Abstract
Brazilian design code ABNT NBR6118:2003 - Design of Concrete Structures - Procedures - [1] proposes the use of simplified models for the consideration of non-linear material behavior in the evaluation of horizontal displacements in buildings. These models penalize stiffness of columns and beams, representing the effects of concrete cracking and avoiding costly physical non-linear analyses. The objectives of the present paper are to investigate the accuracy and uncertainty of these simplified models, as well as to evaluate the reliabilities of structures designed following ABNT NBR6118:2003[1&] in the service limit state for horizontal displacements. Model error statistics are obtained from 42 representative plane frames. The reliabilities of three typical (4, 8 and 12 floor) buildings are evaluated, using the simplified models and a rigorous, physical and geometrical non-linear analysis. Results show that the 70/70 (column/beam stiffness reduction) model is more accurate and less conservative than the 80/40 model. Results also show that ABNT NBR6118:2003 [1] design criteria for horizontal displacement limit states (masonry damage according to ACI 435.3R-68(1984) [10]) are conservative, and result in reliability indexes which are larger than those recommended in EUROCODE [2] for irreversible service limit states.
Highlights
In the design of reinforced concrete structures, it is common practice to use simplified models which penalize the stiffness of structural elements, in order to avoid non-linear material analysis
Results show that ABNT NBR6118:2003 [1] design criteria for horizontal displacement limit states (masonry damage according to ACI 435.3R-68(1984) [10]) are conservative, and result in reliability indexes which are larger than those recommended in EUROCODE [2] for irreversible service limit states
Reliability analyses are performed using rigorous non-linear material analysis and using the simplified models recommended in ABNT NBR6118:2003 [1]
Summary
In the design of reinforced concrete structures, it is common practice to use simplified models which penalize the stiffness of structural elements, in order to avoid non-linear material analysis. The objective of the present article is to investigate the precision of simplified stiffness-reducing models recommended in Brazilian code ABNT NBR6118:2003 [1] in the evaluation of horizontal displacements of plane reinforced concrete frames. The investigation is based on a comparison, for a set of representative frames, of the displacements obtained using simplified models and rigorous physical (material) non-linear analysis. Reliability analyses are performed using rigorous non-linear material analysis and using the simplified models recommended in ABNT NBR6118:2003 [1]. Reliability analyses performed consider uncertainties in loads and in the structural strengths, as well as the uncertainties originated in the use of the simplified stiffness reduction models. Reliability analyses are performed using the StRAnD software (BECK [4])
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