Life Safety Limit State Research Articles

Influence of the strength reduction factor on the seismic reliability of structures with FPS considering intermediate PGA/PGV ratios

This paper deals with seismic reliability of nonlinear structural systems equipped with friction pendulum isolators (FPS), subjected to different natural seismic records with intermediate PGA/PGV ratios. The isolated system is described by a 2DOF system and the FPS behavior is described by employing a velocity dependent model. The friction coefficient at large velocity is considered as a random variable. Employing a set of natural seismic records having intermediate PGA/PGV ratios and scaled to the seismic intensity corresponding to life safety limit state for L'Aquila site (Italy) according to NTC08, the inelastic characteristics of the superstructures are designed by evaluating the ratio between the average elastic responses and increasing strength reduction factors. Incremental Dynamic Analyses (IDA) are developed to evaluate the seismic fragility of both the superstructure and the isolation system assuming different values of the corresponding limit states. Integrating the fragility curves with the seismic hazard curve related to L'Aquila site (Italy), the reliability curves of the inelastic base-isolated structural systems, with a design life of 50 years, are derived.

Friction Pendulum System as a Retrofit Technique for Existing Reinforced Concrete Building

Friction pendulum system (FPS) is one of the most effective base isolation systems, which is widely used for seismic upgrading of existing buildings. FPS is almost always non-linear and often strongly non-linear, so a mechanical model of FPS bearing is of great importance in seismic analysis. In this paper, three different models of FPS bearing are described and a building severely damaged by the L’Aquila earthquake in 2009 is examined according to the latest Italian code. Then, a case study presenting briefly an innovative technique that is able to introduce base isolation on existing buildings is provided with reference to the use of FPS. Different kinds of analyses, including response spectrum analysis and linear and non-linear dynamic analyses, are performed under the life-safety limit state. Storey shear, inter-storey drift, effective stiffness and energy dissipation are the considered parameters to investigate the effectiveness of the retrofitting procedure and analysis techniques.

Shake table tests for seismic assessment of suspended continuous ceilings

After an earthquake, the failure of suspended ceiling systems is one of the most widely reported types of nonstructural damage in building structures. Since suspended ceiling systems are not amenable to traditional structural analysis, full-scale experimental testing is planned and executed. In particular, shaking table tests are performed in order to investigate the seismic behaviour of plasterboard continuous suspended ceilings under strong earthquakes. Two kinds of ceiling systems, named single frame ceiling and double frame ceiling, are tested. A steel test frame is properly designed in order to simulate the seismic effects at a generic building storey. A set of five accelerograms, used as input for the shakings, are selected matching the target response spectrum provided by the U.S. code for nonstructural components. Three limit states (occupancy, damage and life safety limit state) are considered in this study in order to characterize the seismic response of suspended ceiling systems. The tested ceilings show no damage at all intensity levels, evidencing a low fragility. Three main aspects may be the cause of this low vulnerability: (a) the continuous nature of the tested ceilings; (b) the dense steel channel grid that supports the plasterboard panels; (c) the large number of hangers that connects the ceiling system to the roof, avoiding any vertical movement of the ceilings. Finally, an interesting comparison is made with a previous vulnerability study on a different typical U.S. ceiling system.

Probabilistic seismic performance assessment of code-compliant multi-story RC buildings

Fragility analyses are conducted in this study to evaluate the relative seismic safety margins of seismic code-designed multi-story reinforced concrete (RC) buildings with varying input motion intensity, ductility level and configuration. Structural variations are accounted for by using twelve buildings [13] with diverse structural systems, heights and ductile detailing. The design peak ground acceleration (PGA) is also varied. The reference structures also include regular and irregular buildings in order to cover a wide spectrum of contemporary mid-rise buildings. Incremental dynamic analyses (IDAs) are deployed using the twelve inelastic fiber-based simulation models of the reference structures and sixty natural ground motions recorded on different soil conditions with a wide range of spectral amplifications. The regression analyses of the selected response quantities show that the soil condition has a marginal effect on the demand-ground motion intensity relationships when adopting spectral acceleration to characterize the ground shaking intensity. The damage state probabilities of wall-frame structures designed to high PGA and ductility levels do not satisfactorily achieve the most favorable safety objectives. Fragilities are reduced by decreasing the design PGA due to the higher contribution of gravity loads to the details of the building design. Using extensive results from twelve buildings subjected to sixty ground motions, a relationship is proposed to enable the quantifying of the Life Safety limit state probabilities of code-compliant mid-rise RC buildings.

Performance-Based Seismic Design of Reinforced Concrete Beam-Column Joints

A performance-based seismic design approach for reinforced concrete beam-column joints has been proposed in this paper. Instead of adopting empirical analysis such as in ACI 318M-08 building code, the proposed approach is based on rational analysis of the stress-strain state of the joint. Two limit states are considered in the design: serviceability limit state and life safety limit state. Performances of the joint at these two levels are determined respectively, with due considerations of capacity design philosophy and post-earthquake repair requirements. Then stress/strain analyses of the joint panel using the Mohr circle in conjunction with the softened stress-strain relationship of concrete are adopted simultaneously to design the joint to achieve the predetermined performance level. Effectiveness of proposed method are validated by using the model to interpret the behaviors of joints observed in previous experiments.

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

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 μ Δ -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.

Performance-based seismic design of reinforced concrete ductile buildings subjected to large energy demands

Current seismic design codes do not contemplate explicitly some variables that are relevant for the design of structures subjected to ground motions exhibiting large energy content. Particularly, the lack of explicit consideration of the cumulative plastic demands and of the degradation of the hysteretic cycle may result in a significant underestimation of the lateral strength of reinforced concrete structures built on soft soils. This paper introduces and illustrates the use of a numerical performance-based methodology for the predesign of standard-occupation reinforced concrete ductile structures. The methodology takes into account two limit states, the performance of the non-structural system, and in the case of the life safety limit state, the effect of cumulative plastic demands and of the degradation of the hysteretic cycle on the assessment of structural performance.

SEISMIC ANALYSES OF A SOFT-FIRST-STORY RC BUILDING RETROFITTED BY THICK HYBRID WALL TECHNIQUE DURING STRONG EARTHQUAKE EXCITATIONS

Experimental investigations on retrofitting soft-first-story RC frames by thick hybrid wall technique as a strengthening-ductile method are briefly explained. The mathematical models and hysteretic behaviors of the members retrofitted by thick hybrid wall technique were assessed and calibrated through the experimentally observed mechanisms. An existing soft-first-story RC building was selected to evaluate the efficiency of thick hybrid wall method on improving seismic performance of the building during strong earthquake excitations. The building was assumed to be retrofitted by two types of retrofitting methods including installation of shear wall (strengthening type) and application of thick hybrid wall (strengthening-ductile type). The evaluations were implemented approximately by seismic vulnerability assessment based on the guidelines by Japan Building Disaster Prevention Association and exactly by nonlinear dynamic response analysis. The analytical results showed the retrofitted building obtained reasonable performance for design basis earthquake at life safety limit state. Moreover, as a case study, the collapse prevention performance of the building retrofitted by thick hybrid wall was checked under an upper-bound of earthquake excitations based on recorded strong earthquakes (which herein are called maximum capable earthquake), and the obtained results demonstrated an acceptable performance.

Use of Cumulative Ductility Spectra Within a Deformation-Control Format for Seismic Design of Ductile Structures Subjected to Long Duration Motions

Due to the large economical losses derived from recent seismic events, design methodologies that are based in the explicit control of the dynamic response of structures have been formulated. This article introduces, within the framework of performance-based design, a numerical methodology for the seismic design of structures that are expected to undergo severe cumulative plastic demands. The methodology explicitly considers and integrates: (1) the structural and non-structural performance of the building; (2) the life safety limit state; and (3) the prevention of low cycle fatigue.