RETROFITTING OF REINFORCED CONCRETE

BUILDINGS

PRESENTED BY Under Guidance of K.L. KRISHNA Mr. GURUPRASAD (1ga05cv019) Sr. Lecturer

Introduction Methods of increasing resistant capacity of

structure by various techniques is called Retrofitting.

Seismic retrofitting is the modification of existing/damaged structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes.

It can also be defined as increasing the seismic resistant of damaged structure by various techniques as per IS: 13935-1993

Concept of RetrofittingThe basic concept of retrofitting aims at:-

Up gradation of lateral strength of the structure

Increase the ductility of the structure

Increase of strength and ductility

Need for Retrofitting Thus Seismic Retrofitting is required for buildings

under two circumstances:-

Earthquake damaged buildings –Public safety ; structural survivability; for utility and historic significance.

Earthquake vulnerable buildings that has not experienced severe earthquake

Source Of Weakness In RC frames

Discontinuous load path

Lack of deformation compatibility of structural members

Quality of workmanship and poor quality of materials

Classification Of Retrofitting Techniques

Retrofitting Techniques

Global/Structural level Local/member level

Adding Shear Wall

Adding Wing Wall

Adding Infill Wall

Adding Bracing

Wall Thickening

Mass Reduction

Supplement Damping and Base Isolation

Jacketing of Beams

Jacketing of Beam-Column Joint

Jacketing of Columns

Strengthening of Individual Footing

Convectional method

Non-Convectional method

Structural level (or Global) Retrofit methods

1. Conventional methods: based on increasing the seismic resistance of existing structure by eliminating or reducing the adverse effect of design or construction.

• Adding new shear walls• Adding steel bracings• Adding infill walls

2. Non- Conventional methods: based on reduction of seismic demands• Seismic base isolation

a) Adding new shear wall : One of the most common methods of retrofitting is providing the shear wall to increase the lateral strength of the reinforced concrete buildings. A reasonable ductility may also be achieved if the walls are properly designed with a good detailing.

Limitations of this method:• Increase in lateral resistance but it is concentrated at a few places• Increase in overturning moment at foundation causes very high uplift

that needs either new foundation or strengthening of the existing foundation

• Increases the dead load of the structure• Excessive destruction at each floor level results in functional disability of

the building• Possibilities of adequate attachment between the new and the existing

wall• Closing of the formerly open spaces can have major negative impact on

the interior of the building or exterior appearance.

1. Conventional strengthening methods

b) Adding steel bracings: Another method of strengthening is use of steel bracings. These are an effective solution for buildings with large openings.

Advantages:• Higher strength and stiffness can be provided• Opening of the natural light can be made easy• Bracing system adds less weight to the existing structure• Most of the work can be performed with pre-fabricated elements and

disturbance to the occupants may be minimized.• It is cost effective.

Limitations of this method:• Lack of information about the seismic behavior of the added bracing• Undesirable changes regarding original architectural features• Section failure may cause impact on over all performance of the

building• High level workers are necessary for construction• Cost efficiency specially to welding is necessary

STEEL BRACINGS PROVIDED

Effective slenderness ratio 60 to 80 or low

c)Adding infill walls: Adding of infill walls as shown in the figure also helps in increasing the structural strength against seismic force. It is economical and effective method to improve strength and reduce drift of frame.

Limitations of this method:• Some columns in the frame are subjected to large axial tensile forces,

which may exceed the capacity of the column that have been designed for little or no flexure but only for axial and gravity load.

This method aims at reducing the seismic forces on the structure using various techniques of base isolation. It is a powerful method of seismic

retrofitting for buildings.

a)Base isolation: The seismic base isolation technology involves placing flexible isolation systems between the foundation and the superstructure.

Its main advantages are:• Better protection against earthquake due to decreasing of shears• Foundation system will not need any reinforcement to resist overturning moments• Least interrupting the building activities since work is carried out at the basement

Base isolators: Flexible pads and rollers Seismic dampers a) viscous dampers b) friction dampers c) yielding dampers

2. Non-Conventional strengthening methods

Its main advantages are:•Better protection against earthquake due to decreasing of shears

•Foundation system will not need any reinforcement to resist overturning moments

•Least interrupting the building activities since work is carried out at the basement

1.Remove Finishing 2. Encase by half tube steel plate fill concrete

3. Cut off existing column

4. Install Isolator

6. Cut off steel panel

5. Grout mortar

Process of Seismic Retrofitting by Base Isolation in Mid-storey

Base Isolation in Mid-storey

1. Excavate 2.New Foundation 3. Cut off control piles

4. Set isolator base & grout 5. Install Isolator & fix 6. Cut off piles

Process of Seismic Retrofitting by Base Isolation in buildings resting on pile.

Member level (or Local) Retrofit methods

The member level/local retrofit methods are toupgrade the strength of the member which isseismically deficient

The most common methods used are as follows:Jacketing of columnsJacketing of beamsJacketing of beam-column jointStrengthening of individual footing

a) Jacketing of columns:

(i) Reinforced concrete jacketing: Jacketing of columns consist of adding concrete with longitudinal and transverse reinforcement around the existing column .This method of jacketing is the most suitable for reinforced concrete buildings. So it is the widely used and cost effective for RC buildings, as it is familiar to engineers and construction industry, suitable for repair. Also it increases the (i) shear capacity of the columns in order to accomplish strong column and weak beam design (ii) to improve flexural strength by longitudinal steel provided continuously from slab and anchored to the foundation.

Limitation of these method/disadvantages:• They considerably increase member c/s dimension, which may be a serious

draw back in buildings with less floor area.• They disturb the occupants of the building during construction• Production of dust and debris causes pollution and health hazards• They also cause noise pollution at work area.• Rusting of bars takes place.

1. Ties- 70 to 100 mm2. Hooks- 90-135 degree.

CONSTRUCTION TECHNIQUES FOR COLUMN JACKETING

DETAILS FOR PROVISION OF LONGITUDINAL REINFORCMENT.

Properties of jacket

Minimum width of jacket

Minimum area of longitudinal reinforcement

Match with the concrete of the existing structure

Compressive strength greater than that of the existing structures by 5N/mm2 (50kg/cm2),or at least equal to that of existing structure

10 cm for concrete cast-in-place and 4cm for shortcrete

If possible, four sided jacket should be used

A monolithic behavior of the column should be assured

Narrow gap should be provided to prevent any increase in flexural capacity

3A/fy, where , A area of the contact in and fy is in kg/

Spacing should not exceed six times the width of the new element (the jacket in case) up to the limit of 60cm.

Percentage of steel in the jacket w.r.t the jacket area should be limited between 0.015 to 0.04

2cm

Details for reinforced concrete jacketing (UNDP/UNIDO,1983)

2cm

Minimum area of the transverse reinforcement

Shear stress in the interface

Connectors

At least a 12mm bar should be used at every corner of the four sited jacket

Designed and spaced as per earthquake design practice

Minimum diameter used for ties is not less than 10mm or 1/3 of the diameter of the biggest longitudinal bar.

The ties should have 135-degree hook with 10 mm bar diameter anchorage.

Due to difficulty of manufacturing 135-degree hooks at field, ties made up of multiple pieces, can be used.

Provide adequate shear transfer mechanism to assure monolithic behavior

A relative movement between both concrete interface ( between the jacket and the element) should be prevented

Chipping the concrete cover of the original member and roughening its surface may be improve the bond between the new and the old concrete

For the four sided jacket the ties should be used to confine and the shear reinforcement to the composite element

For 1,2,3 sided jacket, special reinforcement should be provide to enhance monolithic behavior

Connectors should be anchored in both the concrete such that it may develop at least 80% of the yield stress.

Distributed uniformly around the interface, avoiding concentration at specific location

It is better to use anchored bar with epoxy resins.

(ii) Fiber Reinforced Polymer: here fiber reinforcement polymer jackets are used.

Advantages:• Carbon fiber are flexible and can be made to contact with the surface

tightly for a high degree of confinement because of its high strength and modulus of elasticity.

• It is light weight • Rusting does not occur

Limitations of this method:• In some cases the presence of beam may require majority of new

longitudinal bars to be bundled in the corner of the jacket• With the presence of the existing column it is difficult to provide cross ties

for new longitudinal bars which are not at the corner of the jacket.• Jacketing is based only on engineering judgment as there are no

guidelines or code recommended for it.

(iii)steel jacketing: It is similar to reinforced concrete jacketing but here steel plates are provided instead of reinforced bar

DRIVING BOLTS THROUGH PLATE

Steel plate thickness

Height of jacket

Shape of jackets

Free ends of jacket

Bottom clearance

Gap between steel jacket and concrete column

Size of anchor bolts

Number of anchor bolts

At least 6mm.

1.2 to 1.5 times the length in case of flexural column

Full height of column in case of shear columns

Rectangular jacketing , prefabricated two L-shaped panels.

Welded throughout the height of the jacket, size of weld 1/4”

38mm (1.5”), steel jacket may be terminated above the top of footing to avoid any possible bearing of the steel jacket against the footing, to avoid the local damage to the jacket and /or an undesirable or un intended increase in flexural capacity.

25mm (1 inch) fill with cementation s grout

25mm (1 inch) in diameter and 300mm (12”) long embedded in 200mm (8”) into concrete column

Blots were installed through pre-drilled holes on steel jacket using an epoxy adhesive.

Two anchor bolts are intended to stiffen the steel jacket and improve confinement of the splice

Details of Steel Jacketing

b) Jacketing of beams: Jacketing beam is recommended for several purposes as it gives continuity to the columns and increases the strength and stiffness of the structure. While jacketing the beam it flexural resistance must be carefully computed in order to avoid the creation of strong beam–weak column system. Due to column jacketing there may be change in mode failure and redistribution of forces which causes beam hinging, so the critical section of the beam and the participation of the existing reinforcement should be taken into consideration. Jacketing of beam is carried out in various ways 3 or 4-sided jackets.

DIFFERENT WAYS OF BEAM JACKETING

•Existing beam•Steel clamp•Steel plate•Nut•Angle profile•welding

Improvement of shear strength using bars

Minimum width of jacket

Longitudinal reinforcement

Shear reinforcement

Depth of jacket beams

8cm if concrete cast in place or 4cm for shotcrete.

Percentage of the steel jacket should be limited to 50 of the total area of the composite section

Ignore the effect of the existing shear reinforcement

New reinforcement should have 135 –degree hook and at each corner of the tie there should be at least one longitudinal bar

The bar used for tie should have at least 8mm diameter

Multiple piece ties can be used, as discussed before for columns

Span/depth ratio

Storey height

Ductile behavior

Reinforcement Detail of Beam Jackets (UNDP/UNIDO, 1983)

c) Jacketing of beam-column joint: Beam column joint forms the critical section failure of reinforced concrete frame structure. Beam column joint can be retrofitted by reinforcement concrete jacketing or FRP jacketing or steel jacketing method as mentioned before.

DIFFERENT TYPES OF BEAM-COLUMN JOINTS

d) Strengthening of Individual Foundation : Strengthening of the foundation of the existing building to resist the seismic effect is known as foundation retrofitting.

Foundation retrofitting are required due to two problems: The change of loads on the foundation by strengthening of the building. The failure of foundation it self due to various reasons.

Retrofitting of the shallow concrete foundations: Reinforced concrete jacketing bolting down the foundation Base isolation

Retrofitting of the pile/deep foundations: Micro pile Methods Screen pipe drain method Super Strengthening Pile Bents method

(a)Beam and column ; (b)Foundation

1. Existing foundation; 2. Existing column; 3.Reinforced jacket; 4. Added concrete; 5.Added reinforcement

RETROFITTING OF FOUNDATION REINFORCED CONCRETE JACKETING

BOLTING OF FOUNDATION

Comparative Analysis of the Methods of Retrofitting

Effects of Various Retrofit Methods on Structural Performance

Factors effecting retrofitting of RC buildings

Technical consideration Cost benefit ratio Load on the structure Materials Design aspect Mode of failure

THANK YOU

A high powered roto-hammer is used to create a pilot hole for the sill plate screws

Next the sill plate bolt is driven into the hole

After the bolts are set, they are torqued down into place

The final hole is then drilled

The final bolt is driven into the foundation wall and torqued down