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RETROFITTING OF REINFORCED CONCRETE BEAM COLUMN JOINTS

Presented byMiss. Mule Disha Pradip

CONTENTSIntroductionBeam column jointsJacketingCase study 1Case study 2Case study 3Concluding remarksReferences

IntroductionRetrofitting is Upgradation of certain building system (existing) to make them more resistant to seismic activity.

Structure can be - Earthquake damaged. - Earthquake vulnerable.

Proves to be better economic and immediate shelter to problems rather than replacement.

There are considerable members of RC structures in India that do not meet the requirement of current design standards because of inadequate design or construction errors which need structural upgradation specially to meet seismic design requirement

In recent earthquakes all over the world has highlighted the consequences of poor performance of beam-column joints.

Failure is because of shear failure in beams, bar slip and shear failure of beam-column joints.

Introduction

Introduction Retrofitting of RC structure 1. Strengthening existing member - RC Jacketing - Steel Plate Bonding and Jacketing, Steel section caging : FRP - Plate Bonding And Jacketing

Introduction 2.Adding new member. - Shear walls. - Frames (moment resisting frames and braced frames) - Bracing. -Buttresses.

Global6.Mass reduction4.Addition of wing wall / buttresses 3.Addition of bracing1. Addition of shear wall5.Wall thick ceiling 7.Supplimental damping and base isolation2.Addition of infillLocal 3. Jacketing of beam column joint2. Jacketing of columns1. Jacketing of beams4. Strengthening individual bldg

Beam column joint Since construction materials have limited strength , the joint have limited force caring capacity

When force larger than this are applied during earthquake , joints are severely damaged.

Repairing damaged joint is difficult , so the damaged must be avoided. Thus the beam column joint must be design to resist earthquake effect.

ObjectiveThe beam-column joint rehabilitation is to- 1. Strengthen the shear and bond-slip resistance in order to eliminate the brittle failure. 2.To ensure that the ductile flexural hinging will take place in the beam.

A joint should maintain its integrity and must be designed stronger than the members framing to it.

JacketingMost popular method for strengthening of building columns.

Purpose of jacketing- 1.To increase concrete confinement.2.To increase shear strength.3.To increase flexural strength.

Concrete jacketinginvolves addition of longitudinal bars, closely spaced ties, and a layer of concrete.

The jacket increases both flexural strength and shear strength of concrete .

The usual practice consists of first assembling the jacket reinforcement cages, arranging the formwork and then placing the concrete jacket.

But it is cost effective.

Concrete jacketingConcrete jacketing disadvantages- 1.Cost effective 2.Requires intensive labour 3.Detailing of steel in the form of digital collars 4.Incrases Dimensions of structure 5.Incrases weight of structure

Steel jacketingSteel jacketing refers to encasing the member with steel plates and filling the gap with non-shrink grout.

The jacket enhances both flexural strength and shear strength of concrete.

Steel jacketing disadvantages 1.Complicated working procedure. 2.Inner surface corrosion . 3. Heavy weight. 4.C/S increases.

Fiber Reinforced Polymer( FRP)Fiber reinforced polymer (FRP) is a composite material consisting of polymeric resin reinforced with high strength fibers.

Composite materials are available in the form of sheets, pre-formed shapes and bars.

The FRP sheets are thin, light and flexible enough to be inserted.

Fiber Reinforced Polymer( FRP)The fibers can be of glass, carbon, aramid.

Glass fibers have lower stiffness and cost compared to carbon fibers.

They are suitable in low cost seismic retrofit applications.

Fiber Reinforced Polymer( FRP)The FRP composites are useful for repair, rehabilitation and retrofit of structures for the following reasons:

1.The FRP sheets are light and flexible, which facilitate installation. It does not need drilling of concrete or masonry.

2.The curing time required is less

3.The sheets are thin and hence there is no marginal increase in the size of retrofitted member.

Fiber Reinforced Polymer( FRP)The material is chemically inert and has resistance against electro-chemical corrosion.

There is good fatigue strength, which is suitable for fluctuating loads.

Case study -1Retrofitting of RC beam column joint using RC jacketing by Tsonos .G (2009).

Objective: To improve strength, stiffness and ductility of the element.

Experimental evidences by Tsonos, G., (2009), shows that strengthening schemes are effective in transforming the brittle joint shear failure mode of reference specimen into a more ductile failure mode in case of the strengthened specimens.

Test specimenThe test specimens are 1:2 scale models of the representative 40 cm x 40 cm beam-column joints.The comparison of the load-deflection curves of the original subassemblage and the retrofitted sub assemblage is done.The dimensions and cross sectional details of the original specimen are as shown in Fig. 1.As shown in Fig. 2, the retrofitted specimen had a four-sided jacket, 14 longitudinal bars at each corner of the column.Which were connected by; 8 supplementary ties at 70 mm.

Fig -1

Fig -2

Experimental setupThe general arrangement of the experimental setup by Tsonos, G., (2009) is shown in Fig.3.

Fig -3

Test ResultsThe original sub assemblage is subjected to a cyclic lateral load history, so as to provide the equivalent of severe earthquake damage.The specimen experienced brittle shear failure at the joint region.Damage occurred both in the joint area and in the critical regions of columns, while the beam remained intact.Failure mode of the strengthened specimen involved the formation of a plastic hinge in the beam near the column juncture. Damage of the specimen occurred mainly in the critical region of the beam and also in the joint area.

Plots of applied shear versus drift angle for the strengthened specimenPlots of applied shear versus drift angle for original specimen

Conclusion of case study-1The control specimen, representing an existing beam-column sub assemblage, performed poorly under reversed cyclic lateral deformations.The connection of the control specimen exhibited premature shear failure during the early stages of seismic loading and damage was concentrated in the joint region.The performance of reinforced shotcrete jacketed sub assemblage showed a vast improvement. Damage to the strengthened specimens is concentrated mainly in the beam critical region and also in the joint area.

Case study -2Strengthing of RC beam column joints using CFRP by Alsayed et al., (2005).

Objective: To upgrade the shear strength and ductility of seismically deficient exterior beam-column joint.

The seismic performance of beam-column joints is investigated and is compared to that of a CFRP strengthened specimen.

Test SpecimensTwo specimens were cast and were subjected to cyclic loading, so as to provide the equivalent of severe earthquake damage.The schematic diagram of the joint specimen is as shown in Fig.4. The damaged specimen is then repaired using CFRP sheets. The repaired specimen was again subjected to the similar lateral cyclic load history.

Fig -4

Fig -5

Figure 5 shows the general arrangement of experimental set up used by Alsayed et al., (2007) for testing of interior beam-column joints.The bottom of the column surface is attached to a base pivot using 4, high strength threaded rods.The base pivot, in turn, is fastened to a strong steel I-beam. To apply the simulated seismic type cyclic load on the specimen, a 500-kN servo-controlled hydraulic actuator is connected to a reaction steel frame, which stands on a strong concrete floor

Test Specimens

Test ResultsThe hysteretic behavior of exterior joints is examined in terms of shear strength and deformation capacity.

The ultimate load for repaired specimen is substantially higher than its corresponding original (before repair) .

This is primarily due to the increased confinement of joint resulting from externally bonded CFRP sheets.

Load-displacement hysteretic plot for CFRP upgraded specimen Load-displacement hysteretic plot for control specimen

Conclusion of case study-2CFRP repaired specimen is compared with its corresponding specimen tested before repair and, in general, it was observed that provision of CFRP sheets improved the shear resistance and ductility of the RC joint to a great extent.

The effectiveness of CFRP sheets in upgrading deficient exterior beam-column joints is established.

Case study-3Strengthening of RC beam column joints using GFRP by El-Armoury and Ghobara, (2002).

Objective: To strengthen the shear and bond-slip resistance in order to eliminate those types of brittle failure and ensure instead that ductile flexural hinges would develop in the beam.

El-Armoury and Ghobara, (2002) have proposed techniques for upgrading reinforced concrete beam column joints using GFRP.

Test specimenIn the experimental study conducted, three reinforced concrete beam column joints were tested.The beam-column joints are designed assuming that points of contra flexure occur at the mid height of columns and the mid-span of beams.No transverse reinforcement was provided in the joint region. The dimensions and reinforcement details of the specimen are as shown in Fig.6.

Fig -6

Test SpecimenThe rehabilitation scheme proposed by the authors consisted of a system for upgrading the shear strength of the joint. The joint is wrapped with two U-shaped composite layers. The first layer was bi-directional sheet and the second was unidirectional sheet. The ends of the sheets are anchored using steel plates and tie rods driven through the joint. Four unidirectional glass fiber sheets were applied to the beam bottom face for a horizontal distance of 1000 mm and extended along the inner column face vertically for a distance of 500 mm, as shown in Fig.7.

Fig -7

Fig -8

Experimental setupThe specimens are tested with the column in the vertical position, hinged at the top and bottom column ends and subjected to a cyclic load applied at the beam tip as shown in Fig.8.

The beam-tip displacement and the column lateral displacement are measured using potentiometers.

Two diagonal linear voltage differential transformers (LVDTs) were attached to the joint to measure the joint shear deformation.

Test resultsThe retrofitted specimen is subjected to the same loading sequence as in the control specimen.

The strain values showed that the fiber sheets attached to the beam face are carrying most of the developed tensile forces, indicating that the glass fiber fabric was working effectively.

Severe pinching and stiffness degradation occurred in the last two cycles following the fracture of the weld

Load-displacement hysteretic plot for retrofitted specimenLoad-displacement hysteretic plot for control specimen

Conclusion of case study-3The control specimen with no shear reinforcement in the joint and with inadequate anchorage for the beam, showed a brittle joint shear failure.

Using GFRP jacketing, the concrete integrity is maintained by confinement.

The ductility and the load-carrying capacity of the rehabilitated joint are significantly improved.

Concluding remarksBy retrofitting, the concrete integrity is maintained by confinement and has significantly improved the ductility and the load-carrying capacity of the rehabilitated joint.Short Crete jacketing improves the strength, stiffness and ductility of the joints. The method is also cost effective. However, jacketing needs skilled and intensive labour due to difficulties in placing of the additional reinforcement. Joint rehabilitation using fiber-reinforced polymers (FRP) has the advantages of simplicity of application, less need for skilled labour, corrosion resistance, not excessively increase the dimensions of the sections of structural elements.

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