The Building

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

an attempt to uncover the truth about September 11th 2001

mirror of “NERDCITIES/GUARDIAN” site : disclaimer

M.I.T., Rotch Visual Collections

Visual Communications in Building Technology Project

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3.3 THE ENVELOPE

The exterior wall, or envelope, of a building is designed to perform a number of functions. The envelope creates one or more barriers against water infiltration, heat transmission, and wind penetration. The envelope provides security for the structure, protecting the interior from undesired intrusions and helping occupants define the limits of their space. The form, color, shape, and texture of the exterior wall help establish a building's aesthetics. The envelope may be part of the load-carrying structure, but in many modern buildings, including Sixty State Street, the envelope is supported by the structure and plays no load-bearing role.

Sixty State Street's exterior wall is composed of three major elements: granite panels hung on triangular steel trusses, forming column covers; granite covered flat frames forming spandrals; and window units composed of extruded aluminum frames supporting tinted and reflective double glazing. Figure 3.14 isolates each of these three elements, and labels the major parts of each. The triangular steel truss strongbacks are fabricated and assembled first and attached to the structural steel perimeter tube. As shown in Figure 3.10, spandrels are then hung from the strongbacks. The aluminum frames are then also hung from the strongbacks, and finally the double glazing is fitted into place.

Figure 3.14 ENVELOPE OF THE BUILDING

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1. Plans of column cladding.
2. Plans of column cladding.

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3. Drawing of window glazing.
4. Detail of window glazing and cladding.

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5. Exterior view of completed cladding.

Figure 3.15 presents an exploded view of a typical column cover. As shown, the element has five major component parts: a triangular galvanized steel truss frame; six rectangular granite panels, quarried in Sweden and cut in Italy, approximately one inch thick, each weighing about two hundred pounds; three pieces of foil backed thermafiber insulation, approximately two inches thick; a 20 gauge galvanized steel firestop, which is clipped to the bottom of the frame; and an extruded anodized aluminum window washing track which slides in at the apex of the triangular frame and completes the chevron shape. A typical column cover is one story high, twelve foot-six inches, approximately five feet wide, two feet deep, and weighs close to 1500 pounds.

Unlike many recent skyscrapers which feature bands or strips of continuous windows or even whole facades of glass, Sixty State Street identifies openings as events in an otherwise opaque wall. A window unit is composed of two major parts: a black anodized extruded aluminum frame; and dual pane reflective and tinted glass. The frame is shipped to the site in pieces, assembled, and hung by a pair of bolts from the base chords of the strongback. The dual glazing units, approximately five feet by seven feet, have a grey heat-absorbing coating on the inner surface of the outer pane and a pink reflective coating on the outer surface of the inner pane (see the schematic heat transmission diagram in Figure 3.16). The glazing is mounted into the frame from inside the building.

Figure 3.15 EXPLODED COLUMN COVER

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1. Exploded view - schematic of column cover.
2. Lab testing architect's model of column cover.

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3. Architects' model for cladding attachment.
4. Transportation of column cover to building site.

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5. Granite slabs awaiting assembly.
6. Column cover frames.

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7. Preparing frame for installation of insulation.
8. Completing frame assembly.

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9. Granite and anchor detail.
10. Close-up of frame and cladding connection.

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11. Detail of panel anchor on building.
12.Detail of panel installation on building.

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13. Aligning column cover panel.
14. Final position of column cover panel.
15. Welding to secure panel.

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16. Several panels in place.
17. Exterior view of process of aligning panel. 18. Close-up of interior connections.

Sixty State Street's envelope was designed to withstand a variety of loading conditions imposed by three types of loads. First, gravity loads imposed by the envelope elements themselves must be carried to the supporting structure. For example, steel strongbacks must carry the weight of the granite and glazing placed upon them and transfer the load to the perimeter steel tube. Second, positive and negative wind pressure loads must be considered. The effects of storm-driven wind and rain, as well as suction forces generated by air flow around the tower, must be estimated and component parts detailed to withstand them. The third type of loads are those created by the structure itself: thermal-induced forces of expansion and contraction, and the racking and bending induced movement of the structure.

The demands of loading conditions were complicated by the need for thermal, vapor, air, water, and acoustic sealing and insulation. What may be necessary to accommodate movement from racking and bending may create a difficult sealing condition.

The dual pane glazing was chosen for its thermal qualities, since it allows much reduced transmission of solar energy and prevents excess heat loss in cold periods. But the granite, so thinly sliced, has little insulative quality. It was therefore necessary to pIace insulation behind the granite panels. Figure 3.17 presents details of the sealing networks.

Figure 3.17 WINDOWS AND SEALANT

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1. Mockup of window wall in Italy.
2. Mockup of window wall in Italy.
3. Completed mockup of window wall.
4. .....5. .....6.
4. Testing mock-up: wind & rain machine.
5. Column cover panel: final position.
6. Welding of column cover assembly.

7. .....8. .....9.
7. Several panels in place.
8. Aligning panel, exterior view.
9. Close-up of interior assembly connection.

10. .....11.
10. Crane for moving window panels within the building.
11. Prepared window panels awaiting installation.

12. .....13.
12. Spandrel attachment detail.
13.Close-up of interior connection.

14. .....15.
14. Site office diagram of cladding sequence.
15. Stacked window frames.

16. .....17.
16. Window frames in crates.
18. .....19. .....20.
18. Attaching window frames.
19. Window frame in place.
20. Exterior view of window frame installation.

21. .....22.
21. Window glass in crate.
22. Interior view of installed window frame.

Two water air seals are employed. The first is a plastic caulking compound, red-colored to complement the granite, which fills joints between graite panels, and between granite panels and window frames. A dark colored caulk does the same job between the window frame and the glazing. This exterior first-line of-defense is subjected to extreme ranges in temperature, and is therefore not counted upon for absolute protection.
23. .....24. .....25.
23. Caulking and backer rod detail.
24. Installing the caulking.
25. Installing the caulking.

26. .....27.
26. Finished window, inside view.
27. Finished window, inside view.

28. .....29.
28. Window washing equipment in use.
29. Cladding placed with straight vertical tracks. Finished exterior.

A second, interior seal was designed to act as a backup. Made of a combination of foam, gaskets, plastic caulk and aluminum tape, this seal lies near the rear face of the strongbacks and window frames. Weepholes for water and air pressure stabilization and drain are located in each window frame. The unusual nature of the exterior wall system at Sixty State Street requires a series of mockups and tests to check the design's performance against expected conditions. (see Figure 3.17). The most extensive tests occurred in Florida, where a full size segment of the exterior wall was built and subjected to wind and water infiltration to evaluate proper methods of fastening and sealing.

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