Design of a 30 Storey Office Building With Reinforced Concrete Design Using Etabs Structural Software (Autosaved)

DESIGN OF A 30 STOREY OFFICE BUILDING WITH REINFORCED CONCRETE DESIGN USING STAAD.PRO STRUCTURAL SOFTWARE DESIGN NAME: AYODELE DINA 070402014

CEG 886: DESIGN OF TALL BUILDING.

DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING

COURSE LECTURER: PROF G L

MARCH 2014

OYEKAN

ABSTRACT. Tall structures have always fascinated mankind. There were buildings stretching well over 100 meters into the sky even before industrialization. But how has the history of the tallest buildings in the world developed? Which structures have played a role in this history? This report considers the tall and multistory structures with a case of study of a proposed 30 storey building to be located at Lekki Peninsular area of Lagos, Nigeria. The building comprises of 30 floors with a lift shaft wall acting as a shear wall in combination of the framed concrete system in resisting lateral forces due to wind. Consideration for seismicity was not considered in this design as Nigeria does not fall in earthquake region. The Proposed building was modeled, analyzed and designed using the Staad.Pro Structural Software, a state of art structural software which is produced by Bentley a leading American company in structural solutions, divers codes of design was factored in to the software of which the case study was designed according to BS8110- 1997 ( Structural use of Concrete). The structure which comprises of Shear Wall( Lift System), Beams , Column pile cap and foundations were designed and the report obtained were included in this report. To reduce the amount of processed data in the design using this software the slab loads were added to the beam using the Yield Line Method of analysis of Slab and then the slab reinforcement were analyzed using RCC spread sheet. More so hand calculation was made for selected element in the building and were compared with that designed in the software. Slabs, two interior beams was designed, a critical column and a pile cap were hand checked and the values are thus: For the slabs designed by hand calculation reinforcement provided for all slabs is Y12@300mm c/c. Due to the amount of data generated and the memory capacity, the yield line method of analysis was used in the application of slab loads in the Staad.Pro Software. The design of 2 critical beams was also designed by hand calculation C (1-7), 3 (A –F) method of analysis used was the Hardy Cross method of Analysis. There were discrepancies in the analysis by hand calculation as against the analysis from the commercial software.

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The critical reaction from the software 12,007KN as compared to that obtained for the ground columns by hand 22,745KN. The discrepancy due to the optimization scheme of the software and the frame/wall action of the software. The core wall also was designed by hand and also with the Staad.Pro software and wind analysis was also designed by hand. The lateral loading of the building was resisted by the combined interaction of the shear wall and the frame system. The pile used in the design of this proposed building is a 1000mm diameter bored pile having a pile capacity of 3000KN

TABLE OF CONTENT 2

LIST OF FIGURES ABSTACT 1.0 1.1

INTRODUCTION TALL BUIDINGS AND EVOLUTION.

1.2

A QUICK OVERVIEW OF THE TALLEST BUIDING BETWEEN 1901 – TILL PRESENT

1.3

EVOLUTION OF SKYLINE EXAMPLES.

2.0

1.3.1

THE NEW YORK SKYLINE

1.3.2

DUBAI SKYLINE.

STRUCTURAL SYSTEM. 2.1

DESIGN PARAMETERS

2.1.1

BUILDING DEFLECTION

2.2

2.1.2

ACCELERATION

2.1.3

SECOND ORDER EFFECTS

2.1.4

UPLIFT

2.1.5

BUILDING DIAPHRAGM

WIND ANALYSIS

3.0 FOUNDATION IN TALL BUILDINGS 3.1 CHALLENGES OF SKYSCRAPER FOUNDATION CONSTRUCTION 4.0 THE CASE STUDY: PROPOSED 30 STOREY BUILDING AT LEKKI PENNISULAR, LAGOS NIGERIA.

5.0

4.1

DESIGN BACKGROUND.

4.2

DESIGN AND RESULTS.

CONCLUSION

6.0. REFERENCES

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LIST OF FIGURES Figure 1: Evolution of Tall Building with Height Figure 2: The evolution of New York City’s skyline from 1879 to 2013. Figure 3: The evolution of Dubai’s skyline from 1879 to 2013. Figure 4 Steel Structural systems Figure 5: Grouping of Structural Systems Figure 6: Core Systems Figure 7: Frame Systems Figure 9: Tube Systems Figure 8: Bearing wall Systems Figure 10: Type of Bracings Figure 11: Shattering of Glass (glass storm) due to wind storm Figure 14: overview of the staad.pro window. Figure 16: The Rendered View Of The 30 Storey Structure Showing The Core Wall. Figure 17: Plan View Of The Rendered Structure. Figure18 :The Slab Loads being inserted in Staad.Pro using Yield Line Method Figure 19: The Designed Beams Showing Green Signifying Design Okay Figure 20: An example of a designed beam Figure 21: Example of application of Lateral Load due to Wind as specified in BS 8110 Figure 22: Von Stresses On The Shear Wall At Post Processing Stage Figure 23:Designed ground columns 4

1.0 INTRODUCTION It is difficult to distinguish the characteristics of a building which categorize it as tall. The outward appearance of tallness is a relative matter. In a typical single-storey area, a five story building will appear tall. In large cities, a structure must pierce the sky around 70 to 100 stories if it is to appear tall in comparison with its immediate neighbours. Tall building cannot be defined in specific terms related to height or number of floors. There is no consensus on what constitutes a tall building or at what magic height, number of stories, or proportion a building can be called tall. Perhaps the dividing line should be drawn where the design of the structure moves from the field of statics into the field of structural dynamics. . From the structural point of view, a building is considered as tall when its structural analyses and design are affected by the lateral loads, particularly sway caused by such loads. In contrast to vertical load, lateral load effects on buildings are quite variable and increase rapidly with increase in height. Under wind load the overturning moment at the base of a building varies in proportion to the square of the height of the building, and lateral deflection varies as the fourth power of the height of the building, others things being the equal. The structure usually accounts for 20 to 30 percent of the cost of a tall building. For building above 50 stories, the cost of a reasonable wind bracing system may work out, at most, to one third of the structural cost. Therefore compared to the total cost of the building, wind bracing costs, which are in the range of 7 to 10 percent, represent far from an overwhelming portion of the total cost. Historically, the unit weight of structural framing members in term of, say, average weight per unit floor area appears to be progressively decreasing over the years. For example, a survey of tall building built in the period 1950 – 1990 will verify that in this period it was possible to build a 100-story building with perhaps no more than 147 kg/m2 of steel as compared to the 205 kg/m2of steel used for the . The reasons for this gradual decrease are manifold, as can be seen from the following list. 5

1. Innovative design concepts. Structural engineers are continually seeking better and more efficient methods of resisting the lateral loads. Some of the common approaches are: i) Increase the effective width of subsystems to resist the overturning moment. ii) Design systems such that the components interact in the most efficient manner. iii) Use interior or exterior bracing for the full width of the building. iv) Arrange floor framing in such a way that all or most of the gravity loading is directly carried by the primary lateral-load-carrying components. v) Manipulate the dispersion of materials in composite construction consisting of concrete and structural steel in a manner such that materials are used to their best advantage. vi) Minimize the bending induced by wind loads in the primary components. vii) Employ truss action to eliminate bending in columns and spandrels. viii) Use rounded plan shapes to reduce the magnitude of the wind pressure. ix) Arrange closely spaced columns at the exterior to support most or all of the gravity loads and all the lateral loads. x) Suspend floors from a central core such that the total gravity load acting on the core will induce enough hold-down forces to counteract the overturning moment. xi) Use an interior braced core that interacts with exterior columns via belt and outrigger trusses.. xii) Use exterior steel plate curtain walls to resist lateral forces. 2. Use of high-strength low-alloy steels. Today it is a common practice to use 345 MPasteel in most composite floor framing systems, gravity columns, and not too infrequently in lateral-loadresisting elements. 3. Increased use of welding as compared to bolting, which effects a saving in the range of 8 to 15 percent in the weight of steel. 4. Increased use of composite construction. 5. Application of computers to both the design and the analytical processes. 6. Gradual increase in the allowable stresses in the materials based on research and successful past performance. 7. A reduction in the weight of other construction materials, like partition and curtain walls. In concrete construction, major factors responsible for reducing the reinforcement and concrete quantities are:

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1. New framing techniques, such as skip joist construction in which every other joist is eliminated, have caught on in high-rise construction with a consequent reduction in the weight of structural frame. 2. Increased use of mechanical couplers in reinforcement for transferring both compression and tensile forces. 3. Use of welded cage for column ties, beam stirrups, etc., which reduces the amount of reinforcement steel. 4. Use of high strength concrete; 40 MPa is quite common, and strengths up to 70MPa are being specified on vertical components of high-rises. 5. Use of lightweight aggregate typically reduces 50 to 100 kg/m2in the dead load of the structure, resulting in savings of approximately 10 to 15 percent in the reinforcement requirement. 6. Most codes do not require as great a thickness of slabs when structural lightweight concrete is used. Typically a thickness of 12 mm of concrete can be taken off from floor slabs without reducing the fire rating. 7. Use of 520 MPa steel reinforcement. 8. Use of the state-of-the-art design methods

Tall buildings have a unique appeal, even a mystery associated with their design. Developments in the last five to seven decades have produced many slender high-rise buildings, demanding that particular attention to be paid to their complex behaviour under lateral loads. High-rise architecture is continuously changing, and prismatic shapes that were once very popular have given rise to terraced, set-back and splayed elevations. Computers have given the structural engineer of today the tools to respond to this changing architecture with daring structural solutions. No longer does the structural engineer require that the building be regular in plan and the interior and exterior columns line up with each other. Ego and competition still play a part in determining the height of a building, but various other social and economic factors, such as increase in land values in urban areas and higher density of population, have led to a great increase in the number of tall buildings all over the world.

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In masonry structures, the percentage of area occupied by the vertical structural elements, i.e., columns, walls and braces, was inordinately large compared to the gross floor area. The area occupied by the walls of the 17-story building in Chicago is 15 percent of the gross are at ground floor with wall thicknesses of 2.1 m. Two technological developments, the elevator and model metal frame construction, removed the prevailing limitations on the height of the buildings, and the race for tallness was on. Today, with the use of computers, buildings are planned and designed which have little or nohistoric precedent. New structural systems are conceived and applied to extremely tall buildings in a practical demonstration of the engineer’s confidence in the predictive ability of the analysis, the methods used, and the reliability of computer solutions. The development of metal trusses made it possible to roof column-free interior spaces easily and economically. The configuration tries to simultaneously satisfy (1) the requirements of site, (2) the requirements of the building program, and (3) the requirements of appearance. For a building to be successful, it should do the following: 1. Create a friendly and inviting image that has positive values to building owners, users, and observers. 2. Fit the site, providing proper approaches to the plaza with a layout congenial for people to live, work and play. 3. Be energy efficient, providing space with controllable climate for its users. 4. For office buildings, allow flexibilities in office layout with easily divisible spaces. 5. Most spaces oriented to provide best views. 6. Most of all, the building must make economic sense, without which none of the modern highrise development would be a reality.

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1.1

TALL BUILDING AND THEIR EVOLUTION

PHIL ADE PHIA CITY HAL L (1901 )

SING ER BUIL DING (1908 )

METR OPOL TAN LIFE TOEW R ( 1909)

WO OLW ORT H BUIL DIN G ( 191 3)

THE TRU MP BUIL DIN G (1930

CHR YSL ER BUIL DIN G (1930

Figure 1: Evolution of Tall Building with Height

EMP IRE STAT E BUIL DIN G

ONE WORLD TRADE CENTE R BUILDI NG (1972)

W IL LI S T O W E R (1 97

PE TR O N AS TO W ER (1 99 8)

T A IP EI 10 1( 20

BUR J KHA LIFA ( 201 0)

There are a few very early examples of architecture reaching far up into the sky. The Pyramid of Khufu for instance, from the 4th Dynasty (2620 to 2500 B.C.), at 139 meters the world's tallest pyramid, or the Pharos of Alexandria, which according to its legend was, at around 140 meters' height, the tallest lighthouse ever built up until the 20th century. And not to forget the Tower of Babel, which according to the Old Testament reached all the way up to Heaven. Since the 19th century, however, it has not just been sacred buildings that have been touching the clouds. Increasing numbers of high-rises with apartments, offices and hotel rooms have been built. Skyscraper construction received its first significant impulse with the invention of the elevator by Elisha Grave Otis in 1852. While it would, from a technical point of view, already have been possible to build tall structures with more than six stories, one would have been unable to find many tenants for them. The invention of steel frame construction was the next important step on the road toward skyscraper architecture. The first building constructed in this way was the Home 9

Insurance Building in Chicago, built in 1885. At a height of 55 meters and ten floors it was the world's first high-rise – and revolutionary for the development of skyscraper construction.

1.2

A QUICK OVERVIEW OF THE TALLEST BUIDING BETWEEN 1901 – TILL

PRESENT Philadelphia City Hall (1901 -1908)

From 1901 onward, Philadelphia City Hall in the American city of Philadelphia, already substantially taller at 167 meters, reached for the skies. In fact, the architecture of this skyscraper with a bell tower was not based on the recently-invented steel frame construction: to this day it is considered the tallest masonry structure in the world Singer Building(1908 – 1909) For seven years, Philadelphia City Hall was the world's tallest building until it was superseded in 1908 by the Singer Building in New York City, which was 20 meters taller. This particular skyscraper was only able to stay in pole position a short time, however. 1913, only four years later, saw the opening of the 213-meter-tall Metropolitan Life Tower, also located in New York City.

Metropolitan Life Tower (1909 – 1913)

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The skyscrapers that followed the Met Life Tower were also located in New York City. After just four years, the Woolworth Building, built in the borough of Manhattan and 241 meters tall, forced the Met Life Tower from the top spot.

Woolworth Building(1913 – 1930)

Construction of the Woolworth Building was only possible thanks to the development of reinforced foundations, technology intended to prevent the skyscraper from leaning too heavily and toppling over in the case of earth movements. Until 1930, the Woolworth Building was the world's tallest skyscraper. The Trump Building (1930 – 1930)

The history of the skyscraper was increasingly concentrated in the decades that followed in the one city – New York City. 1930 saw the completion there, after less than a year's construction, of The Trump Building. At a height of 283 meters, it was briefly the world's tallest building.

Chrysler Building(1930 – 1931)

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The Trump Building unfortunately had to cede its title after just a few weeks to the Chrysler Building, which, at 319 meters, exceeded it by far. The man who had it built, Walter Chrysler, was inspired by the Eiffel Tower in Paris – at that time the world's tallest structure – to want to build the world's tallest skyscraper in the Chrysler Building in New York City. To win the race, Chrysler and his architect William van Alen came up with a sophisticated trick: Van Alen had an additional 56-meter-long spire built, which was then delivered secretly in pieces, put together in the elevator shaft and finally placed on the top of the building after its completion in just 90 minutes.

Empire State Building (1931 – 1972)

Around a year after it opened, the Chrysler Building was superseded by the 381meter-tall Empire State Building. Built in a record time of 18 months, the tower was viewed as the "Eighth Wonder of the World". The weight of its presence in the media and its use as a setting for films such as "King Kong" or "Independence Day" have helped to make the Empire State Building one of the best-known skyscrapers from anywhere around the world. For 41 years it was the world's tallest building – longer than any other of the record-holders.

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One World Trade Center (1972 – 1974)

Only in 1972 was this emblem of New York overtaken by an even-taller skyscraper in the shape of One World Trade Center, which – like so many of its predecessors as record-holder – was built in New York City. Following a construction period of seven years, the skyscraper and its 415-meter-tall twin tower became the two tallest buildings in the world. The terrorist attacks of September 11, 2001, however, led to the World Trade Center being completely destroyed. Willis Tower (1974 – 1998)

In 1974, two years on from the completion of the World Trade Center, the towers were overtaken by Sears Tower in Chicago. For over twenty years, the skyscraper was the world's tallest. In 2009, Sears Tower was renamed Willis Tower after the Willis Group Holding acquired the naming rights to the skyscraper and rented a substantial portion of the office space. Petronas Towers1998 - 2004

In 1998, with construction of the Petronas Towers in Kuala Lumpur, the title of the world's tallest building passed for the first time to a skyscraper outside America. A significant feature of the twin towers is the skybridge between the towers, which is intended to serve as an escape route in the case of emergency. The completion of the Petronas Towers finally made it 13

necessary to set up rules for measuring skyscraper heights, as the towers were only awarded the title following a legal battle. The owner of Willis Tower in Chicago brought the action, since inclusion of the antenna would have meant Willis Tower would have exceeded the architectural height of the Petronas Towers. Since the judgment in favor of the Petronas Towers, television antennas are no longer recognized as an architectural component of buildings.

Taipei 101 (2004 – 2007)

Until completion of Taipei 101 in Taiwan, the towers were considered the tallest skyscrapers in the world on account of the structural height of 452 meters. Taipei 101 took on this title in 2004 and thus became the first title-holder of the 21st century. The skyscraper claimed several records at once: the greatest architectural height, the greatest roof height and the highest occupied floor.

Burj Khalifa (2007 – present)

Even before Taipei 101 was completed, construction work had already begun on the next "supertall", Burj Dubai, today known under the name Burj Khalifa. In order to complete the tower, 2,400 workers were employed, working a three-shift system. On average it only took four days to add a new floor. The tower topped out at 828 meters in December 2008, but the structure had already passed the height of Taipei 101 in July 2007, making Burj Khalifa the world's tallest building – which it remains to this day. It was completed in 2010.

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1.3

EVOLUTION OF SKYLINE EXAMPLES. 1.3.1

THE NEW YORK SKYLINE

Figure 2: The evolution of New York City’s skyline from 1879 to 2013. 1.3.2

DUBAI SKYLINE.

Dubai has 18 completed buildings that rise at least 300 metres !!!!!! This includes the tallest man made structure Burj Khalifa .

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Figure 3: The evolution of Dubai’s skyline from 1879 to 2013.

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2.0

STRUCTURAL SYSTEM.

The structural system of a high-rise building must resist both gravity and lateral loads, due to phenomena such as wind and earthquake. As the height of the building increases, the lateral loads gradually dominate the structural design.

Figure 4 Steel Structural systems Lateral loads due to wind and earthquake produce lateral accelerations. As people normally perceive these accelerations during service conditions, stiffness rather than strength tends to become the dominant factor in buildings of great height. The serviceability limit state can, therefore, be more important than the ultimate limit state. Four overall groupings of structural systems may be identified (Figure 5). They are: a. bearing wall system 17

b. core system c. frame system d. tube system.

Figure 5: Grouping of Structural Systems Each system has different lateral load resisting properties and thus tends to be 'efficient' over a different height range. The bearing wall system due to the self weight of the structural components (usually concrete), normally becomes inefficient for buildings above 15-30 storeys in height. The concrete core system has the same disadvantage as the bearing wall system, namely self weight is a limiting factor. The efficiency of the framed system depends upon the rigidity of the connections and the amount of bracing. Stiffening can be achieved by use of a solid core, shear walls or diagonal bracing. As more bracing is incorporated into the spatial frame, the range of efficient height is increased. The upper limit is in the range of 60 storeys. 18

The tube system can be thought of as a spatial frame with the vertical elements positioned at the exterior. The range of height efficiency is influenced by the type and the amount of bracing employed in the tube. In general a tube structure is considered the most efficient form for the tallest buildings, i.e. above 60 storeys in height. From the four basic structural systems, six secondary systems can be derived from a combination of the basic ones (see Figure ). The four basic systems are assumed as the prime groups which can be associated to the levels of the structural system hierarchy as proposed by Falconer and Beedle. These primary systems are:1. A bearing wall structure is comprised of planar vertical elements, which form all or part of the exterior walls and in many instances the interior walls as well. They resist both vertical and horizontal loads and are mainly made in concrete (see Figure ). 2. A core structure is comprised of load bearing walls arranged in a closed form where the vertical transportation systems are usually concentrated. This arrangement allows flexibility in the use of the building space outside the core. The core can be designed to resist both vertical and horizontal loads. Figure 10 shows some examples of this system. In the upper part of the figure, there is a central core from which floors are either suspended or cantilevered. In the lower part the cores are separated and connected by the floor structures. 3. A frame structure is usually made of columns, beams and floor slabs arranged to resist both horizontal and vertical loads. The frame is perhaps the most adaptable structural form with regard to material and shape, due to the many ways of combining structural elements in order to give adequate support to the given loading. In the examples of Figure 11, steel frames are combined with concrete walls and cores, or with steel bracings and horizontal trusses. 4. A tube structure is normally characterised by closely spaced exterior structural elements, designed to resist lateral loads as a whole, rather than as separate elements. Alternative schemes could include braced tubes and framed tubes (see Figure 12). Besides the simple

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tube, tube-in-tube solutions can be also used. These systems allow for more flexibility in the use of interior space, due to the lack of interior columns.

Figure 6: Core Systems

20

Figure 7: Frame Systems

21

Figure 8: Bearing wall Systems

22

Figure 9: Tube Systems Wall structures as well as cores are usually made of reinforced concrete. 23

Steel frames can be used together with concrete cores, and/or walls, leading to composite structures, which may be called also 'dual structures'. When steel frames are braced, different types of bracing can be used according to structural and functional requirements (Figure 13).

Figure 10: Type of Bracings 24

The most common are: 

single or double diagonal bracing



vertical or horizontal K-bracing



Lattice bracing.

Both K- and single diagonal bracings can be 'eccentric', i.e. the diagonal members do not meet in the nodes. 2.1

DESIGN PARAMETERS

2.1.1

BUILDING DEFLECTION

Is measured by the drift of the building. Inter-story drift =story deflection / story height building drift = lateral sway at the top of the building / building height This first order drifts range from approximately 1/450 to 1/600 under a 50 year wind. It is very important to know the drift of the building for determining how much movement the exterior cladding and other members have to withstand. 2.1.2

ACCELERATION

The building deflection is directly associated with the perception of motion from occupants and therefore with the building acceleration. This acceleration depend on the dynamic behaviour of the building under wind forces. It is very difficult to determine this acceleration and there are different kinds of testing methods used. An example is the wind tunnel testing. 2.1.3

SECOND ORDER EFFECTS

This effect causes additional lateral movement of the building due to gravity loads. If we take for example a steel building with a first order drift of 1 500 than the second order drift is in the range of 20-30% of the first effect. 2.1.4

UPLIFT

If a building is to stiff it causes so much overturning moments to create uplift in the foundation. Most of the foundations have only a limited uplift capacity and therefore some extremely stiff systems cannot be used. 25

To reduce this problem it is recommendable to transfer the gravity loads from interior columns to the exterior columns to counteract the uplift forces. 2.1.5

BUILDING DIAPHRAGM

The lateral loads should be distributed to the lateral systems by floor diaphragms. This diaphragms should be directly connected to columns, beams and to the wind resistive elements. The diaphragm stresses in a normal building are quite small and can be resisted by the floor slabs. It could only be critical if there is a large opening in the diaphragm such as alarge atrium.

2.2

WIND ANALYSIS

Wind load

Strong winds may cause a variety of problems, particularly in tall buildings • Modern tall buildings are even more prone to wind action, due to their lightweight walls and partitions, which reduce the mass and the damping • Even for high seismic areas, for buildings with more then 25-30 stories, the wind load governs the design • Attention should be paid to the following criteria: – Strength and stability – Fatigue of members and connections – Excessive lateral deformations (may cause cracking of claddings or permanent deformations to non structural elements) – Excessive vibrations that cause discomfort to the occupants

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Figure 11: Shattering of Glass (glass storm) due to wind storm Influence of extreme height to building frame In addition to usual checks: 1. Dynamic effects of wind. 2. P - ∆effect (2nd order effect). 3. Influence of member shortening. 4. Static and dynamic rigidity: δmax ≤H/500 a ≤amax≈0,015 g 5. Interaction with ground (especially if H/B > 5). Dynamic effects of wind Generally: • analysis including vibration: - longitudinal (in the wind direction) 27

- lateral (in transversal direction): circular, elliptic shapes: "vortex shedding" rectangular shapes: "galloping" (occurs rarely) Vortex shedding, vortex separation (called also Karman periodic set of whirlwinds) results on condition that:

The first frequency of a building: n ≈46/h Strouhal number: circle St = 0,18 Rearrangement of the building shape wind tunnel, each variation is significant. Longitudinal dynamic wind effects

Wind loading for area A ref according to EN 1993-1-4: - if h ≤100 m and b > 30m, coefficient of the structure cscd= 1; - Otherwise use „detailed method" (depends on natural frequency n, parameters of wind and structure ...) - Eurocode enables to determine even deflection and vibration acceleration P - ∆effect (2nd order effect) Represents effect of horizontal shift on internal forces. Solution: •2ndorder theory (or geometrically nonlinear analysis GNA), 28

If SLS is fulfilled, the approximate guess of V, H (for all building or floor) gives coefficient of 2nd order m. The horizontal loadings then multiply with m:

Influence of member shortening The shortening of member axes is covered by computer FEM analysis!

The stress in diagonals from vertical loading is, therefore, of the same order as in columns! Measures: - Final connection of diagonals not until assembly of all building, - Or pressurising of diagonals to eliminate compression due to vertical loading. Seismic load Many of European areas are under seismic risk • Southern Europe experienced very damaging earthquakes during the last decades. 29

• Many existing structures have inadequate protection against strong earthquakes. The vulnerability is very much increasing, due to the rapid grow of the construction industry. • Seismic loading requires an understanding of the structural behavior under inelastic cyclic deformations • Behavior under such loading is fundamentally different from wind loading (and gravity loading). It is necessary to pay more attention to type of analysis and requirements, in order to assure acceptable seismic performance beyond the elastic range. • Some structural damage in members and connections can be expected under design ground motion, as the majority of modern seismic codes allow inelastic energy dissipation in the structural system

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3.0 FOUNDATION IN TALL BUILDINGS Building construction begins at the base. In ancient times, pyramidal structures were often used to distribute the weight of tall buildings over a large area. In modern times, the advancement of technology and soaring land costs in cities have led engineers to consider the size of a building’s footprint while continuing to think vertically. Creating a large amount of real estate out of a relatively small amount of ground area is very appealing to developers in large cities. Like the roots of a tree, a skyscraper’s foundation is laid below ground to create the most stability. To lay the most stable foundation possible, the bedrock (solid rock underneath the ground’s soil) must be reached. Accessing the bedrock, in most cases, requires significant excavation of soils (sand, clay, etc.) overlying the bedrock. However, building the foundation upon bedrock is necessary given the extremely large loads associated with skyscraper construction. For instance, a typical house might weigh 70 tons; but the Empire State Building in New York City weighs about 350,000 tons! 3.1 CHALLENGES OF SKYSCRAPER FOUNDATION CONSTRUCTION

In some areas, simply accessing the bedrock can be a real challenge. One of the more notable examples occurred during construction of the World Trade Center (WTC) towers in New York City (completed April 4, 1973). The bedrock in this area of New York is approximately 65 feet below the ground surface. However, the WTC construction site was located adjacent to the Hudson River, and just a few feet below the ground surface, the soil was saturated due to the shallow water table. This meant that simply excavating to the bedrock could not be accomplished, as the construction site would flood when the digging began. A novel approach was developed to counter the intrusion of water into the foundation construction site. The construction crew used machinery to dig a 3 foot wide trench around the perimeter of the site. While digging, a mixture of water and bentonite (expansive clay) was piped into the trench and would expand along the sides of the trench, effectively blocking the groundwater. Once a section of the trench was completed, the crew would lower a steel framework into the hole and pump in concrete from the bottom of the hole. Pumping the concrete in from the bottom would displace the water/bentonite mixture, leaving a steelreinforced concrete wall around the perimeter of the construction site; which measured four city blocks by two city blocks upon completion of the wall. This perimeter wall/dam and the 31

construction site contained therein was jokingly referred to as the “bathtub” by the construction crew, and formed a water tight perimeter wall for the towers’ foundation structure to be built. Once the wall was in place, the construction crew could begin digging down to bedrock to lay the buildings’ foundation system. However, as the soil within the construction site was removed, the weight of the soil and water outside the walls would push the walls inward. Thus a series of tiebacks (cables extending from the perimeter walls to the material surrounding the “bathtub” were installed to provide temporary support until the crew could finish a support structure inside the “bathtub”. Using this method, the foundation site could be excavated; culminating in the removal of more than 1 million cubic yards of soil! With the bedrock exposed, the massive foundation structure could be placed. The perimeter footing for the buildings consisted of 60 high strength, load bearing steel columns spaced closely together on each side of the buildings.

The

process

of

foundation

design

is

well-established,

and generally involves the

following aspects: 1.

A desk study and a study of the geology and hydrogeology of the area in which the

site is located. 2. A detailed investigation has to be conducted at a given site only when that site has been chosen for the construction 3. In-situ testing are needed for determining compressibility and shear strength parameters 4.

Laboratory testing to supplement the in-situ testing and to obtain more detailed

information on the behaviour of the key strata than may be possible with in-situ testing. loadings, and the ground conditions. From this stage and beyond, close interaction with the structural designer is an important component of successful foundation design. 8.

Detailed design, in conjunction with the structural designer. As the foundation

system is modified, so too are the loads that are computed by the structural designer, and it is generally necessary to iterate towards a compatible set of loads and foundation deformations. 9.

In-situ foundation testing at or before this stage is highly desirable, if not essential, in

order to demonstrate that the actual foundation behaviour is consistent with the design assumptions. This usually takes the form of testing of prototype or near- prototype piles. If the behaviour deviates from that expected, then the foundation design may need to be revised cater

for

the

observed

to

foundation behaviour. Such a revision may be either positive (a 32

reduction in foundation requirements) or negative (an increase in foundation requirements). In making this decision, the foundation engineer must be aware that the foundation testing involves only individual elements of the foundation system, and that the piles and the raft within the system interact. 10. Monitoring of the performance of the building during and after construction. At the very least, settlements at a number of locations around the foundation should be monitored, and ideally, some of the piles and sections of the raft should also be monitored to measure the sharing of load among the foundation elements. Such monitoring is becoming more accepted as standard practice for high-rise buildings, but not always for more conventional structures. As with any application of the observational method, if the measured behaviour departs significantly from the design expectations, then a contingency plan should be implemented to address such departures. It should be pointed out that departures may involve not only settlements and differential settlements that are greater than expected, but also those that are smaller than expected.

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4.0 THE CASE STUDY: PROPOSED 30 STOREY BUILDING AT LEKKI PENNISULAR, LAGOS NIGERIA. 4.1

DESIGN BACKGROUND.

The design of the structure is carried out using the Staad.Pro Design Structural Software. STAAD.Pro is a general purpose program for performing the analysis and design of a wide variety of types of structures. The basic three activities which are to be carried out to achieve that goal - a) model generation b) the calculations to obtain the analytical results c) result verification - are all facilitated by tools contained in the program's graphical environment. STAAD designs both reinforced concrete and structural steel structures the following can be performed: 

Modelling of structural components, as individual component and holistically for the entire



building frame. Loading and analysis of the whole building structure with the resultant output obtained, i.e

    

bending moments, shear forces, deflections, axial forces and support reactions. Moment releases at joints Modelling of curved members and complex-shaped structures Interface with AutoCAD; importing models and exporting DXF files Load combinations and envelopes Design of the whole structure, for preliminary and final sizing of structural members. Utility



ratios for steel members can be viewed. Standard calculation reporting of designed members.

Applications STAAD.Pro can be used in designing the following structures:      

Storey buildings – high-rise and low-rise buildings Factory buildings, like warehouses and equipment shelters Overhead tanks Telecommunication masts Bridges Bill boards, etc.

3.) Advantages and Disadvantages of STAAD 34

Advantages:      

Speed of design/shortened project delivery time Thoroughness of analysis 3D viewing ability Holistic approach to design Ability to illustrate results with contours and animations Ability to generate standard detail report of designed members

Disadvantages:     

It’s not a substitute for the engineer’s interpretation and judgment May produce lengthy reporting Inability to perform slab detailing It does not perform foundation designs – pad and piles Could produce confusing output if input is incorrect

4.) The sequence of carrying out structural design on STAAD is shown below:

35

Figure 14: overview of the staad.pro window.

MODELLING OF PROPOSED STRUCTURE ON STAAD.PRO

36

Figure 15: THE RENDERED VIEW OF THE STRUCTURE

Figure 16: The Rendered View Of The 30 Storey Structure Showing The Core Wall.

Figure 17: Plan View Of The Rendered Structure.

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4.2

DESIGN AND RESULTS.

SLAB: The thickness of the slab for this project is 150mm, which the concrete unit weight used is 24kg/m3 and the concrete strength used is 25N/mm2 strength at 28 days curing. The loadings of the slab are calculated by hand and placed on the beam by yield line method as shown in the figure below. The reinforcement are adequate as shown to be Y12 - @300mm c/c for the top support and (the span) bottom reinforcements

Figure18 :The Slab Loads being inserted in Staad.Pro using Yield Line Method

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BEAMS

Figure 19: The Designed Beams Showing Green Signifying Design Okay.

Figure 20: An example of a designed beam

Beams were designed in Staad Concrete Designer Suite as shown above, the beams with dimensions 450 x300 was used and formed as continuous members as labeled and designed. Concrete unit weight used is 24kg/m3 and the concrete strength used is 25N/mm2 strength at 28 days curing. Colour Legend of the designed status as shown thus: Green : Passed ( design) 39

Satisfactory, Pink ( Fails in deflection), Black ( Not designed). All the beams showing green showing they are designed and satisfactory. The reinforcement attached to this report shows that most of the top ( support) reinforment showed 2Y 32 but optimizing the software could provide 4Y 20 the bottom reinforcement has 4Y 20 also.

SHEAR WALL/LIFT SHAFT WALL. The lift shaft wall modeled in Staad.Pro using the plate meshing creating a finite element for the wall system. Lateral Loads were applied as simplified method of analysis of Lateral loads as stated in W. H Mosley, the wind pressure calculated from CP3 was multiplied by the storey height taken from the mid of one storey to the other. The result of the stress on the shear wall are shown below using the Von Mises method of stress analyses and the reinforcement are provided as Y 20 @200mm c/c . However the hand calculation for a single system could not be captured as the wind resisting system is a composition of the framed and shear wall system.

Figure 21: Example of application of Lateral Load due to Wind as specified in BS 8110

40

Von Mis Top N/ mm2 =16.7

Y Z X

Load 4

Figure 22: Von Stresses On The Shear Wall At Post Processing Stage

COLUMNS. The columns designed and in Staad.Pro and the result are attached in the appendix. The Concrete unit weight used is 24kg/m3 and the concrete strength used is 25N/mm2 strength, 28 days curing, column sizes varied between group of stories from 1000mm ( circular and rectangular) from ground to 4th floor, 800mm( circular)and 700 x 700 (rectangular) from 4th to 13th floor, 600x600 mm from 13- 21th floor, 500 x500mm from 22 – 25 st floor, 450 x450mm to 26th to 28th and 300mm 29th to roof. The ground column are shown in the figure below. The critical reaction from the software 12,007KN as compared to that obtained for the ground columns by hand 22,745KN. The discrepancy due to the optimization scheme of the software and the frame/wall action of the software.

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Figure 23: Designed ground columns

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5.0

CONCLUSION

It is a stated fact that the use of software for the 21 st century has been an immeasurable help to field of Civil and structural Engineering, however the knowledge of calculation cannot be put aside it is therefore needed that the knowhow of structural design is well harnessed with the use of these commercial software.

STAAD PRO has the capability to calculate the reinforcement needed for any concrete section. The program contains a number of parameters which are designed as per BS: 8110 -1 (1997) Beams are designed for flexure, shear and torsion. Design for Flexure: Maximum sagging (creating tensile stress at the bottom face of the beam) and hogging (creating tensile stress at the top face) moments are calculated for all active load cases at each of the above mentioned sections. Each of these sections are designed to resist both of these critical sagging and hogging moments. Where ever the rectangular section is inadequate as singly reinforced section, doubly reinforced section is tried. Design for Shear: Shear reinforcement is calculated to resist both shear forces and torsional moments. Shear capacity calculation at different sections without the shear reinforcement is based on the actual tensile reinforcement provided by STAAD program. Two-legged stirrups are provided to take care of the balance shear forces acting on these sections. Beam Design Output: The default design output of the beam contains flexural and shear reinforcement provided along the length of the beam. Column Design: Columns are designed for axial forces and biaxial moments at the ends. All active load cases are tested to calculate reinforcement. The loading which yield maximum reinforcement is called the critical load. Column design is done for square section. Square columns are designed with reinforcement distributed on each side equally for the sections under biaxial moments and with reinforcement distributed equally in two faces for sections under uniaxial moment. All major criteria for selecting longitudinal and transverse reinforcement as stipulated by BS 8110 – 1- 1997 have been taken care of in the column design of STAAD

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6.0. REFERENCES [1]Ali, Mir M. (2001), "Evolution of Concrete Skyscrapers: from Ingalls to Jin mao", Electronic Journal of Structural Engineering 1 (1): 2–14, retrieved 2008-11-30 [2]Building Big. PBS. Accessed June 25, 2004. (source of lesson background information) http://www.pbs.org/wgbh/buildingbig/

[3] New Structural Systems for Tall Buildings and Their Scale Effects on Cities, Khan, Fazlur R. "Tall Building Plan, Design and Construction", Symp, Proc, Vanderbilt University, Civ Eng Program, Nashville, Tennessee, 1974. [4] Eurocode Convention of Constructional Steelwork : "Recommendations For Steel Structures in Seismic Zones", ECCS, Publication 54, 1988. [5] Eurocode 8 : "Structures in Seismic Regions - Design", CEN (in preparation) [6] Renolyds reinforced concrete Designers Handbook 11th Edition Charles E. Reynolds BSc (Eng),

CEng, FICE James C. Steedman BA, CEng, MICE, MIStructE and Anthony J. Threlfall BEng, DIC [7] Reinforced Concrete Design W. H. Mosley, J. H. Bungey and R Hulse. [8] Prof. Dr. Zahid A. Siddiqi, UET, Lahore. http://www.pec.org.pk/sCourse_files/CEC5-1.pdf

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