seismic retrofitting options of masonry & concrete structures

5/23/2018 Seismic Retrofitting Options of Masonry & Concrete Structures

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Retrofitting options of masonry

and concrete structures

Presented by:

Armaan Habib, Md. Salauddin and

Dipan Dhali.University of Asia Pacific (UAP)


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Contents.

Basic Concept of retrofitting.

Objective of the present work

Theoretical and numerical background

Experiment and result on masonry model.

Experiment and result on cylinder test.

Experiment and result on beam test.

Comparison of numerical and experimentalresults.

Conclusion.


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Basic concept of retrofitting

The seismic retrofitting of reinforced concrete buildings is the

process of strengthening and/or modifying existing old

(mostly) or new structures; not designed or insufficiently

designed to withstand seismic action.

The retrofitting techniques are mostly aimed at the older

structures due to their increased susceptibility to seismic

actions compared to newer structures even though these

structures meet codal requirements.


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Basic concept of retrofitting (Contd.)

Conventional retrofitting techniques also involve enlargement

of existing structural members e.g.: jacketing of columns

and/or beams.

Other retrofitting techniques may include construction of

shear walls, bracings and cross-lintels in the case of masonry

structures.


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Objective of the present work

Here in this particular research we have focused on the study

of change of strength and stiffness due to retrofitting.

Masonry structure was represented by wood model due to

limitation of shake table.

Concrete cylinders were wrapped with Fiber Reinforced

Polymer (FRP) to check for the change in failure stress due to

compression.

The beams were retrofitted by using epoxy and steel plate

before undergoing a 2-point loading and an axial

compression.


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Two methodologies of retrofitting are available so far, i)Qualitative methods and ii) Analytical methods.

The qualitative methods are based on the background information

available of the building and its construction site, visual inspection

reports and some non destructive tests.

The analytical methods are based on the consideration of the

capacity and ductility of buildings based on the available drawings.


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Using wire mesh in masonry walls is a widespread method of masonryretrofitting. Application of wire mesh increases the lateral strength capacity of

unreinforced masonry walls significantly. It consists of a galvanized iron mesh

fixed to the walls through nails or connector-links drilled through the wall

thickness and the mesh is covered by rich mix of cement-sand mortar in the ratio

of 1:3. Galvanizing of wire mesh is necessary to prevent corrosion.


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In the pictures above we can see the effectiveness of through lintels in

masonry structures under seismic actions. With through lintels the

structure more or less shows a rigid box like behavior.


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bandaging the walls/corners of masonry walls is alsopracticed as retrofitting option.


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(Contd.) The equation of dynamic equilibrium for earthquake forces has the

form in which inertia, damping and restoring forces balance theapplied force,

F(t) = m(t) + c(t) + ku(t)

On the basis of this earthquakes ground shaking magnitude, shake

table was designed; at UAP with a scale factor of 3.68. Therefore, factorized equation

Displacement, umax=El Centro/3.68. (2.2)

Acceleration, amax=aEl Centro* (TEl Centro/TShake)2 (2.3)

Zone Coefficient, Zmax=(a/ag)* (TEl Centro/TShake)2 (2.4)

Weight, Wreq.=Fmax/Zmax (2.5) TEl Centro= 40.96 sec


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(Contd.) The M-relationship of arbitrary RC sections was derived

numerically (Anam & Shoma 2002) by the application of

simple principles of Strength of Materials.

The applied loads are obtained directly from the bending

moment M at the midpoint section; i.e.,

M = PL/6 P = 6M/L

Moreover, the mid-span deflection of the beam is also

obtained from its midspan curvature; i.e., in the fully

elastic range

max= (23/1296) PL

3

/EI = (23/216) maxL2

= (1/9.4) maxL2

Therefore, max=PL/6EI

while in the fully plastic range

max= (1/8) maxL2


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(Contd.)

Effectively confined concrete displays increased strength and

ductility over unconfined concrete. Confined concrete also

has the ability to carry larger stresses and strains at ultimate

strength. This is evident in the stress-strain model proposed

by Saatcioglu and Razvi (1992) shown in Fig.


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Experiment on Masonry Model

Masonry Model Setup:

** The phenomenon of retrofitting on a masonry building wasportrayed onto a wooden model.

**Timber walls (1x1x0.5) were used instead of masonry walls; dueto limitation of shake table.

**Paper tape (3x1) was used to form the bond between the timberwalls.

**Tapes were used in 3 layers and throughout the length of thetimber walls.


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Bonding Test Apparatus:

Bonding of wood and tape was tested using the

bonding test apparatus. The bond force was required tocalculate the weight at which the walls would

dismantle on the shake table.

.


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** In the bond testing apparatus, the bond between wood and tape

required 13.672 lb (including self-weight) to be broken; for 3 layers of

tape.

** For through tapes along the length of the wooden walls, 28.672 lb

(including self-weight) was required to break the bond between tape

and wood.

Bonding Test


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Shake Table Tests:

**Wreq(weight required to be placed on the model walls

theoretically, to fail the bond between wood and tape on the

shake table) was calculated using numerical equations.

For 3 layers of bonding tape

Wreq21.81lb for 15sec shaking. (by using equation )

For Through layer bonding tape

Wreq45.73lb for 15sec shaking. (by using equation )

** Experimentally the values of Wreqwere found to be 25 lb for 3

layers of tape and 50 lb for through layer of tapes.


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(a) (b)Figure: (a) Bond Failure with 3 layers tape (b) Bond failure with through

layers on the shake table.

** Now, the tests were repeated using retrofit options. The methods

of retrofitting used were (i) using through lintels (seismic bands) and(ii) corner sewing of the walls.


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**Retrofitting applied in the

form of through lintels andcorner sewing.


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Results:Applying retrofitting (With Lintel/Seismic Band):

3 layer bonding tape

For 15 sec shaking the bond between the tape and wooden walls did not fail,

under Wreq= 25lb.

Through layer bonding tape


under Wreq= 50lb.

Applying retrofitting (with corner sewing of the walls)3 layers bonding tape


under Wreq= 25lb.

Through layer bonding tape

For 15 sec shaking, the bond between the tape and wooden walls did not fail,under Wreq = 50lb.

The experimental and theoretical values of Wreq were pretty much similar.


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Experiments on Concrete

Cylinders**The experiment works consist of tests on concrete cylinders

and jacketed cylinder; i.e. normal concrete cylinder specimens

were compared with ones retrofitted by FRP and jacketed

cylinders (resulting from jacketing of concrete cubes).

**The results compare the ultimate compressive strengths (fc)

and crushing strains (ult) of retrofitted and un- retrofitted

concrete specimens.

**Fiber Reinforced Polymer was used in two ways. In one way it

was wrapped along the full length of the cylinder and in another

way it was wrapped at only the mid-half length of the cylinder.


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Results:For Normal Cylinders:

fc(ksi)avg= 2.923 ksi and 0(x10-3)avg= 2.473.

For Cylinders Retrofitted with Fiber Reinforced Polymer:

fc(ksi)avg= 6.176 ksi and 0(x10

-3

)avg= 15.092 (fully confined).fc(ksi)avg = 3.283 ksi and 0(x10-3)avg = 2.603 (half confined).

For Jacketed Cylinders Retrofitted with Fiber Reinforced Polymer:

fc(ksi)avg= 4.456 ksi and 0(x10-3

)avg= 17.223 (fully confined).fc(ksi)avg= 2.21 ksi and 0(x10

-3)avg = 3.52 (half confined).


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Experiments on RC Beams

**10 RC beams (42x7x9) were cast withreinforcements 2 #5 bars at bottom and 2 #3 bars at

top. #3 stirrups @ 4c/c were used.


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Sample Arrangements:All 10 beams were tested under 2-point loading but:

**3 samples without retrofitting and without axial force. (S1,

S2, S3)

**2 samples with retrofitting and without axial force. (SR1,

SR2)

**3 samples without retrofitting and with axial force. (SA1,

SA2, SA3)

**2 samples with retrofitting and with axial force. (SAR1,

SAR2)

Steel plates with epoxy resin were used as retrofitting options

for beams as FRP proved to be a costly alternative.


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Load vs Deflection curve for SA2 Load vs deflection curve for SR2(With axial, without retrofitting) (Without axial, with retrofitting)

Load vs. deflection graph and compare (SAR2)

With Axial Force and With Retrofitting


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Conclusion:** Masonry walls should be checked for out-of-plane failure

criteria based on their (h/t) ratio. This is invariably the weakest

link in their seismic resistance. because of the instability causedto them due to the large overturning moments.

**For the case of cylinders, the compressive strength increased by up

to 111% (more than 2 times its initial strength) after full confinementwith fiber reinforced polymer. When the cylinders were confined to

only the mid 1/2 of length, the strength was found to increase by 12%.

Hence partial confinement is not effective.

**The jacketing of concrete cylinders did not produce expectedresults. It was expected that the compressive strength of the cubes

after jacketing would remain same/increase as that of the normal

cylinders. However this did not happen.


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Recommendations:

(i) improved shake table capacity so that larger models can be investigated.

(ii) effect of retrofitting on models having different (h/t) ratios.

(iii) For jacketing purpose the size of aggregate should be reduced.

(iv) The surface where jacketing is to be applied should be more rough so that bonding

improves

(v) The properties concrete used for jacketing purpose can be varied and the effect

studied.

(vi) The price of fiber reinforced polymer although is high, but it is a very effective tool

for retrofitting. However, if the structural member is not fully confined with fiber

reinforced polymer, the results are less effective.


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Recommendations (contd.)

vii)For beam tests, the axial load application system can be improved.

(viii)The beams were cast in a standing position. For beams cast in flat

positions, the results may improve.

(ix)Clear cover for beams cast in a standing position should be more thanwhat was given in this experimental work. This is to ensure proper tamping.

(x)Digital and more sensitive deflection meters.

(xi)Digital UTM machines.

(xii)Ultra slow motion cameras to observe the physical deflection pattern

more carefully.


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THANK

YOU

seismic retrofitting options of masonry & concrete structures

Documents

methodologies of retrofitting

basic concept of retrofitting

masonry model

newer structures

new structures

concrete cylinders

seismic action

lintels inmasonry structures