Oguzhan Bayrak

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.

Plastic hinge length of reinforced concrete columns

Severe damage can be observed in regions subjected to large moments when a column experiences earthquake-induced lateral displacements while supporting gravity loads. These regions, which experience large inelastic curvatures, are referred to as plastic hinges. The inelastic curvatures in plastic hinges are typically assumed to be constant over the plastic hinge length (lp) to simplify the estimation of the tip displacement of a column. Therefore, if the plastic hinge length is known, the tip displacement of a column can easily be obtained by integrating curvatures, and vice versa. In this paper, the effects of axial load and shear span-depth ratio on lp are evaluated experimentally. Results from experimental data on four full-scale concrete column tests indicate that ACI 318-05 provisions for the length of potential plastic hinge rations are slightly unconservative for columns supporting high axial loads. The level of axial load included the length of the plastic hinges that formed in the full-scale column specimens. Specimens tested under high axial loads developed longer plastic hinges than those tested under low axial loads. An equation is developed to estimate the length of the plastic hinges forming in columns supporting a wide range of axial loads.

Seismic performance of full-scale reinforced concrete columns

This paper presents results of simulated seismic tests conducted on five full-scale reinforced concrete columns to investigate the influence of P-Δ effect on the relationships between curvature ductility and displacement ductility or drift capacity. Experimental results show that the shear span-depth ratio at high axial load levels can significantly reduce the member performance of columns due to the P-Δ effect. Findings also indicate that axial load applied on a column plays an important role in determining the plastic hinge length. As the plastic hinge length depends on the axial load, the relationship between the sectional and member behaviors is affected by the axial load. On unexpected and undesirable finding was that the 135-degree hooked anchorages used in the first specimen opened during the scaled column tests, necessitating the need for longer hook lengths in other specimens.

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.

Carbon Fiber-Reinforced Polymer for Continuity in Existing Reinforced Concrete Buildings Vulnerable to Collapse

Carbon fiber-reinforced polymer (CFRP) can provide continuity in reinforced concrete beams and thereby reduce the likelihood of progressive collapse if a supporting column were lost due to extreme loading (blast or impact). Seven half-scale specimens representing two spans of a reinforced concrete (RC) frame with a center supporting column removed were tested. The capacity of vulnerable RC building beams with discontinuous reinforcing steel were evaluated and compared with beams using CFRP to provide continuity, and with beams designed with continuous reinforcing steel. It was found that CFRP is capable of providing sufficient continuity to withstand the loss of a supporting column through either catenary (or cable) action, which reduces material usage, or flexural action, which reduces deflections. Furthermore, beams with continuous reinforcement may not be able to withstand the loss of a center-supporting column due to limited rotational ductility of the beam.

ACI-DAfStb Databases for Shear Tests on Slender Reinforced Concrete Beams with Stirrups (with Appendix)

Nearly all code provisions for the shear capacity of structural concrete members without shear reinforcement have been empirically derived and some were calibrated decades ago. Since then, a significant volume of new test data has been generated that can aid in understanding. To address this issue, a database of 439 shear test results had been created in 2003 that contained the data of beams without shear reinforcement subjected to point loads. The 2003 database was considerably expanded in recent years by a joint ACI-DAfStb (American Concrete Institute - Deutscher Ausschuss fur Stahlbeton, German Committee for Structural Concrete) Group, with new test data including 40 tests on beams with uniformly distributed loads. The newly extended database now contains 784 tests that can be used for the evaluation of shear design provisions. A comparison of ACI 318-11 Eq. (11-3) with this extended database has revealed that current code provisions can be unconservative for large, lightly reinforced members and that the influences of important parameters are not adequately captured.

Behavior and Efficiency of Bottle-Shaped Struts

This paper attempts to determine the strength of compressed struts to be used in strut-and-tie modeling. The strength of the struts were evaluated for safety and accuracy as provided by the American Concrete Institute (ACI) and the American Association of State Highway and Transportation Officials (AASHTO) provision guidelines. In the experiment, 26 concrete panels were tested to failure. The investigation examined various effects, including: reinforcement crossing strut axis, angle between reinforcing steel and strut axis, specimen width, specimen thickness, three-dimensional stress state, bundled versus distributed reinforcement, compression reinforcement and boundary conditions on tests. It appears that the ACI’s specifications demonstrated erratic, but conservative results when compared with the test data, while the AASHTO’s standards were less conservative and more consistent.

Minimum Transverse Reinforcement for Bottle-Shaped Struts

This paper investigates the amount of transverse reinforcement needed to resist the tension developed in a bottle-shaped strut. Bottle-shaped struts are wider at their midpoint than at either end. As the struts widen near the midpoint, tensile stresses transverse to the direction of compression are developed. Reinforcement must be placed within the strut to carry this transverse tension. An expression for the required transverse reinforcement was developed and a database of 476 test specimens was used to evaluate these equilibrium-based equations along with a series of three tests of deep beams. The strut efficiency factors presented in Appendix A of ACI 318-05 were used in this evaluation. The shortcomings of current ACI 318-05 provisions that allow the design of a deep beam without any shear reinforcement are discussed. The amount of transverse reinforcement required to maintain equilibrium in a bottle-shaped strut is proportional to the force applied to the strut and inversely proportional to the slope of the angle of dispersion. These findings suggest that additional research regarding the serviceability of structures designed using strut-and-tie provisions is needed.

Stress Block Parameters for High-StrengthConcrete Members

Early spalling of cover concrete has been reported by researchers in high-strength concrete (HSC) column tests. A rational way to incorporate this phenomenon into structural building codes has yet to be introduced. Moreover, use of ACI 318-02 stress block parameters to estimate the strength of HSC columns results in unconservative predictions for moment capacity in some cases. Hence, modification of ACI 318-02 stress block parameters to incorporate early cover spalling in HSC columns is of great significance. With this in mind, experimental results obtained during the course of this research and those available from the literature were used to study the accuracy and conservativeness of the ACI 318-02 stress block parameters. Based on experimental evidence, it was determined to reduce the ultimate compressive strain limit of ACI 318-02 to 0.0025 for HSC members such that the capacity of these members can be evaluated prior to cover spalling. A new set of stress block factors was analytically derived and validated with experimental data.

Distribution of Stirrups across Web of Deep Beams

Current design codes are inconsistent regarding whether or not the spacing of stirrups should be limited across the web of a deep-beam region. This study evaluates the benefit of distributing stirrups across the web of beams subjected to deep-beam shear. Full-scale beams were fabricated and tested in a total of six tests. Four tests were conducted on specimens with a 21 x 44 in. (530 x 1120 mm) cross section and two tests were conducted on beams with a 36 x 48 in. (910 x 1220 mm) cross section. Experimental variables were the number of stirrup legs distributed across the web and the amount of web reinforcement. The results showed that the addition of closely spaced stirrups did not significantly improve the shear capacity or serviceability performance of the beams. Since web reinforcement is relatively ineffective in a deep beam, limiting the stirrup spacing across the web may be inefficient. In many cases, intermediate stirrup legs are unnecessary in deep beams as wide as 36 inches.