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Multi storey frames

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Multi storey frames

  1. 1. Multi-storey Frames Lalith Kumar Rishav Raj Anmol Chugh Abhigya Sharan
  2. 2. WHAT ARE STEEL STRUCTURES • A structure which is made from organised combination of structural STEEL members designed to carry loads and provide adequate rigidity • Steel structures involve a sub- structure or members in a building made from structural steel.
  3. 3. WHERE STEEL FRAME STRUCTURES ARE USED Steel construction is most often used in - • High rise buildings because of its strength, low weight, and speed of construction • Industrial buildings because of its ability to create large span spaces at low cost • Warehouse buildings for the same reason • Residential buildings in a technique called light gauge steel construction • Temporary Structures as these are quick to set up and remove
  4. 4. ADVANTAGES OF STEEL STRUCTURES • Steel structures have the following advantages: They are super-quick to build at site, as a lot of work can be pre-fabbed at the factory. • They are flexible, which makes them very good at resisting dynamic (changing) forces such as wind or earthquake forces. • A wide range of ready-made structural sections are available, such as I, C, and angle sections • They can be made to take any kind of shape, and clad with any type of material • A wide range of joining methods is available, such as bolting, welding, and riveting
  5. 5. DISADVANTAGES OF STEEL STRUCTURES Steel structures have the following disadvantages: • They lose strength at high temperatures, and are susceptible to fire. • They are prone to corrosion in humid or marine environments. • Heavy and lengthy, not easy to handle • Deflect under loads
  6. 6. DIFFERENT TYPES OF STRUCTURAL FRAMES
  7. 7. BEAM AND COLUMN CONSTRUCTION FRAME • This is often called as “skeleton construction”. The floor slabs, partitions, exterior walls etc. are all supported by a framework of steel beams and columns. This type of skeleton structure can be erected easily leading to very tall buildings. • Generally columns used in the framework are hot-rolled I-sections or concrete encased steel columns. They give unobstructed access for beam connections through either the flange or the web. Where the loading requirements exceed the capacity of available section, additional plates are welded to the section
  8. 8. LONG SPAN BEAMS • The layout of floor beams in buildings depends largely on the spacing of the columns. The columns along the perimeter of the building are generally spaced at 5 to 8 m in order to support the façade elements. In most buildings, the secondary beams are designed to span the longer distance in the floor grid, so that the bending moment they resist is similar to that of the primary beams and therefore they can be of the same depth as the primary beams. • In many buildings, designing longer internal spans creates more flexible space planning. A variety of structural steel systems may be used to provide either long-span primary beams or secondary beams. These long-span systems generally use the principles of composite construction to increase their stiffness and strength, and often provide for integration of services within their depth via openings in the webs of the beams.
  9. 9. TRUSSES • Trusses and lattice girders are used in long span roofing and flooring systems. The term ‘truss’ is generally applied to roofs, which may be pitched, whereas lattice girders are generally used as long-span floor beams which are more heavily loaded and not pitched. • Trusses and lattice girders are often designed to be visible and therefore the choice of the members used and their connections is important to the design solution. • Trusses and lattice girders are triangular or rectangular assemblies of tension and compression elements. The top and bottom chords provide the compression and tension resistance to overall bending, and the inclined bracing elements resist the shear forces.
  10. 10. SPACE FRAMES • A ‘space’ frame is a form of construction that covers large areas using assemblies of small structural components that are connected at pre-formed nodes. They are three-dimensional assemblies that generally consist of tension and compression elements, connected by inclined bracing. Circular hollow sections (CHS) are generally used in space frames as their wall thickness can be varied to suit the forces in the members while maintaining a constant outside diameter. There are three generic forms of support to space frames that determine the forces to which they are subject: • Point support by columns at four or more positions • Multiple supports by rows of columns or ‘column trees’. • Continuous edge support. • An example of the multiple point supports to a double layer space frame over a pedestrian street in Belfast’s Victoria Centre
  11. 11. SHEAR WALL FRAMES • The lateral loads are assumed to be concentrated at the floor levels. The rigid floors spread these forces to the columns or walls in the building. Lateral forces are particularly large in case of tall buildings or when seismic forces are considered. Specially designed, reinforced concrete walls parallel to the directions of load are used to resist a large part of the lateral loads caused by wind or earthquakes by acting as deep cantilever beams fixed at foundation. These elements are called as shear walls. • . The advantages of shear walls are (i) they are very rigid in their own plane and hence are effective in limiting deflections and (ii) they act as fire compartment walls • Generally reinforced concrete walls possess sufficient strength and stiffness to resist the lateral loading. Shear walls have lesser ductility and may not meet the energy required under severe earthquake. A typical framed structure braced with core wall is shown
  12. 12. FRAMED TUBE STRUCTURES • The framed tube is one of the most significant modern developments in high-rise structural form. The frames consist of closely spaced columns, 2 - 4 m between centres, joined by deep girders. The idea is to create a tube that will act like a continuous, perforated chimney or stack. • The lateral resistance of framed tube structures is provided by very stiff moment resisting frames that form a tube around the perimeter of the building.
  13. 13. TUBE IN TUBE FRAME • This is a type of framed tube consisting of an outer-framed tube together with an internal elevator and service core. • The inner tube may consist of braced frames. The outer and inner tubes act jointly in resisting both gravity and lateral loading in steel-framed buildings. However, the outer tube usually plays a dominant role because of its much greater structural depth. This type of structures is also called as Hull (Outer tube) and Core (Inner tube) structures.
  14. 14. BRACED FRAMES • The majority of structural systems used in office construction are braced by one of two methods; • Steel bracing, generally in the form of cross-flat plates or hollow sections that are located in the façade walls, or in internal separating walls, or around service areas and stairs. • Concrete or steel plated cores that enclose the stairs and lifts, service risers, toilets etc. • The choice of this system depends on the form and scale of the buildings. In most buildings up to 6 storeys high, steel bracing is preferred, although its location is strongly influenced by the layout of the building. V or K bracing using tubular sections is often preferred as it is more compact and can be arranged around windows and doors in some cases. X flat bracing is preferred for use in brickwork as it can be located in the cavity between the leaves of the brickwork. • For taller buildings, concrete cores are more efficient and they can either be constructed floor by floor using conventional formwork, or slip- formed continuously. The relative economics is dictated by speed of construction, and slip forming is often used on tall buildings. Steel plated or composite cores are also used where there is need to minimise the space occupied by the core and where it can be constructed in parallel with the steel framework. • The structural design of the steel frame is therefore based on the use of simple shear resisting connections for both the beam to column and beam to beam connections.
  15. 15. CONTINOUS FRAMES • Continuous frames achieve continuity of the beams either by design of the steel structure so that they are multi-span, or by use of moment- resisting connections. • In the Palestra building, the primary beams were arranged in pairs either side of the tubular columns, and the beams were continuous across the building, being spliced only at the quarter span positions from the internal columns where bending moment were low. In that way, the beams are stiffer due to their continuity than the equivalent simply supported beam and so that depth can be reduced. A view of the building during construction is shown. • In buildings up to four storeys in height, it may be economic to design the steel structure as a sway frame to resist lateral loads applied to the building. The connections between the beams and the columns are made moment-resisting by use of extended end plate connections. The columns may be heavier than in simply supported design, but the beams can be lighter, and bracing is eliminated. This may be advantageous in low-rise buildings with highly glazed facades.
  16. 16. LOADS TO BE CONSIDERED
  17. 17. GRAVITY LOADS • Dead loads due the weight of every element within the structure and live loads that are acting on the structure when in service constitute gravity loads. The dead loads are calculated from the member sizes and estimated material densities. Live loads prescribed by codes are empirical and conservative based on experience and accepted practice. • Reduction in imposed load may be made in designing columns, load bearing walls etc., if there is no specific load like plant or machinery on the floor. This is allowed to account for improbability of total loading being applied over larger areas. The supporting of the roof of the multi-storeyed building is designed for 100% of uniformly distributed load; further reductions of 10% for each successive floor down to a minimum of 50% of uniformly distributed load is done. The live load at floor level can be reduced in the design of beams, girders or trusses by 5% for each 50m2 area supported, subject to a maximum reduction of 25%. In case the reduced load of a lower floor is less than the reduced load of an upper floor, then the reduced load of the upper floor should be adopted in the lower floor also.
  18. 18. WIND LOADS • The wind loading is the most important factor that determines the design of tall buildings over 10 storeys, where storey height approximately lies between 2.7 – 3.0 m. Buildings of up to 10 storeys, designed for gravity loading can accommodate wind loading without any additional steel for lateral system. Usually, buildings taller than 10 storeys would generally require additional steel for lateral system. This is due to the fact that wind loading on a tall building acts over a very large building surface, with greater intensity at the greater heights and with a larger moment arm about the base.
  19. 19. SEISMIC LOADS • Seismic motion consists of horizontal and vertical ground motions, with the vertical motion usually having a much smaller magnitude. Further, factor of safety provided against gravity loads usually can accommodate additional forces due to vertical acceleration due to earthquakes. So, the horizontal motion of the ground causes the most significant effect on the structure by shaking the foundation back and forth. The mass of building resists this motion by setting up inertia forces throughout the structure.
  20. 20. INFERENCE Loading on tall buildings is different from low-rise buildings in many ways such as large accumulation of gravity loads on the floors from top to bottom, increased significance of wind loading and greater importance of dynamic effects. Thus, multi-storeyed structures need correct assessment of loads for safe and economical design. Excepting dead loads, the assessment of loads can not be done accurately. Live loads can be anticipated approximately from a combination of experience and the previous field observations. But, wind and earthquake loads are random in nature. It is difficult to predict them exactly. These are estimated based on probabilistic approach.

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