1、1IncreasIng shear capacIty wIthIn exIstIng reInforced concrete structuresKeywords: external steel reinforcement; fiber-reinforced polymer; section enlargement; shear reversal; shear strengthening.IntroductionRehabilitation projects often involve the need to increase the load-carrying capacity of mem
2、bers within existing concrete structures that are deficient due to increased load demand associated with change of use, deficiencies in the original design or construction, or deterioration. Such strengthening often includes increasing shear capacity. Shear strengthening may also be necessary when a
3、 structure requires nullxural strengthening to ensure a ductile failure mechanism in seismic loading situations. nullhe accessibility difficulties normally encountered in concrete structures, as well as general lacnullof design standards, may lead to building official limitations on what strengtheni
4、ng may be accepted.Questionnullat options are available to increase the shear capacity of members within existing reinforced concrete structuresnullAnswernulle following are descriptions and some examples of various tech -niques for shear strengthening. nullote that, unless indicated, the shear stre
5、ngthening measures described are specific to nonseismic applica-tions. nullapter nullof nullnullnullnull-null nullnullnullnull nullommittee nullnullnull nullnullnullnullnull and nullnullnull nullnullnull.nullR-nullnull, nullnullnull.nullR-nullnull, and nullnullnullR-nullnull nullnullnullnull nullomm
6、ittee null nullnullnullnull, nullnullnullnull; nullnullnull nullommittee nullnullnull nullnullnullnullnull provide additional information on structural strengthening. nulll shear strengthening should be performed under the guidance of a licensed engineer familiar with the selected technique.null Ext
7、ernal steel reinforcementnullhe shear capacity of concrete members, such as columns, beams, slabs, and shear walls, can be increased by attaching steel plates nullig. null to the concrete surface with epoxy bonding, mechanical anchorage, or both. Steel rods or rein-forcing bars, either post-tensione
8、d or non-post-tensioned, are another form of external steel shear reinforcement that is well suited to beams nullig. null;null. Section enlargementnullSection enlargement can be accomplished by using concrete, shotcrete, reinforced concrete, or mortar that is bonded to the concrete element nullnulli
9、g. nullnull. nullor example, columns can be strength-ened by using jacnullets nullnullig. null and nullnull, beams by increasing the section, shear walls by increasing the wall thicnullness, and ribbed slabs by filling the open spaces between the ribs. nullhe columns in nullig. null and null were st
10、rengthened to increase the vertical shear capacity of the beam seat, and not the column shear associated with moment interaction;null. Internal steel and FRP reinforcementnullnulldditional steel or fiber-rein-forced polymer nullnullRnullnull reinforcement can be installed by drilling holes, insertin
11、g steel or nullRnull dowels, and grouting nullnullig. nullnull. Several grouting mate-rials have been used successfully to bond the dowels to the concrete;null Near-surface-mounted reinforcementnullear-surface-mounted rein-forcement nullSnullnullinvolves inserting steel or nullnullrods or reinforcin
12、g bars into grooves cut into the surface of the concrete section. nullhe grooves are typically not deeper than the existing reinforcement cover Fig. 1External reinforcement with steel plate.Fig. 2External shear reinforcement with steel rods or reinforcing nullrs.ACI 364.2T-08nullechNoteAmerican Conc
13、rete Institute Copyrighted Materialnullnull.concrete.org2 INCREASING SHEAR CAPACITY WITHIN EXISTING REINFORCED CONCRETE STRUCTURES (ACI 364.2T-08)dimension. nullhe grooves are then filled with a polymer or grout to provide compatibility nullthat is, load transfernull of the reinforcement with the co
14、ncrete. nullhe advantages of this method are easy access, limited chance of disturbing existing reinforcing bar as compared with internal steel reinforcement, and increased protection and bonded surface area of the reinforcement as compared with the use of external shear rein-forcement;null Suppleme
15、ntal memnullrsnullupplemental structural members, such as posts or beams, can be added to reduce shear stress. nullen architectural constraints allow, this can often be a cost-effective and practical solution; andnull. FRP plates or stripsnullnullRnull materials may be used as either exter-nally bon
16、ded plates or strips. nullhe advantage of using strips is that they do not entrap moisture in the structure and more closely resemble conventional shear reinforcement. nullhe advantages of using plates that cover the entire member include the additional effect of shell action, although not usually i
17、ncluded in design, and the ability to use bidirectional fibers to resist shear reversals. nulllthough fibers positioned at an angle may be optimal, a nearly equivalent amount of fibers are often required as compared with vertically arranged fibers when shear reversals are considered. nullhe nullRnul
18、l is typically bonded to the concrete with a structural-grade epoxy. nullhere are many forms of nullRnull suitable for shear strengthening, including fabric sheets impregnated with resin on site using a process called wet layup; preimpregnated sheets of fabric; and precured shapes, such as rods, and
19、 nullat, null-shaped, and null-shaped plates. nullodified approaches are available nullnullnullnull nullommittee nullnullnull nullnullnullnullnull; however, research projects intended for both seismic and nonseismic applications have primarily focused on the nonuniformity of strains in adjacent null
20、Rnull shear reinforcing elements at the intended design limit nullSchuman nullnullnullnull; nullong and nullecchio nullnullnullnull; nullarolin and nullnullljsten nullnullnullnullnull. nullt should also be noted that, along with seismic applications, research has shown that certain conditions, such
21、as at deep beams and negative moment regions, may require anchorage to obtain a measurable strengthening effect nullnullnullnull nullommittee nullnullnull nullnullnullnullnull. nullnchorage comes in many forms, including steel-bonded anchors with a bearing plate, bundled nullRnull fiber anchors that
22、 are integrated into the nullRnull shear reinforcement, fully wrapped sections, and ribbed inserts into the nullange region. nulln example of an external application is the wrap-ping of beams with carbon nullRnull nullnullnullRnullnull null-shaped strips nullnullig. nullnull; this is a good example
23、of how nullRnull anchors can be integrated into the system with minimal visibility and optimum performance. nullultruded null-shaped plate stirrups nullnullig. nullnull are another form of externally bonded nullRnull shear strengthening.Discussionnullhe selection of the most suitable method for a gi
24、ven application depends on many factors, includingnullnull Reason for strengthening nullnote that a majority of the methods will not prevent concrete cracnulls from forming; rather, they will limit the opening and propagation of shear cracnulls after cracnull forma-tion. nullf cracnull prevention is
25、 critical, one should focus on section enlargement, supplemental Fig. nullSection enlargement with supplemental reinforcement conullered nullnull shotcrete nullRaina 1nullnullnullnull.Fig. nullSection enlargement with nullcnullts.Fig. nullnullmpleted section enlargement with nullcnullts.Fig. nullInt
26、ernal steel reinforcement.INCREASING SHEAR CAPACITY WITHIN EXISTING REINFORCED CONCRETE STRUCTURES (ACI 364.2T-08) 3American Concrete Institute Copyrighted Materialnullnull.concrete.orgbuilding operations are shut down during constructionnull;null nullesthetics nullsome methods are less obvious repa
27、irs than others and, as such, might be preferable to the ownernull;null nullvailability of materials, equipment, and qualified contractors; andnull nullost nullinitial and life cyclenull.Shear reversals are common on continuous structures with variable loading patterns. Shear strengthening schemes g
28、enerally require that the additional shear reinforcement remain functional throughout the anticipated failure mecha-nism, which is usually nullexural in nature. nullor bonded reinforcements, this typically requires some form of anchorage to ensure attachment across the shear cracnulls after the init
29、iation of debonding. nullhe debonding mechanism is governed by the tensile strength of concrete, and is difficult to prevent. nulllthough the use of anchorage for bonded shear reinforce-ment provides attachment after debonding, a close examination of strain compatibility with the existing internal s
30、hear reinforcement is required to ensure that all shear reinforcing elements will reach or exceed the intended local design limit when the member reaches the structural capacity. Schuman nullnullnullnullnullnull provided one such anchorage approach for the use of externally bonded nullRnull shear re
31、inforcement. nulln important part of any shear strengthening design is the anticipated sequence of strain generation and attainment of the intended design limits between the conventional internal steel reinforcement and the additional shear reinforcement. nullhe modulus alone will not always dictate
32、 which elements are the most highly strained across a given shear cracnull. Summarynullhere are a variety of materials and methods available to increase shear capacity, including the use of external steel reinforcement, section enlargement, internal steel or nullRnull reinforcement, supplemental mem
33、bers, nullRnull plates and strips, both steel and nullRnull nullSnullR, and external prestressing. nullepending on the type of application, cracnull, cracnull stitching and epoxy injection could be considered shear strength-ening measures, although these methods would typically be described as repai
34、rs rather than strengthening, but are not discussed in this document.members, and tensioningnull;nullgnitude of strength increase required nullome methods can achieve higher relative strength increases than others;null nullecessity to unload the member before performing the shear strengthening repai
35、r nullif the member is unloaded, the repaired portion will be able to resist both future nulllive and superimposednulland present nulldeadnull loads in conjunction with the original member; if the member is not unloaded, then the repair will resist only future loads, and none of the original dead lo
36、adnull;null Sinulle of project nullsome special materials and methods may be less cost effective on small projectsnull;null null-service conditions nullethods using adhesives might be unsuit -able in high-temperature environments; external steel methods can be problematic in corrosive environments,
37、and exposed steel and nullnullmethods can impact the fire rating of strengthened membersnullnull nulln-place concrete strength nullthe effectiveness of methods relying on bond to the existing concrete can be significantly limited by low concrete strengthnull;null nullimensionalnullclearance constrai
38、nts nullsection enlargement might be limited by the degree to which the enlargement can encroach on surrounding clear spacenull;null nullccessibility nullboth for application and maintenancenull;null nullperational constraints nullmethods requiring longer time for construction might be less desirabl
39、e for applications in which Fig. nullFRP shear strengthening example using carnullon FRP nullnullFRPnullnull null-shaped strips with glass nullnuller anchors nullSchuman 2nullnullnullnull.Fig. nullnullnullRP reinforcement with pultruded plate stirrups with nullnullrinulled anchor -age as installed.A
40、merican Concrete Institute Copyrighted Materialnullnull.concrete.org4 INCREASING SHEAR CAPACITY WITHIN EXISTING REINFORCED CONCRETE STRUCTURES (ACI 364.2T-08)Referencesnullnullnull nullommittee nullnullnull, nullnullnullnull, nullStrength nullvaluation of nullxisting nulloncrete nulluildings nullnul
41、lnullnull nullnullnullR-nullnullnull,null nullmerican nulloncrete nullnstitute, nullrmington nulllls, null, null pp.nullnullnullmmittee nullnull nullnull, nulluide for the nullsign and nullnstruction of nullternally nullnded nullnullSystems for Strengthening nullncrete Structures nullnull nullnullnu
42、ll-nullnullnullnullerican nullncrete nullstitute, nullrmington nulllls, null, null pp.nullnullnullmmittee nullnull nullnull, nulluide for the nullsign and nullnstruction of nullncrete Reinforced with nullnullnullrs nullnull.null-nullnullnullnullerican nullncrete nullstitute, nullrmington nulllls, nu
43、ll, null pp.nullnullnull nullommittee nullnullnull, nullnullnullnull, nullnulloncrete Repair nulluide nullnullnull,null nullmerican nulloncrete nullnstitute, nullarmington nullills, nullnull, nullnull pp.nullrolin, null, and nullljsten, null, nullnull, nullehavior of nullncrete Structures Strengthen
44、ed in Shear with nullRnull,null Proceed-ings of the Sixth International Snullposium on FRP Reinforcement for nullncrete Structures nullRPRnull-null, Singapore, nullorld Scientific, pp. nullnullnullnullRaina, null. null, nullnull, nullncrete nullidgesnullInspectionnullRepairnullStrengtheningnullnulle
45、sting and nullad nullpacitnullEnullluation, nullnullaw-nullll, nullw nullornull nullnullpp.Schuman, null., nullnull, nullechanical nullchorage for Shear Rehabilitation of Reinforced nullncrete Structures with nullnull null nullpropriate nullsign nullproach,nullnullnullthesis, nullpartment of Structu
46、ral nullgineering, nulliversity of nullli -fornia at San nullego, null nulllla, null, nullnullpp.nullong, R. S. null., and nullecchio, null. null, nullnull, nullowards nulldeling of Reinforced nullncrete nullmbers with nullternally nullnded nullber-Reinforced nulllymer nullmposites,nullnullI Structu
47、ral nullurnal, null. nullnull null. null nulln.-nullb., pp. null-null.nullnullnullechnulltes are intended for reference for the design and construction of concrete structures. nullis document is intended for the use of individuals who are competent to evaluate the significance and limitations of its
48、 content and who will accept responsibility for the appli-cation of the information it contains. nulle nullerican nullncrete nullstitute disclaims any and all responsibility for the accuracy of the content and shall not be liable for any loss or damage arising therefrom. Reference to this document s
49、hall not be made in contract documents.nullnullnullnullnull-null was adopted and published nullly nullnull.nullpyright nullnullnull, nullerican nullncrete nullstitute.nullll rights reserved including the rights of reproduction and use in any form or by any means, including the manulling of copies by any photo process, or by electronic or mechanical device, printed, written, or oral, or recording for sound or visual reproduction or for use in any nullowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors.Reported by ACI Committee 36
