Advantages of Steel Construction
Steel structures are very reliable. The reasons for this reliability include consistency and uniformity in properties, better quality control because of factory manufacture, large elasticity, and ductility. If different specimens of some type of steel are tested in the laboratory for yield stress, ultimate strengths and elongations, the variation is much lesser then other materials like concrete and wood. Further, because of truly homogeneous and elastic material, steel satisfies most of the assumptions involved in the derivation of the analysis and design formulas and the results obtained and reliable. This may not be the case in concrete structures because of heterogeneous material, cracking and non-linearity of stress-strain relationship.
2. Industrial Behavior
Rolled steel sections are manufactured in factories. Also, the members may be cut and prepared for assembly in factories wile only joining of these components is carried out at the site by installing rivets or bolts and by welding different components. Sometimes parts of the structure are also assembled in the factories, that is, there is a great adaptation to prefabrication. Manual errors reduce greatly in such cases, the speed of construction increases and the total cost reduces.
3. Lesser Construction Time / Greater Erection Speed
Because of the industrial nature of steel construction. Progress of the work is fast making the structures economical. The reason is that these structures can be put to use earlier. The reduction in labor cost and overhead changes and the benefits obtained from the early use of the building contribute to the economy.
4. High Strength And Light Weight Nature
The high strength of steel per unit weight means that the dead loads will be smaller. It is to be noted that dead loads are a bigger part of the total loads on structure. When dead load reduces, the underneath members become still smaller due to less weight acting on them. This fact is of great importance for long-span bridges, tall building, and for structures having poor foundation conditions.
5. Uniformity, Durability And Performance
Steel is a very homogeneous and uniform material. Hence, it satisfies the basic assumptions of most of the analysis and design formulas. If properly maintained by painting, etc. the properties of steel do not change appreciably with time; whereas, the properties of concrete in a reinforced concrete structure are considerably modified with time. Hence, steel structures are more durable.
Steel behaves closer to design assumption than most of the other material because it follows Hooke’s law up to fairly high stresses. The stress produced remains proportional to the strain applied oft the stress-strain diagram remains a straight line. The steel sections do not crack or tear before ultimate load and hence the moments of inertia of a steel structure can be definitely calculated. The moments of inertia obtained for a reinforced concrete structure are rather indefinite.
7. Ductility And Warning Before Failure
The Property of a material by which it can withstand extensive deformation without failure under high tensile stresses is said to be it ductility. Mild steel is a very ductile material. The percentage elongation of a standard tension test specimen after fracture can be as high as 25 to 30%. This gives visible deflections of evidence of impending failure in case of overloads. The extra loads may be removed from the structure to prevent collapse. Even if collapse does occur, time is available for occupants to vacate the building.
In structural members under normal loads, high stress concentrations develop at various points. The ductile nature of the usual structural steel enables them to yield locally at those points, thus redistributing the stresses and preventing premature failures.
8. Additions To Existing Structures
Additions to existing steel structures are very easy to made. Connections between new and existing structures can be employed very effectively. New bays or even entire new wings can added to existing steel frame building, and steel brides may often be widened.
9. Possible Reuse
Steel sections cab be reused after a structure is disassembled.
10. Scrap Value
Steel has a scrap value even though it is not reusable in its existing form.
11. Water-Tight And Air-Tight Constructions
Steel structures provide completely impervious construction and structures like reservoirs, oil pipes, gas pipes, etc. are preferably made from structural steel.
12. Long Span Construction
High-rise buildings, long span bridges and tall transmission towers are made up of structural steel. Industrial buildings up to a span of 90.m can be designed by plate girders or trusses. Bridge spans up to 260.m are made with plate girders. For through truss bridges, Bridge spans of 300.m have been used.
13. Temporary Construction
For temporary structures, steel construction is always preferred. Army constructions during war are mostly made out of structural steel. The structures may be disassembled by opening few bolts, component parts are carried to new places are the structure is easily reassembled.
Disadvantages of Steel Construction
1. High Maintenance Costs And More Corrosion
Most steels are susceptible to corrosion when freely exposed to air and water and must therefore be periodically painted. This requires extra cost and special care. The use of weathering steels, in stable design applications, tends to eliminate this cost. If not properly maintained, steel members can loose 1 to 1.5 mm of their thickness each year. Accordingly such constructions can loose weight up to 35% during their specified life and can fail under the external loads.
2. Fireproofing Costs
Although steel members are incombustible, their strength is tremendously reduced at temperatures prevailing in fires. At about 400ºC, creep becomes much more pronounced. Creep is defined as plastic deformation under a constant load for a long period of time. This produces excessively large deflections/deformations of main members forcing the other members to higher stresses or even to collapse. Steel is an excellent conductor of heat and may transmit enough heat from a burning compartment of a building to start fire in other parts of the building to start fire in other parts of the building. Extra cost is required to properly fire proof the building.
3. Susceptibility To Buckling
The steel sections usually consist of a combination of thin plates. Further, the overall steel member dimensions are also smaller than reinforced concrete members. If these slender members are subjected to compression, there are greater chances of buckling. Buckling is a type of collapse of the members due to sudden large bending caused by a critical compressive load. Steel when used for columns is sometimes not very economical because considerable material has to be used merely to stiffen the columns against buckling.
4. Higher Initial Cost / Less Availability
In few countries, steel is not available in abundance and its initial cost is vary high compared with the other structural materials. This is the most significant factor that has resulted in the decline of steel structures in these countries.
For certain types of buildings, the steel form is architecturally preferred. However, for majority of residential and office buildings, steel structures without the use of false ceiling and cladding are considered to have poor aesthetic appearance. A considerable cost is to be spent on such structures to improve their appearance. Cladding is a covering of metal, plastic or timber put on the surface of a structural member to completely encase it. The cladding not only protects the member but also improves its appearance.