One World Trade Center Case Study: Core, Base and Spire (SOM)

One World Trade Center is a 104-storey office tower in Lower Manhattan by Skidmore, Owings & Merrill (SOM), with WSP Cantor Seinuk as structural engineer, built from 2006 and opened in November 2014. It teaches four things: how a concrete core and a steel perimeter frame share the work in a very tall building, how a security brief produced a fortified base, how a square plan turning 45 degrees makes the tower's eight triangular faces, and how a spire came to decide its official height.

One World Trade Center at a glance

Architect: Skidmore, Owings & Merrill, with David Childs as design partner.
Engineers: WSP Cantor Seinuk, structure; Leslie E. Robertson Associates, peer review (Skyscraper Center); schlaich bergermann partner, spire, rings and cable-net walls; Weidlinger Associates, protective design; Jaros, Baum & Bolles, mechanical.
Construction manager: Tishman Construction (AECOM).
Owner: the Port Authority of New York and New Jersey, with the Durst Organization; Silverstein Properties earlier.
Location: 285 Fulton Street, northwest corner of the World Trade Center site, New York. Site 6,905 m² (74,328 sq ft), per SOM.
Dates: SOM chosen July 2003; redesign released 29 June 2005; construction from 27 April 2006; spire completed 10 May 2013; first tenant 3 November 2014; observatory opened 29 May 2015.
Size: 541.3 m (1,776 ft) to the top of the spire, 546.2 m (1,792 ft) to the tip (CTBUH). SOM counts 104 storeys; the Skyscraper Center counts 94 above ground and 5 below. Gross area 325,279 m² (3.5 million sq ft).
Status: offices, with a public observatory. LEED Gold (core and shell).

Where sources differ, both figures are given. Diagrams are MOA's own schematics.

The brief and the client

The tower stands on the site of the World Trade Center, destroyed on 11 September 2001. In February 2003 the Lower Manhattan Development Corporation and the Port Authority picked Daniel Libeskind's master plan for the site, and in July 2003 David Childs of SOM was chosen to design its tallest tower, then called the Freedom Tower (CNN fast facts). Libeskind's plan fixed the height at 1,776 ft, for the year of the US Declaration of Independence (Joann Gonchar, Architectural Record, 2011).

The brief was an office building with three extra jobs: mark the site on the skyline, meet a security standard far above normal office practice, and give occupants better ways out than most towers. Larry Silverstein ceded control to the Port Authority in April 2006 (Architectural Record), and the Port Authority brought in Douglas Durst in 2010 (Peter Grant, Wall Street Journal, 2021).

Site and context

The tower takes the northwest corner of the site, with West Street, an eight-lane road, along its west side (Newsday, 30 June 2005). The memorial pools lie just south, and the tower's base matches the original towers' footprints at 200 ft square (Metals in Construction, Spring 2015). Below ground, PATH train tracks cross the lowest basement and kept running through construction (Metals in Construction, structure feature). For recording context like this, see our guide to documenting a building for a case study.

One World Trade Center original design and the 2005 change

The scheme revealed in December 2003 had a torqued cable-net exterior and an off-centre spire (Architectural Record). In May 2005 police concerns became public: the tower sat about 25 ft from West Street, too close, they said, to protect against a large truck bomb. Governor George Pataki announced a redesign on 4 May, and the new design came out on 29 June 2005 (CNN fast facts). The Associated Press gave the new setback as an average of 90 ft (27 m) from West Street; Newsday gave 55 to 125 ft from the roads (AP, July 2005). The twist went; the tower became symmetrical, stood on a blast-resistant base and had its spire moved to the centre of the roof. The name changed to One World Trade Center on 26 March 2009 (CNN fast facts).

One World Trade Center design concept

Above a cubic base, the plan changes from a square at the bottom to a smaller square at the top turned 45 degrees. Joining the two with flat planes makes eight tall isosceles triangles and an octagonal plan at mid-height (SOM; Metals in Construction). The 6 ft steel band at the roof runs from 1,362 ft to 1,368 ft, the two original roof heights.

SOM's Kenneth Lewis told Metals in Construction the team wanted a uniform facade that read as aspirational, not defensive. The taper is also practical: in the original towers, upper floors had 70 to 90 ft spans from a small core to the glass, leaving large areas without daylight. Smaller floors towards the top keep people nearer the windows, and the narrowing form reduces wind effects.

One World Trade Center at dusk seen across the water: a tall glass tower with chamfered corners that form long triangular facets, a dark parapet band and a thin steel mast with rings at its foot, rising above the lower office towers of Lower Manhattan lit gold by the low sun — Seen from across the water, the chamfered corners read as long triangles that narrow towards a square top, with the bare steel mast above the parapet. Photo: Olga1969, CC BY-SA 4.0, via Wikimedia Commons

One World Trade Center floor plan

Each office floor is a ring of column-free space around a concrete core. At the base the core is 110 by 110 ft (33.5 m), with walls up to 4 ft (1.2 m) thick; as the tower tapers, some core cells drop off and the walls thin to 2 ft (0.6 m). Outside the core, floors are concrete on composite metal deck on steel beams, spanning up to 47 ft (14.3 m) by Metals in Construction's account and 45 ft by Architectural Record's. That gives a lease depth of about 45 ft on every side.

SOM counts 71 office storeys above the lobby and the podium's mechanical floors, with a three-level observatory on floors 100 to 102 (SOM; ASCE Civil Engineering, 2021). Typical floor plans are printed in the Metals in Construction feature, and the data sheet is on the CTBUH Skyscraper Center.

Schematic plan of One World Trade Center: a 61 m (200 ft) base square in grey with a square concrete core of 33.5 m (110 ft) at its centre; a red square turned 45 degrees with its corners on the midpoints of the base edges marks the 45.7 m (150 ft) top at the parapet; a dashed octagon between them marks the mid-height floor; the four corner triangles are labelled facets; a red arrow marks the column-free span of up to 14.3 m from core to glass. Notes explain the eight isosceles triangles and the core walls, up to 1.2 m thick and thinning to 0.6 m higher up — The floor plate starts as a 200 ft square, becomes an octagon at mid-height and ends as a smaller square turned 45 degrees, around a concrete core. Schematic, not to scale, from published descriptions. Diagram: MOA

One World Trade Center structure and engineering

The structure is a hybrid: a reinforced concrete shear-wall core inside a ductile steel moment frame. The core carries gravity loads, resists wind and earthquake, and holds the lifts and stairs (ASCE). The original Twin Towers put most of the work in a tube at the outside walls. "Having a robust core has a lot of value," Ahmad Rahimian of WSP told Metals in Construction, adding that it securely houses the escape stairs.

The core uses concrete of up to 14,000 psi (about 97 MPa), which ASCE's magazine calls the strongest used in New York City at the time; higher strength let the walls be thinner. Rahimian says the team worked for months with the Port Authority's own materials lab to settle the mixes (Metals in Construction).

The perimeter frame uses about 45,000 tons of high-strength steel. Multi-floor "super diagonals" ring the lower levels like a belt, and outriggers tie the perimeter columns to the core at the top mechanical floors. Lewis explains the frame's second job: if two columns were lost, the perimeter beams would act like a rope in tension and pull loads back to the core (Metals in Construction).

The tower goes 70 ft (21.3 m) down to Manhattan bedrock on spread and strip footings of 60 tons per sq ft or more, reaching 114 tons per sq ft where train lines forced deeper digging, with rock anchors 80 ft (24.4 m) into the rock against overturning (Metals in Construction gives four basement levels, ASCE five).

Schematic elevation of One World Trade Center seen at a corner: a basement 21.3 m deep on bedrock with rock anchors below; a podium about 57 m high wrapped in fins, with thick red concrete walls around a 15.2 m lobby; a tapering glass shaft with a dashed concrete core running from basement to roof; red crossed super diagonals as a belt above the podium and red outriggers near the top tying the frame to the core; a dark parapet band at 417 m and 415 m; three rings and a steel mast rising 124.4 m above the parapet to 541.3 m, with the tip at 546.2 m. Notes give core concrete of up to 14,000 psi, core walls up to 1.2 m thick thinning to 0.6 m, about 45,000 tons of perimeter steel and floor spans up to 14.3 m — A concrete core and a steel perimeter frame share the loads, a fortified podium takes the security brief, and the spire sits on a concrete mat over the core. Schematic, not to scale, from published descriptions. Diagram: MOA

One World Trade Center base and security

The base is a near-cube about 200 ft wide, 186 ft high by Metals in Construction's account and 187 ft by Architectural Record's. Concrete walls 28 in (0.71 m) thick shelter the lobby; Metals in Construction notes they are added protection and do not carry the tower. SOM gives the lobby as 50 ft high, Metals in Construction as 60 ft. Entrances on all four sides are 60 ft tall, with cable-net walls (SOM).

To soften the walls, the podium is faced with more than 4,000 laminated glass fins, each 13 ft 4 in by 2 ft, set at varying angles between stainless steel slats, a pattern that also ventilates the mechanical floors behind. Prismatic glass first planned here bowed during tempering, so the fins replaced it (Architectural Record, 2011).

Sources also list extra-wide pressurised stairs, a stair reserved for firefighters, concrete protection for sprinkler risers, areas of refuge, high air intakes with biological and chemical filters, and stair exits straight to the street (Architectural Record; Metals in Construction; ASCE). Many were first tried at 7 World Trade Center, completed in 2006 by most of the same team.

Looking up at One World Trade Center from beside it: the podium at the bottom is wrapped in vertical glass fins and horizontal steel slats, and above it the glass shaft rises in sharp triangular facets that reflect clouds and blue sky, with a neighbouring glass tower at the right edge — Close up, the podium's glass fins and steel slats sit below the triangular facets of the glass shaft, which carry the edges of the plan's turning square up the tower. Photo: Christian David, CC BY-SA 4.0, via Wikimedia Commons

One World Trade Center spire and the 1,776 ft height

The spire is a steel mast in seven stacked sections, 4.27 m (14 ft) across at its base, with three communication rings at its foot carrying broadcast equipment, lighting and window-washing rigs (schlaich bergermann partner). It is designed to deflect no more than 0.5 degrees in a sustained 50 mph wind, and stands on a concrete mat carried by the core (sbp; Metals in Construction). The mast rises 408 ft (124.4 m) above the parapet; SOM's 441 ft figure is measured from the roof slab to the top of the beacon (Architectural Record). The last pieces went in on 10 May 2013 (CNN fast facts).

The design wrapped the mast in fibreglass cladding. In 2012 the owners dropped it, citing about $20 million in savings and the difficulty of maintaining it; Childs called the change "unfortunate" (C. J. Hughes, Architectural Record, 2012). A bare mast looked like an antenna, which CTBUH does not count in height, and without it the tower measured 1,368 ft, below Chicago's Willis Tower. In November 2013 CTBUH's height committee ruled it a permanent architectural spire, making 1,776 ft the official height and the tower the tallest in the US (ArchDaily, 12 November 2013). The same report gives Burj Khalifa as 828 m (2,717 ft), the subject of our Burj Khalifa case study.

One World Trade Center glass facade and materials

The unitised curtain wall uses low-iron insulated glass units 5 ft by 13 ft 4 in (1.52 m by 4.06 m) that run the full floor-to-floor height with no mullion at the slab. SOM calls them the largest mass-produced for a building of this scale, and Viracon built a special production line for them. The outer pane is at least 3/8 in thick, where 1/4 in is standard, and the inner pane is laminated. Stainless steel marks the chamfered corners, and shorter glass panels leave slots at the mechanical floors for air, so no louvres show (Metals in Construction).

SOM states that the steel was 95% recycled and the concrete used fly ash; ASCE says more than half the cement content was replaced by industrial by-products. Both give LEED Gold; our post on IGBC, GRIHA, LEED and ECSBC compares rating systems for Indian projects. On cost, Architectural Record gave $3.19 billion in 2011 and the Wall Street Journal $3.8 billion in 2021, calling it the most expensive US skyscraper. These are US dollar figures, not converted or adjusted for inflation.

Light, climate and services

The full-height glass and 45 ft lease depth carry daylight to the core (Metals in Construction). The tower has 73 lift cars with a top speed of 10.16 m/s (Skyscraper Center), and the observatory's lifts reach the top in 47 seconds (One World Observatory). SOM says the lift motors recover energy through regenerative braking.

Reception

At the 2005 unveiling Rick Bell of the AIA New York chapter called it "essentially an office building and a good one," while Robert Ivy of Architectural Record asked who would rent the space (Newsday). When it opened, Michael Kimmelman of The New York Times wrote: "It looks as if it could be anywhere, which New York isn't" (quoted by ArchDaily, 1 December 2014). Leasing was slow for years; by 2021 more than 90% was leased, but the World Trade Center complex had not turned a profit since the tower opened (Wall Street Journal). SOM lists a 2024 CTBUH 10 Year Award of Excellence.

What designers in India can take from it

This is our reading, not a claim from the sources.

Plan the core as the escape route first. Here the core was sized around stairs, a firefighters' stair and protected risers, not only lifts. On an Indian high-rise, fix stair widths, the fire lift and refuge areas at concept stage, when moving a wall is cheap. Our post on setbacks, FAR and height limits explains the envelope you start from.
High-strength concrete needs a testing plan. If a core relies on a high grade to keep walls thin, budget time for trial mixes and testing before the pour programme is fixed, as this team did with its lab.
Keep the lease depth, not the glass. A 45 ft core-to-window depth is a useful daylight rule, but four faces of full-height clear glass suit New York better than a hot Indian city. Keep the depth and shade or reduce the east and west glass.
A solid base still owes the street something. Many Indian towers sit on blank parking podiums. SOM used 4,000 glass fins to make a blast-resistant base read as more than a wall. Decide early what a podium shows the street at eye level.

For more tall-building thesis ideas, see our B.Arch thesis topics by typology.

Visiting One World Trade Center

The offices are private, but the One World Observatory is open. Its official site gives the entrance at 117 West Street, at West and Vesey Streets, with seasonal hours and online booking. It puts the observatory about 1,250 ft up; the Skyscraper Center gives 386.5 m (1,268 ft). The plazas and podium can be seen from the street.

Looking for an architect who can work through structure, security and daylight together? Find an architect on MOA.

FAQ

When was One World Trade Center built?

Construction began on 27 April 2006. The spire was completed on 10 May 2013, the first tenant moved in on 3 November 2014, and the observatory opened on 29 May 2015. SOM and the Skyscraper Center give 2014 as the completion year.

Who designed One World Trade Center?

Skidmore, Owings & Merrill, led by David Childs, with WSP Cantor Seinuk as structural engineer and schlaich bergermann partner for the spire and cable-net walls. Daniel Libeskind's master plan set the 1,776 ft height.

How tall is One World Trade Center without the spire?

The roof parapet is at 1,368 ft (417 m), with the band below it at 1,362 ft (415 m), the heights of the two original towers. With the spire it is 1,776 ft (541.3 m), and 1,792 ft (546.2 m) to the tip by CTBUH's measure.

How many floors does One World Trade Center have?

SOM gives 104 storeys. The CTBUH Skyscraper Center counts 94 floors above ground and 5 below. The two count floors differently, so quote the source with the number.

What is One World Trade Center made of?

A reinforced concrete core of up to 14,000 psi concrete, a steel perimeter moment frame of about 45,000 tons, composite floors of concrete on metal deck, and a curtain wall of insulated laminated glass. The base adds thick concrete walls behind glass fins and steel slats.

Sources

SOM: One World Trade Center (accessed 4 Nov 2026)
CTBUH Skyscraper Center: One World Trade Center (accessed 4 Nov 2026)
Metals in Construction (Spring 2015): One World Trade Center Structure (accessed 4 Nov 2026)
Metals in Construction (Spring 2015): One World Trade Center Facade (accessed 4 Nov 2026)
Joann Gonchar, Architectural Record: One World Trade Center, 16 September 2011 (accessed 4 Nov 2026)
Laurie A. Shuster, ASCE Civil Engineering: From the ashes: One World Trade Center, 1 September 2021 (accessed 4 Nov 2026)
schlaich bergermann partner: One World Trade Center, NYC (accessed 4 Nov 2026)
C. J. Hughes, Architectural Record: One World Trade Center's skinny spire redesign might not measure up, 15 August 2012 (accessed 4 Nov 2026)
Karissa Rosenfield, ArchDaily: CTBUH crowns One World Trade as tallest building in US, 12 November 2013 (accessed 4 Nov 2026)
Karissa Rosenfield, ArchDaily: Kimmelman reviews the One WTC, 1 December 2014 (accessed 4 Nov 2026)
Newsday, via The Spokesman-Review: New Freedom Tower plan balances safety with design, 30 June 2005 (accessed 4 Nov 2026)
Associated Press, via Construction Equipment Guide: Freedom Tower redesigned to be world's safest skyscraper, July 2005 (accessed 4 Nov 2026)
CNN, via WRAL: One World Trade Center fast facts, 2018 (accessed 4 Nov 2026)
Peter Grant, Wall Street Journal, via Fox Business: One World Trade Center is 90% filled after cost overruns and delays, 12 September 2021 (accessed 4 Nov 2026)
One World Observatory: home page and Experience (accessed 4 Nov 2026)
Wikimedia Commons: Freedom of panorama, United States (accessed 4 Nov 2026)