Yield Strength Coefficient Research Articles

Residual displacement ground-motion model

Residual displacement (RD), indicative of permanent post-earthquake deformation resulting from nonlinear structural response, plays an important role in assessing structural integrity and seismic safety. This study introduces a ground-motion model (GMM) for predicting RDs in inelastic single-degree-of-freedom systems, utilizing a large database of recorded ground motions. The RD GMM adopts key input parameters including 5%-damped elastic pseudo-spectral acceleration (PSA) over periods from 0.01 to 4.0 s, yield strength coefficient (Cy), earthquake magnitude (M), and closest distance to the fault rupture plane (Rrup). Levels of inelasticity are accounted for by employing the yield strength reduction factor, denoted as (PSA/g)/Cy. The results indicate that while M and Rrup significantly affect RDs, site class and significant duration have relatively less influence. RD GMM coefficients and unconditional standard deviations of the model are provided for specific Cy values across a period range of 0.01–4.0 s. The analyses indicate relatively large standard deviations in the GMM due to inherent variability in residual displacement, highlighting the importance of understanding and accounting for this variability in seismic design and performance assessment. Validation against a multi-degree-of-freedom bridge residual drifts demonstrates the effectiveness of the model in predicting the distribution of residual drifts.

RHEOLOGICAL AND COLLOID-STRUCTURAL BEHAVIOUR OF HIGH-VISCOSITY CRUDE OIL AND EMULSION AFTER WAVE ACTION

One of the main problems in oil production is the formation of stable oil-water emulsions that cause corrosion of pipelines, malfunction of pumping equipment, and poisoning of catalysts at refineries. The features of the viscosity-temperature behaviour of high-viscosity resinous crude oil from the Russkoye field (Yamal-Nenets autonomous district) and its 30 wt% emulsion after exposure to electromagnetic and low-frequency acoustic fields are investigated. Electromagnetic treatment of crude oil leads to a decrease in the phase transition temperature and the yield strength coefficient. Low-frequency acoustic processing of the emulsion is accompanied by a decrease in the effective viscosity and the yield strength coefficient. After a complex wave action, the viscosity-temperature characteristics of crude oil continue to fall, and the stability of the formed emulsions decreases due to the intense coalescence of water-phase droplets. It is shown that the amount of released asphaltenes in oil-containing systems decreases after wave treatment. On the contrary, after a complex impact, their content in crude oil increases, while in the emulsion it continues to decrease. This is likely to occur due to the release of hydrocarbons occluded in the asphaltene structure into the liquid oil phase.

Collapse strength ratios for structures under mainshock-aftershock subduction seismic sequences

The effects of aftershocks on civil structures have called the attention of the earthquake engineering community mainly in the last two decades. However, neither seismic design codes nor seismic assessment guidelines for existing structures worldwide consider the possible detrimental effects of strong aftershocks nowadays. From this aim, this paper presents the results of a statistical study focused on computing the relative lateral strength to avoid collapse, named lateral strength ratio Rc, of degrading (i.e., having stiffness and in-cycle strength degradation) single-degree-of-freedom, SDOF, systems under mainshock-aftershock seismic sequences. For this purpose, the degrading SDOF systems have a trilinear force–displacement backbone mainly defined by their displacement ductility capacity, μc, and the negative slope after peak strength, αc. Therefore, median Rc ratios were computed for a site-specific mainshock-aftershock scenario at rock sites considering as-recorded and artificial seismic sequences. From the results of this investigation, it is shown that median Rc ratios that take into account strong aftershocks are smaller than those computed for only the mainshocks, which means that the yield strength coefficient, Cy, for structures subjected to mainshock-strong aftershock pairs should be greater than that for structures under only the mainshocks. The ordinates of median Rc ratios depend on the site class, period of vibration, T, as well as μc and αc. A predictive equation for obtaining estimates of period-dependent Rc ordinates under mainshock and mainshock-aftershock sequences at rock sites is introduced whose fitted coefficients depend on μc and αc. The predictive equation of Rc allows obtaining estimates of the yield strength coefficient required to avoid collapse, Cyc, under mainshock-aftershock sequences for the design of new structures.

Analysis of the Effects of Different Factors on Damage Potential Ranking

A quantitative evaluation of the damage potential of ground motions to structures can provide a basis for the selection of input ground motions. To determine the main factors influencing the damage potential ranking of ground motions, the corresponding effect factors were analyzed. First, the structural period range from 0.05 to 10 s was divided into three types of period ranges based on an improved Newmark–Hall spectrum. The intensity measures (IMs) that can characterize the damage potential in every period range were determined. Second, the effect of yield strength coefficient (Cy), vibration period (T), and type of site on the damage potential ranking are explained. A recommended damage potential ranking is given in the same period range. Finally, to demonstrate the rationality of the recommended damage potential ranking in this paper, two representative reinforced concrete (RC) shear structure models are established for analysis. For the same type of structures, the damage potential rankings under different Cy and T conditions have high correlation with the recommended damage potential ranking, and the discreteness is very low. When considering the site factors, the corresponding correlation and dispersion change little. Based on the analysis of two typical structural models, the R2 between the recommended damage potential ranking and structural response ranking were 0.89 and 0.94, respectively. It is proven that the methods of Cy, T, and the type of site are reasonable when establishing the recommended damage potential ranking in this paper. This study provides a theoretical basis for simplifying the evaluation of ground motion damage potential and for selecting input ground motions.

Residual storey drift estimation of the MDOF system with the weak storey under seismic excitations using the BP network

This paper focuses on estimating residual storey drifts of multi-degree-of-freedom (MDOF) systems with the weak storey due to seismic excitations. According to the distribution of yield strength coefficients, MDOF systems are categorized into two types: a uniform MDOF system and a non-uniform MDOF system. Through elasto-plastic time history analysis, typical MDOF systems were analyzed considering the P-Δ effect under 60 earthquake records free of near-fault effects. The influences of various structural parameters on distribution characteristics of residual storey drifts along the height of the MDOF systems were investigated in detail. Finally, a back propagation (BP) network model was proposed to estimate residual storey drifts of the MDOF systems with the weak storey under seismic excitations. The results indicate that the maximum residual storey drift usually occurs at the weak storey for a non-uniform MDOF system while at the bottom for a uniform MDOF system. In addition, the larger the overall yield strength coefficient, the yield strength modification factor or the post-yield stiffness coefficient, the smaller the residual storey drift of the MDOF system on the whole. Moreover, the proposed BP model can sufficiently consider the parameters affecting residual storey drifts of the MDOF system with the weak storey, while having a good accuracy and a high efficiency in estimating the residual storey drifts.

The Effect of the Yield Strength Coefficient and Natural Vibration Period on the Damage Potential Ranking of Ground Motions

To analyze the effect of structural parameters on the damage potential ranking of ground motions, the effect of the yield strength coefficient (Cy) and natural vibration period (T) on the damage potential ranking of ground motions is analyzed based on the bilinear model and the modified Clough model, which are the most commonly used hysteretic models in structural dynamic analysis. The displacement response (Sd) spectrum under nonlinear conditions is taken as the damage potential intensity measure (IM) of ground motions, and the effect of the Cy and T on the Sd mean spectrum is also analyzed for comparative analysis. The results show that: (1) in the short-period range, Cy has a great effect on the displacement response ranking. On the other hand, in the medium- and long-period ranges, Cy has little effect on the Sd ranking; (2) with the change of T in medium- and long-period ranges, the variation of Sd values is obvious when the change of T is small, but the variation of Sd ranking is very small. This conclusion can provide a theoretical basis for evaluating the damage potential of ground motions and selecting input ground motions.

Development and Validation of a Modified Equivalent Strut Model of Lightweight Masonry Block Infill Walls for Quasi-static In-plane Cyclic Analysis

ABSTRACT Lightweight masonry blocks (LMBs) are more environmentally favorable than solid clay bricks, and have gradually become the main material for the infill walls of modern buildings. It is of great importance for proposing a suitable hysterical model of LMBs infill walls to accurately predict the seismic responses of modern buildings. However, cyclic loading tests of LMB infill walls demonstrate that the existing formulas of equivalent strut models significantly overestimate the stiffness and underestimate the peak strength. To this end, the force-displacement curves of 24 cyclic loading tests of LMB infill walls sourced from existing literature are statistically analyzed. The modification coefficients of stiffness and peak strength are put forward, and other parameter values such as yield strength coefficient, unloading stiffness coefficient and pinching effect coefficient are also recommended. Then a full-scale cyclic loading test of an LMB infill wall is designed and conducted to verify the accuracy of the modified model. In addition, the applicability of the modified model is also verified by two infill wall tests from existing literature. The results indicate the modified model can accurately simulate the global hysteretic response of LMB infill walls, and the simulation accuracy of the peak strength is increased to more than 90%. This model provides a useful tool for the accurate evaluation of the seismic performance of modern buildings infilled with LMBs.

Damage-based yield point spectra for sequence-type ground motions

Structural damage caused by mainshock can further be aggravated by aftershocks, which can lead to structural collapse. The current practices on the seismic design of structures generally only consider mainshock effects. This manuscript therefore presents the investigation on damage-based yield point spectra (YPS) of a single-degree-of-freedom (SDOF) system under sequential earthquakes. The collected sequence-type ground motions are recorded from 16 earthquake events and classified to four classes. The aftershocks in sequence are scaled according to different relative intensity levels. The modified Park-Ang model, which consists of maximum displacement and hysteretic energy dissipation, is employed to calculate YPS. The effects of period, ductility factor, damage index, site category, aftershock intensity, and structural damping are statistically studied. The results prove that the strong aftershock ground motion has more distinct influences on the YPS. In particular, the yield strength coefficient demand under seismic sequence increases by 10%–50%. The yield strength coefficient demand determined by the damage-based YPS is greater than that determined by the ductility-based YPS—the former is 10%–40% higher than the latter. Finally, the empirical expression of damage-based YPS is established by statistical mean method and regression analysis.

A New Elastoplastic Time‐History Analysis Method for Frame Structures

This study aimed to analyze the formation and application of the time‐domain elastoplastic response spectrum. The elastoplastic response spectrum in the time domain was computed according to the trilinear force‐restoring model. The time‐domain elastoplastic response spectrum corresponded to a specific yield strength coefficient, fracture stiffness, and yield stiffness. However, the force‐restoring models corresponding to different structural systems and the states of the structural systems at different moments were not the same. Therefore, the dynamic characteristics of a particular periodic point corresponding to a particular structure were meaningful for the elastoplastic response spectrum. In addition, the curve in the time‐domain dimension along the periodic point truly reflected the real‐time response of the structure when the structure encountered a seismic load.

Higher modes contribution for estimating the inelastic deformation ratios and seismic demands of structures

In order to estimate the seismic demand by using the nonlinear static procedure, different approximate methods have been developed. One of the most useful methods is called displacement coefficient method (DCM), which is based on some modification factors. One of these coefficients denoted C1, concerns the inelastic deformation ratio and usually depends on either the yield-strength reduction factor or the ductility factor. In general the evaluation of the inelastic deformation ratio is based on the response of single degree of freedom (SDOF) systems, where the response of the structure is mainly controlled by the fundamental mode, knowing that the inelastic deformation ratio will not capture the contribution of higher modes in the overall structural response. A developed theoretical approach with the aim of estimating the inelastic deformation ratio for structures, considering contribution of higher modes of vibration, is introduced. In this assessment, the normalized yield strength coefficient (η) and the post-to-preyield stiffness ratio (α) are key factors. The results are compared to the uncoupled modal response history analysis (UMRHA) procedure and some existing formulations for a nine story building subjected to the El Centro 1940 ground motion. It appears that the new theoretical approach leads to enough accurate estimation of the inelastic deformation ratio compared to the UMRHA one.

Seismic structural demands and inelastic deformation ratios: Sensitivity analysis and simplified models

Modern seismic codes rely on performance-based seismic design methodology which requires that the structures withstand inelastic deformation. Many studies have focused on the inelastic deformation ratio evaluation (ratio between the inelastic and elastic maximum lateral displacement demands) for various inelastic spectra. This paper investigates the inelastic response spectra through the ductility demand µ, the yield strength reduction factor Ry, and the inelastic deformation ratio. They depend on the vibration period T, the post-to-preyield stiffness ratio α, the peak ground acceleration (PGA), and the normalized yield strength coefficient η (ratio of yield strength coefficient divided by the PGA). A new inelastic deformation ratio Cη is defined; it is related to the capacity curve (pushover curve) through the coefficient (η) and the ratio (α) that are used as control parameters. A set of 140 real ground motions is selected. The structures are bilinear inelastic single degree of freedom systems (SDOF). The sensitivity of the resulting inelastic deformation ratio mean values is discussed for different levels of normalized yield strength coefficient. The influence of vibration period T, post-to-preyield stiffness ratio α, normalized yield strength coefficient η, earthquake magnitude, ruptures distance (i.e., to fault rupture) and site conditions is also investigated. A regression analysis leads to simplified expressions of this inelastic deformation ratio. These simplified equations estimate the inelastic deformation ratio for structures, which is a key parameter for design or evaluation. The results show that, for a given level of normalized yield strength coefficient, these inelastic displacement ratios become non sensitive to none of the rupture distance, the earthquake magnitude or the site class. Furthermore, they show that the post-to-preyield stiffness has a negligible effect on the inelastic deformation ratio if the normalized yield strength coefficient is greater than unity.

Inelastic deformation ratio for seismic demands assessment of structures

Modern seismic codes rely on performance-based seismic design methodology which requires from the structures to withstand inelastic deformation. Many studies have focused on the inelastic deformation ratio evaluation for various inelastic spectra. The purpose is to investigate the ratio between the inelastic and elastic maximum lateral displacement demands. Three options are considered in the present work to express the inelastic response spectra in terms of the ductility demand, yield strength reduction factor and inelastic deformation ratio. This later, a new non dimensional parameter called inelastic deformation ratio (Cη), depends on the natural period (T), the post-to-preyield stiffness ratio (a), the normalized yield strength coefficient (t) and the peak ground acceleration (PGA). The inelastic deformation ratio (Cη), is not evaluated for a fixed ductility ratio or for a fixed strength reduction factor. Though the ductility demand and yield strength reduction factor differ greatly for given ranges of T and r“ they take close values for systems for any T value when η > 1. It confirms also previous studies stating that they take unity value for periods T ≥ 1sec.

Simplified procedure for seismic demands assessment of structures

Methods for the seismic demands evaluation of structures require iterative procedures. Many studies dealt with the development of different inelastic spectra with the aim to simplify the evaluation of inelastic deformations and performance of structures. Recently, the concept of inelastic spectra has been adopted in the global scheme of the Performance-Based Seismic Design (PBSD) through Capacity-Spectrum Method (CSM). For instance, the Modal Pushover Analysis (MPA) has been proved to provide accurate results for inelastic buildings to a similar degree of accuracy than the Response Spectrum Analysis (RSA) in estimating peak response for elastic buildings. In this paper, a simplified nonlinear procedure for evaluation of the seismic demand of structures is proposed with its applicability to multi-degree-of-freedom (MDOF) systems. The basic concept is to write the equation of motion of (MDOF) system into series of normal modes based on an inelastic modal decomposition in terms of ductility factor. The accuracy of the proposed procedure is verified against the Nonlinear Time History Analysis (NL-THA) results and Uncoupled Modal Response History Analysis (UMRHA) of a 9-story steel building subjected to El-Centro 1940 (N/S) as a first application. The comparison shows that the new theoretical approach is capable to provide accurate peak response with those obtained when using the NL-THA analysis. After that, a simplified nonlinear spectral analysis is proposed and illustrated by examples in order to describe inelastic response spectra and to relate it to the capacity curve (Pushover curve) by a new parameter of control, called normalized yield strength coefficient (). In the second application, the proposed procedure is verified against the NL-THA analysis results of two buildings for 80 selected real ground motions.

Empirical Correlation between Inelastic and Elastic Spectral Displacement Demands

Inelastic spectral displacement demand is arguably one of the most effective, simplified means of relating earthquake intensity to building damage. However, seismic hazard assessment is typically conducted using empirical ground-motion prediction equations (GMPEs) that only provide indications of elastic spectral response quantities, which an engineer subsequently relates to inelastic demands using empirical relationships such as the equal-displacement rule. An alternative approach is to utilize relationships for the inelastic spectral displacement demand directly within the seismic hazard assessment process. Such empirical relationships are developed in this work, as a function of magnitude, distance, building period, and yield strength coefficient, for four different hysteretic models that are representative of a wide range of possible structural typologies found in practice. The new relationships are likely to be particularly useful for performance-based seismic design and assessment.

Ductility and inelastic deformation demands of structures

Current seismic codes require from the seismically designed structures to be capable to withstand inelastic deformation. Many studies dealt with the development of different inelastic spectra with the aim to simplify the evaluation of inelastic deformation and performance of structures. Recently, the concept of inelastic spectra has been adopted in the global scheme of the performance-based seismic design through capacity-spectrum methods. In this paper, the median of the ductility demand ratio for 80 ground motions are presented for different levels of normalized yield strength, defined as the yield strength coefficient divided by the peak ground acceleration (PGA). The influence of the post-to-preyield stiffness ratio on the ductility demand is investigated. For fixed levels of normalized yield strength, the median ductility versus period plots demonstrated that they are independent of the earthquake magnitude and epicentral distance. Determined by regression analysis of the data, two design equations have been developed; one for the ductility demand as function of period, post-to-preyield stiffness ratio, and normalized yield strength, and the other for the inelastic deformation as function of period and peak ground acceleration valid for periods longer than 0.6 seconds. The equations are useful in estimating the ductility and inelastic deformation demands for structures in the preliminary design. It was found that the post-to-preyield stiffness has a negligible effect on the ductility factor if the yield strength coefficient is greater than the PGA of the design ground motion normalized by gravity.