Chris G. Karayannis

INFLUENCE OF MASONRY STRENGHT AND OPENINGS ON INFILLED R/C FRAMES UNDER CYCLING LOADING

The influence of masonry infills with openings on the seismic performance of reinforced concrete (R/C) frames that were designed in accordance with modern codes provisions is investigated. Two types of masonry infills were considered that had different compressive strength but almost identical shear strength. Infills were designed so that the lateral cracking load of the solid infill is less than the available column shear resistance. Seven 1/3 – scale, single–story, single–bay frame specimens were tested under cyclic horizontal loading up to a drift level of 40%. The parameters investigated are the opening shape and the infill compressive strength. The assessment of the behavior of the frames is presented in terms of failure modes, strength, stiffness, ductility, energy dissipation capacity, and degradation from cycling. The experimental results indicate that infills with openings can significantly improve the performance of RC frames. Further, as expected, specimens with strong infills exhibited better performance than those with weak infills. For the prediction of the lateral resistance of the studied single-bay, single-story infilled frames with openings, a special plastic analysis method has been employed.

EFFECTIVENESS OF RC BEAM-COLUMN CONNECTION REPAIR USING EPOXY RESIN INJECTIONS

The effectiveness of a technique for the repair of reinforced concrete beam column connections, damaged due to cyclic loading, is experimentally investigated. The applied technique is mainly based on the use of thin epoxy resin infused under pressure into the crack system of the damaged joint body. Special attention is given to the examination of the repair efficiency with reference to the shear reinforcement of the joint. To achieve this purpose, the test program included 17 exterior connection specimens covering all commonly used joint reinforcing practices. Specimens were initially subjected to increasing full cyclic imposed deformation until the cycle maximum load decreased to 40% of the yield load level measured in the first cycle. After repair, all specimens were retested in the same way. Conclusions concerning the effectiveness of the applied repair technique, based on maximum cycle loads, loading stiffness, and hysteretic energy absorption capabilities of the tested specimens, are drawn and commented upon. Remarks concerning the influence of different design reinforcement arrangements on the behaviour of the joints are also included. The examined repair technique can be considered to be satisfactory, since all repaired joints exhibited equal or higher response load values and loading stiffness compared to the virgin ones, and tended to undergo more full loading cycles without a significant loss of strength.

Damage Evaluation in Shear-Critical ReinforcedConcrete Beam using Piezoelectric Transducers asSmart Aggregates

Abstract Damage detection at early cracking stages in shear-critical reinforced concrete beams, before further deterioration and their inevitable brittle shear failure is crucial for structural safety and integrity. The effectiveness of a structural health monitoring technique using the admittance measurements of piezoelectric transducers mounted on a reinforced concrete beam without shear reinforcement is experimentally investigated. Embedded “smart aggregate” transducers and externally bonded piezoelectric patches have been placed in arrays at both shear spans of the beam. Beam were tested till total shear failure and monitored at three different states; healthy, flexural cracking and diagonal cracking. Test results showed that transducers close to the critical diagonal crack provided sound and graduated discrepancies between the admittance responses at the healthy state and thedamage levels.Damage assessment using statistical indices calculated from the measurements of all transducers was also attempted. Rational changes of the index values were obtained with respect to the increase of the damage. Admittance responses and index values of the transducers located on the shear span where the critical diagonal crack formed provided cogent evidence of damage. On the contrary, negligible indication of damage was yielded by the responses of the transducers located on the other shear span, where no diagonal cracking occurred.

Effectiveness of Rectangular Spiral Shear Reinforcement on Infilled R/C Frames Under Cyclic Loading

The effect of two types of shear reinforcement, with reference to two types of masonry infills, on the seismic performance of reinforced concrete (RC) frames was experimentally investigated. Six single-story, one-bay, 1/3-scale frame specimens were tested under cyclic horizontal loading, up to a drift level of 40‰. Bare frames and infilled frames with two different infill compressive strengths were sorted into two groups based on stirrups or spirals as shear reinforcement. From the observed responses it can be deduced that rectangular spiral reinforcement is a first experience and more experiments are needed.

ASSESSMENT OF MULTISTORY RC BUILDINGS SUFFERING INTER-STORY POUNDING

Abstract. The pounding of adjacent RC frames with different heights is studied. The main issue behind this investigation is the influence of the local behaviour of the columns that suffer the hit from the slabs of the adjacent building during the seismic excitation. This column’s requirements for ductility and shear capacity have been proven to be the critical parameters for the behaviour and final performance of the whole structure. These parameters can be designed in order the obtained performances to satisfy the seismic demands of the Eurocodes. In this study the critical column’s requirements for ductility and shear capacity are evaluated for three seismic demands based on Eurocode 8-part3: (a) demand for Damage Limitation limit state that corresponds to ground motions with return periods of 225 years (b) demand for Significant Damage limit state that corresponds to ground motions with return periods of 475 years and (c) demand for Near Collapse limit state that corresponds to ground motions with return periods of 2475 years. The evaluated requirements are for seven different seismic excitations that have been properly scaled to fit the Eurocode’s three seismic demands of low zone of seismic hazard. Afterwards, the minimum required gap distances between the adjacent structures are evaluated taking into account the local capacities of the columns that suffer the inter-story pounding effect. These results are compared with the corresponding limitations for adequate gap distances that are provided by the Eurocode 8. The results show that even in the case of Damage limitation seismic demand special measures have to be taken for the column of the multistory RC frame that suffers the hit. The seismic performance of the columns at a specific limit state of the assessment should be considered for the estimation of an adequate gap distance between the adjacent structures. Thus, the minimum gap distance that is required in order to eliminate the possibility for interaction between adjacent structures depends on the limit state of the assessment. Eurocode’s provisions for adequate gap separation do not depend on the seismic demand (limit state) but in any case based on the results of this study these provisions seem to be conservative.

CAPACITY OF RC JOINTS SUBJECTED TO EARLY-AGE CYCLIC LOADING

The influence of cyclic loading applied on T-shape beam-column subassemblies at an early stage of the concrete curing period on the capacity of the joints after the maturing period is experimentally investigated. A series of 20 joints constructed for this purpose were subjected first to an early-age cyclic loading and then retested at the age of 28 days. From the results it is deduced that the early-age loading has significantly influenced the capacity and the overall behaviour of the specimens after the concrete curing period. Specimens subjected to early-age loading 12 or 24 hours after casting exhibited at the age of 28 days peak load values and energy dissipation higher than the ones of the specimens subjected to early-age loading two or seven days after casting, due to the high recoverability of the material at the very early stages of concrete curing period. Emphasis has been placed on the study of the influence of the early-age loading sequence and the slippage of the bars anchorage on the final capacity of the examined specimens. Finally, an attempt is made to assess the damage caused by early-age loading based on the observed stiffness degradation.

The Inter-storey Pounding Effect on the Seismic Behaviour of Infilled and Pilotis RC Structures

The influence of the inter-storey structural pounding on the seismic behaviour of adjacent multistorey reinforced concrete structures with unequal storey heights is studied taking into account the local response of the infill panels. Results of more than 100 dynamic analyses indicate that the most important issue in the interaction between structures is the local response of the column of the tall structure that suffers the hit of the upper floor slab of the adjacent shorter and stiffer structure. This column appears to be in most of the times in a critical condition due to shear action. In this paper, the influence of the infill panels on the pounding problem of adjacent structures is studied. Two types of masonry-infilled structures are considered: (a) infilled frame and (b) infilled frame without infills at the base storey (pilotis frame). Results in terms of inter-storey drifts, shear requirements, ductility requirements and infill local seismic response are presented. The influence of the infill panels on the seismic performance of the critical column that suffers the hit led in all examined cases to an increase of the demands for shear and ductility, when compared to the corresponding values that are developed in the cases that the RC structure is studied without considering the infills. Thus, it can be demonstrated that in all examined inter-storey pounding cases, the presence of the infills was not enough for the amelioration of the excessive demands for shear and ductility of the column that suffers the impact. Nevertheless, the presence of the masonry infill panels has been proved as an important parameter for the safety of the building. Non-linear dynamic step-by-step analyses and special purpose elements are employed for the needs of this study.

Recording and Rehabilitation Procedures for Historic Masonry Buildings

The use of photogrammetric surveys for the recording of the structural system status for historic masonry buildings is a new and sound tool for in-depth pathology understanding and rehabilitation decision-making. In this study, the structural system of two historic masonry buildings is investigated based on the recording data obtained via photogrammetry. The first one is a four-storey traditional tobacco warehouse of the late 19th century in Alexandroupolis, Northern Greece, which has been partially collapsed in 2005 and constituted an extremely dangerous working environment. Therefore, the photogrammetric recording was the only really accurate and safe in situ measurement procedure of the existing damaged structural system of the building. The second one is an old customs house located in the old historic town of Nafplio, Southern Greece, and specifically in front of the central part of the port. It was built around the middle of the 19th century by the well-known Architect S. Kleanthis. Its architectural, structural and morphological characteristics along with its artistic details have been excellently brought out through the advanced method of photogrammetry. Simultaneously, this detailed display of the structural system pointed out its pathology. This way, all the necessary information for the presented structures have been acquired with an appropriate accuracy in order to assist the Engineer to choose the proper repairing and strengthening techniques for the renovation and the re-use of these historic buildings; finally proposed rehabilitation techniques are also presented. The recording procedure used, combines the conventional topographic surveys and the photogrammetric image processing for the formation of all the facades’ orthoimages. The orthoimages were used as background information to digitize details of the buildings facades in a CAD environment. The Photomodeler software and the freeware application Hugin has been used to create respectively a detailed 3D model and an interactive panorama file of the buildings. A proper intervention plan is presented and commented for each building.

REHABILITATION OF A PARTIALLY COLLAPSED MASONRY TRADITIONAL TOBACCO WAREHOUSE OF THE LATE 19TH CENTURY

This study presents morphology, pathology, recording method, applied rehabilitation procedure and constructional problems of a four-storey masonry building in Alexandroupoli which today is partially collapsed. It was constructed before 1900 and enlarged in 1924 to be used as tobacco warehouse. From 1950s onwards, it has been neglected and because of the total lack of maintenance it has sustained structural and architectural deterioration; intervention works for its re-use as Municipal Library began in 2004. The first approved rehabilitation plan based on false strategy adopted the total removal of the interior structural members including wooden diaphragms, beams and columns and the construction of a new internal reinforced concrete frame structure. It proved to be catastrophic since in 2005 a sudden partial collapse of masonry and the roof was caused. The building has remained partially collapsed and difficult to approach due to the danger of a potential further collapsing. Recording of the present state has been based on photogrammetry. A proper intervention plan is applied. This work is a contribution in the field of structural rehabilitation of partially collapsed historic masonry buildings that constitute an extremely unsafe working environment.

STRUCTURAL UPGRADING OF A 3-STOREY HERITAGE STRUCTURE OF 1925 IN THESSALONIKI

Abstract. The structural upgrading and rehabilitation procedure of a historic building representative of 1920s construction in Thessaloniki, Greece is presented. The case study building is a listed three-storey reinforced concrete flat-slab frame structure sited in Nikis Avenue in front of the old seaside of the city of Thessaloniki. Reinforced concrete columns and beams frames along with wide infilled masonry walls are the load-bearing elements of the structure. The building was designed in 1925 without Seismic Code requirements and constructed in 1926. The ground floor was used for many years as a cinema, whereas the upper floors are areas where people may congregate since they are used as assembly halls (dining halls, reading and conference rooms). Concrete core tests and in-situ non-destructive tests were first performed to evaluate the compressive strength of the concrete and to detect determine the existing steel reinforcements of the reinforced concrete members. The concrete strength was low and the steel reinforcement of several columns of the ground floor and slabs of the upper floors was found to be corroded. Analytical evaluations of the original and the strengthened structure were carried out in order to identify the weak members of the structural system and to justify the decisions of the strengthening methods adopted. Comparisons between the capacity of the existing or/and the strengthened members with the design requirements derived from the initial and the strengthened structural system analyses are also presented. Special attention has been given in issues regarding the simulation of actual details encountered in mixed structural system. The upgrading methods used along with the uncovering of latent defects during the strengthening works and how these were managed are also commented in this paper.