GT STRUDL Users Group
22nd Annual Meeting & Training
Seminar
June 24, 2010
Practical Modeling Technique for Transfer Length
Chris Carroll, Ph.D.
Assistant Professor
Department of Civil Engineering
University of Louisiana at Lafayette
Overview
Introduction
Background
Test Speciemens
Top-strand Effect
GT STRUDL Model
Practical Modeling Technique
for Transfer Length
Background
Development length (standard reinforcing steel)
• The length required to anchor the reinforcing to fully
develop the stress in the reinforcing at the nominal
moment capacity of the member (AASHTO)
• The length of embedment required to prevent slip
between reinforcing and the surrounding concrete
when that reinforcing is placed in tension (or
compression)
Practical Modeling Technique
for Transfer Length
Background
Development length (standard reinforcing steel)
Location of the bar
Coating of the bar
Required stress in steel
Size of the bar
 3 f  
y
t e s
Ld  
 40  fc' cb d Ktr
b




 db


Diameter of the bar
Cover and confinement
Effect of lightweight concrete
Concrete Strength
Practical Modeling Technique
for Transfer Length
Background
Development Length
• The length required to
anchor the strand to
fully develop the stress
in the strand at the
nominal moment
fps
capacity of a member
 f ps  f se 
 f 
Ld   se  db  
 db
3000
1000




2 

Ld    f ps  f se  db
3 

fse
ACI
AASHTO
Lt
Practical Modeling Technique
for Transfer Length
Lfb
Ld
Background
Transfer Length
• The bonded length of
strand required to
transfer the prestress
force in the strand to the
surrounding concrete
Lt = 50db
ACI
Lt = 60db
AASHTO
fse
 f 
 f  f se 
Ldt   fsese ddbb   ps
 db
 3000
3000 
1000


Lt
Practical Modeling Technique
for Transfer Length
Background
Transfer Length (Code Provisions)
Unconservative
Unconservative
Practical Modeling Technique
for Transfer Length
Background
Top-strand Effect
Deformed Bar
> 12”
• Provisions exist for
development length of
deformed bars
• Ld multiplied by 1.3 (ACI)
and 1.4 (AASHTO) with
> 12 inches of fresh
concrete below the bar
• Provisions do not exist
for the development or
transfer length of
prestressing strands
Practical Modeling Technique
for Transfer Length
Background
Top-strand Effect
a
a
b
b
Practical Modeling Technique
for Transfer Length
Background
Top-strand Effect
– Is top-strand effect a factor of the amount of
concrete beneath the strand?
– New hypothesis: Top-strand effect may be a factor
of the amount of concrete above the strand rather
than the amount below or a combination thereof
12 ft
12 ft
Block A
Block B
Practical Modeling Technique
for Transfer Length
Test Speciemens
T-beams
30 in.
24 in.
5 in.
24 in.
3 in.
2 in.
4 in.
4 in.
4 in.
17 in.
24 in.
19 in.
2 in.
2 in.
2 in.
8 in.
8 in.
Small
Medium
Large
½” f regular
½” f special
0.6” f
8 in.
Practical Modeling Technique
for Transfer Length
Test Specimens
Inverted
Normal
Normal
Normal
Inverted
A
B
A
B
300 ksi
270 ksi
Inverted
Practical Modeling Technique
for Transfer Length
Test Specimens
T-beams
Practical Modeling Technique
for Transfer Length
Test Specimens
Top-strand blocks
12 ft
12 ft
Block A
Block B
4”
A
2”
5”
4”
B
24”
5”
C
F
5”
D
14”
5”
G
5”
E
2”
5”
H
2”
Practical Modeling Technique
for Transfer Length
2”
Test Specimens
Top-strand blocks
Five
Strand
Blocks
Single
Strand
Blocks
Three
Strand
Blocks
Practical Modeling Technique
for Transfer Length
Test Specimens
Top-strand blocks
Practical Modeling Technique
for Transfer Length
Test Specimens
Top-strand blocks
Practical Modeling Technique
for Transfer Length
Test Specimens
Top-strand blocks
Practical Modeling Technique
for Transfer Length
Test Specimens
Transfer Length
100 mm
spacing
50 mm
spacing
Practical Modeling Technique
for Transfer Length
Test Specimens
Transfer Length
≈ 30,000 measurements
100 mm
spacing
50 mm
spacing
Practical Modeling Technique
for Transfer Length
Test Specimen
Transfer Length
Practical Modeling Technique
for Transfer Length
Test Specimens
Bond/Shear Failure
Practical Modeling Technique
for Transfer Length
Test Specimens
Bond/Shear Failure
Practical Modeling Technique
for Transfer Length
Test Specimens
Bond/Shear Failure
Practical Modeling Technique
for Transfer Length
Test Specimens
Bond/Shear Failure
Practical Modeling Technique
for Transfer Length
Test Specimens
Bond/Shear Failure
Practical Modeling Technique
for Transfer Length
Test Specimens
Bond/Shear Failure
Practical Modeling Technique
for Transfer Length
Top-strand Effect
Transfer Length
•
•
•
•
•
•
•
•
Influence of Release Method
Influence of Strand Strength
Influence of Strand Diameter/Area
Influence of Effective Prestress
Influence of Concrete Strength
Influence of Time
Influence of Casting Orientation
Proposed Transfer Length Equation
Practical Modeling Technique
for Transfer Length
Top-strand Effect
Transfer Length (Influence of Casting Orientation)
Practical Modeling Technique
for Transfer Length
Top-strand Effect
Transfer Length (Influence of Casting Orientation)
Amount of Concrete Above
Amount of Concrete Below
Practical Modeling Technique
for Transfer Length
Top-strand Effect
Transfer Length (Influence of Casting Orientation)
Same Amount of Concrete Above
Same Amount of Concrete Below
Practical Modeling Technique
for Transfer Length
Top-strand Effect
Transfer Length (Influence of Casting Orientation)
Amount of Concrete Above
Amount of Concrete Below
Practical Modeling Technique
for Transfer Length
Top-strand Effect
Transfer Length (Proposed Transfer Length Eq.)
C
Z
1
Transfer Length
12 in.
A
X
f si
f ci'
B
db
dcast  24
dcast  24
Amount of Concrete Above the Strand
Practical Modeling Technique
for Transfer Length
dcast  0
Top-strand Effect
Transfer Length (Proposed Transfer Length Eq.)
Lt  35
f si
f c'
db
 24  dcast 

2
40
=2
1
R2 = 0.206
0.176
Practical Modeling Technique
for Transfer Length
Top-strand Effect
Transfer Length (End-slip)
Practical Modeling Technique
for Transfer Length
Top-strand Effect
Conclusions
• Top-strand effect was more dependent on
the amount of concrete cast above the
strand
• On average Lt increased ½ in. for every 1 in.
reduction in the amount of concrete cast
above the strand
Lt  35
f si
f
'
ci
db
24  dcast 


40
Practical Modeling Technique
for Transfer Length
2
GT STRUDL Model
Practical Modeling Technique
for Transfer Length
GT STRUDL Model
Practical Modeling Technique
for Transfer Length
GT STRUDL Model
Practical Modeling Technique
for Transfer Length
GT STRUDL Model
$$===================================================
$$ CONCRETE ELEMENT DATA
$$===================================================
TYPE PLANE STRESS
GENERATE 6 ELEMENTS ID 'AB-1', 1 FROM 'A1',1 TO 'A2',1 TO 'B2',1 TO 'B1',1
GENERATE 6 ELEMENTS ID 'BC-1', 1 FROM 'B1',1 TO 'B2',1 TO 'C2',1 TO 'C1',1
ELEMENT PROPERTIES TYPE 'IPLQ' THICK 4
'AB-1' TO 'AB-6'
'BC-1' TO 'BC-6‘
CONSTANTS
E 3949 'AB-1' TO 'AB-6‘ –
'BC-1' TO 'BC-6‘
A1
G 1688 'AB-1' TO 'AB-6' 'BC-1' TO 'BC-6‘
B1
POI 0.17 'AB-1' TO 'AB-6' 'BC-1' TO 'BC-6'
A2
AB-1
B2
BC-1
C1
C2
A3
AB-2
B3
BC-2
C3
A4
AB-3
B4
BC-3
C4
Practical Modeling Technique
for Transfer Length
A5
AB-4
B5
BC-4
C5
A6
AB-5
B6
BC-5
C6
A7
AB-6
B7
BC-6
C7
GT STRUDL Model
$$==================================================================
$$ SPECIFY JOINT COORDINATES
$$==================================================================
GENERATE 5 JOINTS ID 'C1',1 X 0. DIFF -1 2 AT 1 2 AT 2. Y 2. Z 0.
C1
C2
C3
C4
C5
(-1,2)
(0,2)
(1,2)
(3,2)
(5,2)
Practical Modeling Technique
for Transfer Length
GT STRUDL Model
$$==================================================================
$$ SPECIFY STRAND PROPERTIES
$$==================================================================
TYPE PLANE TRUSS
GENERATE 4 MEMBERS ID 'STRND-0',1 FROM 'Cd0', 1 TO 'Cd1'
MEMBER PROPERTIES PRISMATIC AX 0.153
'STRND-0' TO 'STRND-3'
Cd0
Cd1
STRND-0
Cd2
STRND-1
Cd3
STRND-2
Practical Modeling Technique
for Transfer Length
Cd4
STRND-3
GT STRUDL Model
$$===================================================
$$ SPECIFY BOND ELEMENT PROPERTIES
$$===================================================
ELEMENT INC
'BOND-1' 'Cd1'
'BOND-2' 'Cd2'
'BOND-3' 'Cd3'
'BOND-4' 'Cd4'
'C1'
'C2'
'C3'
'C4'
NONLINEAR SPRING PROPERTIES
CURVE 'BOND' FORCE VS DISPL
0.0 0.0 -50.0 -1.0
END
250 kip/in.
200 kip/in.
150 kip/in.
ELEMENT PROPS
'BOND-1' TO 'BOND-4' TYPE 'NLS'
NONLINEAR SPRING ELEMENT DATA
STIFFNESS
'BOND-1' TO 'BOND-4' X CURVE 'BOND'
END
Practical Modeling Technique
for Transfer Length
100 kip/in.
50 kip/in.
GT STRUDL Model
P   A   EA
  T
P  T EA


P   1100  6.5 x106  28500  0.153  31.2
$$==================================================================
$$ SPECIFY TEMPERATURE LOADINGS
$$==================================================================
LOADING 'TRANSFER' '-1100 TEMPERATURE CHANGE'
MEMBER TEMPERATURE LOADS
'STRND-0' TO 'STRND-3' AXIAL -1100
Cd0
Cd1
STRND-0
Cd2
STRND-1
Cd4
Cd3
STRND-2
Practical Modeling Technique
for Transfer Length
STRND-3
GT STRUDL Model
•
•
•
•
4x4 in. 12 ft concrete prism (k = 50 kip/in.)
4x4 in. 12 ft concrete prism (k = 50 kip/in.)
4x4 in. 12 ft concrete prism (k = 250 kip/in.)
Excel Spreadsheet
Practical Modeling Technique
for Transfer Length
GT STRUDL Model
99% max force
Practical Modeling Technique
for Transfer Length
GT STRUDL Model
Practical Modeling Technique
for Transfer Length
GT STRUDL Model
Practical Modeling Technique
for Transfer Length
GT STRUDL Model
•
•
•
•
•
4x24 in. 12 ft concrete block (k = variable)
17 in. deep T-beam with eccentric strands
17 in. deep T-beam with eccentric strands
8 ft deep 96 ft long I-beam (End-zone)
8 ft deep 96 ft long I-beam (End-zone)
???
Questions
Practical Modeling Technique
for Transfer Length