Seismic Retrofit of Non-Ductile Concrete Frames

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29 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Seismic Retrofit of Non
-
Ductile Concrete Frames

Using Diagonal
Prestressing


By A.
Molaei

and M.
Navidpour

Supervisor: Dr. Murat
Saatcioglu

Department of Civil Engineering, University of Ottawa

Background

Performance

of

reinforced

concrete

structures

during

recent

earthquakes

indicate

that

the

majority

of

existing

structures

suffer

damage

from

non
-
ductile

reinforced

concrete

frame

structures
.

Most

of

these

structures

were

built

prior

to

the

enactment

of

modern

seismic

codes

or

built

in

locations

were

code

enforcing

is

difficult

to

achieve
.

Since

it

is

not

economically

feasible

to

demolish

and

reconstruct

the

entire

inventory

of

seismically

deficient

structures,

seismic

retrofitting

remains

to

be

the

only

viable

option

to

ensure

satisfactory

performance

of

structures

during

earthquakes
.

Previous Research

Seismic

retrofit

of

non
-
ductile

structures

can

be

achieved

either

through

deformation

control

of

the

entire

system

or

by

retrofitting

individual

elements

for

strength

and

ductility

enhancements
.

An

innovative

technique

for

lateral

bracing

of

reinforced

concrete

structures

is

being

developed

at

the

University

of

Ottawa,

involving

diagonal

prestressing

of

frames

by

means

of

prestressing

strands
.

The

prestressing

ensures

sufficiently

high

diagonal

tension

capacity

against

lateral

forces,

while

releasing

initial

tension

under

diagonal

compression,

thereby

controlling

lateral

drift

demands

effectively

with

little

steel

area

and

without

much

obstruction
.

Initial

pilot

tests

on

reinforced

concrete

frames

with

masonry

infill

walls

have

been

completed
.

The

initial

results

indicated

that

both

strength

and

deformability

of

frames

can

be

enhanced

through

diagonal

prestressing
.

The

technique

requires

further

investigation

prior

to

being

used

in

practice
.


Objectives and Scope

The

objective

of

the

research

project

is

to

develop

diagonal

pressing

as

an

economically

viable

and

structurally

sound

seismic

retrofit

methodology
.

The

scope

involves

tests

of

large
-
scale

reinforced

concrete

frames,

prestressed

with

strands

of

different

prestress

levels

for

lateral

drift

control
.

Two

frames

with

3
.
5
m

span

length

and

3
.
0
m

column

height

will

be

tested

under

simulated

seismic

loading
.

Fig
.

1

illustrates

a

frame

specimen

ready

for

testing
.

The

prestress

level

of

the

diagonal

cables

will

determine

the

drift

level

at

which

the

full

capacity

of

the

frame

is

reached
.


This

method

emphasizes

on

increasing

the

lateral

load

resisting

capacity

and

controlling

the

frame

deformation
.

The

benefit

of

such

a

system

becomes

obvious

in

case

of

retrofitting

an

existing

structure

where

the

cables

could

be

applied

to

certain

bays
.

Unlike

other

retrofit

methods

such

as

FRP

jacketing

that

relies

on

the

ductility

of

entire

structure

which

requires

improvement

of

the

entire

structure
.


Experimental Results

The

experimental

phase

of

the

project

was

finished

in

Summer

2011
.

The

results

indicated

substantial

increase

in

lateral

load

resistance
.

The

magnitude

of

the

added

capacity

had

direct

correlation

with

the

number

of

diagonal

cables

used,

as

shown

in

Figs
.

2

and

3
.

Also,

depending

on

the

prestress

level,

a

higher

load

resistance

could

be

achieved

at

a

lower

drift

by

adding

the

prestress

load
.

The

test

results

showed

an

increase

of

lateral

load

resisting

capacity

from

220

kN

for

a

non
-
retrofitted

frame

to

415

kN

and

525

kN

for

retrof i tted

f rames

wi th

one

and

two

prestressed

di agonal

cabl es

respecti vel y
.

For

the

case

where

the

f rame

was

retrof i tted

wi th

two

non
-
prestressed

di agonal

cabl es,

the

f rame

resi sted

536

kN

lateral

force
.

The

slight

difference

between

the

two

results

could

be

explained

by

the

fact

that

the

cables

ruptured

at

drift

ratios

and

the

second

case

ruptured

at

a

drift

level

closer

to

the

drift

ratio

of

the

peak

capacity

of

the

concrete

frame
.

This

method

showed

satisfactory

results

and

has

the

potential

to

be

used

in

practice
.

The

designer

has

the

option

of

fine
-
tuning

the

behaviour

by

selecting

the

prestress

force
.

Fig. 1 A typical test specimen with
diagonal
prestressing

Fig. 2 Hysteretic response of
retrofitted frame

Fig. 3 Lateral load resisting
components of frames