Sample Essay on Construction Methods of High Rise Buildings
Subject: Construction Methods
Construction of High Rise Building
This paper discusses about construction of a high rise building. High rise is a building whose height creates different conditions in the design, construction and operation because of the influence of its tallness (CTBUH, 1997). It can also be classified as residential or commercial. Therefore, construction of a high rise building requires a comprehensive study and understanding of the plans, design, construction and operation of physical works and facilities which involves different methods and techniques.
The first part deals with construction industry of high rise building, its process, the current state of the industry as well as the codes and regulations that govern planning and erection of a building. It also covers construction management and contracts important in high rise building engineering and the different roles of the designer and the contractor.
The second part discusses the economics of construction of a high rise. This topic is essential in making engineering project decisions. Design and plan of a building involves estimation of costs involved.
The rest of the paper is focused on the construction methods and the technologies involved in the construction of high rise from the structural system to the detailed elements of a high rise including the foundation; loading; steel structure, facade, curtain walling and the different service systems like electrical and HVAC system. Each element has its own construction method with underlying techniques, materials and equipments used for each method. The risks and the problems associated with high rise building construction as well as the techniques to eliminate the risks are also covered in this study.
Generally, this study provides essential information and guidelines in the design and plan of infrastructures that involves different methods. The quality and stability of an infrastructure such as a high rise building all depends on the proper and appropriate used of the different methods as well as on the type of materials used in the design.
Table of Contents
1. The High-Rise Building Construction Industry………………………........5
Codes and Regulations………………………………....………….…..7
2. Economics of Construction ………………………………………………..11
3. Constructions Method……………………………………………………...12
3.1 Construction Equipment………………………………………………….12
Excavation and Loading………………………………………………13
Lifting and Erecting……………………………………………………14
3.2 Elements of High-rise Buildings…………………………………………15
3.2.1 Structural Loads…………………………………………….…..15
3.2.3 Structural System…………………..………………….……..23
3.2.4 The Basic Structural Systems in High-Rise Building.…….26
Steel: Staggered Truss……………………………..………26
Secondary Supporting Structures
Frame and Shear Wall…………………………………….34
3.2.5 Curtain Wall and Façade…………………………..……….35
3.2.6 Service Systems………………………………….....………39
Fire Protection System…………………….………………42
A high-rise building is a building 35 meters or greater in height, which is divided at multiple levels of at least 2 meters in height (EDC, 2006). To be considered as a high-rise, a building must be based on solid ground, and fabricated along its full height through deliberate processes. High-rise buildings can be for residential uses such as condominiums and residential apartments, or for commercial purposes such as hotels and office buildings, occasionally including retail and educational facilities. There are also for both residential and commercial used of high-rise buildings today which accommodates residential, office and commercial spaces. Therefore, high-rise buildings require flexibility in its design and stability on its construction.
High-rise building has been a dominant landmark from the early years from the towers of Antiquity and the Middle Ages in ancient years to the famous Eiffel Tower in Paris, the World Trade Center in the United States and the many buildings of hotels and organizations today. The first high-rise building was an office building and said to be the Home Insurance Building in Chicago. Famous high-rise buildings include Empire State Building which is 381 meters, the United Nations Building, and the Manhattan Building. Construction of high-rise buildings continuously changes as technology change. Evolution of high-rise buildings is brought about by the changes and development of the materials, equipments, technical conditions and methods used in the construction of high-rise buildings.
Construction of high-rise buildings differs from ordinary construction such as residential houses and low-rise buildings with elements which include the foundation, structural systems, enclosure systems, life-safety systems, vertical transportation, plumbing, and electrical systems different from the construction of low-rise buildings. All these considerations require methods and techniques in order to meet the requirements of a high-rise building.
1. The High-Rise Building Construction Industry
The industry consists of private companies and public authorities, with many individuals and organizations involved, from the manufacture of necessary components to the assembly and construction process. High-rise building project requires a registered architect and civil engineer under the direction of a project manager to execute the design and to make sure that it complies with the regulations governing building construction, at the same time conforming to the requirements of the owner (Encarta, 2006). The architects or engineers convert the requirements into a set of drawings and written specifications that are usually sent to contractors for bidding (Encarta, 2006). The successful bidding contractor will be the one responsible for the required manpower such as the plumber, painter, electrician, carpenter and other manpower required for the construction. Contractors carry out the work under the supervision of an architect and engineer who act as agents of the owner (Encarta, 2006).
Scarcity in land urban areas, increasing demand for business and residential space and economic growth are the major reasons behind the increase in demand for high-rise buildings as well as the increasing price of land aside from the above mentioned technological advancements and innovations in structural systems. The rapid growth of population also promoted the construction of high-rise buildings.
One example is the modern Hong Kong which has a land area of 1,037 square kilometers. Hong Kong, due to its small land area has the most number of high-rise buildings in the world with about 7, 254 commercial and residential buildings. This followed by New York City with 5,317 and Singapore with 3, 489 (Emporis, 2004). By region, Asia has the most number of high-rise buildings with about 33.16% of the total high-rise buildings in the world while North America has 31.20% and Europe shares 17.89% (Emporis, 2004) of the total high-rise buildings. Economic growth and resulting demand for office space is a good indication of demand for high-rise buildings (Buyukozturk, 2004).
Other motivations for the demand of high-rise are the desire for aesthetics in urban settings, the concept of city skyline, the cultural significance and prestige and simply the aspiration of human to build higher structures (Buyukozturk, 2004). Most people perceived and associated high-rise buildings to the economic status of a country.
Codes and Regulations
In constructing a high-rise building, there are certain laws and various regulations and standards that must be taken into account and followed. These law and regulations differs in every country and depends on local circumstances but are basically similar in contents. These basic rules are the generally accepted technical rules for construction. Most countries follow or base their local standards to other foreign standards such as the American ANSI Codes and UL Standards, and British Standards (Anonymous, 2000).
Laws and regulations are designed to ensure safety during and after the construction of high-rise and to protect the building against damage and defects. High-rise must be constructed in such a way that the public who will use the building as well as the structures within the vicinity of the building will be safe. Regulations are also enforced to guarantee the owner of the high-rise that the building will be profitable for him in the long run.
Fire protection and security are the basic concerns of many construction regulations. The building should be designed and constructed in a manner that all the people inside can leave the building without the risk of injury and at the shortest possible time once a fire breaks. Generally, it is a standard for high-rise buildings to be constructed with escape routes and fire escapes, fire compartments and with the choice of material. Operational security concerns the safety of elevators and escalators, stairs, railings and parapets and the installation of emergency lightings. Regulations for operational security also include the alarm system for carbon dioxide in underground parking lots; and the non-slip nature of floor in traffic areas, sanitary rooms and kitchens.
Stability and construction physics are also major concerns in standards and regulations. Stability of high-rise buildings depends on how the building is designed and constructed. The designs and construction methods are govern by these regulations; plans and designs which includes stability calculations and material properties and specifications are checked by authorities and should passed their standards. Demonstrations of the internal structural strength of the construction and safe transfer of loads to the subsoil should also be practiced to determine the allowable maximum load that the building can withstand.
Most codes and regulations include protection from natural hazards such as windstorms and earthquakes which are the most serious natural hazards for high-rise buildings. Regulations will specify assumed loads and design rules for the “load cases” of earthquake and windstorm (Anonymous, 2000) to ensure that the building can withstand these natural hazards up to certain limits. Also, the regulations are set to ensure that injuries can be avoided or lessen due to falling parts of the building when these hazards occur.
Other primary considerations in construction regulations and codes are the social aspects and protection of the surroundings. Designs and construction of the high-rise should follow the regulations that protect the surrounding area of the building as well as to prevent any indirect risk or threat to people. Regulations specify the minimum distance of the high-rise to the neighboring buildings or the maximum permissible influence of the building to the surroundings. Location of high-rise building is also govern by rules because it may have effects on air traffic safety on radio communications or the location is not allowed for high-rise building because of its nearness to airports.
Other regulations are for energy used, water system, and electrical system and other standards designed primarily for the safety of the people and for the protection of properties. The need for high-rise safety is increasing as the as the height also increases.
Moreover, construction of high-rise buildings, like any other constructions, requires licensing with all the necessary documents needed such as descriptions, plans, seismic analysis and other documents done by licensed specialists such as architects, engineers and contractors.
Construction planning of high-rise building project involves the weighing of the project costs and determining the most reliable options, making sure that the project is feasible at the most cost-effective time. Construction planning is challenging because of the changes that must be handles as the construction proceeds.
There are several factors to consider evaluating and estimating the construction period of a high-rise building. The scope and the method of construction used in the project decide how long it will take to finish the project. The scope of a high-rise building requires a longer time compared to low-rise structure. A typical high-rise building can take 1 ½ to 3 years construction time (Paxton, 2005) which may vary depending on the location, construction methods and procurement processes involved. Below is a sample construction process breakdown. (source: The Proposed ‘Pandinus Duplex Towers’ Bristol).
2. Economics of Construction
Costs of construction of a high-rise buildings range to hundreds of millions of dollars. It is very rare that an owner can afford the costs without outside assistance such as banks and other financial institutions. Sources of capital to finance the construction of a high-rise depend on the type of high-rise. Financing of high-rise depends on what type of business is going to be established. Basic sources of capital can be through equity finance, debt finance or internally generated funds. Usually, high-rise buildings are owned by large corporations that are financially capable. Residential types of high-rise are also sometimes financed by the government as a housing project. Residential high-rise buildings such as condominiums usually are owned by developers who lease or sell units in the building using the master plan in order to have the amount needed for the construction.
Cost of construction of high-rise building depends on the nature, size, location and management organization of the project. Project managers and designers such as the architect and engineer should realize that the costs are only estimated values and may vary depending on the changes, schedule adjustments and site conditions.
Capital cost for a project includes (Hendrickson, 1998):
· land acquisition, including assembly, holding and improvement
· planning and feasibility studies
· architectural and engineering design
· construction, including materials, equipment and labor
· field supervision of construction
· construction financing
· insurance and taxes during construction
· inspection and testing
· owner’s general office overhead
Engineers and contractors used different types of cost estimation and should be weighed and compared to determine the most cost-effective estimate.
3. Construction Methods
One of the most important factors which influence the cost-effectiveness for the implementation of projects like construction of high-rise building is matching the construction method to the design structure. The type of high-rise should be defined at the start of the design, whether it may be a commercial high-rise such as a hotel or a residential high-rise. Furthermore, the desired quality of the project also depends on the utilization of the best suited system equipment and materials.
3.1 Construction Equipment
In every construction project, it is important that proper equipments and materials are selected carefully for it affects the required amount of time and productivity of a project. The project manager and the architect/engineer must be familiar with the characteristics and uses of equipments commonly used in the construction of high-rise buildings.
In the construction of high-rise buildings, equipments with proper characteristics and size must be chosen carefully. Selection of equipments could be affected by factors (Hendrickson, 1998) such as size of the job; availability of equipment; cost of transportation of equipment; the construction methods to be used; location of dumping areas; and weather and temperature.
Excavation and Loading
One most commonly used equipment for excavation is classified as a crane shovel which consists of three major components: (1) a carrier or mounting which provides mobility and stability for the machine; (2) a revolving deck or turntable which contains the power and control units; and (3) a front end attachment which serves the special functions in an operation (Hendrickson, 1998).
There are types of mounting for different machines which has its own use depending on the job site. Crawler mounting is suitable for crawling over relatively rugged surfaces at a job site while truck mounting and wheel mounting provide greater mobility between job sites but require better surfaces for their operation. The revolving deck has a cab to house the person operating the mounting and revolving the deck. Front end attachments include a crane with hook, clam shall, dragline, backhoe, shovel and pile driver (Hendrickson, 1998).
A tractor which consists of a crawler mounting and a non-revolving cab is used for loading. When an earth moving blade is attached to the front end of a tractor, the assembly is called a bulldozer and when a bucket is attached to its front end, the assembly is called a loader or bucket loader (Hendrickson, 1998). Different types of loaders are used to handle different types of materials of different weights and moisture contents.
Scrapers are used to facilitate the loading and hauling of earthwork and have multiple units of tractor-truck and blade bucket assemblies with various combinations. Types of scrapers include single-engine two-axle scrapers, twin-engine all-wheel drive scrapers, elevating scrapers, and push-pull scrapers; each has different characteristics of rolling resistance, maneuverability stability and speed in operation (Hendrickson, 1998).
Lifting and Erecting
The most commonly used equipment for lifting of materials in building constructions is the derrick which consists of a vertical mast and an inclined boom sprouting from the foot of the mast. The mast is held in position by guys or stifflegs connected to a base while a topping lift links the top of the mast and the top of the inclined boom. A hook in the road line hanging from the top of the inclined boom is used to lift loads. Guy derricks may easily be moved from one floor to the next in a building under construction while stiffleg derricks may be mounted on tracks for movement within a work area (Hendrickson, 1998).
For lifting loads to great heights and to facilitate the erection of steel building frames, tower cranes are used. Horizon boom type tower cranes are most commonly used in high-rise building construction. Inclined boom type tower cranes are also used for erecting steel structures (Hendrickson, 1998).
3.2 Elements of High-rise Buildings
3.2.1 Structural Loads
Loads imposed on a building can either be primary or secondary. Primary loadings includes the materials used in building the structure, the occupants, their furniture, the direct influence of various typical weather conditions as well as unique loading conditions experienced during construction (Luebkeman, 1995) while secondary loads are loads due to temperature changes, construction eccentricities, shrinkage of structural materials, settlement of foundations or other such loads. With its definition, secondary loadings may be analyzed by analyzing the materials used. On the other hand, primary loads can either be dead loads or live loads depending on how they act upon the structural elements.
Dead loads are considered to act permanently on the structure. The largest portion of the dead loads is normally the weight of the structural members and will vary on the materials chosen for the members. The weight of the roofing, flooring, pipes, ducts, interior partition walls, elevator, machineries and other construction systems within the building should all be included in the calculation of the total dead load (Luebkeman, 1995). Error in the computation of the total dead loads should only be + 5%. These loads all vary depending on the properties of materials. Specifications of properties are often provided by books or the manufacturers themselves. Self-load or self-weight of materials are often expressed as force.
Common Unit Weights of Massive Building Materials
Common Unit Weights of Sheet Building Materials
(Source: Luebkeman, 1995)
Live loads are those that can change in magnitude and are transient (Luebkeman et al, 1995) which include all items found within a building such as people, computers, and furniture.
Each part of the building is designed for a specific function such as theater, residential unit, or restaurants, each with expected varying loads. Loads of a residential is different from the load of the restaurant which is expected to have more loads. Therefore, each part of a building is designed to carry the expected loads and some of the columns and the foundation can be reduced for economical purposes. The probable maximum live loads can be determined through research and are usually included in building codes and regulations. Reduction of live loads involves computations with formulas set by building codes.
Lateral loads are loads with horizontal force acting on the structure, typically a wind load against a façade, an earthquake, the earth pressure against a beachfront retaining wall or the earth pressure against a basement wall (Luebkeman et al, 1995). The intensity of lateral loads varies according to the location, structural materials, height and shape of the building. High-rise buildings are prone to these loads due primarily to its height.
Wind load is commonly a wind against a building, applying pressure on the side walls and even on the roof of the building from unexpected directions. Factors that effect the wind load include the geographic location, elevation, degree of exposure, relationship to nearby structures, building height and size, direction of prevailing winds, velocity of prevailing winds and positive or negative pressures due to architectural design features (atriums, entrances, or other openings). All of these factors are taken into account when the lateral loads on the facades are calculated. It is often necessary to examine more than one wind load case (Liebkeman et al, 1995).
Earthquake loads also known as seismic loads are caused by lateral forces or ground movement. This force depends on the intensity of the earthquake, the soil conditions and stiffness of the building. The ability of a structure to withstand an earthquake hinges on whether the structure was properly designed, detailed and constructed to resist the lateral loading created by the earthquake as well on the appropriate quality of the materials used. Maximum seismic force that a building can withstand is usually based on building codes and regulations.
There are also special load cases such as impact loads and blast loads coming from outside sources. One case of impact loads is the impact of the hi-jacked airplane on the Twin Towers in New York.
The foundation consideration of a high-rise building depends on the type and condition of soil on the building site. Geotechnical investigation should be done by soil engineers. Due to the heavier loading, high-rise foundations are a major component of the design. High-rise foundation is a deep-type foundation which includes piles, pile walls, diaphragm walls and caissons. Where soil conditions are poorer, special attention must be made to ensure differential settlement values will not have a detrimental effect (Zils & Viise, 2003).
The foundation of a building is the structure which supports it in the ground. The forms and materials used in building foundations vary according to ground conditions, structural material and structural type (EDC, 2006). The foundation wall or basement of most buildings resists the lateral load of the soil that the basement is constructed in but does not have significant role in carrying the structural loads from the tower (EDC, 2006).
Deep foundation that is the usual and most effective type of foundation for high-rise structure can be piles, piers, or caissons as mentioned above. Pile foundation is the most commonly used foundation when economic, constructional or soil considerations are given much of the importance and when it is desired to transmit loads to strata beyond the practical reach of shallow foundations. Aside from supporting structures, piles are also used to anchor structures against uplift forces and to assists structures in resisting lateral and overturning forces common to high-rise buildings. Therefore, this study is focused on pile foundation; the type of pile most appropriate for high-rise buildings and the method of construction applied for it.
Pile Foundations: Pile foundations consist of vertical structural members that are forced into the ground by impact from a machine called a pile driver. Pile foundations used to carry and transfer the load of the structure bearing ground at some depth below ground surface. The main components of pile foundation are the pile cap and the pile. Piles are long and slender members which transfer the load to deeper soil or rock of high bearing capacity avoiding shallow soil of low bearing capacity (Abebe & Smith, 2005). Pile cap is a thick slab of reinforced concrete poured across the top of a pile cluster to act as a unit in supporting a column or grade beam.
Materials Used for Pile:
Today the most commonly used materials for the foundation of high-rise buildings are steel and concrete. Each has its own advantages and disadvantages over the other and is used depending on the above mentioned factors.
Steel Pile: Steel or iron piles are suitable for handling and driving in long lengths. Their relatively small cross-sectional area combined with their high strength makes penetration easier in firm soil. The piles are easier to handle and can easily be cut to desired length. Steel piles can be driven through dense layers, in hard and in very long lengths. Steel piles can carry heavy loads and can be anchored in clopping rocks. On the other hand, steel piles will corrode and are relatively expensive.
Concrete Pile: Pre-cast or pre-fabricated concrete piles are usually of square. Circle or octagonal section and are produced in short length in one meter intervals between 3 and 13 meters. They are pre-cast so that they can be easily connected together in order to reach to the required length but this will not decrease the design load capacity. Reinforcement is required within the pile to help the pile withstand handling and driving stresses. Pre-stressed concrete piles are becoming gaining popularity than ordinary pre-cast as less reinforcement required (Abebe & Smith, 2005).
Piles are arranged with regards to how they can support load imposed on them. Vertical piles can be designed to carry vertical loads as well as lateral loads and are sometimes required to be combined with raking piles to support horizontal and vertical forces (Abebe and Smith, 2005).
There are factors that affect the choice of piles that must be considered during the design process in order to come up with the most suitable foundation that will make a high-rise building of good quality. These factors include location and type of structure, costs and the type and condition of soil. For heavy structures exerting large foundation load such as a high-rise building, large diameter bored piles are usually the most economical. Heavy structures such as high-rise towers and industrial buildings need foundation piles which can bear a load of several thousand tons, often in unstable or difficult soil conditions.
Large diameter bored piles are non-displacement piles. Non-displacement pile is a type of pile construction wherein soil is removed and the resulting hole filed with concrete or sometimes pre-cast concrete pile is dropped into the hole and grouted in. this method of construction produces irregular interface between the shaft and surrounding soil which affords good skin frictional resistance under subsequent loading (Geotechnical, 2006).
Large boreholes from 750mm up to 30m diameter with 7m under reams are possible by using rotary drilling machinery. A spiral or bucket auger as shown in figure is attached to a shaft known as a Kelly bar, a square section telescopic member driven by a horizontal spinner. Depths of up to 70m are possible using this technique. The use of bentonite slurry in conjunction with bucket auger drilling can eliminate some of the difficulties involved in drilling in soft silts and clays, and loose granular soils, without continuous support by casing tubes. One advantage of this technique is the potential for under reaming. By using an expanding drilling tool the diameter at the base of the pile can be enlarged, significantly increasing the end bearing capacity of the pile. However, under-reaming is a slow process requiring a stop in the augering for a change of tool and a slow process in the actual under-reaming operation. In clay, it is often preferable to use a deeper straight sided shaft (Geotechnical, 2006). In addition to bearing vertical loads, bored piles are a good method for bearing lateral and bending loads such as those induced by wind and seismic events.
3.2.3 Structural System
The structural system of a high-rise building is designed to transmit vertical and horizontal loads from the point of application to the foundations by the most efficient path with minimum impact on the economy and function of the other elements of the building (Anonymous, 2006). The figure below illustrates the typical structural elements of a high-rise building.
The structural engineer must consider many different factors before selecting the final structural system of a high-rise building project. Basic building properties such as height, shape, and usage as well as local economic conditions that affect the materials and labor costs; construction schedule, design loads both vertical and lateral; building behavior and occupant behavior; foundation considerations and coordination with mechanical systems (Cohen, 1986) are all factors that influence the structural system.
Generally, structural systems are flexible and limitations only come from the material used in the construction. Typical materials used for structural systems are steel and concrete.
Steel is a common material used for the construction of large-scale frame system used in high-rise buildings and is popular due to its high strength to weight ration and variety of members can be fabricated from it. Steels can be wide flange sections, open web joists, tube steel, angles, plates and channel sections each one has its uses depending on its characteristics.
Wide flange sections are used for their high strength and easy connectability while open web joists are used for their economic value because of their low weight. Tube steel, also known as Hallow structural section, is used for its high resistance to torsion and is ideal for members that span long, unbraced distances. Angels are most common for beam-to-beam and beam-to-columns connections as well as to form truss structures. Plates are also commonly used for beam-to-beam and beam-to-column connections while channel sections are easy to bend thus commonly use in curved areas (Miller et al, 1999).
Steel frames are fabricated in pieces and then shipped to the building site. The method used is determined by the forces that the connection of pieces or sections must transfer. Beam to column connections usually consists of plates or angels that are bolted or welded to the members. If the members are wide flange sections, the angles or plates are connected directly to the web or flange of the section (Miller et al, 1999). If the members are tube steel section, a slot is cut in the sides of the section and a plate is inserted through the tube. If a pile system is being used, the piles are either driven into the ground using a pile driver, or they are concrete that is poured into a drilled hole. The pile cap is then poured over the piles (Miller et al, 1999).
Concrete is a name applied to any number of compositions consisting of sand, gravel, crushed stone, or other coarse material, bound together with various kinds of cementitious materials, such as lime or cements. Concrete is a combination of aggregate, of which sand is always a part, together with gravel, stone chippings, or crushed slag, and lime or cement to bind the aggregate (Coney, n.d.). It has a high compressive strength and it is often used in buildings that are designed for large vertical loads such as parking garages. There are two major categories of concrete structural components: cast in place and the pre-cast concrete.
Cast in place concrete is concrete that is poured on the building site and cures in place. The steel reinforcing aids the concrete in resisting tensile stresses. The amount of reinforcing and its configuration contributes to the strength of the concrete and the strength of the system as whole. Another method to reinforce concrete is with the use of cables that are placed inside the concrete when it is poured and then are tensioned as the concrete cures. This method is called post-tensioning. The tension cables form a steel net that lends strength to the concrete and act to compress the concrete. This helps tension forces created by loading conditions and increases the bearing capacity of the concrete (Miller et al, 1999). On the other hand, pre-cast concrete is poured into moulds and allowed to cure before it is transported to the building site. The reinforcing is placed in the moulds before the concrete is poured.
3.2.4 The Basic Structural Systems in High-Rise Building.
Steel: Staggered Truss. This system is a basic steel building frame with concrete floor system. The system consists of a series of story-high trusses spanning the total width between two rows of exterior columns and arranged in a staggered pattern of adjacent column lines (Scalzi, 1971) as seen in the figure.
The staggered truss system was developed by the architects and engineers at the Massachusetts Institute of Technology, MIT, to achieve a more efficient structural frame that resists wind loads and provides versatility of floor layout with large column-free areas (Scalzi, 1971).
The total frame of the system act as a cantilever beam when subjected to lateral loads. All columns are placed on the exterior wall of the building and function as the flanges of the beam, while the trusses which span the total transverse width between columns function as the web of the cantilever beam (Scalzi, 1971) as seen on the second figure.
Materials and Fabrication:
The material used for staggered-truss system is steel; truss members are usually rolled shapes, such as wide flange sections, structural tees, channels, or angles. Columns in most buildings used high strength steels. The fabrication of the trusses is by welding and/or bolting. Welding is done in the down hand position for each side of the truss. When one side is completed, the entire fixture holding the truss may be rotated 1800 to enable the welders to continue with down hand position on the newly exposed side (Scalzi, 1971).
Fabrication of staggered-truss system can be done by any shop engaged in structural steel fabrication. Fabrication involves the following components: columns; spandrel beams; trusses; secondary columns and beams; and floor system (Hassler, 1986).
Columns: The columns are required to support the total gravity and wind loads acting on the structure in the transverse and longitudinal directions (Scalzi, 1971). There is no bending of the building columns in the transverse direction because the flow of the transverse lateral loads is through the trusses and the floor system. Drift of the structure in the transverse direction is a function of the floor system and truss stiffness and the column cross-sectional areas (Cohen, 1986). It will be therefore be advantageous to position the columns with their weak axis parallel to the longitudinal direction where the columns bend in their strong axis.
The column section selection is based on the design consideration of all the axial loads and moments acting in the transverse and longitudinal directions of the building. About 90 percent of the load received by the column is from the top chord connection to the truss as shown in the figure at the right. Therefore, when designing the structural system, it assumed that all the loads on the column are applied at the top chord connection of the truss to the column.
For buildings up to 20-storey high, column will be rolled wide-flange up to W14 x 720 with the longest being approximately 25 ft in length and with a total weight of 9 to 10 tons. Columns for buildings up to 20 to 30 stories will be reinforced with flange plates as shown in Fig. 4. Above 30 stories will built-up type columns consisting of three plates of varying thickness (Hassler, 1986).
Spandrel Beams: Spandrel beams, which are designed to resist the wind moment imposed on the end walls in the longitudinal direction, can be steel beams moment-connected by en plate connections or flange plates top and bottom with a web-shear plate (Hassler, 1986). When using end plate connections, the matching holes on the column flange plates or the flange width dimension should be checked and matched with beam to ensure correct center to center of columns due to rolling tolerances. The beam requires to a 2-hour fire rating in accordance with most building codes (Hassler, 1986).
Truss: The story-deep truss is the basic element of staggered truss system, which spans the full transverse width of the building at alternate floors on each column line (Cohen, 1986). The story-deep trusses are also required to support the gravity loads directly, and provide the necessary resistance to the lateral loads (Scalzi, 1971). The truss must provide an opening near the center of span to have a width and height that have sufficient proportions to be used as a corridor. The chords of the truss should be kept to minimum width to have a wall of minimum thickness and at the same time a sufficient width to provide a seat for the floor system (Scalzi, 1971).
Gravity loads from the floor system are applied as concentrated loads at panel points of the top of and bottom chords. In order to design the truss members, whose sizes are determined on the basis of the usual assumptions for pin-connected trusses simply supported at the ends and adjusted for local bending, symmetrical and unsymmetrical live loading should be investigated. The gravity loads are delivered to the truss from the floor slab which spans from top of the chord of one truss to the bottom chord of its adjacent truss. Therefore, each truss is loaded at its top and bottom chord and the total gravity load of the building is transferred to the building’s exterior columns.
The trusses are sized to carry two floors of building gravity loads and span the entire building width. The member trusses are designed to be sufficient enough to provide the required lateral stiffness and due to the one-third increase in allowable stresses, may not have to be increased in weight for the wind forces (Cohen, 1986).
At the top and second level of the building, it is not practical to place a truss in the staggered arrangement. Posts and hangers are used to support the roof and the second floor which are usually placed at the panel points of the trusses in order to avoid secondary moments in the chords. The trusses supporting the roof and the second level of the building is necessary to have heavier members than the other trusses.
The weight of a truss is determined by gravity loads, wind load, penetrations due to corridor, doors, pipes, and window openings (Hassler, 1986) and varies depending on the number of storey of the building. A 43-storey building can weigh of up to 36 tons.
Secondary Supporting Structures: Secondary supporting structures are those necessary to support stair openings, elevator shafts, and other framed openings required for architectural designs (Hassler, 1986). The architect should avoid unnecessary additional framing with his design because additional secondary additional supports add up to the total cost of the project. These supports can be additional steel framing or support steel to resist wind moments (Hassler, 1986).
Floor System: The floor system is an important element for the staggered-truss system to function properly. The floor must function as a shear diaphragm to resist lateral loads. The first procedure is to determine the required thickness of the floor for gravity loads and to verify the in-plane shear capacity and stiffness.
The floor system can be a series of simple spans or continuous for two column spacings. As a continuous system, the floor members rest on the top chord of one truss and extend to the bottom chords of the two adjacent trusses (Scalzi, 1971).
A truss at any level carries the cumulative lateral load from the total building above over a two-bay width (Cohen, 1986). This load must be transfer by the floor area on each side of the truss to the top chord of the adjacent truss in the story below. The floor system must be designed to provide sufficient in-plane diaphragm strength and stiffness to sustain the lateral forces and gravity loads. In general, the total lateral load due to wind forces is distributed equally among the trusses at any given floor level.
The floor system acts as a deep beam and must be designed to resist the in-plane shears and deformations and the resulting in-plane bending moments. The in-plane bending moment at the connection of the floor to the truss is determined on the assumption that a point of inflection occurs at midpoint of the adjacent spans. Its magnitude can be calculated by multiplying the in-plane shear force by the distance from the column line to the midpoint of the adjacent span (Scalzi, 1971).
The connection of the floor system to the chords of the trusses must be capable of transmitting the gravity loads and the in-plane shear loads directly to the chord members. Gravity load are transmitted by direct bearing contact while the in-plane shears are transferred by shear connections such as direct welding when a steel deck is used or by a welded shear plate when concrete slabs or planks are used. The connection to the chord member is made according to the shear distribution along the truss in the transverse direction of the building (Scalzi, 1971).
As noted above, floor systems are designed to deliver the wind load as a diaphragm or shear load to the supporting trusses. Floor systems such as steel deck with infill, steel joists with sufficient concrete topping, concrete slabs, and concrete planks are the ones usually used for staggered-trusses system.
Staggered-truss system is erected on a floor-by-floor sequence; the erection sequence must be programmed to include every structural component as the building rises (Scalzi, 1971). Erection can be done using a tower crane such as the hammerhead or by mobile truck or crawler crane with tower attachment. A tower crane has to be used to reach over the spandrel beams and must have the range and capacity to cover the entire floor area (Hassler, 1986) for the building to remain stable.
A tower crane, self-supported, truck or cat mounted can be used for 20-storey buildings. An external climbing crane supported on steel tower units and connected to the structure of intervals of 75 ft, with the initial tie-in at 160 ft. are used for buildings above 20 stories (Hassler, 1986).
The first step in erection method is to set the columns followed by setting the spandrel beams to tie columns along strong axis. Then, set trusses; connection of the bottom chord should be bolted tight after the dead load is imposed from the floor system. The bottom chord should be shortened to allow for camber reduction and subsequent chord lengthening (Hassler, 1986). The next step is to bolt up and torque high-tension bolts. The floors must be firmly connected to the truss chords before each new set of columns is erected to provide an immediate bracing system as the building becomes taller.
Framed Tube: This system utilized rigid steel or concrete frames with closely spaced wide columns in combination with shear walls at the central elevator core (Cohen, 1986). The overall frame is a cantilever Framed tube fixed at the ground. The effectiveness of the cantilever depends on the minimization of the part of the sway deflection due to the shear frame.
Steel framed tube buildings involve column spacings of 3m to 5 m on the exterior which can be transferred or transitioned into wider spacings, if required, at the lower storeys to integrate street leval activities. This system has been used extensively for structures of 30 to 110 storeys in height such as the World Trade Center in New York. Tubular systems are generally adaptable to prismatic vertical profiles but are disadvantages to varying vertical profiles and buildings involving significant fascia offsets because of the discontinuity required in the tubular frame to achieve the shape (Anonymous, 2006).
Framed tube structure is usually located on the perimeter of the structure and introduces more stiffness because of the added strength of the wide columns and deep beams (Anonymous, 2002). Stiffness is used to overcome the potential problems caused by the horizontal sway in tall buildings which occurs during earthquake.
Frame and Shear Wall: This system is a combination of shear walls and frames to resist lateral wind loads (Cohen, 1986) and seismic lateral forces in proportion to their rigidities. Frames are usually moment resistant frames whose beams are usually welded to columns or bolted solid so that column movement is imparted into the beams as bending moments because of the rigid connection between the two.
Shear wall provides resistance to horizontal forces; it usually constructed of reinforced concrete or steel for high-rise building. The walls provide natural stiffness by limiting deflections and the use of lintels between coupled walls will also provide ductility and energy-dissipating characteristics to help protect the walls and the entire structures from excessive damage (Anonymous, 2002). The frames and shear walls act together to resist lateral loading. The frames are assumed to be coupled to the shear walls and designed for concrete buildings of up to 50 storeys.
3.2.5 Curtain Wall and Façade
Curtain wall is the external wall of a high-rise building with large area of glazed portion that carries no superimposed load except wind load (Wong,). Traditional curtain wall is that of a metal frame system infill with transparent or opaque panels that provide glazing for window openings and to cover-up structures like columns, slabs and beams, or solid wall.
Requirements for Curtain Wall:
· Strength and Stability. Curtain wall must be able to resist wind forces and transmitting them reliably to the building structure. Walling units should be able to take up positive and negative wind pressure and must be able to avoid damage due to deflection or distortion under expected building movements. Members of the walling frames must be strong and durable which should be made of strong and corrosion-resisting materials.
· Weather resistance. Curtain wall system should have the ability to keep out water and wind by using resistant infilling or paneling materials such as glass, metal, plastic sheet, or stone slab. Curtain walls should have appropriately designed jointing or sealing provision.
· Thermal insulation and condensation. High-rise buildings usually have air conditioning for cooling thus need curtain wall that can minimize the loss. Double-glazing of appropriate material and design properly insulated with thermal effective materials under the right design.
· Sound insulation. Many high-rise buildings are situated within congested downtown area and near a super highway, having a background noise of more than 70dB. Curtain wall systems should be designed to reduce the noise entering the building.
· Fire resistance. Curtain wall materials should be fire resistant and by the use of fire resisting construction in the non-glazing portion of the wall, and sealing all voids and gaps between such compartments by fire resisting materials (Wong, )
Aluminum and glass curtain walls are the most commonly used curtain walls in high-rise buildings. The curtain wall is characterized with colored vision and spandrel glass areas, a grid of aluminum caps and most recently with metal or stone spandrel covers. It is also combined with other types of cladding systems such as pre-cast, brick or stone to create attractive and durable building facades (Quirouette, 2006).
Curtain walls can be classified by their method of fabrication and installation. Stick systems and unitized are the general categories of curtain walls. In the stick system, the curtain wall frame, or what is called mullions, and glazing panels are installed and connected together piece by piece (Vigener, 2005) while in the unitized system, the curtain wall is composed of large units
that are assembled and glazed in the factory, shipped to the site and erected on the building.
Unitized System Stick Built System
To connect a curtain wall to any part of the building, the architect/engineer must meet the requirements of the building codes in the location; understand the features of curtain walls as well as the construction methods. Curtain walls must also undergo testing to determine and ensure the strength of the system as well as the failure mode at ultimate load.
The stick built system is installed by hanging the vertical mullion from a floor edge with a steel angle, while sliding the lower end of the vertical mullion over an insert anchor in the vertical mullion. Vertical mullions are spaced from 1.25 meters to about 1.85m depending on the spacing of columns, the wind load and the desired appearance of the facades. The joint between the vertical mullions is also an expansion joint for the floor-to-floor live load deflections, any concrete structure creep movements as well as thermal expansion joint for curtain wall components. The rails or the horizontal mullions are then attached to the vertical mullions to create frame openings; one frame opening for the vision area to receive an insulating glass unit, IGU, and one frame opening to receive the spandrel pane cover to hide the floor edge, perimeter, heating equipment and ceiling plenum areas (Quirouette, 2006).
Unitized curtain wall system has the same components like the stick built curtain wall system. However, unlike the stick built system, instead of assembling the glass and aluminum curtain wall in the site, most of the system components are assembled in a plant under controlled corking conditions, which promotes quality assembly and allows for fabrication lead-time and rapid closure of the building (Quirouette, 2006).
Although the unitized system offers the advantages of quality assembly and speed of on building closure, there is also a disadvantage; it has a third joint at the junction between the half mullion and half rails that increase the potential of air and water leakages by 50% over a stick built system which has only two joints along every mullion and rail (Quirouette, 2006). The stick built system is the most commonly used but due to the requirements for speed of construction today, the unitized system has gained popularity.
3.2.6 Service Systems
Modern buildings today utilize service system and facilities. These facilities are included in the design and construction of the building and are provided for the comfort and safety of the occupants of the building.
Vertical Transportation: One of the features and requirements of high-rise buildings is the vertical transportation system, usually an escalator or an elevator. It is a necessary feature as it is used to move people and materials from one level to another easily. Vertical transportation systems are installed and designed by elevator contractors following the requirements of building codes for vertical transportation.
In modern high-rise buildings, each lift is not usually required to service every level as this would imply a number of stops. This led to the introduction of zoning concepts. Zoning is where a building is divided into so that a lift or a group of lifts is constrained to serve only a designated set of floors (Barney, 2003), making the journey time shorter. For example, in a 30-storey commercial building, one lift may serve the ground floor, grounds 1, 3, 5, 7, 9, and all odd numbered floors while another lift serves the ground floor and all even numbered floors. This concept is also called as interleaved zone.
A stack zone, on the other hand, is where a tall building is divided into horizontal layers, in effect, stacking several buildings on top of each other, with a common ‘footprint’ in order to save ground space. each zone can be treated differently with regard to shared or separate lobby arrangements and grade of service (Barney, 2003).
Double-decker lifts comprise two passenger cars, one above the other, connected to one suspension/drive system (Barney, 2003); the upper and lower decks can thus serve two adjacent floors simultaneously. This kind of lift is common in the United States and other countries and is used in very tall buildings such as the Petronas Towers in Kuala Lumpur Malaysia.
Electrical systems of high-rise buildings should be designed in relation to the anticipated functional needs and life safety and design standards. An electrical engineer is the one responsible for the design of electrical systems including the report and detailed descriptions of the power system, lighting and telephone system as well as the estimated costs for the materials.
Voltage ratings and standards differ from one country to another and also dependent of the type of building. Commercial buildings like hotels, offices, or mixed type of buildings are expected to have more energy consumptions compared to residential types. Therefore, electrical wiring and systems are dependent on the functions of the building.
Generally, electrical system is part of the service system of a building that brings power from the pole or other point on the exterior power distribution line to the point on or inside from which it is distributed to the building circuits. Electrical system consists of the service conductors and the service equipment.
Service conductors supply power from the pole or other point on the exterior distribution system to the building. From the service conductors, electrical power is brought through a service entrance to the service equipment on or inside the building. The service equipment is the necessary equipment, usually consisting of a circuit breaker or switch or fuses, that is located near the entry point of the supply conductors to the building. This equipment is the main control and means of cutting off the power supply to the building (Anonymous, 2006).
HVAC Systems (Heating, Ventilating and Air Conditioning)
HVAC system is a required service of a building. Heating is required to maintain winter indoor space temperatures and during summer for space dehumidification control. Ventilating refers to the proper amount of outside air for occupant comfort and to replace air exhausted from a building. It also refers to the necessary air movement in an occupied space to maintain indoor air quality or IAQ. Air conditioning refers to the processes of heating, humidifying, cooling, dehumidifying, and filtering of air to maintain indoor space comfort and IAQ for occupants; also known as cooling system for a building (AMEC, 2004).
HVAC systems typically exceed more than 30% of the total power consumption in a building. Design and construction of HVAC system should consider some parameters. Building parameters include the location, exterior and interior temperature while the local parameters include the building area, number of stories, ceiling height, plenum height, occupants and the hours of operation. The system parameters include the total airflow and the cooling and heating load. This system is designed and understood by most mechanical engineers.
Fire Protection Systems
Fire Protection systems are designed to prevent personal injury and minimize property damage in a building and must be selected properly based on the potential of a fire, the type of fire expected, the characteristics of the area to be protected and the occupancy of the building. There are two main categories of fire protection system: the detection systems and extinguishing systems (EH&S, 2002).
Detection systems are automatic electronic equipment that use a variety of sensors to detect combustion by-products and heat from fires. The choice and positioning of detectors is based upon room dimensions, use and typical contents (EH&S, 2002). Large detection systems usually are configured into groups of detectors within subdivisions of the building. Detection systems are sometimes connected to elevators and fan systems in order to shut them down in certain situations (EH&S, 2002).
Extinguishing systems are those that can reduce the fire. These may include fire extinguishers, sprinkler systems and pipe systems. Pipe systems are installed with fire-hose connections ate designated locations that provide the fire department with a convenient way to get water into the building (EH&S, 2002).
Construction of high rise buildings has many considerations as well as options for the owner and the designer. Before, when one speaks of high-rise building, steel structure will be recommended and when one to opt for a more economical structure, use of concrete will be recommended. Today, driven by technology and innovations, both concrete and steel materials are used to build a high-rise, making the structure more stable as well as cost-effective. High-rise buildings have unique requirements compared to ordinary infrastructure because of its height that make high-rise buildings prone to risks brought about by different loadings such as wind and earthquake loads.
High-rise buildings are becoming more important today due to scarcity of land aside from its high cost. Also, increase in population and increasing demand for office establishments promoted the utilization of high-rise buildings.
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