SEISMIC RESPONSE OF REINFORCED CONCRETE FRAMES INFILLED WITH WEAKLY REINFORCED MASONRY PANELS

The research work presented in this paper is related to the seismic response of RC frames infilled with weak masonry panels, as it is traditional in many seismic prone countries in southern Europe. More specifically, the benefits derived from the insertion of a light reinforcement, in the mortar layers or in the external plaster, are studied in some detail.

Tests have been performed on different types of single bay, single storey, infilled frames to investigate the in-plane response at different earthquake intensity levels and the out-of-plane strength as a function of the in-plane damage. A series of parametric simulations have then been performed, calibrating the models used in the test results, to evaluate the effects of the different panels characteristics on the response of whole buildings, with different infill patterns. Both in-plane and the out-of-plane response have been considered. The results are described in terms of peak ground acceleration required to induce given limit states of serviceability or damage relatively far from the collapse of the structure, which is governed by the RC frame design more than by the infill panels properties.     

SEISMIC FRAGILITY OF LRC FRAMES WITH AND WITHOUT MASONRY INFILL WALLS

This paper summarises the first phase of the fragility analyses of generic (representative) buildings in the area of Memphis, Tennessee, USA. The study was conducted at Cornell University as a part of the project Loss Assessment of Memphis Buildings (LAMB) for the National Center for Earthquake Engineering Research (NCEER). In this study, the fragility analyses focus on low-rise Lightly Reinforced Concrete (LRC) frame buildings with and without infill walls. The obtained fragility curves are compared with those of ATC-13 for different facility classes. Based on the obtained fragility curves, it is concluded that adding masonry infill wails to low-rise LRC frame buildings significantly reduces the likelihood of seismic damage.     

SEISMIC REHABILITATION OF BEAM-COLUMN JOINTS USING FRP LAMINATES

An innovative and practical technique for the seismic rehabilitation of beam-column joints using fiber reinforced polymers (FRP) is presented. The procedure is to upgrade the shear capacity of the joint and thus allow the ductile ftexural hinge to form in the beam. An experimental study is conducted in order to evaluate the performance of a full-scale reinforced concrete external beam-column joint from a moment resisting frame designed to earlier code then repaired using the proposed technique. The beam-column joint is tested under cyclic loading applied at the free end of the beam and axial column load. The suggested repair procedure was applied to the tested specimen. The composite laminate system proved to be effective in upgrading the shear capacity of the nonductile beam-column joint. Comparison between the behaviour of the specimen before and after the repair is presented. A design methodology for fibre jacketing to upgrade the shear capacity of existing beam-column joints in reinforced concrete moment resisting frames is proposed.     

EXPERIMENTAL BEHAVIOUR OF R/C FRAMES RETROFITTED WITH DISSIPATING AND RE-CENTRING BRACES

An extensive program of shaking table tests on 1/4-scale three-dimensional R/C frames was jointly carried out by the Department of Structure, Soil Mechanics and Engineering Geology (DiSGG) of the University of Basilicata, Italy, and the National Laboratory of Civil Engineering (LNEC), Portugal. It was aimed at evaluating the effectiveness of passive control bracing systems for the seismic retrofit of R/C frames designed for gravity loads only. Two different types of braces were considered, one based on the hysteretic behaviour of steel elements, the other on the superelastic properties of Shape Memory Alloys (SMA). Different protection strategies were pursued, in order to fully exploit the high energy dissipation capacity of steel-based devices, on one hand, and the supple-mental re-centring capacity of SMA-based devices, on the other hand. The experimental results confirmed the great potentials of both strategies and of the associated devices in limiting structural damage. The retrofitted model was subjected to table accelerations as high as three times the acceleration leading the unprotected model to collapse, with no significant damage to structural elements. Moreover, the re-centring capability of the SMA-based bracing system was able to recover the undeformed shape of the frame, when it was in a near-collapse condition. In this paper the experimental behaviour of the non protected and of the protected structural models are described and compared.     

COMPRESSIVE BEHAVIOUR OF CARBON FIBRE COMPOSITE JACKETED CONCRETE WITH CIRCULAR AND NON-CIRCULAR CROSS-SECTIONS

This paper describes the concentric compression test results of carbon fibre reinforced polymer (CFRP) composite sheet jacketed concrete specimens with circular, square and rectangular cross-sections. In the experimental program, pre-damaged specimens and repeated compressive loads were considered as well as undamaged specimens and monotonic compressive loads. The contribution of CFRP composite jackets to the compressive behaviour of the specimens is evaluated quantitatively, in terms of strength, longitudinal and lateral deformability and energy dissipation. Simple analytical expressions are proposed for compressive strength and ultimate axial strain that are valid for CFRP composite jacketed concrete with circular, square and rectangular cross-sections. The analytical results obtained by the proposed expressions are in good agreement with the experimental data obtained in this study, as well as the experimental data available in literature.     

ESTIMATION OF NEAR-SOURCE GROUND MOTION AND SEISMIC BEHAVIOUR OF RC FRAMED STRUCTURES DAMAGED BY THE 1999 ATHENS EARTHQUAKE

On September 7, 1999 an earthquake with magnitude M _W =5.9 occurred close to the city of Athens in Greece. More than 80 buildings collapsed, about 150 deaths and hundreds of injuries were reported. Soon after the event a damage investigation was carried out by two of the authors in the most heavily struck areas. The most serious damages were observed in the northern suburbs of Athens, where reinforced concrete frames and masonry buildings represent the prevalent construction systems. The hysteretic energy demands imposed on RC buildings should have been rather severe considering the structural systems characteristics and the inadequate construction details. However, over-strengths, redundancy and especially the presence of infill walls, provided a significant increase of the seismic capacity and contributed to the survival of many buildings.

The objective of the present work is to reproduce and analyse the response of typical RC frames subjected to the 1999 Athens earthquake in areas where the observed damage was particularly severe but no recordings of the ground motion were available. After a general overview of the seismotectonic environment, seismological data, observed macro-seismic intensities, structural typologies and observed building behaviour, an attempt is made to identify representative excitations in the meizoseismal area. Specifically, the required accelerograms are obtained by modifying available records so as to reproduce a given global energy content and to be consistent with the observed damage. To study the seismic response of RC models, the obtained accelerograms are used to perform nonlinear dynamic analyses.     

A SUMMARY OF RESULTS OF SIMULATED SEISMIC LOAD TESTS ON REINFORCED CONCRETE BEAM-COLUMN JOINTS,BEAMS AND COLUMNS WITH SUBSTANDARD REINFORCING DETAILS

The results of some simulated seismic load tests on reinforced concrete one-way interior and exterior beam-column joints with substandard reinforcing details typical of buildings constructed in New Zealand before the 1970s are described. The tests were conducted using both deformed and plain round longitudinal reinforcement. The interior beam-column joint cores lacked transverse reinforcement and the longitudinal bars passing through the joint core were poorly anchored. The exterior beam-column joint units contained very little transverse reinforcement in the members and in the joint core. In one exterior beam-column joint unit the beam bar hooks were not bent into the joint core. That is, the hooks at the ends of the top bars were bent up and the hooks at the ends of the bottom bars were bent down. This anchorage detail was common in many older buildings constructed before the 1970s. In the other exterior beam-column joint unit the hooks at the ends of the bars were bent into the joint core as in current practice. The improvement in performance of the joint with beam bars anchored according to current practice is demonstrated. In addition, tests were conducted on interior joints with lap splices in the beam longitudinal reinforcement bars near the column face. The tests were conducted using both deformed and plain round longitudinal reinforcement. Tests were also conducted on columns with plain round bar longitudinal reinforcement and inadequate transverse reinforcement.

The reinforcing details were close to identical to those in an existing seven storey reinforced concrete building that was designed and built in New Zealand in the late 1950s.

The test results give an indication of the performance of beam-column joints and members with the above now out-of-date reinforcing details.

The test results reported are a summary of results reported in a number of publications written since 1994.