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Staggered Truss Example

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Totaling approximately 617,000 sq. ft, the mixed-use Embassy Suites hotel development is among the largest of several newly built hotels in the business district adjoining New York’s bustling Wall Street area. The combination hotel-retail-cinema complex was conceived in 1996 as a way to attract people to Battery Park City, located near the southern tip of Manhattan, during the evenings and on weekends, when it is largely empty. Opened to the public in June 2000, the L-shaped hotel is situated on a site measuring 62,500 sq. ft and has two wings connected by a 9,200 sq. ft east-west atrium with skylights and glass walls on its west. The development, which represents an $80 million dollar investment on behave of its developer Forest City Rantner, showcases a 463-room hotel, a 16-screen movie theater, a sizable retail complex and a 13-story- high, glass-walled public atrium forming a centerpiece offering  sweeping views of the Hudson River. C omplex Requirements The functional needs of the pro-  je ct ’s co mp on en ts co ul d not ha ve been more diverse. Movie theaters require long clear spans of up to 60’, with high floor-to-floor heights, typi- cally 32’. In contrast, hotels general- ly need short spans for 13 to 5’ wide rooms and low floor-to-floor heights, normally in the range of 8’ 8” to 10’. Retail spaces offer somewhat more flexibility: they benefit from longer open spaces, however spans of 30 to 40’ are generally acceptable. Space usage also dictated the types of ceilings that had to be used in the project. As large-volume, high-occupancy spaces, movie the- aters need a ceiling plenum for air and service distribution, as well as acoustic enhancement. Where as, hotel rooms are small, private spaces served by individual HVAC units. With bathrooms stacked vertically, Modern Steel Construction / September 2000 Staggered Truss System Proves Economical For Hotels By Aine Br a zil, P.E.
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    Totaling approximately 617,000sq. ft, the mixed-use Embassy Suiteshotel development is among thelargest of several newly built hotelsin the business district adjoining NewYorks bustling Wall Street area. Thecombination hotel-retail-cinemacomplex was conceived in 1996 as away to attract people to Battery ParkCity, located near the southern tip ofManhattan, during the evenings andon weekends, when it is largelyempty.

    Opened to the public in June2000, the L-shaped hotel is situatedon a site measuring 62,500 sq. ft andhas two wings connected by a 9,200sq. ft east-west atrium with skylightsand glass walls on its west. Thedevelopment, which represents an$80 million dollar investment onbehave of its developer Forest CityRantner, showcases a 463-roomhotel, a 16-screen movie theater, asizable retail complex and a 13-story-high, glass-walled public atriumforming a centerpiece offeringsweeping views of the Hudson River.

    Complex Requirements

    The functional needs of the pro-ject s components could not havebeen more diverse. Movie theatersrequire long clear spans of up to 60,with high floor-to-floor heights, typi-cally 32. In contrast, hotels general-ly need short spans for 13 to 5 widerooms and low floor-to-floor heights,normally in the range of 8 8 to 10.Retail spaces offer somewhat moreflexibility: they benefit from longeropen spaces, however spans of 30 to

    40 are generally acceptable.Space usage also dictated the

    types of ceilings that had to be usedin the project. As large-volume,high-occupancy spaces, movie the-aters need a ceiling plenum for airand service distribution, as well asacoustic enhancement. Where as,hotel rooms are small, private spacesserved by individual HVAC units.With bathrooms stacked vertically,

    Modern Steel Construction / September 2000

    Staggered Truss

    System Proves

    Economical ForHotels

    By Aine Brazil, P.E.

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    piping is distributed inside the walls,while the bedroom ceiling is usuallythe soffit of the floor slab above witha directly applied surface finish.Retail spaces will frequently use aceiling plenum for service distribu-tion and the installation of specialfeatures.

    Flouting Tradition

    Certain pieces of conventionalwisdom are generally applied whenchoosing structural systems for a pro-ject, most of them dependent on theprojects location and uses. For aNew York mixed-use project likeEmbassy Suites, such wisdom wouldpoint to a concrete flat-slab systemfor the hotel and a steel-framedstructure for the movie theaters.

    Transferring alternate columnsunder the hotel footprint to providecolumn spacing of up to 30 in orderto accommodate the hotels publicspaces and retail at the base.However, it soon became clear that ifthe project was to survive the

    Thornton-Tomasetti engineers weregoing to need to throw a few tradi-tions out the window.

    Design Dilemma

    During the initial stages of con-cept development, the projects bud-get estimate showed a very tight pro-forma. Unless the construction costper square foot (psf) could bereduced, the project would not befinancially feasible. The conceptual

    cost estimate, based on constructioncosts for similar projects built in theNew York City market, called for acast-in-place concrete system. Thedesign team was charged with thetask of finding significant cost sav-ingsand fresh ideas were needed.

    Thornton-Tomasetti Engineersproposed the use of a staggered trusssystem for the hotel combined with aconventional steel frame and com-posite slab-on-metal deck for the

    theater and retail components. Costestimates by Bovis Lend Lease con-firmed the value of the cost savingsto be in excess of $2 million. Theteam had found a viable and eco-nomic solution, the next was to satis-fy the client of the ability of the sys-tem to meet the programrequirements while still providingthe estimated cost savings.

    Hotels: Concrete vs. Steel

    Why is concrete flat slab so oftenthe system of choice for hotel build-ings? Flat-slab construction providesseveral important benefits:

    Shallow uniform thickness of thestructure facilitates lower floor-to-floor heights, reducing the cost ofthe structure and the area of theexterior wall.

    Flat concrete slab soffit avoids theneed for hung ceilings.

    Modern Steel Construction / September 2000

    Basic Staggered Truss System

    includes:

    Story high trusses span building

    width

    Vierendeel panel permits central cor-

    ridor.

    Trusses Alternate column lines on

    each floor

    Interior is typically column-free

    Floor system alternately supported

    on the top and bottom chords of truss-

    es.

    Trusses resist both gravity and later-

    al loads.

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    Concrete slab satisfies therequired fire rating for floor con-struction.

    Concrete slab provides goodsound insulation and minimalfloor vibration.

    The staggered-truss system used forthe Embassy Suites Hotel had manyof the same advantages:

    The Filigree precast plank floor is8 thick spanning to the staggeredtrusses, which are encapsulated inthe partition walls between hotelsuites and do not affect floor-to-floor heights.

    The soffit of the planks can be fin-ished in the same way as the con-crete slabs, without hung ceilings.

    The Filigree precast plank satisfiesfire rating of the floor system.

    The detail at the ends of each

    plank provides system continuity,resulting in a stiff floor structure.

    The decision to use the staggeredtruss system was, in the end, a finan-cial judgment. For the specific pro-ject and site under evaluation, thesteel system offered significantadvantages:

    The weight of the steel and plankstructure is 15% less than that ofan equivalent concrete frame.Since the project site is located onland that was filled/reclaimed inthe 1960s, deep end-bearing-pilefoundations were required to sup-port the structure. With depth-to-rock averaging 80, the reductionin structural weight resulted inapproximately 15% fewer pilesand smaller pile caps.

    Reduced dead load translates

    directly into reduced seismic load.Since the 14-story buildings later-al systems were governed by seis-mic design, the reduced lateraldesign forces resulted in signifi-cantly lower cost.

    In addition, the mixed-use nature

    of the project strengthened the casefor an all-steel-framed structure.Composite steel framing with slab onmetal deck was a logical, cost-effec-tive choice for the theater clear spansof 40 to 60. For retail uses, steelframing provides maximum flexibili-ty, permitting easy modificationwhen tenants change. The steelweight averaged 7 psf for the hoteland 15 psf for the theaters, whichwhen combined came to a total steeltonnage on the project of 3500 tons.

    In a concrete structure, transfer gird-ers would have been necessary toachieve the longer, more open spansneeded at the lower public spaces.

    Independent Structures or Combined?

    The project footprint is organizedwith the theater in a trapezoidalblock measuring 135 to 170 east-west and approximately 200 north-south, located in the northeast cor-ner of the site with retail beneath.

    The hotel occupies two wings in an Lshape: the west wing is 65170,while the south wing is 45265.The 13-story atrium separates thetheaters and west wing from thesouth wing. The only connectionsare a 40 wide bay of rooms at theeast end of the atrium and a 10 widebridge at each hotel floor at the westend of the atrium. This slot iso-lates the structural systems and tendsto favor a solution using two inde-

    pendent buildings. However, sepa-rating the building into two struc-tures would have led to the use ofundesirable expansion joints.

    To avoid the use of expansionjoints, the building was designed as asingle unit. This presented an inter-esting challenge that was solved bytying the north and south sectionstogether with horizontal K-bracing atthe west end of the atrium at floors

    Modern Steel Construction / September 2000

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    for free when sized for gravityloads. For this project, lateral load-ing (either wind or seismic) did notcontrol the design of any truss mem-

    bers. In the longitudinal direction,conventional Chevron bracing wasprovided in stairwells for lateralresistance. The tube steel is A500Grade B.

    Movie Theaters

    The floor construction consists ofcomposite floor framing construction.Beams were designed using LRFDdesign approach. The grade of steelfor the floor beams, columns and

    bracing was ASTM A572 Grade 50.Cambering of the beams permittedthe use of W18 beams for the 40spans and W27 beams for the 60spans. Floor slabs varied from 2metal deck plus 2 normal weightconcrete to 3 metal deck plus 4normal weight concrete whererequired for acoustic separation.

    Roof framing over the theater

    area consisted of open web joists with1 roof deck.

    Provision was made within thefootprint of each theater for the ten-ant fit-out of a full stadium seatingtheater. Partial mezzanine levelswere provided, framed in structuralsteel, approximately 19 above thefloor, to house projection booths andprovide secondary means of egress.The stadium seating was supportedon light gage steel joists and studwalls sitting on top of the theaterslabs. This solution permitted thedesign and construction of the the-ater fit-out to lag behind the base

    building construction and providedan economical framing method forthe stadium-seating component.

    Foundation System

    The depth-to-rock and thicknessof the fill layer mandated a deepfoundation system. Steel pipe pilesfilled with concrete were chosen.Load tests indicated that a capacityof 200 tons could be achieved with a

    four, eight and twelve. The bracingand open atrium space create animpressive visual effect that high-lights both architectural and engi-neering design.

    The combined structure lacks thenormal symmetry found in a typicalstaggered truss building. With floor-to-floor heights varying significantlyfrom the hotel to the theater (8 9vs. 32), floor diaphragms alignedonly at the lower retail floors.Careful attention was paid to thetransfer of diaphragm forcesthroughout the footprint andbetween the different floor eleva-tions. Precast plank diaphragm forceswere transferred through a fullygrouted joint over each truss chord

    with interlocking rebar. This detailalso helped provide continuity toincrease the floor stiffness and thusreduce vibrations.

    Structural Components

    Hotel Tower

    Story-deep staggered trusses spantransversely across the narrowdimension of each hotel wing. Thetruss spacing matches the suite parti-

    tion locations, typically varying from26 to 30 on center. Staggeringoccurs on alternate lines from onefloor to the next. The 8 deep pre-cast Filigree planks span from thetop chord of one truss to the bottomchord of the adjacent truss. Sincethe trusses are completely encapsu-lated by the partition walls of thehotel, minimal width truss membersare desirable. HSS rectangular sec-tions were determined to be the mostefficient. Top and bottom chordsvary from TS 4 6 to TS416 . Both diagonals andverticals are typically 44,except at the vierendeel bays.

    The trusses frame into the weakaxis of the steel columns with seatedconnections. Lateral resistance inthe transverse direction is providedby the extremely stiff trusses, whichprovide adequate lateral resistance

    Advantages of Staggered Truss System include:

    Improved layout flexibility due to elimination of interior columns & two-bay

    open space in longitudinal direction;

    Large open spaces at base of building;

    Faster erection due to fewer pieces;

    All-dry system speeds winter construction;

    Efficient lateral system due to inherent stiffness; and

    Reduced structure weight compared to concrete construction.

    Modern Steel Construction / September 2000

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    12 -diameter pipe with wall thick-ness of , filled with 8000 psi con-crete. Pile pipe steel was ASTMA252 Grade 3. They were 12 and 3/4by 3/8 piles.

    Though conventional wisdomoften leads engineers in the rightdirection in making design decisions,at times we must not be afraid to takerisks.

    Meeting the objectives of a clientwhether they be related to time, costor functionis the primary goal ofevery project. In the case of theEmbassy Suites Hotel development,the clients cost requirements woulddetermine whether or not the projectgot built at all. In this restrictive cir-cumstance, engineers decided toignore everything that was knownabout typical residential constructionin New York City in favor of the stag-gered steel-truss system. In ademonstration of the rule that somerisks simply must be taken, the resultthey found was exactly what theyand their clientwere looking for.

    Aine Brazil , P.E. is a structuralengineer with Thornton-Tomasetti

    Engineers in New York City, NY.

    Embassy Suites HotelBattery Park City

    Location: New York City

    Owner/Developer: Forest CityRatner Companies, New York City

    Structural Engineer: Thornton-Tomasetti Engineers, New York City

    Architect: Perkins Eastman

    Architects P.C., New York City

    Construction Manager: Bovis LendLease, New York City

    Steel Fabricator: Helmark Steel(AISC member), Wilmington, DE

    Steel Erector: Helmark Steel (AISCmember), Wilmington, DE

    Steel Detailer: Helmark Steel (AISCmember), Wilmington, DE

    Modern Steel Construction / September 2000


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