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Salt River Bridge
he Fort Knox Military Reservation is in north central
Kentucky. It is split by the
Salt River which flows north
through the reservation into the
Ohio River. This isolates a large
area in the northeast part from the
main installation. This tract of
land is a large portion of the total
installation and contains some of
the training areas which have been
accessible only part of the year by
use of an old floating bridge.
For more than 25 years the commanders of Fort Knox have been
trying to get a permanent bridge
over the Salt River to provide yearround access to these training
areas. In 1983 the Corps of Engineers awarded American Engineering Co. a design contract to
develop plans and specifications
for an access road and a high level
bridge across the Salt River.
American Engineering enlisted the
services of Janssen, Spaans and Associates to perform some of the
analysis of the beams.
The Corps of Engineers set several design criteria based on Army
Technical Manual TM 5-312. Five
major loadings would need to be
considered in the design of this
bridge. First, the design should
satisfy AASHTO HS 20 live load.
The second loading was a column
of transporters hauling 100 ton
(91 t) tanks. The third live loading
was a column of combat loaded
100 ton (91 t) tanks in convoy
spacing. Fourth to be considered
was a column of the Army's first
line tanks (the M1E1) which weigh
63 tons (57 t) combat loaded. The
last live load to be considered was
a convoy column of M1E1 's, combat loaded together with an M88
tank retriever towing a disabled
T
William B. Caroland, P.E.
Chief Bridge Engineer
American Engineering Co.
Lexington, Kentucky
David C. Depp, P.E.
Structural Design l:ngineer
American Engineering Co.
Lexington , Kentucky
Describes the design, production and erection of a fivespan post-tensioned concrete
segmental /-beam bridge with
a total length of 542ft (165m),
located at Fort Knox, Kentucky. The 160ft (49 m) central
span was cast in two segments and spliced at the
bridge site. For the deck, stayin-place precast prestressed
concrete panels were used.
108
M1E1 with a 15 ft (4.57 m) cable.
The governing live loading was the
convoy of 100 ton (91 t) tanks
which turned out to be from 2.4 to
2. 7 times as high as the AASHTO
HS 20 truck loading.
Five framing schemes were chosen for preliminary consideration:
A. A five-span V strut post-tensioned concrete segmental 1beam.
B. A three-span continuous composite steel plate girder.
C. A three-span post-tensioned
concrete segmental box beam.
D. A six-span prestressed concrete
1-beam bridge continuous.
E. A five-span post-tensioned concrete segmental 1-beam bridge.
Based on the cost estimates,
Scheme D was the most economical structure, but this scheme
would not allow a 160ft (49 m) unobstructed river channel. The next
most economical bridge was
Scheme A. However, when several
big bridge contractors were approached, although they liked the
bridge, none wanted to build it.
Scheme E was next in line economically; therefore, the preparation of preliminary plans was
based on this scheme, with span
lengths of 85, 105, 160, 105, 85ft
(26, 32, 49, 32, 26 m). A panoramic view of the nearly completed bridge is shown in Fig. 1.
Figs. 2 and 3 show a general plan
and elevation of the bridge. Also
shown are a typical deck section
and special military live loading.
Note that it was necessary to
keep Span 3 160 ft (49 m) long in
order to maintain a clear river
channel. The length of Span 3
made the design more difficult including also the shipping and erection of the beams.
PC! JOURNAL
Since the high water on the Salt
River is controlled by backwater
from the Ohio River, there is a 61
ft (18.6 m) elevation difference between the normal navigation pool
and the 100 year storm high water.
This required the bridge deck at
midstream to be about 87 ft (26.5
m) above normal pool.
When soil boring information
was available, it had a significant
effect on the method chosen to
splice the precast prestressed segmental 1-beams. To splice at or
near points of dead load contraflexure would require falsework
bents with 140 ft (42.7 m) long
piles. The soil on the river bank
was very soft and unstable and
went deep below the river level.
Because of these soil conditions,
it was decided to splice the beam at
the piers which meant the beam
would have to be cast long enough
to span between substructure supports. The 85 and 105ft (26 and 32
m) spans presented no problem;
however, the 160 ft (49 m) span
would be difficult to transport.
The decision was made to cast
the 160ft (49 m) span in two equal
segments and splice them at the
bridge site prior to erection. The
design was completed based on
this choice and the contract to construct this project was awarded to
the E. H. Hughes Company of
Louisville, Kentucky, in late 1986.
Figs. 4 and 5 show the framing
plan and elevation and beam segment details including Segment 3.
Fig. 6 shows the post-tensioning
details.
The extremely poor soil conditions at the river and bridge spill
through slopes prompted the Corps
of Engineers to suggest removing a
sizeable amount of this soft saturated soil down to an elevation 7
ft (2.13 m) above normal pool and
install wick drains and stone columns to speed up consolidation
and stabilize the spill slopes at the
river. The placing of the stone
columns, wick drains and rock fill
proceeded.
The two river piers were started
after the unsuitable material had
been removed and the fills brought
up to a point about 15 ft (4. 57 m)
above normal pool. Even with the
wick drains and stone columns,
there was a soil failure on the south
river bank. Therefore, it was necessary to remove more of the unsuitable soil in the river bed down
to the level of the Pier 2 footing
and place a large rock fill in this
mucked out river channel to add
stability to the embankments.
Settlement plates, inclinometers,
and piezometers were installed in
the fills to monitor the consolidation and give warning of possible
failure. After 6 months of monitoring the fills, the Corps gave the
contractor permission to proceed
with driving piles for Piers 1 and 4
Fig. 1. Panoramic view of Salt River Bridge.
January-February 1990
109
PLAN
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Fig. 2. General plan of bridge.
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Fig. 7. Looking north, Pier 3 is
complete. Pier 4 is being formed .
Fig. 8. Looking down in diaphragm form showing steel tube diaphragms before
casting concrete.
and End Bents 1 and 2. This delay
was necessary because negative
skin friction from the consolidating soil could overload the piles if
consolidation had not reached approximately 90 percent prior to
driving.
Fig. 7 shows a completed pier
and an end pier under construction.
While the substructure was being
built, the segmental I-beams were
being fabricated in Lafayette, Indiana. The fabricator, Construction Products (now Hydro-Conduit), had experience in casting
segments for segmental box girder
bridges and thus understood some
of the problems that lay ahead.
There were seven post-tensioning tendons with nine strands each
in the segments for the 160 ft (49
m) Span 3. All seven tendons were
in the bottom of the beam in the
center portion, thus creating considerable congestion with the pretensioning strand and mild steel
reinforcement.
The precaster elected to design a
concrete mix with Y2 in. (12.7 mm)
coarse aggregate and high range
water-reducing mixture to produce
a concrete with a slump of 8 in.
(203 mm). Using both external and
internal vibration, all the beam
segments were successfully cast
with little or no major problems.
The concrete mix produced compressive strengths in excess of 4000
psi (27.6 MPa) in 18 hours. Therequired 28-day strength was 6000
psi (41.4 MPa).
After all the beam segments were
fabricated, they were loaded on
railroad flat cars and shipped to a
port on the Ohio River some 200
miles (323 km) south of the
fabricating plant. Here they were
loaded on barges. The trip up the
Ohio was about 85 miles (137 km)
to the mouth of the Salt River and
then 1 mile ( 1.6 km) up the Salt
January-February 1990
River to the bridge site.
At the bridge site the contractor
decided to splice the Span 3 segments on the barge rather than
moving them ashore to perform
the splicing. This would prevent
handling the beams several times
since they could be lifted directly
from the barges to the bridge seats
on the piers. While blocking the
beams to the proper position, in
preparation for splicing, it was
noticed that the sun's heat was by
midday raising the middle of the
steel barge deck by 1 Y2 in. (38
Fig. 9. All beams and deck forms are in place.
115
Fig. 10. Side view showing deck forms in place. Piers 1, 2 and 3 can be seen.
Fig. 11 . Span 3 [160ft {49 m)] beam
spliced and ready to be lifted into
place.
mm). To prevent this, the barge
decks were washed continuously
with water to maintain a constant
temperature.
DYWIDAG Systems International, USA, Inc., had the contract
to furnish the post-tensioning
hardware for the job. They took
great pains to design splicing
sleeves to go into the ducts so that
there would be little obstruction
when the 542 ft (165 m) long continuity tendons were pushed
through the ducts. These sleeves fit
so tightly that they were somewhat
difficult to install; however, this
paid off later when the long tendons were pushed through.
Figs. 7 through 18 show various
stages of the bridge construction.
The two segments of the 160 ft
(49 m) span were positioned on the
barge so that the overall length of
the finished beam would be correct. The segments were also
brought to the proper vertical
alignment, and then the tendon
ducts were spliced on all seven tendons. Reinforcing steel was placed
and tied into the 2 ft (0.61 m)
splice, and then the forms were
Fig. 12. End of beams with continuity
tendons sticking out.
Fig. 13. One of Span 3 beams in place on Piers 2 and 3. Note two beams left on
barge; also Span 4 and 5 beams at left on barge.
116
placed in preparation for the placing of concrete.
Concrete was finally placed in
the forms and , within 2 days, the
strength had reached over 7000 psi
(48.3 MPa). This was considerably
above the required 6000 psi (41.4
MPa) strength, which meant that
the stressing of the four tendons
necessary for placing the beam
could proceed.
After the forms had been
stripped, a strongback was placed
on the top flange of each beam to
stabilize these long beams while
placing them on the piers. These
strongbacks remained in place
PCIJOURNAL
until cross bracing and lateral bracing were in place.
Two 250 ton (227 t) cranes with
180ft (55 m) booms were brought
in to set the beams. Both cranes
were required to lift the 160 ft (49
m) span which weighed 185,700
lbs (826 kN). The south river bank
was stable enough to support the
crane and load, but the north bank
was not; thus, steel piles were
driven to bearing and covered with
crane mats to support the second
crane. The cranes had only a 40 ft
( 12.2 m) lifting radius with the
weight of the 160 ft (49 m) beams.
To bring the barge with the
beams close enough for the cranes
to lift the beams off and into place,
it was necessary to remove some of
the river bank. Because of the
depth and length of these precast
prestressed 1-beams, it was realized
that temporary bracing was needed
during erection. It was decided
that, rather than using temporary
bracing which would need removing after the concrete diaphragms
were cast, galvanized steel cross
bracing would be utilized and then
encased in concrete.
The cross bracing was designed
using tubular sections welded and
galvanized. The tube section chosen was a 4 x 3 in. (102 x 76 mm)
with 3/16 in. (4. 76 mm) walls, allowing the cross frames to be cast
into an 8 in. (203 mm) thick diaphragm. The beam splice concrete
required 6000 psi (41.4 MPa) before the tendons could be stressed.
The mix that was finally worked
out yielded concrete strengths in
excess of 7000 psi (48.3 MPa) in 2
days.
Problems were expected in placing and stripping the forms for the
deck casting because of the height
above the river. Therefore, two alternative deck configurations were
designed. The first to be considered used precast prestressed deck
panels as stay-in-place forms. These
had been used very successfully in
Kentucky and elsewhere for a
number of years and offered the
contractor a quick way to form the
deck except the overhangs at the
fascia which he could easily get to
with a rolling scaffold. The second
January-February 1990
Fig. 14. Span 3 beam spliced with strongback in place.
Fig. 15. Blockout in end block of beam with continuity tendon in place.
Fig. 16. Deck panels on blue high density foam prior to grouting.
117
Fig. 17. Deck panel after grouting.
deck alternative was precast prestressed deck segments posttensioned longitudinally.
Both of these alternatives were
designed and detailed on the plans.
Since none of the bridges in Kentucky had used the precast segment
method before, the contractor
chose the more familiar precast
prestressed deck panels. Once all
the beams were in place on the substructure, the splices were cast at
the piers and two of three posttensioning continuity tendons, in
each beam, were tensioned through
the total 542 ft ( 165 m) length of the
bridge.
Next, high density polystyrene
foam was cut to the proper thickness and glued to the top edge of
the beam flanges to support the
precast pretensioned deck panels
which were used as stay-in-place
forms for the concrete deck. The
edges of these panels were grouted
between the bottom of the panel
end and the top of the beam flanges so as to have a positive load
transfer between the deck and top
of beams.
As soon as the panel grouting
was complete and the outside deck
overhangs were formed, the deck
was poured. After the deck had
reached a strength of 4000 psi
(27.6 MPa), the third and last con-
tinuity tendon in each beam was
stressed. The forming and pouring
of the barrier rails followed along
with end bent wing walls. Thus, the
first bridge of this type in the state
of Kentucky was complete (see Fig.
19).
Even though the project has not
had an official opening as yet,
several columns of tanks have
structure. The
crossed
the
contractor's superintendent was
present and reported that the
bridge performed very well. There
was little or no movement or vibration when the tank columns
crossed.
The contractor's final cost for
construction of the bridge was
$2,500,000. The cost of the precast
prestressed beams was $249,896 for
fabrication and shipping to the
river port, and the cost of posttensioning hardware was $40,000.
Credits
Owner: Department of the Army.
In Charge of Project: U.S. Army
Corps of Engineers, Louisville
District.
Design Consultant: American Engineering Co., Lexington, Kentucky.
Sub Design Consultant: Janssen,
Spaans & Associates, Indianapolis, Indiana.
Prime Contractor: E. H. Hughes
Co. , Louisville, Kentucky.
Bridge Contractor: E. H. Hughes
Co., Louisville, Kentucky.
Subcontractor, Marine Piers: Traylor Brothers Co., Evansville, Indiana.
Subcontractor, Erection: Javier
Steel Corp., Louisville, Kentucky.
Beam Fabricator: Hydro-Conduit
(formerly Construction Products), Lafayette, Indiana.
Fig. 18. Between beams with deck in place diaphragm with hole for conduit ducts.
118
Post-Tensioning Supplier and
Consultant: Dywidag Systems
International, Chicago, Illinois.
PC! JOURNAL
January-February 1990
119