First Interstate World Building Los Angeles Floor Plan

(a)

Figure 8.15. Nations Bank Plaza, Atlanta, GA: (a) building elevation; (b) typical framing plan; (c) section.

Fox Plaza Los Angeles Floor Plans
Figure 8.15. (Continued.)

The core columns are braced on all four sides with diagonal bracing as shown schematically in Fig. 8.15c. Since the braces are arranged to clear door openings in the core, their configuration is different on all four sides. Steel girders 36 in. (0.91 m) deep are moment-connected between the composite columns to transfer part of the overturning moment to the exterior columns. Because the girders are deeper than other gravity beams, openings have been provided in the girders to provide for the passage of mechanical ducts and pipes. A diagonal truss is used between levels 56 and 59 to tie the core columns to the perimeter supercolumns. These trusses transfer part of the overturning moment to the perimeter columns and also add considerable stiffness to the building. Above the 57th floor, the building tapers to form a 140-ft (42.68-m)-tall conehead which is used to house mechanical and telecommunication equipment. The structural design is by CBM Engineers, Inc., Houston, TX.

8.1.2.13. First Interstate World Center, Los Angeles

This 75-story granite-clad building (Fig. 8.16a) sports multiple step-backs. The structural system is a dual system consisting of an uninterrupted 73 ft 10 in. (22.5 m) square-braced spine (core) interacting with a perimeter ductile moment-resisting frame. The spine has a two-story-tall chevron bracing core, as shown in Fig. 8.16d.

Figure 8.15. (Continued.)

The 55-ft (16.76-m) span for the floor beams coupled with the two-story-tall free-spanning core loads the corner core columns in such a way that the design is primarily governed by gravity design. To achieve overall economy and take advantage of the increase in allowable stresses permitted under combined gravity and lateral loads, the columns are widely spaced to collect gravity loads from large tributary areas (Fig. 8.16e). The column design is primarily for gravity loads with the additional loads due to seismic activity and wind resisted by the one-third increase in allowable stresses. As required by most seismic codes, the strong column weak/beam concept is maintained in the design of beam-column assemblies of the perimeter frame tube.

The sustained dead weight of the structure is 204,000 kips (927,272 kN), with the fundamental periods of vibration Tx = 7.46 sec, Ty = 6.95 sec, and T2 = 3.57 sec. The interaction between the interior braced core and the perimeter ductile frame is typical of dual systems with the shear resistance of core increasing progressively from the top to the base of the building. Nearly 50% of the overturning moment is resisted by the core. The maximum calculated lateral deflection at the top under a 100-year wind is 23 in. (584 mm).

The structure is founded on shale rock with an allowable bearing capacity of 7.5 tons/ft2 (720 kPa). The building core is supported on an 11.5-ft (3.5-m)-thick concrete mat while a perimeter ring footing supports the ductile frame. Typical floor framing consists

(a)
First Interstate World Center RzsutFirst Interstate Bldg Plan

Figure 8.16. First Interstate World Center, Los Angeles: (a) elevation; (b) plan showing column transfers; (c) composite plan; (d) structural system; (e) framing plan.

Figure 8.16. (Continued.)

of W24 wide-flange composite beams spaced at 13 ft centers, spanning a maximum of 55 ft (16.76 m) from the core to the perimeter. The structural design is by CBM Engineers, Inc., Houston, TX.

8.1.2.14. Singapore Treasury Building

This 52-story office tower, shown in Fig 8.17a, is unique in that every floor in the building is cantilevered from an inner cylindrical, 82-ft (25-m)-diameter core enclosing the elevator and service areas (Fig. 8.17b). Radial beams cantilever 38 ft (11.6 m) from the reinforced concrete core wall. Each cantilever girder is welded to a steel erection column embedded in the core wall. To reduce relative vertical deflections of adjacent floors, the steel beams are connected at their free ends by a 1 x 4-in. (25 x 100-mm) steel tie hidden in the curtain wall. A continuous perimeter ring-truss at each floor minimizes relative deflections of adjacent cantilevers on the same floor produced by uneven distribution of live load. Additionally the vertical ties and the ring beam provide a backup system for the cantilever beams.

Since there are no perimeter columns, all gravity and lateral loads are resisted solely by the concrete core. The thickness of core walls varies from 3.3 ft (1.0 m) at the top to 4 ft (1.2 m) at the sixteenth floor, and remains at 5.4 ft (1.65 m) below the sixteenth floor. The fundamental vibration period of this cylindrical tower is 5.6 seconds. Its foundation has six 8.0-m-diameter reinforced concrete caissons 35 m long, equally spaced on a 23.5-m-diameter circle, which transfer building loads to rock mainly via skin friction. Tops of

Vortex Shedding Structural Engineering

Figure 8.16. (Continued.)

Figure 8.16. (Continued.)

caissons are connected by a 2.9-m-thick reinforced concrete mat. The structural engineering is by LeMessurier Consultants, Cambridge, MA, and Ove Arup Partners, Singapore.

8.1.2.15. City Spire, New York

This 75-story office and residential tower, with a height-to-width ratio of 10:1, is one of the most slender buildings, concrete or steel, in the world today. The critical wind direction for this building is from the west, which produces maximum crosswind response. Wind studies indicated possible problems of vortex shedding as well as occupant perception of acceleration. This possibility was eliminated by adding mass and stiffness to the building.

The main structural system consists of shear walls in the spine connected to exterior jumbo columns with staggered rectangular concrete panels. The structure is subdivided into nine major structural subsystems with setbacks and column transfers as evident from the plans shown in Fig. 8.18. The structural design is by Robert Rosenwasser Associates, New York.

8.1.2.16. 21st Century Tower, China

Designed by the architectural firm of Murphy/Jahn, Inc., Chicago, the exterior form of this proposed building for Shanghai, China, is that of a rectangular tower; however, the building has a setback base and a series of nine-story-high wedge-shaped atria or "winter gardens,"

Treasury Building Singapore

Figure 8.17. Singapore Treasury Building: (a) schematic section; (b) typical floor framing plan.

which run for the full height of the tower. These features effectively remove one corner column over the full height of the building and an opposite column at both the top and the bottom of the tower. The structure therefore takes the form of a stack of nine-story-high chevrons. Other architectural features include a cable-suspended skylight canopy roof over a podium, exposed rod-truss curtain wall supports at the winter gardens, and exposed truss-stringer stairs. Structural elements, most notably the nine-story-high superbraces on each face of the tower, are boldly expressed in red, while blue and green solar glazing covers the office spaces. The winter gardens and the podium are enclosed in clear glass.

Although the building is expressed as a square, it is punctuated by a series of four nine-story-high wedge-shaped winter gardens cut into the northeast corner and two more at the southwest corner (Fig. 8.19a,b). The winter gardens have the effect of dividing the tower, both visually and structurally, into a stack of five modules outlined by the super-braces. At the lowest module, the northeast corner column is eliminated entirely. As a result, the tower has only a single axis of symmetry, which passes at 45 degrees through the corners; in addition, nine different floor plans are required within each module. Plan at a representative floor is shown in Fig. 8.19c.

Figure 8.17. (Continued.)

The design of the tower is driven by the unique architectural treatment to the building envelope, by the arrangement of the winter gardens, and by the high wind speeds of up to 115 mph (185 km/hr) resulting in design pressures as high as 136 psf (6.5 kPa).

The structural solution for the tower consists of a superbrace system on the exterior skin supplemented by an eccentrically braced interior service core. The superbrace system extends for the full width of the tower to achieve maximum resistance to wind loading. Schematic interior core bracing is shown in Fig. 8.19e. Structural action in the primary columns and braces at the base due to lateral loads is shown in Fig.8.19d.

The braces generally consist of heavy W14 sections, field-spliced every three floors and connected at their ends to square steel box columns. The braces also act as inclined columns and carry vertical loads from the secondary columns above them. This arrangement maximizes the vertical load carried by the corner columns and minimizes uplift. Columns vary in size from 20 to 24 in. (520 to 610 mm) with plate thickness up to 5 in. (130 mm). The braces are arranged in five 9-story-high vees with a one-bay gap in the middle of each building face; stiffness in the gap is provided by a one-bay-wide rigid frame. Panel points for the superbraces occur at the ground, 5th, 15th, 24th, 33rd, 42nd, and roof levels. Horizontal members and diaphragms at these levels are stiffened to transfer horizontal brace forces. Service core bracing provides additional overall stiffness and gives lateral support to floors between the superbrace panel points. For architectural reasons,

Cityspire Tower Ground Floor Plan
Figure 8.18. City Spire, New York; floor plans.

braces at the center bay of each core-face are eccentric (Fig. 8.19e). Although most lateral loading is transferred at the ground level to the shear walls, core bracing is extended to the foundation of the substructure. The numerous corner cutouts of the tower structure effectively rotate the principal axes of the structure by 45 degrees, to pass through the corner columns. The lowest two modes of vibration of the tower are single-curvature

(3)

Figure 8.19. 21st Century Tower, China: (a) model photographs (1), (2), and (3); (b) bracing system; (c) framing plan, levels 19, 28, and 37; (d) structural action in primary columns and braces; (e) typical interior core bracing. Architects: Murphy/Jahn Inc., Chicago, structural engineers: John A. Martin & Associates Inc., Los Angeles, and Martin & Huang, International, Los Angeles.

Concentric Braces Passing Steel Decks
Figure 8.19. (Continued.)

bending through these axes; the third mode is torsional. Periods of the first three modes are 4.98, 4.62, and 2.12 sec, respectively. The building's average steel weight is 29 psf (142 kg/in.2). The preliminary design is by John A. Martin & Associates and working drawings are by Martin & Huang, International, both of Los Angeles, CA.

8.1.2.17. Torre Mayor Office Building Mexico City

Constructed in one of the most seismically active regions in the world, this 738-ft (225-m)-tall building, shown in Fig. 8.20a and b, consists of 57 stories including a 13-story parking garage—4 stories below ground and 9 stories above. It rises from a 262.5 x 262.5-ft (80 x 80-m) footprint at the base, decreasing to 262.5 x 213.3 ft (80 x 65 m) in levels 4 through 10, and to 157.5 x 118 ft (48 x 36 m) in levels 11 through 53. The rectangular tower is juxtaposed with a curved facade. The tower is constructed on Mexico City's dry central lake bed or "bowl of Jello" consisting of high water tables and poor alluvial soils. The site is prone to very high seismic activity that can measure 8.2 on the Richter scale.

The design performance criterion established for the structure is that it remain operational immediately following a large seismic event. To fulfill this requirement, fluid viscous dampers have been installed in a structural system consisting of a perimeter moment frame interacting with an interior braced frame. A composite superbrace frame acting in conjunction with a steel tube is present at the perimeter. A braced core at the

Figure 8.19. (Continued.)

building interior completes the lateral resisting systems. The perimeter and core columns are encased in reinforced concrete up to the 30th floor. Viscous dampers, 74 of them in the core and 24 the perimeter framing, are installed to absorb and dissipate seismic energy.

Fluid viscous dampers have been installed in diamond-shaped superdiagonal bracing architecturally expressed on the building's perimeter moment frame (Fig. 8.20c). All four elevations of the building contain superdiagonals configured as diamonds rather than Xs. Broad south and north faces contain dampers, that resist seismic loads in the east-west direction. Each of these elevations has four steel diamonds with 137.8-ft (42-m) legs. The diamonds overlap each other at their peaks and valleys to form three smaller diamonds. Each small diamond has four 1200-kip-capacity dampers, one on each leg near the apex or valley. The building has a total of 98 dampers, 12 on each of the broad faces and 74 in the building core. The incorporation of dampers limits quake-induced damage to hung ceilings, fire sprinklers, partitions, mechanical systems, and cladding. Dampers of 600-kip capacity are used in the core in the north-south direction. Core dampers are located conventionally on diagonals of the vertical trusses that transverse the core, two in the end walls and two in between. Photographs of dampers are shown in Figs. 8.20d and e.

The columns in floors 1 through 10 are composite with structural steel columns encased in concrete, which limits the size of steel members. Damper clusters begin at the 11th floor. Rigid floor diaphragms that connect the perimeter frame to a 90.55 x 49.22-ft

Figure 8.19. (Continued.)

(27.6 x 15-m) structural steel core provide in-plane stiffness that ensures all structural elements respond simultaneously to a seismic event.

The building has a four-story basement that extends 49.2 ft (15 m) below ground. The foundation consists of 3.93-ft (1.2-m)-diameter caissons that extend 164 ft (50 m) to hard rock below the alluvial deposits. A reinforced concrete mat ranging in thickness from 3.28 to 8.2 ft (1 to 2.5 m) links the caissons. The structural engineering is by Cantor Seinuk Group, New York City, and Enrique Martinez Romero, Mexico City.

8.1.2.18. Fox Plaza, Los Angles

The structural system for resisting lateral loads for this 35-story building consists of special moment-resisting frames located at the building perimeter. The floor framing consists of W21 wide-flange composite beams spanning 40 ft (12.2 in.) between the core and the

Figure 8.20. Torre Mayor Office Building, Mexico City: (a) building photograph; (b) plan; (c) schematic elevation showing viscous dampers on front elevation; (d) photograph showing bracing and dampers; (e) close-up view of dampers. (Photographs courtesy of Dr. Ahmad Rahimian, P.E., S.E., President, Cantor Seinuk Group, New York.)

Figure 8.20. Torre Mayor Office Building, Mexico City: (a) building photograph; (b) plan; (c) schematic elevation showing viscous dampers on front elevation; (d) photograph showing bracing and dampers; (e) close-up view of dampers. (Photographs courtesy of Dr. Ahmad Rahimian, P.E., S.E., President, Cantor Seinuk Group, New York.)

perimeters. A 2-in. (51-mm)-deep 18-gauge composite metal deck with a 31/4-in. (83-mm) lightweight concrete topping is used for typical floor construction. A framing plan with sizes for typical members and a photograph of the building are shown in Fig. 8.21. Architects are Johnson, Fain, and Pereira, Inc.; the structural design is by John A. Martin & Associates, Los Angeles.

8.1.2.19. NCNB Tower, North Carolina

This building is an 870-ft (265.12-m)-tall, concrete office building with a 100-ft (30.5-m) crown of aluminum spires (Fig. 8.22). The building has a 12 ft-8 in. (3.87 m) floor-to-floor height and a 48-ft (14.63-m) column-free span from the perimeter to core.

The structural system for resisting lateral loads consists of a reinforced concrete perimeter tube with normal-weight concrete ranging in strength from 8000 psi (55.16 mPa)

Figure 8.20. (Continued.)

near the building's base to 6000 psi (41.37 mPa) at the top. Typical column sizes range from 24 x 38 in. (0.61 x 0.97 m) at the base to 24 x 24 in. (0.61 x 0.61 m) at the top. The floor system (Fig. 8.22b) consists of a 458-in. (118-mm)-thick lightweight concrete slab supported on 18-in. (458-mm)-deep post-tensioned beams spaced at 10 ft (3.05 m) on centers. Lightweight concrete was used to reduce the building weight and to achieve the required fire rating for the floor system.

The tower's columns are spaced 10 ft (3.05 m) on center and are connected by 40-in. (1.01-m)-deep spandrel beams. The building has a square plan at the base, but above the 13th floor it resembles a square set over a slightly larger cross, with the four major corners recessed and its four major faces bowed slightly outward. To maintain

Figure 8.21. Fox Plaza, Los Angeles: (a) building photograph; (b) floor framing plan. Architects: Johnson, Fain & Perei; structural engineers: John A. Martin & Associates Inc., Los Angeles.

Figure 8.21. (Continued.)

tube action between the 13th and 43rd floors, engineers used L-shaped vierendeel trusses to continue the tube around the corners. Instead of transfer girders at the building stepbacks, the building's column-and-spandrel structure is used to create multilevel vierendeel trusses on the building's main facades. These vierendeels transfer loads using another set of vierendeel trusses perpendicular to the facade at the edges of recessed corners. Differential shortening between the core and perimeter columns was a concern during design because the core columns will be under significantly higher stresses than the closely spaced perimeter columns, To compensate for this, the core columns were constructed slightly longer than the perimeter columns.

Both standard and lightweight concrete was used simultaneously. The normal-weight concrete was used for the perimeter columns, which ranged in size from 24 x 38 in. (6.10 x 965 mm) at the bottom to 24 x 24 in. (610 x 610 mm) at the top, as well as for the core columns, ranging from 2 x 18 ft (0.61 x 3.5 m) at the base to 2 x 3 ft (0.61 x 0.92 m) at the top.

Normal-weight concrete was also used for post-tensioned spandrels at the perimeter of each floor, but 5000-psi (34.5-mPa) lightweight concrete was used for the 4%-in. (118-mm)-thick floor slabs and the 18-in. (0.46-m)-deep post-tensioned beams. The two types of concrete were poured in quick succession and puddled to avoid a cold joint.

The foundation system for the Tower consists of high-capacity caissons under the perimeter columns and a reinforced concrete mat for the core columns. The high-capacity

Ground Bearing Pressures
Figure 8.22. NCNB Tower, North Carolina: (a) schematic elevation; (b) typical floor framing plan.

caissons were designed for a total end-bearing pressure of 150 ksf (7182 kN/m2) and skin friction of 5 ksf (240 kN/m2). The high bearing pressure required that the caissons be advanced through the fractured and layered rock zones into high-quality bedrock. Full-length casing was provided to prevent intrusion of soil and ground water into the drilled hole and for the safety of inspectors.

The core columns are supported on a foundation mat bearing on partially weathered rock. The mat dimensions are 83 x 93 x 8 ft (25.3 x 28.35 x 2.44 m). The average total sustained bearing pressure under the mat is equal to 20 ksf (958 kN/m2). The structural design is by Walter P. Moore and Associates, Inc., Houston, TX.

8.1.2.20. Museum Tower, Los Angeles

This 22-story residential building, shown in Fig. 8.23, consists of a tubular ductile concrete frame with perimeter columns spaced at 13-ft (8.96-m) centers interconnected with upturned spandrel beams (Fig. 8.23b). The exterior frame is of exposed painted concrete.

The gravity system for the typical floor consists of an 8-in. (203-mm)-thick post-tensioned flat plate with banded and uniform tendons running in the short and long directions of the building, respectively, as shown in Fig. 8.23c.

Although the building is regular both in plan and elevation and is less than 240 ft (78 m) in height, because of transfers at the base (Fig. 8.23b), a dynamic analysis using site-specific spectrum was used in the seismic design. The dynamic base shear was scaled down to a value corresponding to the static base shear. To preserve the dynamic characteristics of the building, the spectral accelerations were scaled down without altering the story masses. The structural design is by John A. Martin & Associates, Inc., Los Angeles, CA.

Lateral Structural Systems
Figure 8.23. Museum Tower, Los Angeles: (a) building elevation; (b) lateral bracing system; (c) typical floor framing plan. Structural engineers: John A. Martin & Associates, Inc., Los Angeles.

8.1.2.21. Figueroa at Wilshire, Los Angeles

Floor framing plans at various step-backs and notches for the 53-story tower (Fig. 8.24a) are shown in Fig. 8.24d. The structural system, designed by CBM Engineers, Inc., Houston, TX, consists of eight steel supercolumns at the perimeter interconnected in a criss-cross manner to an interior-braced core with moment-connected beams acting as outriggers at each floor (Figs. 8.24b,c). The floor framing is structured such that the main columns participating in the lateral loading system are heavily loaded by gravity loads to compensate for the uplift forces due to overturning. The structural system consists of three major components:

1. Interior concentrically braced core.

2. Outrigger beams spanning approximately 40 ft from the core to the building perimeter. The beams perform three distinct functions. First, they support gravity loads. Second, they act as ductile moment-resisting beams between the core and exterior frame columns. Third, they enhance the overturning resistance of the building by engaging the perimeter columns to the core columns. To reduce the additional floor-to-floor height that might otherwise be required, these beams are notched at the center, and offset into the floor framing, as shown in Figs. 8.24e,f, to allow for mechanical duct work.

3. Exterior supercolumns loaded heavily by gravity loads to counteract the uplift effect of overturning moments.

Tower Overturning Moment
Figure 8.23. (Continued.)

8.1.2.22. California Plaza, Los Angeles

The project consists of a 52-story office tower rising above a base consisting of lobby and retail levels, and six levels of subterranean parking (Fig. 8.25a). A structural steel system consisting of a ductile moment-resisting frame at the perimeter resists the lateral loads. The parking areas outside the tower consist of a cast-in-place concrete system with waffle slab and concrete columns. Figure 8.25b shows a typical mid-rise floor plan for the tower with sizes for typical framing elements. Architects are Arthur Erickson, Inc., and structural design for the building is by John A. Martin & Associates, Inc., Los Angeles.

8.1.2.23. Citicorp Tower, Los Angeles

This 54-story tower rises to a height of 720 ft (219.50 m) above ground level and has a height-to-width ratio of 5.88:1 (Fig. 8.26a). It has two vertical setbacks of approximately 10-ft (3.05-m) depth at the 36th and 46th floors, as shown in the composite floor plan (Fig. 8.26b). As is common to most tall buildings in seismic zone 4, this building was designed for site-specific maximum probable and maximum credible response spectrums, which represent peak accelerations of 0.28 g and 0.35 g, respectively. The corresponding critical damping ratios are 5 and 7.5%. The structural system consists of a steel perimeter tube with WTM24 columns spaced at 10-ft (3.05-m) centers and 36-in. (0.91-m)-deep spandrels. The columns at the setback levels are carried by 48-in. (1.22-m)-deep transfer girders and by vierendeel action of the perimeter frame. Typical floor plans at the setback levels are shown in Figs. 8.26c,d.

The foundation for the tower consists of a 7-ft (2.14-m)-deep mat below a four-story basement. The structural design is by John A. Martin & Associates, Inc., Los Angeles.

Concentric Braces Passing Steel Decks
Figure 8.24. Figueroa at Wilshire, Los Angeles: (a) building elevation; (b) lateral system; (c) section; (d) framing plans; (e) design concepts; (f) reinforcement at beam notches.

8.1.2.24. Taipei Financial Center

The 101-story Taipei Financial Center in Taipei, Taiwan, at 1667 ft (508 m), is the world's record holder for tallness (Fig. 8.27a). The lowest 25 floors of the building taper gradually inward, forming a truncated pyramid. Above is a stack of eight 8-story-high modules with outward sloping wall, creating a "waist" at the 26th floor and setbacks at floors 34, 42, etc. (see Fig. 8.27b). The modules also have double-notched corners. A narrower tower segment and an architectural pinnacle top the eighth module. Because façade slopes and setbacks interrupt vertical continuity of columns, and doubly notched floor plans reduce the efficiency of an exterior moment frame around corners, a perimeter tubular-frame system was not used for this project.

Instead, the lateral bracing consists of a dual system comprised of a braced core interconnected to a planar moment frame at each sloping face, through a system of outriggers and belt trusses. The braced core offers high-shear stiffness with chevron and diagonal braces of I-shaped sections in four planes in each direction. A mix of single-, double-, and triple-story outriggers is distributed every eight to ten floors along the building height (see Fig. 8.27c). Typically on each building face they engage two vertical supercolumns. Below

Outrigger Belt Truss
Figure 8.24. (Continued.)

the 26th floor, additional outriggers engage two more columns on each face with the belt trusses engaging corner columns as well. Steel box-core columns and perimeter supercol-umns are filled with concrete to provide additional stiffness. The size of supercolumns' steel shell at the base is 8 x 10 ft (2.4 x 3.0 m).

The core is designed as a concentric braced frame (CBF). To clear architecturally required openings, some work points for adjacent braces are spread apart, creating eccentric links. But the system is not designed as an eccentric braced frame (EBF). The design is as for a CBF with the braces, not links, controlling the systems strength. Reduced beam section, RBS, also referred to as dogbone connection, is used at locations where the analyses showed plastic rotation demand in excess of 0.005 radian in a 950-year return-period earthquake. For added strength, beam-to-column connections within the braced core system are detailed as moment connections.

Wind engineering studies indicated that accelerations of the building's upper floors would be 30 to 40% higher than desired for this office building. Therefore, to improve the structure's ability to dissipate dynamic energy, a passive damping system consisting

Figure 8.24. (Continued.)

of a 730-ton tuned mass damper (TMD) has been installed near the top of the tower. The TMD consists of a massive steel sphere (Fig. 8.27d) suspended by flexible steel cables.

The design of the TMD is by Motioneering, Inc., a company in Ontario, Canada, that specializes in designing and supplying damping systems for dynamically sensitive structures. The building structural engineers are Evergreen Consulting Engineering, Taipei, and Thornton-Tomasetti Engineers, New York City.

8.1.2.25. World Trade Center Towers, New York

Of the seven buildings of the World Trade Center (WTC) complex of New York City, the WTC towers, known as WTC 1 and WTC 2, were the most visible and recognized tall buildings throughout the world (Fig. 8.28a). Each of the towers encompassed 110 stories above plaza level and seven levels below. WTC 1, the north tower, had a roof height of

Figure 8.25. Cal Plaza, Los Angeles: (a) building elevation; (b) mid-rise floor framing plan. Architects: Arthur Erickson, Inc.; structural engineers: John A. Martin & Associates Inc., Los Angeles.

1368 ft, while WTC 2 stood nearly as tall at 1362 ft. Each building had a square floor plate 207 ft 2 in. (63.14 m) long on each side, with chamfered corners measuring 6 ft 11 in. (2.10 m). A rectangular service core of approximately 137 by 87 ft (41.75 x 26.52 m) was present at the center of each building.

The buildings' signature architectural design feature was the vertical fenestration which featured a series of closely spaced built-up box columns, as shown in Fig. 8.28b. At typical floors, a total of 59 of these columns were present on each of the flat faces of the building, placed at 3 ft 4 in. (1.0 m) on centers. Adjacent perimeter columns were connected at each floor level typically by 52-in. (91.32-m)-deep spandrel plates (see Figs. 8.28d and). In alternate stories, an additional column was present at the center of each of the chamfered building corners. The resulting configuration of closely spaced columns interconnected with deep spandrel plates created a perforated perimeter tube (see Fig. 8.28c).

Twelve grades of steel, having yield strengths varying from 42 to 100 ksi (191 to 455 kN), were used to fabricate the perimeter columns and spandrel plates. In the upper stories of the buildings, plate thickness in the exterior wall was generally 14 in. (6.35 mm), and at the base, column plates as thick as 4 in. (101.6 mm) were used.

The structural system was considered to constitute a tubular system, acting essentially as a cantilevered hollow tube with perforated walls. The side walls acting as stiff webs transfer shear between the windward and leeward walls, thus creating an efficient three-dimensional structure for resisting lateral loads. In the lower seven stories of the

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