Reliability of partially encased steel-concrete composite columns under eccentric loading

Abstract

ResumoThe Brazilian design code for composite buildings (NBR8800) has never been subject to a reliability-based calibration of the partial factors used in design. As a consequence, the level of structural safety achieved by using these factors is, in general, unknown. This paper presents a study of the reliability of partially encased steel-concrete composite columns, subject to compression and bending. Literature statistics are used to quantify uncertainty in strength of structural materials and in loads. Reliability indexes are evaluated, via FORM, for two ratios between dead and live loads, and for some values of load eccentricity. It was found that larger load eccentricity leads to a reduction in reliability indexes. These values are larger than minimum values recommended in international design codes; hence one concludes that design of such columns using NBR8800 leads to acceptable safety levels.Keywords: reliability, composite column, structural safety.A norma brasileira que embasa o projeto estrutural de edifícios contendo elementos mistos (NBR8800) nunca passou pelo processo de calibração, baseada em confiabilidade, dos coeficientes parciais de segurança utilizados em projeto. Como consequência, o nível de segurança obtido com o uso destes coeficientes é, em geral, desconhecido. Este trabalho apresenta um estudo da confiabilidade estrutural de pilares mistos de aço-concreto, parcialmente revestidos e submetidos à flexo-compressão. Estatísticas da literatura são utilizadas para caracterizar as incertezas nas propriedades dos materiais constituintes e nas ações. Índices de confiabilidade (β) são avaliados, via FORM, para duas razões entre ação permanente e ação variável, e para alguns valores de excentricidade da ação vertical. Verifica-se que o índice de confiabilidade dos pilares mistos diminui com o aumento da excentricidade de projeto. Estes valores estão acima de índices de confiabilidade mínimos sugeridos em normas internacionais, o que sugere que o projeto utilizando a norma NBR8800 resulta em colunas mistas seguras.Palavras-chave: confiabilidade; pilares mistos; segurança estrutural.

Reliability-based calibration of main Brazilian structural design codes

“…The reliability indexes found herein for RC columns under eccentric loading are similar to those found by Oliveira et al (2008) and Beck et al (2009) for composite columns under compressive loading: 2.4 to 4,6; and smaller than those found by Pereira et al (2017) for composite columns under eccentric loading: 4.5 to 5.0. These authors considered significantly smaller c.o.v.…”

“…. We notice that the smallest reliability indexes obtained are quite low, and smaller than those found elsewhere (Stucchi and Santos, 2007;Santos et al, 2014;Pereira et al, 2017). This is a consequence of the larger c.o.v.…”

“…Past studies on reliability analysis of steel, concrete and composite structures designed following Brazilian codes have used smaller coefficients of variation (c.o.v.) for live load: Stucchi and Santos (2007), Santos et al (2014Santos et al ( , 2015 and Pereira et al (2017) have used 20% or less; Beck and Dória (2008), Oliveira et al (2008), Beck et al (2009) and Beck and Souza Jr. (2010) used 25%. These papers used as reference the works of Ellingwood et al (1980) and Szerszen and Nowak (2003).…”

“…Studies on reliability of concrete beams and slabs, not addressing calibration, were reported by Stucchi and Santos (2007) and Santos et al (2014Santos et al ( , 2015. Studies on reliability of columns were reported by Oliveira et al (2008), Beck et al (2009) and Pereira et al (2017). The reliability against punching of concrete structures designed to comply with NBR6118 were addressed by Carvalho et al (2017) and da Silva et al (2018).…”

Distribution of load effects and reliability of reinforced concrete frames: intact and with columns removed

“…This may be a consequence of the usual design procedures, where elements are designed on an element-by-element basis. Focusing on results published in Brazil, studies on the reliability of "detached" beam elements include [18]- [23] and "detached" columns include [24]- [29]. Studies addressing "detached" beams and/or columns, based on Brazil and elsewhere, include [30]- [32].…”