Incorporation of Decoupled Damage Index Models in Performance-Based Evaluation of RC Circular and Square Bridge Columns under Combined Loadings

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Title: Incorporation of Decoupled Damage Index Models in Performance-Based Evaluation of RC Circular and Square Bridge Columns under Combined Loadings

Author(s): A. Belarbi, S. Prakash, and P. F. Silva

Publication: Special Publication

Volume: 271

Issue:

Appears on pages(s): 79-102

Keywords: circular columns; combined loadings; damage index; performance-based design; square columns; torsion

Date: 5/24/2010

Abstract:
This paper investigates the performance-based evaluation of reinforced concrete (RC) bridge circular columns under combined bending, shear, axial, and torsion using decoupled damage index models. The main feature of the proposed damage index model is the feasibility of decoupling these combined actions according to various damage limit states. Research has shown that under combined bending, shear, axial, and torsion loads, the main parameters in the structural performance of RC bridge columns that are affected the most are their strength, deformation capacity, and failure mode. Response of RC columns under these combined actions is very complex and requires the implementation of numerical tools that can quantify the progressive nature of damage under the influence of various parameters. A proper damage index should thus include the main parameters that describe the hysteretic behavior under these combined loadings. Existing damage index models are modified to account for these combined actions in a decoupled scenario which are then used to evaluate the progression of damage under the combined loads. Under combined loads damage limit states that can be identified are flexural and/or shear/torsion cracking, yielding of transverse and/or longitudinal reinforcement, spalling of concrete cover, and fracture of transverse and longitudinal reinforcement. The main variables that are considered in the study to characterize the damage index are (i) the ratio of torsion-to-bending moment (T/M) for circular columns and twist-to-displacement (q/D) for square columns, (ii) the level of detailing for high and moderate seismicity (low or high transverse reinforcement ratio) and (iii) level of shear (low or moderate). Progression of damage in RC columns due to the interaction between bending and torsion is also evaluated as a function of the transverse reinforcement ratio. Results show that the columns’ lateral displacement ductility as well as its torsion rotation ductility are decreased under combined loads. The progression of damage is found to be amplified due to the effects of torsion. An important observation from this study that can have a significant impact in the seismic design of RC columns under combined loads is that an increase in the transverse reinforcement ratio helps delay the progression of damage, thereby changing the response of the columns from a torsional response to a predominately flexural response.