Lateral Stability in Multi-story Buildings - Assignment Example

Lateral stability in Multi Storey Buildings Introduction A multi storey building is affected by both horizontal loads and vertical loads. The main components of a basic multi storey building are the foundation, the framework and the floor slabs. The mentioned components can be described to be the main components of steel structures. They are depicted in the figure below; Source: (Hart, 1982) Source: (Hart, 1982) The foundation of the building is made of concrete that has been reinforced. The framework of the building is mainly for the provision of the load bearing resistance and also to support the floor slabs as well ass the cladding (Iyengar et al., 1992). The external loads get transmitted through the steel framework to the foundation. The steel framework is composed of the columns and the beams. The columns are the vertical elements whilst the beams are the horizontal elements (Owens, 1992). Horizontal loads and lateral stability The steel structure is mainly supposed to support the vertical loads. It however needs to have a system for lateral stability that will be used as a guard or a resistor to the horizontal forces. The most common horizontal forces are winds though earthquakes are also factored in some countries (Khan, 1972). Some of the most common stabilizing elements in place today are reinforced concrete, steel bracings and shear walls. The diagrams below indicate some of the common stabilizing elements. Steel bracings Source: (Hart, 1982) Source: (Hart, 1982) The diagrams above show obtaining of lateral stability using steel bracings. Source: (Hart, 1982) Concrete walls Bracing A multi-storey building that has been constructed without lateral bracings has been depicted in the figure below. When a building is connected with simple connections of the beams, there would be no resistance to lateral forces which leads to geometrical instability as in diagram b. This would lead to deflection of the structure as has been depicted in figure (b) (Eurocode). Source: (Hart, 1982) The bracing systems are basically used for resisting horizontal forces which are mainly wind loads and at times earthquakes. They assist in transferring these loads to the foundations. The action of the bracing systems can be best explained using the diagrams below (diagram A); when a particular horizontal load gets concentrated at point on the building’s frontage, the cladding elements transfer this load to the two adjacent floors. Source: (Hart, 1982) In diagram B above, the loads H are acting on the floors. These loads are distributed to the vertical elements that are strategically located on the structure as shown in the diagram above by means of the dotted lines. This process is made possible by the horizontal resisting elements that are in the floor. The elements that provide the vertical support are the vertical bracings whilst the elements that provide the horizontal support are the horizontal bracings. The elements are located on each floor as indicated. The combination of the horizontal elements and the vertical elements make up the bracing system that is hence responsible for transfer of the horizontal forces to the surroundings. Bracings may come in with differing arrangements such as single diagonal. Cross braced, inverted V shape, unsymmetrical, symmetrical and V shaped. In order to resist the lateral deflections, the simplest theoretical method that can be applied is through the application of a full bracing that is diagonal or an X bracing as depicted in the figure (c) below. This bracing is the best for buildings that are from twenty to sixty storeys high though they have a weak point in that they do not offer room for doors and windows. For the provision of room for incorporation of doors and windows, the most common and most effective form of bracing is the K bracing which is shown below: K Bracing Source: (Hart, 1982) Source: (Hart, 1982) When larger openings are required in the buildings, K bracings cannot be used and there comes in the knee bracing systems. This bracing is eccentric and is very efficient in dissipating energy during occurrence of lateral loads such as earthquake and wind loads. It does so by forming a plastic hinge in the beam at the point where there is an intersection of the bracing with the beam. Shear walls There is a general assumption that lateral loads are concentrated at the level of the floor. Rigid floors spread these forces to columns and walls in the buildings. In the case of tall buildings, the effect of wind and seismic forces make these forces quite large. The effect of these forces can be resisted by using reinforced concrete walls that are parallel to the directions of the concrete walls. The concrete walls reduce the wind loads by acting as cantilever beams that are deep and fixed to the foundations. These are the elements that are together referred to as shear walls. In frequent circumstances, buildings have interior walls that are around elevators, service wells and the stairs. These walls are also categorized as shear walls. The main advantage of shear walls is their rigidity in their own planes which makes them very effective in limitation of deflections. They are also advantageous in that they are used as fire compartment walls. In low rise buildings and medium rise buildings, the construction of the shear walls takes longer and with less precision as compared to steelwork. Walls made up of reinforced concrete have enough strength and also stiffness that is necessary in resisting horizontal lateral loads. Shear walls are less ductile and might not be sufficiently adequate in meeting the energy requirements in time of earthquakes that are severe. The diagram below shows a typical shear wall. Source: (Hart, 1982) Central service concrete core This style is used in buildings tat have a plan that is more compact and with a small length to width ratio. This type of a core is very effective in providing lateral stability to the building. In construction of a structure using this method, there is elimination of the vertical steel bracing so as to have a structural economy that is maximized. This core is used to house the stairways and the lifts and any other vertical services that may be needed in a building. The erection of the steelwork is simplified and sped up though quick slip form mode of concrete construction. The diagram below depicts the concrete core. Source: (Hart, 1982) A system which has a single core can be applied to those buildings with a square shape. When the length: width ratio is more than 1:0 and when the core is not centrally located, there needs to be an allowance in the design for the wind load’s torsion effect which happens about the tower’s vertical axis. The connections of the beam ends are simple and quite trouble-free to be erected and fabricated. When it is not possible to connect the beams to the core, it is necessary to have steel cleats that penetrate into the core. The beams are then bolted onto these cleats. Behavior under horizontal loads In common circumstances, moment resistive frames and concentric braced frames have been frequently used to resist horizontal loads. The concentric braced frames are mainly used in steel structures in both normal and seismic resistant cases. Vertical-cantilever trusses are shaped by diagonal bracing elements which have coincident centerlines. They help to resist the lateral forces using the axial forces in bracing elements. This leads to increased stiffness in the elastic range. In such structures as the ones described, the zones of dissipation are located in the diagonals responsible for tensile through an assumption there are buckled compression diagonals. “Moment resisting frames” rely on the fact that the frame can act as a semi rigid frame or a partially rigid frame whilst resisting the impact of the lateral loads. Owing to their flexibility, these frames experience drifts (huge horizontal deflections) in tall buildings and are therefore best in medium rise buildings not exceeding ten storeys. Source: (Hart, 1982) The repetition in buckling of the diagonal members leads to an unsatisfactory performance of the inelastic cyclic in the concentric bracings. The buckling produces reduction of the area of the hysteresis loops progressively. This corresponds to the decrease in the capacity of the structure for energy absorption and dissipation. This condition is illustrated below. Source: (Hart, 1982) Other Stability systems under Horizontal loads The structural system of a certain building must be wholly built in a manner that will maintain stability, strength and stiffness. In order to satisfy these conditions, there needs to selection of suitable members and connection methods as the building is required to resist horizontal loads. This is basically by the structure’s general form, connection methods or addition of bracing elements. The easiest form that can be used to resist wind is by the use of a tripod which is in three dimensions or a triangular form of a tent that is inclined. This is as indicated the figure below which are shapes for sloping structures of roofs: Source: (Eurocode, 1988) Where there are walls and vertical columns, there are three basic options namely; bracings, rigid jointed portals and vertical cantilevers (Hart et al., 1982). Maintaining of stability using these modes is as follows: - Vertical cantilevers are fixed about the foundations. The column is fixed on the base rigidly. The columns are jointed at the top using pins joints that share the applied loads. In such a case, the size of the foundation is bigger in order to limit the pressure of soil in order to limit the bending moment to the foundation. Source: (Eurocode, 1988) Rigid jointed portal frames connected to the beam of the roof as below: - Source: (Eurocode, 1988) Bracings that are pin jointed with compressive or tensile elements. When there are two tensile elements that are crossed, one tends to go into compression and becomes ineffective when there are wind loads. When there is a combination of vertical and horizontal loads, there is a tendency of a horizontal sway. Frames will also tend to sway by the sides when the vertical loading is eccentric. The diagram below shows diagonal bracings (Eurocode, 1988). Source: (Eurocode, 1988) There are additional structures that can be used to resist horizontal load reactions. Some of the reactions are bending, torsion and swaying. Some of the elements are cores and shear walls. In case of tall buildings some other elements that are required and are also used to provide lateral stability are fire escape stair ways and lifts. Shear walls are again incorporated into this design to add on the stability (Ballio and Mazzolani, 1983). References Ballio, G. and Mazzolani, F.M.: "Theory and Design of Steel Structures", Chapman & Hall, London, 1983. BRE The Designer's Guide to Wind Loading of Building Structures Part 1 Butterworths, 1985. Bridge Aerodynamics Conference, Institute of Civil Engineers, Thomas Telford, London, 1981Civil Engineer's Handbook, Butterworths, London, 1974. Eurocode Convention of Constructional Steelwork : "Recommendations For Steel Structures in Seismic Zones", ECCS, Publication 54, 1988 Eurocode 8 "Structures in Seismic Regions - Design", CEN (in preparation). Hart, F., Henn, W. and Sontag, H.: "Multi-Storey Building in Steel" (second edition), Collins, London, 1982. Iyengar, S. H., Baker, W. F. and Sinn, R.: "Multi-Storey Buildings", from Constructional Steel Design, Elsevier, London, 1992. Khan, Fazlur R. "Tall Building Plan, Design and Construction", Symp, Proc, Vanderbilt University, Civ Eng Program, Nashville, Tennessee, 1974. Owens, G. W., Steel Designers' Manual, Blackwell Scientific Publications, Oxford, 1992. . Read More