M. P. Ferreira

Punching of Reinforced Concrete Flat Slabs with Double- Headed Shear Reinforcement

Twelve slabs, 11 of which contained double-headed studs as shear reinforcement, were tested supported by central column and loaded concentrically. Their behavior is described in terms of deflections, rotations, strains of the concrete close to the column, strains of the flexural reinforcement across the slab width, and strains of the studs. All failures were by punching, in most cases within the shear reinforced region. The treatments of punching resistance in ACI 318, Eurocode 2 (EC2), and the critical shear crack theory (CSCT) are described, and their predictions are compared with the results of the present tests and 39 others from the literature. The accuracy of predictions improves from ACI 318 to EC2 to CSCT—that is, with increasing complexity. However, the CSCT assumptions about behavior are not well supported by the experimental observations.

Punção em lajes lisas de concreto armado sem armadura de cisalhamento

Punching strength is a critical point in the design of flat slabs and due to the lack of a theoretical method capable of explaining this phenomenon, empirical formulations presented by codes of practice are still the most used method to check the bearing capacity of slab-column connections. This paper discusses relevant aspects of the development of flat slabs, the factors that influence the punching resistance of slabs without shear reinforcement and makes comparisons between the experimental results organized in a database with 74 slabs carefully selected with theoretical results using the recommendations of ACI 318, EUROCODE 2 and NBR 6118 and also through the Critical Shear Crack Theory, presented by Muttoni (2008) and incorporated the new fib Model Code (2010).

Resistência à punção de ligações laje-pilar interno com conectores de cisalhamento

Punching shear is a brittle failure mode that may occur in slab-column connections, which may be prevented by using shear reinforcement in the slab-column connection. This paper presents comparisons between experimental results of 36 tests in internal slab-column connections with double- headed shear studs, which are largely used in North America, Europe and Asia, with theoretical results using recommendations presented by ACI 318, NBR6118, Eurocode 2 and also the Critical Shear Crack Theory (CSCT).Considering the database used it is possible to observe that ACI 318 presents conservative trends, whereas NBR 6118 showed a low coefficient of variation, but with a large number of unsafe results. Both Eurocode 2 and CSCT showed satisfactory results with Eurocode 2 presenting slightly higher performance.

Bearing Capacity of Partially Loaded Concrete Elements

There are numerous situations where compressive loads are introduced into members over small areas. The stress distribution in these zones, which are depicted D for disturbed, is non-uniform. The flow of compressive stresses into the surrounding member induces transverse tensile stresses which can cause premature splitting failures. D regions are typically designed using the strut-and-tie method (STM). The STM design recommendations provided by ACI 318 (2014), fib Model Code (2010) (2013) and Eurocode 2 (2004), limit bearing stresses to admissible values which depend on a strut efficiency factor (β s ). This paper uses a database with 162 tests on partially loaded concrete prisms without and with crack-control reinforcement to evaluate these provisions and to discuss the influence of parameters like the concrete strength (f c ), the crack-control reinforcement ratio (ρ), concentration ratio (b/a) and aspect ratio (h/b). It also presents results of four tests on diamond-shaped concrete prisms carried out to evaluate the effect on resistance of increasing ρ. Experimental results show that crack-control reinforcement increases both ductility and resistance. Existing design provisions are shown to be conservative.

Effect of wind in the design of reinforced concrete buildings

Resumo This paper presents the results from a parametric study carried in order to quantify how far errors in the design stage related to the consideration of the wind action may put at risk the response and safety of reinforced concrete buildings. Using an architectural model as reference and varying the number of floors of the building, the structural safety was evaluated as a function of the wind action intensity. Results showed that even for low-rise buildings, with 10 floors, ignoring the wind action can significantly jeopardize their behaviour and safety. Yet, for slenderer buildings, up to 30 floors, it can lead to catastrophic results, as the ruin of the structure by progressive collapse.

Influence of the column rectangularity index and of the boundary conditions in the punching resistance of slab-column connections

Resumo Experimental evidence indicates that both the column rectangularity index and the boundary conditions of the connection may affect the ultimate punching resistance. This paper presents general aspects of these topics and, through the analysis of experimental results of tests on 131 slabs, evaluates the accuracy and suitability of recommendations presented by ABNT NBR 6118, Eurocode 2, ACI 318 and fib Model Code 2010. Experimental results showed that the security level of normative estimates trend to reduce as the column rectangularity increases, and in some cases, the punching resistance was overestimated. Finally, adjustments are suggested in equations presented by NBR 6118 and MC2010 in order to eliminate this trend of unsafe results.

Lajes lisas nervuradas bidirecionais com furos

A passagem de tubulacoes em elementos estruturais ainda merece atencao especial dos projetistas, principalmente quando o modo de ruptura dos mesmos pode ser alterado devido as mudancas em seus comportamentos. Este trabalho compara os resultados experimentais obtidos para 06 (seis) lajes lisas nervuradas bidirecionais em concreto armado, com furos retangulares adjacentes aos pilares e sem armadura de cisalhamento, com as estimativas das formulacoes dadas pelas normas ACI 318 e NBR 6118. As lajes foram moldadas e ensaiadas no Laboratorio de Engenharia Civil da UFPA e apresentaram dimensoes de 1.800 mm x 1.800 mm x 150 mm, sendo submetidas a carregamento central em placas quadradas que simularam pilares com 120 mm de lado. O material inerte entre as nervuras foi o EPS e o espacamento entre as mesmas foi de 250 mm, com largura media de 75 mm e altura 130 mm. Os resultados mostraram que as normas convergem em suas estimativas de resistencia ao cisalhamento, quando as nervuras sao tratadas isoladamente como vigas, e tambem ao puncionamento, mas neste caso tendem a ser conservadoras quando as lajes apresentam dois furos.

Análise experimental e numérica de lajes cogumelo de concreto armado

O uso de capiteis e uma das formas de se aumentar a resistencia a puncao de ligacoes laje-pilar. As recomendacoes normativas para a definicao da geometria e verificacao da resistencia ultima de ligacoes com capiteis sao pouco abrangentes, sendo poucas as pesquisas experimentais disponiveis sobre o tema. Este artigo discute aspectos relevantes do desenvolvimento historico das lajes cogumelo e apresenta resultados de 4 ensaios experimentais, sendo 1 laje lisa de referencia e 3 lajes cogumelo com capiteis de secao circular. Estes resultados sao comparados com as estimativas das normas EUROCODE 2 (2004) e NBR 6118 (2007) e com modelos numericos nao-lineares a fim de se compreender melhor a distribuicao das tensoes e o mecanismo de ruptura de lajes cogumelo.