Download High-Rise Buildings

HIGH-RISE BUILDINGS
Introduction
The height of a building has always contributed to the construction’s status. While
India may be far behind when it comes to elevated constructions, in comparison
with many other cities in the world, the country is making a slow but steady ascent
with a number of towering structures that are being built and planned for several
cities.
Status, aesthetic value addition and better view are not the defining factors in the
race to build taller and taller structures. Densification is one of the most important
elements. Vertical expansion is the only option if cities are to grow and flourish. Tall
structures help accommodate more people in the limited resources of land. With
the growing number of immigrant population in the city, if horizontal construction
continues, the city will in no time run out of options for further development.
Tall buildings use lesser ground area and so more land is available for better
infrastructure development such as parking, gardens and other important facilities.
A flat on a higher floor offers residents a significant reduction in the level of smog
and noise. It also provides more light, better ventilation and greater relief from the
heat. In short, better quality of life.
While overall Floor Space Index (FSI) remains low in the country, especially in big
cities, it is also difficult to obtain large-sized plot within cities for construction of
high rise buildings. These are the main reasons for dearth of skyscrapers in India.
There is also the issue of the strain on over-stressed infrastructure. It is argued that
while every new iconic project is important for a city, it alone cannot succeed. It
must have corresponding, complimenting, adequate infrastructure. If not, it would
be a burden on existing resources. Questions arise, “How will the infrastructure
cope with the sudden burden of a large number of people of a tower? What of
their water and drainage needs? What about the hundreds of cars and their
parking?”
Rules for high rise buildings are stringent. Builders must follow norms for setbacks,
elevators, in house fire safety equipment, service ducts, electricity supply, exit
routes, a published fire safety plan and more.
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One of the key factors that decides whether compliance is required is the height of
the building itself; - the height decides whether a building is a high-rise or not.
The two most questioned aspects of high-rise buildings are structural stability and
security. Tall building structures require specialized skill to combat structural,
safety and design challenges. In order to make the high-rise building withstand
natural calamities, the design has to incorporate mechanisms to resist wind force of
a very high intensity. The second important issue is safety in case of fire. For this,
high rise buildings are designed with free spaces for rescue and fire resistant
materials.
Construction
The designer should take basic concepts and methods of construction into account
during the design stage, when he lays out and dimensions the building and
responds to the economy of construction.
The typical construction methods are conventional, industrialized and special
construction techniques. The selection of a construction system includes life cycle
costs, which are to say, not just the initial construction costs, but also the future
maintenance and operating costs or capital investment and maintenance
replacement costs.
Excavator
For earthwork in excavation, excavators are generally used.
A typical modern excavator
An excavator, commonly known as a digger or JCB (the latter actually a proprietary
name, but commonly used generically), is an item of heavy construction equipment
consisting of a boom, bucket and cab on a rotating platform (known as the
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"house"). The house sits atop an undercarriage with tracks or wheels. All
movement and functions of the excavator are accomplished through the use of
hydraulic fluid, be it with rams or motors. Their design is a natural progression from
the steam shovel.
Foundation Systems
Ordinary high- rise building foundations consists of either a collection of individual
rectangular and strip footings, or large mat combining all of the single footings. In
seismic areas, the individual spread and pile foundations will have to be linked and
tied together by bracing struts, so that entire building foundation can act as a unit
in sharing the load resistance. In crowded urban areas, the deep basements for tall
buildings, particularly adjacent to other heavy buildings, cause excavation
problems. In this case, special foundations, such as sheet piling, slurry walls,
bracing of walls, or walls with tie backs, along with underpinning of adjacent
buildings and subways, are required. Pumps may be needed for conditions where
the water table is high, which (in turn) may cause settlement problems with
adjoining buildings.
In order to control settlements and tilting, foundations on compressible soils
should only be concentrically loaded, in other words, the column and wall bases
should not be fixed to the footings, and lateral shear forces should not be
transferred in bending. On the other hand, footings on highly compacted soils may
be loaded eccentrically. Single footings should only be used on soils of low
compressibility, because the independent displacements of the foundations may
cause significant stresses in the superstructure. Columns may be joined by
continuous footings to control vertical differential movements between them.
Mat foundations are most effective in reducing differential movements on
compressible soils.
Mat or Raft Foundation
The mat foundation is basically one large continuous footing upon which the
building rests. In this case, the total gross bearing pressure at the mat-soil interface
cannot exceed the allowable bearing strength of the soil. The system is used when
the soil bearing capacity is low, and it may prove to be more economical when
more than about one-half of the plan area of a building is required for single
footings; it also provides a uniform excavation depth. Mat foundation may be
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useful when individual footings touch, when a concentration of high soil pressure
must be distributed over the entire building area, when small soft soil areas must
be bridged, when compressible strata are located at a shallow depth, so that
settlement will be minimized, when differential settlement of variable soils must be
minimized, since individual footings would create unequal settlement, or when
hydrostatic pressure of groundwater and uplift must be controlled.
The common mat or raft foundations (see fig.)
 A layout Flat plate/slab mats
 Ribbed mat (two way slab on beams) with ribs above or below slab
 Cellular
The mat foundation is designed as an inverted floor structure, but settlements
must also be taken into account where the loading or soil pressure distribution
depends on the layout of the columns or walls, the magnitude and type of the
loads, and on the stiffness of foundation and soil.
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For preliminary design purposes, an average or uniform soil pressure distribution
may be assumed, and the effect of differential settlements may be ignored,
because of the rigidity of the mat. In other words, the contact pressure is
distributed in a straight line, where the centroid of the soil pressure coincides with
the line of action of the resultant vertical loads. This condition actually only applies
for a rigid mat, such as a cellular one on cohesive soil, or a uniformly loaded flexible
mat on an elastic compressible soil.
Floating or Compensated Mat Foundation
When tall buildings are founded on weak soils of very thick deposits of soft
materials, such as compressible clay, the bearing capacity of the soil is extremely
low and controlled by settlement criteria. To control this movement due to heave,
as well as compressive forces and long- term consolidation of the soil stratum, the
fully compensated mat foundation is often used. In this case, so much soil is
excavated that the weight of the soil removed plus any uplift from water pressure
is replaced by the combined gross loading of the substructure and the
superstructure. In other words, the pressure at the base of the excavated soil will
not change, the pressure of the displaced soil will be equal to the pressure caused
by the building, thus theoretically resulting in no settlement. The structure seems
to float on soil like a ship in water, as caused by a buoyant force equal to the
weight of the soil displaced by the basement volume, balancing the weight of the
floating structure according to the Archimedes principle.
Naturally, this is only a theoretical model, since some settlement of the mat will
occur due to the change of live loads and the water table, non-homogeneity of the
ground, and due to recompression of excavation heave with subsequent
settlement, realizing that, as a result of the removal of the soil overburden,
pressure heave of the bottom of the excavation has occurred.
For tall buildings, this floating foundation concept may require several basements,
which may not take just the form of a box or pedestal, but rather a stepped root
like extension into the ground. These deep substructures, in turn, generate
extremely large loads, clearly indicating that the building should be a lightweight
structure. Hence, it may be more economical to use a partially compensated mat
foundation rather than a fully compensated one, when some settlement due to the
net bearing pressure of building weight minus the weight of the soil excavated at
the mat-soil interface is tolerable.
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Because the floating mat is constructed deep into the ground, groundwater may
have to be considered, especially the high water table during rainy season, thus
requiring a watertight box-type foundation. In this instance, the buoyancy effect
and the lateral pressure must be considered in the design.
Deep Foundations
Deep foundations are used when adequate soil capacity is not available close to the
surface and loads must be transferred to firm layers substantially below the ground
surface. When settlement is a primary problem, then a pile length must be selected
to minimize differential settlement.
The common deep foundation systems for buildings are piles and piers (caisson
piles). While the small-diameter slender piles are normally driven into the ground,
the large diameter piers are placed by first excavating a hole; this distinction,
however, may not always be that clear. Other deep foundation systems
occasionally used are slurry walls (i.e., a method of construction for earth retaining
walls) and caisson foundations, which are generally used for the construction of
bridge piers and abutments. A caisson is a massive, cellular hollow box structure
that is sunk into position, and also provides the bracing for the excavation. The
three major types are the box caisson or floating caisson (open at top and closed at
bottom), the open caisson (open at top and bottom), and the pneumatic caisson
(closed at top, open at bottom, and filled with compressed air to prevent water
from entering the working chamber) as may be used for constructing an
underground garage.
In the following paragraphs, the most common deep foundations for buildings are
briefly discussed.
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Pile Foundations
Piles are usually driven by hammers. They are made of treated timber, steel, cast-in
place concrete, precast, pre-stressed concrete or composite material; they are
manufactured in various shapes of solid or hollow configuration. The bearing
capacity of a pile depends on the strength of the pile and the supporting strength
of the soil. The estimation of the bearing capacity of piles is quite complex; it is
determined by static analysis, dynamic analysis, or pile load tests. In static analysis,
which is often used for preliminary design purposes, the pile-bearing capacity
depends on the soil-to-pile connection (i.e., soil properties and pile geometry), i.e.,
the sum of end bearing resistance and the screen friction, as well as on group
effect. For long slender piles, the pile-tip resistance becomes insignificant, so that
they act mainly as friction piles, although in clayey soils the resistance is primarily
provided by adhesion. In dynamic analysis, so-called pile formulas have been
developed where the pile capacity is directly related to the resistance offered to
driving with a hammer. The estimation of the pile length is not always easy. When
point bearing-piles are supported directly on rock-like material, the pile length is
known for this condition, the pile is treated as a short column braced by the soil,
hence assuming it is not surrounded by soft mud; its size is dependent on the load
bearing capacity of the base material and on the strength of the pile. Occasionally,
it may be the case that the bedrock is stronger than the concrete. For the opposite
condition, where there is no firm soil available at a reasonable depth, friction piles
must be used. In this case, the length of the piles depends on the piles size and the
skin friction along the pile, which is derived, in turn, primarily from the shear
strength of the soil or the adhesion on the pile face. Often, piles pass through a soft
soil layer where they are supported by skin friction but then must be extended
several feet into firm soil to act as end-bearing friction piles. The evaluation of skin
friction through layered soil systems, and hence the determination of the pile
length, is extremely difficult. Where settlement is a serious problem, the pile must
be long enough to withstand differential settlement. Short piles may be driven in
granular soils to compact the soil close to the ground surface; they are called
compaction piles.
Piles are generally used in groups, such as at least three piles to support a major
column. A concrete cap is always necessary to distribute the loads from the super
structure to the piles. Pile-cap footings are designed like spread footings but for
concentrated pile loads. Pile clustered may be of any arrangement below column,
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wall, or combine footings; usually, the location of the resultant pile load coincides
with the resultant applied load. The pile should be spaced far enough from each
other, so that the load-bearing capacity of each individual pile is not reduced,
otherwise group behavior must be taken into account, which not only results in less
soil resistance, but also larger settlements.
Where pile group are subject to lateral forces, and the lateral resistance of the
vertical piles in bending (as partially supported cantilevers) is exceeded, then
inclined or batter piles must be employed. When in addition, an overturning
moment is applied, then some of the piles may have to act in tension to resist
uplift.
Pier Foundations
Piers or drilled piers are large-diameter piles with slenderness of less than 10,
placed vertically. They are sometime called caissons, but should not be confused
with caissons sunk into position. For the construction of piers or caisson piles, a
hole is usually drilled with machines, or circular steel shells are driven into the
ground and the soil inside is excavated. Piers can be of any cross section. They are
large enough so that a single pier can replace a group of piles. Piers can penetrate
dense soil, which piles may not. Piers may be belled or straight shafted, they usually
are supported by end bearing, and occasionally may be supported in addition, by
skin friction. They are classified according to material as concrete, concrete in steel
pile and concrete plus steel core piers.
The size of a typical concrete pier is determined from the soil capacity in bearing
and sometimes in skin friction, while the pier itself is designed as a compression
member. It is assumed that it does not have to resist lateral forces, which are
usually absorbed by shear resistance at the building base and passive earth
pressure on basement walls. Occasionally, piers must resist uplift forces, as may be
the case in core columns and corner columns of trussed tubes. Here belled piers act
in tension, or piers are post-tensioned and anchored in bedrock.
Choosing a Foundation System
The selection of a foundation system depends on the type of superstructure, that
is, the location, magnitude, and kind of forces to be transmitted to the ground, and
also on the subsurface conditions, the bearing capacity and settlement
characteristics of the soil, as well as the groundwater conditions. Usually, there are
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several possible economical solutions for a given situation. For example, when
forces are concentrated locally, such as for a lateral-force resisting central core, it
may be more economical to use piles or piers directly under the core rather than a
thick mat to redistribute the loads over the footprint of the building. For complex
subsurface conditions, such as non-uniform soils across the building site or other
underground interferences, it may be necessary to adjust the layout of the
superstructure.
In this context, only larger, heavy buildings are considered, where load resistance
is critical. The selection of foundations for small buildings is usually based on local
practice. These buildings are light and cause hardly any bearing pressure, so that
the design of their foundations depends less on loads and more often on resistance
to movements in the soil.
A building can be founded on soil or, directly or indirectly, on rock. While New York
City sits directly on rock, the tall buildings in Chicago are carried by caisson piles
roughly 30 m down to bedrock. Naturally, the ideal situation exists when the
bedrock is located near the surface and seismic action is nonexistent, so that
shallow foundations can be supported directly and settlements are usually not a
problem. However, when this bedrock is at great depth, then it depends on the
nature of the overlying soil and the magnitude of the loads to determine whether
the building should be founded directly on soil or indirectly on rock. When buildings
are founded on soil rather than rock, the selection of the foundation type depends
-on the bearing capacity and settlement characteristics, as well as the necessary
compatibility with the superstructure. For a thick, firm stratum, individual shallow
foundations may be satisfactory, or mat foundations (for tall buildings). However,
for the other extreme, where there is only a thick stratum of weak, soft soil
present, such as in Houston, tall buildings cannot be founded just on mat
foundations because of excessive settlement problems, unless it is placed in a deep
excavation to behave like a floating mat. Softer soils may have to be stabilized by
friction piles, which carry the mat, as is the case in New Orleans. For the condition
where weak soil is overlying firm soil at a reasonable depth, piers or end-bearing
piles may be used, although it may be more economical to use spread footings and
stay closer to the surface, than to go deeper and take advantage of higher bearing
values, assuming differential settlement is tolerable. On the other hand, when firm
soil is over a soft stratum, mat foundations may be required for heavy loads,
possibly together with piles, to control settlements, while individual shallow
footings may be satisfactory for light flexible buildings. Hybrid foundations are
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required, for example, when the bedrock underlying a site is sharply sloped and has
an overlay of loose sand and a wedge of clay. For this situation, a foundation mat
may be used beneath a portion of the building, while the remainder is founded on
caisson piles drilled into the bedrock.
Particular attention must be given to building foundations on sloping ground. For a
firm soil and a stable slope, step footings transverse to the slope or trenched
footings parallel to the slope with transverse grade beams may be used. For sloping
soft ground or semistable slopes, slope stability must obviously be considered. In
this case, the building may be supported by piles (poles) anchored into a firm layer
and cantiIevered above the ground. There are several methods for the prevention
of a potential landslide, which include: soil stabilization (e.g., chemical grouting,
compaction, reinforced earth), retaining structures, tiebacks, surface and
subsurface drainage, and flattening the slope, possibly with a series of terraces.
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Water Proofing Treatment for Basement
Sometimes proper care is not taken for construction of the basement, and this
leads to tremendous problems later on. If care is taken initially, a lot of problems
can be avoided.
Entry of outside water, rise in water level, seepage from side structure including
leakage in pipe and sewer line etc. are some of the problems that one may face. If
proper care is not taken for such problems, they may endanger the structure
including basement and floors above.
The following method can be applied for water proofing of basement during
construction.
Basement where adequate space is available for excavation
The excavation of ground for basement is carried out in such a manner that
working space of at least 60 cm is available around external walls. If water level is
high, the excavated area should be kept dry by continuous pumping.
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The following operations are executed in water proofing of basement.
1) On dry and leveled ground 75 to 100 mm thick lean concrete is laid to serve as a
leveling
course
for
water
proofing.
2) The bitumen based primer is applied with brush on entire area at 0.24 to 0.30
liter
per
sq.
meter
after
cleaning
the
surface.
3) Bitumen compound polyester membrane is fixed in hot bitumen of grade 85/25
at the rate of 1.5 kg. per square meter. This layer should be protected by
construction
of
flat
brick
flooring.
4) After construction of structural slab and walls, water proofing treatment on
external
face
which
is
in
contact
with
earth is
done.
5) After
the
surface is
cleaned,
bitumen
primer
is
applied.
6) Self adhesive S B S rubber bitumen membrane with high density polyethylene
film
is
applied
on
vertical
walls.
7) Before back filling the soil an outer protective wall leaving a space about 100
mm around should be made and the space should be grouted subsequently.
Injection Grouting
A. Vertical Surfaces
Water proofing treatment is provided and laid by chemical injection grout process
in basement using 20 mm dia MS nozzles of minimum 25mm deep in walls placed
and fixed @1.2m distance in both direction in the wall and @ 0.75m c/c along
construction joints, consisting of injecting cement slurries of different viscosities
under pressure by pump using acrylic based water proofing chemical mixed with
neat cement slurry and sealing off nozzles after the injection operation with
suitable admixture including providing and applying two coats of acrylic based
water proofing chemical mixed with neat cement slurry as per manufacturer’s
specifications and providing 12-15mm thick neat finished cement plaster 1:3
(1 cement : 3 coarse sand) added with acrylic based water proofing chemical as per
manufacturers specification.
B. Horizontal Surfaces
Providing and laying water proofing treatment by chemical injection grout process
on basement, base of underground water tank or other area as per specifications
comprising of two layers of Cement mortar 1:3 (1 cement : 3 coarse sand) mixed
with Acrylic water proofing compound, providing and fixing 20 mm pipe sleeves at
1.2m C/C and grouting acrylic based water proofing chemical mixed with neat
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cement slurry through the pipes with complete with cutting & sealing of the pipes
as per specification and direction.
Box Method Waterproofing to Basement
External Tanking i.e the waterproofing specification is to be laid underneath the
raft on the lean concrete and on the exterior surface of the walls. This is carried out
in two operations. In operation number one, the waterproofing specification is
laid on the base and for this a brick wall of 230mm thickness is constructed at the
perimeter with inside face plastered with cement/sand mortar 1:5( 1 part cement :
5 parts coarse sand) of half inch thickness, the height of the brick wall be the same
as that of RCC slab. The waterproofing specification is taken up the brick wall. In
operation number two, after the RCC slab has been laid and walls casted, with an
overleap from the waterproofing specification laid on the brick walls, the
specification is taken up on the exterior surface of the RCC walls upto a height of
300mm above the finished ground level.
However in case there are individual footings, columns coming inside the
basement, the waterproofing specification has to go underneath the columns and
brought up and joined with the waterproofing specification laid on the lean
concrete laid on the base of basement.
Specification:
(A)
For Base:
(a) A layer of cement slurry mixed with integral water proofing compound
added by the quantity recommended by the manufacturer and
conforming to IS:2645 or equivalent laid over the lean concrete bed.
(b) A layer of cement sand mortar 1:3 of about 25mm thickness (1 part
cement & 3 parts of coarse sand) mixed with integral water proofing
compound conforming to IS: 2645 or equivalent and added by the
quantity recommended by the manufacturer and laid over the slurry laid
bed as above.
(c) A layer of rough red sand stone of about 25mm to 35mm thickness laid in
the above mortar and the joints of stone sealed with 1:2 cement sand
mortar ( 1 part cement & 2 part coarse sand ) mixed with integral water
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proofing compound conforming to IS: 2645 or equivalent and added by
the quantity recommended by the manufacturer .
(d) A layer of cement/sand mortar 1:4 of 20-25mm thickness
( 1 part cement :4 parts coarse sand ) mixed with integral water proofing
compound conforming to IS: 2645 or equivalent and added by the
quantity recommended by the manufacturer with stone grit of about
15mm size sprinkled over it.
(B) FOR WALLS :
(a) A layer of rough red sand stone of about 25mm to 35mm thickness laid
with a gap of about 12mm between the stone and walls and joint of stone
sealed with neat cement mixed with integral water proofing compound
conforming to IS: 2645 or equivalent and added by the quantity
recommended by the manufacturer.
(b) A layer of cement sand mortar 1:4 ( 1 part cement & 4 part coarse sand )
mixed with integral waterproofing compound conforming to IS: 2645 or
equivalent and added by the quantity recommended by the manufacturer
splashed on the exterior surfaces of stone.
(c) A layer of cement slurry mixed with integral water proofing compound
conforming to IS: 2645 or equivalent and added by the quantity
recommended by the manufacturer filled in the gap between the stone
and wall.
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