Alexandria University Faculty of Engineering Civil Engineering Department
Third Year – Year – First First Term
Presented By: Ahmad Said Helmy Al-Abassy Section: 1 Seat Number: 36
Steel Structures
Contents 1. Introduction 1.1 Benefits of Structural Steel 1.1.1. Erected by Skilled Specialists 1.1.2. Rapid Waste-Free Assembly Assembly 1.1.3. Offsite Fabrication for Precision Parts 1.1.4. Reduce Loads on Foundations 1.1.5. Economic Benefits 2. Bridges 2.1. Types of Steel Bridges 2.1.1. Suspension Bridges 2.1.2. Cable-Stayed Bridges 2.1.3. Arch Bridges 2.1.4. Truss Bridges 2.2. Span of Bridges 3. High-Rise Buildings 3.1. Structural Steel Framing Options 3.1.1. Rigid Frames 3.1.2. Semi-Rigid Frames 3.1.3. Braced Frames 3.1.4. Rigid Frame and Br aced Frame Interaction 3.1.5. Outrigger and Belt Truss Systems 3.1.6. Tube Structures 3.1.6.1. Framed Tube 3.1.6.2. Truss Tube 3.1.6.3. Bundled Tube 3.1.7. Other Steel High Rise Buildings 4. Heavy Duty Plants 4.1. Anatomy of Structure 4.1.1. Main and Secondary Beams 4.1.2. Connections 4.1.3. Bracing, Stiff Walls or Cores 4.1.4. Loading 4.1.4.1. Process Plant and Equipment 4.1.4.2. Lateral Loadings from Plant 4.1.4.3. Wind Loadings 4.1.4.4. Blast Loadings 4.1.4.5. Thermal Effects 5. Masts and Towers 6. Long Span Structures 6.1. Classification of Long Span Structures 6.1.1. Beam Structures 6.1.2. Portals and Arches 6.1.3. Masted Structures 6.1.4. Space Frames 6.1.5. Umbrella Structures 6.2. Claddings 7. References
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1. Introduction Structural steel is an essential component of most stadia, shopping centres and commercial developments; steel cladding systems adorn iconic, landmark structures worldwide. Steel is one of the most sustainable construction materials. Its strength and durability coupled to its ability to be recycled, again and again, without ever losing quality make it truly compatible with long term sustainable development. It provides light, open, airy spaces, making it ideal for reconfiguring, extending or adapting with minimal disruption, and without costly and sometimes harmful demolition and redevelopment.
1.1. Benefits of Structural Steel
1.1.3. Offsite fabrication for precision parts Fabrication of the individual steel elements takes place offsite under controlled, highly regulated factory conditions where the use of leading edge fabrication systems delivers precisionengineered components. With so much work carried out offsite, the on site construction programme is reduced and the building programme is relatively unaffected by adverse weather conditions. Furthermore, steel components can be pre-assembled or fabricated into modules either offsite or at low level, which reduces the need for working at height. Steel can be delivered to site as and when it is required, reducing the need for potentially hazardous on site storage.
1.1.1. Erected by skilled specialists Steelwork contractors are highly skilled specialists, trained in a single discipline. They must hold industry recognised qualifications in the design, fabrication and erection of steel-framed structures. That means the steel elements are quickly and accurately assembled on site following proven, reliable and safe techniques. What’s more, unlike concrete, steel frames are full strength as soon as they are completed, enabling stairs to be fitted and providing safe access to the structure for other trades straight away. Steel decking for composite slabs also provides a safe platform after installation, as well as protection to lower storeys. 1.1.2. Rapid, waste-free assembly All steel structural elements can be very precisely fabricated to tight tolerances before delivery to site, facilitating rapid and waste-free assembly. From the very beginning, specialist designers work with the construction team to ensure that the steel frame design can be manufactured and erected safely. Erection procedures can be planned in detail using 3D models and, for the most complex structures, trial assemblies can be staged to ensure everyone understands the required procedure.
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The Hearst Tower is a building located at 300 West 57th Street, 959 8th Avenue, near Columbus Circle in Midtown Manhattan, New York City, New York.
1.1.4. Reduce loads on foundations Cost savings in steel buildings start at the foundations, where the loads imposed by a steel frame are much less than those of a concrete alternative. That means foundations can be smaller and therefore cost considerably less. Foundations are a major component of overall building costs, so lighter foundation loads can have a big impact on costs. 1.1.5. Economic benefits Fabrication in controlled factory conditions results in high quality, defect free components that produce very little waste during the construction process. Long span steel sections enable large open plan, column free spaces to be created inside buildings. Such ‘future-proofing’ means that the building’s use can be changed and the layout adapted many times – extending the lifetime of the structure. Short construction periods leads to cost savings in site preliminaries, earlier return on investment and reduced interest charges. Time related savings can easily amount to 3-5% of the overall project value, reducing the client’s requirements for working capital and improving cash flow.
The San Francisco – Oakland Bay Bridge. A complex of bridges spanning San Francisco Bay of the U.S. state of California. As part of Interstate 80 and the direct road route between San Francisco and Oakland.
2. Bridges For short and medium span highway bridges composite deck construction is economic because the slab contributes to the capacity of the primary members. For continuous spans it uses the attributes of steel and concrete to best advantage. In cases where construction depth is restricted, for example in developed areas, then half-through or through construction is convenient; this is common for railway and pedestrian bridges.
2.1. Types of Steel Bridges 2.1.1. Suspension Bridges Suspension bridges are used for the longest spans across river estuaries where intermediate piers are not feasible. The cables form catenaries supporting both sides of the deck and are tied to the ground usually by gravity foundations sometimes combined with rock anchors. Thus
ground conditions with firm strata at or close to the surface of the ground are essential. Towers are usually twin steel or concrete box members which are braced together above the roadway level. They are designed so as to be freestanding under wind loading during construction until the cables are installed. The construction process for suspension bridges is more time consuming than for other types because the deck cannot be installed until the towers, anchorages, cable and hangers are constructed. Depending upon ground conditions, the cables can be catenaries supporting side spans. Cables may alternatively be straight from tower top to the ground anchorages and merely support a main span, side spans being non-existent or formed as short-span viaducts. Decks are either trusses with a steel orthotropic plate floor spanning between or an aerofoil box girder. Footways are often cantilevered outside the two sets of cables.
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Aerodynamic behaviour must be considered in design because of the tendency for the deck and cables to oscillate in flexure and torsion under ‘vortex shedding’ and other wind effects. This is due to the flexible nature and light weight of suspension bridges. Suspension bridges behave as non-linear structures under asymmetric deck loading so that deflections may be significant. Behaviour under such loading depends upon the combined gravity stiffness and the flexural rigidity of the deck or stiffening girder. The type is less suitable for heavy loading such as railway traffic, especially for short spans. Suspension bridges are sometimes suitable for medium spans carrying pedestrian or light traffic.
The George Washington Bridge is a double-decked suspension bridge spanning the Hudson River, connecting the Washington Heights neighborhood in the borough of Manhattan in New York City to Fort Lee, Bergen County, New Jersey, in the United States.
The Humber Bridge, near Kingston upon Hull, England, is a 2,220 metres single-span suspension bridge, which opened to traffic on 24 June 1981. It is the seventh longest of its type in the world.
The Verrazano – Narrows Bridge is a double-decked suspension bridge that connects the boroughs of Staten Island and Brooklyn in New York City at the Narrows.
Hardanger Bridge is a suspension bridge between Ullensvang and Ulvik in Hordaland, Norway. The bridge is 1,380 m long, with a main span of 1,310 m. Sailing height is 55 m and the towers reach 200 m above sea level. The Golden Gate Bridge is a suspension bridge spanning the Golden Gate, the opening of the San Francisco Bay into the Pacific Ocean.
The Akashi Kaikyō Bridge links the city of Kobe on the mainland of Honshu to Iwaya on Awaji Island, in Japan. The bridge has the longest central span of any suspension
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2.1.2. Cable-Stayed Bridges Cable-stayed bridges are of a suspension form using straight cables which are directly connected to the deck. The structure is selfanchoring and therefore less dependent upon good foundation conditions, but the deck must be designed for the significant axial stress from the horizontal component of the cable forces. The construction process is quicker than that of suspension bridge because the cables and deck are erected at the same time and the amount of temporary works is reduced. Either twin sets of cables are used or alternatively for dual carriageways a single plane of cables and tower can be located in the central reserve space. Two basic forms of cable configurations are used,
either ‘fan’ or ‘harp’. A fan layout minimizes bending effects in the structure due to its better triangulation but anchorages can be less easy to incorporate into the towers. The harp form is often preferred where there are more than four cables. The number of cables depends on the span and cable size, which i s often selected such that each fabricated length of deck contains an anchorage at one end to suit a cantilever erection method. Bridges either have two towers and are symmetrical in elevation or have a single tower as suited to the site. Floors are generally an orthotropic steel plate but composite slabs can be used for spans up to about 250m. A box girder is essential for bridges having a single plane of cables to achieve torsional stability, but otherwise either box girders or twin plate girders are suitable. Aerodynamic oscillation is a much less serious problem than with suspension bridges but must be considered. It is essential to use cables of maximum strength and modulus at a high working stress so that sag due to self-weight, which produces non-linear effects, is negligible. During erection the cable lengths are adjusted or prestressed so as to counteract the dead load deflections of the deck arising from extension of the cables.
The Incheon Bridge is a newly constructed cable-stayed bridge of length 21.38 kilometres in South Korea with 6-lane motorwa .
The Pont de Normandie is a cable-stayed road bridge that spans the river Seine linking Le Havre to Honfleur in Normandy, northern France. Its total length is 2,143.21 metres – 856 metres between the two piers.
2.1.3. Arch Bridges Arch bridges are suitable in particular site conditions. An example is a medium single span over a ravine where an arch with spandrel columns will efficiently carry a deck with the horizontal thrust taken directly to rock. A tied arch is suitable for a single span where construction depth is limited and presence of curved highway geometry or other obstruction to the approaches conflicts with the back stays of a cable-stayed bridge.
The Zolotoy Bridge is cable-stayed bridge across the Zolotoy Rog bay in Vladivostok, Russia. Built in preparation for the 2012 APEC summit.
The Silver Jubilee Bridge is an arch bridge with a main span of 1,082 feet crosses the River Mersey and the Manchester Shi Canal at Runcorn Ga in Cheshire, En land.
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Kishiwada Bridge is a steel arch bridge located in Osaka, Japan. It has main span of 225m and length of 445 m.
The Xinguang Bridge is an arch bridge located in Guangzhou, Guangdong, China. Opened in 2008, it has a main span of 428 metres making it one of the ten longest arch bridge spans in the world.
2.1. 4. Truss Bridges
The Chaotianmen Bridge, is a road-rail bridge over the Yangtze River in the city of Chongqing, China. The bridge which opened on 29 April 2009 is the world's longest arch bridge.
The Sydney Harbour Bridge is a steel through arch bridge across Sydney Harbour that carries rail, vehicular, bicycle and pedestrian traffic between the Sydney central business district and the North Shore.
Bayonne Bridge is the fifth-longest steel arch bridge in the world, and was the longest in the world at the time of its completion. It connects Bayonne, New Jersey with Staten Island, New York.
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Through trusses are used for medium spans where a limited construction depth precludes use of a composite deck bridge. They are suitable in flat terrain to reduce the height and length of approach embankments and for railway bridges where existing gradients can not be modified. A truss may be unacceptable visually; a bowstring truss is an alternative solution. For short spans and medium spans up to 50m, trusses are generally less economic than plate girders because of higher fabrication cost. They are therefore adopted only where the available construction depth is not sufficient for composite beams.
The Warren truss consists of longitudinal members joined only by angled cross-members, forming alternately inverted equilateral triangle-shaped spaces along its length, ensuring that no individual strut, beam, or tie is subject to bending or torsional straining forces, but only to tension or compression.
The through truss bridge has a truss on each side that is connected across the top and bottom forming a box through which vehicles can pass. Iron plates are riveted together to create linear steel members joined to make triangular sections, which are fastened into rectangular sections, producing a strong web-like structure.
2.2. Spans of Bridges Factors which decide choice of span f or viaducts
Parker truss bridge is a Pratt truss design with polygonal upper chord. A ‘camelback ’ is a subset of the Parker type. The upper chord consists of exactly five segments. An example of Parker truss is Traffic Bridge in Saskatoon, Canada.
A Whipple truss is usually considered a subclass of the Pratt truss because the diagonal members are designed to work in tension. The tension members are elongated, usually thin, at a shallow angle and cross two or more bays (rectangular sections defined by the vertical members).
The Baltimore truss is a subclass of the Pratt truss. It has additional bracing in the lower section of the truss to prevent buckling in the compression members and to control deflection. It is mainly used in train bridges.
Town's Lattice Truss bridge is an alternative to heavy-timber bridges where planks arranged diagonally with short spaces in between them.
Factor
Reasons
Location of obstacles
Pier positions are often dictated by rivers, railway tracks and buried services
Construction depth
Span length may be limited by the maximum available construction depth
Relative superstructure and substructure costs
Poor ground conditions require expensive foundations; economy favours longer spans
Feasibility of constructing intermediate piers in river
(a) Tidal or fast-flowing rivers may preclude intermediate piers (b) For navigable waterways, accidental ship impact may crossings preclude mid-river piers
Height of deck above ground
Where the height exceeds about 15 m, costs of piers are significant, encouraging longer spans
Loading
Heavier loadings such as railways encourage shorter spans
The Fink truss bridge designed by Albert Fink of Germany in the 1860s.
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3. High-Rise Buildings 3.1. Structural Steel Framing Options Today there are innumerable structural steel systems that can be used for the lateral bracing of buildings. The different structural systems that are currently being used in the design for buildings are broadly divided into the following categories
Rigid frames Semi-rigid frames Braced frames Rigid frames and braced frame interaction Belt and outrigger truss systems Tube structures
All the systems have a theoretical maximum height at which point they become inefficient at transferring lateral loads. Many attempts has been made to loosely define the maximum heights associated with each system. Descriptions of each system and its range of applicability are detailed in the following sections.
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3.1.1. Rigid Frames The use of portal frames, which consist of an assemblage of beams and columns, is one of the very popular types of bracing systems in building design because of minimal obstruction to architectural layout created by this system. Rigid frames are most efficient for low rise to mid-rise buildings that are not excessively slender. To attain maximum frame action, the connections of beam to columns are required to be rigid. Rigid connections are those with sufficient stiffness to hold the angles between members virtually unchanged under load. It gets strength and stiffness from the nondeformability of joints at the intersections of beams and columns, allowing the beam in reality to develop end moments which are about 90 to 95 % of the fully fixed condition. Rigid frames generally consist of a rectangular grid of horizontal beams and vertical columns connected in the same plane by means of rigid connections. Because of the continuity of members at the connections, the rigid frame resists lateral loads primarily through flexure of beams and columns.
Rigid frames can have good ductile characteristics if detailed properly. Performance is very sensitive to detailing and craftsmanship of the connections. Designers have numerous options available for plastic design of rigid frames including elastic perfectly plastic and elastoplastic analyses. Plastic hinges (or equivalent elasto-plastic zones) form at the base of columns, beam-column connections, and in beam spans. Failure occurs if a mechanism of fully plasticized zones has developed. The number of fully plasticized zones depends on the redundancy of the frame. Ductility provides large capacities in the system, but typically their low system stiffness results in large deflections which can l ead to high non-structural damage in heavy winds or seismic activity. The rigid frame can prove to be quite expensive. Resisting the lateral load through bending of the columns exhibits inefficiency in the system and requires more material than would another structural system. Rigid frames also require laborintensive moment resisting connections. Limited field welding is desirable by using bolted connections where possible; however, achieving full rigidity of a connection with bolts only is nearly impossible. 3.1.2. Semi-rigid Frames Rigid frames require certain boundary conditions to develop its full frame lateral resistance potential. In such frames rigid connections are specified to assure the stiffest building frame. However, in other situations rigid framing may not yield the optimum solution. This is because heavier connection elements, along with fully developed welds or large connections, are needed to obtain the desired fixity. In addition the gravity moment induced in external or unsymmetrical loaded interior columns may offset the advantage of reduced beam bending requirements and their attendant economic reduction in beam weight. At the other end of the spectrum is the simple framing with very little resistance to bending (usually referred to as a pin connection). This framing requires some other provision for carrying lateral loads in building; shear wall, braced frames, or some other lateral
bracing system is required in the planning and design of the building. Semi-rigid connections can be defined as those connections whose behavior is intermediate between fully rigid and simple connections. Such connection offers substantial restraint to the end moment and can affect sufficient reduction in the mid-span moment of a gravity loaded beam. However, they are not rigid enough to restrain all rotation at the end of the beam. Although the actual behavior of the connection is complex, in practice simplified approaches are used in the design of such connections. 3.1.3. Braced Frames Rigid or semi-rigid frames are not efficient for buildings higher than 30 storeys because the shear racking component of deflection produced by the bending of columns and gi rders causing the building drift to be too large. A braced frame attempts to improve upon the efficiency of pure rigid frame action by providing a balance between shear racking and bending. This is achieved by truss members, such as diagonals, between the floor systems. The shear is now primarily absorbed by the diagonals and not by the girders. The diagonals carry the lateral forces directly in predominantly axial action, providing for nearly pure cantilever behavior. All members are subjected to axial loads only, thereby creating an efficient structural system.
Efficient and economical braced frames and compact braced frames can lead to lower floorto-floor, which can be an important economic factor in tall buildings, or in region where there are height limits.
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Any rational configuration of bracing can be used for bracing systems.
deep spandrels and additional columns on the building’s exterior because additional columns will not interfere with interior planning or circulation and the depth of spandrels may not present any problems for HVAC and additional mechanical and electrical ducts. As an alternative to perimeter frames, as et of interior frames can be made to act in conjunction with the core bracing. Yet another option would be to simply provide rigid girder connections between the braced core and perimeter columns. For slender building it becomes less practical to use interaction system of moment frames and braces whose depths are limited by the depths of building cores. An economical structural solution is to increase the bracing system the full width of the building. If done properly it would not compromise the architecture of the building. 3.1.5. Outrigger and Belt Truss Systems Traditional approaches to wind bracing for midheight structures is to provide trussed bracing at the core or around stair wells and to supplement the lateral resistance by providing additional moment connected frames at the other convenient plan locations. However, as building height increases, the core, if kept consistent with the elevator, stair well and other mechanical requirements does not have sufficient stiffness to keep wind drift at acceptable levels.
3.1.4. Rigid Frame and Br aced Frame Interaction Unreasonably heavy columns can result if wind bracing is confined to building’s braced service core because the available depth for bracing is usually limited. In addition, high uplift forces occurring at the bottom of the core columns can present foundation problems. In such instances an economical structural solution can be arrived at by creating rigid frames to act in conjunction with the core bracing system. Although deep girders and moment connections are required for the frame action, rigid frames are often preferred because they are the least objectionable from an architectural planning perspective. Although each building has its own set of criteria, many times architecturally it may be tolerable to use
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One way of limiting drift is a technique of using a cap truss on a braced core combined with perimeter columns. I this system columns are tied up to the cap truss through a system of outrigger and belt trusses. In addition to traditional function of supporting gravity loads, the columns restrain the lateral movement of the building, when the building is subjected to lateral forces, the cap truss restrains the bending of the core by introducing a point of inflection in its deflection curve. This reversal in curvature reduces the lateral movement at the top. The belt trusses act as horizontal stiffeners engaging the exterior columns which are not directly connected to the outrigger trusses. This system can improve stiffness up to 25 to 30 percent over the same system without outrigger trusses.
The revolutionary design of the tubular system essentially strives to create a rigid wall around the structure’s exterior. Since all lateral loads are resisted by the perimeter frame, the interior floor plan is kept relatively free of bracing and columns thus increasing rentable value. A tradeoff for structural efficiency is the reduction in window wall space of larger and closely spaced exterior columns. Maximum efficiency for lateral strength and stiffness using the exterior wall alone is achieved by making the entire building act as a hollow tube cantilevering out of the ground. The tubular action produces uniform axial stress on the flanges and triangular distribution on the webs, an efficient configuration for a cantilever structure as building. 3.1.6.1. Framed Tube
3.1.6. Tube Structures Tube design can be defined simply as a structural system that prompts the building to behave as an equivalent hollow tube. In past decades, tubular structures were several of the world’s tallest buildings. The tubular concept is credited to Dr. Fazlur Khan. Tubular system are so efficient that in most cases the amount of structural material used is comparable to that used in conventionally framed buildings half the size. Their development is the result of the continuing quest for structural engineers to design the most economical yet safe and serviceable system. Until the development of tubular structures, buildings were designed as an arrangement of vertical columns and horizontal beams and girders spanning between the columns. Lateral loads were resisted by various connections, rigid or semi-rigid, supplemented where necessary by bracing and truss elements. Further improvement in the structural economy was achieved by engaging the exterior frame with the braced service core by tying the two systems together with outrigger and belt trusses. This was perhaps the beginning of tubular behavior since the engagement of exterior columns is similar to that of the tube structures.
The method of achieving the tubular behaviour by using closely spaced exterior columns connected by deep spandrel beams is the most used system because rectangular windows can be accommodated in the design.
One World Trade Center, New York City, USA. Designed using tube-frame structural system. Framed tube structure is defined as a three dimensional space structure composed of three, four, or possibly more frames, braced frames, or shear walls, joined at or near their edges to form a vertical tube-like structural system capable of resisting lateral forces in any direction by cantilevering from the foundation.
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The framed tube requires large columns and deep beams with welded connections for rigidity. When such frames are provided on all four faces of a building. One obtains a hollow tubular configuration. The use of fabricated framed tube elements where all welding can be performed in a controlled environment has made the framed tube more practical and efficient. The prefabricated elements are then erected by bolting at mid-span of the beams.
The World Trade Center towers used high-strength, loadbearing perimeter steel columns called Vierendeel trusses that were spaced closely together to form a strong, rigid wall structure, supporting virtually all lateral loads such as wind loads, and sharing the gravity load with the core columns.
One negative aspect of the frame tube design is a phenomenon commonly referred to as shear lag. Shear lag is essentially a nonlinear stress distribution across the flange and web sections of a beam. Design of the tube structure assume a linear distribution and shear lag results in corner columns experiencing greater stresses than central perimeter columns. This shear lag is a result of local deformation of beams which leads to a reduction of axial stress near the center of the flange. The distribution of these axial loads result in the corner perimeter columns becoming overstressed. Designer should be prepared for this increased stress in the corner columns and adjust the design as necessary. 3.1.6.2. Truss Tube The stiffness and the strength of the framed tube reside in the rigidity of the connections between closely spaced columns and spandrels that require welding connections at the joints. Even with its rigid connections the framed tube is still somewhat flexible. The frames parallel to the wind essentially act as multi-bay rigid frames with bending moment in the columns and beams becoming controlling factors in the design. As much as 75% of the total lateral sway results from racking of the frame as direct consequence of
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shear lag. Because of the racking of the frame, the columns at the corners of the building take more than their share of the load, while columns in between do less work than an ideal tube. One method of overcoming the problems resulting from the framed tube is to stiffen the exterior frames with diagonals or trusses. The resulting system is commonly known as trussed tube. This system creates problems in terms of window wall details because of the large number of joints between the diagonals resulting in odd shape windows and façade elements. 3.1.6.3. Bundled Tube A bundled tube structure is essentially a structure resulting from combining two or more independent tube structures designed to act as one. The idea of a bundled tube is that the individual tube can be terminated at any desired level, creating a variety of floor plans for a building. This becomes a distinct advantage because the tubes can be assembled in any configuration and terminated at any level without loss of structural integrity. This allows the architect to create multiple floor plans within the
same building. Of course the obvious disadvantage to the bundled tube concept is each individual tube needs to be completely framed as a tube, resulting in columns that invade the floor plans. The structural concept behind the bundled tube is that the interior columns from the individual tube act as internal webs of the cantilever structure this results in a substantial increase in shear stiffness over the other tubular designs with no lateral resisting interior frames or columns. Increases shear resistance results I a reduction in the shear lag effect. The decrease in shear lag improves torsion and bending behaviour of the structure. Interior frames of the individual tube provide this additional bending resistance. Because more frame elements are resisting lateral loads in both shear and bending. The column spacing of the bundled tubular building can be increased considerably over the perimeter column-to-column spacing of the framed tube. 3.1.7. Other Steel High Rise Buildings
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4. Heavy Duty Plants Structural steelwork for industrial use is characterized by its function, which is primarily concerned with the support, protection and operation of plant and equipment. In scale it ranges from simple support frameworks for single tanks, motors or similar equipment, to some of the largest integrated steel structures, for example complete electric power-generating facilities. Floors are provided to allow access to and around the installation, being arranged to suit particular operational features. They are therefore unlikely to be constructed at constant vertical spacing or to be laid out on plan in any regular repetitive pattern. In industrial steelwork two factors must be taken into consideration. First, floors cannot automatically be assumed to provide a horizontal wind girder or diaphragm to distribute lateral loadings to vertical-braced or framed bays: openings, missing sections or changes in levels can each destroy this action. Secondly, column design is similarly hampered by the lack of frequent and closely spaced twodirectional lateral support commonly available in normal multi-storey structures.
4.1.2. Connections
4.1. Anatomy of structure
Connections must be designed to suit both the member type and the design assumptions about the joints. For particularly deep beam members, where plate girders are several times deeper than the column dimensions, assumed pin or simple connections must be carefully detailed to prevent inadvertent moment capacity. If this care is not taken, significant moments can be introduced into column members even by notional simple connections due to the relative scale of the beam depth.
4.1.1. Main and secondary beams
4.1.3. Bracing, stiff walls or cores
Since large plant items normally impose a line or point loading there are clear advantages in placing main or secondary beams directly below plant support positions. Brackets, plinths or bearings may be fitted directly to steelwork, and for major items of plant this is preferable to allowing the plant to sit on a concrete or steel floor. Where plant or machinery requires a local floor zone around its perimeter for access or servicing, it is common practice to leave out the flooring below the plant for access or because the plant protrudes below the support level.
Horizontal loads can be satisfactorily transferred to braced bays or other vertical stiff elements. Basic means of transferring horizontal loading to foundation level must be decided where discrete braced bays or stiff walls or cores are being utilized, then initially a geometric apportionment of the total loading should be made by an imaginary division of the structure on plan into sections that terminate centrally between the vertically stiff structural element. The loadings thus obtained are used to design each stiff element.
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4.1.4. Loading 4.1.4.1. Process plant and equipment Particular examples of structural requirements which are not easily recognized are as follows: The physical space requirements of bracing members and the fact that they cannot be moved locally to give clearances. The actual size of finished steelwork taking account of splice plates, bolt heads and fittings projecting from the section sizes noted on drawings. The fact that a steel structure is not 100% stiff and that all loads cause deflections. The fact that a steel structure may interact with a dynamic loading and that dynamic overload multipliers calculated or allowed on the assumption of a fully rigid or infinite mass support are not always appropriate. 4.1.4.2. Lateral loadings from plant Lateral loadings imposed by plant on the structure derive from three sources. These are considered separately although there is a common theme throughout that the operating process undergoes a change in regime which is the cause of loading. Temperature-induced restraints Restraint against rotational or (more rarely) linear motion Restraint against hydraulic or gaseous pressures. 4.1.4.3. Wind loadings Wind loadings on fully enclosed industrial structures do not differ from wind loadings on conventional structures. The only special consideration that must be given applies to the assessment of pressure or force coefficients on irregular or unusual shaped buildings. 4.1.4.4. Blast loadings Varying requirements exist for blast loadings. Typical examples are transformers, where the requirement is usually to deflect any blast away from other vulnerable pieces of plant but where frequently one or more walls and the roof are open, serving to dissipate much of the energy discharged dust or fine particle enclosures, which are often wholly inside enclosed buildings. 4.1.4.5. Thermal effects The key to avoiding damage or problems from thermal movements is to consider carefully the detailing of vulnerable finishes (for example, brickwork, blockwork, concrete floors, large
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glazing areas and similar rigid or brittle materials). Provided that conventional guidance is followed in the movement provisions for these materials, then structural joints in steel frames can usually be avoided unless there is particularly severe restraint between foundations and low-level steelwork.
5. Masts and Towers Masts and towers are, typically, tall structures designed to support antennas (also known as aerials) for telecommunications and broadcasting, including television. They are among the tallest man-made structures. Similar structures include electricity pylons and towers for wind turbines. Consideration of a masted solution arises from the need to provide a greater flexibility in the plan or layout of the building coupled with its aesthetic value to the project as a whole. At the same time it offers the opportunity to utilize structural materials in their most economic and effective tensile condition. The towers or masts can also provide high-level access for maintenance and plant support for services. The plan form resulting from a mast structure eliminates the need for either internal support or a deeper structure to accommodate the clear span. By providing span assistance via suspension systems the overall structural depth is minimized giving a reduction in the clad area of the building perimeter.
The concentration of structural loads to the mast or towers can also benefit substructure particularly in poor ground conditions where it is cost effective to limit the extent of substructures. However, differential settlement can have a significant effect on the structure by relaxing ties on suspension systems. The consequent load redistribution must be considered. Most tension structure building forms consist of either central support or perimeter support, or a mixture of the two. Any other solutions are invariably a variation on a theme. Plan form tends to be either linear or a series of repetitive squares. In all cases it is advantageous but not essential that forces are balanced about the mast. Out-of-balance loads will obviously generate variable horizontal and vertical forces, which require resolution in the assessment of suitable structural sections. These forces require careful consideration, often necessitating additional restraints in the roof plane in either the open sections or top chord of any truss.
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Suspension ties must be designed to resist not only tension but also the effects of vibration, ice build-up and catenary sag. Ties induce additional compressive forces in the members they assist. Longitudinal stability is created either by twinning the masts and creating a vertical truss or by cross bracing preferably to ground. If outriggers are used, as is the case with the majority of masted structures, then the lateral stability of the outrigger can be resolved in a similar form to the masts by a stiff truss or Vierendeel section in plan or by plan or diagonal bracing at the extremity.
6. Long Span Structures The development of these buildings has closely followed technological progress and really started in the nineteenth century with the advent of the railways, which generated the need for long span enclosures at a time when the technology of cast iron structures was sufficiently advanced to be able to provide them. Progress from cast iron to wrought iron, then steel in quick succession provided the means to build longer and larger structures and created a new architectural vocabulary. The 1850s in Britain saw the construction of the Crystal Palace by Paxton and Paddington Station by Brunel. These were two fine buildings of different use which exemplified the spirit of a new age of architectural engineering; two vast sheds of lightweight construction which were functional, economical and which expressed a simplicity of form and clarity of structure. Large single volume sheds can be conveniently classified according to their structural forms. For simplicity
6.1.Classification of Long Span Structures 6.1.1. Beam structures Beam structures, consisting of beams supported on columns, are of common usage both in single span and multi-span form.
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A major advantage is that they lend themselves to prefabrication in elements for ease of transportation and site erection. This form of flat beam structure is most appropriately used where the minimum volume of space is required within the desired clear height. The achievable span is then directly related to the depth of the beams which, for normal loadings, would require a depth to span ratio of approximately 1:15 for solid web steel beams.
increased with the introduction of new, trussed beam solutions were more favoured. There has been an increasing demand for longer column free spaces to provide maximum flexibility of the interior space. The need for column free space may require much longer spans. Modified truss forms can provide improved design solutions. In some instances the use of triangular beams may increase the efficiency of the longer span structures. Additional advantages are that potential buckling due to compressive forces can be shared between two (usually top) booms, and spans for cladding support are reduced. 6.1.2. Portals and Arches
Beams may be solid web or truss forms, offering a variety of opportunities for architectural design. Although solid web beam structures have the depth to span advantage they have a high selfweight and they often lose to the trussed solutions in terms of services co-ordination where the open webs provide valuable routes for the services. Later, as the level of building services
Arches, which can take a variety of forms, are efficient structures for long span roofs. For the large single clear span, the need for internal columns can be eliminated by the use of arched vaults or portals. Difficulty in terms of loading calculations and structure erection in heavy fixed arch were simplified in later arched roofs with the introduction of 'pinned joints'. There are many other alternative solutions available, of which the steel portal frame is perhaps the most common. Portal frame structures, where standard beam sections are bolted together with a range of joint types, are
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commonly used for routine applications requiring a long span single storey building. Different spans and clear heights are achieved from a set of standard components with the advantage of speed and economy. Despite their simplicity and efficiency, providing good workable enclosures for manufacturing, storage and a whole range of other activities.
6.1.3. Masted structures
Masted structures have been used in bridge building and tented structures for decades but only more recently as a means of providing lightweight structures for general use. This solution
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was derived from the need for extremely high levels of mechanical services structure required in the production process. 6.1.4. Space frames Space structures are three-dimensional assemblies of linear members in which the interconnections are such that a load at any point is distributed in all directions throughout the assembly. With the loads spread, member sizes can be decreased which makes them an efficient and economic solution to long spans. They can take the form of flat double-layer grid structures or braced domes and vaults.
6.1.5. Umbrella structures The final option for consideration is the umbrella or tree structure in which the roof cantilevers from a central column and can be repeated and joined to other similar assemblies at each or any side to form a continuous structure.
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6.2. Claddings The enclosures to the large sheds require particular attention to achieve performance and economy. Large uninterrupted areas of wall cladding demand clear simple statements and precise detailing. Profiled sheeting is an obvious choice of material for cladding large areas because of the size of sheets which are manufactured, the weathering qualities and the cost. Used vertically or horizontally, the depth of profile is chosen to suit the required span and the shape to create the preferred visual effect. The choice has increased dramatically since the early days of corrugated iron to offer intricate steel profiles which catch the light on the one hand and deep troughs which can span up to 12m on the other. Profiled sheeting can be used in a range of ways to create different aspects and panel systems can also be used to form the enclosure. Other popular options tend to be panel based systems of which there are many available. These range from rain screen systems to bonded panels, and from smooth skins to rough textures. It is up to the designer to choose the most appropriate cladding to suit the particular requirements.
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7. References Websites http://dspace.mit.edu/handle/1721.1/34634#files-area http://www.tatasteelconstruction.com/en/reference/teaching_resources/architectural_studio_reference/design/lo ng_span_structures/ http://www.campbellreith.com/projects/heathrow-worldwide-distribution-centre/ http://www.steelconstruction.info/Energy_from_waste_facility,_La_Collette,_Jersey http://www.campbellreith.com/projects/jersey-efw/ http://www.steelconstruction.info/The_case_for_steel#Fire_performance http://amsections.arcelormittal.com/fileadmin/redaction/4-Library/1Sales_programme_Brochures/Sales_programme/ArcelorMittal_EN_FR_DE.pdf
http://en.wikipedia.org/wiki/List_of_spans http://www.skyscrapercenter.com/List/Tallest-100-Buildings http://en.wikipedia.org/wiki/Cable-stayed_bridge http://en.wikipedia.org/wiki/Radio_masts_and_towers http://en.wikipedia.org/wiki/List_of_longest_suspension_bridge_spans http://en.wikipedia.org/wiki/List_of_longest_arch_bridge_spans http://en.wikipedia.org/wiki/Suspension_bridge#Longest_spans http://en.wikipedia.org/wiki/List_of_largest_cable-stayed_bridges http://www.tatasteelconstruction.com/file_source/StaticFiles/section_plates_publications/sections_pub lications/Advance%20to%20BS5950%20Sept%2013.pdf http://amsections.arcelormittal.com/fileadmin/redaction/4-Library/1Sales_programme_Brochures/American_Structural_Shapes.pdf http://www.burohappold.com/ http://www.sals.org.cn/teaching/zhaosir/StructuralSteel-6.pdf http://workgroups.clemson.edu/AAH0503_ANIMATED_ARCH/M.Arch%20Studio%20Documents/Designin g%20for%20Long%20Spans-2.pdf Books Jason A. Cook, Structural Steel Framing Options for Mid- and High Rise Buildings, Massachusetts Institute of Technology. Buick Davison & Graham W. Owens, Steel Designer’s manual 6th Edition, Blackwell Publishing.
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