Brisid Isufi

Beams with width greater than their depth and depth equal to the depth of the slab (concealed wide beams) are widespread in Reinforced Concrete (RC) buildings in Albania. A large number of RC buildings with wide beams were hit by a strong M_w6.4 earthquake on 26 November 2019. The earthquake hit two of the most densely populated areas in Albania and caused widespread damage to these buildings. This paper aims to provide an updated view on the seismic performance of buildings with wide beams in light of the new field data following the 26 November 2019 Albania earthquake. To this end, a thorough literature review including experimental and field observations from past earthquakes is presented and data from field visits in Albania are described. It was found that damage to the joists and wide beams themselves was limited, even when the buildings suffered significant non-structural or structural damage in other elements as a result of the earthquake. A discussion on the behavior of wide beam–column frames based on nonlinear structural analyses and tests from the literature is presented. Furthermore, the implications of the results of the analysis for the seismic performance of RC buildings are discussed and confronted with observations from the 2019 Albania earthquake. Based on the literature review, further experimental research on wide beams with longer and more realistic span lengths is recommended.

Numerous studies have shown that incorporating fibers in concrete can improve its mechanical properties, including tensile strength and toughness. Due to these outstanding properties, the use of fiber‐reinforced concrete (FRC) composites has been investigated for applications in structural members subjected to shear, in which brittle failure mode may occur. An example of this type of application is the use of high/ultrahigh‐performance fiber‐reinforced concrete (HPFRC/UHPFRC) in flat slab‐column connections as an alternative to shear reinforcement. The experimental results revealed that the use of advanced concrete materials led to an increase in the punching shear and deformation capacities. Despite these advantages, the use of HPFRC/UHPFRC in practical design is somehow limited by the lack of guidelines in codes for this type of new material. Furthermore, the study of constitutive models for nonlinear finite element analysis is also necessary. In this regard, this article addresses the simulation of two‐way slabs with HPFRC, subjected to concentrated monotonic vertical loading, tested in a previous experimental study, where the HPFRC was used only in a limited slab region around the column, and the rest of the slab was cast with normal strength concrete. The numerical models were developed in the commercial finite element software DIANA. The concrete nonlinear behavior was modeled by a total strain‐based constitutive model, in which the HPFRC softening is represented by a tensile stress‐crack width (σ − w) relationship. The validity of the numerical model is checked by the comparison with the experimental results and a punching shear mechanical model, leading to the formulation of a closed‐form equation. Finally, a parametric study was conducted to extrapolate the results obtained in the experimental campaign and to optimize the use of HPFRC to obtain a more economical and sustainable solution.

Strengthening of flat slab-column connections to resist imposed lateral drifts is often required for older reinforced concrete structures in which the flat slabs were typically designed to resist gravity load only or have been designed for less stringent requirements. The complex stress state around the columns means that strong experimental evidence is required for the repair and strengthening techniques to be adopted with confidence in practice. Although a significant amount of research has been devoted to strengthening of slab-column connections, a series of limitations still remain. A thorough literature review revealed that only relatively thin and isolated specimens have been tested under seismic-type loading and realistic strengthening scenarios. To overcome these limitations, a full-scale two-story flat slab building specimen was tested at the European Commission’s Joint Research Centre – ELSA Laboratory in Ispra, Italy. This paper describes and analyzes the results obtained from the last of a series of phases, on repaired and strengthened internal and external slab-column connections using post-installed bolts. Compared with previous tests, the full-scale building specimen has a thicker slab (200 mm), realistic slab continuity conditions and the connections were strengthened after they suffered damage from gravity loading and previous lateral loading phases. The results show that post-installed bolts are efficient even under the described conditions (i.e. damaged and repaired specimens with realistic thickness), and punching shear failure was prevented in the strengthened connections.

Replicating boundary conditions of slab–column connections under seismic loading in a realistic manner is difficult, and different test setups have been used throughout the years. An innovative test setup has been developed at NOVA School of Science and Technology, and it has been used to test various punching shear behavior enhancement methods, including stirrups, headed studs, fiber‐reinforced concrete (FRC), high‐strength concrete (HSC), and post‐installed bolts under similar conditions. The aim of this paper is to compare the results from these different solutions for seismic loading. It is shown that relatively large drift ratios at failure can be achieved in slabs containing shear reinforcement. However, less conventional methods such as FRC and HSC are shown to be promising alternatives. Compared to reference specimens that failed at 1% drift, the other specimens with various enhancement methods achieved significantly higher drifts, ranging from 2.5% to above 6.0%.

WHY 1CroCEE The March 22, 2020 magnitude ML5.5 Zagreb earthquake painfully brought to light inadequate earthquake preparedness and hazard resilience of Croatia’s capital. This seismologically moderate event caused significant disruption to life in the capital and induced major damage to the building stock of the administrative, cultural, scientific and economic center of the country. At the same time, the prospect of an expected magnitude 6.5 event on the Medvednica fault zone still looms...

Fiber reinforced polymer (FRP) composites can be efficient for flexural strengthening of flat slabs if debonding of the FRP is postponed. However, with the increase of the flexural capacity, the flat slab becomes more susceptible to punching shear failure. In this context, four flexural or simultaneous flexural and punching shear retrofitting systems are investigated in this study to strengthen a flexure‐deficient flat slab. Externally Bonded Reinforcement on Grooves (EBROG) and externally bonded reinforcement (EBR) methods are used for flexural strengthening in two cases: slabs without punching shear reinforcement and with post‐installed shear bolts as shear reinforcement. According to the results, flexural strengthening of the slab using the EBR and EBROG techniques increased its load capacity by 12% and 21%, respectively. Simultaneous flexural and shear strengthening of the slab using the EBROG technique was the most effective, leading to a 57% enhancement of the load capacity. For specimens whose failure was governed by punching, comparing the results with code predictions showed that Eurocode and ACI (and the respective guide documents fib bulletin 90 and ACI 440.2R) overestimated the capacity of these specimens. In cases where failure was governed by flexure, a simple application of the yield line theory predicted reasonably well the load capacity of the specimens.