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Lateral steel in columns is generally required for shear resistance, confinement of concrete and prevention of buckling of longitudinal bars. This paper is limited to the design of lateral steel for confinement and bar buckling only. The provisions of the ACI 318-89 code (1989) related to this area are critically examined and changes are suggested to these provisions to make the design more rational. Both the tied and the spirally reinforced columns are considered and seismic as well as non-seismic design provisions are discussed.
The performance of bridges during past earthquakes has clearly indicated that many structural failures could be attributed to inadequate seismic design of existing bridge structures. This paper reports on experimental research conducted to investigate the effectiveness of carbon fiber reinforced polymer (CFRP) jackets in retrofitting bridge columns with splice deficiencies. The test procedure is explained, and the test results, along with conclusions drawn from them are presented.
The corrosion of reinforcing steel in reinforced concrete (RC) bridge decks due to the application of de-icing salts in winter has been recognized as one of the major causes of highway bridge deterioration in North America. Corrosion-induced damage is usually manifested by longitudinal cracking, spalling, and/or delamination of the concrete cover due to the expansion of corrosion products accumulating around the reinforcement. This damage leads to reduction or loss of serviceability, safety, and service life of RC bridge decks. This paper presents finite element analyses of the behaviour of the concrete bridge deck cover subjected to reinforcing steel corrosion. The prediction of the damage caused by corroding reinforcing bars is established by calculating the induced stresses in the surrounding concrete. The numerical model is used to conduct a parametric investigation of several design variables. It is found from the analyses that different failure mechanisms govern depending on the geometry and configuration of the reinforcing bars in the concrete cover of the bridge deck. Finally, the impact of concrete overlays on the governing failure modes of cracking, spalling and/or delamination of the concrete cover is investigated by using the model.
A simplified nonlinear sectional analysis approach (NLSA) of aged reinforced concrete beam-columns based on numerical integration, enhanced inspection, material testing and/or empirical estimation of materials deterioration due to reinforcement corrosion is proposed. The proposed procedure is an iterative nonlinear technique that uses numerical integration of the sectional stresses and satisfies force equilibrium in every load increment step. The NLSA model incorporates corrosion-induced damage by reducing the steel cross section and ductility, removing the loss of concrete cover, and accounting for the loss of local bond in corrosion-damaged zones. Two case studies are presented to estimate the effects of corrosion on the structural behaviour of reinforced concrete beam-columns by taking into account critical damage states. The model results show good agreement with test results, high numerical stability and consistent convergence for all examined loading cases.
The use of fibre reinforced polymer (FRP) reinforcement in buildings is gaining acceptance in the construction industry due to their superior corrosion resistance, durability and higher strength. Of the various types of fibres used in producing FRP reinforcement, carbon fibers provide higher elastic modulus. Therefore, carbon-fibre reinforced polymer (CFRP) bars and grids are more suitable for use in building construction. Research on seismic performance of FRP reinforced concrete buildings is currently limited worldwide. The paper reports on dynamic response of two CFRP reinforced concrete buildings designed for Vancouver, Canada, following the seismic requirements of the National Building Code of Canada (NBCC 2005) and the Canadian Standards Association (CSA) S806-02 (2002) for “Design and Construction of Building Components with Fibre-Reinforced Polymers”. A computer program was developed for dynamic response history analysis, incorporating a hysteretic model for FRP reinforced concrete elements. The buildings were analyzed under NBCC compatible earthquake records to establish design force and deformation demands. The results indicate that FRP reinforcement can be used to reinforce concrete buildings in seismically active regions with structural elements designed not to suffer from the rupturing of CFRP prior to the onset of concrete crushing. FRP reinforced concrete buildings designed to respond in the elastic mode of deformations remain within the force and deformation demands indicated in building codes. Inelastic response of buildings under amplified ground excitations indicate that it is possible to reduce design force levels through limited inelasticity provided by confining the compression concrete in members and eliminating tension failure in FRP reinforcement.
This paper presents results of shake-table testing on a single-story and single-bay reinforced concrete (RC) frame which was strengthened by a newly designed buckling restrained brace (BRB). The objective of strengthening by BRB was to increase the lateral strength and stiffness, and energy dissipation of the frame. A companion frame without strengthening was used as the control specimen and was tested along with the BRB-strengthened frame. Different levels of the 1995 Kobe earthquake record were employed to impose the dynamic loading on the frames up to a peak ground acceleration (PGA) of 0.679g. The implementation of the BRB increased the frame lateral stiffness, resulting in an approximately 80% reduction in the initial fundamental period of the frame. Concrete in the plastic hinge region at base of the columns in the control frame crushed at PGA = 0.679g after yielding of the longitudinal reinforcement. The BRB-strengthened frame remained within the elastic range at all stages of load application, exhibiting high lateral strength with some reduction in stiffness at 0.679g. The lateral force sustained by the BRB-strengthened frame at PGA of 0.679g was nearly 215% higher (on average) as compared to the maximum force sustained by the control frame. The displacement ductility ratio for the control frame was 2.42.
Seismic resiliency of new buildings has improved over the years due to improved seismic codes and design practices. However, vulnerability of seismically deficient older buildings, designed and built on the basis of older codes of practices, poses a significant threat to life safety and survivability of buildings. It is economically not feasible to retrofit the entire inventory of seismically deficient buildings. Therefore, there is need for a comprehensive plan to identify critical buildings and prioritize their retrofit and upgrading requirements. A risk-based evaluation technique is proposed in this paper to quantify seismic assessment and develop a ranking scheme for reinforced concrete buildings. The seismic hazard, building vulnerability and consequence of failure are handled in hierarchical structures. Some of the input risk parameters, expressed as qualitative and quantitative quantifiers, are transformed into commensurable values. A knowledge-based fuzzy rule base modelling is developed and verified through the use of 1994 Northridge Earthquake data on seismic damage of reinforced concrete buildings.
A hysteretic model is proposed for shear response of reinforced concrete members subjected to shear force and bending moment reversals. The model consists of a primary shear force‐shear displacement curve and unloading and reloading branches under cyclic loading. The primary curve is the envelope curve for the hysteretic relationship, and can be considered the same as the force displacement relationship under monotonic loading up to the onset of strength decay. A primary curve established by any acceptable procedure, with well‐defined cracking and yield points, can be used in constructing the model. The rules proposed for unloading and reloading branches of the model were obtained from a large number of test data. Full‐scale column tests, under inelastic load reversals, were used for this purpose. These rules were developed to predict experimentally observed stiffness degradation and pinching of hysteresis loops. Empirically derived expressions were included to introduce the effects of axial load, deformation level, and the number of deformation cycles. The model was employed to reproduce the results of large scale tests of reinforced concrete elements. The comparisons include columns, beams, and structural walls, tested by different investigators. Individual hysteresis loops of small and large deformation cycles are compared with those produced by the model. The results show a good agreement between the model and the experimental data.
Strength and ductility of high-strength concrete columns were investigated by examining available test data. The results are summarized in terms of load and ductility capacities of columns. Experimental observations indicate that strength and ductility of high-strength concrete columns can be improved to levels that are usually expected of normal-strength concrete columns, provided that the core concrete is adequately confined by transverse reinforcement.
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An analytical procedure was developed to model formation and progression of flexural hinging in reinforced concrete columns. The procedure was employed to demonstrate second order inelastic analysis of concrete columns under monotonically increasing lateral loads. Other relevant features of inelastic column analysis were also incorporated, including concrete confinement, reinforcement strain hardening, bar buckling and anchorage slip. The analysis procedure has been verified extensively against experimental data, and used to illustrate a displacement-based design procedure for column confinement. Furthermore, a set of column analyses was conducted to develop design aids for column confinement. The amount, grade, spacing and arrangement of confinement reinforcement can be selected from these design aids for expected levels of lateral drift and axial compression.
