9 - 1 1 R e s e a r c h

an attempt to uncover the truth about September 11th 2001
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Multi-Storey Buildings in Steel, Godfrey, GB (Editor); Second Edition; Collins, London, England, 1985


Architectural design: M. Yamasaki and Associates, Troy, Michigan; E. Roth and Sons, New York.
Structural design: Skilling, Helle, Jackson, Seattle.
Built: 1966/1973.

The complex comprises two tower office blocks, each 110 storeys high, together with four 8-storey and 10-storey ancillary buildings which are grouped around the towers and enclose a plaza. The complex provides for a total staff of 50,000 people but it also receives up to 80,000 visitors daily. It accommodates the premises of business firms, insurance companies, banks and public authorities and includes a hotel.

The 110 upper floors contain open-plan offices, free from internal supports, each with 2,900 m2 of effective floor area. On five basement floors are stations served by underground railway and rapid-transit lines, underground parking space for 2,000 cars, and service facilities of various kinds. Below these, on a sixth basement floor and in four double-height storeys distributed over the height of the building, are air-conditioning plant and other technical services.

Each of the tower blocks has 100 passenger lifts and four goods lifts. A lobby on the ground floor and two sky-lobbies, on the 44th and 78th floor respectively, subdivide the building into three circulation zones. Each sky-lobby is connected by eleven or twelve lifts to the ground floor. From each of the three lobbies 24 local lifts give access, to the floors above. Furthermore, five express lifts go non-stop from the ground floor to the 107th and 110th floors. As three local lifts, one above the other, can operate in the same shaft, only 56 shafts are needed to accommodate the 104 lifts in the building (the liftshafts occupy 13% of the area of each floor). Maximum transit time, including change of lift, is 2 minutes. In an emergency, a fully occupied tower block (assuming it to contain 55,000 people, including visitors) can be emptied in 5 minutes.

Diagram (left): Vertical section through a tower block

1 Ancillary building
2 Plaza level
3 Sky-lobby
4 Technical services
5 Underground car park


Two towers, each 411 m high, 63.5 x 63.5 m square on plan, core 24 x 42 m.
Storey height 3.66 m, ceiling height 2.62 m.
Height of entrance hall 22.3 m.

Structural features

The structural design of the two towers is determined by the method of absorbing and transmitting the wind forces. On each of the facades a Vierendeel girder type wall is formed by 59 box-section columns (spaced at 1.02 m centres) which are rigidly connected to spandrel panels at each floor level. At the corners of the building these walls are interconnected to transmit shear, so that, together with the floors of the building, they form a torsionally rigid framed tube which is fixed to the foundations and transmits all wind loads. The floors span without intermediate columns between the external columns and the core, the 44 box-section columns of which have to carry vertical loading only.

External framework and facade

The external columns are of constant overall cross-section, 450 x 450 m. The spandrel panels interconnecting them comprise steel plates, 1.32 m deep. 12 m above the entrance level the columns are combined in groups of three to form single base columns, spaced at 3.05 m centres and with an overall cross-section of 800 x 800 mm.

Diagram (above): Framed tube construction principle: load-bearing external walls stiffened by the floors to form a torsionally rigid tube

13 Load-bearing external wall
17 Core box column
20 Floor slab

Note that the buildings are stiffened by the composite steel-concrete floors. The floors are an integral part of the structural system. Without the composite floor slabs, the buildings would soon collapse.

The wall thickness and grade of steel in the external columns are varied in successive steps in the upward direction: wall thickness decreasing from 12.5 to 7.5 mm, yield point of the steel from 70.0 to 29.5 kg/mm2. To ensure that the floors remain plane, i.e., free from warping distortion, the external columns are so designed that the stresses, and therefore the strains, produced in them by vertical loads are equal to those produced in core columns (mild steel with yield point of 24 kg/mm2). The reserve stress capacity in the external columns which is provided by the progressively graded qualities of steel serves to absorb wind load. The design value adopted for wind pressure over the entire height of the building is 220 kg/in2. The calculated maximum deflection at the top of the building is 28 cm.

Although the calculated maximum deflection at the top of the building may be 28 cm, the measured deflections were considerably larger. In Robertson [1] we find information concerning the World Trade Center when subjected to a 95 mph (153 kph) wind. It turns out that the static deflection is 45 inches (114 cm) from the vertical and that the building then oscillates some 33 inches (84 cms) either side of the point of static deflection (with a period of eleven seconds). Thus a 95 mph (153 kph) wind induces a maximum deflection of 78 inches (198 cms) from the vertical.

The external framework was erected using prefabricated three-storey units, each comprising columns interconnected by spandrel panels. These units, ranging in weight from 22.3 to 6.0 tonnes, were fitted together, alternately staggered in one storey heights, and spliced with high-strength friction-grip bolts.

Diagram (above): Horizontal section through an external column with window frame connection

36 External column
38 Vermiculite plaster
39 Special fire-resistant plaster
40 Aluminium sheet
41 Guide rail for window-cleaning equipment
42 Special tinted glass
43 Aluminium window frame

The external cladding to columns and spandrels consists of aluminium sheet. The window openings, 1.98 x 0.48 m, are infilled with bronze-tinted solar-heat rejecting glass fitted into aluminium frames and sealed with Neoprene gaskets. Automatic window cleaning is by means of rotating brushes guided along rails fitted on the column cladding.

Diagram (above): Structural system for typical floor

13 Load-bearing external wall
14 Bar joist 900mm deep
15 Secondary joist
16 Horizontal bracing
17 Core box column


Composite floors comprise 900mm deep bar joists (spaced at 2.04 m centres and braced transversely by secondary joists) and a 10 cm thick lightweight concrete slab laid on steel trough decking as permanent formwork. Composite action between the concrete and the steelwork is ensured by extending the diagonal web members of the joists through the steel decking and embedding them in the slab. Dead weight of floor 50 kg/in2, imposed load 488 kg/in2.

Diagram (above): Constructional features of a prefabricated floor unit

21 Floor covering
22 In-situ concrete
23 Trough decking
24 Bar joist
25 Electrical services duct
26 Air-conditioning duct

Each upper floor comprises 32 prefabricated units spanning between core and external columns.

This is (partly) wrong. It turns out that 18 floors have heavy steel beams instead of trusses. Consider the following quote from Engineering News-Record, January 1, 1970.

On the 41st and 42nd floors, both towers will house mechanical equipment. To accommodate the heavy loads, the floors are designed as structural steel frame slabs. All other floors from the ninth to the top (except for 75 and 76, which will also carry mechanical equipment) have typical truss floor joists and steel decking.

Typical office floors have 4-in. thick slabs of composite construction using top chord knuckles of the joists (trusses), which extend into the slab, as shear connectors. On mechanical floors, composite action is provided by welded stud shear connectors.

So the first 8 + 6 = 14 stories, and the 41st, 42nd, 75th and 76th floors, used solid steel beams in place of trusses. Also, the top stories had special steel reinforcing diagonals called outrigger trusses.

These units are of two sizes: 18.3 x 6.0 m along the longitudinal faces of the core and 10.7 x 4.0 m along the transverse faces. Additional beams are provided to strengthen the four corner bays.

Diagram (above): Shock absorbers between floor girders and external wall: plan and elevation

29 Visco-elastic pads
30 High tensile steel wire
31 Steel plates

Oscillations due to wind are absorbed by visco-elastic shock absorbers installed between the floor beams and the external columns. Electricity and telephone cables and air-conditioning ducts are incorporated in the floor units, being installed prior to erection.

Fire protection of the steelwork is provided by 3 mm thick sprayed vermiculite plaster. The core has been designed as a safety zone with emergency stairs for escape and with hydrants for active fire-fighting operations. Water for extinguishing fires is available in tanks, each of 18,500 litres capacity, which are installed on the technical services floors. Additional riser pipes are installed in the core.


Rock with a permissible bearing pressure of 39 kg/cm2 occurs at a depth of 22.5 m. The excavation for the basement and foundations, area 440,000 in2, is enclosed by a 90 cm thick reinforced concrete diaphragm wall anchored back into the surrounding ground. The column foundations comprise two-layer grillages which transmit the loads through a concrete base slab, 2.1 m thick, to the underlying rock.

Diagram (below left): Plan of foundation for a tower block

Diagram (below right): Grillage foundation under a core column

33 Concrete base slab
34 Grillage beams
37 Core column

Diagram (above): Shock absorbers between floor girders and external wall: plan and elevation

32 Rock
33 Concrete base slab
34 Grillage beams
36 External column
37 Core column


The whole complex of buildings is fully air-conditioned. In the tower blocks, air-conditioning units are installed in front of the windows around the perimeter of the floor. The cooling plant at bottom basement level obtains cooling water from the adjacent Hudson River at a rate of 330,000 litres/mm. There are seven cooling units, each with a capacity of 21,000 Mcal/h. The cooled water (at a temperature of 3.3 degrees C) is distributed through a total of 20 distributing stations.

The top part of each tower (59th to 110th floor) has a high-pressure air-conditioning system comprising two distributing stations on the 75th floor and two on the 108th. On the floors below and in the ancillary buildings a low-pressure system is provided, with a total of 16 distributing stations located in the basement and on the 7th and 41st floors. Heating is by means of steam generated in the central air-conditioning plant.

Location plan of the complex

1. WTC 1 - North Tower - 110 floors
2. WTC 2 - South Tower - 110 floors
3. WTC 3 - Hotel - 22 floors
4. WTC 4 - South Plaza Building - 9 floors
5. WTC 5 - North Plaza Building - 9 floors
6. WTC 6 - US Custom Building - 8 floors
7. WTC 7 - World Trade Center Seven - 47 floors

Diagram (above): Plan of typical floor

10 Open-plan office
11 Express lifts
12 Local lifts

Assembly of the external wall units (alternately staggered in one-storey heights) and floor units

Areas and volume (per tower)

gross area: 418,000 m2
area on plan: 4,032 m2
effective floor area: 319,000 m2
volume: 1,754,000 m3

Quantities of steel (structural steelwork in one tower)

total: 78,000 tonnes
per square meter gross area: 166.6 kg
per cubic meter: 44.5 kg
per square meter effective floor area: 244.5 kg


Architectural Forum, 4/1 964. p. 119.
Engineering News-Record. 9/1964. p. 36; 11/1971.
Der Stahlbau, 11/1964. p. 350; 4/1970. p. 123.
Der Baningenleur. 9/1965. p.373:11/1967, p. 421.
Bauwelt. 32/1 966. p. 909.
Acier-Stahl-Steel. 12/1966, p. 556:6/1970. p. 273.

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