Ph.D. Theses

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A method of evaluation of relationship between the safety management and overall safety performance in construction industry
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2010., 2010.) Metinsoy, Tamer.; Aşkar Altay, Gülay.; Say, Arzu İşeri.
Safety is crucial in construction industry. From beginning to end, the construction process is complex and characterized by a number of uncertainties and a high fatality rate. But despite its high fatality rate constructions can be a safe occupation when there is an effective safety management and workers and employers are aware of hazards and prevent the probable risks. In this context, the objective of this study is to propose a new methodology that determines the overall safety performance of the construction site by including safety management performance and on-site safety performance assessment to improve construction safety. The study is particularly based on getting data from the construction sites via developed questionnaire and checklist and creating software called SME (safety management Evaluator) working on the basis of fuzzy logic approach that has the capability to evaluate construction contractors’ safety management performance and onsite safety performance and notify the safety management deficiencies. The survey data were collected from a sample size of 30 on-going building type construction firms and the responders of the questionnaire were safety managers. Checklist was applied on-site by author in order to observe the real situation of safety. The study includes factor analysis and correlation of safety management components and descriptive statistics in order to uncover the correlation patterns among the different variables. The purpose of this evaluation is to establish a base point against which construction contractors can be classified by an index of safety developed for the Turkish construction sector. By developing an index of safety, which includes safety management and on- site performance “defectiveness of the safety management and the overall safety performance of a specific construction company regarding the safety index scale for Turkey” is determined.
Development of fragility curves for seismic vulnerability assessment of high-rise R/C buildings with added viscous dampers
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2007., 2007.) Güneyisi, Esra Mete.; Aşkar Altay, Gülay.
The fragility curves are used to represent the probabilities that the structural damages, under various level of seismic excitation, exceed specified damage states by means of earthquake intensity-damage relations. This, in turn, can be utilized for prioritizing retrofit, pre-earthquake planning, and loss estimation tools such as cost-benefit analysis. Therefore, the fragility analysis is also particularly useful in regions of moderate and high seismicity, such as İstanbul. In this study, a twelve storey non-ductile R/C office building, which represents the typical properties of the high rise office buildings located in Mecidiyeköy region, was selected and fragility curves were developed for the economic loss estimation as well as a basis for assigning retrofit prioritization for the R/C office buildings in Mecidiyeköy region. For the purpose of seismic retrofitting, external passive fluid viscous damper systems were designed to provide the structure with different effective damping ratios (ζ) of 10, 15 and 20 per cent; and the improvement of the system reliability achieved through the use of the dampers was assessed through fragility, risk and cost-benefit analysis. In order to be able to compare the innovative technique applied with the conventional retrofitting techniques, the building was also retrofitted by shear walls. In the fragility analysis, a set of 240 artificially generated earthquake ground motions compatible with the design spectrum selected to represent the variability in ground motion were developed to study nonlinear dynamic responses of the structures before and after retrofit. Fragility curves represented by lognormal distribution functions were constructed as a function of peak ground acceleration, spectral acceleration and displacement. By combining fragility analysis results with the seismic hazard risk of the region, the effectiveness of viscous damper systems and shear walls in the retrofit of the high rise R/C building is further investigated based on economic point of view by performing seismic risk analysis and cost-benefit analysis.
Earthquake protection of Beylerbeyi Palace by reversible mixed technologies
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2007., 2007.) Aras, Fuat.; Aşkar Altay, Gülay.
The historical building stock and seismic risk of Anatolia draws considerable attention of the engineers for studying the earthquake performance of these structures and investigating appropriate protection techniques. In this respect, this study aims to examine the earthquake performance of the historic Beylerbeyi Palace which is a representative of the great buildings in the late of 19th century in Istanbul and elsewhere in the Ottoman Empire and search the possible retrofit techniques by means of reversible mixed technologies within the framework of FP6-Earthquake Protection of Historical Structures by Reversible Mixed Technologies (PROHITECH). Starting with the mineralogical structure of the mortar and ending with mechanical properties of masonry, material identification process has been carried out. Later on Ambient Vibration Survey, (AVS) was conducted in the structure with the University of "St. Cyril and Methodius", Institute of Earthquake Engineering and Engineering Seismology. Numerical model of the palace has been constructed and calibrated according to results of AVS. Response spectrum analyses have been carried out by using three different earthquake cases, defined by accounting the site specific soil and seismic characteristics of the site. The performance of the structure was assessed as weak. Furthermore the vulnerable parts of the structure were revealed by nonlinear analyses. Finally according to results of the analyses and considering the historical and architectural perspective of Beylerbeyi Palace three different retrofit strategies have been proposed and discussed in details. According to order of presentation, the first proposed strategy is the use of fiber reinforced polymers. The second strategy is the installation of the base isolation and the last one is the consolidation of the roof level to have the rigid diaphragm behavior with the application of fiber reinforced polymers to the required walls.
Experimental and numerical study on the cyclic behavior of connections in storage rack structures
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2015., 2015.) Shahxhenas, Rahman.; Aşkar Altay, Gülay.
Contrary to buildings which serve live loads lower than the self-weight, storage racks support much higher live loads. In unbraced frames of storage racks, the lateral sti ness and the stability along down-aisle direction are provided by means of beam-toupright and base connections which are neither pinned nor xed connections. Beamto- upright connections govern the structural performance and stability of the whole system. Therefore, estimating the behavior of a connection, having knowledge about the failure modes and rotation capacity are of great importance. This study presents the main results of an experimental program on the response of beam-to-upright connections of racks commonly used in the Turkish market. To create reliable references for comparing the results, beam-depth and end-connector types were tested for 2 different types of specimens, altogether 18 specimens were tested. Monotonic and slow quasi-static reversed cyclic tests were performed. Interlocked boltless beam-to-upright connections cause di culties in collecting the data and measuring the parameters; therefore the testing procedures and originally developed set-up for beam-to-upright connections were proposed and modeled with FE models developed in SAP2000 v.15 software to establish reliable references. Numerical investigations were used to verify experimental results and to introduce a model of such connections. To simulate the test and to model the connection, ANSYS v.14 software was used. The semi-rigid nature of this type of connections which primarily is due to the unpredictable distortions at di erent components of the joint requires a nonlinear simulation through advanced analyses. ANSYS software results showed a good match with moment-rotation curve results with respect to initial sti ness, ultimate strength and the level of ductility.
Experimental study on the behaviour of an i-beam to SHS-column by t-stub bolted connection
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2007., 2007.) Yemez, Kağan.; Aşkar Altay, Gülay.
In 80’s and 90’s hollow sections (HS) were mainly used in compression and tension elements of truss systems. Nowadays, architects are more and more using hollow sections in building applications where limited amount of research is available especially considering the economics of prefabrication to produce connections with acceptable ductility performance. In most of the examined joint types, the connection elements were welded on the tubular sections. This research is focused on the behaviour of I-beam to HScolumn t-stub bolted connections under monotonic and cyclic loading conditions. In view of easy and economic bolted field application and transportation without damage by leaving the column exterior without projections, in proposed joint type t-stubs are bolted by using long partially threaded studs through HS column. Four full scale beam-column specimens have been tested in two groups. As a difference the rear face of the HS column at the connection area has been reinforced by backing plate in the second group of specimens. Considering the cyclic performances, both joints maintain high plastic rotations in adequate resistance levels with acceptable energy dissipation capacities; consequently are suitable for use as semi-rigid partial strength joints in simply designed braced steel frames in seismic areas or in unbraced steel frames in less seismic areas. Additional research on the connection components has also been performed to observe the component behaviour of the column face in bending. Applicability of the component method described in Eurocode 3 part 1-8 with proposed design approach is checked by comparing with the test results both in resistance and stiffness point of view.
Fiber reinforced plastic overlay retrofit of hollow clay tile masonry infilled reinforced concrete frames
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering , 2002., 2002.) Hanoğlu, Kemal Burak.; Aşkar Altay, Gülay.
Stiffness and strength contribution of hollow clay tile infills to ductile and non-ductile reinforced concrete frames were evaluated using 1:3 scale specimens tested under quasi-static reversed cyclic seismic loads. Four ductile and four non-ductile framed single-bay-single-story specimens with and without infill panels were tested in tandem specimen configuration simulating the seismic actions on lowest interior spans of typical of low-rise infilled frame structures. During the tests on masonry infill material, the testing procedures originally developed for solid brick masonry were shown unsuitable for the hollow clay tile masonry testing. A new method for tensile strength testing of the hollow clay tile units was proposed and used coupling with finite element models to establish a tile tensile strength estimate. Infilled frames with plain hollow clay tile infill were shown to have failure loads well in excess of the bare frames. Glass fiber woven sheet and carbon fiber reinforced plastic laminates were used to confine and brace the hollow clay tile infill. Addition of glass-fiber overlays and carbon fiber laminates increased the strength of the infilled frames above the conventionally infilled frames. However, the maximum displacement capacity of the new system reduced due to low compressive strength of the tile infill, which confined the failures and deformations to infill corners and column mid-height. A new finite element modeling approach was developed based on the use of plane-framework analysis methods for plane stress analysis. Two different model scales were considered to show the capabilities of the proposed approach and the possibilities of simplification for engineering office use. A standard finite element analysis package with point-wise nonlinear element capability was used to calculate the pushover curve of the tested specimens. The results showed a good agreement with the test results for the detailed model, and the engineering model was regarded amenable for further calibration.
Hybrid tracking controller with attached TMD
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2009., 2009.) Bozer, Ali.; Aşkar Altay, Gülay.
Supplemental damping systems can be substitute for hysteretic or kinetic damping and eliminate the need for additional strength and stiffness for desired performance levels. Supplemental dampers dissipate the energy mechanically and activated through movements of main structural system. Tuned Mass Dampers are examples of motionactivated devices in which flow of energy in the structure is disturbed through the vibration of a secondary system. In the essence tuned-mass dampers (TMDs) or vibration absorbers are mass-spring-dashpot systems that are tuned to a particular vibration mode of the structure on which they are installed. Under a dynamic excitation, the TMD resonates at the same frequency as the main structure but out-of-phase from it, thereby diverting the input energy from the main structure into itself. The environmental loads, such as wind and earthquake loads, possess many frequency components. Therefore, tuned mass dampers are optimized to render them effective for such large bandwidth inputs. To broaden the operational range of a TMD some damping is introduced in the optimization procedure. Although this added damping term smoothens the sharp peaks of frequency response function out of the operating range it also considerably decreases the effectiveness of TMD in the operating range. Thus, in this study it is aimed to constraint the response of the structure in the boundaries of the operating range of TMD, then the requirement for additive damping will be eliminated and TMD can be used in full efficiency. As a consequence a hybrid system is proposed in which the combined plant/nonoptimal TMD structure is equipped with an active controller such that the closed-loop system tracks the response of an oscillator with natural frequency set to the operating frequency of TMD unit. In the proposed system closed-loop states are not regulated to zero. Instead controller tries to help TMD to suppress vibration effectively thus lesser control effort is needed. This is further verified by numerical simulations which show the performance of the proposed approach under wind and earthquake loads.
Optimization modeling for a steel structure project with multiple sourcing points and stochastic delivery times
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2011., 2011.) İleri, O. Serkan.; Aşkar Altay, Gülay.; Aras, Necati.
Due to the consolidation trends in steel industry and advancements in logistics, steelmakers now face the challenge of choosing the most cost-effective sourcing network for a steel structure project. This research deals with the problem of determining the optimal trade-off between the cost of material supplies and the tardiness cost of a project. The aim of this research study is to provide the mathematical background of the sourcing optimization models for steel structure projects. Another objective is to come up with an efficient solution method for this mathematical model. Due to the stochastic nature of the delivery times of steel components, the optimization problem is solved by stochastic dynamic programming, which is a well-known technique in operations research. The model is applied to an on-going real project carried out in Siberia, and 9.54% and 19.99% savings are obtained in terms of total project cost. A C++ code is developed for the solution of the optimization problem. The required computation time is quite short, which implies that the proposed method can also be utilized on larger project networks. The main research contribution is the formulation of stochastic dynamic programming model that optimizes the cost of sourcing. Furthermore, a fit-for-purpose computer code has been developed for the solution process. In addition, standing on the intersection of civil and industrial engineering disciplines, this study stimulates further studies on project scheduling and cost optimization, and use of linear, dynamic and stochastic programming methodologies on different civil engineering subject matters. Further research can be conducted on the optimization model that is formulated in this study – i.e. resource or capacity constraints can be introduced, optimization can be re-conducted at each stage, partial sourcing for activities can be allowed, continuous probability distribution can be used for activity durations.
Static and dynamic analysis of super-elliptical plates
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2005., 2005.) Altekin, Murat.; Aşkar Altay, Gülay.
This study reports the static and dynamic analysis of super-elliptical plates ofuniform thickness. There are mainly three distinct boundary conditions (clamped/simplysupportedalong the contour and point supported on the contour) to be examined in the dissertation. In static analysis the plate is subjected to constant lateral load. On the otherhand, in dynamic analysis the undamped free vibration is concerned. The plate perimeteris defined by a super-elliptic function with a power corresponding to the shape rangingfrom an ellipse to a rectangle. The Kirchhoff fourth-order plate theory is employed for the isotropic and the functionally graded elastic plate. Numerical examples for plates withdifferent shapes are solved by the Ritz method and the methods of weighted residuals. Theresults are compared with known ones where possible.
Statics and dynamics of super elliptical homogeneous and FGM plates
(Thesis (Ph.H..)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2007., 2007.) Çeribaşı, Seyit.; Aşkar Altay, Gülay.
In this study super elliptical plates are statically and dynamically analyzed. This class of plates includes a wide range of shapes varying from an ellipse to a rectangle. Although studies on the extreme boundaries of the super elliptical shapes (circle, ellipse, and rectangle) are extensive, contributions on the mid shapes are very limited. The studies are mostly concentrated on dynamics, thermal, and buckling behavior of the plates but not on static behavior. Kirchhoff plate model with isotropic and homogeneous material is studied. The first part of this study is concentrated on static behavior of the super elliptical plates under uniformly distributed surface load. For static analyses clamped boundary is assumed. The partial differential plate equations are solved by Galerkin method. The study is conducted for 10 super elliptical shapes and 14 central plate dimension ratios. The second part of the study deals with the dynamic behavior of super elliptical plates. Free vibration of simply supported plates is investigated and Ritz method is employed for the solution of the governing plate equations. The effect of Poisson’s ratio, which can not be neglected for round edged simply supported plates, is examined in detail. The results of the study are presented in graphical and tabular forms and compared with previously obtained results of the available literature. In the last part of the study, functionally graded material (FGM) Kirchhoff plates have been investigated. For static analysis of the FGM plates, uniformly distributed surface load and clamped boundary is assumed and Galerkin method is employed. In addition to this, free vibration characteristics of simply supported FGM plates are studied by using Ritz method. The Poisson’s ratios of the plates are assumed to be constant, but their Young’s moduli vary in the thickness direction functionally. The objective of this part is to investigate the influence of volume fractions and the component materials on static and dynamic behavior of this class of plates.
Thermal buckling of FGM thin plates with variable thickness
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2010., 2010.) Sanrı, Işıl.; Aşkar Altay, Gülay.
The main focus of this study is the thermal buckling analysis of plates. In particular, the studies on the problem of the stability of various shapes of plates are extensive but quite little has been reported for the thermal buckling problem of plates. In this thesis, thermal buckling of rectangular and elliptical plates is examined. Plate theories are investigated and Kirchhoff plate theory is applied to derive and formulate thermal buckling problem. Boundary conditions are assumed to be simply supported and clamped in the whole study. Rayleigh-Ritz method is used to solve the energy equations and to obtain the critical buckling loads. The study consists of three main parts. The first part focuses on the thermal buckling of rectangular and elliptical Kirchhoff plates with constant thickness. Critical buckling temperatures for the homogenous, isotropic plates are obtained; the results are presented in graphical and tabular forms and compared with previously obtained results of the available literature. The second part of the study covers the thermal buckling analyses of functionally graded material (FGM) Kirchhoff plates with constant thickness. FGM plates are advanced composites with properties that vary continuously through the thickness of the plate. Metal-ceramic FGM plates are proposed for the use in thermal analyses where a metal-rich interior surface of the plate gradually transitions to a ceramic-rich exterior surface of the plate. The effect of the FGM on the thermal buckling of rectangular and elliptical plates is discussed. In the last part of the study, elliptical and rectangular FGM Kirchhoff plates with variable thicknesses are studied. The effect of parabolic thickness variation on the critical buckling load parameter is investigated. Then, parametric studies are carried out to examine the effects of material properties and thickness variation on the critical buckling temperatures. As a consequence, since the exact solution of thermally induced general buckling problems of plates is very difficult and in many cases even impossible, thermal buckling problem of plates is simplified and the critical buckling temperatures for homogenous and FGM plates with constant thicknesses and FGM plates with variable thicknesses are presented.

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