Calibration of AASHTO LRFD for Filled Grid Decks Based on Historical Performance Christopher Higgins and O. Tugrul Turan Christopher Higgins and O. Tugrul Turan School of Civil and Construction Engineering School of Civil and Construction Engineering Oregon State University Oregon State University and and Mark Kaczinski and Phil Gase Mark Kaczinski and Phil Gase Bridge Grid Flooring Manufacturing Association Bridge Grid Flooring Manufacturing Association International Bridge Conference June 8, 2011
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Calibration of AASHTO LRFD for Filled Grid Decks Based on Historical Performance Christopher Higgins and O. Tugrul Turan School of Civil and Construction.
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Calibration of AASHTO LRFD for Filled Grid Decks Based on Historical Performance
Christopher Higgins and O. Tugrul TuranChristopher Higgins and O. Tugrul TuranSchool of Civil and Construction EngineeringSchool of Civil and Construction Engineering
Oregon State University Oregon State University andand
Mark Kaczinski and Phil GaseMark Kaczinski and Phil GaseBridge Grid Flooring Manufacturing AssociationBridge Grid Flooring Manufacturing Association
International Bridge ConferenceJune 8, 2011
•Widely used in practice
•Light weight compared to conventionally reinforced decks
•Two way bending (orthotropic behavior)
Source: www.bgfma.org
Source: www.bgfma.org
Introduction & Background
Main Bars (Strong Direction) Cross Bars (Weak Direction)
Introduction & Background
4 4 4
4 2 2 42 ( , )x y
w w wD H D p x y
x x y y
1 2 xyH D D
2 2
12 2( )x x
w wM D D
x y
2 2
12 2( )y y
w wM D D
y x
2
2xy xy
wM D
x y
•Orthotropic Thin Plate Theory
•Non-homogenous biharmonic equation.
•Stiffnesses can be determined experimentally
1
: Flexural rigidity in the strong direction
: Flexural rigidity in the weak direction
: Torsional rigidity contribution from
the strong and the weak direction rigidities
: Torsional rigidity
( ,
x
y
xy
D
D
D
D
w x ) : Deflection
( , ) : Applied transverse load in
the Cartesiancoordinate system
y
p x y
Introduction & Background
4 4 4
4 2 2 42 ( , )x y
w w wD H D p x y
x x y y
1 2 xyH D D
2 2
12 2( )x x
w wM D D
x y
2 2
12 2( )y y
w wM D D
y x
2
2xy xy
wM D
x y
• D = 0, plate acts like a one –way slab or beam.• D = ∞, plate behaves like a collection of separate strips.
D = 0D = 0 D = ∞D = ∞
D = 2D = 2
Introduction & Background
AASHTO-LRFD (2004) section 4.6.2.1.8
Higgins 2003, Higgins 2004
x yH D Dx yH D D x yH D D, ,
Introduction & Background
•One-way slab, (Prior to AASHTO-LRFD, 1994)
•Orthotropic Thin Plate Theory (AASHTO-LRFD, 1994) , Single patch at the center
•Orthotropic Thin Plate Theory (AASHTO-LRFD, 2004),
Tandem axle and multiple patches,
Fatigue Limit State
Deflection equations
0.25 20ln 12.0 35transverseM ClpD S
0.29 0.46150 ln 12.0 1908parallel
lM Cp D S D
1100 2.5
IM Pl
Introduction & Background
0.25 20ln 12.0 35transverseM ClpD S
0.29 0.46150 ln 12.0 1908parallel
lM Cp D S D
1100 2.5
IM Pl
:Strong direction moment,
m. bars transverse to traffic dir.transverseM
:Strong direction moment,
m. bars parallel to traffic dir.
parallelM
: X
Y
DDD
: Continuity Factor (0.8 for continuous spans)
(1.0 for simply supported)
C
L, S: Span Length
C=0.8
C=1.0
Introduction & Background
•Many of the decks were constructed more than 30 years ago and AASHTO-LRFD(2004) not calibrated against historically successful performance
•BGFMA selected 26 decks, design details and supporting information provided
•Min. 10; max. 51 years in service.
Introduction & Background
Comparison of AASHTO LRFD 2004 and AASHTO LRFD 1994
Moment main bars transverse to traffic
X
Y
DD
D Region generally used in practice
Strength Limit State Comparison
Comparison of AASHTO LRFD 2004 and AASHTO LRFD 1994
Moment main bars parallel to traffic
X
Y
DD
D Region generally used in practice
Strength Limit State Comparison
Comparison of AASHTO LRFD 2004, AASHTO LRFD 1994 and Table A4
AASHTO-LRFD (2004), AASHTO-LRFD (1994) moment values for D=1.0 and C=0.8, and AASHTO-LRFD (2004) deck slab design table positive moment values (A4)
Span Length (in)
Mo
me
nt
(kip
-in
/in)
40 60 80 100 120 140 160 1802.5
5
7.5
10
12.5
15
17.5
20AASHTO LRFD Table A4-1 (Multiplied by =1.75)AASHTO LRFD 2004(Perpendicular to traffic) (C=0.8)AASHTO LRFD 2004 (Parallel to traffic) (C=0.8)AASHTO LRFD 1994 (Perpendicular to traffic) (C=0.8) (Design Truck)AASHTO LRFD 1994 (Parallel to traffic) (C=0.8) (Design Truck)
Comparison of AASHTO LRFD 2004, AASHTO LRFD 1994 and Table A4
AASHTO-LRFD (2004), AASHTO-LRFD (1994) moment values for D=1.0 and C=0.8, and AASHTO-LRFD (2004) deck slab design table negative moment values (A4).
Span Length (in)
Mo
me
nt
(kip
-in
/in)
40 60 80 100 120 140 160 1800
5
10
15
20
25AASHTO LRFD Table A4-1 (Multiplied by =1.75)AASHTO LRFD 2004(Perpendicular to traffic) (C=0.8)AASHTO LRFD 2004 (Parallel to traffic) (C=0.8)AASHTO LRFD 1994 (Perpendicular to traffic) (C=0.8) (Design Truck)AASHTO LRFD 1994 (Parallel to traffic) (C=0.8) (Design Truck)
Comparison of AASHTO LRFD 2004 and AASHTO LRFD 1994
Strength Limit State Comparison, 26 Decks
AASHTO-LRFD-1994 Moment (Kip-in/in) (Design Truck)
Theoretical spans were determined•Strength: C=1.0; M+ only with first yeild limit
•Deflection: AASHTO-LRFD Prescribed deflection
•Fatigue: C=1.0; AASHTO-LRFD Prescribed fatigue SR (Strength/3) to limit of 5 ksi
•L/800 was the most conservative
•New service level stresses were determined for L/800
Limits on Possible Span Lengths
• Current AASHTO-LRFD moment provisions are not substantially higher than those specified for RC decks in traditional design
• Suite of decks not controlled by the strength or positive fatigue moment
• All 26 decks are limited by negative fatigue moment
• Negative fatigue moment can be reduced by a factor of 2.2 (for design say 2) for transverse to traffic and 2.8 (for design say 2.5) for parallel to traffic cases
• Additional analyses and/or tests around the negative moment region may help identify additional load distribution that may reduce stress range over the support for fatigue design
• Design approach would be: use the current design for Strength I with C=1.0, detail to obtain infinite life for positive fatigue moment, and limit the service level deflections to L/800