STATISTICAL SEISMIC VULNERABILITY ASSESSMENT OF EXISTING REINFORCED CONCRETE BUILDINGS IN TURKEY ON A REGIONAL SCALE

A statistical procedure, called discriminant analysis, is used to develop a model for the preliminary assessment of the seismic vulnerability of low- to medium-rise (2-7 storey) reinforced concrete buildings. The earthquake damage data compiled in Düzce province after the 12 November 1999 Düzce earthquake formed the damage database. Number of storeys, minimum normalised lateral stiffness index, minimum normalised lateral strength index, normalised redundancy score, soft storey index and overhang ratio are selected as the basic damage inducing variables. Two discriminant functions are derived in terms of these variables considering immediate occupancy and life safety performance levels. In the proposed preliminary seismic vulnerability assessment model, the discri-minant scores obtained from these two discriminant functions are combined in an optimal way and axe used to classify existing buildings as “safe”, “unsafe” and “requires further evaluation”. The optimality criterion imposed into the model is the minimisation of the misclassification rate of damage states causing collapse. The validity of the proposed model is checked by using the seismic damage data associated with recent earthquakes that occurred in Turkey. The consistency between the observed damage distribution and the predictions of the proposed model supports the effectiveness of the proposed model.     

SEISMIC PROTECTION OF LIGHT SECONDARY SYSTEMS THROUGH DIFFERENT BASE ISOLATION SYSTEMS

The objective of the present work is to examine advantages and drawbacks of different types of isolation systems, when seismic isolation is used as a protection strategy against damage to internal equipment and contents. The starting point of the study is the big experimental program of table tests on reduced-scale R/C structural models, carried out within the MANSIDE (Memory Alloys for New Seismic Isolation DEvices) project. Seven identical l:3.3-scaled, 3-storey frames were tested, including two fixed-base models and four base-isolated models with different isolation systems, namely: (1) rubber isolators, (2) steel-hysteretic system and (3), re-centring SMA (Shape Memory Alloy) system. In this study the internal equipment is regarded as an elastic single degree of freedom, with 2% equivalent viscous damping. Therefore, the capability of fixed-base and base-isolated models with different isolation systems to protect light secondary systems is evaluated by comparing the floor response spectra obtained from the storey accelerations recorded during shaking table tests. Three different PGA's are considered, about 0.15g, 0.3g and 0.5g, respectively. All the shaking table tests are also simulated with an accurate numerical model, to validate and better understand the experimental results. It is found that each type of isolation system reduces considerably the seismic effects on internal equipments in wide frequency regions. However, tuning effects may arise in specific frequency ranges, corresponding to the first mode in structures equipped with quasi-elastic (rubber) isolation systems, and to higher modes in structures equipped with elasto-plastic (steel) and nonlinear re-centering (SMA) isolation systems.     

STRONG GROUND MOTIONS AND DAMAGE PATTERNS FROM THE 1999 DUZCE EARTHQUAKE IN TURKEY

The M _w , 7.1 Duzce earthquake occurred on 12 November 1999 along the North Anatolian Fault in northwestern Turkey. This paper documents observations from a field reconnaissance team, addressing two principal aspects of this significant earthquake: the recorded ground motions and the distribution and severity of the earthquake effects on the built environment. In general, the recorded ground motions from this earthquake were smaller than predicted by ground motion predictive equations available at the time of the event. One anomalous recording is presented and potential causes for this irregular motion based on observations from field reconnaissance are discussed. The effects of rupture directivity on the near-fault recordings are assessed and the effects of soil conditions on the recorded ground motions are examined. The patterns of building damage based on post-earthquake reconnaissance are presented for the most strongly shaken cities in the near-fault region: Duzce, Kaynasli, and Bolu. Damage in Duzce was concentrated in the southern part of the city, which is underlain by softer sediments. Damage in Bolu was distributed evenly throughout the city; whereas damage was concentrated on more recent alluvial sediments in Kaynasli. No evidence of liquefaction or ground failure was observed in the populated areas surveyed after the earthquake.     

A COMPARATIVE STUDY ON THE ACTUAL AND ESTIMATED SEISMIC RESPONSE OF KIRALKIZI DAM IN TURKEY

Kiralkizi Dam, a 120 m high earthfill dam located in Diyarbakir city, Turkey, was shaken by a moment magnitude, M _w =4.6 earthquake at an epicentral distance of 8 km, on December 24, 2000, at 13:31 local time. The seismic response of the dam was assessed by using spectral ratios between (i) available crest and foundation records (C/F), (ii) horizontal and vertical components of the recorded motions (H/V), (iii) by performing 2 dimensional finite difference-based seismic response analyses (Flac-2D), and (iv) ID elastic shear beam solutions. First mode of vibration of the dam in the transverse direction by all four methods were estimated in the range of 0.55 to 0.62 second. Similar close agreement was not observed in higher modal periods estimated by H/V technique as compared to the predictions by C/F, Flac-2D, shear beam analysis techniques. Thus, H/V technique was concluded to be useful for the estimation of the fundamental resonance frequency of a soil structure, but not for its higher harmonics as consistent with available limited literature. In the longitudinal direction, natural period of the dam was estimated as 0.28 and 0. 82 second by H/V and C/F techniques, respectively. Such disagreement was explained by (i) differences in the definitions of the estimated periods, (ii) internal impedance contrast of the dam, (iii) contributions of 3D valley effects. Single seismometer record obtained from crest level was found to be inadequate for reliably assessing the response of a dam in the longitudinal direction, and it is recommended to install multiple seismometers both within dambody and the abutments. Last but not least, the results of these analyses were further compared by available accelograms recorded at three earthfill and rocknll dams from Japan. In general, it was concluded that the seismic response of Kiralkizi Dam is comparable and within the prediction ranges of available analyses methods and is consistent with the expected response of a dam this height.     

NONLINEAR SHAKE TABLE IDENTIFICATION AND CONTROL FOR NEAR-FIELD EARTHQUAKE TESTING

The primary focus of a structural shake table system is the accurate reproduction of acceleration records for testing. However, many systems deliver variable and less than optimal performance, particularly when reproducing large near-field seismic events that require extreme table performance. Improved identification and control methods are developed for large hydraulic servo-actuated shake table systems that can exhibit unacceptable tracking response for large, near-field seismic testing. The research is presented in the context of a 5-tonne shake table facility at the University of Canterbury that is of typical design. The system is identified using a frequency response approach that accounts for the actual magnitudes and frequencies of motion encountered in seismic testing. The models and methods developed are experimentally verified and the impact of different feedback variables such as acceleration, velocity and displacement are examined.

The methods show that shake table control in testing large near-field seismic events is often a trade off between accurate tracking and nonlinear velocity saturation of the hydraulic valves that can result in severe acceleration spikes. Control methods are developed to improve performance and include both acceleration and displacement feedback to reduce the acceleration spikes, and record modification, where the reference signal is modified to conform to the shake table's operational parameters. Results show record modification gives exact tracking for near-field ground motions, and optimal system response for reference signals with velocity components greater then the system capabilities. Overall, the research presents a methodology for simple effective identification, modelling, diagnosis and control of structural shake table systems that can be readily generalised and applied to any similar facility.     

MODIFICATION OF SITE RESPONSE IN PRESENCE OF LOCALISED SOFT LAYER

Experimental and numerical simulations are performed to evaluate the modification of ground response resulting from either the presence of soft layers or occurrence of partial liquefaction. Results from two densely instrumented dynamic centrifuge tests are presented to show the ambiguous role played by the presence of a soft layer. It was found that the lateral extent of the soft layer has significant influence on the overall response of the layered strata and any structure founded on it. The experimental observations are supported by simplified numerical analysis. The amplification or deamplification of the input motion is found to be a function of the ratio of the width of soft layer to the wave length. Based on the numerical analysis, a general function describing the site amplification is presented which may be used as a guide in seismic design of foundations in such layered strata.     

RESPONSE OF UNREINFORCED MASONRY BUILDINGS

Abstract

A summary of dynamic measurements are presented that illustrate relations between linear seismic demand and true nonlinear response of unreinforced masonry buildings with flexible diaphragms and rocking piers subjected to a series of simulated earthquake motions.     

SEISMIC DESIGN AND RESPONSE OF BARE AND MASONRY-INFILLED REINFORCED CONCRETE BUILDINGS. PART I: BARE STRUCTURES

Abstract

Eurocode 8 is applied for the complete design of 26 multi-storey reinforced concrete buildings to study its operationally and compare the implications of trading strength for ductility through designing the same structure for a different Ductility Class. The difference between the conventional full Capacity Design of columns in bending and the relaxed one allowed by Eurocode 8 is quantified, and the implications on the column capacities are examined. About half of the designed buildings, representative of the class of regular frames, are subjected to nonlinear dynamic response analyses to spectrum-compatible motions with intensities up to twice that of the design motion. Nonlinear modeling is very simple, but gives satisfactory agreement with available quasistatic or pseudodynamic test results on full scale structures. Results show that the three Ductility Classes of Eurocode 8 are essentially equivalent in terms of material quantities and seismic performance. Within the limitations of the nonlinear modelling, the response results suggest very satisfactory performance of structures designed to Eurocode 8, even under twice the design motion intensity. Softening of the structure due to concrete cracking and steel yielding significantly reduces the seismic force demands and contributes to the satisfactory performance, despite the increased P — 6 effects. Another important contributor to the good performance is the significant overstrength of the members considered in the analyses with their average as-built properties. Beam overstrength due to the contribution of the slab to flexural capacity is large enough to overcome the effects of the application of the relaxed Capacity Design rule to columns in bending. However, the resulting column plastic hinging does not lead to drift concentrations suggesting formation of storey-sway mechanisms.     

ENGINEERING EVALUATION OF THE 1 OCTOBER 1995 DİNAR EARTHQUAKE (ML = 5.9)

In 1995, a swarm of earthquakes affected the city of Dinar, Turkey, which is located in Southwest Anatolia and has a population of 35 000. The mainshock having a local magnitude of 5.9 occurred on 1 October 1995. It was preceded by foreshocks in the previous four days, the largest one with a magnitude of 4.7. These foreshocks initiated structural damage in many buildings, which was then severely aggravated by the mainshock and a strong aftershock two hours later, with a magnitude of 5.0. Numerous aftershocks were recorded throughout the following three months. Strong ground motions were recorded within the city. The mainshock produced peak horizontal accelerations of 0.28 g and 0.29 g. The Dinar earthquake caused a death toll of 92 and more than 200 injuries. The economic losses due to structural damage alone are estimated at 250 million USD.     

SEISMIC DESIGN AND RESPONSE OF BARE AND MASONRY-INFILLED REINFORCED CONCRETE BUILDINGS. PART II: INFILLED STRUCTURES

The effects of masonry infills on the global seismic response of reinforced concrete structures is studied through numerical analyses. Response spectra of elastic SDOF frames with nonlinear infills show that, despite their apparent stiffening effect on the system, infills reduce spectral displacements and forces mainly through their high damping in the first large post-cracking excursion. Parametric analyses on a large variety of multi-storey infilled reinforced concrete structures show that, due to the hysteretic energy dissipation in the infills, if the infilling is uniform in all storeys, drifts and structural damage are dramatically reduced, without an increase in the seismic force demands. Soft-storey effects due to the absence of infills in the bottom storey are not so important for seismic motions at the design intensity, but may be very large at higher motion intensities, if the ultimate strength of the infills amounts to a large percentage of the building weight. The Eurocode 8 provisions for designing the weak storey elements against the effects of infill irregularity are found to be quite effective, in general, for the columns, but unnecessary and often counterproductive for the beams.