E. Yuksel

Estimation of the Fundamental Vibration Period of Existing RC Buildings in Turkey Utilizing Ambient Vibration Records

Earthquake design codes generally provide empirical formulae or various approximations for determining fundamental periods of buildings for the evaluation of statically equivalent seismic forces. It is thus important to use realistic periods for the design and performance assessment of existing buildings. This study deals with the evaluation of fundamental vibration periods of reinforced concrete (RC) buildings having a frame-type structural system, and considers the effects of infill walls. The problem is evaluated by utilizing experimental and analytical approaches. A height-dependent relationship to estimate the fundamental period of vibration of Turkish RC moment-resisting frames is derived for a fully elastic condition. Furthermore, a constant parameter is defined to magnify the empirical relationship in order to attain code-specified periods obtained by considering moderate-intensity earthquakes.

SEISMIC RETROFIT OF INFILLED REINFORCED CONCRETE FRAMES WITH CFRP COMPOSITES

Retrofit of existing poor reinforced concrete frame structures is a major concern for Turkey. Considering the large stock of existing poor buildings and economical situation of the country, research on economical and easily applicable retrofitting techniques is important. Although the infill walls are not considered as structural members during design, the observations after earthquakes have shown that they are important sources of strength, stiffness and damping during earthquakes. Consequently, keeping infill walls in their locations during earthquake by preventing premature damage either due to in-plane or out-of-plane loads can help the structure maintain a significant amount of its unaccounted strength, stiffness and damping characteristics. In this study, 6 reinforced concrete frames, two bare, two infilled and two with CFRP retrofitted infill walls were tested under constant axial load and reversed cyclic lateral loads. At the end of the tests, it was seen that retrofitting of infill walls with CFRP composites in diagonal direction provided significant enhancement in lateral strength and stiffness.

Retrofitting of Vulnerable Reinforced Concrete Frames with Wet-Mixed Shotcrete Panels

In this paper, wet-mixed shotcrete panels are proposed instead of reinforced concrete (RC) shear walls for the retrofitting of vulnerable RC frames. The general behavior of the retrofitted frames responding in-plane lateral loading is investigated in detail. The experimental work is composed of retrofitting of undamaged and damaged RC frames. Nearly 1/2 scale, one bay and one story specimens were tested under external loads simulating earthquake effects. To assess the effectiveness of the technique, the results of the retrofitted frames were also compared with a bare frames and a conventional shear wall retrofitted frames. The experimental study shows that the wet-mixed shotcrete panels added to vulnerable RC buildings increases the lateral load carrying capacity, the lateral rigidity and the energy dissipation capacity of the system. It is considered that the technique can be used as an efficient, practical and cost effective method in retrofitting of existing vulnerable RC buildings.

Development of Earthquake Energy Demand Spectra

Current seismic codes are generally based on the use of response spectra in the computation of the seismic demand of structures. This study evaluates the use of energy concept in the determination of the seismic demand due to its potential to overcome the shortcomings found in the current response spectra–based methods. The emphasis of this study is placed on the computation of the input and plastic energy demand spectra directly derived from the energy-balance equation with respect to selected far-field ground motion obtained from Pacific Earthquake Engineering Research (PEER) database, soil classification according to National Earthquake Hazards Reduction Program (NEHRP) and characteristics of the structural behavior. The concept and methodology are described through extensive nonlinear time history analyses of single-degree-of-freedom (SDOF) systems. The proposed input and plastic energy demand spectra incorporate different soil types, elastic perfectly plastic constitutive model, 5% viscous damping ratio, different ductility levels, and varying seismic intensities.

Improved Infill Walls and Rehabilitation of Existing Low-Rise Buildings

Five to 10% of buildings in earthquake prone areas, with structural deficiencies and non-structural partitioning walls are expected to collapse totally during a severe earthquake. Relying on the encouraging early test results, transforming a selected number of non-structural partitioning walls to structural walls has been considered as one of the realistic preventive measures if sufficient reliability is achieved both experimentally and theoretically. The major part of the recent experimental and theoretical works of The Structural and Earthquake Engineering Laboratory of Istanbul Technical University (ITU) has been devoted to achieve better understanding of the seismic behavior of brittle partitioning walls which are generally ignored in the design, rehabilitation design or evaluation stages of ordinary low-cost, low-rise, reinforced concrete relatively old buildings. In this chapter the complementary tests and analytical works carried out for this purpose are summarized to come up with a cost-effective prescriptive solution to prevent the total collapse of buildings. The proposed retrofitting technique is exemplified through the mathematical models of strengthened buildings offered by codes which are reviewed as well. The experimentally developed data such as the modulus of elasticity of clay brick walls, damping ratios, shear strength of improved partitioning walls and earthquake load reduction factors are referred in the analyses.

Behaviour of steel cushions subjected to combined actions

Mild steel is relatively low-cost and easily accessible material to fabricate some structural members. It would be a significant advantage if seismic energy dissipaters that are used in structures constructed in the earthquake prone areas, could also be produced on site. In this paper, a promising seismic energy dissipater made of mild steel, so-called steel cushion (SC) is presented. It is provided experimental and analytical responses of SCs subjected to bi-axial loadings. SC rolls under the lateral loading that allows relocation of the plasticized cross-section. Henceforth, SC dissipates considerable amount of seismic energy. A series of tests were performed to achieve experimentally the behavior of SC subjected to longitudinal and transversal loading. Finite Element Models (FEMs) were also generated to reproduce the experimental backbone curves and to predict the bi-directional response properties for discrete transversal forces and plate thicknesses. Closed-form equations were derived to determine yield and ultimate forces and the corresponding displacements as well as location of the plasticized sections. The behavior of SC could either be projected by the FEMs with the exhibited parameters or by means of the proposed closed-form equations and the normalized design chart.

Earthquake performance improvement of low rise RC buildings using high strength clay brick walls

Large number of vulnerable reinforced concrete (RC) buildings exists in earthquake prone areas. These low cost residential and/or commercial buildings, which are three to seven-stories high, usually do not receive essential engineering services during the construction phase. Finding cheap, easily applicable and occupant friendly retrofitting techniques are extremely important to reduce the seismic risk of these buildings. As an attempt to this, a particular type of high strength clay brick is studied to evaluate its potential for the structural retrofitting. A set of experiment was conducted to assess the important mechanical characteristics of the infill walls made from these bricks. Also the performance of two RC frames retrofitted with these walls, having different connection details between the wall and RC members was examined experimentally. The analytical nonlinear static analyses of these specimens have been performed using SeismoStruct to achieve some model parameters for representing the “infill wall model” in the program. Adaptive pushover and nonlinear time history analyses were conducted to investigate the performance of a six storey representative RC frame retrofitted with these walls. Evaluation of the results obtained in these analyses prove that this retrofitting technique introduces important strength and stiffness increments to the structure, regarding its seismic demands, which are similar to the results obtained from the experiments.

Structural Behaviour of Ordinary RC Bare and Brittle Partitioned Frames with and without Lap Splice Deficiency

In the framework of an experimental program supported by a NATO project, it is aimed to examine the performance of a new strengthening technique for existing buildings using FRP sheets. For this purpose, three preliminary 1/2 scale tests were carried out in Istanbul Technical University. Since lap splicing deficiency in columns has been selected as one of the major factors causing early collapse of buildings, two similar sets of one bay-two story specimens have been fabricated with and without lap splicing deficiency. The early results achieved at the end of tests, and the theoretical predictions are presented in this paper.

Spring tube braces for seismic isolation of buildings

A new low-cost seismic isolation system based on spring tube bracings has been proposed and studied at the Structural and Earthquake Engineering Laboratory of Istanbul Technical University. Multiple compression-type springs are positioned in a special cylindrical tube to obtain a symmetrical response in tension and compression-type axial loading. An isolation floor, which consists of pin-ended steel columns and spring tube bracings, is constructed at the foundation level or any intermediate level of the building. An experimental campaign with three stages was completed to evaluate the capability of the system. First, the behavior of the spring tubes subjected to axial displacement reversals with varying frequencies was determined. In the second phase, the isolation floor was assessed in the quasi-static tests. Finally, a ¼ scaled 3D steel frame was tested on the shake table using actual acceleration records. The transmitted acceleration to the floor levels is greatly diminished because of the isolation story, which effects longer period and higher damping. There are no stability and self-centering problems in the isolation floor.

A capacity curve model for confined clay brick infills

Experimental studies have proven that clay brick infills, confined with carbon-fiber-reinforced polymers (CFRP) in reinforced concrete (RC) frames, have some advantages in terms of stiffness, strength, energy dissipation capability and damage intensity. Owing to these advantages, existing infill walls in RC frames may be retrofitted with CFRP strips, especially in low-rise buildings in earthquake-prone areas. There is a gap in the literature concerning their behavior model, for use in structural analysis. A piecewise linear capacity curve model called “DUVAR” is proposed here, which estimates the envelope of force-vs.-displacement hysteresis, depending on the data compiled from the literature and the completed experimental studies. A nonlinear shear spring element is utilized in the model to represent the bare and retrofitted infills. The ultimate shear strength and the corresponding displacement, the ratio of cracking stiffness to initial stiffness, the ratio of ultimate strength to cracking strength, and the ductility ratio are the five key parameters of the model. The model is validated against the experimental results of two sovereign studies. Finally, the model is employed in the performance evaluation of an existing three-story RC building to exemplify its straightforward application.