SEISMIC RELIABILITY ASSESSMENT OF EXISTING REINFORCED CONCRETE BUILDINGS

Structures designed according to earlier codes with inadequate seismic provisions have not performed satisfactorily during recent earthquakes. The seismic performance of an existing three-storey reinforced concrete building designed according to the 1963 ACI 313-63 is evaluated and compared to the performance of a similar frame designed according to current code provisions. Non-linear static and dynamic analyses of the reinforced concrete frames are conducted. In this study, a probabilistic approach is adopted where a large number of artificially generated ground motion records is used as input motion to the structure. The results of the analysis indicated the probability of various degrees of damage to be expected when the existing frame is subjected to different ground motion levels. This information is useful in the design of the required rehabilitation scheme to provide an identified level of protection.     

FRAGILITY ASSESSMENT OF SLAB-COLUMN CONNECTIONS IN EXISTING NON-DUCTILE REINFORCED CONCRETE BUILDINGS

Fragility functions that estimate the probability of exceeding different levels of damage in slab-column connections of existing non-ductile reinforced concrete buildings subjected to earthquakes are presented. The proposed fragility functions are based on experimental data from 16 investigations conducted in the last 36 years that include a total of 82 specimens. Fragility functions corresponding to four damage states are presented as functions of the level of peak interstory drift imposed on the connection. For damage states involving punching shear failure and loss of vertical carrying capacity, the fragility functions are also a function of the vertical shear in the connection produced by gravity loads normalised by the nominal vertical shear strength in the absence of unbalanced moments. Two sources of uncertainty in the estimation of damage as a function of lateral deformation are studied and discussed. The first is the specimen-to-specimen variability of the drifts associated with a damage state, and the second the epistemic uncertainty arising from using small samples of experimental data and from interpreting the experimental results. For a given peak interstorey drift ratio, the proposed fragility curves permit the estimation of the probability of experiencing different levels of damage in slab-column connections.     

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.     

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.     

REHABILITATION OF REINFORCED CONCRETE COLUMNS USING CORRUGATED STEEL JACKETING

An experimental investigation was conducted to study the failure mode of existing reinforced concrete columns designed during the 1960s. The effectiveness of using corrugated steel jackets for enhancing the seismic flexural strength and ductility of these types of columns was examined. Three large-scale columns were tested under cyclic loading. The three columns represent existing column, current code-detailed column and rehabilitated column. The variables in the test specimens include the amount of column transverse reinforcement and jacketing of the column. The corrugated jacket was found to be effective in the rehabilitation of the selected existing structure, which does not meet the current seismic code requirements. A method is proposed for the design of the corrugated steel jacket to enhance the lap splice capacity and ductility of the column.     

ON THE OPTIMAL LOCATION OF SENSORS IN MULTI-STOREYED BUILDINGS

A new approach has been developed for finding optimal location of sensors in 3D multi-storeyed buildings. This approach is based on the compact probabilistic representation of acceleration response in terms of its covariance matrix. For a specified number of sensors, the optimal location has been taken to be the one for which the computed covariance matrix is closest to the exact covariance matrix of the random field constituted by the acceleration response process. It has been found that the determined sensor locations match favourably with those predicted by earlier studies for the special case of shear buildings. Further, the optimality of the determined sensor locations has also been verified by identifying the system parameters from the time series data and comparing them with those of the Finite Element Model.