Hysteretic Damped Braces Research Articles

Combination of different types of damped braces for two-level seismic control of rc framed buildings

Hysteretic damped braces (HDBs) represent an effective and low-cost solution for the seismic retrofitting of reinforced concrete (RC) framed structures. However, notable damage to nonstructural elements can be observed at the serviceability design earthquake (SDE), when activation forces of the HDBs are set too high in order to attain structural safety (SS) performance level at the basic design (BDE) and/or maximum considered (MCE) earthquakes. Moreover, HDBs designed at the immediate occupancy (IO) performance level prevent nonstructural damage of MIs under SDE but may collapse and/or induce structural damage for high intensity seismic loads, due to early activation which renders them unable to develop large energy dissipation. On the other hand, as regards significantly increasing dissipation without extra stiffness, nonlinear fluid viscous damped braces (VDBs) offer a promising solution provided that limit states of the VDs are not exceeded. This work proposes a two-level displacement-based design procedure, where HDBs, ensuring SS at the MCE, are combined with HDBs and VDBs, calculated for IO at the SDE. Gap-hook elements attached in series or in parallel to the HDBs are also examined, with the aim of preventing their deformation at MCE and delay their activation at SDE, respectively. A six-storey RC framed structure designed in L'Aquila (Italy) for moderate seismic loads is to be retrofitted in a high risk-seismic region. The OpenSEES platform is considered for the numerical modelling of the original and retrofitted structures. Nonlinear multi-stripe analyses are carried out with reference to actual real records scaled to the SDE, BDE and MCE seismic intensity levels assumed in the current Italian code.

Damage protection of earthquake resistant structures by means of damped braces

Supplementary energy dissipation is an efficient technique for the retrofitting of as built framed buildings as long as the loss of capacity due to previous seismic degradation is evaluated properly. Most of the displacement-based design procedure of damped braces only considers the capacity curve of an initially undamaged structure, while the hysteretic energy is accounted for by an equivalent viscous damping. However, structures may be initially damaged by previous earthquakes, so the proportioning of the damped braces should include the effects of the cumulative damage caused by previous loading cycles. A displacement-damage-based design procedure is proposed and implemented by means of a computer-aided tool named DAMPERS (DAMage Protection of Earthquake Resistant Structures). The residual capacity that a building might experience when subjected to more than one earthquake during its nominal life is taken into account. The loss of capacity of the original structure is evaluated by a combined plastic-damage model, obtained as an in-parallel combination of elastic-perfectly plastic and elastic-softening damage mechanisms, and a damage index, accounting for the cyclic damage of each pair of mechanisms. An archetype six-storey reinforced concrete framed building, representative of the Italian residential housing stock constructed during the 1990s, is retrofitted with hysteretic damped braces designed assuming different damage evolution laws during previous earthquakes. Three configurations of masonry infills (MIs) are considered: bare frame, with nonstructural MIs; infilled frame, with a uniform in-elevation distribution of structural MIs; pilotis frame, with no MIs at the ground floor and structural MIs at the other floors. OpenSees is the computational platform for the nonlinear seismic analysis of the unbraced and damped braced structures, with reference to records scaled in line with the hypotheses adopted and considering failure modes of structural and nonstructural elements.

In-plane and out-of-plane nonlinear seismic response of masonry infills for hospitals retrofitted with hysteretic damped braces

The present work studies the combined in-plane (IP) and out-of-plane (OOP) collapse of unreinforced masonry infills (MIs) of strategic infrastructures such as hospitals retrofitted by hysteretic damped braces (HYDBs), where immediate occupancy (IO) and structural safety (SS, e.g. life-safety or collapse prevention) are the performance levels required after low-to-high amplitude earthquakes. Despite many advances in the development of new supplementary energy dissipation systems, few investigations have evaluated their effectiveness in improving the IP-OOP response of MIs. Because of the complex articulation of medical centres, reference is made to a five-storey pavilion of the hospital campus of Avellino in Campania (Italy). Bare and infilled structural models of this reinforced concrete pavilion are designed in a medium-risk seismic zone, in order to fulfill the corresponding IP drift ratio thresholds provided by a former Italian code for limiting nonstructural damage of MIs at the serviceability limit state. The IO and SS performance levels are checked in a displacement-based design procedure of the HYDBs at the Serviceability (SDE) and Basic (BDE) Design Earthquakes. An optimization procedure is considered, taking into account that an anticipated (delayed) activation of the yielding devices may give rise to concerns about their suitability with respect to high (low) intensity seismic loads. The demolition and reconstruction of MIs, which generally goes hand in hand with the introduction of damped braces in existing frames, is avoided. To this end, exterior, with diagonal damped braces in the window bays, and interior, with less-intrusive chrevron damped braces, retrofitting systems are adopted. Two aspect ratios of MIs are analysed assuming corner arrangements along the perimeter. Nonlinear time-history analyses for the four reference seismic levels assumed in the current Italian code highlight that IP and OOP failures of MIs can be avoided when the activation of the HYDBs at the SDE can be attained without jeopardising structural safety at the Maximum Considered Earthquake (MCE).

Effects of Fire Duration on the Seismic Retrofitting With Hysteretic Damped Braces of r.c. School Buildings

School buildings are susceptible to high incidents of fire because of carelessness, faulty electrical installation and arson, raising the attention on their seismic retrofitting after fire exposure. Hot and residual mechanical properties of a reinforced concrete (r.c.) structure exposed to fire depend on duration of the heating and cooling phases. As a consequence, seismic retrofitting of a fire-damaged framed structure may not be effective when the peak temperature during a fire is considered. For a successful retrofit, ultimate capacity resulting from residual properties after cooling needs to be taken into account. To this end, the state secondary school Collina-Castello of Bisignano (Italy), a three-storey r.c. framed structure designed in a medium-risk seismic region to comply with a former Italian seismic code, is considered as test structure. Thermal analysis of r.c. frame members is preliminarily carried out for two fire scenarios, on the assumption that the fire compartment is confined to the ground (i.e. F0) and first (i.e. F1) levels. Moreover, four fire-damage cases are examined, considering only the heating phase, at 45 minutes of fire resistance, and the overall fire cycle, for fast, medium and slow phases of cooling. Afterwards, the school is supposed to be retrofitted with hysteretic damped braces (HYDBs), in order to achieve the performance levels imposed by current Italian code in a high-risk seismic zone. Nonlinear static and dynamic analyses of the unbraced and damped braced structures are carried out, with reference to the degradation of r.c. frame members for different fire durations in the design procedure of the HYDBs.

A simplified retrofitting method based on seismic damage of a SDOF system equivalent to a damped braced building

A new Displacement-Based Design (DBD) procedure to proportion hysteretic damped braces (HYDBs) is proposed in order to take into account the effects of the seismic degradation of a structure that needs to be retrofitted. To this end, a hysteretic model based on plastic and damage mechanisms is adopted to describe the inelastic response of reinforced concrete (r.c.) frame members. Then, nonlinear seismic analysis of a single-degree-of-freedom system, equivalent to a multi-degree-of-freedom model of the structure, is used to generate the capacity boundary curve by means of the hysteretic model defined starting from the initial backbone curve. Two-, four- and eight-storey r.c. framed structures are designed for a medium-risk seismic zone of a former Italian code. These are then to be retrofitted by inserting HYDBs to attain performance levels imposed by current Italian code in a high-risk seismic zone. Different retrofitting solutions are compared, considering for each retrofitted structure: (i) variable damper ductility coupled with constant frame ductility; (ii) variable frame ductility coupled with constant damper ductility. The effects of different damage levels and different damage evolution laws are also investigated. Finally, pushover curves of the unbraced and damped braced structures are carried out, with and without considering degradation of the hysteretic response of r.c. frame members on the DBD procedure of the HYDBs.

Seismic retrofitting of gravity-loads designed r.c. framed buildings combining CFRP and hysteretic damped braces

Despite the effectiveness of carbon fibre reinforced polymers (CFRPs) and steel braces equipped with hysteretic dissipative devices (HYDBs) to improve the seismic performance of r.c. framed structures, the only use of CFRPs may be unsatisfactory for the retrofitting of buildings designed for gravity loads only while HDYBs may be unsuitable without a preliminary upgrading. The use of CFRP and HYDB as a combined technique for the seismic retrofitting of reinforced concrete (r.c.) buildings is investigated herein. To this end, two-, four- and eight-storey r.c. framed structures are designed with reference to the structural codes for r.c. buildings in force in Italy before and after 1971. A computer code for the nonlinear static and dynamic analysis of r.c. framed structures is modified to include CFRPs and HYDBs. Firstly, the nonlinear static analysis of the original test structures is carried with the aim of improving preliminarily strength and displacement capacities by applying CFRPs laminates, at the top and bottom sides of r.c. frame members, and CFRPs wraps, at the critical end zones of columns, respectively. Then, a displacement-based design procedure of HYDBs is adopted to complete the seismic retrofitting, starting from capacity curves of the upgraded test structures. To check the reliability of the combined CFRP–HYDB technique, nonlinear dynamic analysis of the original and retrofitted structures is performed considering two sets of seven near- and far-fault ground motions scaled to the seismic design level. Results highlight that the insertion of the HYDBs is effective in reducing the seismic demand of previously upgraded CFRP structures, with Post71 types generally performing better than Ante71 ones.

Shear modelling of the beam-column joint in the nonlinear static analysis of r.c. framed structures retrofitted with damped braces

The adoption of a reliable shear model for predicting brittle failure modes of a reinforced concrete (r.c.) beam-column joint, beyond ductile flexural mechanisms at member level, is essential to retrofit r.c. framed buildings properly. The retrofitting of existing structures by means of the insertion of hysteretic damped braces (HYDBs) turns out to be a highly effective means of improving seismic response. In the present work, a Displacement-Based Design (DBD) procedure to proportion the HYDBs to attain, for a specific level of seismic intensity, a designated performance level has been revisited in order to take into account the effects of the nonlinear shear response of beam-column joints. To this end, two-, four- and eight-storey r.c. framed structures, representative of low-, mid- and high-rise r.c. framed buildings, are designed in line with a former Italian seismic code for a medium-risk seismic zone. These are then to be retrofitted by inserting HYDBs to attain performance levels imposed by the current Italian code in a high-risk seismic zone. A computer code for the nonlinear static analysis of r.c. framed structures has been developed, involving local shear response of beam-column joints. A path-following analysis based on the arc-length method has been adopted to obtain the pushover curves of primary and retrofitted test structures, with and without nonlinear shear modelling of the beam-column joints, and the HYDB response is idealized by a bilinear law on the assumption that buckling of the steel braces is prevented.

Effectiveness of damped braces to mitigate seismic torsional response of unsymmetric-plan buildings

The seismic retrofitting of unsymmetric-plan reinforced concrete (r.c.) framed buildings can be carried out by the incorporation of damped braces (DBs). Yet most of the proposals to mitigate the seismic response of asymmetric framed buildings by DBs rest on the hypothesis of elastic (linear) structural response. The aim of the present work is to evaluate the effectiveness and reliability of a Displacement-Based Design procedure of hysteretic damped braces (HYDBs) based on the nonlinear behavior of the frame members, which adopts the extended N2 method considered by Eurocode 8 to evaluate the higher mode torsional effects. The Town Hall of Spilinga (Italy), a framed structure with an L-shaped plan built at the beginning of the 1960s, is supposed to be retrofitted with HYDBs to attain performance levels imposed by the Italian seismic code (NTC08) in a high-risk zone. Ten structural solutions are compared by considering two in-plan distributions of the HYDBs, to eliminate (elastic) torsional effects, and different design values of the frame ductility combined with a constant design value of the damper ductility. A computer code for the nonlinear dynamic analysis of r.c. spatial framed structures is adopted to evaluate the critical incident angle of bidirectional earthquakes. Beams and columns are simulated with a lumped plasticity model, including flat surface modeling of the axial load-biaxial bending moment elastic domain at the end sections, while a bilinear law is used to idealize the behavior of the HYDBs. Damage index domains are adopted to estimate the directions of least seismic capacity, considering artificial earthquakes whose response spectra match those adopted by NTC08 at serviceability and ultimate limit states.

Nonlinear seismic analysis of r.c. framed buildings with setbacks retrofitted by damped braces

Current seismic codes allow the use of damped braces (DBs) for the seismic retrofitting of reinforced concrete (r.c.) framed buildings, but simplified design procedures need to be developed in the case of structures with in-elevation irregularities due to setbacks. In this work, an improved Displacement-Based Design (DBD) procedure of hysteretic damped braces (HYDBs) is proposed to take into account the higher vibration mode effects along the elevation. In line with the extended N2 method, this iterative procedure combines the usual nonlinear static analysis with an elastic modal analysis. Two in-elevation irregular six-storey r.c. framed buildings, characterized by one setback (at the third-storey) and two setbacks (at the second- and fourth-storey), are to be retrofitted by inserting of HYDBs to attain performance levels imposed by the Italian code (NTC08) in a high-risk zone. Local and global displacement-based demand parameters are considered to compare the nonlinear dynamic response of the original and retrofitted structures, considering sets of ground motions whose response spectra match those adopted by NTC08 at serviceability and ultimate limit states. R.c. frame members are idealized by a two-component model, assuming a bilinear moment-curvature law whose ultimate bending moment depends on axial load, while the response of the HYDBs is idealized by a bilinear force-displacement law to prevent yielding and buckling of steel braces.

Nonlinear seismic analysis of unsymmetric-plan structures retrofitted by hysteretic damped braces

A displacement-based design procedure using hysteretic damped braces (HYDBs) is proposed for the seismic retrofitting of unsymmetric-plan structures. An expression of the viscous damping equivalent to the hysteretic energy dissipated by the damped braced frame is proposed under bidirectional seismic loads, where corrective factors are assumed as a function of design parameters of the HYDBs. To this end, the nonlinear dynamic analysis of an equivalent two degree of freedom system is firstly carried out on seven pairs of real ground motions whose displacement response spectra match, on average, the design spectrum proposed by the Italian seismic code for a high-risk seismic zone and a medium subsoil class. Then, the extended N2 method considered by the European seismic code, which combines the nonlinear static analysis along the in-plan principal directions of the structure with elastic modal analysis, is adopted to evaluate the higher mode torsional effects. The town hall of Spilinga (Italy), a reinforced concrete (r.c.) framed building with an L-shaped plan, is supposed to be retrofitted with HYDBs. Six structural solutions are compared considering two alternative in-plan distributions of the HYDBs, to eliminate (elastic) torsional effects, and three design values of the frame ductility combined with a constant design value of the damper ductility. To check the effectiveness and reliability of the DBD procedure, the nonlinear static analysis of the test structures is carried out, by evaluating the vulnerability index of r.c. frame members and the ductility demand of HYDBs for different in-plan directions of the seismic loads.

Seismic vulnerability and retrofitting by damped braces of fire-damaged r.c. framed buildings

There is a lack of knowledge regarding the seismic response of fire damaged reinforced concrete (r.c.) buildings; moreover, an amplification of the damage is to be expected for such structures in the event of an earthquake. To evaluate the nonlinear seismic response following a fire, a numerical investigation is carried out with reference to a five-storey r.c. framed building, which, designed according to the previous Italian seismic code for a medium risk zone now, needs to be considered as in a high-risk seismic zone in line with the current Italian seismic code. More specifically, the nonlinear seismic response of the test structure in a no fire situation is compared with that in the event of fire, at 45 (i.e. R45) and 60 (i.e. R60) minutes of fire resistance, assuming damaged (i.e. DS) and repaired (i.e. RS) stiffness in combination with damaged strength conditions. Then, the fire-damaged test structures are retrofitted by the insertion of hysteretic damped braces (HYDBs), placed only at the storey where the fire compartment is hypothesized. Five fire scenarios have been considered on the assumption that the fire compartment is confined to the first level (i.e. F1), the first two (i.e. F1/2) and the upper (i.e. Fi, i=3–5) levels, with the parametric temperature–time fire curve in accordance with Eurocode 1. The nonlinear dynamic analysis is performed through a step-by-step procedure based on a two-parameter implicit integration scheme and an initial-stress-like iterative procedure. At each step of the analysis, plastic conditions are checked at the critical (end) sections of the girders and columns, where a thermal mapping with reduced mechanical properties is evaluated in accordance with the 500°C isotherm method proposed by Eurocode 2, while the behaviour of a HYDB is idealized through the use of a bilinear law provided that buckling is prevented.

Comparative study of the seismic response of RC framed buildings retrofitted using modern techniques

The main purpose of this work is to compare different criteria for the seismic strengthening of RC framed buildings in order to find the optimal combinations of these retrofitting techniques. To this end, a numerical investigation is carried out with reference to the town hall of Spilinga (Italy), an RC framed structure with an L-shaped plan built at the beginning of the 1960s. Five structures are considered, derived from the first by incorporating: carbon fibre reinforced polymer (FRP)-wrapping of all columns; base- isolation, with high-damping-laminated-rubber bearings (HDLRBs); added damping, with hysteretic damped braces (HYDBs); FRP-wrapping of the first storey columns combined with base-isolation or added damping. A three-dimensional fibre model of the primary and retrofitted structures is considered; bilinear and trilinear laws idealize, respectively, the behaviour of the HYDB, providing that the buckling be prevented, and the FRP-wrapping, without resistance in compression, while the response of the HDLRB is simulated by using a viscoelastic linear model. The effectiveness of the retrofitting solutions is tested with nonlinear dynamic analyses based on biaxial accelerograms, whose response spectra match those in the Italian seismic code.

Displacement-based seismic design of hysteretic damped braces for retrofitting in-elevation irregular r.c. framed structures

A displacement-based design procedure is proposed for proportioning hysteretic damped braces (HYDBs) in order to attain, for a specific level of seismic intensity, a designated performance level of a reinforced concrete (r.c.) in-elevation irregular framed building which has to be retrofitted. To check the effectiveness and reliability of the design procedure, a numerical investigation is carried out with reference to a six-storey r.c. framed building, which, originally designed according to an old Italian seismic code (1996) for a medium-risk zone, has to be retrofitted by inserting of HYDBs to attain performance levels imposed by the current Italian code (NTC08) in a high-risk zone. To simulate a vertical irregularity, a change of use of the first two floors, from residential to office, is also supposed; moreover, masonry infill walls, regularly distributed along the perimeter, are substituted with glass windows on these floors. Nonlinear dynamic analyses of unbraced (UF), infilled (IF) and damped braced infilled (DBIF) frames are carried out considering sets of artificially generated and real ground motions, whose response spectra match those adopted by NTC08 for different performance levels. To this end, r.c. frame members are idealized by a two-component model, assuming a bilinear moment–curvature law whose ultimate bending moment depends on the axial load, while the response of an HYDB is idealized by a bilinear law, to prevent buckling. Finally, masonry infills are represented as equivalent diagonal struts, reacting only in compression, with an elastic–brittle linear law.

Displacement-based seismic design of hysteretic damped braces for retrofitting in-plan irregular r.c. framed structures

A displacement-based design procedure is proposed for proportioning hysteretic damped braces in order to attain, for the in-plan least seismic capacity direction and a specific level of seismic intensity, a designated performance level of a reinforced concrete (r.c.) irregular framed building to be retrofitted. To this end, a computer code for the nonlinear static analysis of spatial frames is developed to obtain the pushover curve for an assigned in-plan direction of the seismic loads. The town hall of Spilinga (Italy), a two-storey r.c. framed structure with an L-shaped plan built at the beginning of the 1960s, has been considered as case study. Four alternative structural solutions are examined, derived from the first one by the insertion of hysteretic damped braces, considering: the extended N2 and the extended pushover methods combined with a proportional and an inversely proportional in-plan stiffness distributions of hysteretic damped braces. To check the effectiveness and reliability of the design procedure, the nonlinear static response of the unbraced and damped braced frames is compared for different in-plan directions of the seismic loads. Frame members are simulated with a lumped plasticity model, including a flat surface modeling of the axial load-biaxial bending moment elastic domain, while the behavior of a hysteretic damped brace is idealized through the use of a bilinear law. Vulnerability index domains are adopted to estimate the directions of least seismic capacity at the ultimate (i.e. life-safety and collapse prevention) limit states prescribed by Italian and European seismic codes.

Equivalent viscous damping for displacement-based seismic design of hysteretic damped braces for retrofitting framed buildings

A displacement-based design (DBD) procedure aiming to proportion hysteretic damped braces (HYDBs) in order to attain, for a specific level of seismic intensity, a designated performance level of a structure is proposed for the retrofitting of framed buildings. A key step for the reliability of the DBD procedure is the selection of the equivalent viscous damping in order to account for the energy dissipated by the damped braced frame. In this paper, expressions of the equivalent damping are obtained considering the energy dissipated by the HYDBs and the framed structure. To this end, dynamic analyses of an equivalent single degree of freedom system, whose response is idealized by a trilinear model, are carried out considering real accelerograms matching, on the average, Eurocode 8 (EC8) response spectrum for a medium subsoil class. Then, a three-storey reinforced concrete (r.c.) framed structure of a school building, designed in a medium-risk seismic region according to the Italian code in force in 1975, is supposed as retrofitted as if in a high-risk seismic region of the current seismic code (NTC08) by the insertion of HYDBs. Nonlinear static analyses are carried out to evaluate the vulnerability of the primary structure, characterized by the lack of interior girders along the floor slab direction, and to select optimal properties of the HYDBs. The effectiveness of the retrofitting solutions is checked referring to nonlinear dynamic analyses, considering artificially generated accelerograms whose response spectra match those adopted by NTC08 for the earthquake design levels corresponding to the serviceability and ultimate limit states.

Nonlinear seismic analysis of RC framed structures equipped with hysteretic damped braces

The insertion of damped braces proves to be very effective for the seismic retrofitting of framed buildings. In the last few decades several applications have been used in many countries, adopting damping systems with different characteristics, depending on both the arrangement of the damped braces and kind of damping device. However, for a widespread application of this technique, practical design procedures and simple numerical models are needed. In this paper, attention is focused on the modelling and nonlinear seismic analysis of framed structures equipped with friction, metallic yielding, viscoelastic and viscous dampers. A design procedure is proposed for proportioning damped braces in order to attain, for a specific level of seismic intensity, a designated performance level of the structure. A six-storey reinforced concrete (RC) framed building, designed in a medium-risk seismic region, is supposed to be retrofitted as in a high-risk seismic region. Two different criteria are followed for distributing the stiffness and strength properties of dissipative braces, over the whole of each storey and among the single braces. A numerical investigation is carried to study the nonlinear dynamic behaviour of the designed structures under real and artificially generated ground motions. The results show that the proposed design procedure is effective and reliable.