Flexural Modeling of Reinforced Concrete Walls-Experimental Verification

pThis study presents detailed information on the calibration of a nonlinear wall macromodel by comparing model results with experimental results for slender reinforced concrete walls with rectangular and T-shaped cross sections. Test measurements were processed to allow for a direct comparison of the predicted and measured flexural responses. Responses were compared at various locations on the walls. Results obtained with the analytical model for rectangular walls compare favorably with experimental responses for flexural capacity, stiffness, and deformability, although some significant variation is noted for local compression strains. For T-shaped walls, the agreement between model and experimental results is reasonably good, although the model is unable to capture the variation of the longitudinal strains along the flange.

Keywords: flexure; stiffness; strain; walls.

INTRODUCTION

Prediction of the inelastic response of reinforced concrete (RC) structural walls and wall systems requires accurate, effective, and robust modeling and analysis tools that incorporate important material characteristics and behavioral response features such as neutral axis migration, tensionstiffening, progressive gap closure, confinement, nonlinear shear behavior, and the effect of fluctuating axial force on strength and stiffness. Effective analytical models should be relatively simple to implement and reasonably accurate in predicting the hysteretic responses of RC walls at both local and global levels, as well as capturing the interaction of the walls with other structural members.

Various phenomenological macroscopic models have been proposed to incorporate such response features in predicting the inelastic response of RC structural walls. The multiplevertical- line-element model (MVLEM) proposed originally by Vulcano et al.1 has been shown to successfully capture important response characteristics by the simplicity of a macroscopic model. Yet, the model has not been implemented into widely available computer programs and has not been sufficiently calibrated with and validated against extensive experimental data at both local and global response levels.

Given these shortcomings, a research project was undertaken to investigate and improve the MVLEM for RC wall systems, as well as to calibrate and validate it against experimental data. A description of the improved model, implementation of detailed cyclic constitutive relationships, and the sensitivity of the model predictions to both model and material parameters are presented by Orakcal et al.2 This paper emphasizes the accuracy and limitations of the model by comparing model results with experimental results. The study presented herein focuses on modeling and simulation of the axial-flexural response; an improved model for shear behavior as well as ways to incorporate axial-shear-flexure interaction will be presented in a follow-up paper.

RESEARCH SIGNIFICANCE

The use of structural walls for earthquake resistance is common; therefore, wall models that accurately capture cyclic wall responses, yet are simple enough for design office applications, are needed. An MVLEM is able to capture important response features; however, detailed comparisons between model results and experimental results are not available. In this paper, detailed comparisons are made between results obtained with a MVLEM and results obtained in experimental studies. Comparisons allow not only for a better understanding of the inelastic behavior of RC walls, but also identify model capabilities and as well as ways to improve the model.

ANALYTICAL MODEL

The model in Fig. 1(a) is an implementation of the generic MVLEM for structural walls. A horizontal spring placed at the element center of rotation (at relative height ch) simulates the shear response of the wall element. Flexural and shear modes of deformation of the wall element are uncoupled (that is, flexural deformations do not affect shear strength or deformation), which is a very commonly used assumption. A structural wall is modeled as a stack of m elements, which are placed one upon the other (Fig. 1(b)). The flexural response is simulated by a series of n uniaxial elements (or macrofibers) connected to infinitely rigid beams at the top and bottom (for example, floor) levels. The primary simplification of the model involves applying the planesections- remain-plane assumption (validated experimentally by Thomsen and Wallace3,4 for the walls investigated in this study) in calculating the strain level in each uniaxial element. The stiffness properties k^sub i^ and force-displacement relationships of the uniaxial elements are defined according to constitutive stress-strain relationships implemented in the model for concrete and steel and the tributary area assigned to each uniaxial element. The strains in concrete and steel are assumed equal (perfect bond) within each uniaxial element. Although the model could be modified to incorporate slip of reinforcing bars, the present model does not consider slip, in part because the model was calibrated using tests results where negligible slip was observed.3,4