Seismic FRP Retrofit of Bond-Critical Regions in Circular RC Columns: Validation of Proposed Design Methods

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					  ACI STRUCTURAL JOURNAL                                                                                            TECHNICAL PAPER
Title no. 105-S70


Seismic FRP Retrofit of Bond-Critical Regions in Circular
RC Columns: Validation of Proposed Design Methods
by Mohamed H. Harajli and Zeinab Khalil

A general expression was derived for designing the thickness of
external fiber-reinforced polymer (FRP) jackets required for bond
strengthening of spliced reinforcement within the critical hinging
region of reinforced concrete (RC) columns. The reliability of the
expression has been verified against recent experimental data of
rectangular column sections. The objective of the current investigation
is to evaluate the application of the corresponding design expression
for circular column sections compared with other design methods
proposed in the literature. Full-scale circular columns with spliced
reinforcement were tested under lateral load reversals. It was found
that the mechanism by which the FRP confinement improves the
bond strength of spliced bars in circular columns is similar to
that in rectangular columns. All of the unconfined columns
suffered premature splitting bond failure leading to almost total
strength and stiffness degradation. Confining the splice zone with
external FRP jackets within the range of thicknesses predicted by the
proposed design expression under evaluation lead to a substantial
improvement in seismic performance.                                                 Fig. 1—Splice parameters used in Eq. (3), (4), and (13).

Keywords: bond strength; cyclic load; fiber-reinforced polymer; seismic retrofit.   in which nf and tf are the number of layers and thickness per
                                                                                    one layer of the FRP sheets, respectively; D is the diameter of
                        INTRODUCTION                                                the circular column section; Ef is the modulus of elasticity of
   One of the most important characteristics that control the                       the FRP; fh is the horizontal stress level provided by the
behavior of reinforced concrete (RC) members in areas of                            existing hoop reinforcement in a circular column at a strain of
seismic hazard is ductility. Ductility, or the ability of a structure               0.1%; and fl is the lateral clamping pressure over the lap splice
or a structural member to absorb and dissipate energy                               length Ls provided by the FRP jacket, given as (refer to Fig. 1)
through post-elastic deformations, can best be improved
through adequate detailing of the steel reinforcement and                                                                            Ab fy
confinement of the concrete at the locations where plastic                                                 f l = -------------------------------------------------         (2)
hinges would potentially develop. For building or bridge                                                               p-
                                                                                                                    -------- + 2 ( d b + c ) L s
structures that are designed for gravity load, it is a common                                                       2n st
practice to splice the vertical reinforcement at the base of the
pier with starter bars projecting above the foundation footings                     where p is the perimeter line in the column cross section
in bridge structures or at the junction of the floor slab and                       along the lap-spliced bar locations; nst is the total number of
columns in building structures. In the absence of adequate                          spliced/doweled bars in the column section along perimeter
confinement by transverse steel ties, the spliced reinforcement is                  p; Ab is the area and db is the diameter of one main column
likely to fail in splitting mode leading to premature flexural
                                                                                    spliced reinforcing bar; fy is the yield stress of the spliced
failure, followed by a quick bond and stiffness degradation
                                                                                    reinforcement; and c is the concrete cover to the column
under cyclic loading, and possibly causing complete failure
                                                                                    reinforcement. Equations (1) and (2) were derived with the
of the structural member.
                                                                                    assumption that the onset of lap-splice debonding or relative
   Several experimental studies have been carried out to                            slippage starts when the measured hoop or dilation strain
evaluate the prospects of using external fiber-reinforced                           levels are between 1000 and 2000 με and consequently
polymer (FRP) jackets for improving the bond strength of                            limiting the dilation strain level to 1000 με. For the purpose
spliced column reinforcement when subjected to tension
                                                                                    of the current study, Eq. (1) and (2) can be combined to
stresses. Some of the first such studies have been undertaken
                                                                                    derive an expression for calculating the thickness of the FRP
by Priestley et al. (1996) and Seible et al. (1997) in which the
                                                                                    jacket nf tf required to develop a desired splice stress fs.
following expression for evaluating the jacket thickness nf tf
to ensure proper lap splice clamping in circular columns (in                        Replacing fs for fy and the value of Ab = πdb2/4 in Eq. (2), and
SI system of units) is suggested
                                                                                      ACI Structural Journal, V. 105, No. 6, November-December 2008.
                                                                                      MS No. S-2007-179.R3 received October 11, 2007, and reviewed under Institute
                                      D ( fl – fh )                                 publication policies. Copyright © 2008, American Concrete Institute. All rights reserved,
                        n f t f = 500 ----------------------                 (1)    including the making 
				
DOCUMENT INFO
Description: A general expression was derived for designing the thickness of external fiber-reinforced polymer (FRP) jackets required for bond strengthening of spliced reinforcement within the critical hinging region of reinforced concrete (RC) columns. The reliability of the expression has been verified against recent experimental data of rectangular column sections. The objective of the current investigation is to evaluate the application of the corresponding design expression for circular column sections compared with other design methods proposed in the literature. Full-scale circular columns with spliced reinforcement were tested under lateral load reversals. It was found that the mechanism by which the FRP confinement improves the bond strength of spliced bars in circular columns is similar to that in rectangular columns. All of the unconfined columns suffered premature splitting bond failure leading to almost total strength and stiffness degradation. Confining the splice zone with external FRP jackets within the range of thicknesses predicted by the proposed design expression under evaluation lead to a substantial improvement in seismic performance. [PUBLICATION ABSTRACT]
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