Sunshine Skyway Bridge Ship Impact Design of Low Level Approaches

Home , Foundation (engineering), Skyway, Sunshine Skyway Bridge

Vijay Chandra Vice President Parsons Brinckerhoff Quade Douglas, Inc. New York, N.Y.

George Szecsei Project Engineer Parsons Brinckerhoff Quade Douglas, Inc. New York, N.Y.

afety was in everyones mind when a of structure traditionally designed for its S new Sunshine Skyway Bridge - own live and dead loads, but not ship an innovative cable stayed prestressed impacts. concrete structure — was authorized This made the design of the low level across Floridas lower Tampa Bay. In approaches a new type of challenge, and 1980 a main span of one of the older our firm, Parsons Brinckerhoff Quade twin bridges had collapsed during a Douglas, Inc., was asked to take up the catastrophic ship impact. The risk of a task. We discovered that protection similar impact to the new bridge was could be provided, at only a modest cost evident due to wayward vessels not only increase over traditional design, by ex- in the area of the central channel but all ploiting the elasticity of prestressed along the crossing, thus creating an es- concrete to absorb and transfer impact sentially shore-to-shore ship impact risk loads. As a result, for apparently the first over a 4.18 mile (6.73 km) length. Dur- time in the United States, a major ing design, a decision was made that ad- crossing (see Fig. 1) would be designed vanced todays trends toward incor- for a ship impact anywhere on its length porating safety, reliability, and ease of —the first shore-to-shore protection. maintenance into design criteria. This article describes the design ap- That decision was to extend ship im- proach and construction of the low level pact criteria beyond the channel spans approaches, and the application of pre- normally protected, to include also the stressed concrete to solving the complex lengthy low level approaches — a type problem of ship impact requirements.

96 Presents the design-construction method used for the low level approaches together with the application of prestressed concrete in solving the unprecedented shore-to-shore ship impact requirements of the Sunshine Skyway Bridge. The results show that prestressed concrete is an excellent material for resisting impact.

THE NEED sisted of twin two-lane bridges with bal- anced cantilever truss main spans. The In 1980 the phosphate freighter original of the pair, designed by Parsons Summit Venture struck the southbound Brinckerhoff in 1954, had been a (western) bridge of the Sunshine Sky- trendsetter in its day: the most massive way, causing a widely publicized failure application yet of prestressed concrete and creating the need for the new Sun- for an American bridge. Originally a shine Skyway Bridge. two-way bridge, it had been converted Before the impact, the crossing con- in 1971 to northbound-only to share the

Fig. 1. A finished view of the new bridge with old bridges in the background. This is the first time a major crossing has been designed for ship impact along its entire span.

PCI JOURNAUJuly-August 1988 97 CD co N

NORTH ABUTA BU I SOUTH ABUTMENT NEW BRIDGE o -f

CHANNEL DOGLEG ^^^^ ^ _" I i` f 1

NAVIGATIONAL INTRACOASTAL CHANNEL WATERWAY

4283 2430 _860540 1200 T40 , 860 I2430 I 87 37 _^

LOW LEVEL APPROACH HIGH LEVEL MAIN SPAN AREA HIGH LEVEL LOW LEVEL APPROACH APPROACH APPROACH TWIN STRUCTURE SINGLE STRUCTURE TWIN STRUCTURE

NOTE: I fool • 30.48 cm Fig. 2. Layout of new Sunshine Skyway Bridge. traffic with its newer twin; it was this than the old, the skew to the channel 1971 bridge that was struck by the could be reduced and more space pro- Summit Venture, decommissioning the vided between bridge and dogleg. This southbound crossing, reducing the arrangement offered better maneuver- four-lane total to two, and disrupting the ability to ship captains trying, some- local economy and commuters. The times in inclement weather, to navigate original 1954 crossing was undamaged, the dogleg in the channel and then get and again carried traffic in both direc- their ships properly aligned before tions during construction of the new passing between the bridge piers. This cable stayed bridge. Both older bridges realignment supplements the impact will eventually be demolished, with protection built into the new design (see sections perhaps left (depending on the Fig. 2). outcome of current studies) for a new life as fishing and recreational piers. Greater Clearances Fig. 1 shows the new cable stayed Sunshine Skyway crossing in its fin- By nearly all measures, the new ished state, with the two older truss bridge is larger than either of its twin crossings in the background. Thus the predecessors and gives ships greater new crossing, opened to traffic in mid- clearance. For the record, some of the 1988, was born not only to replace the particulars of the new bridge are: previous crossings and improve the re- • Total length of crossing = 21,880 ft gional economy, but to provide an im- (4.18 miles) (6.73 km) proved, safer structure — better aligned, Low level approaches (twin road- more generous in navigational clear- ways): north = 4283 ft (1.31 km) ances, and with new criteria to resist the each; south = 8737 ft (2.66 km) impact forces of any aberrant marine each traffic. With its striking cable stayed High level approaches (twin road- main span designed by Figg and Muller ways): north and south = 2430 ft Engineers and its lengthy low level ap- (0.74 km) each proaches designed by Parsons Main span area (single wide road- Brinckerhoff, this new 4.18 mile (6.73 way): 4000 ft (1.22 km) km) crossing with shore-to-shore ship (Cable stayed spans = 540-1200- impact protection is at the forefront of 540 ft) (165-366-165 m) todays prestressed concrete bridges. • Main channel width = 500 ft (152 m) • Main span horizontal clearance = 350 ft (106.7 m) on either side of chan- THE NEW, SAFER BRIDGE nel [versus 182 ft (55.5 m) in the old bridges] The new bridge provided superior • Vertical clearance: advantages in the form of better align- Over the channel = 175 ft± (53.3 m) ment, greater clearances and more effi- At low level approaches = 20 ft ± (6.1 cient structural configuration. m) • Water depth: Better Alignment Low level approaches = 0 to 24 ft (0 Safety planning began at the most to 7.3 m), mostly 12 to 24 ft (3.7 to basic level: where to place the bridge. 7.3 m) There is a dogleg in the Tampa Bay High level approaches = 24 to 30 ft Channel west of the bridge where the (7.3 to 9.1 m) Intracoastal Waterway Channel meets Main span area = 30 ft (9.1 m) the Tampa Bay Channel. By putting the Channel = 43 ft (13.1 m) new bridge 1000 ft (304.8 m) farther east • Tides:

PCI JOURNAL/July-August 1988 99 Tidal range: mean = 1.3 ft (40 cm); parallel two-lane structures, one for extreme = 3.8 ft (116 cm). northbound and the other for Current: 0.8 to 1.0 knot (1.48 to 1.85 southbound traffic. Single cell, km per hr); 3.0 knots (5.56 km per precast post-tensioned, continuous hr) possible in outgoing tidal concrete box girders, supported on stream. concrete piers, are used. The • Wind = 50 knots (92.66 km per hr) or length of each span is 135 ft (41.2 more, 65 knots (120.46 km per hr) m). The foundations are supported maximum recorded. by 24 in. (61 cm) square, precast Hurricane possible every other year prestressed concrete piles. within 60 nautical miles (111.19 • Main span area. This consists of a km). single structure with a single cell, precast post-tensioned, concrete box superstructure that carries Structural Configuration northbound and southbound traf- From shore to shore, the new bridge fic. The substructure consists of complex consists of three distinct types post-tensioned concrete piers sup- of prestresssed concrete structures, each ported by 24 in. (61 cm) square with appropriate considerations for im- precast prestressed concrete piles. pact risk assessment and protection. The The main piers support 432 ft high dual roadway structures of the low level (131.7 m) cable stayed pylons. The north and south approaches stretch from main and flanking spans are cable the shores, rising into high level ap- stayed, with a single plane of stays proaches that lead in turn to the main at the center of the two roadways. span area. In the main span area, the parallel approach roadways merge into a single wide roadway with a central COLLISION RISK cable stayed main span and two flanking spans (see Fig. 2). These are the The risk assessment and impact characteristics of the structural types: criteria are now discussed. • Low level north and south approaches. These consist of two Risk Assessment parallel two-lane structures, one for northbound and the other for At the initial stage of the project, the southbound. The superstructure Florida Department of Transportation consists of a four-span continuous (FDOT), through Figg and Muller En- reinforced concrete deck slab sup- gineers, requested COWlconsult of ported on precast prestressed con- Denmark to perform a ship collision risk crete AASHTO Type IV girders. assessment study. The study is based on The substructure consists of rein- a mathematical risk assessment model, forced concrete wall type piers using available background data. The founded on 20 in. (51 cm) square study considers the various types of ma- precast prestressed concrete piles. rine traffic using lower Tampa Bay and The length of each span is ap- identifies the main reasons for ship col- proximately 100 ft (30.5 m). The lisions with bridges: piers of the parallel roadways are • Human error (misjudgment, negli- connected across between the two gence, etc.) structures by precast prestressed • Mechanical failure (engine or concrete frangible struts. steering failure) • High level north and south ap- • Environmental conditions (cur- proaches. These also consist of two rents, wind, fog, etc.)

X61 • Alignment of the bridge with re- should be applied: spect to the entrance channel Main span area = 4000 kips (17.79 The report stated that the designers MN), except 12,000 kips (53.38 could use probability to optimize safety MN) at the cable stayed pylon and cost by distributing the risk level piers. along the bridge structure and by ac- High level approach = 2000 kips cepting a tolerable level of risk. The (8.90 MN). following practical protective measures The ship impact load and the perma- were suggested: nent dead loads were not to be factored. • Design the piers and pier shafts All the pier elements were to be de- along the entire length for certain signed for ultimate loading conditions magnitudes of static ship impact only. force since superstructures them- Based on the recommendations of the selves rarely have a substantial re- risk assessment study, the following im- sistance to horizontal loads. The provements over the old bridge design point of impact should correspond were made: to the height of the hull of relevant • Increased navigational horizontal ships. and vertical clearance of main • Raise the roadway level along the span. entire crossing. • Protection of main and flanking • Protect the main and flanking piers piers with sand islands and dol- with sand-filled rock islands and phins. with dolphins. • Shift the alignment farther east to increase the distance from the 18 IMPACT EVALUATION degree dogleg in the navigation channel west of the old bridges. Since the risk assessment indicated • Consider aberrant barges as the that barges and other vessels might most likely vessels to hit the strike any portion of the bridge, the ship bridges. impact criteria had to be applied for the entire length of the bridge, including the approaches. The design team for the Impact Criteria lengthy low level approaches, therefore, As a result of the ship collision risk faced a perhaps unprecedented task in assessment study, FDOT set up the fol- protecting a type of structure normally lowing criteria for the design of the new not protected against ship impact. Sunshine Skyway Bridge, in addition to The remainder of this paper describes AASHTO loads and their loading com- how the Parsons Brinckerhoff team ar- binations: rived at the design of the ship impact • Low level approaches. The ship protection for these low level structures. impact load of 1000 kips (4.45 The key to our design proved to be MN) ultimate static load, com- linking the twin structures so that the bined with the dead load, should force from an impact would be resisted be applied at 0 to 30 degree skew by the piers for both roadways working and at the water level of the outer together like interconnected springs. pile caps only. • High level approaches and main Concepts and Alternatives span area. The following ultimate static ship impact loads, A ship impact force of 1000 kips (4.45 applied at 0 to 30 degree skew, MN), even when it is applied at the and combined with the dead load water level, is a large force for a low

PCI JOURNAL/July-August 1988 101

0 N)

I000k Iii SIDE VIEW CONCRETE PILES (TYP)

BETWEEN BATTERED STRUCTURES PILES 0:3) VERTICAL PILES 30 ^^ IOOOk•ill I ______ /

PILE PLAN

• 2.54 cm Fig. 3. Typical cross section of low level approaches. level structure to resist. Measures such Geotechnical Investigation as protective nets, pile barricades, or Evaluation of a bridges structural dolphins along the entire length of such system for ship impact forces is influ- a long crossing are extremely expensive enced by the soil characteristics below and, in many instances, impractical. the river bottom and the resulting de- Therefore, it was necessary to make an flection of the piles. The stiffer the soil, evaluation and arrive at a rational solu- the smaller the deflection of the top of tion satisfying the stringent design the pile. In the area of the low level ap- criteria. proaches, as is usual in long approaches, The following design criteria for the the soil stratification and composition low level approaches were set prior to vary (Fig. 4). this evaluation: In the area of the north approaches, • The previously approved pier the water depth varies from 0 to 30 ft (0 shape had to be maintained. The to 9.1 m), with a deep sand and shell pier consisted of two columns con- layer around elevation –55.00 ft (-17 nected with a pier cap, diaphragm m). Below the sand and shell layer, clay, walls, strut above the footings, and with intermittent silt and limestone individual footings under each col- pockets, predominates. umn (Fig. 3). In the area of the south approaches, • The superstructure consisting of the water is shallower and varies in AASHTO Type IV beams and a depth from 0 to 25 ft (0 to 7.62 m), with a deck with no intermediate dia- thin layer of sand and shell underlain phragms was to be used. mostly with clay. Sandstone and dolo- The following limitations were ap- mite lenses are present in the clay at a plied to the ship impact forces: few locations. • Ultimate load of 1000 kips (4.45 The projects geotechnical consul- MN) was to be combined only with tants, Schmertmann and Crapps, Inc., unfactored dead load. performed an extensive geotechnical • Ship impact load was to be as- field investigation, including an elabo- sumed to be a static load applied at rate field testing program. As a result of the water level. this program, the safe working load level • Ship impact load was to be applied for the 20 in. (51 cm) square prestressed only at the outer, not inner, pile concrete piles was established at 150 caps (those that are exposed to an tons (1.33 MN), with an ultimate factor aberrant barge or ship). of safety of 2.25. • Ship impact angle was to be within Vertical load on a pile was only one ±30 degrees measured in a hori- factor in the puzzle to design a structure zontal plane from the pier center- for ship impact criteria. The large hori- line (see Fig. 3). zontal forces of a ship impact to any • The superstructure was not to be given pier also had to be accounted for. designed to resist direct ship im- Either the piers would have to be very pact forces. massive, or, conversely, they would • It was assumed that if the ship im- have to be slender and flexible and so pact force to one of the roadways linked that a group of piers shared the (northbound or southbound) were ship impact force. The flexible linking greater than 1000 kips (4.45 MN), it of adjacent piers with a precast pre- would damage the impacted road- stressed concrete frangible strut was the way, but the adjoining roadway key. would remain undamaged and To simplify the analysis and at the open to traffic. same time simulate the interaction of

PCI JOURNAUJuly-August 1988 103

r2 0 0 . Mean Sea Level

—60 6 —80

—120

NORTH APPROACH SOUTH APPROACH

WATER SANDSTONE

SAND AND SHELL ;;;+:LIMESTONE

. CLAY DOLOMITE

®SILT CLAYSTONE

NOTE: I foot - 30.48 cm. Fig. 4 Soil stratification beneath the low level approaches. soil and structure, the "equivalent point used as a frangible strut between the of fixity" approach was used to model northbound and southbound roadway the foundation system. Using method- piers. ologies described in articles by Davis- The authors believe the following ad- son (1965, 1970), 1.2 Reese and Matlock vantages make prestressed concrete the (1956),3 and Penizen (1970) and the suitable material to resist ship impact: finite difference computer program de- • The high load resistance of the pre- veloped by Reese and his colleagues, stressed concrete piles, both in the design team determined equiva- compression and in tension. lent points of fixity for a unit pile ex- • Ductility of the prestressed con- pressed as a depth below the mud line crete piles to transmit the excess beyond which the pile could be consid- loads to adjoining foundation ele- ered fixed. Equivalent points of fixity ments through the concrete piers. were evaluated based on an estimation • Ability of the stiff precast pre- of the moment curvature and deflection stressed superstructure element to of a unit pile. transmit the unresisted loads to the Equivalent points of fixity for the low adjoining piers, without which, re- level approaches of the structure can be sistance to ship impact forces summed up as follows: would have been very difficult. • 15 ft (4.57 m) below mud line for • Simple transfer connection both at piers located in less than 15 ft (4.57 the foundations and at the bearing m) of water. levels to transfer the high ship im- • 12 ft (3.66 m) below mud line for pact forces. piers located in greater than 15 ft • Saving in costs due to the elimina- (4.57 m) of water. tion of intermediate diaphragms The above values are an average for because of the stiff prestressed all the low level approach piers. How- concrete beams. ever, piers in the north approach had • Greater safety for the bridge not deeper points of fixity than south ap- struck, since the precast pre- proach piers because of different soil stressed concrete frangible strut conditions. can be designed to fail before Individual pile deflections tend to be transmitting too much load from greater than a pile groups deflection. the struck bridge to the adjacent The final results of the frame analysis, one, endangering both. Thus, the using the equivalent points of fixity ap- strut acts as a fail-safe mechanism. proach, were compared to the individual pile deflections obtained from the pre- Pile Layouts viously mentioned programs and were found to be less than the deflection ob- Several pile group layouts were tained for the individual piles. thoroughly investigated. They ranged from six to ten piles at a pier, with the piles at several horizontal angles and Advantages of Prestressed also at different slopes. Most alterna- Concrete to Resist Ship Impact tives failed, due to either high compres- Throughout the low level approaches, sion or high tension loads in the piles. precast prestressed concrete piles sup- Other alternatives, though theoretically ported a reinforced concrete pier. The feasible, had to be discarded as imprac- piers supported precast prestressed tical since they were difficult to concrete I-beams, and a concrete deck construct and were uneconomical. With was poured on top (Fig. 3). In addition, a the introduction of a precast prestressed precast prestressed concrete beam was concrete frangible strut between the

PCI JOURNAL/July-August 1988 105

O NORTHBOUND ROADWAY rn PILE GROUP PILE GROUP j BETWEEN 8 COLUMN B COLUMN STRUCTURES 13-9 1 13-9" 12-6 3-10 1 3-IO" I

BATTERED PILES ON - - - r------PRECAST PRESTR SSED 3p,• CONCRETE FRANGI E STRUT 1000^ N o I -

500 N N I I —STRU Ti i L------J ihM - I-10}" 2-9 2 9 2-9" I 10 VERTICAL PILES 6-0" 6-0 2-3" PLAN EXTERIOR SUPPORT INTERIOR SUPPORT

------}------^ STRUT I IPRECAST PRESTR^SSED — -. -- — — — —4------_ l CONCRETE FRANGIBLE STRUT 9 • TH. CONC. I ° I I I SEAL --. •--• EL. I^f1T1i T -^-r-i ^ — — ^^^ , f 20 SO. PRECAST PRESTRESSED PILE I 1

ELEVATION

foot • 30.48 cm : I inch • 2.54 cm Fig. 5. The constructed foundation system. Fig. 6. Precast prestressed concrete piles being driven using template.

adjoining structures, a practical solution was achieved. Using the frangible strut, two work- able pile layouts were developed for consideration, a six-pile system in shallow water and a seven-pile system in deeper water. The proposed six-pile system had four piles at the exterior and two piles at the interior supports, whereas the proposed seven-pile system had four piles at the exterior supports and three piles at the interior supports (see Fig. 5). Finally, after much evaluation and discussion, a seven-pile-per-pier arrangement (in both shallow and deeper water) was selected for uniformity as well as for its increased safety in low water areas. This scheme was accept- able to the FDOT, the Federal Highway Administration (FHWA), and the contractors. The piles are shown being driven and after cut-off in photographs taken after construction was started Fig. 7. View of precast prestressed (Figs. 6 and 7). piles after cut-off.

PCI JOURNAL/July-August 1988 107 Analytical Models structure, connected through the precast prestressed concrete frangible struts Three levels of modeling were used, (Fig. 8). representing first a single pier, then a series of piers (from a single roadway In all of the above, the piles were structure), and finally two parallel modeled as individual columns, fixed multi-pier roadway structures. top and bottom. The battered piles in The first level was a simplified two- the exterior pile caps intersected the dimensional model of a single pier with vertical center of gravity of the cap in its foundations and did not include the order to reduce the moments in the superstructure. This model was used piles. The diaphragm walls between the only to help in designing the larger columns were idealized into a grid. In three-dimensional models. the second and third levels, the com- The second level was a three-di- posite section of the superstructure was mensional model of a four-span con- modeled as a series of line elements. tinuous superstructure with the five The transverse continuity of the deck piers of a single roadway. The indi- was simulated by the diagonal bracing vidual piers were modeled as in the elements, which helped to create the first-level model. No frangible strut effect of a horizontal truss for transfer of connection was included in this second loads from one pier to the next. At the model. The results of the computer run bearing levels only the lateral and lon- of the second-level model clearly gitudinal moments were released. The showed that a single structure was un- behavior of the elastomeric bearing pads able to resist the ship impact forces. was considered as a longitudinal spring, At the final third level, the second- but since the effect was minimal, it was level model was extended to include the discarded. piers and superstructure of the adjoining Fig. 9 shows the plan view of the

= : ,•. x;11 i • • • •0 •

•

Fig. 8. Isometric view of final computer model.

108 C C- 0 PIER I PIER C PIER PIER . PIER I, t z 2.40 1.60 1.60 C- C 3Ok C 48k T 325k T 367k 365k T 323k 48k T 3ok C C CDC c

COm

1.60" 1.60" 60k C ^\\ 60k C 7 k C 329k C 370k C k k 73k C 240 BETWEEN _ FRA NGI BLE STRUCTURES STRUT (TYP.) 2.51 (DEFLECTIO -TYP.) 442k 29k 49k G 1.56 I.56"Y 50k 28k 7 372k 439k 375k

56k C 49k C 48k C 55k C 378k C 441k C 446k C 381k C 1.56" 1.56 I I 2.51

T TENSION t 1000k C COMPRESSION PLAN OF DECK

NOTE: I kip 4.448 kN : I inch • 2.54 cm Fig. 9. Distribution of forces and deck lateral deflection from a ship impact at continuous deck pier. BETWEEN STRUCTURES ESUPERSTRUCTURE = I22 KI L11J L11ESUPERSTRUCTURE 36K

d 43

MUD LINE i

PIES • 7 9 K II I PjL E S 79K CROSS-SECTION NOTE: I kip = 4.448 kN Fig. 10. Force transfer in the ship impacted pier.

model for a four-span continuous unit utilizing additional simplified models. between deck expansion joints. The Fig. 12 also shows the deflections and center pier was subjected to ship impact forces in the superstructure resulting force. The lateral springs at the two ends from a ship impact load at the expansion simulated the resistance from the adja- joint pier. The force transfer in the sub- cent units through the pier shear blocks. structure is very similar to that at a con- Fig. 9 also shows the deflections and tinuous deck pier, as shown in Fig. 10, forces that resulted in the superstruc- but with minor variations in the values. ture. Fig. 10 shows the force transfer in the impacted substructure elements, and Fig. 11 shows the force transfer in Evaluation of the Analytical Results the adjoining substructure elements. A careful evaluation of the various The model similar to that in Fig. 8 was computer outputs verified that when the created to simulate an expansion joint ship impact angle was zero, all the piles pier under ship impact. Fig. 12 shows encountered axial compression along the plan view of this model. A discon- with lateral moment, but piles of the tinuity was introduced in the interior pile cap experienced lower superstructure at the expansion joint magnitude forces than piles of the ex- pier, and also at each end of the four- terior pile cap. span model. Longitudinal and lateral When the ship impact angle was springs were introduced at these ends to changed laterally to ±30 degrees, the simulate a continuous deck behavior. piles experienced similar but smaller The spring constants were evaluated forces in the lateral direction than for

OM Fig. 11. Force transfer in the pier adjoining the snip impaciea pier.

Table 1. Pile loads at the impacted pier.

Impact angle Exterior pile cap piles Interior pile cap piles (Impact pier) (Impact pier) Maximum Maximum Maximum Maximum axial axial axial axial tension compression tension compression

0 degrees — 324 kips — 181 kips 30 degrees 148 kips 619 kips — 213 kips Note: 1 kip = 4.448 kN. the zero ship impact angle. However, the piles in the exterior pile caps the longitudinal component of the 30 underwent axial tension. However, only degree ship impact force was seen to two piles in the group suffered axial ten- cause tension in two of the four piles of sion, while the other two piles were the exterior pile caps and compression subjected to axial compression. In the in the other two. Table 1 shows the re- interior pile caps, all piles were sub- sulting pile load values for the deeper jected only to axial compression. water area. The shallower water area The moments were large in the lateral pile forces were less critical. direction (along the axis of the pier). The As can be observed in Table 1, only longitudinal component of the ship im-

111 PCI JOURNAL/July-August 1988 PIER PIER PIER PIER PIER 2.98 2.98 1.48"

497k C 451, T 180k C 146k C 142k C 175k 443k C 489k C

• I

1.48 1.4 8 493k T 446k T 178k T 143k T 139k T 170k T 441k T 487k T FRANGIBLE _ 2.98 +2..1 8 BETWEEN STRUT (TYP.) 3.122" STRUCTURES 60k C 52O,5, 28k C 184k C 18k C 234k C 1.44" 529k C 568k C 1.44

556k T 517k 225k T 14k 179k T 231k 526k T 566k T 1.44" 3.12 3.12 1.44

T TENSION C COMPRESSION f 1000k PLAN OF DECK

NOTE: I kip 4.448 kN : I inch = 2.54 cm Cn 1) n;—:L... _r s -• ^• uu• ai u uc^^ gat WI ueiiecuun arum a snip impact at expansion point pier.

CONCRETE DECK

CONSTRUCTION o SHEAR JOINT – K

PIER CAP

DIAPHRAGM WALL

PILE GROUP PILE GROUP AND COLUMN I AND COLUMN

NOTE: I foot = 30.48 cm ; I inch - 2.54 cm Fig. 13. Typical pier top detail.

pact force was mostly resisted by the with bending moments along both the A-frame action of the piles in the ex- principal axes. In general, buckling is terior pile cap, and moments in the critical in piles not only during high longitudinal direction (perpendicular to compression loads coupled with biaxial the axis of the pier) were minimal. moments but also during low compres- The model showed that a420 kip (1.87 sion loads coupled with relatively MN) force from the impacted pier, at the higher biaxial moments (Hawkins 1977,5 foundation level, would be transferred Nathan 1983, 6 Park and Falconer 1983, to the pile of the adjacent structure Sheppard 19838). In our case, the trans- through the frangible strut. The unre- verse moment along the pile, starting sisted horizontal force at the foundation from the top, decreased gradually before level would be transferred to the super- reaching the point of inflection ap- structure elements through deflection of proximately at the mud line and then the piers. The force transfer from sub- gradually increased up to the equivalent structure to superstructure was accom- point of fixity at about 10 to 15 ft (3.1 to plished through shear blocks at the top 4.6 m) below the mud line. Thus, the of the piers (Fig. 13). The combined re- entire length of the pile up to the sponse of the beams with the end dia- equivalent point of fixity was critical for phragms would transfer the force into buckling. the deck. The deck would act as a horizontal diaphragm, and by lateral bending transfer the unbalanced load into the PROTECTIVE DESIGN adjoining piers. In the adjoining piers, the load transfer path would be re- We expected the plastic hinge forma- versed, running from the deck level to tion in the piles to occur just below the the foundations. pile cap. In order to provide additional As a result of the various analyses, the fixity at the pile cap level, the piles em- 20 in. (51 cm) square battered piles were bedment into the caps was increased to determined to experience maximum 1 ft 3 in. (38.10 cm) (Fig. 14). Drilled-in compression or tension loads, together reinforcement was provided at the top of

PCI JOURNAL/July-August 1988 113 }m SPIRAL 20 S0. PILE

O . 20 HOLE

1g 12-}• 270k STRANDS #10 DOWELS

PLAN

TOP OF PILE CAP

x10 DOWELS

5 TURNS PILE PENETRATION 0 1" PITCH

BOTTOM OF PILE CAP

CONCRETE SEAL 2 PITCH SPIRAL

DRILLED HOLES

12 -4♦ 270k STRANDS

SECTION

NOTE: I kip • 4.448 kN I foot • 30.48 cm : I inch 2.54 cm

Fig. 14. Precast prestressed concrete pile details.

114

7-2 " 4-0" 7 -2 .. TOP OF CAP " ul N

B 2j 4 N LEVELING ELEVATION GROUT

41-3-}"0 270k P 3 STRANDS A-A B-B

NOTE: I kip ° 4.448 kN : I foot • 30.48 cm : I inch 2.54 cm

Fig. 15. Precast prestressed concrete frangible strut.

the pile and additional spiral reinforce- critical buckling load is theoretical, and ment was provided in the upper 10 ft can vary due to actual in-place strength (3.1 m) of the pile to increase ductility of concrete, elastic modulus, etc., it was and reduce any damage due to plastic decided to test a few of the struts with hinge formation. We expected also that minor variations to optimize their cross the plastic hinge formation would occur section. The successful strut shape was only in the battered piles of the exterior then finalized (Fig. 15). pile caps. In our opinion, at the onset of All the various pier elements such as plastic hinge formation, redistribution of the strut above the footings, columns, forces between the piles in the exterior diaphragm wall, pier cap, and shear pile caps and also to a certain extent blocks were carefully designed to resist with the other piles along the entire the ship impact transfer force. cross section would take place, reducing Strengthening of the end diaphragms the damage. Any such damage inflicted and additional reinforcement at the top on the battered piles of the exterior pile and bottom of the deck slab were re- caps would be minor and could be required to transmit the horizontal forces. paired. Fig. 13 shows the details at the top of a The precast prestressed concrete pier and Fig. 16 shows a typical con- frangible strut connecting the two ad- tinuity diaphragm and deck detail. joining piers was carefully designed for At the expansion joint pier, an addi- a critical buckling load (Michalos and tional element of horizontal restraint Wilson 1971 9) of 450 kips (2.00 MN) was provided, and the end diaphragm [slightly higher than the 420 kips (1.87 was strengthened to resist the longi- MN) transfer force]. The strut carries a tudinal tension force (Fig. 17). The minimum amount of prestress force to longitudinal restrainer system was de- eliminate hairline cracking due to its signed to accommodate the normal own dead load deflection. Since the thermal movements of the structure.

PCI JOURNAL/July-August 1988 115

DECK SLAB

' 5 I I •8 I TH. PREMOLDED FILLER

SHEAR BLOCK II I I

1 1

NOTE: I inch 2.54 cm

Fig. 16. Continuity diaphragm detail.

x 10 2-6" — THREAD BAR SHEATHING (TYP.) I}"0 H.S.

BARTBAR (UNSTRESSED) (STEE ifItiSJ/flEt• 2 PER BAY (2 BAYS) X506" STEEL ANCHOR PLATE 6 (CONT.) x 8 (CONT.) ) NEOPRENE PAD

L --- SHEAR BLOCK GAP TO ACCOMMODATE SHRINKAGE. CREEP, AND THERMAL MOVEMENTS.

NOTE: I foot = 30.48 cm : I inch • 2.54 cm

Fig. 17. Expansion joint diaphragm detail.

116 Fig. 18. Pile caps after construction.

CONSTRUCTION OF field. During the pier construction, as is APPROACHES usual, some piles were out of place after they were driven. Construction of the piles and of the This was corrected by either adding beams and deck are now described. additional piles to the bent if the error was too much; or, if the error was minor, the bent was reevaluated, and in most Piles instances construction was allowed to The construction of the low level ap- proceed. However, a continuous com- proaches was started in late 1983 and munication channel was open during finished by late 1986. The precast pre- construction between the designer and stressed piles for each pier were driven the field personnel in order to cut delays in position using a template (as shown when problems occurred. in Fig. 6). The batter piles and the vertical piles were then cut perpendicular Beams and Deck to their axes (Fig. 7) prior to pouring the After a few piers were constructed, pile caps (Fig. 18). The strut and pier construction then followed (Figs. 19 and the erection of the prestressed concrete 20). Careful attention was paid to pre- beams followed (Fig. 21). At the same fabricating reinforcing bar cages, off- time, additional piles were being driven site, so that they could be slipped into and piers constructed farther up the their respective forms. This made the line. Fig. 22 shows a continuity dia- construction of the pier units easier and phragm after construction. more economical since minimal tying of The reinforced concrete deck was reinforcing steel was necessary in the poured after a few spans were erected

PCI JOURNAL/July-August 1988 117 Fig. 19. Pier under construction. Strut on top of pile caps in the background.

Fig. 20. Piers after construction.

118 Fig. 21. Precast prestressed concrete beams after erection.

(Fig. 23). The continuous deck slab was poured as one unit from expansion joint to expansion joint, starting from one end and proceeding to the other. Fig. 24 shows the precast prestressed concrete frangible strut in place between the piers. Fig. 25 shows the completed view of the Sunshine Skyway Bridge. During construction, close contact was kept between the designers and field personnel. All field problems were solved within a short period of time and the structure was successfully completed.

COSTS OF MEETING IMPACT CRITERIA

The need to meet the ship impact criteria does increase the cost of a structure, both during design and during construction. The design cost increase is Fig. 22. Deck continuity diaphragm.

PCI JOURNAL/July-August 1988 119 Fig. 23. Birds eye view of deck slabs after pour.

Fig. 24. Precast prestressed concrete frangible strut in place.

120 Fig. 25. Completed view of Sunshine Skyway Bridge.

difficult to quantify, but it is minimal. CONCLUDING REMARKS The construction cost can be more read- ily estimated. The ship impact criteria The stringent criteria for the design of required the following additional items: the low level approaches necessitated • Additional precast prestressed evaluating several configurations before arriving at a practical engineering solu- concrete piles. • Increased reinforcement in the tion. Several alternative solutions were strut between the pile caps. feasible in principle but involved dif- • Introduction of shear blocks at the ficult construction. The results, pre- tops of the piers. sented in this paper, were thoroughly • Additional reinforcement at both scrutinized and were found satisfactory faces of the deck to resist longi- by FHWA and FDOT. Several discus- tudinal bending (in plan) of the sions preceded the final decision. For the designers of future pre- deck. • Introduction of precast prestressed stressed concrete crossings over navi- concrete frangible struts between gable waterways, the Sunshine Skyway project suggests that ship impact pro- adjoining roadways. • A complicated interconnection of tection can effectively be extended to the expansion joint diaphragms. the entire crossing, including low level The above items added about 15 to 20 approaches. The elasticity of pre- percent to the construction cost of the stressed concrete enables the designer project when compared to a structure to meet impact criteria without massive designed conventionally without ship pier structures or costly protective impact criteria. structures surrounding the piers. De-

PCI JOURNALJJuly-August 1988 121 signers using the Sunshine Skyway ex- still a structure was designed and suc- perience as a model should bear in mind cessfully built for the ship impact that it was a pioneering effort; the de- criteria. The increase in cost over a con- signers of the low level approaches were ventional design is small when com- limited to those solutions that were pared to the protection attained and the compatible with the basic design con- reduction in potential disruption to the cepts already established. areas commerce. For the Sunshine Skyway Bridge — a long structure in an area where aberrant marine traffic is frequent — ship impact ACKNOWLEDGMENT resistance was a critical design require- ment. Impact resistance was success- We wish to offer our sincere appreci- fully achieved in the design by utilizing ation to all engineers, reviewers and the elastic and ductile properties of the contractors who assisted during the de- various elements and letting the struc- sign and construction of this state-of- ture deflect and thus relieve the energy the-art concept. We particularly wish to due to any possible ship impact. Though acknowledge the advice and sugges- the design was complex and many trial tions of FDOT and FHWA (Tallahassee and error procedures were necessary, and Washington, D.C., offices).

REFERENCES

1. Davisson, M. T., and Robinson, K. E., tures," PCI JOURNAL, V. 22, No. 6, No- "Bending and Buckling of Partially Em- vember-December 1977, pp. 80-110. bedded Piles," Proceedings, Sixth Inter- 6. Nathan, N. D., "Slenderness of Pre- national Conference on Soil Mechanics stressed Concrete Columns," PCI JOUR- and Foundation Engineering, V. 2, 1965, NAL, V. 28, No. 2, March-April 1983, pp. pp. 243-246. 50-77. 2. Davisson, M. T., "Lateral Load Capacity 7. Park, R., and Falconer, T. J., "Ductility of of Piles," Highway Research Record, No. Prestressed Concrete Piles Subjected to 333, 1970, pp. 104-112. Simulated Seismic Loading," PCI JOUR- 3. Reese, L. C., and Matlock, H., "Non-di- NAL, V. 28, No. 5, September-October mensional Solutions for Laterally Loaded 1983, pp. 112-144. Piles with Soil Modulus Assumed Propor- 8. Sheppard, D. A., "Seismic Design of Pre- tional to Depth," Proceedings, Eighth stressed Concrete Piling," PCI JOUR- Texas Conference on Soil Mechanics and NAL, V. 28, No. 2, March-April 1983, pp. Foundation Engineering, 1956,41 pp. 20-49. 4. Penzien, J., "Soil-Pile Foundation Inter- 9. Michalos, J., and Wilson, E. N., "Buck- action," Chapter 14, Earthquake Engi- ling, Stress Intensification and Second- neering, Robert Wiegel (Editor), Prentice ary Stresses," Chapter 12, Structural Hall, 1970. Mechanics and Analysis, Gene Nord- 5. Hawkins, N. M., "Seismic Resistance of by (Editor), The Macmillan Compa- Prestressed and Precast Concrete Struc- ny, 1971.

122 MAJOR PARTICIPANTS

Florida DOT Owner.

Parsons Brinckerhoff Low level approach designers and Quade Douglas, Inc. constructibility reviewers for the entire structure.

Figg Muller Engineers High level approach and main span area designers.

SKYCEI, an association of Construction managers. Parsons Brinckerhoff Construction Services H. W. Lochner, Inc. DRC Consultants, Inc. Kissinger Campo and Associates Corp.

Paschen Contractors, Inc. High level approach and main span area general contractor.

Ballenger Corporation Low level approach general contractor.

NOTE: Discussion of this article is invited. Please submit your comments to PCI Headquarters by April 1, 1989.

PCI JOURNAUJuly-August 1988 123