K. C. Stylianidis

Development of Seismic Risk Scenarios Based on a Hybrid Method of Vulnerability Assessment

A hybrid methodology of vulnerability analysis is presented, involving elements from both empirical and theoretical methods. A model for correlating analytically calculated structural damage indices to loss (in monetary terms) is also proposed and calibrated against available statistical data. Probability damage matrices derived using this methodology are incorporated into a cost-benefit model tailored to the problem of estimating the feasibility of seismically rehabilitating the existing stock of reinforced concrete buildings in Thessaloniki, Greece. Losses calculated using the suggested procedure are found to be in good agreement with losses incurred during the 1978 Thessaloniki earthquake. The results of the present study also indicate that benefit/cost ratios for reinforced concrete buildings are quite low. Hence, it appears that a pre-earthquake strengthening programme is not economically justifiable.

ANALYTICAL MODELS FOR BRICK MASONRY INFILLED R/C FRAMES UNDER LATERAL LOADING

Proposed in this paper are two analytical models for predicting the inelastic response of unreinforced brick masonry infills in reinforced concrete frames subjected to monotonic and reversed cyclic loading. The first model is based on the traditional diagonal strut concept, while the second one is a simple isoparametric element with shear deformation only. All the essential characteristics of the hysteretic behaviour of the panel, including strength and stiffness degradation, pinching and slippage, are explicitly taken into account. The models are implemented in a general-purpose program for the inelastic time-history analysis of structures, and are used for studying the seismic behaviour of typical multistorey frames with various arrangements of infill panels, including structures with an open ground storey. The results of the analysis are in agreement with both experimentally observed behaviour and with experience regarding seismically damaged buildings.

Experimental Investigation of Masonry Infilled R/C Frames

Although it is well documented that infills significantly affect the dynamic characteristics and the seismic re- sponse of the bare basic structural system, codes are reluctant to encourage consideration of infills as main structural ele- ments, mainly due to structural uncertainties and computational complexities. Part of the uncertainties are due to the very many parameters affecting the behavior of the system, such as infill materials, reinforcing of infills, connection to the sur- rounding frame, geometry, relative stiffness and strength, local construction techniques etc. In the present paper three suc- cessive experimental programs, conducted at the R/C and Masonry Structures Laboratory of the Aristotle University, are described, commented and discussed. All of them refer to single-storey one-bay 1:3 scale R/C moment resisting frames. The first program consisted of 18 specimens, 2 bare and 16 unreinforced masonry (URM) infilled. The second program was an extension and a supplement of the previous one. It consisted of 20 more specimens, 6 bare and 14 URM infilled. The third program was directed towards the investigation of quick and low cost strengthening methods of R/C frames damaged by earthquakes, using several infill techniques. For this purpose 10 of the damaged specimens of the first pro- gram were repaired and strengthened. The significant change of the static and dynamic charac- teristics of the bare basic structural system by the incorpora- tion of infills is a fact stated by many authors and experi- enced in every day praxis. Older versions of codes (1, 2) provide specific instructions for the design and construction of infilled structures, recommending two alternatives: either an effective isolation of the infills from the surrounding frames, so that their structural effects can correctly be ne- glected, or a tight placing of the infills so that their interac- tion with the frames should be properly considered in the design, detailing and construction, especially for seismic excitations. In the first case of the isolation of the infills, the struc- tural system is clear and relatively reliable but, as separation at the bottom of the infill and adequate resistance to out of plane seismic loads are difficult to achieve, this can be haz- ardous (3). Furthermore, the infills add a significant undesir- able mass. In the second case of the tight placing of the in- fills, the increased strength, stiffness and energy dissipation capacity seem to improve the seismic behavior of the regular buildings although base shear may be increased because of the increase of stiffness (4). Recent codes (5) give special attention to the consequences of irregularities in plan and in elevation produced by the infills, which should be taken into account.

Assessment of the seismic vulnerability of unreinforced masonry buildings

A methodology for seismic vulnerability analysis of URM buildings is presented. It includes the estimation of capacity curves for typical building classes, as well as vulnerability (fragility) curves in terms of peak ground acceleration (PGA). The methodology for developing vulnerability curves is based on a hybrid approach, combining statistical data with appropriately processed results from nonlinear static analyses that permit extrapolation of statistical data to intensities for which no data are available. The data used are statistical, derived from Greek earthquakes, the Thessaloniki 1978 and the Aegion 1995 events, with some additional data from the Pyrgos 1993 earthquake used for comparison purposes. The databases of the first two earthquakes are briefly presented and processed using a filtering technique. The resulting hybrid vulnerability curves correlate PGA to the probability that a building type exceeds a particular damage state.

Comparison of damage data from questionnaires and field survey: the case of the June 20, 1978 Thessaloniki (northern Greece) M6.5 earthquake

The earthquake of June 20, 1978 (M6.5) near Thessaloniki, Greece, and the abundant information on damage distribution provided researchers with an opportunity for a more detailed study of earthquake effects. The damage on buildings caused by that earthquake was recorded in several ways. In this paper two of them will be presented and discussed: First, the use of questionnaires filled in by citizens and second, the in situ inspection of the buildings by a team of expert engineers, enriched by data of retrofitting costs, where available. In the current study, the damage data derived from the questionnaires are compared against the ones given by the engineers after they have been both converted to the European Macroseismic Scale 1998 (EMS-98) Damage Grades. It is observed that for EMS-98 damage grades equal and larger than 2 the questionnaire method overestimates damage while for lower grades both approaches provide comparable results.

Irregularities In The Seismic Response Of R/CBuildings Due To The Presence Of Masonry Infills

A refined phenomenological model for masonry infill panels surrounded by R/C frames and subjected to transient cyclic loading is used to investigate the influence of the presence of regular and irregular patterns of masonry infills on the seismic behaviour of multistorey frames. It is seen that regular patterns of infills (not necessarily in all bays) can lead to a behaviour generally superior to that of corresponding bare frames. Open storeys in infilled buildings lead to damage concentration in these storeys and failure can only be avoided through heavy confinement in R/C members, as that specified in Eurocode 8.

EXPERIMENTAL AND ANALYTICAL INVESTIGATION OF AN UNCONVENTIONAL, ROCKING TYPE RESPONSE MECHANISM FOR EARTHQUAKE-RESISTANT BRIDGES

Abstract. In the present study a new technique is investigated, which takes advantage of the rocking principle for the treatment of the two ambivalent demands in bridge designing, i.e. due to seismic and serviceability actions. The idea is based on the formation of cracks only in two positions on each bridge pier, which are the pier cross-sections in contact with the deck and the pile cap respectively, excluding the possibility of cracking in the mid-length of the piers. The possibility of flexural crack formation in one of these cross-sections was examined experimentally in order to reach the above objective. This was achieved through partial suppression of the longitudinal reinforcement bond with the surrounding concrete. In order to confirm the feasibility of the above objective and to verify the influence of the bond suppression on all three response indexes, namely strength, stiffness and ductility, eight experimental specimens in the form of cantilevers, representing ductile bridge piers, were prepared. The varying parameter was the shear span ratio. Useful results were observed regarding the three response indexes and interesting conclusions were obtained.