RETROFITTING REINFORCED CONCRETE FRAME

STRUCTURES

Prachand Man Pradhan, PhD

Act. Head, Department of Civil & Geomatics Engineering,

Kathmandu University21 June, 2015

Introduction

RC Buildings are Popular in Nepal

But most are seismically vulnerable

Basic Reasons

Design done by Arch./Er. not formally trained in EQRD & construction.

Masons involved in construction unaware about EQRD technology.

Big EQ can cause structure damage/collapse Large loss of life/property

Vulnerable bldgs need strengthening

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Retrofitting RC structures_Prachand

Need of Retrofitting

Retrofitting may be done:

Before an EQ to prevent future damage

After an EQ to strengthen and repair the damaged components

Retrofitting is cheaper than new construction

If the retrofitting cost is more than 35% of building cost -> better to demolish

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What kind of structs. need retrofit?

• Structures designed with old codes

• Structures designed with new codes but deficient in design and construction,

• Important bldgs whose service is essential after EQ, [hospitals, w/s, monumental, schools etc.]

• Purpose altered buildings,

• Buildings which resisted past EQ (become weak)

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Criteria of Retrofitting

Technical Criteria

• Upgrading the lateral strength of structure

– Shear wall

– Jacketing

– grouting

• Increasing ductility and strength

– Bracing

– EDU

– Base isolation

General Criteria

• Cost/Benefit analysis

• Available material & workmanship

• Aesthetics

• Architectural Identity

• Duration of project

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Principles of Retrofitting

removal of irregularities and asymmetry

increasing the strength and stiffness

enhancement of deformation capacity (or ductility), &

earthquake demand reduction by Base-isolation or supplemental energy dissipation (EDU)

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Materials for retrofitting

• Cement-grouts mixed with admixtures

• Concrete need at least 5 MPa stronger

• Polymers (FRP) CFRP, SFRP, GFRP etc.

• Epoxy stronger than concrete

• Base isolations (Elastomeric rubber or neoprene pads)~ costly

• Supplemental EDU (braces which work as viscous dampers) ~ costly

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Methods

• Jacketing of beams, columns using FRP, steel plates or reinforcement.

• Over-laying or under-laying of slabs~ shotcretes or FRP

• Introducing shear walls wherever required, configuring new plan

• Use of viscous dampers diagonally in the frames (EDU)

• Use of Base isolators (rubber pads) in footing

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Jacketing columns with steel

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Contd… Column Jacketing

100

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100mm

Contd…Column Jacketing

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FRP Strengthening of Columns

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Plate Bonding of beams

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Beam under-laying

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Slab over-laying/under-laying

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Shear Keys in Hollow slab

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FRP on slab

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Base Isolation

2005 NPEEE Earthquake Desi gn Concept : Lecture 25: Retroffiting & Base-Isolation 22/25

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Configuring new plan

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Increasing lateral load resistance

• Bracings

2005 NPEEE Earthquake Desi gn Concept : Lecture 25: Retroffiting & Base-Isolation 11/25

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New Shear wall connection

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Guidelines for retrofitting

• IS 15988:2013 Seismic Evaluation and strengthening of existing RC Buildings-Guidelines

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How the retrofit design is done?

• The structure is re-modeled and analyzed for its existing condition regardless of damage

• Area of steel is calculated for columns and beams

• The calculated steel area are compared with the structural drawing of existing building

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Contd…

• If the steel is insufficient in old building, additional steel is provided with sufficient jacketing in columns and beams.

• If walls are cracked with bricks crushed, cement grout with admixture is provided

• Beams may be FRP wrapped or steel jacketed.

• If footing size is found insufficient (for settlement), then area of footing is increased

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Contd…• If settlement is seen, mass on floors are to

be reduced

• If floors are found soft, additional walls are provided

• If beam is cracked with hairlines, epoxy grout is sufficient.

• If slab is cracked, epoxy grouting or FRP can be provided

• Mixes of concrete and mortar shall be advised as per need

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The End.

THANK YOU.

Photo & drawing sources

Guidelines for Retrofitting of buildings, UNDP, India

Agarwal, P., Shrikhande, M., Earthquake Resistant Design of Structures, Prentice Hall of India P. Ltd.

Paul, D.K., Singh, Y., Ruhela, M.K., Guidelines for Earthquake Resistance Buildings, Department of Earthquake Engineering, IIT Roorkee.

IS 15988:2013

2005 NPEEE Earthquake Design concept, Lecture 25

Sources from internet