Ahmet Yakut

Displacement-Based Fragility Functions for Low- and Mid-rise Ordinary Concrete Buildings

Fragility functions are determined for low- and mid-rise ordinary concrete buildings, which constitute the most vulnerable construction type in Turkey as well as several other countries prone to earthquakes. A hybrid approach is employed where building capacities are obtained from field data and their dynamic responses are calculated by response history analyses. Field data consists of 32 sample buildings representing the general characteristics of two- to five-story substandard reinforced concrete buildings in Turkey. Lateral stiffness, strength, and deformation capacities of the sample buildings are determined by pushover analyses conducted in two principal directions. Uncertainties in lateral stiffness, strength, and damage limit states are expressed by using statistical distributions. The inelastic dynamic structural characteristics of the buildings investigated are represented by a family of equivalent single-degree-of-freedom systems and their seismic deformation demands are calculated under 82 ground-motion records. Peak ground velocity (PGV) is selected as the measure of seismic intensity since maximum inelastic displacements are better correlated with PGV than peak ground acceleration (PGA). Fragility functions are derived separately for different number of stories, which is a prominent parameter influencing the vulnerability of existing substandard concrete buildings.

A screening procedure for seismic risk assessment in urban building stocks

An effective step for seismic risk mitigation in large urban areas under high seismic risk is to identify the most vulnerable buildings that may sustain significant damage during a future earthquake. Once they are identified properly, existing seismic risks may be reduced either by retrofitting such buildings, or by replacing them with new buildings in view of a particular risk-mitigation planning strategy. A fast and simple seismic risk-assessment procedure for vulnerable urban building stocks is proposed in this study. It is basically a sidewalk survey procedure based on observing selected building parameters from the street side, and calculating a performance score for determining the risk priorities for buildings. Statistical correlations have been obtained for measuring the sensitivity of damage to the assigned performance score by employing a database consisting of 454 damaged buildings surveyed after the 1999 Düzce earthquake in Turkey. The results revealed that the proposed screening procedure provides a simple but effective tool for selecting those buildings that have significant damage risk. These buildings have to be subjected to a more detailed assessment for a final decision on their seismic risk level.

Analytical Fragility Curves for Ordinary Highway Bridges in Turkey

This study focuses on the development of analytical fragility curves for the ordinary highway bridges constructed after the 1990s. Four major bridge classes were employed based on skew angle, number of columns per bent, and span number (only multispan bridges). Nonlinear response-history analyses (NRHA) were conducted for each bridge sample using a detailed 3-D analytical model subjected to earthquake ground motions of varying seismic intensities. A component-based approach that uses several engineering demand parameters was employed to determine the seismic response of critical bridge components. Corresponding damage limit states were defined either in terms of member capacities or excessive bearing displacements. Lognormal fragility curves were obtained by curve fitting the point estimates of the probability of exceeding each specified damage limit state for each major bridge class. Bridges with larger skew angles or single-column bents were found to be the most seismically vulnerable.

Spectral Ground Motion Intensity Based on Capacity and Period Elongation

Ground motion intensity parameters are used to express the relationship between expected structural damage and the seismic forces imposed. The graphical representation of damage probability as a function of ground motion intensity leads to fragility curves that are generally used in loss estimation studies. The most typical parameters used to represent the ground motion intensity are peak ground acceleration, peak ground velocity, spectral acceleration, and spectral displacement. Other parameters obtained from the ground motion trace and response spectra have been recommended in literature, but no consensus on which intensity parameter to use exists because of the various drawbacks of these ground motion intensities. A new spectrum ground motion intensity parameter that relies on the expected elongated period of the structure under seismic forces has been developed. This intensity measure takes into account the approximate yield capacity of the structure and the area between the fundamental and elongated ...

Deformation Limits for Structural Walls with Confined Boundaries

For accurate analytical assessment of performance and damage in reinforced concrete members, well-defined deformation limits at particular damage states are required. With advanced and computationally intensive finite element analyses, we establish deformation limits at yield and ultimate limit states for adequately confined rectangular reinforced concrete structural walls in terms of drift ratio, plastic rotation, and curvature. To investigate the deformation limits of structural walls, a parametric study on isolated cantilever wall models is performed. The primary variables of the parametric study are the shear-span-to-wall-length ratio, wall length, axial load ratio, normalized shear stress, the amount of horizontal web reinforcement, and the amount of longitudinal reinforcement at the confined boundary of structural wall models. Expressions and limit values are proposed for yield and ultimate deformation capacity of structural walls, based on the most influential parameters. The proposed equations are found to be promising when compared to results of experiments.

Preliminary Seismic Vulnerability Assessment of Existing Reinforced Concrete Buildings in Turkey

The vulnerability assessment method, described in the companion paper [1], relies on a damage score, which is compared with an appropriate cutoff value to identify the buildings as “safe”, “unsafe” or “intermediate”. The cutoff values are considered to be valid for damaging earthquakes and regions similar to DUzce, where the data was gathered. To generalize the procedure, the variability of ground motion with respect to soil properties and the distance to source needs to be incorporated. This was done by modifying the cutoff values based on the above factors. Sites are classified according to the Turkish Seismic Code’s [2] definitions based on the shear wave velocity. Various attenuation relations are used to account for the variation of the ground motion with distance and the soil type.

Ground-motion characterization for the probabilistic seismic hazard assessment in Turkey

This study describes the methodology implemented to establish the ground-motion logic-tree for national probabilistic seismic hazard map of Turkey for shallow active crustal regions. The presented procedure provides quantitative information to guide the hazard experts while establishing the logic tree to capture the epistemic uncertainty in ground-motion characterization. It uses non-data-driven and data-driven testing methods to identify and rank candidate ground-motion prediction equations (GMPEs) under a specific ground-motion database. The candidate GMPEs are subjected to visual inspection and are classified into center, body and range (CBR) spectral estimates for a proper consideration of epistemic uncertainty. The GMPEs classified into CBR are then used in a suite of seismic hazard sensitivity analysis to establish the most suitable GMPE logic-tree whose spectral estimates are not biased by any one of the GMPEs in the logic-tree structure. The sensitivity analysis considers normalized spectral ordinates and is not manipulated by the spectral amplitudes. The proposed procedure is inherited from the relevant studies of the Earthquake Model of the Middle East (EMME; www.efehr.org:8080/jetspeed/portal/emme.psml) regional seismic hazard project. This paper also highlights the similarities and differences in ground-motion characterization between EMME and our approach.

Capacity Related Properties of RC Frame Buildings in Turkey

The capacity curves of various structural systems that are used in the HAZUS methodology to determine seismic vulnerability were developed based on the seismic design codes. These capacity curves are, therefore, very idealistic and do not take into account variations in the regional design practice. This article presents the results of nonlinear static analyses of representative reinforced concrete frame buildings located in Turkey to obtain their capacity curves. A representative set of buildings was selected from existing RC buildings. The parameters such as yield over-strength ratio, fundamental period, post elastic stiffness, yield and ultimate drift ratios, and yield base shear coefficient have been obtained from the idealized capacity curves. These parameters are used in HAZUS to determine seismic response of buildings under a given hazard level. This seismic response is then used in predetermined fragility curves to estimate the expected loss for the particular building type. The capacity curves developed herein for Turkish buildings were compared with other studies and HAZUS recommendations.

A DETAILED SEISMIC PERFORMANCE ASSESSMENT PROCEDURE FOR RC FRAME BUILDINGS

Reinforced concrete buildings constitute the majority of the building stock throughout the world, especially in developing countries. Due to their poor performance under major earthquakes occurring in the last decade, significant research has been initiated to assess the vulnerability of the existing building stock and to provide means of improving the expected seismic performance of the vulnerable buildings. The available seismic performance assessment procedures are classified based on the degree of sophistication involved, in addition to the quality of data in hand and the purpose of the assessment. Quick performance assessment procedures using only superficial data that reflect the general properties of the building, mostly architectural features, provide a crude index that is used to rank a group of buildings to determine their priority for further evaluation. An improved but preliminary step is to use additional information including certain structural features of the building to assess its vulnerability by identifying unsafe buildings using simple and practical procedures. The buildings identified as vulnerable in the preliminary or quick assessment phases require more thorough evaluations before a mitigation measure is proposed. Detailed assessment procedures generally require a full blown seismic analysis of the building to determine the forces and deformations experienced by its components under a presumed level of earthquake intensity. A number of widely used such procedures (FEMA 356, ATC-40, EUROCODE 8) compare these demands with the recommended values of member capacities varying with the level of the performance objectives employed. Each member is classified as either force-based or displacement based, depending on its mode of behavior. The outcome of the evaluation is a decision on the capacity of the member as such it meets the criteria of the target performance level or not. No guidelines are provided on how to make a decision on the building as a whole. In other words, only member level information and evaluation is carried out. This article presents a detailed assessment procedure that is applied to individual buildings. The procedure is based on the behavior of the individual components comprising the structural system. The component vulnerabilities are expressed in the form of a damage curve that relates a response parameter to the physical damage. Columns, beams and masonry infill walls are considered as structural components that contribute to the lateral load capacity of the RC frame buildings, so damage curves have been developed for these components only. The response parameters that have been used as pertaining to seismic damage are the interstory drift ratio for columns and infill walls and the chord rotation for beams. For each type of component numerical analyses and experimental test results have been used to develop damage curves. Extensive numerical analyses have been carried out to determine parameters affecting damageability of RC members. For the columns and beams, damage curves for shear behavior and three ductility levels of flexural behavior have been developed. Figure 1 shows damage curves for a RC column. The building to be evaluated is modeled and analyzed to determine nonlinear deformation demands under a prescribed seismic intensity. Quantitative component damage states are computed from these demands using developed damage

Assessment of Simulated Ground Motions in Earthquake Engineering Practice: A Case Study for Duzce (Turkey)

Simulated ground motions can be used in structural and earthquake engineering practice as an alternative to or to augment the real ground motion data sets. Common engineering applications of simulated motions are linear and nonlinear time history analyses of building structures, where full acceleration records are necessary. Before using simulated ground motions in such applications, it is important to assess those in terms of their frequency and amplitude content as well as their match with the corresponding real records. In this study, a framework is outlined for assessment of simulated ground motions in terms of their use in structural engineering. Misfit criteria are determined for both ground motion parameters and structural response by comparing the simulated values against the corresponding real values. For this purpose, as a case study, the 12 November 1999 Duzce earthquake is simulated using stochastic finite-fault methodology. Simulated records are employed for time history analyses of frame models of typical residential buildings. Next, the relationships between ground motion misfits and structural response misfits are studied. Results show that the seismological misfits around the fundamental period of selected buildings determine the accuracy of the simulated responses in terms of their agreement with the observed responses.