29-08-2014, 09:53 AM
MULTI – STOREY BUILDINGS
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Introduction
In developed countries a very large percentage of multi-storeyed buildings are
built with steel where as steel is not so commonly used in construction of multi-storeyed
frames in India even though it is a better material than reinforced concrete. The use of
steel in multi-storey building construction results in many advantages for the builder and
the user. The advantages of using steel frames in the construction of multi-storey
buildings are listed below:
• Steel, by virtue of its high strength to weight ratio enables large spans and light
weight construction.
• Steel structures can have a variety of structural forms like braced frames and
moment resistant frames suitable to meet the specific requirements.
• Steel frames are faster to erect compared with reinforced concrete frames resulting
in economy.
• The elements of framework are usually prefabricated in the factory under effective
quality control thus enabling a better product.
• Subsequent alterations or strengthening of floors are relatively easy in steel frames
compared with concrete frames.
• The steel frame construction is more suitable to withstand lateral loads caused by
wind or earthquake
Structural configurations
The structural components in a typical multi-storey building, consists of a floor
system which transfers the floor loads to a set of plane frames in one or both directions.
The floor system also acts as a diaphragm to transfer lateral loads from wind or
earthquakes. The frames consist of beams and columns and in some cases braces or
even reinforced concrete shear walls. As the height of the building increases beyond ten
stories (tall building), it becomes necessary to reduce the weight of the structure for
both functionality and economy. For example a 5% reduction in the floor and wall weight
can lead to a 50% reduction in the weight at the ground storey. This means that the
columns in the lower storeys will become smaller leading to more availability of space
and further reduction in the foundation design.
Floor systems
Since concrete floors are functionally more suitable, have less vibration and more
abrasion and fire resistance, the usual tendency is to make them act either with profiled
steel decks and/or with steel beams to give a light weight floor system. Similarly
masonry walls may be replaced with glazing and curtains or blinds to reduce the weight.
The different types of floors used in steel-framed buildings are as follows:
a) Concrete slabs supported by open-web joists
b) One-way and two-way reinforced concrete slabs supported on steel beams
c) Concrete slab and steel beam composite floors
d) Profiled decking floors
e) Precast concrete slab floors
Concrete slabs supported with open-web joists
Steel forms or decks are usually attached to the joists by welding and concrete
slabs are poured on top. This is one of the lightest types of concrete floors. For
structures with light loading, this type is economical. A sketch of an open-web joist floor
is shown in Fig.3.1.
One-way and two-way reinforced concrete slabs.
These are much heavier than most of the newer light weight floor systems and
they take more time to construct, thus negating the advantage of speed inherent in steel
construction. This floor system is adopted for heavy loads. One way slabs are used
when the longitudinal span is two or more times the short span. In one-way slabs, the
short span direction is the direction in which loads get transferred from slab to the
beams. Hence the main reinforcing bars are provided along this direction. However,
temperature, shrinkage and distribution steel is provided along the longer direction.
The two-way concrete slab is used when aspect ratio of the slab i.e. longitudinal
span/transverse span is less than 2 and the slab is supported along all four edges. The
main reinforcement runs in both the directions. A typical cross-section of a one-way slab
floor with supporting steel beams is shown in Fig.3.2. Also shown is the case when the
steel beam is encased in concrete for fire protection.
Precast concrete floors
Precast concrete floors offer speedy erection and require only minimal formwork.
Light-weight aggregates are generally used in the concrete, making the elements light
and easy to handle. Typical precast concrete floor slab sections are shown in Fig.4.4. It
is necessary to use cast in place mortar topping of 25 to 50 mm before installing other
floor coverings. Larger capacity cranes are required for this type of construction when
compared with those required for profiled decking. Usually prestressing of the precast
elements is also done