Shamim A. Sheikh

CONFINED CONCRETE COLUMNS WITH STUBS

The critical sections of columns in a framed structure subjected to earthquake loads are invariably adjacent to the beam-column/slab joints or the footings. External restraint provided by the heavy elements alters the behavior of the adjacent sections and may not always prove conservative. To simulate this behavior, 6 reinforced concrete columns were tested under cyclic flexure and shear while simultaneously subjected to a constant axial load. The variables in the test program were the amount of lateral steel, steel configuration, and level of axial load. From a comparison of the results with those from similar prismatic specimens tested earlier, it was concluded that the stub enhanced the adjacent sections flexural strength by more than 20%. The details of the study are described, and these and other findings are presented and discussed.

Retrofit of Square Concrete Columns with Carbon Fiber-Reinforced Polymer for Seismic Resistance

This paper investigates the prospect of strengthening deficient and repairing damaged square reinforced concrete columns with carbon fiber-reinforced polymer (CFRP) jackets. 8 specimens representative of structural members in bridges constructed prior to 1971 consisted of a 305 x 305 x 1473 mm column connected to a 508 x 762 x 813 mm stub. Each 900 kg specimen was tested under lateral cyclic displacement excursions and simultaneous constant axial load to simulate seismic forces. Results indicate that added confinement with CFRP at critical locations enhanced ductility, energy dissipation capacity, and strength of all substandard members. A positive relationship prevailed between favorable behavior and increasing reinforcement layers while improvements realized through CFRP repair declined as damage level prior to retrofit increased. Appropriately strengthened specimens also exceeded the performance of comparable columns with adequate seismic lateral reinforcement.

Seismic Behavior of Concrete Columns Confined with Steel and Fiber-Reinforced Polymers

This paper presents results from an experimental program in which 12, 356-mm diameter and 1473-mm long columns were tested under constant axial and reversed cyclic lateral load simulating forces from an earthquake. Each specimen consisted of a column cast integrally with a 510 x 760 x 810 mm stub representing a beam-column joint area or footing. Test specimens were divided into 3 groups: the first consisted of 4 columns conventionally reinforced with longitudinal and spiral steel reinforcement; the second group contained 6 reinforced concrete columns strengthened with carbon fiber-reinforced polymers (CFRP) or glass FRP (GFRP) before testing; and the last group included 2 columns that were damaged to a degree, repaired with FRP under axial load, and then tested to failure. Variables investigated were axial load level, spacing of spirals, thickness, and type of FRP. Test results indicate that CFRP and GFRP can be used effectively to strengthen deficient columns such that their behavior under simulated seismic loads matches or exceeds the performance of columns designed according to the seismic provisions of the 1999 ACI code. The use of FRP significantly enhances strength, ductility, and energy absorption capacity of columns.

REINFORCED CONCRETE COLUMNS CONFINED BY CIRCULAR SPIRALS AND HOOPS

Twenty seven short concrete columns reinforced with longitudinal steel and circular spirals or hoops were tested to failure under monotonic axial compression. Effects of different variables, such as amount and type of lateral steel, lateral steel spacing, etc., were investigated. The relation between lateral pressure on concrete and concrete strength enhancement and the variation of spiral steel stress and confinement effectiveness coefficient k with respect to the amount of spiral steel were also investigated. Requirements of the ACI 318-89 Building code related to the minimum volumetric ratio ofspiral reinforcement and the maximum spiral pitch of 80 mm were critically examined. An increase in the volumetric ratio of spiral steel was found to significantly improve strength and ductility of confined concrete, the effect on ductility being more pronounced. These and other study results are presented and discussed.

Experimental Study of Normal- and High-Strength Concrete Confined with Fiber-Reinforced Polymers

The experimental program reported here was conducted to gain insight into the behavior of concrete confined with fiber-reinforced polymers (FRPs). A total of 112 cylindrical concrete specimens, each 150 mm in diameter, 300 mm in height, and concrete strength up to 112 MPa, were tested under monotonic uniaxial compression. Test variables included amount of FRP, strength and stiffness of FRP, concrete strength, and the health of concrete at the time of strengthening. Results showed that, with an increase of the unconfined concrete strength, the strength enhancement, energy absorption capacity, ductility factor, and work (energy) index at rupture of FRP jackets all decreased remarkably. A positive correlation was found between concrete ductility and FRP rupture strain. A gradual post-peak failure of the specimens, observed previously from FRP-confined concrete columns tested at the University of Toronto, was also observed in some of the current tests. This ductile failure, attributed to the gradual unzipping failure of FRP jacket, is related to specimen size and is explained in terms of various confinement parameters.

Seismic upgrade with carbon fiber-reinforced polymer of columns containing lap-spliced reinforcing bars

Most reinforced concrete columns designed and constructed prior to 1970 may have inadequate seismic resistance. This study evaluates the effectiveness of carbon fiber-reinforced polymer (CFRP) jackets in the strengthening and repair of such columns under simulated earthquake loading. Six circular and 6 square columns were constructed and tested. The columns were 1.47 m (58 in.) long and had a 510 x 760 x 810 mm (20 x 30 x 32 in.) stub at one end with a construction joint at the interface and spliced longitudinal bars in the columns. The variables studied in this program included effect of the presence of lap splices, the effectiveness of CFRP in pre-earthquake strengthening and post-earthquake retrofitting of deficient columns, as well as effects of level of axial load, shape of column cross section, and transverse steel reinforcement details. Findings suggest that the CFRP retrofitting technique was effective in enhancing the seismic resistance of the columns and resulted in more stable hysteresis curves with lower stiffness and strength degradations compared with the unretrofitted columns. Ductility improvements in the square columns with lap splices as a result of CFRP retrofitting were significantly lower than that of comparable circular columns.

Performance of concrete structures retrofitted with fibre reinforced polymers

Retrofitting with fibre reinforced polymers (FRP) to strengthen and repair damaged structures is a relatively new technique. In an extensive research program underway at the University of Toronto, application of FRP in concrete structures is being investigated for its effectiveness in enhancing structural performance both in terms of strength and ductility. The structural components tested so far include slabs, beams, columns and bridge culverts. Research on columns has particularly focussed on improving their seismic resistance by confining them with FRP. All the specimens tested can be considered as full-scale to two-third scale models of the structural components generally used in practice. Results so far indicate that retrofitting with FRP offers an attractive alternative to the traditional techniques. In many circumstances, it can provide the most economical (and superior) solution for a structural rehabilitation problem. Selected results from experimental and analytical research are presented in this paper.

Seismic Resistance of Square Concrete Columns Retrofitted with Glass Fiber-Reinforced Polymer

The capacity of key structural members, particularly columns, to absorb and dissipate energy without severe strength degradation dictates the survival of structures during a major earthquake. Reinforced concrete columns with inadequate confinement do not possess the necessary ductility to dissipate sufficient seismic energy. This research evaluates the effectiveness of glass fiber-reinforced polymer (GFRP) wraps in strengthening deficient and repairing damaged square concrete columns. Each of the eight specimens tested, representing columns of buildings and bridges constructed before 1971, consisted of a 305 x 305 x 1473 mm column connected to a 508 x 762 x 813 mm stub. Specimens were tested under constant axial compression and cyclic lateral displacement excursions simulating earthquake loads. Test results reveal that retrofitting with GFRP wraps significantly enhanced ductility, energy dissipation ability, and shear and moment capacities of deficient columns. Cyclic behavior progressively improved as the number of GFRP layers increased, causing both stiffness degradation and strength reduction rates to decrease. Improvements observed following GFRP repair of damaged columns depended mainly on the extent of damage stained. GFRP-confined columns exceeded the performance of similar columns that contained transverse steel reinforcement in accordance with the seismic provisions of the current North American codes.

HIGH-STRENGTH CONCRETE COLUMNS UNDER SIMULATED EARTHQUAKE LOADING

The main objectives of this research were to evaluate performance of high-strength concrete (HSC) columns for ductility and strength and to critically examine ACIs Code requirements for confinement steel. Results from four HSC specimens with concrete strength of 72 MPa tested under simulated earthquake loading are presented and compared with similar specimens made of normal strength concrete (NSC). Each specimen consisted of a 305 x 305 x 1473 mm column and 508 x 762 x 813 mm stub that represented a discontinuity like a beam column joint or a footing. The variables studied are the concrete strength, steel configuration, axial load level, amount of lateral steel, and the presence of a heavy stub. As in the case of NSC, an increase in the amount of lateral steel, reduction in axial load, and increased effectiveness of the lateral support provided to longitudinal bars resulted in increases in energy absorption and dissipation capacity as well as ductility. For a specified column performance, the required amount of lateral steel appears to be proportional to the strength of concrete, in the 30-72 MPa strength range considered in this study.

A PERFORMANCE-BASED APPROACH FOR THE DESIGN OF CONFINING STEEL IN TIED COLUMNS

A review of the development over the years of the ACI Code provisions for confinement is presented. Based on the available experimental evidence, the current Code requirements for the amount of confinement steel in tied columns are critically evaluated. It was concluded that the behavior of columns designed according to the ACI Code may vary from unacceptably brittle to very ductile. While the amount of Code-required steel can be reduced in many cases, much larger amounts of lateral steel are needed in other cases. A new design procedure is proposed in which the amount of lateral steel required is a function of the column ductility performance. The lateral steel content increases with an increase in the level of axial load, and depends on steel distribution and the extent of lateral restraint provided to the longitudinal bars. For any specific steel configuration, the procedure lends itself to a simple design chart. The proposed method when applied to realistically-sized specimens tested by different investigators yielded excellent agreement with the experimental results.