Behavior and Design of Link Slabs for Jointless Bridge Decks
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Jointless Bridge Decks
Tags
Transcript
ehavior
and Design
of
Link
Slabs
for
Jointless
ridge Decks
Alp Caner Ph.D.
Structural Engineer Parsons Brinckerhoff
Quad
e and
Doug
l
as
N
ew
York
New
York
Paul Zia Ph.D.
P E
Distinguished University Professor
of
Civil
Engineering
Eme
ritu
s)
North
Ca
rolina State University Raleigh
North
Carolina
68
Maintenance
of
bridge
deck
joints
is a
costly
problem
Debris accumulation
in
the joints can restrain
deck
expansion caus
in
g
und
es
ir
able forces
in
the
deck and
damage to the structure. Water leaking through the joints is a major
ca
use
for
the deterioration
of
bridge girder bearings
and
supporting
structures Therefore
elimination
of
deck joints at the supports
of
multispan bridges wi
ll
reduce the cost
of
const
ru
ction and maintenance.
This
paper presents the results
of
a test program to
in
vestigate the behavior
of
link slabs connecting two adjacent simple-span girder
s
and
proposes a simple
method
for
designing the link sl
ab.
To
illustrate the proposed design method three design examples
are
included.
M
any
highway bridges
are
designed as multiple simple
span composite structures that
utilize either steel
or
prestressed con
crete girders
and
a
cast in place
concrete deck spanning from one pier
(or bent) to another.
At
each end
of
the simple-span deck, a
joint
is provided for
deck movement
due to temperature, shrinkage, and creep effects. Bridge
deck joints
are a persistent
and
costly maintenance
problem.
Water leaking through the joints is a
major
cause
for
the
deterioration
of
bridge girder bearings and supporting
structures. Debris accumulation in the
joints
restrains
deck expansion
and
causes
damage
to the bridge.
Joints
and bearings are expensive to install and maintain. Therefore, the
cost
of
construction
and
maintenance
for
a
bridge
can be greatly reduced
if
the number
of
deck
joints
in multi-span bridges can be minimized.
It should
be noted
that when the
deck joints are removed and replaced
by a jointless deck, fine cracks can be
expected
to
develop
in
the
jointless deck and
in many
cases water
may
still leak through the fine cracks
. However, the situation
s
preferable to that
of
jointed decks. During the last several years, bridge
engineers have designed
different
types
of
jointless bridge decks. Integral bridges are jointless and designed
as single-
or
multiple-span continuous
bridges
with
capped pile stub type
abutments. Burke -
3
has discussed the
attributes
and
limitations
of
integral bridges. The use
of
prestressed
con-
crete pile
s
in
integr
al
abutment
PCI
JOURN L
bridges has been discussed
by
Kamel et al. Jointless bridge decks with continu
ous girders are
commonly
used in many states. Wasserman
5
and Loveall
6
have described their extensive experience with such bridges in Tennessee.
Their experience included both bridge rehabilitation and new construction.
A comprehensive study
of
jointless
decks with continuous girders
was
conducted
by
Oesterle et
aU
In their study, the precast, prestressed girders
were made continuous to
resist
live load by the use
of
continuity steel and end diaphragms at the bridge piers.
Their design recommendations have been used
by
many state highway departments; however, the required deck reinforcement tends to be excessive. In actual practice, most bridge engi
neers have used a smaller amount
of
reinforcing steel based on their own
judgment and experience. In addition,
the end diaphragm is a difficult con
struction detail to execute
in
the field. Bridge deck joints can also be elimi
nated
by
making the deck continuous while keeping the girders as simplespans.
The
section
of
the
deck
connecting the two adjacent simple-span
girders
is
called the link slab.
8
Jointless bridge decks supported by simple-span girders have been used both
in
the United States and abroad.
9ã
Two such examples are shown
in
Fig. 1. It
is
noted that end diaphragms sep
arated
by
two layers
of
smooth roofing paper are used
by
the Florida DOT but no diaphragms are used
by
the Texas
DOT.
By
eliminating
the
end
di-
aphragms, this construction detail is greatly simplified.
In
1981, Zuk
12
studied the concept
of
jointless bridge decks built on multiple
simply-supported girders. He analyzed the effects
of
expansion and contraction
of
the jointless deck and considered the interactive forces between the
girders
and
the
deck.
Although
the concept seemed promising, it was not
used
in
any actual applications.
In
the late 1980s, Gastal and Zia
13
described the results
of
a finite element method
of
analysis for jointless
bridge decks
supported
by
simple-
span girders.
The
analysis accounted for the nonlinear material properties, cracking
of
concrete, creep, shrinkage,
May June
1
998
Prestressed
Concrete
Panel
Interior
Bent Center
Line
a)
Texas
·
-----
r_
_____
r
8
4
Deck
Slab
with
li
Bus
6
o.
.
top
t
bottom
End
or
1
Presu-essed
I
Beam =
F
I.
I
I · I
r
I
enter
line
:
:
11C
i
aphraJm
I I
I
epuate
Di
ap
Beams
with
t
0 3
lbslsq.
hragms and
wo
layers
of
d.
smooth
roofin1
paper
b) Florida
ig
. 1. Typical jointless bridge deck.
Note:
1
in.=
25.4 mm; 1 lb per
sq
yd
= 0.543 kg/m
2.
temperature effects and various loading conditions. For
Jack
of
experimental data, the computer solutions were validated by comparisons with the re
sults of several different tests
of
simplysupported beams (without a jointless
bridge deck) that were reported
in
the literature. El-
SaftyB
modified Gastal s finite element program
14
by
incorporating an optional analysis for partial debonding
of
the
deck
from
the
supporting
beams. He also
introduced
the
assumption
of
constant
strain through
the
depth
of
the link slab,
whereas
Gasta
l
assumed
a
linearly varying
strain through the depth
of
the link slab.
Richardson
15
also
studied
there-
moval
of
expansion
joints
from
bridges using continuous and partially debonded decks and developed a sim
plified design procedure.
Computer
programs were developed to predict
the crack
width and
spacing
in
the
deck and to calculate the vertical de
flection
of
the structure.
These analytical studies notwith-
standing, no experimental validation
of
the concepts
of
analysis and design for jointless bridge decks supported
by
simple-span girders can be found in
the literature. This paper presents the results
of
a test program to investigate the
behavior
of
the
jointless bridge
deck, and proposes a simple design
method for the
link
slab.
16ã17
Three numerical design examples are included to
demonstrate
the proposed design method.
TEST
PROGRAM
The test program included two large test specimens
of
composite construc
tion one being
a
continuous
rein-
forced concrete deck slab cast on two
simple-span steel beams, and the other
being a similar slab cast on two simple
span
precast reinforced concrete
69
beam
s.
Fig. 2 shows the details
of
the two test specimens. The material and geometrical properties
of
both specimens
are
given
in
Table
1
Fig
. 3 shows a general view
of
the test setup for the concrete bridge under the ultimate load test. Detailed descriptions
of
instrumentation
can be found in Refs.
16
and
17
. It should be noted that even though this test program used steel and pre
cast
reinforced concrete beams, the concept being evaluated should be directly applicable to bridge deck rehabilitation with existing precast, prestressed concrete girders. The concept is also applicable to new bridge construction using precast, prestressed concrete girders
if
the effects
of
creep
and
shrinkage
are adequately
ac
counted for.
Steel Bridge
The first specimen represented
a steel bridge with a jointless composite concrete deck. Two simply-supported
W
12x26 steel beams, each 20
.5 ft (6.25 m) long, were aligned with a
2 in. (50.8 mm) gap between the adjacent ends
of
the two beams. These two steel beams were then joined with a continuous concrete deck reinforced with three #6 epoxy coated longitudinal bars, thus creating a
41
ft 2 in. (12.55 m)
long two-span structure
with a jointless deck supported by two simple-span beams (see Fig. 2). A row
of
0.
75
in. (19 mm) shear connectors
spaced at
17 in.
432
mm)
were
welded to the top
of
the steel beams to develop composite action with the reinforced concrete deck, except in the
deck debonded zone
as
described
below.
The concrete deck
was 2 ft (610 mm) wide, 4
in.
(102 mm) thick, and
41
ft
2 in. (12.55
m)
long. At the two adjacent ends
of
the steel beams, the concrete deck was debonded from each steel beam for a distance
of
12
in.
(305 mm), which was 5 percent
of
the span
of
each beam. The 5 percent debonding length was based
on
the results
of
theoretical studies that showed that the load-deflection behavior
of
the struc
ture would
not
be affected
by a debonding length
of
up to 5 percent
of the
span
length
.
The purpose
of
debonding is to reduce the stiffness
of
70
1
p
26
Debonding
1
p 4
+ +
~
plaJr/.{///.(71r W/~9
5'-8
20'-0 40'-8
(a) Elevation
of
Test Specimen
[i
2
(b)
Cross-section
of
Steel Bridge
. c
0 ãããã0
4
ã
ã
'· . . ã
0
0
(3) #6 Bars
Wl2x26
(4) #8
(c) Cross-section
of
Concrete Bridge
Fig.
2
Details
of
test specimens.
Note
1 in.= 25.4
mm
. Table 1. Material and geometrical properties
of
steel and concrete bridges.
Properties
Steel
bridge
Concrete bridge
Compressive strength
of
concrete deck 4200 psi 5670 psi Compressive strength
of
girder
-
4580 psi Girder yield strength 52,000 psi
-
Girder modulus
of
elasticity 30,500,000 psi
-
Girder reinforcement
-
(4) #8 Girder reinforcement yield strength
-
62,000 psi Girder reinforcement modulus
of
elasticity
-
29,550,000 psi Girder cross-sectional area (gross)
7.65 sq in. 96 sq
in.
Girder moment
of
inertia (gross) 204
in.
ã 1152
in
4
Deck width 24
in
. 24 in. Deck
th
ickness 4
in.
4
in.
Link slab reinforcement (3) #6 (3) #6
Link slab reinforcement yield strength 63,600 psi 72,400 psi Link sl
ab
reinforcement modulus
of
elasticity 28,500,000
psi
30,300,000 psi
Note: I
in
. = 25.4 mm; I sq
in.=
645.2
mm
2;
I psi = 0.006895
MPa.
PCl JOURNAL
the link slab so that the stress devel
oped
in
the link slab can be minimized.
The continuity
of
the deck
rein
forcement
in
the debonding zone was developed
by
a lap splice
17
in.
( 432 mm) long to
simulate
the
situation
where a
damaged
joint of
a
bridge
deck would be removed and replaced
by
a jointless deck. The computed mo
ment capacity
of
the composite section was 247 ft-kips (335 kN-m), based on
the actual material properties given
in
Table
1.
C
crete ridge
The second specimen represented a concrete bridge with a jointless deck.
First, two 20.5 ft (6.25
m
long reinforced concrete beams were precast
in
the laboratory. The beams were
12
in. (305 mm) high and 8 in. (203 mm)
wide. Each beam was reinforced with
four 8 bars and thirty-one 3 stirrups.
A deck
of
the same size as that
of
the first specimen was cast
on
the precast beams when the concrete
of
the beams had gained enough strength. As before, the
jointless
deck was also debonded from each beam for a
distance
of
12
in.
(305 mm) at the cen
ter
of
the specimen. Debonding was
achieved
by
omitting the stirrups and by placing two layers
of
plastic sheet between the beam and the deck. The
longitudinal reinforcement
in
the deck consisted
of
three
4
bars which, in turn, were lap spliced with three
6
epoxy coated bars at the center
of
the specimen. The computed moment ca
pacity
of
the composite section was 185ft-kips (251 kN-m), based on the
actual
material
propertie
s
given
m Table l.
TEST
PRO EDURE
The
test
procedure
used for
both
specimens was similar. The steel bridge
was tested with four different support
configurations: HRRH, RHRH, RRRR and RHHR, where H stands for hinge
and R stands for roller.
In
each configuration, the first and fourth letters rep
resent the two exterior supports. The
second and third letters represent the
two
interior supports.
A hinge
was
provided
by
using a 1.5
in
. (38 mm) diameter steel pin between two 1.5 in. (38 mm) thick bearing plates, each with
May-June
1998
Fig.
3
Concre
te
bridge during ultimate load test.
a V -groove. A roller was provided by using the same size pin and plates but
without V -grooves. The concrete bridge was tested with
the same support conditions, except
for the RRRR configuration, which is an unlikely support condition
in
the field. The goal in testing different sup
port conditions was to observe
if
there were any differences
in
the behavior
of
the jointless deck (i.e., link slab)
under different support conditions, as
previously predicted
by
El-Safty's
computer model.
8
In
all cases, tests were carried out to
no more than
40
percent
of
the estimated ultimate load capacity
of
each test specimen to observe the behavior
in the elastic range. The load was applied on each span in increments. For each load increment, data for the
steel and concrete strains, loads, crack growth and deflections were collected. The final ultimate load test was per
formed with the support configuration
of
RHHR and a complete set
of
data on strains, loads, crack widths, and de
flections was collected.
TEST
RESULTS
A discussion
of
the test results will be given first for the steel bridge and
then for the concrete bridge.
Steel ridge
The steel bridge was tested with four
different support configurations
, namely, HRRH, RHRH, RRRR, and RHHR. Initially, the load was applied
up to 17.4 kips (77 .4 kN) on each span
to observe the behavior
in
the elastic range. Within this elastic range, the load-deflection behavior was comparable for all the four test cases,
as
shown by the measured load-deflection relationships
in
Table
2.
In addition, the
load-deflection behavior was almost
identical
for
both spans
of
the
test specimen as required by symmetry. It is noted that the measured slopes
of
the load-deflection curves are comparable to the theoretical value. The theoretical value
is
obtained by using the average
of
the moment
of
inertia
of
a fully composite section and the moment
of
inertia
of
the steel beam
Table
2.
Slope
of
load-defl
ec
tion curve kips/in.).
Support
configuration Steel bridge
I
Experimental Theoretical
T
HRRH
RHRH RRRR RHHR
f
Note: I kip/m.
=
0.175
kN
/
mm
.
55.8 58.7 49.6 54.8
t
526
j
52.6 52.6 52.6
1
Co
ncrete bridge
Experimental
J
Theoretical 57.6 1 52.3
55
.0
l
52.3
l
4.8 52.3
-
71
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