Design & Construction Team
Project Name: The New York Times Building
Owners: The New York Times Company, New York; and Forest City Ratner Companies, New York
Structural Engineer: Thornton-Tomasetti Engineers, New York
Architects: Renzo Piano Building Workshop, Paris, France; and Fox & Fowle Architects, New York
Contractor: RAMEC, New York
At the new headquarters building for The New York Times in New York City, expressed and exposed structural steel framing forms key elements of a complex and challenging architectural design. Framing configurations, member sizes, and fabrication details reflect not just demands of strength, stiffness, cost, and practicality, but also of architectural proportion, alignment and visual order, weather-tightness, fire protection, thermal control, and special occupant needs. This interplay of architectural, environmental, and user requirements posed some unusual structural design demands.
The project team and design goals
For this project, two different user groups are involved: journalists and other New York Times newspaper staff in the lower half of the building and general office tenants in the upper half. The New York Times Company wants a headquarters building that befits its position in the newspaper industry, in the city, and in the world. The company also has a long-term perspective; it has occupied its current building for a century. Its partner in creating this project, New York-based real estate development firm Forest City Ratner Companies, is familiar with local market needs and the financial requirements for a viable project. The design side has its own pairing: the Renzo Piano Building Workshop has created cutting-edge architecture around the world, and Fox & Fowle Architects has designed beautiful and innovative buildings in many cities.
Designers for this 52-story building wanted to create an impression of lightness. A light exterior tone is enhanced by a fine filigree screen comprised of horizontal, closely spaced, 1 5/8-inch diameter ceramic rods mounted outboard of the glazed weather envelope. The screen extends upward around rooftop equipment, with rod spacing gradually increasing to transition gently from building to sky. A 300-foot-tall tapering rooftop mast completes the transition (see Figure 1).
On the long east and west faces, perimeter columns are pulled several feet into the building space, and the north and south faces have cantilevers that extend a full bay beyond the end columns. The resulting overhangs contrast with the glazed steel storefront, which is pulled inward at ground level to create an impression of office floors floating above the lobby. Large corner notches complete the tower composition, creating a cruciform building plan that shortens the apparent width of building faces and signals a change of façade treatment (see Figure 2).
The end façade screens extend slightly past the outer notch corners to soften the plane change. Screens are omitted on notch facades to create a sense of openness and transparency consistent with journalistic ideals. Also, the notches expose major structural framing to the weather and to public view, creating a dramatic shift of scale from thin-screen wall tubes to one- and two-story-high diagonals and X-braces. The structural engineering challenges of this exceptional building will be presented from inside out, much as the design itself was developed. Structural design efforts During the preliminary design phase, both steel and concrete framing systems were studied. Steel was selected based upon the large, open office bays the owners desired, as well as for future flexibility, local familiarity with the material, and construction cost and speed.
Accommodating the users—The two user groups require different floor systems. The New York Times Company desired a raised floor to accommodate both wiring distribution and an under-floor air supply. The top-of-slab elevations are depressed 1 foot, 4 inches below the top of the finished (raised) floor. In many locations, beams extend through the façade to connect to the exposed columns. For protruding beam stubs to cross spandrel panels properly, special “cranked” or offset end details poke above the floor slab just inside the glass line (as a result of the large, raised floor dimension). They fit below the raised floor and are coordinated with mechanical systems in that space. Core girders are depressed for a different reason: return air to fan rooms flows between filler beams that pass over them.
At the tenant spaces on floors 29 through 50, floors and core girders accommodate a 6-inch raised floor, but are not depressed. Generous story heights allow for ducted air supply and return above suspended ceilings with a 9 foot, 7 inch minimum clear height. Where coordinated with the steel framing, a 10-foot clear height can be provided and girder penetrations are included where necessary.
Vertical transportation also is affected by the dual users’ needs. To separate The New York Times space and upper floor traffic, 12 passenger cabs are assigned to the lower floors. Above a mid-height mechanical room at Level 29, those elevators terminate and the space is available for lease above. In turn, the elevator layouts affect the lateral load-resisting system.
Lateral system components—A central core 90-feet long and 65-feet wide was chosen for the building design, as it provides continuous bays more than 45-feet deep along either side of the core—a practical distance for modern office plans—and 30-feet deep at each end. Additionally, a braced-core lateral system was selected for perimeter transparency, construction simplicity, and economy. Seismic forces are less than wind forces but stiffness is needed for occupant comfort. The braced core alone would be unacceptably flexible; therefore steel outrigger trusses that engage perimeter columns and improve lateral stiffness were designed to cross at mid-height and at rooftop mechanical floors. A two-way grid of trusses engages every perimeter column, improving efficiency.
The core width accommodates four lines of passenger elevators, each with seven shafts. The lower half of the building has steel braced frames surrounding the core. Additionally, the two north-south (or longitudinal), lower brace lines stop at the mid-height mechanical room, and a single, new brace line continues up from there.
Elevators also affected the east-west bracing. With a broader wind face and a narrower core dimension, four lines of bracing initially were considered to meet wind drift and comfort requirements. However, service cabs with reverse facing doors would require crossing one of the four bracing lines, rendering it less effective. The solution was Xbraced bays in the perimeter notches that work in tandem with the core (see Figure 3). These braces, however, brought their own architectural and structural challenges.
Bracing challenges—One X-brace design challenge was fire resistance. Exposed perimeter bracing could be used as a design feature, but it would be vulnerable to fire, and the design team did not consider traditional protection systems to be appropriate. Conventional spray-on and mineral wool fire protection with cladding creates unacceptable bulk, and intumescent fire protection varies with thermal mass and exposed surface. A reasonable thickness of intumescent coating can provide an acceptable fire resistance rating on massive building columns, but not on smaller bracing rods. Therefore, to avoid rod fire protection, the lateral system was designed twice. The first check ignores the perimeter bracing when checking building structural safety and stability under wind and earthquakes. A second design check, for occupant wind comfort only, includes the X-bracing enhancement to the building stiffness. It reduces sway under design wind loads from height/350 to height/450. With the X-bracing, Guelph, Ontairo-based wind tunnel consultant Rowan, Williams, Davies & Irwin, Inc., determined that the peak total acceleration at a top-floor corner location during a 10-year, nonhurricane windstorm is less than 25 milli-g (11 percent larger under hurricanes), which is an acceptable office condition.
A second X-brace challenge was pre-tensioning. In chevron or V-braced bays, one brace is in tension when the other is in compression. The result is stocky members that are sized for compression. On the contrary, X-bracing can be designed for single-diagonal, tension-only conditions, assuming the other diagonal simply buckles out of the way if compressed. However, this is inefficient since only half the braces work at a time. If the braces could be pretensioned so that neither one goes slack nor into compression, both diagonals would contribute to strength and stiffness. The designers determined that turnbuckles are architecturally unacceptable, and twisting a turnbuckle is ineffective to generate more than minimal tension because thread friction builds rapidly. Therefore, European-style, high-strength, steel rods that have thin, sleek sleeve nuts for length adjustment and a cone-shaped lock nut for each end of the sleeve were selected. The lock nuts also work with a special hydraulic jack system to apply jacking loads with just 2-percent force deviation. For economy, tensioning should be a single step, not a prolonged “piano tuning” process. But during construction, and even during wind or seismic load conditions, column shortening occurs. Chevron or V-braces are affected minimally since crossbeam flex accommodates the slight change in story height. However, X-braces experience compression as columns shorten, so pretensioning performed during construction must compensate for this specifically, based on a particular erection sequence. Forces will significantly exceed the target final pretensioning values, but they are still less than final pretensioning plus maximum wind force. Therefore, construction pretensioning does not control the design of the bracing rods, fittings, and connections.
The third design challenge is the appearance of the X-braces and all exposed framing along the building notches. Beams, columns, and bracing rods of uniform size would increase steel tonnage significantly, adding to project costs. The upper exposed columns also would act at much lower stresses than upper interior columns, causing differential shortening that complicates floor levelness. Instead, the sizes vary with building height at regular intervals. For example, exposed columns are built-up steel plate boxes with 30- x 30-inch outside dimensions and web plates slightly inset. Flange thickness varies gradually from 4 to 2 inches. The X-bracing rod diameters closely follow the flange thickness on each floor. The locations of size changes suit both structural and architectural needs. Where beam or column properties must change between these steps, the thickness of web plates is varied because webs have no architectural impact.
Detailing also reflects input from the architect, structural engineer, and contractors. Singlerod, X-bracing would require large member sizes, and they would have to be offset to allow crossing rods to clear, which would induce column torsion. Using pairs of rods can raise questions about load sharing, particularly during pretensioning, but the availability of highly accurate jacking systems resolved this concern. Rod pairs are used with one pair in an X-brace oriented side-by-side to clear the other pair.
Another major design question was the decision between uniform and alternating X-brace patterns. The architect chose a uniform pattern, with all over-and-under pairs running in the same direction. While rod bracing systems have standard details for sleeve nuts, forked ends, and spade ends, the gusset plates to which they attach are project-specific. The architect and engineer tried a variety of shapes before deciding on gently curved gussets. Reinforcing plates make up the necessary thickness to work with forks and spades, and to carry pin loads. The configuration also was reviewed for constructibility issues such as practical weld lengths and jacking access. (The team created and studied wooden models.) While X-bracing rod lengths are adjustable, horizontal struts between the braced columns are not. The team developed a practical detail with adjustability that meets the architect’s aesthetic requirements. Strut ends join fabricated knuckles on columns through field-bolted end plates with provisions for shims at gaps. The knuckles then are boxed in with field-welded closure plates.
Gravity framing —Appearance and constructibility also play key roles in the structural framing of the other notch faces—the cantilevered bays. Three framing lines extend out from the building, one on each side of the cantilevered bay and one down its center. The side framing lines support the cantilever through multiple load paths. A diagonal rod at each floor “hangs” the outer tip of floor beam from the supporting column. A continuous, vertical member connects multiple beam tips together and is available to act as a post or hanger to redistribute load in the event that one or more rods fail, as in a fire. And the tapered floor beams themselves are moment-connected to the supporting column with sufficient capacity to cantilever on their own, though with excessive deflection. Thus, these multiple load paths permit exposed steel to be used, but the central framing line uses a different system—a ladder Vierendeel truss with floor beams moment-connected to both the supporting column and to the cantilever tip vertical tie member. With both ends of the floor beam restrained against rotation, it has strength and stiffness to carry the floor loads.
While load paths for the completed cantilevered bays are straightforward, their construction is not. The design team used sequential construction computer models to limit the forces going to framing, to determine strut design forces, to establish removal timing, and to validate removal methods. Based upon currently anticipated erection dates, the struts will be removed when end-bay framing reaches the mid-height outrigger level.
Unique design considerations
Designing for appearance and reviewing for constructibility are positive steps, but the goal of the architect and owner is a completed project of acceptable quality. When assembling large, heavy, built-up and hot-rolled steel members, some dimensional variation in both shop fabrication and field erection is inevitable. The owners approached this proactively well before construction began. They commissioned the fabrication of a full-size, steel joint that includes some intentional fit-up deviations to illustrate the types and magnitudes of misalignments one reasonably could expect (see Photo 1). This helped the architect and owner consider acceptable tolerances and appropriate, remedial measures. And it alerted potential fabricators and erectors to the complexity of the work and the high standards that would apply.
Thermal changes—The mix of exposed and interior structure, and the beams that transition from inside to outside, required extra attention. Differential strain between inside and outside columns because of thermal changes can affect member and connection forces, floor levelness, and local joint behavior. To study these effects, Thornton-Tomasetti first established a design temperature differential of +70°F to -80°F based on historical daily maximums and minimums for New York City, which were then modified by recommendations in the National Building Code of Canada (NBC) to reflect radiant heating and cooling effects. The designers then determined a floor slope criterion of less than span/300 between any two adjacent columns, following the NBC approach. For a 70°F temperature change, an unrestrained, 650-foot-tall steel column supporting the top office floor will grow about 3.5 inches, while a 30-foot span can tolerate only 1.2 inches of differential motion. The solution was to recognize that the exterior column is not unrestrained, and the first interior column is not stationary.
Wind-resisting outrigger trusses are supplemented with “thermal trusses” that link exposed and interior columns. By pushing down on a “hot” column and simultaneously pulling up on the adjacent interior column, these trusses cut maximum growth of exposed columns in half and cut the differential between columns by a factor of three. Of course, the forces necessary to do this “pushing and pulling” also must be considered in the design of members and connections. Load combinations, including thermal effects, generally govern over those with wind load. The ends of beams connecting inner and outer columns could experience significant daily and seasonal rotations because of column temperature swings, potentially causing joint noises and “sawing” of bolts. A moment connection at the outer connection provides sufficient strength to resist flexure induced by gravity plus thermal movements. A deep shear tab at the inner connection has slip-critical bolts. Thornton-Tomasetti studies show that this connection is unlikely to slip under the range of anticipated forces.
Further, any project that mixes interior and exterior steel must address thermal bridge, condensation, and weather-tightness issues. Determination of steel frame temperature gradients and appropriate measures for thermal performance and weather-tightness is not the responsibility of the structural engineer. However, members designed by the engineer are involved, so an understanding of the issues is important. Beams penetrating the façade of The New York Times Building have their exposed stubs fireproofed, insulated, and clad up to the intersection with the exterior column. Because thermal conductivity is much greater along the beam than across the insulation, a thermal gradient is established along the stub. In this way, condensation on the beam is avoided. Stiffeners welded to the beam where they cross the façade act as a collar to which the façade is sealed for weather-tightness.
The New York Times Building design illustrates the ways in which architectural, environmental, structural, construction, and user issues affect each other in cutting-edge building design. The construction is underway currently and the team is targeting a “move-in” date during the first quarter of 2007.
Thomas Z. Scarangello, P.E., is a managing principal in the New York office of Thornton-Tomasetti Engineers. He can be reached at email@example.com. Leonard M. Joseph, S.E., P.E., is a principal in the Tustin, Calif., office and can be reached at firstname.lastname@example.org. Kyle E. Krall, P.E., is vice president in New York.