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W/PLATES 1

(b) MID-RISE FLOOR

Figure 8.25. (Continued.)

towers where there are fewer columns, vertical diagonal bracing in the building cores provided the lateral stiffness.

Floor construction typically consisted of 4 in. (101.6 mm) of concrete on 1V2-in. (38.1-mm)-deep, 22-gauge noncomposite metal deck. The slab thickness was 5 in. (127 mm) in the core area. Outside the central core, the floor deck was supported by a series of composite floor trusses, 29-in. (0.74-m)-deep, open-web joist-type trusses with ASTM A36 steel chord angles and steel rod diagonals. Composite behavior of the truss with the floor slab was achieved by extending the diagonal truss members above the top chord so that they would act much like shear stud (see Figs. 8.28g,h,i,j). Detailing of these trusses was similar to that typically used in open-web joist fabrication, but the floor system design was not typical of open-web-joist floor systems. It was considerably more redundant and was well-braced with transverse members. Trusses placed in pairs, at 6 ft-8-in. (2.03-m) spacing spanned approximately 60 feet (18.29 m) to the sides and 35 ft (10.67 m) at the ends of the central core. Metal deck spanned parallel to the main trusses and was supported by continuous transverse bridging trusses spaced at 13 ft 4 in. (4.06 m) from the transverse trusses.

In approximately 10,000 locations in each building, viscoelastic dampers extended between the lower chords of the trusses and gusset plates attached to exterior columns (see Fig. 8.28i). These dampers were provided to reduce occupant perception of wind-induced motion. Pairs of flat bars extended diagonally from the exterior wall to the top of chord of adjacent trusses. These diagonal flat bars, which were typically provided with shear studs, provided horizontal shear transfer between the floor slab and exterior wall, as well as out-of-plane bracing for perimeter columns not directly supporting floor trusses.

Figure 8.26. Citicorp Tower, Los Angles: (a) building photograph; (b) composite plan; (c) 36th floor framing plan; (d) 47th-52nd floor framing plan.
Figure 8.26. (Continued.)
Figure 8.27a. Taipei 101 Financial Center. (Photograph courtesy of Mr. Hung Lee of John A. Martin & Associates, and Mr. David Lee.)

The core framing consisted of 5-in. (127-mm) concrete fill on metal deck supported by floor framing of rolled structural shapes, in turn supported by wide-flange shape and built-up box section columns. Some of these columns measured 14 by 36 in. (0.35 x 0.91 m). For the upper levels these box columns transitioned into wide-flange shapes. At the top a total of 10 outrigger trusses were present, six extending across the long direction of the core and four extending across the short direction (see Fig. 8.28f). In addition to providing support for a transmission tower (WTC 1 had a transmission tower; WTC 2 did not, but was designed to support such a tower), this outrigger system provided stiffening of the frame for wind resistance.

Prior to construction, the site was underlain by deep deposits of fill material, placed over a period of several hundred years to reclaim the shoreline. In order to construct the towers, perimeter walls for the subterranean structure were constructed using slurry wall and tieback technique. Tieback anchors were drilled diagonally down through the wall and grouted into position into the rock deep behind the walls. (For more information see Ref. 97).

Floors within the substructure were of reinforced concrete flat-slab construction, supported by structural steel columns. These floors also provided lateral support for the perimeter walls, holding back the earth and water pressures from the unexcavated side of the excavation. The tiebacks, which had been installed as temporary stabilizers, were decommissioned by cutting off their end anchorage hardware and repairing the pockets in the slurry wall where these anchors had existed.

In slurry wall construction, a trench is dug in the eventual location of the perimeter walls. A bentonite slurry is pumped into the trench as it is excavated, to keep the trench open against caving of the surrounding earth. Prefabricated reinforcing steel is lowered into the trench, and concrete is placed through a tremie to create a reinforced concrete

Figure 8.27b. Framing plan for level 50, Taipei Financial Center. The structure consists of a dual system of a braced core connecting to a perimeter sloping frame at each sloping face. The core diagonal and chevron braces are interconnected to vertical supercolumns via outrigger and belt trusses. The supercolumns at the base are 2.4 m X 3.0 m (approximately 8 ft X 10 ft).

Figure 8.27b. Framing plan for level 50, Taipei Financial Center. The structure consists of a dual system of a braced core connecting to a perimeter sloping frame at each sloping face. The core diagonal and chevron braces are interconnected to vertical supercolumns via outrigger and belt trusses. The supercolumns at the base are 2.4 m X 3.0 m (approximately 8 ft X 10 ft).

wall around the site perimeter. After the concrete is cured, excavation of the substructure begins. As the excavation progresses below surrounding grade, tiebacks are drilled through the exposed concrete wall and through the surrounding soil into the rock below to provide stability for the excavation.

Tower foundations beneath the substructure consisted of massive spread footings, socketed into and bearing directly on the massive bedrock. Steel grillages, consisting of layers of orthogonally placed steel beams, were used to transfer the column loads in bearing to the reinforced concrete footings.

On September 11, 2001, two commercial airlines were hijacked, and one was flown into each of the towers. The structural damage sustained by each tower from the impact, combined with the ensuing fires, resulted in the total collapse of both buildings. The north tower was struck between floors 94 and 98, with the impact roughly centered on the north face. The south tower was hit between floors 78 and 84 toward the east side of the south face. Both planes banked steeply with estimated speeds of 470 mph and 590 mph at the time of impacting the north and south towers, respectively. The population on September 11, 2001, of the seven buildings of the WTC complex has been estimated at 58,000 people. Almost everyone in WTC 1 and 2 who was below the impact area was able to evacuate the buildings, due to the length of time between the impact and collapse of the individual towers.

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208 ft

Figure 8.27c. Taipei 101 Office Building schematic cross section.

Figure 8.27d. Tuned mass damper (TMD), Taipei Financial Center, currently the world's tallest building at 1667 ft (508 m). The 730-ton TMD, consisting of a steel sphere, is suspended by steel cables from level 92. In addition to the TMD for the tower itself, two additional TMDs have been installled for the 197-ft (60-m) spire. Structural engineering by Thornton-Tomasetti Engineers, New York, NY, and Evergreen Consulting Engineering, Inc., Taipei, Taiwan, Republic of China. TMD design by RWDI and Motion Engineering, Guelph, Ontario, Canada.

Figure 8.27d. Tuned mass damper (TMD), Taipei Financial Center, currently the world's tallest building at 1667 ft (508 m). The 730-ton TMD, consisting of a steel sphere, is suspended by steel cables from level 92. In addition to the TMD for the tower itself, two additional TMDs have been installled for the 197-ft (60-m) spire. Structural engineering by Thornton-Tomasetti Engineers, New York, NY, and Evergreen Consulting Engineering, Inc., Taipei, Taiwan, Republic of China. TMD design by RWDI and Motion Engineering, Guelph, Ontario, Canada.

In each case, the aircraft impacts resulted in severe structural damage, including some localized partial collapse, but did not result in the initiation of global collapse. In fact, WTC 1 remained standing for a period of 1 hour 43 minutes following the initial impact, and WTC 2 for approximately 56 minutes. The fires heated the structural systems and, over a period of time, resulted in additional stressing of the damaged structure, as

Figure 8.28. World Trade Center (WTC) Towers, New York: (a) photographs (1)-(6): photo (2) is a view looking from inside (photograph courtesy of Andrew Besirof, John A. Martin & Associates, Los Angeles, CA); (b) framing plan; (c) column axial loads due to wind force; (d) prefabricated column and spandrel assembly; (e) 1. Section A through spandrel, 2. Section B through perimeter column; (f) outrigger truss at tower roof, plan, and section; (g) floor framing system; (h) typical floor truss; (i) detail A, exterior wall end detail; (j) detail B, interior wall end detail; (k) height comparison of some contemporary tall buildings.

Figure 8.28. World Trade Center (WTC) Towers, New York: (a) photographs (1)-(6): photo (2) is a view looking from inside (photograph courtesy of Andrew Besirof, John A. Martin & Associates, Los Angeles, CA); (b) framing plan; (c) column axial loads due to wind force; (d) prefabricated column and spandrel assembly; (e) 1. Section A through spandrel, 2. Section B through perimeter column; (f) outrigger truss at tower roof, plan, and section; (g) floor framing system; (h) typical floor truss; (i) detail A, exterior wall end detail; (j) detail B, interior wall end detail; (k) height comparison of some contemporary tall buildings.

well as additional damage and strength loss to initiate a progressive sequence of failures that culminated in total collapse of both structures.

Design experts from ASCE and FEMA who investigated the WTC destruction have agreed that it would be futile to create a "terror code" to try to out-design terrorists. The WTC buildings were not required to protect their occupants during the disaster, but on 9/11 did so stunningly. Despite being subjected to stresses that never could have been anticipated, the structural design of the towers kept them standing long enough for more than 20,000 people to evacuate.

Figure 8.28. (Continued.)
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