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 BasementExternal 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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