Instability of buildings during seismic response (original) (raw)
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CHAPTER 3 REVIEW OF LITERATURE 3.1 SEISMIC RESPONSE OF BUILDINGS
One of the major areas of research in the field of earthquake engineering has been the development of the method of evaluating the earthquake response of buildings under static nonlinear analysis, popularly known as the push over analysis. It was in the year 1996 that ATC 40 (Applied Technology Council document No. 40) [01] titled as "Seismic Evaluation and Retrofit of Concrete Buildings" was published. It emphasizes the use of available simplified nonlinear static procedures like the capacity spectrum method, the displacement coefficient method and the secant method and focuses on the capacity spectrum method (CSM) which uses the intersection of the capacity (pushover) curve and a reduced response spectrum to estimate maximum displacement. This document is a comprehensive guide for implementing the Static Non Linear analysis procedure along with the other two important documents FEMA 273 and 274 [02]
The effects of repeated earthquake ground motions on the non-linear response of SDOF systems
Earthquake Engineering & Structural Dynamics, 2003
In many parts of the world, the repetition of medium-strong intensity earthquake ground motions at brief intervals of time has been observed. The new design philosophies for buildings in seismic areas are based on multi-level design approaches, which take into account more than a single damageability limit state. According to these approaches, a sequence of seismic actions may produce important consequences on the structural safety. In this paper, the e ects of repeated earthquake ground motions on the response of single-degree-of-freedom systems (SDOF) with non-linear behaviour are analysed. A comparison is performed with the e ect of a single seismic event on the originally non-damaged system for di erent hysteretic models in terms of pseudo-acceleration response spectra, behaviour factor q and damage parameters. The elastic-perfect plastic system is the most vulnerable one under repeated earthquake ground motions and is characterized by a strong reduction of the q-factor. A moment resisting steel frame is analysed as well, showing a reduction of the q-factor under repeated earthquake ground motions even larger than that of an equivalent SDOF system. This in practice means a two-level veriÿcation, serviceability and ultimate limit states. Mazzolani and Piluso [2], and Anastasiadis et al. proposed three levels of veriÿcation: serviceability, damageability and survivability limit states, with a return period of the earthquake ground motion for each level equal to 10, 50 and 450 years, respectively. In the Vision 2000 Committee of SEAOC [4], four levels of structural performance were considered: fully operational, operational, life safety and near collapse for frequent, occasional, rare and very rare earthquake ground motions, respectively. In the ATC40 proposition [5], a complete seismic design approach based on performance philosophy is presented. Dual or multi-level performance objectives are considered (ÿve levels are generally recommended) and they can involve both structural and non-structural performance levels. Structural performance levels can require the immediate occupancy, damage control, life safety, limited safety and structural stability of the building.
Inelastic Behavior of Buildings Under Repeated Vrancea Earthquakes
2007
In the current seismic design format, the key issue in establishing realistic seismic loads is the behavior factor. It accounts for all the dissipative mechanisms that a structural system may develop under a strong ground motion, however not clearly enough stated yet. It corresponds to the performance level associated to the ultimate limit state (i.e. life safety), related to a 100 years mean return interval of earthquake ground motion with a prescribed peak acceleration of ground. The paper investigates the effect of repeated Vrancea strong ground motions on the behavior factors and the related parameters that accounts for cyclic structural deterioration due to inelastic response. A large number of integrated analyses, nonlinear response analyses and energy balance-based analyses were carried out and estimates were made on the behavior factors for inelastic SDOF systems controlled by flexure with stiffness degrading. The correlation between behavior factors and damage level are inv...
Effect of second-order forces on seismic response
Canadian Journal of Civil Engineering, 2006
In a building structure subjected to seismic forces, the gravity loads acting through the lateral displacements lead to additional shears and moments. This is generally referred to as the P–Δ effect; it tends to reduce the capacity of the structure to resist the seismic forces and may lead to instability. It has been suggested that an increase in structural strength, in stiffness, or in both would mitigate the P–Δ effect and ensure stability of the structure. It is shown here that instability results when the P–Δ effect causes the stiffness of the structure to become negative in the post-yield range, in which case increasing the strength, the stiffness, or both does not ensure stability. In a single-storey structure, stability can be ensured if there is sufficient strain hardening that the post-yield stiffness is positive even in the presence of the P–Δ effect. For a multistorey building the vulnerability of the structure to P–Δ instability can be judged by obtaining a pushover curv...
Critical response of structures to multicomponent earthquake excitation
Earthquake Engineering & Structural Dynamics, 2000
This paper aims to develop an improved understanding of the critical response of structures to multicomponent seismic motion characterized by three uncorrelated components that are deÿned along its principal axes: two horizontal and the vertical component. An explicit formula, convenient for code applications, has been derived to calculate the critical value of structural response to the two principal horizontal components acting along any incident angle with respect to the structural axes, and the vertical component of ground motion. The critical response is deÿned as the largest value of response for all possible incident angles. The ratio rcr=rsrss between the critical value of response and the SRSS response-corresponding to the principal components of ground acceleration applied along the structure axes-is shown to depend on three dimensionless parameters: the spectrum intensity ratio between the two principal components of horizontal ground motion characterized by design spectra A(Tn) and A(Tn); the correlation coe cient of responses rx and ry due to design spectrum A(Tn) applied in the x-and y-directions, respectively; and ÿ = ry=rx. It is demonstrated that the ratio rcr=rsrss is bounded by 1 and (2=1 + 2 ). Thus the largest value of the ratio is √ 2, 1.26, 1.13 and 1.08 for = 0, 0.5, 0.75 and 0.85, respectively. This implies that the critical response never exceeds √ 2 times the result of the SRSS analysis, and this ratio is about 1.13 for typical values of , say 0.75. The correlation coe cient depends on the structural properties but is always bounded between −1 and 1. For a ÿxed value of , the ratio rcr=rsrss is largest if ÿ = 1 and = ± 1. The parametric variations presented for one-storey buildings indicate that this condition can be satisÿed by axial forces in columns of symmetric-plan buildings or can be approximated by lateral displacements in resisting elements of unsymmetrical-plan buildings.
Probabilistic assessment of dynamic instability of frame structures under seismic excitations
Mitigation of collapses of structural systems caused by a strong earthquake shaking is crucial to reduce the potential casualties, injuries and economic losses. Hence, accurate risk assessment of structural collapse under seismic excitations is critical in efforts to promote hazard-resilience of the society. This paper summarizes the authors' recent efforts for accurate prediction of structural collapse with systematic incorporation of uncertainty. Computational simulation models are developed for collapse test frames in the literature and validated using experimental data. Through the validated computational simulations of collapse, alternative collapse criteria are proposed in terms of dynamic instability, i.e. the loss of the ability to sustain the gravity loads. Using the new collapse criteria, key parameters that govern collapse capacity and collapse limit state functions are identified for more effective risk assessment. A probabilistic framework is also being developed for systematic treatment of uncertainties in the ground motion time histories and for risk-informed design of frame structures under earthquake hazards.
Earthquakes results from the sudden movement of tectonic plates in the earth's crust. The movement takes place at fault lines, and the energy released is transmitted through the earth in the form of waves that causes ground motion many miles from the Epicentre. Regions adjacent to active fault lines are the most prone to experience earthquakes. These waves arrive at various instants of time, have different amplitudes and carry
MODIFICATION OF THE BUILDING DYNAMIC RESPONSE DUE TO STRUCTURAL DAMAGING DURING STRONG EARTHQUAKEs
The paper is an analysis of buildings behavior during strong earthquakes, considering the modifications of the structural dynamic characteristics due to strong earthquake -generated damages. The study is focused on the difference in a building behavior, function of the mode the structure is situated relative to the seismic ground motion from the spectral point of view: above resonance or below resonance. Experimental and simulation results are presented for simplified mechanical models of building structures with stiffness degradation. As a convenient measure of the effect of duration and severity of the building seismic loads, the total energy dissipated through hysteresis is considered.
EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS
How to select a limited number of ground motion records (GMRs) is an important challenge for the nonlinear analysis of structures. Since epsilon (ε Sa ) is an indicator of spectral shape, which has a significant correlation with the non-linear response of a structure, the selection of GMRs based on the hazard-related target ε Sa is a reasonable approach. In this paper, an alternative indicator of spectral shape is proposed, which results in a more reliable prediction of the non-linear response for the structures with the natural period of 0.25 to 3.0 s. This new parameter, named eta ( ), is a linear combination of ε Sa and the peak ground velocity epsilon (ε PGV ). It is shown that , as a non-linear response predictor, is remarkably more efficient than the well-known and convenient parameter ε Sa . The influence of -filtration in the collapse analysis of an eight-story reinforced concrete structure with special moment-resisting frames was studied. Statistical analysis of the results confirmed that the difference between ε-filtration and -filtration can be very significant at some hazard levels. In the case of this structure, the resulting annual frequency of collapse was found to be lower in the case of -based record selection, in comparison with the ε-based record-selection approach.
Dynamic Instability of Simple Structural Systems
Journal of Structural Engineering-asce, 2003
Lateral strengths required to avoid dynamic instability of single-degree-of-freedom systems are examined. Oscillators with a bilinear hysteretic behavior with negative postyield stiffness are considered. Mean lateral strengths normalized by the lateral strength required to maintain the system elastic are computed for systems with periods ranging from 0.2 to 3.0 s when subjected to 72 earthquake ground motions recorded on firm soil. The effect of the period of vibration and postyield stiffness are investigated. Results indicate that mean normalized lateral strengths required to avoid dynamic instability decrease as negative postyield stiffness increases and that the reductions are much larger for small negative postyield stiffness than for severe negative postyield stiffnesses. It is concluded that there is a significant influence of the period of vibration for short-period systems and for systems with mildly negative postyield stiffnesses. Dispersion of normalized lateral strengths required to avoid dynamic instability are found to increase as the negative postyield stiffness decreases and as the period of vibration increases. Simple equations that capture the effects of period and postyield stiffness to aid in the evaluation of existing structures are obtained through nonliner regression analyses.