Mohamed H. Harajli

Stress-Strain Model for Fiber-Reinforced Polymer Jacketed Concrete Columns

Fiber-reinforced polymer (FRP) jackets are often used to confine and reinforce concrete columns. This article reports on a study of the stress-strain behavior of FRP-confined concrete columns that focused on rectangular column sections. The authors developed a new design-oriented model of the stress-strain response of FRP confined columns. Their test variables included the volumetric ratio of the FRP jackets, the aspect ratio of the column section, and the area of longitudinal and lateral steel reinforcement. Results showed that jacketing rectangular column sections with FRP sheets increases their axial strength and ductility. FRP jackets can be used to prevent premature failure of the concrete cover and buckling of the steel bars, leading to substantially improved performance. The authors include a discussion of the main parameters that control the stress and strain characteristics of FRP-confined rectangular column sections. They propose a general design model of the stress-strain response of FRP-confined concrete. The authors conclude that the results predicted by this model showed very good agreement with other test data of FRP-confined circular and rectangular columns reported in the literature.

EFFECT OF CONFINEMENT OF BOND STRENGTH BETWEEN STEEL BARS AND CONCRETE

*ACI COMM 408, 1990, 4081R90 ACI; Darwin D, 1996, ACI STRUCT J, V93, P347; Eligehausen R, 1983, UCBEERC8323 U CAL; Esfahani MR, 1998, ACI STRUCT J, V95, P272; Filippou F, 1983, UCBEERC8319; HAMAD BS, 2002, IN PRESS ACI STRUCTU; Harajli M, 2002, J MATER CIVIL ENG, V14, P503, DOI 10.1061-(ASCE)0899-1561(2002)14:6(503); HARAJLI M, 2002, BOND CONCRETE RES ST, P570; HARAJLI MH, 2004, J MAT CIVIL ENG ASCE, V16; HARAJLI MH, 1995, ACI MATER J, V92, P343; Harajli MH, 2002, ACI STRUCT J, V99, P509; Orangun C, 1975, 1543F U TEX AUST CTR; Orangun C. O., 1977, ACI J, V74, P114; Zuo J, 2000, ACI STRUCT J, V97, P630

Effect of Confinement Using Fiber-Reinforced Polymer or Fiber-Reinforced Concrete on Seismic Performance of Gravity Load-Designed Columns

*ACI, 2001, 440101 ACI; *ACI, 1982, 5441R82 ACI; *ACI ASCE, 1991, 352R91 ACI ASCE; *ACI INN TASK GROU, 1999, ACC CRIT MOM FRAM BA; [Anonymous], 1996, 440R96 ACI; [Anonymous], 1999, 31899 ACI; Harajli M, 2002, J SEISMOL, V6, P257, DOI 10.1023-A:1015687602473; HARAJLI MH, 1994, RES REPORT; HARAJLI MH, IN PRESS ACI STRUCTU; Harajli MH, 2002, ACI STRUCT J, V99, P509; *INT C BUILD OFF, 1997, 1997 UN BUILD COD ST, V2; NAAMAN AE, 1990, SHRPCWP90004 NAT RES; Sheikh SA, 2002, ACI STRUCT J, V99, P72

Effect of fibers on the punching shear strength of slab-column connections

Abstract Twelve sets of small scale fiber reinforced concrete (FRC) slab-column connections were tested to investigate the effect of fiber reinforcement on the punching shear resistance of flat slabs. The parameters studied included type, content and aspect ratio of fibers, and span-depth ratio of the slab. The results indicate that the use of hooked steel fibers improves substantially the ductility of shear failure of slab-column connections, modifies their failure mode, favorably, from pure punching to flexural, and leads to a significant increase in their ultimate shear capacity. The increase in ultimate shear resistance varied almost linearly with the steel fiber content. While polypropylene fibers increase the ductility of shear failure, they are not as effective as steel fibers in increasing the punching shear resistance. Based on evaluation of the current test results and other experimental data reported in the technical literature, a design equation is proposed to predict the ultimate punching shear strength of slab-column connections containing deformed steel fiber reinforcement.

EFFECT OF SPAN-DEPTH RATIO ON THE ULTIMATE STEEL STRESS IN UNBONDED PRESTRESSED CONCRETE MEMBERS

The effect of member span-depth ratio on the predicted steel stress f sub ps in unbonded prestressed concrete members at their nominal moment resistance is implemented in an approximate manner of ACI 318-83. The recognition of member span-depth ratio and its implementation as an independent design parameter was made based on comparison of limited experimental results. However, there is not yet a clear phenomenological explanation that helps in understanding the mechanism of this parameter and its level of influence on the predicted f sub ps at ultimate flexural strength.

Seismic Strengthening of Bond-Critical Regions in Rectangular Reinforced Concrete Columns Using Fiber-Reinforced Polymer Wraps

ACI Committee, 2002, 31802 ACI; ACI Committee 440, 2002, 4402R02 ACI; *ACI INN TASK GROU, 1999, ITGT1199 ACI; Darwin D, 2005, ACI STRUCT J, V102, P892; Harajli MH, 2005, J COMPOS CONSTR, V9, P4, DOI 10.1061-(ASCE)1090-0268(2005)9:1(4); HARAJLI MH, 2005, FIBER REINFORCED POL, P579; Harajli MH, 2004, ACI STRUCT J, V101, P47; Harries KA, 2006, ACI STRUCT J, V103, P874; ORANGUN CO, 1975, 1543F1975 U TEX AUST; Sause R, 2004, ACI STRUCT J, V101, P708; Seible F, 1997, J COMPOS CONSTR, V1, P52, DOI 10.1061-(ASCE)1090-0268(1997)1:2(52); Sheikh SA, 2002, ACI STRUCT J, V99, P72; Zuo J, 2000, ACI STRUCT J, V97, P630

On the Stress in Unbonded Tendons at Ultimate: Critical Assessment and Proposed Changes

This paper provides a comprehensive assessment of the main parameters that influence the stress in unbonded tendons at ultimate flexural strength of post-tensioned members. Most of the previously proposed expressions and code design equations on this topic differ substantially in the way in which the important parameters are accounted for and encounter significant scatter in the prediction of test results. In this paper, using a physical model of increase in stress in prestressed steel above effective prestress (Δfps) together with a large database corresponding to simply supported and continuous members prestressed with internal or external unbonded tendon systems, an accurate expression for evaluating the equivalent plastic hinge length, which has a great influence on Δfps, is generated. Three possible design alternatives based on this expression were introduced for calculating the stress, and the sensitivity of each to commonly used simplifying assumptions is illustrated. Based on comparative assessment of design equations, it was reiterated that the ACI Building Code Eq. (18-4) and (18-5) ignore most of the critical parameters, leading to inaccurate predictions of Δfps. The fact that the inaccuracy of the ACI Code approach is shielded by being overconservative does not prevent unsafe calculation of Δfps, particularly for continuous members. While the AASHTO LRFD equation is more rational than the ACI Code equation, the closed-form solution proposed in the commentary of AASHTO LRFD code for more accurate evaluation of Δfps is simply unconservative. In considering the effect of continuous members on the stress in unbounded tendons, it is safe and rational to consider the actual collapse mechanism. Using conservative collapse mechanisms that involve a single span for all loading cases may lead to design inconsistencies. The three alternative methods presented in this study showed very good accuracy and should be considered as a base for modifying the ACI Building Code Eq. (18-4) and (18-5) for calculating the stress in unbonded tendons.

Evaluation of Bond Strength of Steel Reinforcing Bars in Plain and Fiber-Reinforced Concrete

This study aimed to analytically evaluate the development/splice strength of reinforcing bars in tension by incorporating a local bond-stress slip law more accurately derived by using beam specimens. The influence of several parameters, such as concrete compressive strength, on the development and bond properties of reinforcing bars embedded in conventional concrete, and the effect of fiber reinforcement on bond performance in comparison with plain unconfined concrete and concrete confined by transverse reinforcement, were investigated. Based on results of this analytical study and available experimental data, a design expression was proposed to evaluate the contribution of steel fiber reinforcement to the development/splice strength of reinforcing bars in tension.

Response of Externally Post-Tensioned Continuous Members

Results from experimental and analytical evaluations of the behavior and strength characteristics of continuous concrete beams prestressed using external tendons are presented. Similarly to beams with internal unbonded tendons, the stresses in the prestressing steel at nominal flexural strength were below or slightly exceeded yield. As a result of the change in tendon eccentricity with increasing beam deflection, specimens with undeviated tendons mobilized relatively smaller load capacities and post-elastic deformations. Increasing the area of internal bonded reinforcement resulted in better crack distribution and, consequently, a more ductile mode of flexural failure. A comprehensive analytical model based on the concept of plastic hinge length was developed to predict the flexural response characteristics of continuous concrete members post-tensioned with external or internal unbonded tendons, with appropriate consideration given to the influence of the 2nd-order effects and the rotational capacity in the plastic region. The important parameters affecting the behavior are evaluated and discussed.

Seismic Repair and Strengthening of Lap Splices in RC Columns: Carbon Fiber–Reinforced Polymer versus Steel Confinement

A design approach, developed specifically for seismic bond strengthening of the critical splice region of reinforced concrete columns or bridge piers, is presented and discussed. The approach is based on providing adequate concrete confinement within the splice zone for allowing the spliced bars to theoretically develop enough postelastic tension strains demanded by large earthquakes before experiencing splitting bond failure. The accuracy of the approach was validated experimentally by evaluating the seismic behavior of full-scale gravity load-designed (as-built) rectangular columns that were strengthened or repaired in accordance with the proposed approach. Three types of confinement were used and compared, namely, internal steel ties, external fiber polymer reinforced jackets, and a combination of both. The repaired/strengthened columns developed sizable postyield strains of the spliced bars, considerable increases in the lateral load and drift capacities, and much less concrete damage within the splice zone when compared with the as-built columns. As a further support of the adequacy of the design strengthening approach, the backbone lateral load-drift response of the strengthened columns showed a good agreement with the envelope response generated using nonlinear flexural analysis assuming perfect bond between the column reinforcement and concrete.