capacity of strengthened reinforced concrete columns
TRANSCRIPT
Port Said University
Faculty of Engineering
Civil Engineering Department
Capacity of Strengthened Reinforced
Concrete Columns
By
Eng. Khaled Mohamed Mahmoud Ahmed
Senior Engineer, Suez Canal Authority
Email: [email protected]
Tele: +201003915088
Prof. Dr. Kamal Gad SharobemProfessor of properties and strength of materials
Vice President of Suez Canal University for Community Service
and Environmental Development - Suez Canal University
Prof. Dr. Hassan Mohamed Hassan IbrahimProfessor of concrete structures
Faculty of Engineering
Port Said University
SUPERVISORS OF THESIS
Introduction Column definition : Structural element which transfers the loads
from slabs to foundation.
The reasons of columns strengthening:
1) Error at design or bad detailing.
2) Improper quality control measures (poor construction
materials and workmanship).
3) Unexpected loading, like earthquakes, strong winds,
impact and explosive loading.
4) Functional changes in the service of the structure.
5) Deterioration by time.
The difference between : repair-strengthening-repair and strengthening.
Concrete jacket Steel jacket FRP jacket
The techniques of columns strengthening:
The most common methods for columns' strengthening in Egypt are
concrete or steel jackets.
Structural engineers don't prefer using FRP jackets because of :
1-high cost 2-lacking of trained workers 3-weakness of fire protection.
There is still a need of design equations to refine the design of
strengthening.
So this work focused on the analysis of concrete and steel jackets to :
1- Get simple equations for design.
2- Perform experimental investigation.
3- Compare theoretical analysis by experimental work to validate the
refined equations.
The two types of columns jackets were used at sites :
Concrete jacket The cage
Reinforced concrete jacket:
Advantages:
economy, compatibility with the original concrete
substrate, and the ability to enhance durability and impact
fire protection.
Disadvantages:
the loss of floor space due to enlargement of the column
cross-section, and difficulties in casting and compacting.
Steel jacket:
Advantages:
increase the confinement, capacities, ductility and
stiffness. Steel jackets/cages have also advantageous due
to their light weight and insignificant increase in cross
sectional area of the column compared to concrete
jackets.
Disadvantages:
Compared to concrete jacket: More expensive and
weaker at fire protection.
The main objectives of this research are:
Study analytically and experimentally the efficiency of RC
columns strengthened by concrete and steel jackets.
To study the effect of direct and indirect load applied to jackets
(i.e. to simulate the situation where it is not feasible to connect
the jacket to the roof slabs/beams).
To define the expected failure modes.
To compare the predicted load carrying capacity with that
obtained from the current experimental works and that performed
by others.
To refine the design equations for concrete and steel jackets.
Outlines of Thesis
Chapter 1 Introduction.
Chapter 2 Literature review.
Chapter 3 Strengthening design methods.
Chapter 4 Experimental investigation.
Chapter 5 Theoretical investigation and
parametric study.
Chapter 6 Conclusion and recommendations.
Load transmitting between column and jacket.
Load transmitting between column and concrete jacket
Load transmitting between column and jacket.
Load transmitting between column and steel jacket
- 800 mm height.
- 100x100 mm cross section.
- The main reinforcement is 4Ø6
mm.
- seven square stirrups Ø4 mm
spread along the column height
starting at 50 mm from both ends
were used.
Experimental study
Concrete original column (core) :
Materials
Concrete and mortar :
- Concrete strength was 36 MPa .
- Mortar strength was 25.9 MPa .
Steel :
Nominal
diameter
(mm)
Yield strength
(MPa)
Tensile strength
(MPa) Elongation (%)
4 461.8 652.9 6.25
6 323 410.5 27
External angles 298.7 402.7 28.26
First trial
Experimental Program for Concrete Jacket:
Group Specimen No.Column
dimensions(mm)
Main
reinforcementStirrups
O 1,2,3 100x100 4Ø6 7Ø4
Specimens details of group (O)
Specimens details of group (A)
Experimental Program for Concrete Jacket:
Gro
up
Specimen No.
Column dimensions
after strengthening
(mm)
Jacket main
reinforcement
Jacket
stirrupsRemark
A 4,5,6 150x150 4Ø6 7Ø4 Square stirrups
Specimens details of group (B)
Experimental Program for Concrete Jacket:
Gro
up
Specimen No.
Column dimensions
after strengthening
(mm)
Jacket main
reinforcement
Jacket
stirrupsRemark
B 7,8,9 150x150 4Ø6 9Ø4 Square stirrups
Specimens details of group (C)
Gro
up
Specimen No.
Column dimensions
after strengthening
(mm)
Jacket main
reinforcement
Jacket
stirrupsRemark
C 10,11,12 150x150 4Ø6 7Ø4Jacket height shorter
than core by 50mm
Experimental Program for Concrete Jacket:
Failure mode
Original column failure Jacket failure of group (A)
Experimental Program for Concrete Jacket:
Failure mode
Jacket failure of group (B) Jacket failure of group (C)
Experimental Program for Concrete Jacket:
Strengthening
type Gro
up
Specimen numberMaximum capacity
( KN )
Experimental average
maximum capacity
( KN )
Percentage of
capacity increase
Reference
column (core)O
1 249
253.3 NA2 239
3 272
Concrete jacket
A
4 464
446.6 76.3%5 424
6 452
B
7 472
473.3 86.85%8 481
9 467
C
10 380
382 50.81%11 384
12 382
Experimental results of concrete jackets
Experimental Program for Concrete Jacket:
Strain measurement
Experimental Program for Concrete Jacket:
0
50
100
150
200
250
300
350
400
0.000 0.001 0.002 0.003 0.004 0.005 0.006
Strain
Lo
ad
,KN
original
concrete jacket
Axial load – Vertical strain relationship
Group ( C )
Experimental Program for Concrete Jacket:
0
50
100
150
200
250
300
350
400
0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014 0.0016
Horizontal strain
Lo
ad
,KN
original column
concrete jacket
Axial load – Horizontal strain relationship
Ultimate load ratio of concrete jacket
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Original group (A) group (B) group ( c )
Pu
/Pre
f
Experimental Program for Concrete Jacket:
Experimental Program for Steel Jacket (Cage)
Group Specimen No.Column
dimensions(mm)
Main
reinforcementStirrups
O 1,2,3 100x100 4Ø6 7Ø4
Specimens details of group (O)
Specimens details of group (D)
Gro
up
Specimen No. CoreCorner angles
size (mm)Strips plates(mm) No. of strips
Rem
ark
.
D 13,14,15As columns
at group O30x30x3
4 plates
100x30x36
Experimental Program for Steel Jacket (Cage)
Specimens details of group (D”)
Gro
up
Specimen No. CoreCorner angles
size (mm)Strips plates(mm) No. of strips
Rem
ark.
D” 19,20,21As columns at
group O30x30x3 4 plates 100x30x3 6
Jacket height
shorter than
core by 10mm
Experimental Program for Steel Jacket (Cage)
Gro
up
Specimen No. CoreCorner angles
size (mm)Strips plates(mm) No. of strips
Rem
ark
.
E 16,17,18As columns
at group O30x30x3 4 plates 100x30x3 8
Specimens details of group (E)
Experimental Program for Steel Jacket (Cage)
Failure mode
Jacket failure of group (D) Jacket failure of group (E)
Experimental Program for Steel Jacket (Cage)
Strengthening
type Gro
up Specimen
number
Maximum
capacity
( KN )
Experimental average
maximum capacity
( KN )
Percentage of capacity
increase
Original column
(core)O
1 249
253.3 NA
2 239
3 272
11 384
12 382
Steel jacket
(cage)
D
13 235
264.7 4.5%14 295
15 264
D" D" 340 340 34.22%
E 16 453 465.7 83.85%
Experimental results of steel jackets
Experimental Program for Steel Jacket (Cage)
Strain measurement
Experimental Program for Steel Jacket (Cage)
0
100
200
300
400
500
600
700
0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010
Strain
Lo
ad
,KN
original
steel jacket
Axial load – Vertical strain relationship
Group ( D” )
Experimental Program for Steel Jacket (Cage)
0
100
200
300
400
500
600
700
0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004
Horizontal strain
Lo
ad
,KN
original column
steel jacket
Axial load – Horizontal strain relationship
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Original group (D) group (D") group ( E )
Pu
/Pre
f
Ultimate load ratio of steel jacket
Experimental Program for Steel Jacket (Cage)
Refined Equations
Concrete jacket
By first principle method:
Pu = (4*w1*L + 4*w2*L*n)*tan
ᶱ
Steel cage
Adam Equation:
Pu = 0.85 * Ac* fc+ As * fys
+ K * fl* Ac+ NL
Where
w1 confinement pressure at strip 1
w2 confinement pressure at strip 2
L original column width
n number of strips
Tan (ᶱ) friction factor, its range between
1.1 and 1.5
Where
Ac cross section area of concrete
As cross section area of longitudinal
reinforcement
Fc compressive strength of concrete
Fys yield strength of reinforcement steel
F1 confinement pressureN
L axial load supported by the cage
Comparison of experimental and theoretical results using concrete jackets :
Strengthening
type Gro
up
Experimental
average maximum
capacity
( KN ) Per
cen
tag
e o
f
cap
acit
y
incr
ease Analytical
capacity (KN)
Pexperimental
PtheoreticalRemarks
Original
column (core)O 253.3 NA NA NA NA
Concrete
jacket
A 446.6 76.3% 373.7 1.2 20%
additional
capacityB 473.3 86.9% 392 1.2
C 382 50.8% 368.8 1.03F.O.S
needed
Comparison of experimental and theoretical results using steel jackets :
Strengthening
type Gro
up
Experimental
average maximum
capacity
( KN )
Analytical
capacity
(KN)
Per
cen
tag
e o
f
cap
acit
y
incr
ease
Remarks
Original
column (core)O 253.3 NA NA NA
Steel jacket
(cage)
D 264.7 386.6 4.5% 0.68Buckling
failure
D" 340 346 22.4% 0.98 F.O.S needed
E 465.7 464.8 24.4% 1 F.O.S needed
luanalytica
erimentalu
P
P exp
Analytical parameter study1- Concrete jacket :
0.00
20.00
40.00
60.00
80.00
100.00
120.00
140.0030*3
0
40*4
0
50*5
0
60*6
0
70*7
0
80*8
0
90*9
0
100*1
00
110*1
10
120*1
20
130*1
30
140*1
40
150*1
50
160*1
60
170*1
70
180*1
80
190*1
90
200*2
00
Concrete jacket dimension,cm
Eff
icie
ncy%
c20*20 c25*25 c30*30
c40*40 c50*50
40
60
80
100
120
140
160
180
200
Ø6 Ø8 Ø10 Ø12
Diameter of stirrups at upper region of column
Eff
icie
ncy%
20*20
25*25
30*30
40*40
20
30
40
50
60
70
80
90
100
6Ø6 8Ø6 9Ø6 10Ø6
stirrups of jacket upper region
eff
icie
ncy (
%)
20*20 25*25
30*30 40*40
85
90
95
100
105
110
1.1 1.2 1.3 1.4 1.5
tan (angle of fricition)
Eff
icie
ncy %
Stirrups of jacket near loaded zone Diameter of stirrups near loaded zone
2- Steel jacket :
0
500
1000
1500
2000
40x4
0x4
50x5
0x5
60x6
0x6
70x7
0x7
80x8
0x8
90x9
0x9
100x
100x
10
angles of the cage, mm
Pu
, K
N
1100
1110
1120
1130
1140
1150
1160
1 3 5 8 10
Thickness of cage batten plates, mm
Pu
, K
N
1050
1100
1150
1200
1250
1300
1350
100 200 300 410 500
Distance between batten plates of cage, mm
Pu
, K
N
1080
1100
1120
1140
1160
1180
1200
1220
0.1 0.2 0.3 0.4 0.5
Friction coefficient
Pu
, K
N
Analytical parameter study
Conclusions & Recommendations
1- The length of concrete jacket should by equal to the original column's length if possible to
achieve the best benefit of the concrete jacket. and the capacity of original column will be
increased up to 87% of the original column capacity by confinement.
1- The length of cage's angles should be shorter than original column's length to achieve the
benefit of cage. If the length of cage angles is equal to column length, the applied load will
transmit to the angles as a vertical load and this lead to angle yielding by buckling and
cause failure before benefiting of cage confinement.
2- The stirrups near loaded area should be intensified to a distance of (1-1.5) the jacket
width. The efficiency of jacket will be increased by decreasing the distance between
stirrups for the tested specimens.
2- The recommended design equation for steel cage is Adam’s equation:
Pu = 0.85 * Ac* fc+ As * fys + K * fl* Ac+ NL
3- The increase of stirrups (number/diameter) is more important than (increase of longitudinal
bars or increase of jacket cross-section).
4- The recommended design equation for concrete jacket is : Pu = (4*w1*L + 4*w2*L*n)*tan ᶱ
For concrete jackets:
For steel cages:
Recommendations and Future Work
As a result of this work, several items should be considered in future
research :
1- Study the design of concrete and steel jackets for rectangular and
circular columns as well as L and T – section columns.
3- Study strengthened columns which subjected to eccentric loads.
5- Performing experimental study to validate the present parameter
study in this thesis.
6- Study the efficiency of strengthening of loaded columns by
percentage from ultimate capacity using jackets without removing its
loads during strengthening process.
2- Study experimentally the strengthening using full scale or nearly full
scale specimens.
4- Study jacketing from 2 sides or 3 sides of strengthened columns.