Safety evaluation of slender high-strength concrete columns under sustained loads Sofia M.C. Diniz a , Dan M. Frangopol b, * a Department of Structural Engineering, Federal University of Minas Gerais, Belo Horizonte, MG 30110-060, Brazil b Department of Civil, Environmental and Architectural Engineering, University of Colorado, Boulder, CO 80309-0428, USA Received 23 July 2002; accepted 19 January 2003 Abstract A realistic analysis of the reliability of slender high-strength concrete (HSC) columns must include the long-term effects of sustained loads. In this paper, the long-term reliability of eccentrically loaded slender HSC columns under sustained loads is evaluated. The reliability of slender reinforced concrete columns representing normal-strength concreteandHSCisinvestigated.Sincemostofthevariablesinvolvedincolumndesign(materialproperties,geometric characteristics, loads, etc.) are random, probabilistic methods are used in the analysis. The effects of the concrete compressivestrength,amountoflongitudinalsteel,loadeccentricity,andslendernessratioonthecolumnreliabilityare investigated. It was found that all these factors have a considerable impact on the resulting column reliability. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: High-strength concrete; Monte Carlo simulation; Reinforced concrete columns; Slenderness; Structural reliability; Sustained loads 1. Introduction High-strength concrete (HSC), i.e., concrete with compressive strength exceeding 70 MPa [1] has been a major breakthrough in the concrete industry. It has founditsmainapplicationinthelowercolumnsofhigh- rise buildings. In this case, the large sections and small load eccentricities make the slenderness effects negligi- ble. However, economic studies have shown that HSC columns are also feasible for low- and medium-rise buildings, especially if a minimum amount of longi- tudinal steel is used [2]. Here, smaller sections, larger load eccentricities, and smaller amounts of longitudinal steel may lead to instability problems, which in turn may overcome the advantages of a higher strength ma- terial [3]. The structural behavior of a slender column is the result of geometric nonlinearities as well as both mate- rial strength and deformability. In many aspects HSC behaves differently (e.g., linearity of the stress–strain diagram, higher tensile strength, higher modulus of elasticity, and less creep) from normal-strength concrete (NSC). Regarding the short-term behavior, it has been shown that stocky HSC columns display relatively smaller strength ratios (test to predicted values) as compared to their NSC counterparts [3,4]. Also, a pre- vious study on the short-term behavior of slender HSC columns has shown that column reliability is largely affected by the load eccentricity, ranging from higher reliabilities at small eccentricities, to very low reliability levels at large eccentricities [3]. In the case of reinforced concrete (RC) columns, a large portion of the total load may be expected to be sustained, and it is well known that the strength of concrete under sustained loads is smaller than that ob- tained in short-term tests. Previous studies on the strength and reliability of stocky RC columns have * Corresponding author. E-mail addresses: dinizs@dedalus.lcc.ufmg.br (S.M.C. Di- niz), dan.frangopol@colorado.edu (D.M. Frangopol). 0045-7949/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0045-7949(03)00085-3 Computers and Structures 81 (2003) 1475–1486 www.elsevier.com/locate/compstruc