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2021
Eccentric punching strength of continuous flat slabs—Analysis of different experimental setups, Secci, Lorenzo, Lapi Massimo, Teoni Emanuele, Ramos Antonio Pinho, and Orlando Maurizio , Structural Concrete, Volume n/a, Number n/a, (2021) AbstractWebsite

Abstract This paper analyses the performance of the experimental setups to assess the punching strength of slab-column connections in continuous flat slabs under vertical and horizontal loading. In the last years, several experimental campaigns have been performed to investigate the punching strength of slab-column connections, but most of the experimental tests concerned isolated slab-column connections. Among the few setups aimed at reproducing the eccentric punching failure in continuous flat slabs, the setup developed at the NOVA School of Science and Technology in Lisbon is considered in this paper. The performance of the Lisbon setup is assessed through nonlinear finite element analyses, calibrated on experimental data, by comparison with numerical results of a theoretical continuous setup. Then, the performance of the isolated setups, used in many researches and at the base of some international codes, is also evaluated through the same finite element model. Numerical analyses highlight that the setup developed in Lisbon could provide reliable ultimate rotations of continuous flat slab connections, but it underestimates the punching strength. Despite isolated setups lead to similar results when compared with the Lisbon setup, the latter seems to provide a better representation of a continuous slab-column connection. The numerical analyses presented in this paper have been performed assuming monotonic lateral loading.

A review of tests on slab-column connections with advanced concrete materials, Isufi, Brisid, and Ramos António Pinho , Structures, Volume 32, p.849-860, (2021) AbstractWebsite

Advances in concrete technology during the last decades have resulted in the development of materials with enhanced mechanical properties, such as High Strength Concrete (HSC), Fibre Reinforced Concrete (FRC) and Ultra-High Performance Fibre Reinforced Concrete (UHPFRC). The application of these materials in flat slabs, which are a popular structural solution in Reinforced Concrete (RC) buildings worldwide, has the potential of significantly reducing raw material consumption by enabling the design of slenderer and therefore lighter structures. However, flat slabs are susceptible to punching shear failure, which is a complex phenomenon that remains challenging, even though significant efforts have been made to experimentally study it. For advanced concrete materials (HSC, FRC and UHPFRC), the challenge is further accentuated by the continuous and rapid development of these materials. With the purpose of identifying and highlighting gaps in the published literature, a review of tests with HSC, FRC and UHPFRC slab-column connections in non-seismic and seismic loading applications is presented in this paper. It is shown that future research directions in this field include, among others, testing thicker slabs, HSC slabs with higher concrete compressive strength, HSC combined with FRC and several more cases related to seismic loading conditions.

Shear and flexural strengthening of deficient flat slabs with post-installed bolts and CFRP composites bonded through EBR and EBROG, Torabian, Ala, Isufi Brisid, Mostofinejad Davood, and Ramos António Pinho , Structural Concrete, Volume 22, Number 2, p.1147-1164, (2021) AbstractWebsite

Abstract 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.

2020
Rational Use of High-Strength Concrete in Flat Slab- Column Connections under Seismic Loading, and Micael Inácio, Brisid Isufi, Massimo Lapi António Pinho Ramos , Volume 117, Issue 6, (2020) Abstract

High-strength concrete (HSC) slab-column connections with relatively low concrete strengths compared to today’s capabilities have been tested under seismic-type loading in the past. Herein, the hybrid use of HSC with compressive strength approximately 120 MPa and normal-strength concrete (NSC) is investigated through three reversed horizontal cyclic-loading tests with different geometries of the HSC region and a reference NSC specimen. The results show that HSC applied in the vicinity of the column can significantly enhance the seismic performance of slab-column connections. The best result in terms of drift capacity and economic use of HSC was achieved in the case of full-depth HSC extended from the column’s face up to 2.5 times the effective depth. Drift ratios up to 3.0% were achieved. A comparison with previous tests showed that the hybrid use of HSC and NSC can achieve similar results to the provision of punching shear reinforcement.

Shear and flexural strengthening of deficient flat slabs with post-installed bolts and CFRP composites bonded through EBR and EBROG, Torabian, Ala, Isufi Brisid, Mostofinejad Davood, and Ramos António Pinho , Structural Concrete, Volume n/a, Number n/a, (2020) AbstractWebsite

Abstract 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.

A state of the art of flat-slab frame tests for gravity and lateral loading, Coronelli, Dario, Muttoni Aurelio, Pascu Ion R., Ramos Antonio P., and Netti Teresa , Structural Concrete, Volume n/a, Number n/a, (2020) AbstractWebsite

Abstract This paper presents a critical review of the state of the art of experimental research concerning the seismic response of reinforced concrete flat slab frames. After a summary of tests on connections, the paper examines tests carried out on frames with gravity and cyclic lateral loading, and shake table tests; scaled specimens and one real scale study are included. A discussion of the results reached so far is provided focusing on the global response, the different load types and effects; the ultimate rotations at failure in relation to the gravity shear and a classification of different failure modes for different types of connections. Based on this analysis, the research needs are highlighted. An experimental program launched to address these open questions is described. Further open topics are highlighted.

Behavior of RC flat slabs with shear bolts under reversed horizontal cyclic loading, Almeida, André F. O., Ramos António P., Lúcio Válter, and Marreiros Rui , Structural Concrete, Volume 21, Number 2, p.501-516, (2020) AbstractWebsite

Abstract An experimental work on reinforced concrete flat slab specimens to test the efficiency of postinstalled bolts, as punching shear reinforcement in resisting vertical and cyclic horizontal loads, was conducted and is presented in this paper. The test protocol consisted in increasing horizontal drifts combined with constant vertical load until failure. Two different detailing solutions for the shear reinforcement were considered, one using a radial distribution around the column and another using a cross distribution, being the results compared with a previously tested reference specimen. The dimensions of the specimens were 4.25 x 1.85 x 0.15 m3. The test setup used for these tests was developed by the research team and simulates the boundary conditions with already recognized good results. Postinstalled steel bolts were proven to be an efficient solution for strengthening of existing structures, improving the structural behavior, and the punching resistance.

Behaviour of reinforced-concrete flat slabs with stirrups under reversed horizontal cyclic loading, Almeida, André F. O., Alcobia Bruno, Ornelas Miguel, Marreiros Rui, and Ramos António Pinho , Magazine of Concrete Research, Volume 72, Number 7, p.339-356, (2020) AbstractWebsite

This paper describes the experimental campaign to study the behaviour of reinforced-concrete flat slab structures with steel stirrups as punching shear reinforcement, under combined vertical and horizontal cyclic loading. The vertical load was first applied and kept constant during the test, while, regarding the cyclic horizontal loading, imposed cyclic drifts were increased until failure. Four slab specimens with shear reinforcement were tested and the results compared to a control slab specimen without shear reinforcement. The studied variables were different shear reinforcement ratios and the number of stirrup layers. The slabs were 4·15 × 1·85 m2 and 0·15 m thick, connected to two steel half-columns. The test setup used was developed by the research team and aimed to simulate the boundary conditions of a flat slab, representing the slab between middle spans in one direction and between zero bending moment points in the other direction. Results show that the use of steel stirrups as shear reinforcement is very effective, increasing shear, drift and energy dissipation capacities. The obtained results were also compared to the provisions given by European and American codes.

Flexural strengthening of flat slabs with FRP composites using EBR and EBROG methods, Torabian, Ala, Isufi Brisid, Mostofinejad Davood, and Ramos António Pinho , Engineering Structures, Volume 211, p.110483, (2020) AbstractWebsite

One of the major disadvantages of conventional fibre-reinforced polymer (FRP) strengthening techniques is the premature debonding of the FRP, leading to an underutilization of the materials. The externally bonded reinforcement on grooves (EBROG) method, which has been proven successful in postponing debonding in several structural applications, is examined in this study for the first time for realistic conditions in flat slabs. To this end, two different layouts of the strengthening solution are tested under concentric monotonic loading: one representing roof-level slab-column connections in which carbon FRP (CFRP) sheets are laid on top of the joint region (cross layout); and another one representing intermediate floors, in which the aforementioned layout is not possible due to the presence of the column (grid layout). For each layout, two FRP bonding techniques are used: conventional externally bonded reinforcement (EBR) and EBROG. Another specimen, without FRP strengthening, is used as a reference. It is shown that the EBROG technique is effective in postponing debonding for both layouts. Compared to the specimens in which EBR was used, the load capacity was increased in case of EBROG by 36% when FRP sheets were bonded on top of the joint (cross layout) and by 15% when sheets were attached outside the joint region (grid layout). Debonding strains are shown to be significantly higher in the case of EBROG compared to EBR. The experimentally observed debonding strains were compared with code provisions and predictions of models from the literature. A simple calculation method giving reasonably good results for the load capacity of the FRP-strengthened specimens is presented.

A hybrid method for the calibration of finite element models of punching-shear in R/C flat slabs, Lapi, Massimo, Secci Lorenzo, Teoni Emanuele, Ramos Antonio Pinho, and Orlando Maurizio , Computers & Structures, Volume 238, p.106323, (2020) AbstractWebsite

The paper is focused on the calibration of non-linear 3D finite element (FE) analyses to simulate punching failure of R/C flat slabs. The calibration procedure is developed with reference to the code ABAQUS, which is one of the most used computer codes in nonlinear modelling of R/C structures. Generally, the calibration of a nonlinear FE model is grounded on one test only, so its reliability could be limited. Here a hybrid method for the calibration of FE models of R/C flat slabs failing in punching is proposed and discussed. The method consists in calibrating input data by comparison of finite element model (FEM) results with both experimental data and predictions provided by analytical models. The procedure allows for a consistent calibration to be performed, valid for a wide range of longitudinal reinforcement ratios, from 0.5% to 2.00%, and concrete grades, from C20/25 to C50/60. A case study is investigated using the proposed method. Results show that calibrated values of the fracture energy lie between those provided by Model Code 1990 and Model Code 2010. From the new calibration procedure, a relationship between fracture energy and concrete compressive strength is also derived and blind analyses are performed to check its reliability against experimental results.

Influence of the top reinforcement detailing in the behaviour of flat slabs, Faria, Ricardo, Marreiros Rui, Ramos António Pinho, and Jesus Catarina , Structures, Volume 23, p.718 - 730, (2020) AbstractWebsite

This paper studies the influence of different longitudinal top reinforcement detailing on the behaviour and punching capacity of flat slabs. Experimental and numerical studies were carried out. The experimental campaign consisted in testing three reinforced concrete slabs 2.20 m wide with a thickness equal to 0.15 m. The numerical study was conducted using the non-linear finite element software ATENA 3D. Using numerical models, which were calibrated using the experimental tests, a parametric study was carried out to include other design solutions beyond the tested ones. The parameters that were changed in this parametric study were the steel reinforcement ratio, the concrete strength and the detailing of the top steel reinforcement, namely using a solution with uniform distribution and other with a higher concentration of reinforcement near the column. Finally, the results were compared with predictions obtained using some of the existing codes (Eurocode 2 and Model Code 2010). From this study it can be concluded that concentrating the top flexural reinforcement, keeping the same total amount of top longitudinal reinforcement, results in a stiffer response, an increment in the punching resistance, and a decrease in the maximum vertical displacement.

Post-earthquake Performance of a Slab-Column Connection with Punching Shear Reinforcement, Isufi, Brisid, Ramos António Pinho, and Lúcio Válter , Journal of Earthquake Engineering, p.1-23, (2020) AbstractWebsite
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Punching of reinforced concrete flat slabs – Rational use of high strength concrete, Inácio, Micael M. G., Lapi Massimo, and Ramos Antonio Pinho , Engineering Structures, Volume 206, p.110194, (2020) AbstractWebsite

This paper deals with punching of reinforced high strength concrete (HSC) flat slabs. Despite the use of HSC increased significantly in the last years, the experimental research on punching behavior of HSC slabs is still limited. Furthermore, most of this past research adopted concrete compressive strength lower than 90 MPa. In a previous work by this research group three specimens with concrete compressive strength around 120 MPa and one with normal strength concrete (NSC) were tested. The present work represents the continuation of the previous activity and it is focused on the rational use of HSC. Four specimens with HSC and one of NSC were tested under monotonic vertical loading. The HSC was placed only in the slab-column connection region and it was limited to a thin layer in the compressive zone, in order to have a more economical and sustainable solution. This rational use of the HSC showed excellent results in terms of punching strength. Limiting the HSC to a thin layer in the compressive zone resulted in an almost equal punching strength to that obtained with the slab entirely casted in HSC.

Role of punching shear reinforcement in the seismic performance of flat slab frames, Isufi, B., Cismasiu I., Marreiros R., Pinho Ramos A., and Lúcio V. , Engineering Structures, Volume 207, (2020) AbstractWebsite
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2019
Behavior of RC flat slabs with shear bolts under reversed horizontal cyclic loading, Almeida, André F. O., Ramos António P., Lúcio Válter, and Marreiros Rui , Structural Concrete, (2019) AbstractWebsite

Abstract An experimental work on reinforced concrete flat slab specimens to test the efficiency of postinstalled bolts, as punching shear reinforcement in resisting vertical and cyclic horizontal loads, was conducted and is presented in this paper. The test protocol consisted in increasing horizontal drifts combined with constant vertical load until failure. Two different detailing solutions for the shear reinforcement were considered, one using a radial distribution around the column and another using a cross distribution, being the results compared with a previously tested reference specimen. The dimensions of the specimens were 4.25 x 1.85 x 0.15 m3. The test setup used for these tests was developed by the research team and simulates the boundary conditions with already recognized good results. Postinstalled steel bolts were proven to be an efficient solution for strengthening of existing structures, improving the structural behavior, and the punching resistance.

Behavior of RC flat slabs with shear bolts under reversed horizontal cyclic loading, Almeida, André F. O., Ramos António P., Lúcio Válter, and Marreiros Rui , Structural Concrete, Volume n/a, Number n/a, (2019) AbstractWebsite

Abstract An experimental work on reinforced concrete flat slab specimens to test the efficiency of postinstalled bolts, as punching shear reinforcement in resisting vertical and cyclic horizontal loads, was conducted and is presented in this paper. The test protocol consisted in increasing horizontal drifts combined with constant vertical load until failure. Two different detailing solutions for the shear reinforcement were considered, one using a radial distribution around the column and another using a cross distribution, being the results compared with a previously tested reference specimen. The dimensions of the specimens were 4.25 x 1.85 x 0.15 m3. The test setup used for these tests was developed by the research team and simulates the boundary conditions with already recognized good results. Postinstalled steel bolts were proven to be an efficient solution for strengthening of existing structures, improving the structural behavior, and the punching resistance.

Flat slab strenghtening techniques against punching-shear, Lapi, M., Ramos A. P., and Orlando M. , Engineering Structures, Volume 180, p.160-180, (2019) AbstractWebsite

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Resposta da ligação pilar-laje fungiforme sujeitas a ações horizontais cíclicas reforçadas com estribos, Almeida, André, Ramos António, Marreiros Rui, Lúcio Válter, and Faria Ricardo , Revista Portuguesa de Engenharia de Estruturas, Volume Série III, Issue 9, p.67-74, (2019)
Assessment of SFRC flat slab punching behaviour – part I: monotonic vertical loading, Gouveia, Nuno D., Faria Duarte M. V., and Ramos António Pinho , Magazine of Concrete Research, Volume 71, Number 11, p.587-598, (2019) AbstractWebsite

This paper presents a study of the behaviour and load capacity of steel-fibre-reinforced concrete (SFRC) flat slabs under monotonically increased concentrated vertical loads. The SFRC was used only in the local region of the slab–column connection, as the rest of the slab was cast using normal concrete without fibres. The six experimental test specimens had a thickness of 150 mm with different longitudinal reinforcement ratios, using a non-uniform distribution over the slab width. The concretes used were made with different Dramix 4D 65/60 BG steel fibre contents (0, 0·5, 0·75 and 1·0% volume content). The slab tests were complemented by flexural tests on notched beams. This made it possible to determine the tension behaviour of the different concretes used, through a linear post-cracking behaviour and inverse analysis. The inverse analysis made it possible to define the stress–crack opening relationship that characterises the tension behaviour of SFRC and to relate it to the observed behaviour and load capacity of the tested slabs. The tests results show that the tensile behaviour of the SFRC plays an important role in the behavioural and load capacity of the slabs and that it can be considered relevant to physically based models.

Assessment of SFRC flat slab punching behaviour – part II: reversed horizontal cyclic loading, Gouveia, Nuno D., Faria Duarte M. V., and Ramos António Pinho , Magazine of Concrete Research, Volume 71, Number 1, p.26-42, (2019) AbstractWebsite

This paper presents an experimental study of four flat slab specimens subjected to combined vertical and horizontal cyclic loading. Steel fibre-reinforced concrete (SFRC) was used only in the local region of the slab–column connection, while the rest of the slabs were cast using normal concrete. The specimens measured 4·15 m × 1·85 m × 0·15 m and were connected to two steel half columns by 0·25 m × 0·25 m rigid steel plates, prestressed against the slab using steel bolts, to ensure monolithic behaviour. The specimens were tested using an innovative test setup system that accounted for important factors, such as the ability of bending moment redistribution, line of inflection mobility and assured equal vertical displacements at the opposite slab borders, and symmetrical shear forces. Results show that the presence of SFRC in the slab–column connection region is effective in increasing the deformation capacity of slab–column connections, allowing the increase of horizontal drift ratios.

Behavior of thin lightly reinforced flat slabs under concentric loading, Torabian, Ala, Isufi Brisid, Mostofinejad Davood, and Ramos António Pinho , Engineering Structures, Volume 196, p.109327, (2019) AbstractWebsite

The current research aims to study the behavior of thin reinforced concrete (RC) slabs under concentrated loads as well as to investigate the application of Critical Shear Crack Theory (CSCT) to such slabs. For this purpose, four square 100-mm-thick slabs were cast and subjected to concentrated punching monotonic loading. The experimental parameters were the flexural reinforcement ratio, 0.38% and 1.00%, and the presence or absence of shear headed stud reinforcement. It is shown that the failure criteria of CSCT describe reasonably well the observed failure modes and the ultimate loads of the specimens. However, attention is brought to some peculiarities in the analytical derivation of the load-rotation curve for thin lightly reinforced flat slabs, in which large deformations are experienced. Results showed that in such slabs, the behavior can be highly influenced by the post-yield stress-strain curve of the flexural steel reinforcement. As a result, the constitutive law of steel reinforcement should be explicitly taken into account in such cases. The versatility of CSCT to adapt to these conditions is demonstrated.

Finite element analysis of punching shear of R/C slabs: A hybrid approach for model calibration, Secci, L., Teoni E., Lapi M., Orlando M., and a Ramos , Proceedings of the fib Symposium 2019: Concrete - Innovations in Materials, Design and Structures, p.643-650, (2019) Abstract
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Post-earthquake strength and deformation capacity of a flat slab specimen with shear studs, Isufi, B., Lúcio V., and Ramos A. P. , Proceedings of the fib Symposium 2019: Concrete - Innovations in Materials, Design and Structures, p.1684-1691, (2019) Abstract
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Reversed horizontal cyclic loading tests of flat slab specimens with studs as shear reinforcement, Isufi, Brisid, Ramos António Pinho, and Lúcio Válter , Structural Concrete, Volume 20, Number 1, p.330-347, (2019) AbstractWebsite

The results of a series of experiments on four reinforced concrete flat slab specimens with shear studs and a control specimen without any shear reinforcement are presented. The specimens were tested under constant gravity loads and reversed horizontal cyclic displacements. The main test variables were the applied gravity load and the number of perimeters of studs. One of the specimens was tested in two phases to study the postearthquake behavior. Results showed a considerable improvement of the deformation capacity of specimens with studs compared to the reference specimen. In agreement with previous research, increasing the applied gravity shear ratio resulted in a lower experimental drift capacity. It is shown that a better explanation of the observed ultimate drifts can be made by considering also the flexural capacity and the extent of shear reinforcement. The specimen tested in two phases exhibited considerable residual capacity, even after severe horizontal loading.

Static and seismic behaviour of R/C slabs with openings adjacent to columns, Lapi, M., Orlando M., Spinelli P., and a Ramos , Proceedings of the fib Symposium 2019: Concrete - Innovations in Materials, Design and Structures, p.1795-1802, (2019) Abstract
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