27-08-2014, 02:14 PM
Design of Tall Buildings Preliminary Design and Optimization Seminar Report
Design of Tall Buildings.pdf (Size: 1 MB / Downloads: 64)
Introduction
The design of tall buildings essentially involves a conceptual design, approximate analysis,
preliminary design and optimization, to safely carry gravity and lateral loads. The design criteria are,
strength, serviceability, stability and human comfort. The strength is satisfied by limit stresses, while
serviceability is satisfied by drift limits in the range of H/500 to H/1000. Stability is satisfied by sufficient
factor of safety against buckling and P-Delta effects. The factor of safety is around 1.67 to 1.92. The human
comfort aspects are satisfied by accelerations in the range of 10 to 25 milli-g, where g=acceleration due to
gravity, about 981cms/sec^2. The aim of the structural engineer is to arrive at suitable structural schemes,
to satisfy these criteria, and assess their structural weights in weight/unit area in square feet or square
meters. This initiates structural drawings and specifications to enable construction engineers to proceed
with fabrication and erection operations. The weight of steel in lbs/sqft or in kg/sqm is often a parameter
the architects and construction managers are looking for from the structural engineer. This includes the
weights of floor system, girders, braces and columns. The premium for wind, is optimized to yield drifts in
the range of H/500, where H is the height of the tall building. Herein, some aspects of the design of gravity
system, and the lateral system, are explored. Preliminary design and optimization steps are illustrated with
examples of actual tall buildings designed by CBM Engineers, Houston, Texas, with whom the author has
been associated with during the past 3 decades. Dr.Joseph P.Colaco, its President, has been responsible for
the tallest buildings in Los Angeles, Houston, St. Louis, Dallas, New Orleans, and Washington, D.C, and
with the author in its design staff as a Senior Structural Engineer. Research in the development of
approximate methods of analysis, and preliminary design and optimization, has been conducted at WPI,
with several of the author’s graduate students. These are also illustrated. Software systems to do
approximate analysis of shear-wall frame, framed-tube, out rigger braced tall buildings are illustrated.
Advanced Design courses in reinforced and pre-stressed concrete, as well as structural steel design at WPI,
use these systems. Research herein, was supported by grants from NSF, Bethlehem Steel, and Army
Floor Systems
The floor system carries the gravity loads during and after construction. It should be able to
accommodate the heating, ventilating and air conditioning systems, and have built in fire resistance
properties. These could be classified as two-way systems, one-way systems and beam and slab systems.
Two -way systems include flat plates supported by columns, flat slabs supported by columns with capitals
or drop panels. Large shears and moments will be carried by the latter. Slabs of constant thickness are also
used. Slabs with waffles are also used. Two-way joists are also used. One-way systems include following -
slabs of constant thickness, with spans of 3m to 8m. Closely spaced joists could also be used. Beam and
slab systems use beams spaced of 1m to 4m.Lattice floor joists and girders are useful to have ductwork
inside of them. Floors of small joists are also used, in addition to integral floor slabs which house piping.
The IBM Mutual Benefit Life building, in Kansas City, MO illustrates the one way and two way joist
systems. It also has shear walls for lateral resistance
Concrete Floor Systems
In concrete floor systems, slabs of uniform thickness are often used with spans of 3m to 8m. One
way or two way systems are used. Concrete joists or ribs are used in one way or two way systems, called
pan joists are also used. One Shell Plaza, in Houston, TX uses this. Beam and slab system is used with
beams spaced at 3m to 8m. Beam depths of L/15 to L/20 are used.
Steel Floor Systems
In steel floor systems, we use reinforced concrete slabs on steel beams. Thickness of slabs is in the
range of L/30 to L/15 of the span. Pre-cast concrete slabs are also used with some shear connectors,
grouted. Spans vary from 1.2m to 9m. Concrete slabs on metal decking are often used, with shear
connection. For steel beams, wide flange shapes are used. Welded plate girders, latticed girders, and
vierendeel girders are also used, which house ducts. Castellated beams and stub girders, developed by
Colaco (1970), are also used, which allow mechanical ductwork to be placed between short stubs, welded
on top of these girders. The stub lengths are 1.5m to 2m long. Stub girders are of composite construction.
Vertical Framing Systems
Vertical framing elements are columns, bearing walls, hangers, transfer girders, and suspended
systems such as cable suspended floors. Structural steel, reinforced concrete and composite columns are
used. Bearing walls carry loads in compression, and sometimes, like staggered trusses between floors.
Transfer girders are used to bridge large openings at lower levels of a tall building. Suspended systems use
massive structure at top, with many floors suspended below by using cables. Lower floors are column free.
Federal Reserve Bank in Minneapolis, MN is an example. Mercantile Bank building, Kansas City, has
space truss as a transfer truss to carry loads to 5 columns at the first level.
Framed Tube Systems
Framed tubes are 3-dimensional space frameworks made by connecting intersecting plane frames
at the corners by stiff corner columns. Framed tubes behave like giant flange frames and perpendicular web
frames carrying axial loads and shear. The flange frames are normal to wind, while web frames are parallel
to the wind. The axial forces in the columns in the flange frames are obtained by beam theory. However,
due to flexibility of spandrel girders, and columns, there is a shear lag effect, in the box beam cantilever,
with a hyperbolic type stress distribution in web frames. In the flange frames the column axial stresses are
magnified also in a parabolic type stress distribution. Thus the corner columns may have almost 4 times the
axial stress as in an ideal cantilever tube. Framed tubes have columns fairly closely spaced with variations
from 1m to 3m. This allows stiff spandrel beams to be designed to enable lateral resistance. Shear lag
effects are thus reduced. The overturning resistance of the overall tube is increased. Braced tubes are three
dimensional diagonal braced or trussed system, acting like a giant space frame. The 100 – story John
Hancock Center, designed by Fazlur R.Khan, Hal S.Iyengar, and Joseph P.Colaco, in Chicago, is the best
example of a diagonal trussed tube. Its natural frequency is 0.125 hertz, giving a stiff system at about 30 psf
steel for its structural weight. Shear wall tubes are made up of four shear walls connected at corners. Tube
in tube system is designed by using interior core shear-walls combined with exterior framed tube. One
Shell Plaza in Houston is one such example. Bundled tubes are made with multiple tubes sharing common
interior side frames. Sears Tower in Chicago is an example of nine framed tubes to make a bundled tube,
with belt and outrigger trusses at different levels. This is the tallest in the US, at 110 stories, and was
designed by the same engineers as John Hancock. This has about 33psf steel and a frequency of 0.125
hertz. One Shell Plaza, Houston and Boatmen’s Tower, St. Louis, illustrate framed tubes designed by CBM
and GCE Consultants. The structure weight is about 13 to 14 lbs/sft for a 32 story building, increasing to
about 30 lbs/sft for a 90 story building. Tall Building Monographs (1978) have typical values, in the
Systems and Concepts, Volume I.
Preliminary Design and Optimization
The structural design of a tall building involves conceptual design, approximate analysis,
preliminary design and optimization, followed by detailed and final design. Codes and standards are used
effectively to match limiting stresses, displacements and accelerations. Risk analysis with safety and
reliability, is often included in arriving at suitable factors of safety in sliding and overturning. Tall narrow
buildings develop uplift in the foundations, which should be designed for suitably. The initial selection of a
structural system involves architectural, mechanical and electrical requirements. Different floor systems are
studied, in combination with 3 to 4 lateral systems, with consequent structural schemes, almost 15 of them,
for various combinations between gravity and lateral. Preliminary design and optimization of various
schemes follows, in an iterative fashion by satisfying drift and acceleration limits. Often simple software
systems are used in this stage, such as frame and shear-flexure cantilever beam, and cantilever box beam
models. The first is for moment frames, while the second is for shear wall-frame buildings, and the third for
framed tubes respectively. Methods developed by Fazlur Khan and Sbarounis (1964), Heidebrecht and
Bryan Stafford Smith (1973) and Coull (1974) are used for shear wall-truss frame interaction, while the
latter is used for framed tubes. Goldberg (1975) also has approximate methods of analysis for tall buildings
composed of frames, shear wall-frames and framed tubes. Displacements and member forces are obtained,
and corresponding components designed at different levels. The author and his students have developed
some software systems based on these techniques. They are used to model frames, shear wall-frames,
framed tubes and outrigger braced tall buildings. Herein, a review of these techniques is made. Sequence of
design calculations is examined to assess procedures for preliminary design.
Comparison of Systems Efficiency
Plane frames (Type I), the important variables are span, bay lengths, and member depths. The
spans vary from 6.1m to about 15.2m (20ft to 50ft). Shear trusses (Type II) improve the efficiency of plane
frames considerably. Here, the distance between the chords and the number of trusses are important
parameters. An optimum combination of trusses and frames yield efficient system. In concrete, it is an
optimum combination of shear walls and frames. In framed tubes, the equivalent cantilever behavior of the
tube dominates the efficiency. The overall length and width of the building determines its stiffness. The
equivalent tube moment of inertia depends on column areas and chord distances of these columns from the
centroid of the building. The systems with outrigger and belt trusses are more efficient than ones with shear
truss only. The outrigger trusses increase the system efficiency by 20 to 25%. This is accomplished by
engaging the exterior columns along with the core shear trusses. They develop overturning resistance. In
system 5 and 7, the interior trusses interact with equivalent end cantilever channels. These are often termed
partial tubes (Type III). Band trusses added to these will improve stiffness further. The full framed tube
(Type IV) is more efficient. Bundled tube is used in Sears Tower, Chicago. Diagonal truss tube is used in
the John Hancock Center, Chicago. Diagonal and tapered tubes, even though highly efficient, may have
increased costs, of 15% or so, due to connections for diagonals and the tapered columns. Framed tubes with
only perimeter frames are less efficient, due to shear lag effects. Tapered tubes have less shear lag. The
tapered tube being designed in India, at Jabalpur, at 667m height and 334m width is highly efficient, with a
ratio of only 2. Minoru Yamasaki and Associates are the architects, to be completed in 2008. This will be
the tallest in the world. Fig.2 is an illustration of the various systems
Initial Selection of Structural System
Several different structural schemes are examined, for the initial selection of systems. Knowledge
of behavior each structural system, rapid preliminary design methods, approximate analysis and
optimization techniques are necessary to achieve this balance in design. Often 15 structural schemes are
studied, with various combinations of gravity and lateral systems. Starting with a basic plan size, and
height, each scheme is developed with a candidate structural system. In order to compare systems, different
column spacing, member sizes, truss and other subsystem dimensions such as outriggers, and diagonal truss
system should be carefully examined. Optimization can then be made with one or two story sub assemblies,
at different heights of the building, in 2 to 3 iterative cycles, for given drift. Interpolation, often linear,
could be made from these different level optimizations, for member sizes and moments of inertia, at
intermediate levels. This is then used in an overall stress analysis, using large structural analysis software
systems, such as Strudl, Sap4, Etabs and Drain2D. This will enable rapid final design and detailing.
Otherwise, the initial sizes may not be very efficient and convergence to drift and acceleration limits will
take many more iterative cycles. The optimum design of a tall building is an art and science, with the
accumulated years of experience by the structural engineers, with techniques of stress analysis, structural
design and detailing, put to judicious use at the right time and place. The detailed steps are as follows
(Iyengar, 1972, Colaco, 1975).
Summary and Conclusions
The design issues for preliminary design and optimization have been briefly summarized, and a rational
methodology of design was shown. This enables optimization of initial structural systems for drift and
stresses, based on gravity and lateral loads. Some insight into the design of many types of tall building
structural systems and their subsystems was provided based on past experience in tall building design.
The design issues are efficiency of systems, stiffness, member depths, balance between sizes of beam and
column, bracings, as well as spacing of columns, and girders, and areas and inertias of members. Drift and
accelerations should be kept within limits. Good preliminary design and optimization leads to better
fabrication and erection costs, and better construction. The cost of systems depends on their structure
weight. This depends on efficient initial design. Efficient structural design also leads to a better foundation
design, even in difficult soil conditions. The structural steel weight is shown to be an important parameter
for the architects, construction engineers and for fabrication and assembly. Optimization fine tunes this