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EXTERNAL REINFORCEMENT SYSTEMS –CONCRETE REPAIR, STRENGTHENING &SEISMIC

EXTERNAL REINFORCEMENT SYSTEMS –CONCRETE REPAIR, STRENGTHENING &SEISMIC

 EXTERNAL REINFORCEMENT SYSTEMS CONCRETE REPAIR, STRENGTHENING &SEISMIC

The principles behind externally bonding FRP plates or wraps to concrete

structures are very similar to the principles used in application of bonded steel plates. In general, the member’s flexural, shear, or axial strength is increased or better mobilized by the external application of high tensile strength material.

Reasons for applying FRP systems as an external reinforcement for bridge structures:

– Capacity upgrade due to a change in use of a structure

– Passive confinement to improve seismic resistance

– Crack control

– Strengthening around new openings in slabs

– FRP composite systems have been applied to many structural elements including beams, columns, slabs and walls as well as many special applications such as chimneys, pipes and tanks. More recently this technology has been applied to infrastructure security applications relating to hardening and blast mitigation of structures.

See Fig (7-16,17,18).

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.7.3.2 FEATURES AND BENEFITS

STRENGTHENING

FRP composite systems can be used to strengthen undamaged concrete structures that require greater load capacity due to functional changes, additional loads, code changes or other reasons. The FRP is placed on tensile surfaces in a manner similar to steel plate bonding for strengthening or embedded into saw cut grooves near the concrete surface. FRP composite systems can add shear and flexural strength to beams and slabs for both positive and negative moment conditions. Strengthening of existing concrete structural members with FRP composites is accomplished by utilizing the tensile strength and stiffness of the composite and the strain compatibility of the composite to the existing member. The design must include proper selection of the adhesive used to bond the FRP reinforcement to the surface of the concrete to be strengthened. As in repair, the type of composite, the number of layers, the orientation of fibers, and the preliminary work and surface preparation all depend on the design goals and type of structural element as determined by the project.

SEISMIC RETROFIT

FRP composite systems have been used extensively in seismic zones for confinement of concrete columns and walls. Improvements in ductility factors of up to 10 fold have been realized through the use of FRP column wrapping. Specific FRP systems, offered by some of the manufacturers referenced below, address seismic requirements according to the load capacities anticipated and geometric considerations of the building structure. In addition, FRP systems can be used for stabilizing hollow clay tile, brick and other unreinforced and lightly reinforced masonry walls in life-safety applications where vital egress and exit paths in buildings are required.

 

7.8.1 SHEAR WALL SYSTEM

It works as vertical cantilever walls, designed to receive lateral forces from floor diaphragms and to transmit them to the ground.

The size and location of shear walls is extremely critical. Plans can be conceived as collections of resistant elements with varying orientations to resist translation forces, and placed at varying distances from the center of rigidity to resist torsion forces. A typical layout of a building utilizing shear walls is shown in Fig 7-19.

Shear walls have three basic failure modes that are sliding, rocking and bending Fig 7-20.

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Increasing the gravity load on the wall can minimize failure. Increasing the dead load generally does this. However as the dead load is increased the ductility of the structure as a whole reduced.

7.8.2 DIAPHRAGM FLOORS

The term ‘diaphragm’s used to identify horizontal resisting elements,

(Generally floors and roofs) that transfer lateral forces between vertical resisting elements (shear walls or frames).The diaphragm acts as a horizontal beam: the diaphragm itself acts as the web of the beam, and its edges act as flanges, Fig 7-21.

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When diaphragms form part of a resisting system, they may act either in a flexible or stiff manner.

7.8.2.1 MOVEMENT JOINT

A joint which is formed to accommodate relative movement between adjoining parts of a structure or diaphragms is a called a movement joint. It is caused by shrinkage, temperature changes, creep and settlement. It divides the structure into a number of individual sections and passes through the structure above the ground in one level. For reinforced concrete structures, movement joints are at least 25mm wide normally provided at 25m longitudinally and transverse.

7.8.3 FRAMED SYSTEM

Cast in-place frames with monolithic columns and beams have a natural rigid frame action. For seismic resistance both columns and beams must be specially reinforced for the shears and torsions at the member ends. The Joints become highly stressed due to seismic forces.

7.8.4 COMBINED SHEAR WALL AND FRAMES

The combined shear walls and frames is designed in a way that the shear wall takes the lateral load, resulting from an earthquake, in one direction and the frames takes the lateral load from the other direction as shown in Fig 7-22or any other arrangement. Since solid walls tend to be quite stiff in there own planes, requires the use of separation joints or flexible connections that will allow the frame deform as necessary under the lateral loads.

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