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.