Details of structural members in concrete and steel buildings - Essay Example

Details of structural members in concrete and steel buildings The common materials used in the construction industry are concrete and steel. Each of these materials has cons and pros that make them fit for construction in this industry. From tests and studies conducted, the weaknesses attributed to these materials have been used to look for ways of improving them. Designs drawn by engineers have therefore focused on improvement of structural members in concrete and steel buildings. The structural members play key roles to the durability, safety and costs of structures. Introduction Concrete and steel have been used for many years in construction industry. Their demand in the industry has grown widely due to the properties they hold. Some properties that steel hold that makes it preferred for construction are; it is environmental friendly because it is recyclable and unlike wood it has no warping and twisting properties. Steel is not adversely affected by weather, easy to erect, strong and durable with low weight. Its main function is in the formation of a skeleton that holds the whole structure. The components for concrete, that is, cement, sand and aggregates are easily available and affordable worldwide making allowing preference for concrete structures. From research and studies, concrete has been realized to be brittle, strong in compression and weak in tension. From these properties, modalities of improving it have been designed. The most currently used modality is its reinforcement. Reinforced concrete therefore has been realized to withstand tension and compression leading to its durability and use in many structures. Literature review The study and test of steel for industrial construction has been done for many years. Its properties led to its popular use since the early 1990’s. Due to this demand, designs for steel structures have been implemented. The main factor that has driven the designs is earthquake disasters. Its design has evolved from stiffer approach to flexibility and ductility. During the 1970’s, proven formulas were used to design steel structures. Advanced technology through Computer Aided Design has improved designing work with building codes controlling the steel industry. Working stress design was the main factor for consideration in concrete members since 1960s. Strength design method was adopted by the International Building Code (IBC) in its 1956 edition. The concepts considered are strength needed inclusive of the load and strength of the design with an aim of achieving durable structures. The designs lead to increased costs in construction with reduced maintenance costs. Due to low maintenance costs, projects like highway infrastructure and industrial buildings make use of reinforced concrete. Maintenance costs are likely to be incurred due to misuse of the structures and poor workmanship that are unpredictable (Berman, Gary 8). Details of structural members in concrete and steel buildings The success of concrete and steel structures is pegged on durability, economical factors and strength of structures. These attributes are achieved through proper design of structural members to support the weight of the structure and bear both the dead and live loads and lateral loads exposed to it. Live loads refer to inhabitants in the structure and dead loads are items attached to the structure while lateral loads arise from wind and or earthquakes. The purpose of members in steel and concrete structures is to unite and support all loads transferred in the structure via the allocated load paths to foundation members. Soil and or rock where the structure’s foundation is laid finally support the load. Structural members work as one unit using the joints to transfer bending moments, shear and axial forces. Structural members discussed in this article are; beams and one-way slab, two-way slabs, columns, walls and foundations. One-Way Systems In this type of floor or roof system, all members run in one direction with a common flexural reinforcement. They are also referred to as flexural members because they are exposed to bending moments and shear forces. They are occasionally positioned horizontally apart from the roof level where they are sloped. The sloping is due to the drainage factor considered in the roof of the structure. Transfer of loads to the beams is perpendicular to the slab. The loads are then transferred to girders then the columns and finally to the footing which release them to the soil. The figure below shows the one-way slab system (6). Two-Way Slab From the name, this type of floor or roofing system transfers loads in two directions. Flexural reinforcement is placed in the two directions. The load is transferred in the column-line beams that deliver it to the columns. Columns This structural member is responsible for supporting axial loads that come from the roof and members in the floor and delivers them to the floor of the structure. They are vertically positioned unless an orientation is provided. They are also exposed to bending moments that result from lateral and gravity loads. Walls This member is vertically positioned, separating spaces in a structure. They fall in both categories, that is, as load-bearing and non load-bearing. They are referred to as non load-bearing when they support their own weight and as load-bearing when they support loads arising from the floor as well as the roof. Shear walls are used to resist wind and earthquake effects and auxiliary gravity loads. Other walls like Basement or foundations walls oppose gravity loads and lateral pressure that act perpendicularly to the face of the wall. The figure below is a reinforced concrete foundation wall. It resists axial loads that result from reinforced concrete wall and lateral pressure from the soil. Foundations The purpose of the foundation is the transfer of loads from the building to the soil or rock beneath. They are of two types; shallow and deep foundations. Shallow foundations include footings and mats. For footings, loads from the structure are spread evenly to the stress in the soil because it is not greater than its bearing capacity. Mat foundation is constructed below the pillar walls. Its design does not allow the maximum soil pressure to exceed the soil bearing capacity. The deep foundations like piles and drilled piers support the columns and walls of the structure. They are installed on a firm base in the soil beyond the weak strata. The firm base enables the foundations to support loads. For steel structures, there are primary and secondary members. Primary members are beams, columns, trusses and girders. When erected, they are the skeleton of the structure that bears loads exposed to the building. The secondary members include bracers, stairs and the deckers. They are designed to bear specific loads. Bracers provide additional support in load area to reduce bending moments at a joint. The joints or connections transfer loads among the members. Steel structures are commonly used in industrial constructions, suspension bridges, go-downs, skyscrapers and other big structures. They are highly preferred to these structures because of the functions allocated to them. Structural structures are sensitive to hostile weather and face the risk of rusting. Reinforced concrete is mainly used in structures like bridges, hospitals, high rise buildings and residential buildings. Despite its popular use, it has deteriorated in the 20th century. The reason behind deterioration is corrosion of reinforcement bars. This corrosion is further divided into two categories, that is, corrosion resulting from carbonation and that from chloride ions. Deterioration of concrete is visible through cracks, stains of rust and spalls. Before any remedy is sought, it is wise that the cause for these deteriorations be determined. Though it is durable, this property is likely to be affected by poor workmanship, use of low quality materials and poor designs and lack of maintenance. These weaknesses enhances carbon dioxide penetrate the concrete. Exposure of the reinforcement bars promotes its rusting and final deterioration. The surface of concrete is exposed to damage by frost and leaching. Construction of steel and concrete buildings The main steps involved in construction of steel structures are; engineering, detailing, fabrication and erection. Structural engineering refers to the combination of science and mathematical calculations in designing a structure. The design must comply with the American Institute for Steel Construction (AISC). The design drawings are then converted into shop drawings. This is also called detailing. This step assists the fabricator in singling out every component of the structure as per the specifications. The size, shape and material of the framework is considered. Fabrication follows to produce the steel pieces according to the detail drawings. This step involves core activities that will eventually produce the desired product. The activities are systematic. They include drilling, grinding, punching, bending, burning and welding. A steel inventory guides the fabricators to produce the required design according to the templates moulded. Fitters join the pieces together to form shipping pieces. The shipping pieces are transported to site, unloaded, sorted and erected. The final step is erection of the steel pieces. During erection, the shipping pieces are joined together on site. This step is fully dependent on accuracy of the fabricated pieces of steel. Columns are first erected, followed by trusses, girders, beams, bracings, stairs and finally, any other pieces of steel. Columns are held in position by anchor bolts found in the foundation of the structure made of concrete. The steel pieces are placed in their respective positions and held together by use of bolts. Decking, welding of the deck, installation of studs and stairs follows. This process is referred to as buttoning-up of the structure (Berman, Gary). Construction procedures followed in construction of concrete buildings are; erection of formwork, placement of reinforcement, placement of concrete, formation of strip forms and finally provision of reshores. These activities are repeated for each floor until completion of the structure. Formwork determines the quality of the structure. Its holds fresh concrete until it sets, taking the required shape of the structure. The processes involved are; assembly of formwork elements and shoring to provide supports for fresh concrete and loads. The next step is installation of reinforcement. This is done under the directives of placing drawings. Columns are made of longitudinal bars with reinforcement in form of ties. All reinforcement in beams, slabs and other members are constructed and put in the formwork. Concrete is then mixed, transported and placed in the formwork. The class of concrete will depend on the specifications given. Compaction is then done manually or by mechanical vibrators to eradicate honey combs that will weaken the structure. Finishing follows thereafter depending on texture required. Curing follows with immediate respect to ensure that cement hydrates and gains the required strength. Slump cone test is done during construction to check on workability of concrete. Cubes of 150mm by 150mm by 150mm are set aside to check the strength of concrete after seven and twenty eight days consecutively. Curing will depend on weather conditions that the concrete is exposed to. When the concrete has set, the formwork is stripped. It is best done when concrete has achieved at least 70% of its compressive strength. Strength and weakness of steel and reinforced concrete buildings A close look at the collapse of World Trade Centre can give a clear picture in comparison between steel and reinforced concrete structure. WTC was made of steel structures that were light in weight and concrete covering to the joists. The impact created by the airplane weakened its columns. It induced fire that caused distortion and loss of strength of the steel structure and buckling of the structure. The fire caused weakness in joints and collapse of the columns. The weakness was created by the fact the design of the Gerber beams connection was far away from the columns hence there was no shear connection through the columns. After the collapse, the steel was recycled (JOM 8-11 and Structure 2008). From this case study, advantages and disadvantages of steel structures are evident. The advantages are; steel structures have high strength, provide uniformity in structure, it is ductile and tough. Additions are easily made to the existing structure. Some of the disadvantages are; high maintenance costs because they easily rust, loss of strength when exposed to high temperatures increasing costs incurred in fire proofing, it buckles easily with increase in length and decrease in thickness ( University of Maryland 2002). Reinforced concrete is used in construction of hospitals, bridges and residential houses. The reasons behind this preference is that concrete is affordable and easy to work with. Other attributes related to reinforced concrete structures are simplicity in distribution of services below the flat soffits, ability to resist fire, good performance under vibration and adaptability features. One main characteristic of concrete is its ability of high compressive strength and minimal tension resistance. This makes concrete to crack when exposed to an extensive load. The tensile force is increased through reinforcement because the materials, that is, concrete and reinforcement bars have a strong bond. These bonds refer to; one, lack of movements of bars and concrete around it and close thermal expansion coefficients. Concrete’s long period of curing enables it gain the required strength. In comparison to steel structures, they are cheap. Its large dead mass enables it produce stable structures. Conclusion In general, the members of concrete and steel structures are almost the same. The difference arises in the designs of the structures and materials used in their construction. For reinforced concrete structures, reinforcement bars, cement, aggregate and sand are required. These components are easily available and affordable hence the wide use of concrete. Steel structures need steel as the main material. Steel is modified to meet various standards making it expensive. Its design requires a lot of expertize, heavy machinery and labour during erection. However, they remain to be more durable unless exposed to hostile conditions. Many studies and tests have improved these structures to enable their use in most structures. Works Cited Assakkaf, Ibrahim. Introduction to Structural Steel Design, Third Edition, University of Maryland, USA. Department of Civil and Environmental Engineering, 2002. Berman, Gary, Structural Steel Design and Construction. North America, Greyhawk. Eagar, Thomas and Musso Christopher, Why Did the World Trade Center Collapse? Science, Engineering, and Speculation. JOM, 53(12), 2001. Print. LaMalva, Kevin, Barnett, Jonathan and Dusenberry, Donald, World Trade Center 5 Failure Analysis, Structure. 2008. Print. Read More