Investigation of the sway characteristics of reinforced concrete frames subjected to lateral loads

Abstract

Sway of reinforced concrete (RC) frames subjected to lateral load is an important design parameter from serviceability point of view. A reasonably accurate determination of sway still lags proper guideline. The widely followed code of American Concrete Institute (ACI)as well as Bangladesh National Building Code (BNBC)does not provide any specific guideline in this regard. These codes only suggest the use of eff~ctivemoment of inertia in plane frame idealization of RC frames using linear frame members and homogenous material properties. Such approximation of RC frames may be considered as over simplification by present day standards with the availability of versatile computational techniques. Thus there is scope of investigation and improvement of the methods for sway estimation. In this thesis an investigation based on finite element (FE) modeling of RC frame is carried out to study the sway behaviour of RC frame. In the FE modeling the heterogeneity of RC is maintained by separate modeling of concrete, steel and bond-slip mechanism using different elements. Thus the RC frame is modeled as two-dimensional plane stress problem in which bending as well as shear deformations are included. Flexural cracks are also incorporated. Thus developing a finite element model that is closer to reality, an extensive investigation is performed under various parametric conditions to determine the deformation and sway characteristics of RC frames. The investigation is based on two models. The first model consists of two columns and a beam representing an exterior joint of a RC frame. Investigation of deformation characteristics of this frame by FE modeling as well as ACI guideline established the fact that ACI method differs from the FE analysis in terms of deflections calculated. The second FE model was a simple portal frame subjected to a lateral load. The same frame was analyzed according to ACI guideline and the results are compared with those of FE analysis. From the comparison, different parameters, which influence the sway, are identified. It was observed that in most of the cases ACI method tends to overestimate the sway by considerable amount, establishing the fact that there is scope of modification in the ACI guideline. In the last phase of the work investigation was carried out to identify possible areas of modification in the ACI formula and proposals have been made for calculation of effective moment of inertia using which more reasonable estimation of sway can be made. Finally the performance of the proposals are checked through an example.