strengthening techniques of buildings

8/20/2019 Strengthening Techniques of buildings

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STRENGTHENING OF R.C. COLUMNSStrengthening of reinforced concrete columns is needed when:

1. The load carried by the column is increased due to either increasing the number of floors

or due to mistakes in the design.

2. The compressive strength of the concrete or the percent and type of reinforcement are

not according to the codes’ requirements.

3. The inclination of the column is more than the allowable.

4. The settlement in the foundation is more than the allowable.

There are two maor techniques for strengthening reinforced concrete columns!

1 REINFORCED CONCRETE JACET

The si"e of the acket and the number and diameter of the steel bars used in the acketing

process depend on the structural analysis that was made to the column.

#n some cases$ before this technique is carried out$ we need to reduce or even eliminate

temporarily the loads applied to the column% this is done by the following steps!

• &utting mechanical acks between floors.

• &utting additional props between floors.

'oreover$ in some cases$ where corrosion in the reinforcement steel bars was found$ the

following steps should be carried out!

• (emove the concrete cover.

• )lean the steel bars using a wire brush or sand compressor.

• )oat the steel bars with an epo*y material that would prevent corrosion.

• #f there was no need for the previous steps$ the acketing process could start by the

following steps!

1. +dding steel connectors into the e*isting column in order to fasten the new stirrups

of the acket in both the vertical and hori"ontal directions at spaces not more than

,-cm.Those connectors are added into the column by making holes 34mm larger than

the diameter of the used steel connectors and 1-1,cm depth.

2. /illing the holes with an appropriate epo*y material then inserting the connectors

into the holes.

3. +dding vertical steel connectors to fasten the vertical steel bars of the acket

following the same procedure in step 1 and 2.


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4. #nstalling the new vertical steel bars and stirrups of the acket according to the

designed dimensions and diameters.

,. )oating the e*isting column with an appropriate epo*y material that would guarantee

the bond between the old and new concrete.

0. &ouring the concrete of the acket before the epo*y material dries. The concrete used

should be of low shrinkage and consists of small aggregates$ sand$ cement and

additional materials to prevent shrinkage.

The previous steps are illustrated in Fig 1.

Fig:1! Incre"sing the cross#section"l "re" of column $% RC &"c'eting!

( STEE) JACET


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This technique is chosen when the loads applied to the column will be increased$ and at the

same time$ increasing the cross sectional area of the column is not permitted.

This technique is implemented by the following steps as shown in Fig 2!

1. (emoving the concrete cover.

2. )leaning the reinforcement steel bars using a wire brush or a sand compressor.

3. )oating the steel bars with an epo*y material that would prevent corrosion.

4. #nstalling the steel acket with the required si"e and thickness$ according to the design$

and making openings to pour through them the epo*y material that would guarantee the

needed bond between the concrete column and the steel acket.

,. /illing the space between the concrete column and the steel acket with an appropriate

epo*y material.


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Fig:(! Incre"sing the cross#section"l "re" of column $% steel &"c'eting!

#n some cases$ where the column is needed to carry bending moment and transfer it

successfully through the floors$ one should install a steel collar at the neck of the column by

means of bolts or a suitable bonding material.

Fig:3. shows a column which was strengthened with steel angles.

STRENGTHENING OF R.C.BEAMS

(einforced concrete beams need strengthening when the e*isting steel bars in the beam are

unsafe or insufficient$ or when the loads applied to the beam are increased. #n such cases$

there are different solutions that could be followed!

I#ADDIN* REINFORCE+ENT STEE) ,ARS TO T-E +AIN STEE) .IT-O/T

INCREASIN* T-E ,EA+0S CROSS SECTIONA) AREA

This solution is carried out when the reinforcing steel bars are not capable to carry the

stresses applied to the beam. The following steps should be followed!

1. The concrete cover is removed for both the upper and lower steel bars.

2. The steel bars are well cleaned and coated with an appropriate material that would

prevent corrosion.


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3. oles are made$ in the whole span of the beam under the slab$ as shown in /ig.1$ 1,

2,cm apart$ a diameter of 1.3cm and e*tend to the total width of the beam.

.

Fig!1: -oles in the s"n of " $e"m

4. The holes are filled with an epo*y material with low viscosity and installing steel

connectors for fastening the new stirrups.

,. teel connectors are installed into the columns in order to fasten the steel bars added to

the beam.

0. The added stirrups are closed using steel wires and the new steel is installed into these

stirrups.

. The surface is then coated with a bonding epo*y material.

. The concrete cover is poured over the new steel and the new stirrups.

The previous steps are illustrated in /ig 2.


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Fig!(# Strengthening " $e"m without incre"sing cross section"l "re"!

II#INCREASIN* ,OT- T-E REINFORCIN* STEE) ,ARS AND T-E CROSS

SECTIONA) AREA OF CONCRETE

This solution is chosen when both the steel and concrete are not able to carry the additional

loads applied to the beam. #n such cases the following steps should be followed as in Fig 3.

1. (emoving the concrete cover$ roughing the beams surface$ cleaning the reinforcement

steel bars and coating them with an appropriate material that would prevent corrosion.

2. 'aking holes in the whole span and width of the beam under the slab at

1,2,cm.


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3. /illing the holes with cement mortar with low viscosity and installing steel connectors for

fastening the new stirrups.

4. #nstalling the steel connectors into the columns in order to fasten the steel bars added to

the beam.

,. )losing the added stirrups using steel wires and the new steel is installed into these

stirrups.

0. )oating the concrete surface with an appropriate epo*y material that would guarantee

the bond between the old and new concrete$ e*actly before pouring the concrete.

. &ouring the concrete acket using low shrinkage concrete.

Fig!2: Strengthening of $e"m $% incre"sing the cross#section"l "re" "nd $"rs

III#ADDIN* STEE) 3)ATES TO T-E ,EA+


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5hen it is required to strengthen the beam’s resistance against the applied moment or

shear stress$ steel plates are designed with the appropriate si"e and thickness.

Then those plates are attached to the beam as follows!

1. (oughing and cleaning the concrete surfaces where the plates will be attached.

2. )oating the concrete surfaces with a bonding epo*y material.

3. 'aking holes in the concrete surfaces and plates.

4. &utting a layer of epo*y mortar on top of the plates with a ,mm thickness.

,. +ttaching the steel plates to the concrete using bolts.

The previous steps are illustrated in /ig 4.

Fig!4: Strengthening of ,e"m $% "dding steel l"tes


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#n some cases$ it is needed to reduce the load on the beam that needs strengthening before

implementing the previous steps$ either partial or complete unloading.

This is made by putting steel beams on top or below the concrete beams$ as shown in Fig5.

Fig!5: Reducing the lo"d on the $e"m using steel $e"m!

The following photos (8-11) were taken during strengthening an e*isting building% they present

the practical method of implementing some strengthening techniques.


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STRENGTHENING TECHNIQUES- R.C. SLAB•

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'any strengthening techniques are available depending on the purpose needed fromstrengthening. ome of those techniques are e*plained in details in the following sections.

.h% do we strengthen or retrofit $uildings6

• #ncrease in the applied loads.

• 'istakes or unsafe design.

• (.steel corrosion or insufficient number of bars


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• )racks in concrete or stress less than design stress.

• The settlement in the foundation is more than the allowable.

STREN*T-ENIN* OF R!C!S)A,S

#n some cases$ and due to increasing the applied loads on slabs or their unsafe design$ or

corrosion of the reinforcing steel bars$ or cracks in the slabs$ one of the following solutions

should be made!

1. #f the slab is unable to carry the negative moment and the lower steel is sufficient$ upper

steel mesh should be added with a new concrete layer.

2. #f the slab is unable to carry the positive moment or when the dead load 6that will be

added to the slab7is much less than the live load carried by the slab $ a new concrete layer

on the bottom of the slab should be added.

#n order to implement the previous solutions$ the following steps should be made as shown

in Fig 1and Fig 2!

1. (emoving the concrete cover.

2. )leaning the reinforcing steel bars using a wire brush or a sand compressor.

3. )oating the steel bars with an epo*y material that would prevent corrosion.

4.#f a high percent of corrosion was found in the steel bars$ a new steel mesh$ designed

according to the codes’ requirements$ must be added.

,.The new reinforcing steel mesh is then installed and fastened vertically to the slab of the

roof and hori"ontally to the surrounding beams $using steel dowels.

0. )oating the concrete surface with an appropriate epo*y material that would guarantee

the bond between the old and new concrete.

.8efore the epo*y dries$ the concrete is poured with the required thickness .+dditional

materials that would lower the shrinkage should be added to the concrete.


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Fig!1: Strengthening " sl"$ $% incre"sing its deth from $ottom


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Fig!(: Strengthening " sl"$ $% incre"sing its deth from to!

There are also some other techniques used for strengthening a reinforced concrete slab

such as!

• #ncreasing the shear bearing capacity of the slab by adding steel plates strengthened

by vertical screw bolts.

• trengthening of the slab by post stressed reinforcement.

• +dding steel beams.


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Fig1! Strengthening of RC ."lls

STRENGTHENING OF FOUNDATIONS•

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)olumns foundations need strengthening in the case of applying additional loads. 5idening

and strengthening of e*isting foundations may be carried out by constructing a concrete

acket to the e*isting footings. The new acket hould be properly anchored to the e*isting

footing and column neck in order o guarantee proper transfer of loads. The si"e of the

9acket9 shall be selected such that the average ma*imum foundation pressure does not

e*ceed the recommended allowable value. +ttention shall be given during construction in

rder that the e*cavations for the new 9ackets9 do not affect the e*isting dacent

foundations.

+n isolated footing is strengthened by increasing the si"e of the footing and the

reinforcement steel bars as follows!


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1. :*cavating around the footing

2. )leaning and roughening the concrete surface.

3. #nstalling dowels at 2,3-cm spacing in both directions using an appropriate epo*y

material.

4. /astening the new steel bars with the dowels using steel wires. The diameter and number

of steel bars should be according to the design.

,. )oating the footing surface with a bonding agent in order to achieve the equired bond

between old and new concrete.

0. &ouring the new concrete before the bonding agent dries. The new oncrete should contain

a nonshrinkage material.

The previous steps are illustrated in Fig 1.


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/ig.1! teps for strengthening foundations

The following photos (13-16) illustrate the practical way of acketing a footing by reinforced

concrete.

strengthening techniques of buildings

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