i.e. low lateral resistance and limited translation ductility; hence an effective strategy scheme

Acknowledgements

This work was financially supported by the Italian Consortium Tecnologie per il Recupero Edilizio (Technologies for the Restoration of Structures), under the project TELLUS-STABILITA (Testing of innovative technologies and devices to protect the structures from the environmental-induced vibrations, with emphasis on earthquake loading), funded by the Ministry of Education, University and Research—FAR art.5 D.M.8/8/2000, no. 593. Any opinions, findings and conclusions or recommendations expressed

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