In the previous sections, we discussed tubular systems generally applicable to prismatic profiles including a variety of nonrectilinear plans, such as circular, hexagonal, triangular, and other polygo-nal shapes. However, for buildings with significant vertical offsets, the discontinuity in the tubular forms introduces serious inefficiencies. A bundled tube that can be configured with multiple cells, on the other hand, provides for vertical offsets without much loss in efficiency. Additionally, it allows for wider column spacings that would be possible with a single cell tube. In principle, many building shapes can be configured using bundled tubes (Figure 1.75).
1.13.1 Behavior
The structural principle behind the bundled tube concept is that the interior rows of columns and spandrels act as interior webs minimizing shear lag effects. Without their beneficial effect, as stated earlier, the exterior columns in a framed tube toward the center of the building would be of little use in resisting the overturning moment. The bundled tube system may therefore be thought of as an extension of the perimeter tubular system with stiffened interior frames.
Cell 1
X-bracing
Moment-connected spandrels
Cell 3 Cell 2
Cell 1
Cell 2 Semicircular
tube
Rectangular tube Semicircular
tube
FIGURE 1.75 Bundled tubes.
Lateral Load Resisting Systems for Steel Buildings 75
The individual cells of a bundled tube may be curtailed at different heights without diminishing structural efficiency. The torsional loads are readily resisted by the closed form of individual cells, permitting greater spacing of columns, and shallower spandrels. Larger window openings are pos-sible. The shear lag experienced by conventional framed tubes is reduced by the addition of interior framed “web” panels across the entire width of the building. When subjected to lateral forces the presence of interior web reduces the nonuniformity of column axial forces caused by shear lag. This effect is shown schematically in Figure 1.76.
1.14 ULTIMATE HIGH-EFFICIENCY SYSTEMS FOR ULTRA TALL BUILDINGS The concept behind high-efficiency systems, first envisioned by master builder, the late Dr. Fazlur Khan of Skidmore, Owings & Merrill, is based on three requisites:
1. Transfer as much, if not all of the gravity load into those columns that resist over-turning moments caused by lateral loads.
2. Position these columns as far back as possible, from the geometric center of the building.
3. Interconnect these columns with a structural system capable of resisting shear forces due to lateral loads.
The first principle is not entirely new. The idea of transferring additional gravity loads into columns
Compression
Tension
Compression
Tension
FIGURE 1.76 Bundled tube behavior.
76 Structural Analysis and Design of Tall Buildings: Steel and Composite Construction This is so particularly when the width of the lateral bracing system is narrow compared to the build-ing height. Coaxbuild-ing additional loads into these columns helps in anchorbuild-ing the buildbuild-ing firmly at the base.
The reason for the second requirement is to maximize the bending rigidity of the building. By placing the columns at the periphery of the building, we are in effect, maximizing the bending rigidity of the building.
The need for interconnecting the columns with a shear-resisting system is to coerce them to func-tion as tension-and-compression-resisting columns of a giant brace, and not so much as individual bending members. Without the presence of an interconnecting shear-resisting system, the columns would function as individual bending members with the resistance merely equal to the sum of the independent resistances of the individual columns.
To explain the concept further, let us consider a tall building, square in plan, some 120-stories tall, with closely spaced columns and deep spandrels at the perimeter. The system commonly referred as tube system, has been the workhorse of tall building construction for the past 30 years.
Referring to our tall building shown in Figure 1.77, one obvious option for channeling all gravity loads into the perimeter columns would be to eliminate interior columns all together. This would leave a column-free volume inside of the building but would be prohibitively expensive to clear-span
5 at 25 ft = 125 ft
5 at 25 ft
= 125 ft
Four corner columns Zone 1 Zone 2 Zone 3 Zone 4
Transfer truss
FIGURE 1.77 Structural concept for ultra-high-rise building.
Lateral Load Resisting Systems for Steel Buildings 77
the entire floor area at each level. Therefore, we need to figure a way of transferring the entire grav-ity load into the perimeter columns without incurring a significant premium in the framing the floor system. This is considered next.
An obvious solution for achieving this goal is to provide transfer-floors recurring at selected lev-els, say, for example, at every 12th-floor for the 120-story building considered here. At these levlev-els, a system of one or two-story-deep Vierendeel trusses are used to clear span the entire area within the perimeter columns. Where appropriate, the transfer levels would be used as sky lobbies or other common usage areas.
The transfer trusses are then used to support interior columns within certain designated zones between the transfer levels. In our building, with transfer levels at every-12th-floor, we would be picking up interior columns that are 11-story tall—a relatively economical task given the presence of one story-deep, two-way Vierendeel trusses. One advantage of the system is that the interior col-umns do not have to line-up for the entire height of the building—within any given zone between two consecutive transfer levels, the columns may be placed to suit the interior space planning desired for specific occupancies. Yet another benefit is it lends itself to opening up certain floors without any interior columns. This is possible because floors above the no-column floors may be hung from the Vierendeel trusses above, while those below the no-column floor are supported on the transfer trusses below. The obvious advantage is that we are able to achieve channeling of the entire gravity load to the four corner columns without paying a significant penalty in steel tonnage.
The third requisites, as stated previously, is to mobilize the corner columns as chords of a