Design-Construction of Precast Prestressed Concrete Floating Wave Attenuator

Roderic A. Ellman, Jr., P.E. Precast prestressed concrete was used imaginatively Senior Associate Mueser Rutledge to build the first phase of a 960 ft (293 m) floating Consulting Engineers wave attenuator in Flushing BaYt New York. The New York, New York precast system proved to be cost effective and efficiently constructable, while also solving the site constraints and environmental issues. The attenuator features three 76 ft (4.9 m) wide, 320 ft (97.5 m) long sections comprising 8 ft (2.4 m) wide, 40 ft (7 2.2 m) long precast, prestressed polystyrene-filled Simon Harton, P.E. concrete box units transversely post-tensioned Ch ief Structural Engineer together. This article provides the conceptual LEAP Associates International, Inc. system, bidding process, design criteria and Tampa, Florida structural design considerations of the project as well as highlights of the manufacture, transportation, and erection of the precast segments.

he World's Fair Marina is located southeast of La Guardia Airport on Flushing Bay in New York. A runway extension project for the airport required re­ Harold E. Wescott T moval of an existing rubble mound breakwater that pro­ General Manager vided wave protection to the marina. Mueser Rutledge Precast Structures, Inc. Auburn, Maine Consulting Engineers (MRCE) was retained by the Port Authority of New York and New Jersey to design a re­ placement breakwater for the facility. Selection of a suitable replacement breakwater was based on the need to meet the performance criteria of the marina, site constraints and environmental issues. MRCE performed a geotechnical investigation that revealed subsurface condi­ tions to include approximately 50ft (15.2 m) of soft organic silt above the glacial outwash sand. Environmental regula­ tions prohibited any filling, which ruled out a replacement rubble mound breakwater. Also, the navigable area was lim­ ited between the marina and an adjacent Federal Anchorage

30 PCI JOURNAL Fig. 1. Aerial view of floating wave attenuator.

Area, restricting breakwater location and movement. MRCE designed a cantilevered steel pipe pile anchorage system that was capable of developing the required lat­ eral resistance in the soft subsurface soils while maintaining the structure's ! location within acceptable limits. Pre­ dicted wave forecasts and required wave protection within the marina led to the selection of a precast, pre­ stressed, post-tensioned, prismatic concrete floating wave attenuator. The total planned length of the new wave attenuator is approxi­ mately 1650 linear ft (503 m). The project was phased to accommodate future marina development with the first stage comprising 960 linear ft (293 m), constructed in three 320 ft Fig. 2. Location plan of floating wave attenuator.

July-August 1997 31 F L U S H N G 8 A Y

190'-0" 20'-G 7-SPACES 0 40'- 0" • 280'- 0" UNE OF FEDERAL • ~CHORAGE AA/ fSTEEL PIPE t /24PILE (TYPICAL) ~------+-~----~------~+------/~------4--H----- 10' -0" _] T+-----16-,-_-0"-__1-.---rlt•::t::jt::;;::-1""'0:...:---....,-r--- _-:i•==~;::::-~:::;:::oi::::;;:::Jo~== ;;::J•~==:.==~•i: ::t{~::;;::...z:....,.,...O --, 0 0 • • • • s·-O"....J t I L MC SERIES PRESTRESSED CONC. BREAKWATER (lYPICAL.) 8-SPACES 0 40'-0" • 320'-0" 120'-(]

4'-Cf

PLAN 40'-- 20-' 0 40'- 80'

Fi g. 3. Plan of floating wave attenuator (for Section A-A, see Fig. 4) .

(97 .5 m) sections. The Stage 1 struc­ DESIGN CRITERIA AND miles per hour (96 km!hr). Tidal varia­ ture, described in this paper, was STRUCTURAL DESIGN tion measured approximately 17 ft completed in March 1996 (see Fig. 1). (5 .2 m) between mean low water and CONSIDERATIONS the 100-year flood tide. Historical data Several wave attenuator systems for the original rubble mound break­ BIDDING PROCESS were evaluated for use on this project water indicated a maximum transmit­ Due to the specialized aspect of the including timber, timber/concrete ted wave height of 2ft (0.61 m). floating wave attenuator structure, it composites, timber/steel composites Based on the design criteria, site was decided to use a proprietary sys­ and prestressed concrete. The wave at­ conditions and required attenuation tem. MC Series prestressed concrete tenuator had to meet the following de­ performance, the required nominal di­ floating segments - designed jointly sign criteria: mensions of the floating wave attenua­ by Marina Components, Inc. and Pre­ • Provide a minimum 50 percent wave tor were 4 ft 3 in. deep by 16 ft wide cast Structures, Inc., engineered by attenuation performance of the spec­ (1.3 x 4.9 m). The anchorage system LEAP Associates International, Inc. and ified design wave and furnish a min­ included 24 in. (610 mm) diameter manufactured by Precast Structures, imum 1 ft (0.30 m) of free board open ended steel pipe piles spaced 20 Inc. - were selected for this project. above the still water level. ft (6.1 m) on centers. Contract documents indicated de­ • Simultaneously resist all vertical Figs. 3 and 4 show the plan and sec­ sign and performance criteria for the and horizontal forces generated by tion of the floating wave attenuator. floating wave attenuator and required the specified design wave and dis­ the use of the MC Series prestressed placements of the anchorage system. concrete floating segments. The final • Allow ease of assembly and trans­ STRUCTURAL SYSTEM design and detailing of the segments port to the site. The precast, prestressed wave atten­ was performed by LEAP Associates • Provide long-term durability with uator structure is designed and built for the successful bidder. Bids were simple maintenance in a marine en­ based on the principles of segmental received from several marine contrac­ vironment. construction. Individual 8 ft wide, 4 ft tors with a fixed price for floats. Gen­ For this particular site (see Fig. 2), 3 in. deep, 40 ft long (2.4 m wide, 1.3 eral Contractor Simpson & Brown, the design wave height was 3.5 ft (1.1 m deep, 12.2 m long) segments were Inc. was the low bidder and was se­ m) with a period of 3.0 seconds and a prefabricated and shipped to the job­ lected to install the floating wave wave length of 50ft (15.2 m). The de­ site. At the jobsite, the units were set attenuator. sign wind speed was established as 60 in the water and connected together by

32 PCI JOURNAL F L U S H N G 8 A Y

8 ---SPACES 0 40' o" =320' o" ~ - ... 0 0 0 0 0 0 0 • • • 0 • tJ 7-SPACES 0 40'-o" • 280'-o" 20'-0

b I 40'- o" io ('.1

~

8-SPACES 0 40'-o"=320'-o" I • + • • • + • + • + • • tl 7- SPACES 0 40'-o"• 280'-o"

Fig. 3 (cont.). Plan of floating wave atten uator (for Section A-A, see Fig. 4) .

post-tensioning rods to build three 16 ft wide, 4ft 3 in. deep, 320ft long (4.9 PROlECTIVE PILE Cfof· m wide, 1.3 m deep, 97.5 m long) structures. The segments are EL.+20.0' polystyrene-filled, thin walled con­

crete boxes for the permanent floating El.+16.7'= EHW requirement. The exterior walls (sides) ( 100 YR . FLOOD) and flanges (top and bottom) of the in­ MONO- STRAN D POST TENSIONING CABLES dividual units are prestressed for (TOP SLAB ONLY) strength and crack control. POST TENSIONING CABLES The method of connecting the indi­ vidual segments into the continuous structure is somewhat unique. The EL. +6.8' - MHW structure consists of two rows of 40 ft (12.2 m) long precast segments placed

side by side with alternately overlap­ CONCRETE ping butt joints at 20 ft (6.1 m) inter­ 16'- 0" ENCASEMENT EL. 0.0'= MLW vals. At both sides of each splice joint there are two post-tensioned rods

placed transversely through 4 in. (102 EXISTING BAY BOTIOM 3 El.-7' ± mrn) diameter sleeves located 5 / 4 in. ( 146 mrn) from the top and the bottom of the segments (see Figs. 5 and 6). In­ stead of being post-tensioned longitu­ dinally, as is common practice in seg­ mental construction, the segments are SECTION A-A post-tensioned transversely. This method of assembly proved to be very MIN . TI P EL. - 60.0' D D convenient during construction, en­ abling a relatively simple operation in Fig. 4. Cross section of wave attenuator (for plan of structure, see Fig. 3) .

July-August 1997 33 24"f SI[[L PIPE PILES 20'-o" 20'-o· 20'-0" 20'-o·

1/2" \>------=J9:.__'-_::.II"~(_::.PRE::::C::.:AS:.__T),______,f-r------'J"-9'--'-11-" -';-(PRE_:c:._CA_:_ST.:_) ______--fl/2"

1/2" C.L. POST- TENS ION ING (P.T.) 2'-1 l/ 8" 2'- 1 7/8"

L__, SI:QUENCE Of POST TENSI ON ING SI:E SCHEDULE I CEN)[ RU NE OF ~ 24" o STEEL PIPE

PILE. 19' - II" (PRECAST) 39'-11" (PRECAST)

Fig. 5. Detailed plan of wave attenuator structure showing precast, prestressed concrete segments and sequence of post-tensioning.

the water assembly of each 320 ft (97.5 m) structure. EPOXY GROUT As mentioned above, the continuity (m>.) of the structure is achieved by assem­ 0 bling the units in a staggered pattern fr···~ ( so the units are spliced to each other ' . ~ at the butt joints (see Fig. 7). The L J loads that occur in the units at the butt 1;;. -'7. joints must be transferred into the ad­ [ ,.; jacent units to provide continuity. The l transfer of these loads is provided by ~ the lateral post-tensioning. The hori­ \__ P.1. BOTTOM zontal moment capacity is provided r I K£YWAY [!! i! by the moment couple equal to the 1-n Sf! 1-n S/8" I I ~"' tension capacity of the post-tension­ ~ ~ ing rods multiplied by the horizontal "' distance between them. The vertical moments are transferred similarly, ex­ Fig. 6. Precast segments showing epoxy grout joint. cept that here the moment couple is provided by the shear-friction capac­ ity of the shear key. The torsional continuity is provided by the moment resistance of the post­ tensioning rods in the vertical direc­ tion. The rods are designed to resist the combined effects of all of the above mentioned loads. At each set of post-tensioning rods, 8'-0" X 4'- 3" X 40'- 0" there is a 6 in. (152 mrn) wide keyway PRECAS T UN IT (TYPICAL) between the precast segments to trans­ fer vertical shear forces. The back surface of the keyway is formed to make an undulating surface in the concrete to ensure a definite shear connection after the shear key is Fig. 7. Schematic view of wave attenuator showing method of sp licing precast units grouted (see Fig. 8). and structu ral components.

34 PCI JOURNAL t -n 5/8' PI![ CAS!

,.------___ _.

UJ I.[ Of'RENE STRIP (CONT.)

+; e BACKER ROO (EPOXY IN TO PLACE)

LONGITUDINAL SECTION

Fig. 8. Longitudinal secti on and plan of keyway joint

I 1/4'f DYWIDAG THREADBAR WITH NUT, P.T. TENDON

DYWIDAG M

~· · DAYTON SUPERI OR F- 42 S.S. FERRULE INSE RT OR EQUAL

MAS TI C CORROSI'<{ INH IB ITOR CORRUGATE D PLASTIC SHE ATHING (GROUT FILLED)

~~~ ~ SH£ R L5 (4 REQ'D.)

Fig. 9. Detail of post-tens ion ing rod.

2'- 0' STEEL 2' · 0' STEEL PIPE PILE PIPE PILE

100 Y£AR FLOOO 100 YEAR fLOOO (+)16.7' $ - (+)16.7'

HI GH WAtrR H!GH WATER (+) 6.8' $ (+) 6.8' $

lOWWATER LOWWATER (+) o.o· $ (+) 0.0' $

BAS[ BASE (- ) 7.0' $ (· ) 7.0' $

Fi g. 10. Attenuator at high water level. Fi g. 11 . Attenuator at low water level.

July-August 1997 35 LEGEND @Place longitudinal post-tensioning mono strand CD Welded wire and mild steel ®Additional tie-in reinforcement reinforcement (!) Attach form top tie with void hold­ @ Pretensioned bottom and side strand down @ Place first layer of concrete to proper @ Place concrete on each side of void elevation in lifts @ Place voids to proper elevation ® Place top layer of concrete and finish

'-~------~~ (a) Initial setup of casting showing ~~ side forms, reinforcement, ; ; ; ; ; v-7 pretensioned bottom and side r------,-6

strand. · ~5

~ -'T77777777?777777777). -;:::;---4 n

8

(b) Phase 1 of casting operation. (c) Phase 2 of casting operation. (d) Phase 3 of casting operation. First layer of concrete is placed to Setup shows voids, additional tie­ Quality control checks. proper elevation. in reinforcement and form top tie Continuous monitoring of with void hold-down. Quality concrete placement, control checks. consolidation and void stability.

Fig. 12 . Casting sequence of segment manufacture.

The post-tensioning was executed in lated in cement grout and corrugated the wave will not act uniformly on the two stages. First, a fraction of the plastic sheathing. The anchorage ends, structure. Because the supports are post-tensioning force was applied including the bearing plates and the flexible, the structure has to span over against the neoprene pads and foam nuts, are recessed in a pocket and multiple supports to distribute the lo­ backer rods in place to seal the key­ encapsulated in a mastic corrosion calized lateral wave actions. Due to way for grouting. The keyways were inhibitor (see Fig. 9). The pocket is the variation in the water elevation, grouted with epoxy grout. The main covered with stainless steel plates fas­ the cantilever lengths varied, so the reason for using epoxy grout instead tened with bolts to confine the mastic. stiffness of the pile supports for the of portland cement grout was that at structure also varies. this first stage of construction, the seg­ This change of the support stiff­ ments were held together through the DESIGN LOADS nesses has a significant effect on the neoprene pads while floating on the The principal loads the structure has horizontal bending moment in the water. Therefore, there could be some to resist are the horizontal wave loads. structure. The waves can approach the movement in the joints during the set­ To resist these loads, the structure is structure at any angle and at any part ting of the grout, which would have supported by steel piles that are driven of the structure. The structure was an­ affected the portland cement grout. into the sea bed, acting as cantilevers alyzed with waves acting at its end After the grout had set, the full post­ to resist the lateral loads. Infinitely and middle at different water eleva­ tensioning force was applied. stiff pile supports could make the tions, as well as with waves acting at Prestressing of individual segments structural analysis identical to a con­ short and long distances. in combination with high quality [8000 tinuous beam with sixteen 20 ft (6.1 Since the structure is freely floating psi (55 MPa)], low water-cement ratio m) long spans. The bending moments on the water, when the cross-direc­ concrete and calcium nitrate corrosion could be minimal because of the rela­ tional waves are passing they lift the inhibitor provided the necessary dura­ tively short spans. structure, generating uplift forces. bility for exposure in a marine environ­ A structure with flexible supports Figs. 10 and 11 show the attenuator at ment. To provide the same level of and a uniform lateral load on the entire high and low water level, respectively. protection for the post-tensioning rods, length of the structure could have the Because the structure is 16 ft (4.9 m) the main body of the rod is encapsu- same effect. As in the actual structure, wide, the wave side of the structure

36 PCI JOURNAL 7 1 lt \

Fig. 13. Reinforcement cage showing welded wire fabric and Fi g. 14. Setting polystyrene core in casting bed . Note epoxy-coated mild reinforcing steel. pretensioning steel in side wall . will be lifted first, then the entire cross ing bed for maximum economy. The plates held in place by threaded rods section, and finally the leeward side, bottom flange and sidewalls were supported by the top tie system (see generating torsional and vertical bend­ stressed with pretensioned strand. Due Figs. 12d). ing moments across the structure. to the casting sequence, it was neces­ The 8000 psi (55 MPa) normal Similarly as for the lateral loads, these sary to post-tension the top flange. weight high performance concrete vertical wave actions were also ap­ The sequence of casting each segment used in this project consisted of Type plied at various wave lengths and lo­ was critical and required considerable II cement, ledge rock coarse aggregate cations along the structure. coordination in placing the concrete with a Los Angeles Wear ASTM For the preliminary design, the and voids and completing the installa­ Cl31 less than 0.19, high range water structure was assumed to be 640 ft tion of mild steel reinforcement (see reducer, corrosion inhibitor and a (195 m) long. During the final design, Fig. 12) water-cement ratio less than 0.35. Air when the computer model was final­ Rigid steel forms were absolutely content was 4 (±I) percent. The con­ ized and all of the various load cases necessary to maintain dimensions crete was produced in a fully auto­ were calculated, it was found that by within acceptable tolerances. The bot­ mated central mix concrete plant and making two 320 ft (97.5 m) long seg­ tom pallet, side forms, end bulkheads delivered to a casting bed by ready­ ments, the forces in the structure were and all block-outs were fabricated mix concrete trucks. reduced and the factor of safety of the using steel. Coordination of the casting se­ structure could therefore be increased. The thin wall and flange sections quence, concrete placement, void dictated the use of galvanized welded placement, additional installation of MANUFACTURE OF wire reinforcement (WWF) fabricated reinforcement and final placement of PRECAST /PREST RESSED to overlap intersecting adjacent sheets. concrete required precise planning and The WWF was placed outside of the close coordination of all personnel SEGME NTS pretensioned strand to provide a com­ (see Fig. 14). The precast, prestressed concrete plete envelop around the concrete seg­ Side forms, bulkheads, post-ten­ segments were manufactured by Pre­ ment. Additional reinforcement was sioning ducts, inserts, block-outs and cast Structures, Inc. at their plant in provided by epoxy-coated reinforcing other fixed items were placed and Auburn, Maine. The company is a bars to distribute the concentrated pre­ continually checked by quality control PCI Certified Producer Member with tensioning and post-tensioning forces personnel. Bottom and side welded more than 35 years of proven reliabil­ (see Fig. 13). wire reinforcement WWF and other ity and service. Two large 1 lb (0.45 kg) density reinforcement were then placed. Pre­ The manufacture of the precast/ polystyrene voids, fabricated within stressing strand was then placed and prestressed concrete floating breakwa­ ±1/s in. (±3 .2 mm) tolerance, provided tensioned. ter segments is similar to the manufac­ the interior float dimensions. Each Daily turn over of the casting bed re­ ture of standard AASHTO bridge box void weighed 800 lbs (363 kg) and quired 5000 psi (34 MPa) concrete in beams. The concrete float segments, had to be placed with precision after 12 hours. Accelerated heat curing was however, have a larger cross section the bottom layer of concrete had been provided by steam at a temperature not and thinner walls and flanges. The de­ cast. A crane was required to handle to exceed 160°F (71 °C). Polystyrene sign of the floating concrete segments the voids due to their size, weight and can melt if temperatures exceed 160°F required pretensioning the walls placement. The voids were held in (71 °C), so the heat of hydration of the (sides) and flanges (top and bottom). place horizontally with spacer plates concrete had to be carefully monitored It was decided to manufacture four attached to the side forms. Void hold­ with thermocouples and time tempera­ segments per cast in a long-line cast- downs were provided with galvanized ture recorders.

July-August 1997 37 MATERIAL SPECIFICATIONS forced post-tensioning ducts and tensioning of the staggered segments block-outs. provides a stable and totally pre­ Long-term durability of floating The following are other material stressed float in all directions. A structures requires careful considera­ specifications: schematic diagram showing the longi­ tion of material specifications. High tudinal pretensioning envelope and performance concrete encompasses • ASTM A416 - Uncoated seven­ transverse post-tensioning is shown in not only high strength but also dura­ wire strand for prestressed concrete Fig. 7. bility. Proper material specifications • ASTM A 775 - Epoxy-coated rein­ The polystyrene voids guarantee produce high strength, low permeabil­ forcing steel flotation if the floats are damaged by ity and low shrinkage. Corrosion pro­ • ASTM A185 -Welded wire rein­ ship impact. This unique design al­ tection is provided by galvanized forcement (WWF) lows dismantling and repair of dam­ and/or epoxy-coated mild reinforce­ • ASTM A153- Hotdip galvanized aged segments and placement at an­ ment and a corrosion inhibitor chemi­ • ASTM A666 - Stainless steel, other location or site in the same or cal admixture. Corrugated plastic Type 304 varying configuration. sheathing (grout filled) encases the • ASTM D2842- Polystyrene 1 • ASTM A 722-90 - Uncoated high 1 / 4 in. (32 mm) 150 ksi (1034 MPa) post-tensioning bars. strength steel bar for prestressing TRANSPORTATION The 8000 psi (55 MPa) concrete concrete mix design used for the Port Authority • ASTM C150-86- Portland cement AND ERECTION project included cement meeting Type grout Figs. 17 to 20 show various erection • ASTM A36 - Structural steel II ASTM C150 with a maximum C3A phases of the precast concrete structure. of 8 percent and a water-cement ratio • ASTM D1784- Rigid polyvinyl The completed segments were less than 0.35. The aggregates, meet­ chloride (PVC) trucked from Auburn, Maine, to Port ing ASTM C33, consisted of natural • Dywidag PTI mastic corrosion Elizabeth, New Jersey, for assembly. sand fine aggregate and quarried inhibitor The segments were prepared on land ledgerock coarse aggregate with a Los The design required special forming to receive pile guides and waterproof Angeles Wear of less than 0.19. Ad­ for the end blocks and pile guides, sealing strips around shear keys and mixtures included ASTM 260 for air shear key and block-outs for the post­ along the top side longitudinal joint entrainment and ASTM C494 for a tensioning hardware (see Figs. 15 between segments. The segments were high range water reducer (HRWR). and 16). then lowered into the water and posi­ HRWR was used to provide a work­ Prestressing provides strength and tioned two abreast to receive post­ able slump to enhance placement and watertightness for the structure. Lon­ tensioning threadbar. This required the consolidation of concrete in the thin gitudinal prestressing envelops each use of a diver to install below water wall sections, around heavily rein- concrete segment. Transverse post- thread bars.

Fig. 16. Shear key detail showing gaskets and post-tensioning bars prior to Fig. 15 . Completed casting of 8 x 40ft (2.4 x 12.2 m) segment. Note block-out for placement in water, post-tensioning and shear keys. applying epoxy grout.

38 PCI JOURNAL An initial post-tensioning force was applied to seal joints and shear keys filled with epoxy grout. After the grout sufficiently cured, final post­ tensioning forces were applied. Threadbar head assemblies were packed with grease after stressing. The completed 320 ft (97 .5 m) sections were towed from Port Newark to Flushing Bay. At Flushing Bay, the wave attenuator sections were care­ fully positioned by survey and the steel pipe pile anchorage installed. Fig. 21 shows a portion of the com­ pleted floating wave attenuator. The total cost for the installed sys­ tem was approximately $2 million. The precast work, including post­ Fig. 17. Setting precast concrete segments and post-tensioning threadbar. tensioning threadbar, pile guides, seal­ ing strips and rubbing blocks, was ap­ proxjmately $1.5 rrullion. Thjs project won an Honorable Men­ tion in the 1996 PCI Design A wards Program. The jury citation read: "The precast, prestressed concrete system used on this project was an innovative solution which can be replicated in other marine applications."

CLOSING REMARKS The precast, prestressed system used on tills project was an ideal solution to a challenge which would have been cost prohibitive using any other build­ ing material. With the experience gained, it will be possible to econorru­ cally build even larger floating wave Fig. 18. Post-tension ing hardware and diver equipment. attenuators in the future. Floating wave attenuators can pro­ vide the necessary wave protection in a variety of situations where rubble mound or other filled-type structures are impractical. This includes areas where environmental regulations pro­ hibit filling and locations containing soft mud bottom (compressible soils), mabng filled structures impractical or cost prohibitive. Precast, prestressed concrete sys­ tems can provide the required struc­ tural resistance and attenuation perfor­ mance while providing ease of installation and environmental com­ patibility. Due to their inherent stabil­ ity, they can be used as floating dock structures, providing both wave pro­ tection and usable berthing space for Fig. 19. Epoxy grouting shear keys. boats and other marinas.

July-August 1997 39 Fig. 20. Two completed 16 x 320ft (4.9 x 97 m) precast sections ready for towing from Port Newark to Flushing Bay.

Fig. 21. Part of completed floating wave attenuator in final position.

CREDITS Owner: City of New York Parks & Recreation, Flushing, New York Engineers: - Mueser Rutledge Consulting Engineers, New York, New York - LEAP Associates International, Inc., Tampa, Florida General Contractor: Simpson & Brown, Inc., General Contracting, West Cranford, New Jersey Precast Concrete Manufacturer: Precast Structures, Inc., Auburn, Maine Supplier: Marina Components, Inc., Jupiter, Florida

40 PCI JOURNAL