Abstract:
The skyscraper as a concept is a product of the industrialized age,
made possible by cheap energy and raw materials. The amount of steel, concrete and
glass needed to construct a skyscraper is vast, and these materials represent a
great deal of embodied energy. Tall skyscrapers are very heavy, which means that
they must be built on a sturdier foundation than would be required for shorter,
lighter buildings. Building materials must also be lifted to the top of a skyscraper
during construction, requiring more energy than would be necessary at lower heights.
Furthermore, a skyscraper consumes a lot of electricity because potable
and non-potable water must be pumped to the highest occupied floors, skyscrapers
are usually designed to be mechanically
ventilated, elevators are generally used instead of stairs, and natural lighting
cannot be utilized in rooms far from the windows and the windowless spaces such
as elevators, bathrooms and stairwells.
Despite these costs, the size of skyscrapers allows for high-density
work and living spaces, reducing the amount of land given over to human development.
Mass transit and commercial transport are
economically and environmentally more efficient when serving high-density
development than suburban or rural development.
Also, the total energy
expended
towards waste disposal and climate control is relatively lower for a given number of people occupying a skyscraper than that same
number of people occupying modern housing.
CONCRETE FOUNDATIONS IN SKYSCRAPERS
Due to the great
height of skyscrapers, huge foundations are needed to
support these structures. First, a large
hole is dug into the ground to reach a point of stable soil (often bedrock). After some stability is reached, large steal footings
are placed, and from these, vertical steal beams are placed along with a network
of rebar. This is an image in actual proportion;
the foundation of the CN Tower is 50 ft (15m) deep, to support the 1,815 ft (553m)
structure. The foundation itself is almost
five floors deep! Most house foundations
are only one to two feet deep. Compare that to a skyscraper! But this is nothing
compared to the worlds deepest foundation! The Petronas
Towers has a foundation that is 394 ft (120 m) deep!
SKYSCRAPERS DESIGN AND CONSTRUCTION:
The design and construction of skyscrapers involves creating safe,
habitable spaces in very tall buildings. The buildings must support their weight,
resist wind and earthquakes, and protect occupants from fire. Yet they must also
be conveniently accessible, even on the upper floors, and provide utilities and
a comfortable climate for the occupants. The problems posed in skyscraper design
are considered among the most complex encountered given the balances required between
economics, engineering, and construction management.
BASIC DESIGN CONSIDERATIONS:
Good structural design is of importance in most building design, but
especially among skyscrapers since even a small likelihood of catastrophic failure
is
unacceptable given the number of individuals served by skyscrapers
and the resulting price of failure. This presents a paradox to civil engineers:
the only way to assure a lack of failure is to test for all modes of failure, in
both the laboratory and the real world. The only way to know of all modes of failure
is to learn from previous failures. In this way, no engineer can be absolutely sure
that a given structure will resist all loadings that could cause failure, but can
only be sure, that given large enough margins of safety, that a sufficiently small
percentage of the time will a failure ever occur. When buildings do fail, engineers
question if the failure was due to some lack of foresight on their part or some
unknowable factor that would have never been expected to have been designed for.
LOADING AND VIBRATION :
The load a skyscraper experiences is largely from the force of the
building material itself. In most building designs, the weight of the structure
is much larger than the weight of the material that it will support beyond its own
weight. In technical terms, the dead load, the load of the structure, is larger
than the live load, the weight of things in the structure (people, furniture, vehicles,
etc). As such, the amount of structural material required within the lower levels
of a skyscraper will be much larger than the material required within higher levels.
This is not always visually apparent, or borne out visually.
The wind loading on a skyscraper is also considerable. In fact, the
lateral wind load imposed on super-tall structures is generally the governing factor
in the structural design. Wind pressure increases with height, so for very
tall buildings, the loads associated with wind are larger than dead or live loads.
STEEL FRAME:
When one thinks of a skyscraper, the steel frame design comes to mind.
This design is characterized by a large steel
box, containing smaller steel boxes inside. This 3D grid is simple and efficient
for most low-rises, but has its’ drawbacks for high-rise structures. As the building's height increases, the space between
steel beams must decrease to compensate for the extra weight, resulting in less
office space and the need for more material.
Tube Frame:
The tube design is a recent innovation used to maximize floor space
and increase resistance to lateral force in any direction. The buildings skin (outside) consists of closely
aligned supporting columns. This design only
leaves about one- half of the building’s
exterior left for windows. Depending on the
designer’s outlook, this can be an advantage or disadvantage. The decreased window space helps those who suffer
acrophobia (a fear of heights) comfortably occupy the space; however, it decreases
the visibility and openness offered by other designs.
Concrete Core:
This is the most common design for modern skyscrapers as it is fast
to build and provides a strong center. All
the utilities, elevators, and stairwells are centralized in this design, making
it easier for building modifications and repair. This design can be dangerous. If a part of the core is damaged, everything above
that section will be cut off from ground access. This happened in the WTC during the September 11,
2001 terrorist attacks, making it impossible for many people to escape the burning
towers
DESIGN:
Designing a low-rise building involves creating a structure that will
support its own weight (called the dead load) and the weight of the people and furniture
that it will contain (the live load). For a skyscraper, the sideways force of wind
affects the structure more than the weight of the building and its contents. The
designer must ensure that the building will not be toppled by a strong wind, and
also that it will not sway enough to cause the occupants physical or emotional discomfort.
Each skyscraper design is unique. Major structural elements that may
be used alone or in combination include a steel skeleton hidden behind non-load-
bearing curtain walls, a reinforced concrete skeleton that is in-filled with cladding
panels to form the exterior walls, a central concrete core (open column) large
enough to contain elevator shafts and other mechanical components, and an array
of support columns around the perimeter of the building that are connected by horizontal
beams to one another and to the core.
Download :
Download :