Florida`s Long Span Bridges: New Forms, New Horizons

Florida's Long Span Bridges:
New Forms, New Horizons
I
Antonio M. Garcia, P.E.
Structures Design Engineer
Florida Department of Transportation
Tallahassee , Florida
Antonio M. Garcia, structures design
engineer, with the Florida Department
of Transportation, is currently in
charge of in-house design of major
bridges. During his tenure as the state
bridge engineer, many of the bulb-tee
bridges discussed in this article were
designed and constructed. Mr. Garcia
also served as the FOOT's district
director and project manager bringing
the Sunshine Skyway Bridge to
completion . Prior to coming to the
FOOT in 1984, he was with
consultants on various international
and domestic structural projects. He is
a registered engineer in Florida and
has a BCE from the University of
Florida and a Masters degree in
structural engineering from The City
College of New York.
34
After intense developmental work, an innovative continuous
post-tensioned bulb-tee girder system was ready to be
applied. A full-scale prototype structure - the Eau Gallie
Bridge near Melbourne, Florida - was constructed and
tested. The test results confirmed the overall structural
efficiency, strength and ductility of the bulb-tee girder system.
Also, the method of construction proved to be fairly routine
and economical. The new bulb-tee girder has revolutionized
bridge design in Florida, allowing longer spans and reducing
the amount of substructure and foundation work. The
system's efficiency and wide top flange also allow wider girder
spacings while keeping deck construction costs down. Used
with spans of up to 145ft (44 m), the system has become a
very potent and competitive option for segmental bridge
construction. Several projects, either built or in construction,
are presented to show the effectiveness and aesthetic beauty
of this new generation of bulb-tee bridges.
y the 1980s, the technology that
spawned the original AASHTO
1-beams was 30 years old. The
beams had more than met their intended goals, but times were changing:
sophisticated structural analysis, improved materials and fabricating techniques, and advanced constructio n
methods were being introduced at a
rapid pace.
In 1983, patterned after work done
by Washington State's Department of
Transportation, Arvid Grant and Associates, Inc., Construction Technology
Laboratories and other firms, Henry T.
B o llm an n, c hief of th e Bureau of
Structures , Florida Department of
B
Transportation (FDOT), and Paul F.
Csagoly, then chief structural analyst,
FDOT, introduced a modified bulb-tee
girder combined with a staged posttensioning system. The potential saving s were apparent: continuity exte nd ed the girder's effective spa n
range, allowed for wider girder spacings and provided redundancy.
The FDOT' s three current bulb-tees
(FBT) and a modified bulb-tee (MBT),
discussed in the next section, are shown
in Fig. 1. Table 1 compares the beam
properties of all beams currently listed
in Florida's standards plus the MBT.
The time was ripe in Florida for
developing the post-tensioned bulbPCI JOURNAL
tee bridge. Engineering talent was
there, the FDOT administration was
receptive to the idea, and the need
and demand were evident. All that
was needed was an opportunity to
apply the new system.
4'-0"
"'2
E:
Q)
THE CHALLENGE
The tragic events of May 9, 1980,
when the bulk carrier Summit Venture
destroyed the southbound Skyway
bridge over Lower Tampa Bay with a
loss of 35 lives, spurred stringent ship
impact requirements. The added design load required increasing the structure's inherent load resistant capacity
and/or reducing the risk by using
fewer piers and longer spans.
The Intracoastal Waterway (ICWW)
winds its way around the Florida penninsula through many of the wide bays
separating the mainland from barrier
islands. Except for dredged channels,
Florida' s coastal waters are relatively
shallow, ranging in depth between 10
to 15 ft (3.0 to 4.6 m). Many fixed and
movable bridges over the ICWW connect the barrier islands with the mainland. The fixed bridges usually consist
of AASHTO 1-beam trestle portions
[with spans of 50 to 80ft (15 to 24m)]
and a three-span, contin uous, steelplate girder portion over the channel.
Where right-of-way and other geometric requirements can be accommodated, high-level, fixed bridges are replacing the movable bridges. But with
the ship impact requirements , simple
span bridges are not as structurally efficient nor as economical as continuous, long span bridges. For Florida' s
shallow water s and limited heavy
commercial shipping (both in numbers
of ships and in dead weight tonnage),
a 130 to 160 ft (40 to 49 m) span
range is usually most economical.
Florida's first segmental bridges ,
built in the Florida Keys and completed in the early 1980s, have spans
between 118 to 135ft (36 to 41 m). At
about 145 to 150 ft (44 to 46 m), the
segmental bridges' span-by-span erection method approaches its upper economical limit. Beyond that range ,
longer spans are achieved by balanced
cantilever construction, although considerably increasing the structures '
cost. Steel superstructures can compete in the longer span ranges but are
July-August 1993
1--
Q)
Lt..
<oI
6Y2"
'%
'-!-
""
I
'~
"'
.§
~
2'-6"
2'-6"
FBT 54
FBT
5' - 0
63
11
4'-0"
3"
6)12"
-t-1---
7"
bI
<o
10"
2'-6"
2'-4'
FBT 72
MBT
Fig. 1. Cross section of Florida DOT standard bulb-tee girders and the nonstandard modified bulb-tee (MBT).
Table 1. Florida DOT standard beam properties.
Depth
(in.)
Weight
(lb per ft)
Area
(in.')
Moment of
inertia
(in.')
Yt
Yb
(in.)
(in.)
Type II
36.0
384.4
369.0
50,979.0
20.17
15.83
Type III
45 .0
582.3
559.0
125,390.0
24.73
20.73
Type IV
54.0
821.9
789.0
260,74 1.0
29.27
24.73
Type V
I
63 .0
1055.2
1013.0
521 ,163.0
3 1.04
31.96
Type VI
I
72.0
1130.2
1085.0
733,320.0
35.62
36.38
FBT 54
54.0
817.6
784.91
311 ,765.0
28.11
25.89
FBT63
63 .0
878.6
843.4 1
458,52 1.0
32.88
30. 12
FBT72
72.0
938.8
901.26
638,672.0
37.64
34.36
MBT
78.0
1137.5
1092.00
930,538.0
37.2 1
40.79
Bea m
type
Notes:
MBT is the modified bulb-tee de veloped by Janssen & Spaans Engineering, Inc., for Odebrecht Contractors of
Florida on the Golden Glades HOY Viaduct project in Miami , Florida. It is not a Florida DOT Standard Beam.
Metric (Sl) equi valents: I in . = 25 .4 mm, 1 in.' = 645.2 mm' , 1 in.' = 416,23 1 rnm' , I lb per ft = 1.488 kg!m.
35
0
TALLAHASSEE
JACKSONVILLE
Clearwater Pass
()
FT. MYERS
Fig. 2. Location of Florida's bulb-tee girder bridges (as of May 1993).
less desirable in Florida's extremely
aggressive coastal environments. Simple span AASHTO 1-beam bridges
cannot compete in span ranges of over
150ft (46 m).
After Illinois, Florida was the next
state to use post-tensioning combined
with pretensioning on a limited number of projects in the late 1970s. Having designed AASHTO 1-beams in
continuous post-tensioned bridge applications, accepting a more efficient
beam was a logical sequence. Hence,
the field was narrowed to concrete
segmental bridges, using the span-byspan method, and to continuous posttensioned bulb-tee bridges.
TESTING
After developing the bulb-tee design
on paper, the opportunity to test a fullscale prototype and a completed
36
bridge became a reality on the Eau
Gallie Bridge project over the Indian
River near Melbourne, Florida. The
tests proved the system' s innovation,
efficiency and cost effectiveness.
The project was selected because:
• As a state-funded project, the state
had full and sole control of the project' s design (by in-house staff) ,
testing (by FDOT personnel) and
construction (supervised by FDOT
personnel).
• Its size, 2900 ft (884 m) with a 91 ft
(28 m) wide deck , allowed the
amortizing of the new form costs
without severely impacting the persquare-foot cost of the bridge.
The construction contract called for
testing a prototype "bridge" prior to
the girder's full production. The prototype, tested at Gates Concrete Products Co. yard in Jacksonville, Florida,
consisted of two full y instrumented
girders, fabricated as they would be
for the project and placed on simulated bridge supports, with a partial
composite deck section cast-in-place,
with the post-tensioning applied as
called for in the bridge' s design.
Two tests were performed: the first,
on June 11 , 1985, tested the girders
for design or service loads and to 80
percent of factored or ultimate load;
the second , on December 5, 1985 ,
tested the girders to failure. The second test was described in Engineering
News-Re cord' and later in Concrete
International.2
After the Eau Gallie Bridge was
constructed, it was load tested by the
FDOT Structure Research and Testing ' s personnel and equipment in
1990. The results were published in a
FDOT report by Mohsen A. Shahawy. 3
Both the prototype and the full
bridge load tests, discussed in this artiPCI JOURNAL
Concrete Blocks
Test Live Loads (TypJ
290.00' (2 Span Continuous unitJ
145.00'
145.00'
48 .33'
48.33'
48.33'
Fig. 3. Elevation of prototype bridge showing location of test live loads. Note: 1 ft = 0.3048 m.
cle, validated the girder's efficiency
and guided the development of standards for major bridges in the state.
Fig. 2, a map of Florida, shows the location and distribution of the FDOT
bulb-tee bridges as of 1993.
14 00'
I .33'
July-August 1993
I .33'
I
~ 100 TonJocko--~
Pre-Construction Tests
The prototype bridge consisted of
two 145 ft (44 m) post-tensioned
girders with a 10.33 ft (3.15 m) wide,
7 .5 in. (191 mm) thick slab. Concrete blocks, each weighing 2150 lb
(975 kg) , simulated superimposed
dead loads that the actual girders
would experience.
Live load was applied by four 100
ton (91 Mg) capacity hydraulic jacks
placed at the third points of each span.
Figs. 3 and 4 show the elevation and
section, respectively , of the live load
test arrangement. Fig . 5 shows the
prototype bridge being tested.
Load cells monitored the applied
loads at the supports, embedded strain
gauges monitored flexural strains ,
Whittemore gauges measured multidirectional strains at gauge points
glued to each girder' s web, and thermocouples measured the temperature
in the girders. Electronic deflectometers measured small deflections while
optical methods were used for the
larger deflections during the final test
to failure.
All electronic data were gathered in
a computer-controlled data acquisition
system housed in a mobile van parked
adjacent to the test site.
II .34'
I
II
II
II
II
- - - - -u
~r-----
~Concrete
Blocks
IL----..-----+i--~----JI
w
. .______ : r--'
H
7.5"
I
I
I
+I
Q
"'
- t-
10 .33'
aq
r-
(\J
Ground Line
·~~::f/
14 .00'
1.33
8
2
1.33
11.34'
I
§L
I
I
I
I
I
I
I
I
I
I
I
I
I
Fig. 4. Section through prototype bridge girder showing concentrated live load test
assembly.
37
Fig. 5. Test setup at Gate Concrete Products Co. yard, Jacksonville, Florida, June 1985- Eau Gallie Bridge prototype.
June Test
The first test simulated the design or
service conditions required by the
1977 AASHTO Standard Specifications for Highway Bridges. These
were:
Live Load Factors
Distribution factor
(wheel line)
Impact, I
= 1.722
= 0.185
Design Moments, ft-kips (kN-m)
Live load
= 1805 (2447)
Impact
334 (453)
LL + I
= 2139 (2900)
Dead load
= 4979 (6752)
DL + LL +I
= 7118 (9652)
After the test, again, no cracking or
signs of distress were found .
December Test
The second test carried the prototype bridge to failure, which occurred
in compression in the bottom flange
at the intermediate support. When
initial crushing began, the girders had
midspan deflections exceeding 18 in.
(457 mm), evidence of a highly ductile system. A close-up of a deflected
girder is shown in Fig. SA .
At failure , the maximum applied
load at the hydraulic jacks measured
185 kips (823 kN), corresponding to a
live load moment of 8843 ft-kips
(11990 kN-m) or 4.90 times the 1805
ft-kips (2447 kN-m) design live load
without impact. The corresponding
AASHTO requirement is 2.57 times
(i.e., 2.17 x 1.185). The factored shear
was 473 .5 kips (2106 kN) at the support, 1.13 times higher than the 420 kips
(1868 kN) as determined by AASHTO.
Factored Moments, ft-kips (kN-m)
Live load
3911 (5304)
Impact
=
723 (980)
LL +I
4634 (6284)
Dead load
64 73 (8777)
DL + LL +I
= 11,107 (15061)
The bulb-tee girders carried the full
AASHTO design load with no cracking or any signs of distress. After the
design load test, the girders were taken
to 80 percent of factored loads in anticipation of the later test to failure.
38
Fig. 5A. View of deflected girder- Eau Gallie Bridge prototype.
PCI JOURNAL
@@
4.50'
@@
I
I 4.50' I
21.50'
15.50'
45 .50'
15.630 lb
83 .720 lb
104 ,650 lb
Weights : 72 ballast blocks
Equipment
Trailer
Tractor
154,800 lb
8 ,200 lb
24 ,000 lb
17,000 lb
Toto!
Load Transfer:
Steering axle
Drive tandem
Trailer tandem
/5,630 lb
83,720 lb
104 .650 lb
204,000 lb
Note : All weights and dimensions ore
approximate and f or information only.
/000 /b = 4.448 kN
Fig. 6. Bridge testi ng vehicle.
Post-Construction Tests
The post-construction tests on the
co mpleted Eau Galli e Bridge were
conducted using the test vehicles of
th e FDOT Structures Research and
Testing.• Fig. 6 shows the vehicles ' dimensions and load distribution, Fig. 7
show s the Eau Gallie Bridge under
construction and Fig. 8 shows the vehicles in position. The ballast blocks
allow incremental loading up to the
maximum AASHTO allowable lo ad
Fig. 7. Eau Gallie Bridge under construction.
July-Aug ust 1993
39
Table 4 lists the physical characteristics of the tensioning systems used
(either pre- or post-), and Table 5 Jjsts
the current total number of bulb-tee
girders at various stages. Table 6 lists
the total cost per square foot for the
bridges, including substructure and superstructure, plus the bid cost per lineal foot of girder. Costs are from the
contractor's bid tabulations as of the
bid dates shown.
BULB-TEE VS.
SEGMENTAL BRIDGES
Fig. 8. FOOT's Structures Research and Testing test vehicle in position to test the
Eau Gallie Bridge.
without endangering the structure. As
with the prototype test, all electronic
data were collected by the data acquisition system in the mobile van.
Girders in the first two spans on the
east end of the bridge had been instrumented with Carlson strain gauges .
Vertical deflection was measured .at
the bottom of the girders with linear
vertical transducers (LVTDs) mounted
on an independently supported aluminum frame.
The two test vehicles were placed at
six different locations and ultimate live
load moments and shears were measured. The field test data were compared to an analysis of the bridge using
BRUFEM (Bridge Rating Using Finite
Element Methods). The finite element
computer program was developed at
the University of Florida, Gainesville.
The maximum measured deflection
was approximate'iy 1 in. (25 mm) compared to a calculated value of 1.35 in.
(34 mm) , or about 75 percent of the
predicted value. Maximum measured
stresses were 76 percent of the analytical prediction and no shear cracks or
other distress signs were observed.
loads and small deflections in carrying
actual loads.
The testing gave the FDOT the impetus for developing standards and
promoting the system's usage on later
bridges.
IMPACT OF
BULB-TEE GIRDERS
IN FLORIDA
Test Conclusions
The results of the pre-construction
.
(prototype) and post-constructwn
(completed bridge) testing indicated
excellent performance of the system,
extreme ductility in carrying very high
40
0
The continuous post-tensioned bulbtee girder has revolutionized bridge
design in Florida. First, it allows
longer spans and reduces the amount
of substructure and foundation work.
Second, its efficiency and wide top
flange allow wider girder spacings yet
keep deck construction costs down.
The post-tensioned bulb-tee bridge
used with spans up to about 145 ft
(44 m) became a very potent and very
competitive ~ ption for the segmental
bridges available for those spans. In
addition, the girders are more economical, as prestressed simple span girders, than the Type V and VI AASHTO
I-beams.
Tables 2 and 3 list Florida's current
bulb-tee bridges for post-tensioned
"continuous spans and simple spans, respectively. In some cases, simple span
I-beams or' bulb-tees are used for the
approaches while the channel section
is a three-span, continuous unit with a
drop-in segment.
As of 1993, five alternate designs of
bulb-tee and span-by-span segmental
concrete bridges have been let. In all
cases the bulb-tee was the low bid,
thus confirming our belief in the girders' competitiveness. The results have
been conclusive, allowing us to forego
alternate design s and thu s adding to
the savings.
The Howard Frankland Bridge over
Upper Tampa Bay, which connects
Tampa with St. Petersburg, is 16,000 ft
(4880 m) long with a 70.83 ft (22m)
wide deck. The project was bid as
three alternate de signs : continuous
post-tensioned bulb-tee, span-by-span
segmental, and a combination of continuous post-tensioned bulb-tee approaches and a three-span, continuous
steel channel section. Except for the
steel channel alternate, all designs
used 143 ft (44 m) spans. There were
four bidders, one for the segmental (at
5 percent above the low bulb-tee bid),
three for the bulb-tee, and none for the
steel channel alternate.
At the time, the FDOT design criteria for the segmental option called for
internal post-tensioning.
The second project, the Thomas A.
Edison Bridge in Ft. Myers, Florida,
consists of two separate bridges, each
about 5000 ft (1520 m) long with
143 ft (44 m) typical spans, with girders spaced at 10 and 11 ft (3.05 and
3.35 m). The span -by-span concrete
segmental alternate also used 143 ft
(44 m) spans but with external posttensioning. Eight bids were received,
with only one contractor bidding the
segmental (at 15 percent above the
low bulb-tee bid).
The third bridge, not a Florida DOT
project, was the 49th Street Causeway
PCI JOURNAL
in Clearwater, Florida. In this case, a
simple span bulb-tee design was the
low bid over a number of other option s, including a span-by-span concrete segmental bridge with external
post-tensioning.
The fourth bridge, the Golden Glades
Viaduct in Miami, Florida, is entirely
over land and was bid as a span-byspan segmental concrete bridge vs . a
simple span AASHTO Type VI beam
bridge. The project's strict maintenance
of traffic requirements, at a location
where 1-95 and the Florida Turnpike
are intertwined with a number of other
major state and local roads, gave the
segmental design an advantage because
it could employ "from the top" construction; yet none of the bidders chose
the segmental design.
The AASHTO Type VI I-beam aJternate was the low bid, but the contractor submitted a Value Engineering
Change Proposal (VECP) for a modified bulb-tee (MBT), shown in Fig. 1.
Although not a true comparison between two similar designs (a simple
span girder bridge without post-tensioning vs. a post-tensioned continuous segmental bridge), the contractor
saw his bulb-tee system as even more
economical and efficient than the
AASHTO 1-beam.
The fifth bridge , the MacArthur
Causeway Bridge in Mjarru , is part of
1-195 and consists of twin bridges,
each about 2200 ft (671 m), over the
ICWW connecting Mjami and Miami
Beach. The new bridge will replace an
existing two-leaf bascule bridge. The
de s ign alternates were a bulb - tee
bridge and a span-by-span concrete
segmental bridge. Both alternates used
144 ft (44 m) span arrangements with
a smaller three-span section at both
ends. There were eight bidders and all
bid the bulb-tee bridge alternate.
CONSTRUCTION
Web Width and Duct
Characteristics
The web width and duct configuration have evolved from the Eau Gallie
Bridge until the most recent project,
and is a case study of how design and
construction are interrelated. Each step
in the girder's fabrication, transportation and erection was considered in
July-August 1993
Table 2. Post-tensioned, continuous bub-tee bridges.
Location
Eau Gallie
Number
of spans
Span
length
(ft)
20
1-15
Total
length
(ft )
2900
Deck
width
(ft )
Girder
spacing
90.64
10.33
(ft)
---~
Howard Frankland
110
143
15.873
AP
42
142
5964
c
3
160.
200. 160
520
NB
36
143
SB
32
3
Choctawhatchee
70.83
10.00
46.83
9.76
51-18
59.08
10.00
143
4576
6508
11.00
196.
250. 196
642
47.08
9.50
142
3834
49.04
10.00
140.5
3372
54.04
11 .00
130. 145
2130
68.08
10.00
43.08
11 .33
85.08
9.56
Edison
Highland View.
channel span
NB
27
SB
27
Merrill Barber
Anna Marie
24
EB
15
MacArthur
WB
AP
90
144
12.960
c
3
144,
234.5 ,
144
522.5
3
160,
200, 160
3
122.25
8
137.0
8
148.0
8
149.0
16
132
Gandy Bridge
St. John's River.
channel span
520
11.25
Vilano
3839
53.04
9.00
AP
EB
2 11 2
10.00
49.04
c
Flagler Beach
3
c
161.
230. 161
8.00
552
8.50
WB
43.08
AP
16
132
2 11 2
11 .33
Notes:
C denotes a continuous, three-s pan channe l unit in which the centra l unit is a drop-in segment. AP denotes the approaches which may be made up of groups of three of four continuous spans.
NB = northbound, EB =eastbound, WB = westbound , SB = southbound.
l ft = 0. 3048 m.
achieving an optimal and efficient design. Each preceding project contributed lessons for the next project
until we arrived at our current standard - and we continue to improvise
certain details, as di scussed in the following paragraphs.
A critical issue is that post-tension-
ing systems require close attention to
detail s. Actual dimensions should be
used rather than nominal dimensions.
For example, for reinforcing bars, the
out-to-out dimension at the bar deformations is used rather than the nominal bar diameter. Duct out-to-out dimensions, including the corrugations
41
Table 3. Simple span bub-tee bridges.
Number
of spans
location
Span
length
(ft)
Total
length
113, 122*
2834
(ft)
Deck
width
(ft)
Girder
spacing
43.08
8.75
(ft)
--
25
SB
Structure "A"
NB
EB
WB
1
9.50
I
116
2088
3
108
324
I
52.54
3
White City,
approaches
4
6
I 21
65
1
112.56
I
10.00
I
1-
1133
116.05
112.58
120
-
12.00
338
113.33
-
10.75
68.08
10
Golden Glades
56.04
2834
18
St. John's River
Clearwater Pass
I
113,122*
--+---25
West Bay,
approaches
MacArthur
--
I
-
85.08
9.56
1140
46.03
9.25
2520
59.00
I
--
10.00
:
I
135
to 150
6308
Vary
Vary
Notes:
The * denotes the channel span length.
NB = northbound, EB =eastbound, WB =westbound. SB =southbound.
I ft = 0.3048 m.
and manufacturing tolerances, must be
used. Duct splice locations within the
beam need to be carefully considered
because the splice constitutes a thickening of the duct.
Also, placement tolerances allowed
in both the side form manufacture and
installation, and placement of the reinforcement cages and ducts, must be
taken into consideration. The saving
grace is that the tolerances are not cumulative since the ducts and reinforcement are all tied together prior to setting the side forms. Once the forms
are set in place, the fit is easier to
achieve. However, there is little tolerance for sloppy workmanship, thus requiring the best from the fabricator
and the inspectors.
The Eau Gallie Bridge design used
four circular ducts in a 6.5 in. (165 mm)
web, #4 (#15) vertical web reinforcement and a minimum 1.5 in. (38 mm)
of cover. After the job was awarded, the
contractor proposed three circular ducts
and a modified post-tensio ning sequence. One less duct and anchorage
was a real savings in both material and
labor in manufacturing the girders and
in the post-tensioning.
The fewer ducts required parti al
post-tensioning (tendons stressed to 50
percent of design) at the first stage ,
42
thus exposing the stressed tendons to
the elements for an extended period of
time prior to grouting. Although subsequent tests showed that the tendons
were sati sfactory, the FDOT now requires 100 percent of the specified
post-tensioning force at each stage and
fully grouted ducts as soon as possible.
For the Howard Frankland Bridge
design, the web was increased to 7 in.
(178 mm) to accommodate increased
cover requirements [from 1.5 to 1.75 in.
(38 to 44 mm)], three round ducts, and
#4 (#15) vertical reinforcement. External vibration was required in all areas
except in the anchor block region
where the wider web could accommodate internal vibrators.
However, the ducts , as designed ,
provided a net area of twice the tendon
area as allowed by Section 9.24.4.1 of
the AASHTO Specifications and the
Post-Tensioning Institute (PTI) - yet
the post-tensioning specifications for
the segmental alternate called for 2.5
times the tendon area. The contractor
pointed out that he could not concurrently meet the larger net area requirements and the placement tolerances
and, as a solution, proposed a 7.5 in.
( 191 mm) web which was accepted by
the FDOT.
For the Choctawhatchee Bay Bridge,
designed entirely by the FDOT Structures Design Office staff, a new duct
shape was introd uced that wou ld
achieve all the requirements within the
7 in. (178 mm) web. The oval duct has
become the standard in all subsequent
post-tensioned bulb-tee bridges.
Several manufacturing methods for
the oval duct were considered, with an
extruded corrugated polyethylene duct
meeting the design requirements. The
supplier had first tried compressing a
round galvanized metal duct into the
oval shape, but the results were not acceptable bec ause sharp edges were
formed at the top and bottom of the
duct.
The corrugated polyethylene duct
require s close wall thickness tolerances. If the walls are too thick, placement tolerances are reduced ; if they
are too thin, they will collapse under
the wet concrete's hydrostatic pressure . A test was devised whereby a
small section of duct is placed in a
form and concreted to a level above
the top of the duct representing the hydrostatic press ure expected in girder
production. After the concrete has set,
a "torpedo," with dimensions slightly
less than the duct but providing the desired gross area, is run through the
duct. If the torpedo passes through, it
indicates that the duct wall s have
minimal deformation in resisting the
hydrostatic pressures.
For the Highland View Bridge (see
Fig. 17) under construction in May
1993, the contractor proposed an oval
metal duct whose manufacturing process eliminated the unacceptable sharp
edges mentioned earlier. Some projects bid in May 1993 have requested
substituting the metal oval duct for the
polyethylene shown in the plans while
projects under design as of May 1993
will show the metal oval duct.
L imiti ng the number of splices is
important and achieved with very flex ible ducts that can follow the desired
tendon trajectory with few splices.
Both the polyethylene and the new
metal oval ducts can easily accommodate the trajectories called for in the
bulb-tee. The metal oval duct can be
formed to the required radii without
crimping the surfaces.
Also, each splice must be considered a potential leakage point and the
PCI JOURNAL
Table 4. Physical characteristics of tensioning systems.
Post-tensioning tendons
Pretensioning strands
I
Concrete strength
I
Number
of ducts
Strands
per duct
Force
(kips per
strand)
Duct
shape
4
6 each
41.5
RS
I
Project
Eau Gallie
Howard Frankland
Approach
3
7 each
2,1
8 each, 12
45
Choctawhatchee
I
I
46.9
Channel
RS
12, 9, 10
NB
3
9 each
SB*
I, 2
10, II
122
3
7 each
46.9
137
2, I
9 each , 7
46.9
3
8 each
46.9
llighland View*
3
14 each
46.9
OS
Anna Mariat
3
10 each
46.9
OS
EB
2, I
8 each, 13
WB
2, I
7 each, 12
Vi lano*
46.9
I
I
Flagler Beach*
148, 149
Pier
St. John ' s River
3
8 each
I
12
3
9 each
9 each
9/16
38.88
36
1/2
31.0
26
1/2
31.0
24
1/2
J
I
Girder
(psi)
r
3400
3400
6000
I
6000
4500
6500
31.0
3400
6000
1/2
38.89
3400
6500
24
1/2
3 1.0
3400
6500
28
J/2
31.0
3400
6500
1/2
30.99
3400
6500
36
1/2
31.0
5500
6500
26
1/2
31.0
3400
6500
32
1/2
31.0
3400
6500
22
4 and 26
OS
28
B 18
OS
B 30
45.4
3
24
!
Deck
(psi)
T26
I
46.9
MacArthur*
I
I
Merrill Barber
41.36
I
13, 9, 8
Drop-in
9/16
OP
46.9
3
22
(kips)
26
Back span
I
Size
(i n.)
I Force
of strands
OP
I, I, I
Edison
I Number
I
OS
OS
I
Notes:
Continuous bulb-tee usage is listed (May 1993). The * denotes under construction and the t denotes in design.
All strands are 0.6 in. diameter, 270 ksi low-relaxation strands except Eau Gall.ie, which are 9/16 in. diameter.
RS refers to round steel ducts, OP refers to oval polyethylene ducts, and OS refers to oval steel ducts.
NB =northbound, EB =eastbound, WB =westbound, SB = southbound.
l in . = 25.4 mm, 1000 psi = 6.895 kPa, I kip= 4.448 kN.
duct corrugations must be such to
allow placement of a tight sleeve. It is
important to prevent duct di splacement during casting by securely tying
it off at close intervals along the
girder length. Inspectors a nd yard
personnel should understand the importance of the duct' s placement and
trajectory .
The indiscriminate use of internal
vibrators can physically damage the
duct itself as well as affect its horizontal positioning within the web. Di splacements which create kink s can
damage the girder once it is post-tenJuly-August 1993
sioned. Only external vibration should
be allowed along most of the beam
length, and internal vibration should
be allowed only in the thicker anchorage areas. In all cases, once a girder is
cast, a torpedo s hould be pulled
though the duct and all interferences
removed before the girder is accepted
and shipped to the job site.
The task is not as complicated as it
may appear. Its economical success is
evident in the number of successful
bridges built in Florida, and in the
even deeper and thinner webs used in
other parts of the United States and
Canada. There have not been any reported problems in placing tendons in
the oval ducts , in providing the required tolerances or in consolidating
the concrete.
Pretensioning
The bulb-tees are pretensioned for
their own se lf-weight plus an allowance, ranging between 30 and 40
percent of the girder's self-weight, for
handling , shipping and construction
loads. Others may find that their fabrication and construction requirements
43
Table 5. Number of bub-tee bridges- various stages.
Total
span
Girders
length I per
(ft)
span
Bridge
Eau Gallie
Total number of girders
I
Duct
shape
Under
In
I
construction design
I
Built
2900
9
RS
180
15.873
7
RS
I 777
6484
5
OP
325
5148
6
OP
216
4576
6
OP
642
5
OS
3834
10
OS
3372
5
OS
~~
Howard Frankland
I
Choctawhatchee
j
I
-
-Edison
-': j
~
B
Highland V1ew
l
192
15
-
I
-
_..,.;u """'=
I
Anna Maria
270
~
120
1463
Vilano
I
I
I
5
OS
6
OS
5/6
OS
'---
2376
I
55
I
96
-~~--
Flagler Beach
I :~t 2664
2664
4/5
OS
2834
617
n/a
2834
617
n/a
2088
5
n/a
1653
9
n/a
2520
7
n/a
Vary
n/a
7
OS
6
OS
4
OS
98
I
-
I
79
I
-
NB
Structure "A"
-
-
I SB
r--------
l
I
West Bay
~
I
---
St. John ' s River
-
1-
I
90
6308
EB
- WB
Total
r-
-
90
147
I
r
2454
I
13,483
88,638
I
266
126
~
2468
Gandy Bridge
I
-
Golden Glades
MacArthur
175
~
+
Clearwater Pass
-
175
I
I 1763
_==r
1008
-
108
-
829
Notes:
Girders per span is the number of girder lines per span. Where two numbers are given, the first refers to the approaches and the second to the channel spans.
Duct shape is the post-tensioning duct shape used. RS is for round, steel; OP is for oval, polyeth ylene; and OS is for
oval, steel.
NB =northbound, EB = eastbound, WB = westbound, SB = southbound.
I ft = 0.3048 m.
differ from these. Nevertheless, some
allowance should be used.
In addition, the pretensioning force
causes a slight positive camber to the
beam. The result is a nearly flat profile
without the noticeable upward bowing
common to AASHTO 1-beams. Because the pretensioning is only for
slightly more than the girder weight,
camber growth is reduced, thus eliminating problems when decks are cast.
44
Post-Tensioning and
End Anchorage
Post-tensioning should proceed simultaneously from both ends of the
unit. We have found that a reasonable
strand pull is 580ft (177m) [i.e. , four
spans of 145 ft (44 m) each] . Longer
pulls, the equivalent of five 145 ft
(44 m) spans, may be possible but
have not been used yet. For longer
pulls, post-tensioning wobble and fric-
tion losses may significantly affect the
girder' s load carrying capacity at interior locations.
The post-tensioning sequences are
s hown for the Howard Frankland
Bridge in Fig. 10, the Cboctawhatchee
Bay Bridge in Fig. 11 and the Edison
Bridge in Fig. 12. Generally, post-tensioning is done in the following stages:
First Stage: After the pretensioned
beams are placed on their supports and
end diaphragms are cast, the continuity
post-tensioning, top- or bottom-most
duct, is completed. This is followed by
the middle duct post-tensioning, designed to carry the form and deck concrete loads.
Second Stage: The third duct posttensioning force acts on the composite
section and is designed to resist live
load and secondary dead load effects.
Grouting is done after completing
each post-tensioning operation . The
post-tensioning sequence should start
with the middle girder line and proceed
outward toward the fascia girder lines.
In dimensioning the girder's anchor
block-outs , the dimensions of the
stressing equipment and clearances to
adjacent girders are important. Early
anchorage layouts had them coplanar
with the web and exiting through the
girder' s top flange. The exposed anchorages will collect debris, dirt and
water. It is important to seal the duct
openings after girder fabrication and
to provide some means of positive
drainage at the low points. The evolution of the anchorage layout from project to project can be seen in Figs. 9
through 13.
For the Eau Gallie Bridge, a rectangular opening was left exposed (see
Fig. 9) which was concreted at completion of the post-tensioning. For the
Howard Frankland Bridge, the entire
deck width between the expansion
joint and the third tendon's anchorage
was left open (see Fig. 10) until after
the post-tensioning was completed .
The deck reinforcement was then
placed and the last of the deck concrete
poured. The advantage was that the
jacking operation could proceed uninterrupted by any reinforcement; the
disadvantage was that the screeds had
to be brought back to finish the deck.
For the Choctawhatchee Bay Bridge,
a small rectangular area (see Fig. 11)
PCI JOURNAL
Table 6. Unit costs for completed Florida bulb-tee bridges (as of bid date).
Bridge
Eau Gal!ie
Bid date
Bridge
($per sq ft)
Girder
($per ft)
June 1984
33.37
104.24
Howard Frankland
May 1987
35.33
109.51
Choctawhatchee
June 1988
55.56
109.08
Edison, NB
August 1990
46.58
139.70
Edison, SB
Augu st 1990
47.05
147.04
White City*
October 1989
55.53
92.00
West Bay *
March 1990
52.63
125.00
Highland View
May 1991
46.93
119.10
Structure "A." NB*
July 1991
55.82
87.11
Structure ''A," SB *
July 1991
54.60
87.11
June 1992
62.9 1
St. John 's River
I
I
143.94
Notes:
Simple span bridges are denoted with*; tbe others are continuous systems. The girder costs include all the
post-tensioning costs, i.e., material and installation labor.
NB =northbound, SB =southbound.
$ 1 per sq ft = $ 10.76/m' , $ 1 per ft = $3.28/m.
exposed only the last anchor. This was
accomplished by stressing tendons two
and three in Stage 1 and tendon one in
Stage 2. The advantage was that only a
small pour remained; the disadvantage
was that the previously placed reinforcement had to be bent out of the
way , bent back in place, and then
lapped prior to pouring the concrete.
An innovative anchoring arrangement was developed by the consultant
for the Edison Bridge. Only one anchorage exited through the girder' s top
flange, while the other two exited at a
sloped cut in the web near the end of
the girder. Figs. 12 and 13 show the arrangement. The advantage was that
only a small pour was required, and it
could be done in conjunction with placing the expansion joint. This arrangement appears to be the best, although a
variation on this would place all three
anchors in the sloped girder surface.
before the release of the pretensioned
strand. In any case, curing is very important and each location needs to use
the best method possible.
AESTHETICS
The longer spans achieved with the
bulb-tee result in a very slender looking bridge. Al so, the rounded comers
where the web meets the flanges eliminate horizontal lines that would normally detract from the fascia's aesthet-
Aggregate Size
Smaller aggregates [ ~ and 7:1 in. (13
and 19 mm)] are preferable, along with,
in some cases, superplasticizers. It is important that concrete fills all the voids,
especially in the areas between ducts and
in the commonly congested anchorage.
ics. The rounded corners also facilitate
the concrete pour by eliminating air
pockets, thus reducing honeycombing.
Several other items have helped improve bridge aesthetics and some have
improved the rideability. For example,
since the girders are pretensioned for a
little more than their dead load, they
do not have the "scalloped" appearance of simple span A;.SHTO 1beams that are pretensioned for all the
loads . The added benefit is an improved rideabi lity in contrast to the
camber growth common with time in
AASHTO 1-beams. Expansion joints
are a never ending maintenance problem that, in time, will leak. Post-tensioning eliminates at least half the
number of expansion joints and the attendant problems.
One other consideration that affects
the overall appearance is the continuity of the superstructure' s soffit for the
full length of the bridge. That is , rather
than use a variety of spans with changing girder depths, we encourage designers to keep the span lengths constant from end to end.
Proper deck drainage is very important. Some Florida bridges allow water
to fall freely to the ground. If deck
scuppers exit the bottom of the deck
near the girder, we extend the lip 1 in.
(25 mm) below the deck soffit to prevent unsightly stains along the girder' s
fascia. Drainage requirements can be
very costly if, for example, an expensive collector system is used. Using
larger deck drains with down spout
into the first interior bay and piped
down through the pier to an outfall
near the ground or water surface is a
viable solution in Florida. Our climate
allows us this luxury that would cause
severe problems in areas of the country where freezing is a common winter
occurrence.
The piers, however, offer the most
exciting potential for improving the
aesthetic qualities of our bridges. The
Edison Bridge (see Fig. 14) is an example of what can be done . In this
particular case, a series of precast concrete segmental pier units* have been
designed that, together, offer increased
Curing
Wet curing with burlap and soaker
hoses is very effective in eliminating
the shrinkage cracks that can develop
July-August 1993
Fig. 9. Anchor block-out in the Eau
Gallie Bridge deck.
* "Precasting the Edi son Bridge" by Theun is A. van
der Veen (HDR, Inc.), paper presented at the PC!
Convention, Nashvi lle, Tennessee, October I 992.
45
I
I
L -1£-~-~~ ~ --------------
------+------------ ~
-----·----- 1~ -------- - -----
DECK PLAN
I'
I'
I'
i ~-~~?Z~~~~~~==~
I
#'0
#2
; ~--------------------~
Tl
~~
if
Expansion joint
m
;......___if
Pier
BEAll ELEVATION
I . Placed stee.tes on ducts
2.
Pour closure ond diapllrO(}m corcrete
3 . Stress tendons #f and #2 and grout
4.
Pour deck up to tendon #3 onchorO()e. Note all ducts at intermediate pier
located in or below the beam top flange
5 . Stress tendon #3 and grout
6. Pour rerooinder of deck leaving opening for expansion joint
Fig. 10. Post-tensioning sequence for Howard Frankland Bridge.
I
rl"7"ti'z'+::.~::-:: : .: .· -- --- - --L - ~~~ -- ~~ --------
ii.,.<-H;.,.<-,.+':.:. -~:!- ~: :':~ - - -- - - - - -
-- tzc~~;~~+~~~~~~~--~--~~
- - ---------~-------------1
'
!
DECK PLAN
#3 "~""""'"""'....
#2 ~.,..,..,........_
#t
m
f-.-___ I£
Pier
BEAU ELEVATION
I.
Placed slee.tes on ducts
2.
Pour closure and diophrO(}m corcrete
3.
Stress tendons #2 and #3 and grout. Exposed tendon #3 slee.te at intermediate pier was difficult to seat becouse corrugated CNol duct connection could
not be properly aligned between two ends
4.
Pour deck leoving tendon #f orchorO(}e exposed. SletNes for tendon #3
ore thus ercosed
5. Stress tendon #t and grout
6. Pour rerooinder of deck t00o1ing opening for expansion joint
Fig. 11 . Post-tensioning sequence for Choctawhatchee Bay Bridge.
46
savings and an aesthetically pleasing
solution . Both the pier columns and
the pier cap are precast, shipped to the
site, placed, post-tensioned vertically
and grouted. It is a quick, efficient, effective and elegant solution.
Bridge aesthetics is in revival and
many articles, publications and conferences are addressing the issue. Some
states, such as Maryland, are preparing
extensive guidelines addressing aesthetics; other states such as California,
have for many years made aesthetics
an integral part of design.
FURTHER
DEVELOPMENT NEEDS
A series of issues, related to design,
specifications, construction and maintenance, have developed since we first
began working with the post-tensioned
bulb-tee. Some are presented for further consideration, but not in any particular order of importance:
1. Vibrations -- Vibrations have
become more noticeable with the advent of longer span bridges. Maintenance inspectors have begun questioning this common phenomenon and we
are considering means of measuring,
quantifying and comparing the vibrations experienced on these bridges
with that of other bridges. During the
early development of the bulb-tee, the
calculations showed a potential for a
low and acceptable frequency.
2. Negative Camber -- On the
Howard Frankland Bridge, negative
camber, between 0.5 and 1.5 in. (13
and 38 mm), was evident in many
girders at the time they were placed on
their bearings. This indicates that a
higher pretensioning force might have
solved the problem.
3. Top Flange Edge -- Initially ,
there was concern that the unreinforced top flange edges would be easily broken off. The edge, 2 in. (51 mm)
thick, is vulnerable to numerous actions during fabrication, yard movement, transportation and erection. Of
all the girders used to date, very few
have experienced damage and none so
severe that corrective action or rejection was necessary.
4. Discoloration -- This phenomenon is sometimes seen as a darkening that accentuates the duct profile.
PCI JOURNAL
/ ~Beam
I
f
-- z -==-=-~=-----~- -==-=-~=- - ~- -==-===----~--,
--------------~ --- ------- ---1
I
I
I
DECK /"LAN
m
1---....__
~ Pier
BEAll ELEVATION
I.
Placed sleetes on ducts
2.
Pour C1osure and diaphragm roxrete
3.
Stress tendons #! and #z and grout
4.
Pour deck except for cross-hatched area ixluding rendon #J axlrJroge
5. Stress tendon #3 and grout
6. Pour remainder of deck IB<Ning opening for ex{XJnsion joint
Fig. 12. Post-tensioning sequence for Edison Bridge.
This does not occur in all post-tensioned girders but appears randomly.
Its cause and elimination are still to be
determined.
5. Specifications - Although the
1989 AASHTO "Guide Specifications
for Design and Construction of Segmental Concrete Bridges" covers posttensioned bulb-tee girder bridges (Section 1.1, General), on some issues we
rely on our research and experience.
There are some issues applicable to
post-tensioned segmental box girder
bridges that are questionable for the
bulb-tee. One is the net duct area,
mentioned earlier. Both AASHTO and
PTI recommend that the net duct area
be twice the strand area. For segmental bridges with larger tendons and a
tendon profile that varies both vertically and horizontally, the 2.0 value
may cause undue tendon placement
problems and 2.5 is usually required.
But on bulb-tee bridges, with a tendon
profile that varies only in the vertical
plane, we have not seen any problems
in placing tendons. This may suggest
that a value of 2.0 is applicable to
bulb-tees.
July-August 1993
6. Fabrication- Unquestionably,
good workmanship is imperative for
these girders; if it is not available or
enforceable, problems will arise. A
conscientious workforce at the yard is
essential and personnel must understand what and why they need to do
what they must do. As in all work of
this nature, whether it be precast or
cast-in-place, segmental or bulb-tees,
good workmanship is essential to a
successful job. There is little room for
sloppy work.
7. Ducts- Some problems with the
polyethylene ducts have surfaced.
They relate, however, to thin walls
and improper storage that result in
"hour-glass" shapes that create blockages in the trajectory. Artificial means
of maintaining the oval shape, such as
inflatable hoses, have had limited success and should not be relied on in
place of properly manufactured ducts.
Of all the girders cast and in place
(see Table 5), four have each developed a crack following the tendon trajectory, extending from the web transition area (the area where the web
thickness begins to neck down) into
the 7 in. (178 mm) web area for a few
feet. The cracks were not noticed until
the final cleanup work had begun.
An analysis of these girders by an
independent laboratory has shown that,
aside from the visible cracks, some
vertical delamination occurred in the
webs. Tests along the remaining length
of each girder showed neither an external nor internal crack. Why these four
girders, out of the more than 408
placed, should have cracked has not
yet been determined. Excessive grout
pressure resulting from a blockage
may be a possibility. A series of tests
Fig. 13. Post-tensioning anchorages are located in sloped portion of the end
blocks of the Edison Bridge.
47
have been proposed to resolve and answer questions raised by this phenomenon. In the meantime, the girders
were analyzed and found to be acceptable after pressure grouting the cracks.
EXAMPLES
OF FLORIDA'S
BULB-TEE BRIDGES
Fig. 14. The completed north bound Edison Bridge.
A sampling of bulb-tee bridges in
service or at various stages of construction as of March 1993 is included. These are:
Fig. 14 shows the Thomas Alva Edison Bridge across the Caloosahatchee
River in Ft. Myers, Florida. The completed northbound bridge is shown;
the southbound bridge was still under
construction in early 1993.
Fig. 15 shows the Choctawhatchee
Bay Bridge under construction and
Fig. 16 shows the completed bridge. It
was Florida's first drop-in design with
a center span of 200ft (61 m).
Fig. 17 shows the Highland View
Bridge, near Port Saint Joe, Florida,
under construction in March 1993. At
the time of construction , it was the
longest - 250 ft (76 m) - drop-in
span bridge in Florida.
CONCLUSION
Fig. 15. Channel span erection for the Choctawhatchee Bay Bridge, back span
support and hangers for drop-in segment shown .
Fig. 16. Completed channel span and approaches for the Choctawhatchee Bay
Bridge.
48
The continuous post-tensioned bulbtee bridge has become common in Florida. Many fabricators have purchased
forms and many more designs are
being produced (see Tables 2 and 3).
Our consultants have gained the design experience necessary to produce
the contract plans and our field forces ,
both those in the yards and at the job
sites, have gained much experience
with the bulb-tee girders.
Costs have been very competitive
and are not much more than the costs
for simple span AASHTO 1-beam
bridges. Table 6 lists some of the cost
data available as of March 1993.
All post-tensioning work requires a
high level of care in design, fabrication
and construction . The bulb-tee is no
exception; it is not "idiot proof' - no
system can be. Yet when proper care
and attention is given to the details,
when the fabricator and the suppliers
perform acceptable work, and when
the contractor pays attention to his
PCI JOURNAL
in some of the later testing and offered
invaluable assistance in interpreting the results. Douglas Edwards ,
FHW A's Florida Division Bridge Engineer, has listened to many of our arguments and agreed with most. Others
in the FDOT Structures Design Office
in Tallahassee, such as Clark Williams,
and members of his Consultant Plans
Review Group, helped gather the data,
review the document and, more often
than not, kept me in line. To all of
them I respectfully acknowledge their
assistance and offer my thanks.
REFERENCES
Fig. 17. Highland View Bridge under construction, May 1993, showing 250 ft
(76 m) drop-in section in place.
work, the results are beneficial to all.
In Florida, we have seen the results
of good work in the many bulb-tee
bridges successfully built and in service. There will be the occasional bad
duct, poor consolidation or cracks, as
seen in every system used ; but this
system is viable, economical and aesthetically pleasing.
We are approaching the era when
even longer spans, in the 150 to 160ft
(46 to 49 m) range, should be considered. Also, with the proliferation
July-August 1993
of the drop-in segment design, spans
of 250 ft (76 m) may become more
common.
ACKNOWLEDGMENT
There are many individuals who
have , in one way or another, contributed to the preparation of this
paper. Henry Bollmann and Paul
Csagoly were instrumental in implementing the girder's use in the first
place. Mohsen Shahawy was involved
1. "Test Girder Shows Strength," Engi-
neering Ne ws-Record, December 19,
1985, p. 29.
2. Csagoly, Paul F. , and Nickas, William
N. , "Florida Bulb-Tee and Double-Tee
Beams," Concrete International, V. 9,
No. 11 , November 1987, pp. 18-23 .
3. Shahawy , M. , and Garcia , A. M .,
" Structural Research and Testing in
Florida ," Transportation Research
Record 1275, Transportation Research
Board , National Research Council ,
Washington, D.C. , 1990.
4. Shahawy , M. , " Strength Evaluation
and Load Testing of the Eau Gallie
Bridge," Structures Design Office Report, Florida Department of Transportation, Tallahassee, FL, 1991.
49