Structural Design or Engineering - Essay Example

Running head STRUCTURAL DESIGN STRUCTURAL DESIGN Structural Design Introduction Structural design entails the design and development of structures that aid in the support of building and resists the imposed loads on the building. Structural engineers play a major role in designing and appraising the architect design. They transform the architect work to a reality. Structural engineering is based on the engineers knowledge and understanding of the laws of physics as well as the empirical knowledge on structural performance of most building materials. Most of the complex structural systems are design using the basic structural elements and are either in tension or compression. Proper engineering design is required for all structural work as inferior designs have resulted to many catastrophic accidents such as the collapse of buildings, bridges underground tunnels, roofs and other structures. These collapse lead to heavy loses in terms of materials, finances and loss of lives. It is therefore important to consider all aspects of a building especially in terms of applied loads so as to design a sufficient structure to resist these forces. There are many design consideration that must be taken into account when designing the structural details of any building. After the structural design, the buildings are then constructed. There are many types of materials that are used in the building construction industry. Some of the most common construction material includes wood, steel/iron, masonry and concrete. Plastics are now gaining importance in building and construction especially for interiors design or supporting at points where minimum load resistance is required. Background study The main structural elements of any building are; The foundation: this structural element supports the whole building. It carries the whole load of the building and distributes the stress to soil or rock layer beneath it. The foundation must be well constructed in order to aid in support of whole structure. Most of the foundation elements are made using reinforced concrete, metal or Masonry. Concrete is highly preferred due to it durability and ability to withstand heavy compressive forces. The type of foundation is greatly determined by type of soil, the size of the building, ground water present and other special design considerations. Some of the common types of foundations include; Pile foundation Continuous or spread footing Foundation walls Matt foundation Different foundations have got different footings. The type, size and materials used for construction of the footing depend upon the type of soil and the load bearing capacity of that soil. The main types of footing commonly used include; Spot footing Continuous spread footing Grade beam footing The walls Most of the building walls are made of masonry, bricks, wood, glass and concrete. The walls are mostly supported by concrete. The floors The main structural elements used for most of the floors are the concrete. Reinforced concrete is used so as to impact strength on the floor. The steel bars support the concrete and prevent it from cracking. A layer of an impervious material is applied such as PVC in very wet soils to prevent the influx of water. The roofs The roofing consists of the structural support and the roof sheets or the roofing material. There are many different types of roofing. The type of roof is greatly influenced by the wind speed, the size of the building, the client’s specifications, aesthetic values and the architect design. Some of the most commonly used roofing styles include; Gable roof Cross gable Flat roof Mansard roof Hipped roof Cross hipped roof Pyramidal roof Shed roof Gambrel roof Salt box roof Materials and elements of building and design Concrete Concrete is a structural material composed of cement, cementituos materials, sand, aggregate, water and other chemical additives. All the materials that make up concrete are mixed with water after which solidification occurs. The solidification process is a hydration chemical process that results in the formation of a hard mass. In building and construction industry concrete is used more than any other material. Concrete has high compressive strength and low tensile strength. However large-scale application of concrete requires that they be reinforced. The reinforcement of the concrete is done using steel bars usually referred to as steel rebar structure. The form of this structure is a mesh, bars or spirals. The steel bars used are either twisted or have projection to increase the surface area and hence enable the concrete to transfer more stress to the steels bars. The advantages of reinforced concrete are derived from the fact that the concrete-steel bond transfers most of the stress to the steel which has high tensile and compressive strength. Also the steel bars are protected from environmental damage due to corrosion. Differential expansion rate of both the concrete and steel can separate both layers. One major weakness of concrete is cracking. This is usually overcome by the use of fiber reinforced concrete or the use of larger metal reinforcement techniques. Steel structures Steel is an alloy of carbon and iron. Carbon is used for the purposes of hardening steel. This is coupled with other heat treatment processes. Other chemical are also added to improve the structural qualities of steel. The steel used for construction varies in size and cross-section area. Different crossectional have differing strengths. The most common structural steel profiles are; I-beam section: it is also referred to as the universal beam. It profile assumes “H” shape. It has very good structural properties and it is the most common and widely used beam. Hollow section: the have several shape and are hollow shape. They include the rectangular hollow section which is in the form of a rectangular tube, round hollow section which is tubular. Square hollow sections that are square and hollow and elliptical hollow section Angle sections: They are L shaped Channel shaped cross-section- they are channel shaped. T section; have a T-shaped cross section Rail way profile- railway rail, flanged T rail, grooved rail, Vignoles Bar; this is a solid metal that has a rectangular cross sectional. They are wide and long but they are not as wide as the sheets Rod: round or square. Plate: these are sheet metal with a thickness Structural elements There several structural elements used for the purposes of building and constructions. Columns: These are structural element that carries axial forces. There are also refereed to as beam columns. The most important aspect when designing the columns is the axial capacity and the buckling capacity. Columns tend to buckle when subjected to compressive forces. Beams: this is a structural element with one dimension being greater than the other two dimensions. Beams are only subjected to pure bending only. The applied load is perpendicular or normal to the main axis. Based on their support beams can be classified as; Cantilever beams: these are supported at one side Simply supported beams; there are supported at both ends. The support is vertical. At one end the beam is made in such a way that it can move horizontally to cater for thermal expansion. Continuously supported beams: they are supported at several points. Truss unit The truss unit is comprised of the struts and ties. Struts; this is a lightweight structure which is able to withstand compressive forces. Ties; These are slender units that make up the truss unit and are able to withstand tension forces. Arches Most of the arch units are made from concrete or masonry and they carry forces in compression only. Catenaries These structural units carry pure tension forces and transverse forces. Their strength is determined by their form Shells Their strength is derived from their form and they carry forces in compression. Some of the shell design is the dome shaped structure Design of office building This section stipulates the design aspects of the building under study. The whole structure is divided into five main sections. These are Foundation design The walls The roof Fire proofing methods. Other building specification The foundation design The ground conditions are specified as; 0 – 4 m fill 4.0 – 5.5 m sand and gravel Below 5.5 m stiff clay From the soil types outlined above, the building can only be constructed on the clay soils. Mat foundation and foundation friction piles can be used. The Mat foundation. Diagram 1 showing the Mat foundation. In the diagram above the foundation is made as a continuous bock such that the all the columns seem as if the float on the foundation. This kind of structure is used in very poor and loose soils. In this design the soil condition are poor, however the design limitation of the river flowing below the building limits the use of this foundation as there should be no structural loading on the culvert. The method is also expensive considering the size of the building Use of foundation pier Figure 2 showing the friction pier with a spread footing The friction piers are considered as the most viable solution in this design. Continuous spread footing This means that the foundation must be located on the clay soil. However clay expands and contracts on application of water. This result to cracking of structural elements located within it. Steel cannot be a viable solution due to corrosion and effects caused by water which will accelerate the collision. The design will involve excavating soil 15 meters below the ground level, the soil is the extracted. Sand and gravel are then be used to fill the excavated site, the sand and gravel must be mixed with lime. This will prevent the upward flow of clay to the upper layers. The soil surrounding the foundation is also treated with lime to improve clay properties. Two types of foundations can be used. These are the mat foundation and the pier foundation; the type of footing used is the spread footing that runs along the length of the buildings. Along section A-A, spot footing are used as the structural load cannot be applied on the culvert. The spread footing run perpendicular to section A-A Diagram 4 showing the foundation details The floor They are constructed using reinforced concrete. The basement floor is lined with PVC material to prevent it from soaking water thus weakening it. For the first, second and third floor the PVC layer is omitted as there is not water flow. The walls There are two main viable structural designs these are (1)The structural steel framework The building is constructed using steel frame work. The columns are set 2m meters between their centers. I beams or universal beam are joined to form a truss unit, this unit is then used as a columns to support the build. After setting the columns, a layer of concrete is the applied round the columns. In between the columns, layers of masonry can be constructed or glazing can be used to fill the space between the structures. (2)The use of reinforced concrete The columns are set to 0.6 m in length and 0.2 meters thick. The long side of the column is set so that it is aligned along the wall. Reinforced concrete is used to construct the column units. Steel rebar structure is first constructed. Then a wooden framework is build round the column. Concrete is then poured. On solidification the wooden framework is removed. Figure 5 showing the structural elements along section A-A The relative distance between the columns is Figure 6 showing the column spacing Materials and size of the columns The columns are placed two meters apart and run along the length of the building. The size of the columns is 0.6 meters in length and 0.3 meters wide. To prevent the columns from protruding in the house the longer part of the column is made to face the wall. The area between the columns is filled with masonry. For the second solution, the columns and the beams are constructed using steel structures that are coated with concrete. The second solution reinforced concrete is used. Steel bars are formed into rebar’s and concrete is poured to form the columns. The truss beams are made of H sections. Angle plates are used for the ties and struts. The plan view Figure 7 showing the plan view for the building The spread footing run along the building and is at a distance equal to 2 meters apart. At the light wheel the spread footing are at the two extreme ends and the truss units run in a direction parallel to section A-A. Fire proofing All the walls of the building are coated with vermiculite material. This material prevents the structural failure of concrete when exposed to the fire. For the steel bars used in construction, they are fire proofed by spraying a layer of heat insulation paint. The paint prevents thermo softening of the steel bars in cases of fire. Comparison of the two designs From the two designs it can be seen that the steel structure is more cumbersome to design and construct, furthermore the effects of fire cause thermo softening of the steel structures. Even though the steel members are coated with a fire proofing materials. They cannot withstand fire for a long period. The most viable solution is the use of reinforced concrete members for this design. The inherent advantages of using reinforced concrete include Concrete is resistant to fire and can withstand fire for a long time. Coupled with a suitable fire proofing material it can last for long periods. Without the steel rebar structure softening Concrete is more cheap It is easier to construct the house using concrete Concrete members withstand compressive forces. Better than the steel structures Roofing Truss units are used running along the length of the building. The truss unit structural details are as shown in the figure below. INCREASING THE FLOOR PLAN AREA BY REMOVING THE LIGHT WELL. By removing the light well and incorporating the floors, horizontal beam spanning across the building have to be added for each floor. Due to the length of the building at the light well section, it is necessary to provide extra support for floors, this is done using Arch shaped concrete structure can be incorporated as shown in the diagram below. SUPPORT FOR THE CONICAL STRUCTURE For the conical structure, two types of supports can be used for the support Using a 16 sided bracket This involves welding a 16 sided plate with bolt holes on all the sixteen sides. Universal beams are then attached to this unit. The beams can also be welded on all the sides. Shaping the beams to form an apex At the end of the beams, vertical straight beams are welded to the universal beam. The vertical plates can then be joined to form an apex for the conical structure. References Kosmatka, S.H.; Panarese, W.C. (1988). Design and Control of Concrete Mixtures. Skokie, IL, USA: Portland cements Association. Gibson, L.J. & Ashby, M.F. 1999. Cellular Solids: Structure and Properties; 2nd Edition (Paperback), Cambridge: Cambridge Uni. Press Oakes, C.; Leone, L.; Gunn, Craig J. (2001), Engineering Your Future, Great Lakes Press Read More